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Dyes and Pigments 113 (2015) 239e250

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Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

Photochromic bi-naphthopyrans

Stuart Aiken a, Christopher D. Gabbutt a, B. Mark Heron a, *, Suresh B. Kolla b

a Department of Chemical Sciences, School of Applied Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UKb School of Chemistry, The University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

a r t i c l e i n f o

Article history:Received 16 July 2014Received in revised form19 August 2014Accepted 20 August 2014Available online 28 August 2014

Keywords:SynthesisPhotochromismNaphthopyranWeinreb amideBi-naphthopyranFunctional dye

* Corresponding author. Tel.: þ44 (0)1484 471728.E-mail addresses: m.heron@hud.ac.uk, b

(B.M. Heron).

http://dx.doi.org/10.1016/j.dyepig.2014.08.0180143-7208/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

A series of novel 3-aryl-(3,3-diaryl-3H-naphtho[2,1-b]pyran-8-yl)-3H-naphtho[2,1-b]pyrans has beenaccessed from 6-bromo-2-naphthol via a four step transformation. Acylation of the dianion derived fromthe treatment of 6-bromo-2-naphthol with n-butyllithium with Weinreb amides and subsequent reac-tion with a 1,1,-diarylprop-2-yn-1-ol gave 8-aroyl-3H-naphtho[2,1-b]pyrans in good yield. Addition oflithium trimethylsilylacetylide to the foregoing 8-aroylnaphthopyrans proceeded smoothly with base-mediated desilyation to afford the target bi-naphthopyrans upon acid-catalysed reaction with 2-naphthol. Preliminary evaluation of the photochromic response of the new bi-naphthopyrans revealedreversible independent naphthopyran ring-opening leading to a complex photochromic signature.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

There has been sustained interest in the design and synthesis ofphotochromic naphthopyrans for commercial application inophthalmic lenses for at least the last 20 years [1]. The synthesisand photochromic response (in a variety of solvents and polymersand at differing temperatures) of an increasing number of morestructurally complex naphthopyrans has appeared in the scientificliterature over the last few years [2e5]. Amongst these examplesare systems in which two 3H-naphtho[2,1-b]pyran units are linkedtogether through one of the C-3 substituents e.g. 1 [6], 2 [7], 3 [8]and 4 [9] (Fig. 1) to afford bi-naphthopyrans.

Two examples of symmetrical bi-naphthopyrans linked throughthe 8,80-positions (5 and 6) have been described in the literature todate (Fig. 2). In 5 competition between fluorescence and photo-chromismwas observed which was dependent upon the number ofthienyl units [10]. The latter weakly photochromic example, 6, wasaccessed by an electrochemical dimerisation [11].

Attracting somewhat lesser attention, perhaps as a consequenceof their relative inaccessibility, are bi-naphthopyrans in which thetwo naphthopyran units bear different geminal aryl groups andthus provide the opportunity to develop different coloured

_mark_heron@hotmail.co.uk

photomerocyanines upon irradiation. Furthermore, as far as we areable to ascertain there appear to be no examples of bi-naphthopyrans in which one naphthopyran unit serves as the ‘3-aryl substituent’ on a second naphthopyran unit.

Previous studies of substituent effects on the photochromism of3H-naphtho[2,1-b]pyrans have revealed that the presence of amethoxy group at the 8-position leads to a bathochromic shift inabsorption maximum of the photomerocyanine coupled withenhanced intensity [3,4]. Given this interesting photochromicresponse of 8-substituted naphtho[2,1-b]pyrans we were interestedinexamining the influenceon thephotochromicpropertiesof linkingtwo naphthopyran units together involving the 8-position of onenaphthopyran and the para-position of one of the geminal arylgroups of the second naphthopyran. This study predominantly de-scribes our investigation of the synthesis and a preliminary survey ofabsorption properties of a series of novel linked naphthopyranswherein the typical C-2 aryl substituentof a 3H-naphtho[2,1-b]pyranis replaced bya 3,3-diaryl-3H-naphtho[2,1-b]pyran-8-yl unit leadingto a molecule that contains two different photoactive pyran rings.

2. Experimental

2.1. Equipment

Unless otherwise stated, reagents were used as supplied. NMRspectrawere recorded on a Bruker Avance 400MHz instrument (1HNMR 400 MHz, 13C NMR 100 MHz) for sample solutions in CDCl3

Fig. 1. Selected examples of bi-naphthopyrans.

Fig. 2. Symmetrical 8,80-bi-naphthopyrans.

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250240

with tetramethylsilane as an internal reference. FT-IR spectra wererecorded on a Perkin Elmer Spectrum One spectrophotometersystem equipped with a golden gate ATR attachment (neat sample).UVevisible spectra were recorded for spectroscopic grade CH2Cl2solutions of the samples (4 min activation with UV irradiation,10 mm pathlength quartz fluorescence cuvette, PTFE capped, ca.1 � 10�4 e 10�6 moldm�3) using a Cary 50 Probe spectrophotom-eter equipped with a single cell Peltier temperature controlled (10and 22 �C) stirred cell attachment with activating irradiation pro-vided by a Spectroline 8 W lamp. All compounds were homoge-neous by TLC (Merck TLC Aluminium sheets, silica gel 60 F254) usinga range of eluent systems of differing polarity. Mass spectra wererecorded independently at the National EPSRC Mass SpectrometryService Centre, Swansea. 1-Phenyl-1-(4-pyrrolidinophenyl)prop-2-yn-1-ol 11a [12] 1-(2-fluorophenyl)-1-(4-pyrrolidinophenyl)prop-2-yn-1-ol 11b [12] and 1-(4-fluorophenyl)-1-(4-phenyl)prop-2-yn-1-ol 11c [6] were prepared according to our published procedures.

2.2. Preparation of Weinreb amides 7a, b

4-Fluorobenzoyl chloride (15 g, 94.6 mmol) and N,O-dime-thylhydroxylamine hydrochloride (10.1 g, 104 mmol) were

dissolved in CHCl3 (200 mL) and stirred at room temperature. Thesolution was cooled to 0 �C and pyridine (17.3 mL, 230 mmol) wasadded. The mixture was warmed and stirred at room temperaturefor 1 h and then poured into aq. sat. NaCl solution (300 mL). Theorganic layer was separated and the aqueous layer extracted withCH2Cl2 (3 � 100 mL). The combined organic layers were washedwith water (3 � 50 mL), dried (anhyd. Na2SO4) and then evapo-rated. The crude 4-fluoro-N-methoxy-N-methylbenzamide 7a waspurified via distillation under vacuum to afford a colourless liquid(96%), bp ¼ 120 �C at 0.3 mmHg (lit. b.p. 70 �C at 0.1 mmHg [13]);nmax 583, 905, 918, 1262, 1375, 1508, 1582, 1630, 2972, 3274 cm�1;dH 3.34 (3H, s, CH3), 3.52 (3H, s, OCH3), 7.08 (2H, m, AreH), 7.73 (2H,m, AreH).

2.2.1. 2,4-diflouro-N-methoxy-N-methylbenzamide 7bFrom 2,4-difluorobenzoyl chloride using an identical protocol

as for amide 7a, as a colourless liquid after vacuum distillationin 89% yield, bp ¼ 125 �C at 0.3 mmHg; nmax 489, 583, 679, 852,967, 984, 1099, 1207, 1613, 2939 cm�1; dH 3.35 (3H, s, CH3), 3.56(3H, bs, OMe), 6.91 (2H, m, AreH), 7.51 (1H, m, AreH). Key 1HNMR signals were in agreement with those reported in theliterature [14].

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250 241

2.3. Preparation of 6-benzoyl substituted 2-naphthols fromWeinreb amides

A solution of n-BuLi (11.76mL,18.82 mmol, 1.6 M in hexane) wasadded dropwise to a cold (�78 �C) stirred solution of 6-bromo-2-naphthol (2 g, 8.96 mmol) in anhydrous THF (50 mL). After30 min 4-fluoro-N-methoxy-N-methylbenzamide 7a (1.640 g,8.96 mmol) in anhydrous THF (20 mL) was added slowly over20 min. The resulting mixture was stirred for 2 h whilst warming toroom temperature. Water (100 mL) was added and the organiclayer was separated. The aqueous layer was extracted with ethylacetate (3 � 50 mL), and the combined organic extracts were driedover anhydrous Na2SO4, removal of the solvent under vacuumafforded the crude 6-(4-fluorobenzoyl)-2-naphthol 8a. The crudeproduct was purified by crystallisation from ethyl acetate andhexane, 52% yield, m. p. 142e143 �C; nmax 531, 769, 804, 1127, 1299,1458, 1648, 3271, 3550 cm�1; dH 5.97 (1H, s, OH), 7.19 (4H, m,AreH), 7.75 (1H, d, J¼ 8.6 Hz, 4-H), 7.83 (1H, d, J¼ 8.7 Hz, 8-H), 7.88(3H, m, AreH), 8.17 (1H, d, J ¼ 1.0 Hz, 5-H); dC 109.61, 115.53 (d,J ¼ 21.7 Hz), 119.10, 126.36, 126.81, 127.34, 131.54, 132.17, 132.27,132.63, 132.72, 134.18 (d, J ¼ 3.1 Hz), 137.09, 156.21, 165.30 (d,J ¼ 254 Hz), 195.84; dF �106.58; Found [MþH]þ ¼ 267.0821,C17H11FO2 requires [MþH]þ ¼ 267.0816.

2.3.1. 6-(2,4-difluorobenzoyl)-2-naphthol 8bFrom 2,4-difluoro-N-methoxy-N-methylbenzamide 7b and 6-

bromo-2-naphthol in a similar manner to 8a, as pale purple col-oured crystals in 44% yield after crystallisation from methanol, mp132e133 �C; nmax 624, 967, 1267, 1428, 1496, 1575, 1604, 1647, 3058,3431 cm�1; dH 5.68 (1H, bs, OH), 6.95 (1H, m, AreH), 7.03 (1H, m,AreH), 7.17 (2H, m, AreH), 7.63 (1H, m, AreH), 7.74 (1H, d,J¼ 8.7 Hz, 4-H), 7.82 (1H, d, J¼ 8.8 Hz, 8-H), 7.92 (1H, m, AreH), 8.17(1H, bs, 5-H); dC 104.49,104.75, 105.00, 109.66,111.77 (d, J¼ 3.7 Hz),111.98 (d, J ¼ 3.7 Hz), 118.94, 123.58, 123.70, 125.60, 126.86, 127.55,131.88, 132.34, 132.38, 132.48, 132.51, 137.56, 156.25, 160.84 (dd,J¼ 253.4, 11.9 Hz), 164.74 (dd, J¼ 252.1, 11.9 Hz), 192.27; dF�106.85,-104.53; Found [MþH]þ ¼ 285.0727, C17H10F2O2 requires[MþH]þ ¼ 285.0722.

2.4. Preparation of 1-(4-fluorobenzoyl)-2-methoxynaphthalene

4-Fluorobenzoyl chloride (3.25 g, 20.5 mmol) was added drop-wise to a suspension of AlCl3 (2.7 g, 20.5 mmol) and 2-methoxynaphthalene (2.5 g, 15.8 mmol) in CH2Cl2 (40 mL) at0 �C. The reaction mixture was allowed to warm to room temper-ature and was stirred for ca. 2 h, poured into ice water (150 g)containing conc. HCl (40 mL). The organic layer was separated andthe aqueous phase was extracted with CH2Cl2 (3 � 50 mL). Thecombined organic layers were dried over anhyd. Na2SO4 andremoval of the solvent under reduced pressure to afford the 1-(4-fluorobenzoyl)-2-methoxynaphthalene 10 as off-white micro-crys-tals in 62% yield, mp 129e130 �C; nmax 501, 678, 899, 920, 1158,1503, 1623, 1655, 2939, 3073 cm�1; dH 3.83 (3H, s, OMe), 7.08 (2H,m, AreH), 7.34 (1H, d, J¼ 9.0 Hz, 4-H), 7.38 (2H, m, AreH), 7.48 (1H,m, AreH), 7.86 (3H, m, AreH), 7.96 (1H, d, J ¼ 9.0 Hz, 8-H);dF �104.99; dC 56.48, 112.94, 115.63, 115.85, 122.41, 123.88, 124.15,127.51, 128.17, 128.74, 131.35, 131.56, 132.35, 134.35, 153.98, 164.76,167.30, 196.21. Found C 77.35, H 4.65. C18H13FO2 requires C 77.13, H4.67%.

2.5. Preparation of 6-(4-fluorobenzoyl)-2-methoxynaphthalene18a

A solution of n-BuLi (11.76mL,18.82 mmol, 1.6 M in hexane) wasadded dropwise to a cold (�78 �C) stirred solution of 6-bromo-2-

methoxynaphthalene (4.24 g, 17.90 mmol) in anhydrous THF(75 mL). After 30 min 4-fluoro-N-methoxy-N-methylbenzamide 7a(1.64 g, 8.96mmol) in anhydrous THF (15mL)was added slowlyover10 min. The resulting mixture was stirred for 2 h whilst warming toroom temperature.Water (100mL)was added and the organic layerwas separated. The aqueous layer was extracted with ethyl acetate(3 � 50 mL), and the combined organic extracts were dried overanhydrous Na2SO4. Removal of the solvent under vacuum affordedthe crude 6-(4-fluorobenzoyl)-2-methoxynaphthalene 18a as anoff-white solid in 26% yield after crystallisation from ethanol, mp113e115 �C; nmax 660, 893, 918, 960, 1151, 1225, 1388, 1504, 1619,1648, 2921 cm�1; dH 3.97 (3H, s, OMe), 7.19 (4H, m, AreH,1-H), 7.81(1H, d, J ¼ 8.7 Hz, 4-H), 7.82 (1H, d, J ¼ 8.5 Hz, 8-H), 7.88 (3H, m,AreH), 8.18 (1H, d, J ¼ 1.2 Hz, 5-H). Found [MþH]þ ¼ 281.0973.C18H13FO2 requires [MþH]þ ¼ 281.0972.

2.6. Preparation of 6-(4-pyrrolidinobenzoyl)-2-naphthol 8c

6-(4-Fluorobenzoyl)-2-naphthol 8a (3.0 g, 11.3 mmol) washeated in neat pyrrolidine (15 mL) for 2 h. Upon completion of thereaction (TLC), the mixture was diluted with water (50 mL), aq. HCl(20 mL, 2 M) and extracted with CH2Cl2 (2 � 20 mL). The organiclayers were combined and dried (anhyd. Na2SO4). Removal of sol-vent afforded the title compound as yellow crystals after elutionfrom silica (70% CH2Cl2 in hexane) in 89%, yield, mp 211e212 �C;nmax 460, 632, 755, 918, 1216, 1472, 1527, 1567, 1612, 2838,3140 cm�1; dH (d6-DMSO) 2.03 (4H, m, (CH2)2), 3.34 (4H, m,N(CH2)2), 6.63 (2H, d, J 8.9 Hz, AreH), 7.17 (1H, dd, J 8.8, 2.4 Hz, 3-H),7.20 (1H, d, J 2.3 Hz,1-H), 7.67 (3H, m, AreH), 7.79 (1H, d, J 8.8 Hz, 4-H), 7.92 (1H, d, J 8.8 Hz, 8-H), 8.09 (1H, d, J 0.8 Hz, 5-H), 10.1 (1H, bs,OH). Found C 79.75, H 5.95, N 4.3. C22H21NO2 requires C 79.47, H6.03, N 4.41%.

2.7. Synthesis of 6-(4-pyrrolidinobenzoyl)-2-methoxynaphthalene18b

6-(4-Fluorobenzoyl)-2-methoxynaphthalene 18a (3.0 g,10.7 mmol) was heated in neat pyrrolidine (20 mL) until TLC ex-amination indicated that no starting ketone remained (ca. 2 h), themixture was diluted with water (100 mL), aq. HCl (20 mL, 2 M) andextracted with CH2Cl2 (3 � 20 mL). The organic layers were com-bined and dried over anhyd. Na2SO4. Removal of solvent gave thetitle compound 18b, as yellow crystals in 85% yield after elutionfrom silica with 80% CH2Cl2 in hexane, mp 108e112 �C; nmax 459,585, 688, 747, 921, 1018, 1388, 1506, 1634, 2838, 2952 cm�1 dH 2.04(4H, m, (CH2)2), 3.40 (4H, m, (NCH2)2), 3.96 (3H, s, OMe), 6.56 (2H,m, AreH), 7.18 (2H, m, AreH), 7.83 (5H, m, AreH), 8.14 (1H, d, J1.0 Hz, 5-H). Found C 79.65, H 6.31, N 4.00. C22H21NO2 requires C79.73, H 6.39, N 4.23%.

2.8. Acidic alumina method for the preparation of 8-aroylnaphthopyrans 12a, c, d

A stirred solution of 6-(4-fluorobenzoyl)-2-naphthol 8a (1.0 g,3.7 mmol) and 1-phenyl-1-(4-pyrrolidinophenyl)prop-2-yn-1-ol(1.0 g, 3.7 mmol) in toluene (50 mL) was warmed to 50 �C. Acidicalumina (1.0 g) was added and themixturewas heated under refluxuntil TLC examination indicated that none of the prop-2-yn-1-olremained (ca. 1 h). The mixture was cooled to ~50 �C, filtered andthe alumina was washed with hot toluene (2 � 30 mL). Removal ofthe toluene from the combinedwashings and filtrate gave a viscous,brown coloured, oil that was eluted from silica (25% EtOAc inhexane). Crystallisation from acetone and MeOH gave 8-(4-fluorobenzoyl)-3-phenyl-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran 12a as pale blue crystals in 58% yield, mp 164e165 �C;

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250242

lmax 558 nm (PhMe); nmax 611, 699, 811, 853, 938, 999, 1152, 1229,1339, 1465, 1521, 1896 cm�1; dH 1.96 (4H, m, (CH2)2), 3.25 (4H, m,N(CH2)2), 6.28 (1H, d, J ¼ 9.9 Hz, 2-H), 6.48 (2H, m, AreH), 7.18 (2H,m, AreH), 7.23e7.35 (7H, m, AreH), 7.49 (2H, m, AreH), 7.70 (1H, d,J ¼ 8.8 Hz, 6-H), 7.87 (3H, m, AreH), 8.03 (1H, d, J ¼ 8.9 Hz, 10-H),8.09 (1H, d, J ¼ 1.6 Hz, 7-H); dC (all signals) 25.46, 47.48, 83.31,110.90, 113.99, 115.33, 115.55, 118.44, 119.57, 121.80, 126.41, 126.82,127.33, 127.86, 128.04, 128.42, 128.86, 130.72, 131.17, 131.97, 132.18,132.48, 132.57, 134.33, 145.29, 147.31, 153.02, 195.09; Found[M]þ ¼ 525.2092C36H28FNO2 requires [M]þ ¼ 525.2099.

2.8.1. 8-(2,4-difluorobenzoyl)-3-(2-fluorophenyl)l-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran 12c

From 6-(2,4-difluorobenzoyl)-2-naphthol 8b and 1-(2-fluorophenyl)-1-(4-pyrrolidinophenyl)prop-2-yn-1-ol, in 30%yield as green microcrystals after elution from silica (30% EtOAc/hexane), mp 148e150 �C; lmax 570 nm (PhMe); nmax 519, 678, 732,852, 999, 1134, 1246, 1371, 1463, 1519, 1607, 1656, 2965 cm�1; dH1.96 (4H, m, (CH2)2), 3.25 (4H, m, N(CH2)2), 6.38 (3H, m, AreH, 2-H),6.93 (1H, m, AreH), 7.02 (2H, m, AreH), 7.13 (1H, m, AreH), 7.28(5H, m, AreH), 7.62 (1H, m, AreH), 7.68 (1H, m, AreH), 7.72 (1H, d,J ¼ 8.8 Hz, AreH), 7.93 (1H, dd, J ¼ 8.8, 0.8 Hz, AreH), 8.02 (1H, d,J ¼ 9.2 Hz, AreH), 8.10 (1H, s, AreH); dC (all signals) 25.49, 47.46,81.76, 104.46, 104.71, 104.96, 111.00, 111.78, 111.99, 113.82, 116.29,116.51, 118.69, 119.38, 121.91, 123.70, 123.74, 125.63, 127.08, 127.12,127.85, 127.88, 128.05, 128.09, 129.35, 129.46, 131.53, 131.64, 132.38,132.47, 132.92, 147.55, 152.96, 157.74, 159.56, 160.20, 191.90;dF �104.71, -106.79, -110.40; Found [M]þ ¼ 561.1905C36H26F3NO2requires [M]þ ¼ 561.1910.

2.8.2. 3-(4-fluorophenyl)-3-phenyl-8-(4-pyrrolidinobenzoyl)-3H-naphtho[2,1-b]pyran 12d

From 6-(4-pyrrolidinobenzoyl)-2-naphthol 8c and 1-(4-fluorophenyl)-1-phenylprop-2-yn-1-ol as pale yellow microcrys-tals in 56% yield, mp 174e175 �C; lmax 428 nm (PhMe); nmax 411,546, 697, 755, 957, 1084, 1156, 1216, 1284, 1379, 1505, 1594, 2842,2968, 3056 cm�1; dH 2.06 (4H, m, (CH2)2), 3.93 (4H, m, N(CH2)2),6.25 (1H, d, J¼ 9.9 Hz, 2-H), 6.55 (2H, app. d, J¼ 8.8 Hz, AreH), 7.00(2H, m, AreH), 7.22 (1H, d, J ¼ 8.8 Hz, AreH), 7.28 (1H, m, AreH),7.35 (3H, m, AreH), 7.46 (4H, m, AreH), 7.73 (1H, d, J ¼ 8.8 Hz,AreH), 7.81 (2H, m, AreH), 7.86 (1H, dd, J ¼ 8.7, 1.7 Hz, AreH), 8.02(1H, d, J ¼ 8.7 Hz, 10-H), 8.09 (1H, d, J ¼ 1.5 Hz, 7-H); dC (all signals)25.47, 47.59, 82.43, 110.61, 113.97, 114.90, 115.11, 118.88, 119.52,121.28, 124.55, 126.85, 127.20, 127.74, 127.77, 128.19, 128.23, 128.90,128.98, 131.18, 131.23, 132.89, 134.53, 140.42, 144.46, 150.79, 151.83,163.91 (d, J ¼ 251.2), 194.74; dF �115.19; Found[MþH]þ ¼ 526.2169C36H28FNO2 requires [MþH]þ ¼ 526.2177.

2.9. Toluene sulfonic acid method for the preparation of 8-aroylnaphthopyran 12b

A stirred solution of 6-(4-fluorobenzoyl)-2-naphthol 8a (1.0 g,3.7 mmol) and 1-(2-fluorophenyl)-1-(4-pyrrolidinophenyl)prop-2-yn-1-ol (1.1 g, 3.7 mmol) in toluene (50 mL) was warmed to 50 �C.4-Toluenesulfonic acid (0.1 g) was added and the mixture washeated under reflux until TLC examination indicated that none ofthe prop-2-yn-1-ol remained (ca. 1 h). The cooled reaction mixturewas washed with water (2 � 50 mL) and the toluene layer dried(anhyd. Na2SO4) and evaporated to afford a viscous, gum that waseluted from silica (80% CH2Cl2 in hexane) to give 8-(4-fluorobenzoyl)-3-(2-fluorophenyl)-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran 12b as pale blue microcrystals in 58% yield, mp173.0e175.0 �C; lmax 580 nm (PhMe); nmax 611, 879, 1152, 1228,1375, 1464, 1520, 1598, 1612, 1895 cm�1; dH 1.96 (4H, m, (CH2)2),3.25 (4H, m, N(CH2)2), 6.48 (1H, d, J ¼ 10 Hz, 2-H), 6.49 (2H, m,

AreH), 7.01 (1H, m, AreH), 7.16 (3H, m, AreH), 7.21e7.31 (5H, m,AreH), 7.68 (1H, m, AreH), 7.73 (1H, d, J¼ 8.8 Hz, 6-H), 7.86 (3H, m,AreH), 8.04 (1H, d, J ¼ 9.2 Hz, 10-H), 8.11 (1H, d, J ¼ 1.6 Hz, 7-H);dF �110.10, �106.92; dC all signals, 25.48, 47.45, 81.67 (d, J¼ 2.7 Hz),110.99, 113.79, 115.34, 115.56, 116.27, 116.49, 118.72, 119.35, 121.82,123.69, 123.72, 126.48, 127.07, 127.12, 127.86, 127.90, 127.97, 128.05,129.32, 129.41, 129.50, 131.22, 131.56, 131.92, 132.31, 132.49, 132.58,134.28, 134.31, 147.54, 152.58, 158.98 (d, J ¼ 246.6 Hz), 165.23 (d,J¼ 252.0 Hz) 195.04; Found [M]þ ¼ 543.2000C36H27F2NO2 requires[M]þ ¼ 543.2004.

2.10. General method for the preparation of propynols 16 and 19

n-Butyllithium (1.6 M in hexanes, 1 equiv.) was added slowly viasyringe to a cold (�10 �C), stirred solution of trimethylsilylacety-lene (1.0 equiv.) in anhydrous tetrahydrofuran (50 mL) under anitrogen atmosphere. On completion of the addition (ca. 5 min) thecold solution was allowed to stir for 1 h. The ketone 12 (0.9 equiv.)was added in a single portion and the mixture stirred at roomtemperature until TLC examination of the reaction mixture indi-cated that none of the ketone remained (ca. 2 h). The reactionmixture was re-cooled to 0 �C and a solution of methanolic po-tassium hydroxide [(from potassium hydroxide (2.0 equiv.) inmethanol (10 mL)] was added in a single portion. The cooling bathwas then removed and the mixture warmed to room temperature,after ca. 20 min TLC examination indicated that desilylation wascomplete. The mixture was neutralised to pH ~7 using glacial aceticacid and then poured into water (250 mL). The organic layer wasseparated and the aqueous layer extracted with ethyl acetate(3 � 50 mL). The organic phases were combined, washed withwater (2 � 50 mL) and dried (anhyd. Na2SO4). Removal of the sol-vent gave the propynol which was sufficiently pure by TLC and 1HNMR spectroscopy to be used directly in the subsequent step. Thefollowing propynols were obtained in this manner.

2.10.1. 1-(4-fluorophenyl)-1-{3-phenyl-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl}-prop-2-yn-1-ol 16a

From 8-(4-fluorobenzoyl)-3-phenyl-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran 12a as green micro-crystals in 98% yield,dH 1.95 (4H, m, (CH2)2), 2.80 (1H, s, alkyne-H), 2.91 (1H, s, OH), 3.23(4H, m, N(CH2)2), 6.22 (1H, d, J¼ 9.9 Hz, 2-H), 6.46 (2H, d, J¼ 8.6 Hz,AreH), 7.01 (3H, m, AreH), 7.18e7.30 (4H, m, AreH), 7.50 (4H, m,AreH), 7.57 (3H, m, AreH), 7.63 (1H, d, J ¼ 8.8 Hz, AreH), 7.89 (1H,d, J ¼ 8.9 Hz, AreH), 8.00 (1H, bs, AreH).

2.10.2. 1-(4-fluorophenyl)-1-{3-(2-fluorophenyl)-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl}-prop-2-yn-1-ol16b

From 8-(4-fluorobenzoyl)-3-(2-fluorophenyl)-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran 12b as blue micro-crystals in 97% yield, dH 1.95 (4H, m, (CH2)2), 2.86 (1H, bs, OH),2.90 (1H, s, alkyne-H), 3.25 (4H, m, N(CH2)2), 6.42 (1H, dd,J ¼ 10.0, 4.5 Hz, 2-H), 6.47 (2H, app. d, J ¼ 8.7 Hz, AreH), 6.99(3H, m, AreH), 7.10 (1H, m, AreH), 7.20e7.27 (5H, m, AreH), 7.49(1H, m, AreH), 7.58 (2H, m, AreH), 7.69 (2H, m, AreH), 7.89 (1H,d, J ¼ 8.9 Hz, 10-H), 8.01 (1H, t, J ¼ 2.2 Hz, AreH).

2.10.3. 1-(2,4-difluorophenyl)-1-[3-(2-fluorophenyl)-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl]prop-2-yn-1-ol16c

From 8-(2,4-difluorophenyl)-3-(2-fluorophenyl)-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran 12c as pale blue crys-tals after washing with ethyl acetate and hexane in 93% yield, dH1.94 (4H, m, (CH2)2), 2.89 (1H, s, alkyne-H), 2.10 (1H, bs, OH), 3.37(4H, m, N(CH2)2), 6.42 (1H, dd, J¼ 9.9, 2.4 Hz, 2-H), 6.47 (2H, app. d,

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250 243

J¼ 8.5 Hz, AreH), 6.74 (1H, m, AreH), 6.89 (1H, m, AreH), 6.99 (1H,m, AreH), 7.10 (1H, m, AreH), 7.25 (5H, m, AreH), 7.52 (1H, m,AreH), 7.74 (3H, m, AreH), 7.91 (1H, d, J ¼ 8.8 Hz, AreH), 8.00 (1H,dd, J ¼ 4.0, 1.8 Hz, AreH).

2.10.4. 1-[3-(4-fluorophenyl)-3-phenyl-3H-naphtho[2,1-b]pyran-8-yl]-1-(4-pyrrolidino phenyl)prop-2-yn-1-ol 16d

From 8-(4-pyrrolidinobenzoyl)-3-phenyl-3-(4-fluorophenyl)-3H-naphtho[2,1-b]pyran 12d, as pale blue micro-crystals in 97%yield, dH 1.97 (4H, m, (CH2)2), 2.71 (1H, s, bs, OH), 2.86 (1H, s,alkyne-H), 3.23 (4H, m, N(CH2)2), 6.19 (1H, d, J ¼ 9.9 Hz, 2-H), 6.49(2H, m, AreH), 6.98 (3H, m, AreH), 7.12 (1H, J ¼ 8.8 Hz, AreH), 7.28(3H, m, AreH), 7.42 (6H, m, AreH), 7.57 (1H, dd, J ¼ 8.9 Hz, 1.5 Hz,AreH), 7.67 (1H, d, J ¼ 8.8 Hz, AreH), 7.87 (1H, d, J ¼ 8.8 Hz, AreH),8.02 (1H, d, J ¼ 1.8 Hz, AreH).

2.10.5. 1-(4-fluorophenyl)-1-(6-methoxynaphthalen-2-yl)-prop-2-yn-1-ol 19a

From 6-(4-fluorobenzoyl)-2-methoxynaphthalene 18a, in 96%as colourless crystals from ethyl acetate and hexane, dH 2.93 (1H, s,alkyne-H), 2.97 (1H, s, OH), 3.92 (3H, s, OMe), 7.03 (2H, m, AreH),7.11 (1H, d, J ¼ 2.4 Hz, 1-H), 7.17 (1H, dd, J ¼ 8.8, 2.5 Hz, 3-H), 7.50(1H, dd, J ¼ 8.8, 1.8 Hz, 7-H), 7.60 (2H, m, AreH), 7.66 (1H, d,J ¼ 8.5 Hz, 4-H), 7.74 (1H, d, J ¼ 8.5 Hz, 8-H), 8.07 (1H, d, J ¼ 2.0 Hz,5-H).

2.10.6. 1-(6-methoxynaphthalen-2-yl)-1-(4-pyrrolidinophenyl)prop-2-yn-1-ol 19b

From 6-(4-pyrrolidinobenzoyl)-2-methoxynaphthalene 18b, aspale yellow micro-crystals in 97%, dH 1.98 (4H, m, (CH2)2), 2.75 (1H,bs, OH), 2.89 (1H, s, alkyne-H), 3.26 (4H, m, N(CH2)2), 3.92 (3H, s,OMe), 6.49 (2H, m, AreH), 7.01 (2H, m, AreH), 7.43 (2H, m, AreH),7.55 (1H, dd, J ¼ 8.6, 1.9 Hz, 7-H), 7.68 (1H, d, J ¼ 8.8 Hz, 4-H), 7.75(1H, d, J ¼ 8.8 Hz, 8-H), 8.08 (1H, d, J ¼ 1.6 Hz, 5-H).

2.11. Method for the preparation of naphthopyrans 17a and 17b

A stirred solution of 2-naphthol (1.0 equiv.), the appropriateprop-2-yn-1-ol either 16a or 16d (1.0 equiv.) and 4-toluenesulfonicacid (0.1 g) in toluene (50 mL) was heated under reflux until TLCexamination indicated that none of the prop-2-yn-1-ol remained(ca. 1 h). The mixturewas cooled to room temperature and the darksolution washed with water (2 � 50 mL). The toluene solution wasdried (anhyd. Na2SO4) and evaporated to afford a multicomponentgum that was further purified.

2.11.1. 3-(4-fluorophenyl)-3-{3-phenyl-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl}3H-naphtho[2,1-b]pyran 17a

From 1-(4-fluorophenyl)-1-[3-phenyl-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl]prop-2-yn-1-ol 16a, as blue crystalsin 5.4% yield after elution from silica with 5% EtOAc/toluene and2 � crystallisation from acetone/methanol, mp 234e235 �C; lmax481 nm (PhMe); nmax 551, 611, 758, 832, 1056, 1227, 1339, 1408,1486, 1598, 1613 cm�1; dH 1.94 (4H, m, (CH2)2), 3.23 (4H, m,N(CH2)2), 6.20 (1H, d, J¼ 9.9 Hz, 2-Hz), 6.24 (1H, d, J¼ 9.9 Hz, 20-Hz),6.44 (2H, m, AreH), 6.99 (2H, m, AreH), 7.14 (1H, d, J ¼ 8.8 Hz,AreH), 7.17e7.32 (8H, m, AreH, 10-H), 7.35 (1H, d, J ¼ 9.8 Hz, 1-H),7.45 (6H, m, AreH), 7.57 (1H, d, J ¼ 8.8 Hz, AreH), 7.64 (1H, d,J ¼ 8.8 Hz, AreH), 7.69 (1H, d, J ¼ 7.8 Hz, AreH), 7.79 (1H, d,J¼ 1.5 Hz, 70-H), 7.89 (1H, d, J¼ 9.0 Hz,10-Hy), 7.95 (1H, d, J¼ 8.4 Hz,100-H. Found [MþH]þ ¼ 678.2793. C48H36FNO2 requires[MþH]þ ¼ 678.2803. Pairs of signals marked (z) and (y) may beinterchanged.

2.11.2. 3-(4-fluorophenyl)-3-{3-(2-fluorophenyl)-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl}3H-naphtho[2,1-b]pyran 17b

From 1-(4-fluorophenyl)-1-{3-(2-fluorophenyl-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl}prop-2-yn-1-ol16b, as pale blue micro-crystals in 17% yield after elution from silicawith 40% CH2Cl2 in hexane, mp 182e183 �C; lmax 564 nm (PhMe);nmax 408, 516, 686, 725, 786, 893, 944, 1104, 1337, 1376, 1484, 1505,1608, 2835, 3056 cm�1; dH 1.94 (4H, m, (CH2)2), 3.22 (4H, m,N(CH2)2), 6.24 (1H, d, J ¼ 10 Hz, 2-H), 6.41 (1H, dd, J ¼ 10.4 Hz, JH-F ¼ 4.6 Hz, 20-H), 6.45 (2H, m, AreH), 7.00 (3H, m, AreH), 7.08 (1H,m, AreH), 7.15e7.25 (6H, m, AreH), 7.31 (1H, m, AreH), 7.33 (1H, d,J ¼ 10 Hz, 1-H), 7.44e7.51 (4H, m, AreH), 7.59 (1H, d, J ¼ 8.8 Hz,AreH), 7.65 (3H, m, AreH), 7.80 (1H, s, 70-H), 7.89 (1H, d, J ¼ 9.2 Hz,10-Hz), 7.95 (1H, d, J ¼ 8.8 Hz, 100-Hz). Found [M]þ ¼ 695.2634.C48H35F2NO2 requires [M]þ ¼ 695.2630. Signals marked (z) andmaybe interchanged.

2.12. Method for the preparation of naphthopyrans 17c and 17d

A stirred solution of 2-naphthol (1.0 equiv.) and the appropriateprop-2-yn-1-ol either 16c or 16d (1.0 equiv.) in toluene (50mL) waswarmed to 50 �C. Acidic alumina (1.0 g) was added and the mixturewas heated under reflux until TLC examination indicated that noneof the prop-2-yn-1-ol remained (ca. 1 h). The mixturewas cooled to~50 �C, filtered and the alumina was washed with hot toluene(2 � 30 mL). Removal of the toluene from the combined washingsand filtrate gave a multicomponent gum that was further purified.

2.12.1. 3-(2,4-difluorophenyl)-3-{3-(2-fluorophenyl)-3-(4-pyrrolidinophenyl)-3H-naphtho[2,1-b]pyran-8-yl}3H-naphtho[2,1-b]pyran 17c

From 1-(2,4-difluorophenyl)-1-[3-(2-fluorophenyl)-3-(4-pyrrolidnophenyl)-3H-naphtho[2,1-b]pyran-8-yl]prop-2-yn-1-ol16c and 2-naphthol, as pale green micro-crystals in 13% yield afterelution from silica with 5% EtOAc/hexane and crystallisation fromacetone/methanol, mp 254e256 �C; lmax 490, 570(sh) nm (PhMe);nmax 518, 660, 707, 741, 852, 1008, 1096, 1168, 1214, 1336, 1503, 1517,1608, 2835, 2965, 3059 cm�1; dH 1.94 (4H, m, (CH2)2), 3.23 (4H, m,N(CH2)2), 6.41 (2H, m, 2-H, 20-H), 6.47 (2H, m, AreH), 6.76 (1H, m,AreH), 6.88 (1H, m, AreH), 6.94 (1H, m, AreH), 7.08 (1H, m, AreH),7.17 (3H, m, AreH), 7.34 (2H, m, AreH), 7.27 (2H, m, AreH), 7.48(1H, m, AreH), 7.54 (1H, d, J ¼ 8.4 Hz, AreH), 7.59 (1H, d, J ¼ 8.4 Hz,AreH), 7.73 (6H, m, AreH), 7.90 (1H, d, J ¼ 8.2 Hz, 10-Hz), 7.99 (1H,d, J ¼ 8.0 Hz, 100-Hz). Found [M]þ ¼ 713.2542C48H34F3NO2 requires[M]þ ¼ 713.2536. Signals marked (z) and may be interchanged.

2.12.2. 3-(4-pyrrolidinophenyl)-3-{3-phenyl-3-(4-fluorophenyl)-3H-naphtho[2,1-b]pyran-8-yl}3H-naphtho[2,1-b]pyran 17d

From 1-[3-(4-fluorophenyl)-3-phenyl-3H-naphtho[2,1-b]pyran-8-yl]-1-(4-pyrrolidinophenyl)prop-2-yn-1-ol 16d and 2-naphtholas pale yellow micro-crystals in 19% yield after elution from silicawith 20% EtOAc/hexane and crystallisation from hexane/EtOAc, mp235e236 �C; lmax 434 nm (PhMe); nmax 517, 601, 684, 725, 944,1082, 1217, 1337, 1505, 1518, 1609, 2835, 2965, 3056 cm�1; dH1.96(4H, m, (CH2)2), 3.23 (4H, m, N(CH2)2), 6.18 (1H, d, J ¼ 9.9 Hz, 2-Hz)6.25 (1H, d, J ¼ 9.9 Hz, 20-Hz), 6.47 (2H, d, J ¼ 8.7 Hz, AreH), 6.96(2H, m, AreH), 7.13 (1H, d, J ¼ 8.8 Hz, 5-Hx), 7.16 (1H, d, J ¼ 8.8 Hz,50-Hx), 7.23e7.31 (8H, m, AreH), 7.40 (5H, m, AreH), 7.56 (1H, dd,J ¼ 8.8, 1.8 Hz, AreH), 7.60 (2H, m, AreH), 7.68 (1H, d, J ¼ 8.1 Hz,AreH), 7.84 (1H, d, J ¼ 1.6 Hz, 70-H), 7.87 (1H, d, J ¼ 8.8 Hz, 10-Hy),7.94 (1H, d, J ¼ 8.4 Hz, 100-Hy). Found [M]þ ¼ 677.2726. C48H36FNO2requires [M]þ ¼ 677.2725. Pairs of signals marked (z), (x) and (y) maybe interchanged.

Scheme 1. Preparation of 6-benzoyl-2-naphthols 8.

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2.13. General method for the preparation of photochromicnaphthopyrans 20a and 20b

A stirred solution of 6-(benzoyl)-2-naphthol (4.0 mmol) and the1-(aryl)-1-(6-methoxynaphthalene-2-yl)prop-2-yn-1-ol(4.0 mmol) in toluene (75mL) was warmed to 50 �C. Acidic alumina(1.5 g) was added and the mixture was refluxed until TLC exami-nation indicated that none of the prop-2-yn-1-ol remained (ca.1.5 h). The mixture was cooled to ~50 �C, filtered and the aluminawas washed with hot toluene (2 � 30 mL). Removal of the toluenefrom the combined washings and filtrate gave a viscous browncoloured oil that was eluted from silica to afford the naphthopyran.

2.13.1. 3-(4-fluorophenyl)-3-(6-methoxynaphthalen-2-yl)-3H-naphtho[2,1-b]pyran 20a

From 1-(4-fluorophenyl)-1-(6-methoxynaphthalene-2-yl)prop-2-yn-1-ol 19a and 2-naphthol in 36% yield as a colourless solid afterelution from silica with 5% EtOAc in toluene, mp 151e152 �C; lmax464 nm (PhMe); nmax 474, 688, 946, 955, 1173, 1244, 1385, 1504,1628,1913, 2939 cm�1; dH 3.89 (3H, s, OMe), 6.27 (1H, d, J¼ 10.0 Hz,2-H), 6.99 (2H, m, AreH), 7.11 (2H, m, AreH), 7.20 (1H, d, J ¼ 8.8 Hz,AreH), 7.33 (1H, m, AreH), 7.36 (1H, d, J ¼ 9.9 Hz, 1-H), 7.45 (4H, m,AreH), 7.67 (4H, m, AreH), 7.85 (1H, d, J¼ 1.5 Hz, AreH), 7.97 (1H, d,J ¼ 8.4 Hz, 10-H); dC 55.31, 88.29, 105.50, 114.01, 114.84, 115.06,118.25, 118.98, 119.85, 121.30, 123.68, 125.59, 125.79, 126.69, 126.97,127.45, 128.10, 128.52, 128.91, 129.00, 129.35, 129.81, 129.96, 133.89,139.46, 140.61, 150.39, 158.00, 160.86, 163.31. Found[MþH]þ ¼ 433.1605. C30H21FO2 requires [MþH]þ ¼ 433.1598.

2.13.2. 3-(4-pyrrolidinophenyl)-3-(6-methoxynaphthalen-2-yl)-3H-naphtho[2,1-b]pyran 20b

From 1-(4-pyrrolidinophenyl)-1-(6-methoxynaphthalene-2-yl)-prop-2-yn-1-ol 19b and 2-naphthol, as colourless micro-crystals in 40% yield, after elution from silica with 2% EtOAc intoluene, mp 230e231 �C; lmax 553 nm (PhMe); nmax 520, 744, 856,1029, 1162, 1241, 1372, 1459, 1518, 1604, 1628, 2837, 3059 cm�1; dH1.95 (4H, m, (CH2)2), 3.25 (4H, m, N(CH2)2), 3.89 (3H, s, OMe), 6.29(1H, d, J¼ 9.9 Hz, 2-H), 6.49 (2H, m, AreH), 7.08 (1H, bs, AreH), 7.10(1H, d, J ¼ 2.5 Hz, AreH), 7.20 (1H, d, J ¼ 8.8 Hz, AreH), 7.32 (4H, m,AreH), 7.42 (1H, m, AreH), 7.51 (1H, dd, J ¼ 8.6, 1.6 Hz, AreH), 7.63(1H, d, J ¼ 8.8 Hz, AreH), 7.67 (3H, m, AreH), 7.90 (1H, d, J ¼ 1.7 Hz,AreH), 7.96 (1H, d, J ¼ 8.4 Hz, 10-H); dC 25.46,47.51, 55.30, 82.88,

105.51, 110.94, 113.93, 118.50, 118.69, 119.03, 121.31, 123.34, 125.56,125.91, 126.42, 126.66, 128.23, 128.45, 129.22, 129.57, 129.84,131.19, 133.70, 140.51, 147.22, 150.81, 157.76. Found[MþH]þ ¼ 484.2274C34H29NO2 requires [MþH]þ ¼ 484.2271.

3. Discussion

The most convenient method to access the target linked naph-thopyrans relies upon access to a 6-aroyl-2-naphthol from whichan 8-aroylnaphthopyran can be accessed directly upon acid cata-lysed condensation with a 1,1-diarylprop-2-yn-1-ol [15] and thearoyl group of which may be subsequently transformed into anprop-2-yn-1-ol through addition of lithiumtrimethylsilylacetylide[16] and then condensed with an alternative naphthol (Scheme 1).

The addition of organolithium reagents to aromatic nitriles is auseful protocol to access symmetrically and asymmetricallysubstituted ketones [17]. Thus our initial attempt to access 6-aroyl-2-naphthols 8 relied upon the generation of the dianion from 6-bromo-2-naphthol by treatment with 2.1 equivalents of n-BuLiat �78 �C in THF solution. Quenching this anion with 4-fluorobenzonitrile failed to afford any of the expected 6-(4-fluorobenzoyl)-2-naphthol 8a and instead gave a multi-component mixture on an examination by TLC.

In our second approach to access 8 metallation of commerciallyavailable 6-bromo-2-naphthol was again investigated followed byaddition of a Weinreb amide (Scheme 1). The amides 7a,b wereconveniently accessed using the standard protocol of acylation ofN,O-dimethylhydroxylamine hydrochloride [18] with either 4-fluoro- or 2,4-difluoro- benzoyl chloride in 96 and 89% yield,respectively after distillation. Thus treatment of 6-bromo-2-naphthol with two equivalents of n-BuLi at low temperature fol-lowed by addition of the amide 7 gave the requisite 6-aroyl-2-naphthols 8a and 8b in 52 and 44% yield, respectively. The incor-poration of the aroyl group was confirmed by the presence of asignal at d 195.8 for the C]O group with a 19F signal at dF �106.58for 8a and a signal at d 192.3 (C]O) with 19F signals at dF �106.85and dF �104.53 for 8b.

As an alternative to the use of the foregoing metallation e

Weinreb amide protocol examination of the literature revealed that2-methoxynaphthalene undergoes FriedeleCrafts acylation withacetyl chloride in nitrobenzene to afford the 6-acetyl-2-methoxynaphthalene [19]. Thus the FriedeleCrafts benzoylation

Scheme 2. Alternate route investigated to 6-benzoyl-2-naphthols.

Scheme 3. Synthesis of 8-aroylnaphthopyrans 12.

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250 245

of 2-methoxynaphthalene with 4-fluorobenzoyl chloride wasexamined with a view to obtaining 6-(4-fluorobenzoyl)-2-methoxynaphthalene 9 which could be subsequently demethy-lated. In marked contrast to the reported acylation, benzoylationafforded the 1-benzoylated product 10 in 62% yield (Scheme 2);none of the desired 6-benzoylated product could be detected in thereaction mixture. The absence of a signal in the 1H NMR spectrumof 10 that could be attributed to 1-H, which appears at d 7.05 in 2-methoxynaphthalene, affirmed the regioselectivity of thebenzoylation.

Treatment of the foregoing 6-aroyl-2-naphthols 8aec with the1,1-diarylprop-2-yn-1-ols 11aec in toluene containing either acidic

Fig. 3. Absorption spectra of 12a and 12d.

alumina or 4-TsOH as the acidic catalyst gave the 8-aroylnaphthopyrans 12aed in 30e58% yield (Scheme 3). Con-struction of the pyran ring was confirmed by 1H NMR spectroscopy[3] which provided a doublet (J ¼ 9.9 Hz) for 2-H of the pyran ringat ca. d 6.3 for 12a and 12d. However for 12b and 12c a complexmultiplet was present in their 1H NMR spectra at ca. d 6.5 ac-counting for three protons which was attributed to 2-H togetherwith the phenyl ring protons that are ortho to the pyrrolidine ring.

The absorption spectra of the aroylnapthopyrans 12 wereexamined in dilute toluene solution prior to and immediately afteractivation by UV irradiation at either room temperature (22 �C) orca. 5 �C. Absorption spectra for 12a and 12d are included as ex-amples in Fig. 3, with absorption maxima (lmax) for all of the 8-aroylnapthopyrans provided in Table 1.

Comparison of the lmax data for 12 (Table 1) with that of thesimple 8-unsubstituted compounds 13 [12], 14 [12], 15 [6] (Fig. 4)merits some comments. Comparison of 12awith 13 and of 12bwith14 revealed that the 8-(4-fluorobenzoyl) group induced a bath-ochromic shift in lmax of 20 nm and 26 nm, respectively. Compar-ison of the absorption data of 12b with 12c which differ by thepresence of a 2-fluorine atom in the 4-fluorobenzoyl moiety at C-8revealed a 10 nm hypsochromic shift in lmax of 12c. Interchange ofthe fluorine atom and the pyrrolidine group of 12a lead to 12dwhich had a significantly hypsochromically shifted absorbptionmaxima to 428 nm relative to that of 12a of 558 nm, and which was

Table 1Absorption maxima for photomerocyanines derived from naphthopyrans 12.

W X Y Z lmax(nm)PhMe

12a H H F Pyrrolidinyl 55812b F H F Pyrrolidinyl 58012c F F F Pyrrolidinyl 57012d H H Pyrrolidinyl F 428

Fig. 4. Structures and absorption maxima of reference naphthopyrans 13e15.

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250246

identical to that of the simple naphthopyran 15. Thus for 12d it isconsidered that the electron donating pyrrolidine unit diminishesthe electron withdrawing influence of the 8-aroyl unit.

Propynols 16aed were cleanly accessed in high yields by theaddition of lithium trimethylsilylacetylide to the 8-aroylnaphthopyrans 12aed with an in situ hydroxide ion medi-ated desilylation (Scheme 4). The presence of a sharp singlet in therange d 2.8e2.9 for the acetylene proton confirmed the success ofthis transformation.

Scheme 4. Synthesis of b

Direct reaction of the foregoing propynols 16aed with 2-naphthol in refluxing toluene containing either a catalyticamount of 4-TsOH or acidic alumina gave the (naphthopyran-8-yl)naphthopyrans 17aed in disappointingly low yields (5e19%)(Scheme 4). The low yields were a consequence of difficultiesexperienced with solubility and resolution during purification ofthe crude mixtures by column chromatography. The 1H NMRspectra of the naphthopyrans 17a and 17d were similar with eachcompound affording two doublets in the narrow range d 6.18e6.24

i-naphthopyrans 17.

6.206.306.406.50

2-H

2'-H

Fig. 5. Partial 1H NMR spectrum (400 MHz, CDCl3) of 17b.

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250 247

for the two distinct pyran ring protons (2-H and 20-H) with J ~ 10 Hz[3]. The 1H NMR spectrum of 17b, which contains an o-fluorophenyl20-C substituent, exhibited a doublet at d 6.25 for 2-H (J ¼ 10 Hz)and a dd for 20-H at d 6.41 with J¼ 4.6 and 10.4 Hz (Fig. 5); the latterdd arising as a consequence of coupling of 20-H to 10-H and theproximal fluorine atom; the latter JH-F coupling in naphthopyranshas been previously documented [6,12]. The 1H NMR spectrum of17c, which contains two o-fluorophenyl moieties, was more com-plex with 2-H and 20-H giving rise to a complex signal at d 6.46. Ineach of the foregoing naphthopyrans 17 the protons of the pyrro-lidine ring afforded a multiplet at d 1.9 ((CH2)2) and at d 3.2(N(CH2)2); two low field signals at ca. d 7.9 and d 8.0 were readilyassigned to 10/100-H [3].

With the series of naphthopyrans 17 to hand their absorptionproperties before and after irradiation in toluene solution wassurveyed. Irradiation of a colourless solution of 17a resulted in thedevelopment of an orange e red shade as a consequence of arelatively broad absorption band with lmax at 481 nm (Fig. 6).Whilst simple fluorine substituted naphthopyrans such as 15 giverise to photomerocyanines that absorb at ca. 415 nm and are rela-tively long-lived [6], photomerocyanines derived from 13 arelonger wavelength absorbing, ca. 538 nm and fade relatively rapidly[12], the absorption band of 17a appears someway between thesetwomaxima. In order to better examine the absorptionmaxima thetoluene solution of 17a was cooled to ca. 10 �C and then irradiated.At lower temperature a much more intense absorption band was

Fig. 6. Absorption spectra of 17a.

noted, as a consequence of the decrease of the rate of the thermalback reaction of the T-type naphthopyran, with a maximum ab-sorption at ca. 480 nm. However this new absorption band tailedappreciably over the longer wavelength range 525e675 nm andthis tail may be due to a greater contribution of the merocyaninecontaining the pyrrolidine donor group.

Given that the absorption maxima of the photomerocyaninederived from UV-irradiation of 17a falls between those of referencecompounds 13 and 15 it was thought of interest to examine theinfluence of a 2-methoxy-6-naphthyl unit, which is structurallysimilar to the naphthopyran unit in that the oxygenation pattern isretained, in two additional model compounds 20a and 20b(Scheme 5). Thus quenching the anion derived from LieBr ex-change in 6-bromo-2-methoxynaphthalene with Weinreb amide7a gave 6-(4-fluorobenzoyl)-2-methoxynaphthalene 18a in 26%yield. Nucleophilic displacement of fluoride ion from 18a on heat-ing in neat pyrrolidine was readily accomplished in 85% yield toafford 18b. The foregoing 6-aroyl-2-methoxynaphthalenes 18a and18b were efficiently converted into propynols 19a and 19b in highyields (>96%) using our standard protocol. The acid-catalysedcondensation of 19a and 19b with 2-naphthol gave the modelphotochromic naphthopyrans 20a (36%) and 20b (40%), respec-tively. UV-irradiation of a toluene solution of 20a and of 20bresulted in a facile electrocyclic ring-opening to afford photo-merocyanines with absorption maxima at 464 nm and 553 nm,respectively. Notably, these absorption maxima are shifted bath-ochromically relative to the analogous phenyl substituted com-pounds 13 and 15; a feature which implies that the 3,3-diarylnaphthopyran-8-yl unit, which is structurally similar to the2-methoxy-6-naphthyl unit, can be expected to induce a similarbathochromic effect. Thus comparing the absorptionmaxima of 17awith that of 20a it is highly probable that the short wavelengthmaxima at 481 nm is due to themerocyanine chromophore bearinga 4-fluorophenyl and naphthopyran-8-yl unit. Thus, at lower tem-perature, it would appear that each of the separate napthopyranunits can undergo electrocyclic ring-opening contributing to theoverall colour observed.

The absorption spectrum of 17b (Fig. 7) displays a very broadabsorption band with a maximum at 558 nm with the absor-bance band tailing to shorter wavelength. In this instance theabsorption maximum arises from the merocyanine derived fromthe pyran unit substituted with the pyrrolidinophenyl and 2-fluorophenyl moieties as a consequence of the 2-fluorine atomhindering the ring closure of this merocyanine [12]. The tailingof the absorption band to shorter wavelength arises from acontribution from the merocyanine containing the 4-fluorophenyl ring [6].

Scheme 5. Preparation of 2-methoxynaphthyl substituted naphthopyrans.

Fig. 7. Absorption spectra of 17b.

Fig. 8. Absorption spectra of 17c.

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250248

The influence of the 2,4-difluorophenyl unit on the absorption of17c can be clearly seen (Fig. 8). Here a very broad asymmetric ab-sorption band results with the maximum absorption maximumappears at 492 nm due to the merocyanine containing the 2,4-difluorophenyl unit however, on closer inspection of the spec-trum a shoulder is apparent at ca. 575 nm which is due to themerocyanine substituted with the pyrrolidinophenyl and 2-fluorophenyl moieties. Perhaps the absorption spectrum of 17d isthe most interesting (Fig. 9) since there is an appreciable differencein the spectra recorded at room temperature and recorded cold. Thespectrum recorded at room temperature has a maximum absorp-tion at ca. 431 nm which must be a consequence of the relativelypersistent merocyanine bearing the 4-fluorphenyl group since 15,bearing the same diaryl groups absorbs at 428 nm [6]. However,irradiation of a cooled solution leads to the expected intensificationof the absorbance but with the noticeable emergence of a shoulderat ca. 560 nm as a consequence of a greater proportion of themerocyanine bearing the pyrrolidinophenyl unit. Here the

Fig. 9. Absorption spectra of 17d.

Scheme 6. Interrelationship of photomerocyanines derived from 17.

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250 249

magnitude of the long wavelength maxima aligns well with that of20b bearing a pyrrolidinophenyl unit and an alkoxynaphthaleneunit.

The interrelationships between the pyran and merocyanineunits in, for example, 17c are presented in Scheme 6. From theabsorption spectra of 17c it is apparent that UV-irradiation inducesring opening of both of the pyran rings to afford species 21 and 22.However, with the data acquired to date, it cannot be stated thatboth pyran rings of a single bi-naphthopyran molecule opensimultaneously to afford 23. The observed colour of the solutionsresults from additive mixing of the colours resulting from thephotomerocyanines with the greater contribution (absorbance)resulting from the more persistent photomerocyanine which iscontrolled by a combination of electronic and steric factors asso-ciated with the substituents and their locations.

4. Conclusions

The synthesis of a series of new 8-aroyl-3H-naphtho[2,1-b]py-rans 12 has been accomplished in good yield through the aroylationof 6-bromo-2-naphthol using Weinreb amide chemistry. The 4-fluorobenzoyl group induces a bathochromic shift in lmax relativeto the non-benzoylated compound. However, the influence of astrong electron donating pyrrolidine unit in the 4-position of thebenzoyl ring serves to diminish the electron withdrawing effect ofthe benzoyl group and its associated bathochromic effect. Whilst

these naphthopyrans 12 can be smoothly transformed into propy-nols 16 using standard acetylide chemistry their subsequent con-version into novel bi-naphthopyrans 17was particularly inefficient.A preliminary examination of the absorption properties of 17 pre-and post UV irradiation revealed a complex interplay between thetwo pyran units. Interestingly recording the absorption spectra atlower temperature led, in the case of pyrans 17c and 17d, to theemergence of an appreciable shoulder on themain absorption bandaccompanied by a change in colour; it could be envisaged thatrevealing an additional low temperature change in hue may beattractive as a ‘hidden’ authentication feature. The naphthopyran-8-yl unit was shown, by comparison with model compounds pos-sessing a 2-methoxy-6-naphthyl unit, to exert a useful bath-ochromic shift. A detailed examination of the full photochromicresponse of these complex bi-naphthopyrans 17 including anexploration of the irradiation time, matrix temperature and kineticanalysis of the fade rates complete with TD-DFT computations ofthese complex bi-naphthopyrans that possess conjugated mer-ocyanine dye units in their irradiated forms, will be presented indue course.

Acknowledgement

James Robinson Ltd., (Huddersfield) and Yorkshire Forward arethanked for a studentship to SBK (2004-07). The EPSRC are thankedfor access to the National Mass Spectrometry Service (Swansea).

S. Aiken et al. / Dyes and Pigments 113 (2015) 239e250250

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