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Synthesis of a Fully Unsaturated "Molecular Board"** Matthias Loffler, Arnulf-Dieter Schluter," Katrin Gessler, Wolfram Saenger, Jean-Marc Toussaint. and Jean-Luc Bredas Dedicated to Profl.xsor Henry Hall, J r . , on the occasion of' his 70th birthria.).

Our current research is focused on the synthesis ofstructural- ly perfect, fully unsaturated double-stranded polymers.['] These are potentially interesting as materials for application in areas such as electrical conductivity. optical nonlinearity, electro- luminescence, and photovoltaics.['~ '1 Unsaturated polymers of this kind should have a planar, boardlike structure, a feature which inevitably renders them insoluble. For the synthesis of insoluble and yet well-defined polymers, precursor concepts have proved very successful for those cases in which the first step is the preparation of a characterizable and processable polymer which then, in a second step. is converted into the desired poly- mer. Here we report the synthesis of a double-stranded Diels- Alder (DA) polymer and its polymer-analogous conversion into the unsaturated, all-carbon polymer 1 (Scheme 1 ) . The back- bone of 1 consists of a regular sequence of typical substructures of fullerenes, like the pyracylene A and the oligoacene B (n7 = 2). Polymer 1 is the second member of a new class of

t

I-

0 LL., , J J L ~ > U . L A L I I , . I I , , , , I , , . >.I. 1 1

-6 -5 -4 -3 -2 -1 0 1

log c - Fig 3 Relati\e fluorcscence intensity I,,, of 2 a s a function of the glucose concen- tration lo: < w i t h and without 0.1 mM 11-galactose or u-fructose at 2 5 ' C : 1 . 0 ~ t o - ' hi of 2 in 33.3% MeOH'H20 buffer at pH 7.77. j.L" = 370 nm. i,,,, = 413 11111. i,-gnlactose,'u-glucose. i,-fructose:i,-glucose. o u-glucoac.

ethylenegl>col.

observed stability (log K,) is the same as that obtained with D-glucose alone (Fig. 3).

In conclusion. a PET sensor has been designed that is selective for glucose. We believe that it will be possible to construct other saccharide-selective sensors if particular attention is paid to the spatial disposition of the two boronic acid moieties with respect to the hydroxyl groups of the target saccharides.

Received: April 5. 1994 Revised version: July 2, 1994 [Z 6823 IE]

German version. dnpcii.. C/iiw. 1994. 106. 2287

[I] Revic\\\ A. J. Bryiin. A. P. de Silva. S. A. de Silva. R. A. D. Rupasingha. K . R A S Sandanayake. Bio.srn.\or.s 1989, 4, 169; R. Bissel. A. P de Silva. H. 0 N Gunaritna. P L. M. Lynch. G. E. M. Maguire. K. R. A. S. San- danayakc. Chwn. So<,. Rcr.. 1992, 21. 187.

121 S. Shinkai. ti. Tsukagoshi. Y. Ishikawa, T. tiunitake, J C ' h i w i . So?. Chon. C . n r i i n i I i i i . 1991. 1039.

[3] K. Tsukagoshi. S. Shinkai: J Org. Cherii. 1991. 56. 4089. [4] K. Kondo, Y. Shiomi. M . Saisho. T. Harada, S. Shinkai. Terraheihwi 1992, 4X.

[5] Y. Shioiiii. K tiondo. M. Saisho, T. Harada. K. Tsukagoshi. S . Shinkai.

161 Y. Shiomi. M . Saisho, ti. Tsukagoshi. S. Shinkai. J . Chein. Snc. Pwkbi Trini.\.

[7] G. Deng. T. D. .lames. S. Shinkai. .I h i . C'hrrn Sot.. 1994. 116. 4567-4572. [XI T. D. .I.imes. T. Harada, S. Shinkai. J Chem. Sor. Chiwn. C'onnnun 1993. 857.

191 T. D. I:imcs. ti. Murata. T. Harada, K . Ueda. S Shinkai. Chiwi. Lrir 1994, 273. [lo] R. Luduig. T. Harada. ti. Ueda, T. D. lames, S. Shinkai, J. Chcin Soc. Pcv-Xiri

T i - i r f r > . 2 1994. 4. 697. [I I ] T. D. Junes. ti. R. A. S. Sandanayake. S. Shinkai. J Cheiii. So<. C h ~ ~ n i . C'oni-

n i r i i i . 1994. 477 [I?] J. Yoon. A W. Czarnik. J . h i . C'heni. So?. Chrni. Conini~in. 1994. 477. 11.31 L. K . Mohler. A. W. Czdrnik. .I h i . Cheni. Soc. 1993. / / 5 , 7037. 1141 L. ti. Miohler. A. W. Czarnik. J Am. Chen?. So?. 1993, 11.5. 2998. [l5] G. Wulff. B. Heide. G. Helfmeier. J h i . Chern. So?. 1987. IOK, 1089. 1161 G. Wulff. H:G. Poll, Mokroiiioi. C ' h ~ w . 1987, IKX, 741. 1171 Review: G. Wull'f. Pirrr .4pp/. Chcm. 1982. 54, 2093. [ I X ] From oiii- work and that of others [1 171 it IS known that the boronate ester

is rapid11 and rcvcrribly formed under basic conditions. The samc rapid cqui- libriuin condit~on also exists in ortho aminoboronic acids at neutral pH [I 1, 161 Non-co~ iilent interactions are described with such terms as "recognition". "complex". and "binding constants". These terms will be used to describe the equilihriuin betueen covalent boronate ester and free boronic acid under these conditions.

[I91 "Clefi" type receptors have been extensively studied by J. Rebek J r . , et al. J. Rebek, .Ir . .A", Cheni. RPS 1990, 23, 399 and references therein.

1201 R T Ilawkin\. H. R. Snyder, J . Am. Chein. Soc. 1960. X2. 3863; H. R. Snydei-. R . T. 1i:iwkins. W J. Lennorr. il>ril. 1960. 82. 3053: H. R. Snyder, M. S . Ko- neck). W _I. Lennorz. Fhd. 1958. 80. 361 I .

8239.

.Siiprnniii/. 1993. 2. I 1

1 1993. 7111

I176 ~corrigendum).

n

A B

rn = 0. 1, 2,

C

1

Scheme 1. Some fullerene substructures A, B. and C and the btrnctui-e of the rully unsaturated, double-stranded polymer 1.

[*] Prof. Dr. A.-D. Schluter. M. Loffler lnstitut fur Organische Chemie dcr Freien Universitit Takustrasse 3, D-14195 Berlin (FRG) Telefax: Int. code +(30) 8383357 K . Gessler. Prof. Dr. W. Saenger Institut fur Kristallographie der Freien Universltiii Berlin (FRG) Dr. J:M. Toussaint. Prof. Dr. J.-L. Bredas Universite de Mons-Hainaut Service de Chimie des Materiaux Nouveaux. Mons (Belgium)

[**I This work wa? supported by the Bundesministerium fur Forschung und Tech- nologie. the Fonds der Cliemischen Industrie, and the Deutsche Forschungsge- meinschaft. We also thank W. Lamer for the SEC measurements, 0. Klein and G. Buntkowski for recording "C CP.'MAS spectra. and C. Bartel for help with some of the experiments. The work in Mons was partially supported by the Belgian Government "Pole d'Attraction lnteruniversitaire en Chimie Supramoleculaire a t Catalyse" and the Belgian "Programme d'Impulslon en Technologie de I'lnformalion" (contnict ITiSC:22). FNRS-FRFC, an IBM Academic Joint Study. and the Commission of Eui-opean Community through the ESPRIT Basic Research Action TOPFiT Project 7282.

D-69451 Weinheirn, 1994 0570-0~33'94~'2121-2209 3 10.00 + .25:0 2209

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polymers composed of substructures of fullerenes.[6] It closely resembles the open-chain analogue C of the belt-region of C6,,. We also discuss the results of calculations on the elec- tronic structure of the parent compound of 1, which bears no hexyl substituents. These calculations are aimed at provid- ing a description of the interactions between these substruc- tures.

The selection of the precursor polymer and the kind of chem- istry needed for its conversion into the target structure resulted from the key observation that compound 2, regardless of its actual stereochemistry (six diastereoniers) can be dehydrated with p-toluenesulfonic acid (p-TsOH) to benzodifluoranthene 3 in yields of virtually 100'Yi (Scheme 2).[71 To our knowledge.

2 3

Scheme 2. Dehydration of the model compound 2.

this is the first and to date only successful way of converting precursor molecules by dehydration into polycyclic aromatic compounds, which proceeds reproducibly and cleanly. The dehydration does not seem to be catalyzed by p-TsOH: thus, it should be possible to control the number of water molecules removed by stoichiometry (see below). These features make a polymer with the structural subunits of 2 an ideal candi- date for a polymer analogous application. We, therefore, synthesized polymer 6 and subsequently dehydrated i t with

The synthetic route to 6 involves the DA-polyaddition of the in situ prepared bisdiene 4 with the bisdienophile 5 (Scheme 3).

p-TsOH.

[ 0*0] C6H13

4 5 l a . ,

6

6 I - ( H A L 6 I 1

Scheme 3. Synthesis of the precursor polymer 6 and I t s conversion into polymer 1 : a ) 4 (583 mp. 0.51 inmol). 5 (410 mg: 0.51 mmol). 110 C . toluene (50 mL) . 24 h. precipitation into methanol:hexane; b) 6 (24.3 mg). pTsOH (5.4 mg. 1.3 equiv: rcpeat unit). 110 C . toluene (15 mL). 24 h, precipitation into methanol; c) 6 (4X.6 mg). p-TsOH (15 mg, 4.0 equiyrepeat unit). 110 'C. toluene (20 mL), 24 h. precipitation into methanol.

"Monomer" 4 has proved very versatile in countless poly-DA- additions;"] monomer 5 deserves some comments.[*] First, 5 is bridged by flexible alkyl chains ("loops"). This should help to increase the solubility of the precursor and the target polymer. The loops, like straight alkyl chains,['] are effective solubilizers for entropic reasons, but are also good for enthalpic reasons because they disturb the packing of "flat boards". Second, this allows one to obtain a polymer with repeating units. which already contain extended aromatic regions. Therefore only a few dehydration steps are necessary to generate large conjugat- ed areas. As to the question of solubility, the crystal struc- ture"". of a diastereomer of model compound 7[*] provides an insight into how the "molecular boards" would most pro- bably pack if alkyl loops were not present (Fig. 1 ) . The

7

Fig. 1. Model compound 7 and its crystal structure (ORTEP)

diastereomer of 7 shown forms sandwichlike structures in which the planar regions are arranged almost on top of each other at van der Waals distance. This situation cannot occur in the poly- mer because of the randomness of the relative orientations of the

Polymer 6 was obtained as a yellow-orange material in 8 5 - 95% yield. The molecular weight of 6, in contrast to other DA-ladder polymers, has a pronounced effect on its solubility in common organic solvents at room temperature. The ratio of soluble, yet relatively low-molecular-weight material to insol- uble but high-molecular-weight material can be adjusted by changing the conditions. For example, heating a 0.005 M mix- ture of the monomers 4 and 5 to 110 "C for 24 h in toluene gave a material which was completely soluble in benzene [size exclu- sion chromatography in T H F versus polystyrene: M,, = 5000 ( p , =4-5);M,.=11100(P,, =lO);D=2.2] . I f thesameexper- iment was carried out a t higher monomer concentration (0.135 M). the amount of soluble material was reduced to

D = 4.21. In this context it is important to note: a) A sample of the soluble polymer remained completely soluble if it was heated in toluene under reflux for 3 d . Cross-linking does, therefore, not seem to play a role, b) The formation of insoluble material can be prevented if dodecyl instead of hexyl chains are used as solubilizers. The determination of the structure of polymer 6 is based chiefly upon the 'H and 13C N M R spectra for the soluble and the I3C CPiMAS solid-state N M R spectrum for the insol-

loops.

25-30% [ M , ~ 7 2 0 0 (Pa = 6-7); M,,, = 30200 (Px, 126):

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A

! I

n

I I I I 200 150 100 50 0

-6

Fig. 2. "C CP:MAS N M R spectra (300 Mkl r ) or polqmera 6 (A) ( a = C-arom..

CH,) and 1 (C). and " C N M R spectrum (270 MH7. C.DC13) of partially dehydrated polymer 6 I B ) . Signals due to residual b- and j.-C iitomb m d tlic d v e n t are marked with a r r o h i a n d ,in a a t e r i k

[j = C - 0 . = H-C'-C-O. 6 = CH,. i. =

uble part. The latter is shown in Figure 2 to- gether with a signal as- signment which is based on the chemical shifts of four diastereomers of model compound 2. The 'H N M R signals of 6 are relatively broad ; therefore a de- tailed stereochemical (sequence) analysis was not undertaken. The C,H values from ele- mental analysis match the theoretical values within 0.5%. The ex- cellent match between the UV spectra of 3 and 6 (Fig. 3) is also in ac- cordance with the pro- posed structure.

In order to establish the point a t which the polymer becomes insol- uble. the initial dehy- dration studies were aimed at a partial de- hydration. Preliminary experiments were done on a low-molecular- weight fraction (M,2 =

5000. M , =10500)of6 by heating in toluene under reflux for 24 h in the presence of 1.3 equivalents of p-TsOH per mole repeat unit. The dehydration was accompanied by a color change from intense orange to red. The reaction mixture remained homogeneous. A representative UV spectrum of the material obtained shows that the i.,,, value is shifted bathochromically from 500 to 600 nm. The degree of dehydration has not yet been determined quantitatively. A com- parison of the 13C N M R spectrum (Fig. 2B) with that of the starting material, however. suggests that more than 80% of the oxygen bridges as well as the adjacent hydrogen atoms have been removed. Only low intensities for the signals assigned to

Xinm - Fig. 3 . U \ VIS spectrii of model cornpound 7 in chloroform (2max(lgc) = 497 (4700)) (crocked line) and of polyiner 6 (dotted h e ) . partially dehydrated 6 (dashed line). and fully dehydrated I (solid line). A = absorption (arbitrary units).

the ether (/I) and bridgehead ( 7 ) carbon atoms are observed. The fact that the partially dehydrated materials are still soluble in common organic solvents. even though a large proportion of the kinks have been removed, underlines the effectiveness of the alkyl substituents as solubilizers. The second step involved the attempt to drive the dehydration to completion. For this the same experiments were carried out with an excess of p-TsOH. The material obtained in this way was only sparingly soluble. Its UV spectrum (Fig. 3) shows no further bathochromic shift, but instead a significant increase in the intensity of the long wave- length absorption bands. This is a clear indication that more water molecules have been removed from the backbone of the polymer. Furthermore, the 13C CP!MAS NMR spectrum of this material (Fig. 2C). does not indicate the presence of any "residual water" in the structure. The data from elemental anal- ysis match the calculated values within 1.0%. The soluble high- molecular-weight fractions of 6 gave comparable results.

Parallel to the synthetic work, theoretical investigations were undertaken on the unsubstituted form of 1 to determine the electronic structure and to outline the origin of the band gap[41 in this polymer. The structure of the trimer was first optimized by the semiempirical AM1 technique, a method well-known for its reliable determinations of geometries on conjugated com- pounds.[12] Then, in a second step, the central part of the opti- mized trimer was used as input for band-structure calculations with the valence effective Hamiltonian (VEH) technique;[l3] this method provides accurate determinations of essential elec- tronic parameters in organic compounds. such as ionization potential, electron affinity. and band-gap values. From the opti- mized geometric structure of 1. it appears that the polymer backbone can be simply envisioned as a regular succession of naphthalene and anthracene units that are linked to each other by single carbon-carbon bonds. The bond lengths and angles along the conjugated backbone obtained from the calculations are indeed almost identical to the ones optimized in discrete naphthalene and anthracene molecules. The most important electronic parameters calculated for the unsubstituted form of polymer 1 are given in Table 1 together with those obtained for

Table 1 , VEH-culculated values of solid-state ionization potential ( IF) . electror affinity (EA). and cnergy gap (Eg) for the unsubstituted polymer 1. naphth;ilenc. and anthracene Values in eV.

Compound IP EA Ep

Polymer 1 [a] Naphthalene Anthracene

4.96 - 2.81 2.15 6.70 - 1.60 4,611 5.65 - 2.17 3.4X

[a] Parent compound without hexyl suhstituents

naphthalene and anthracene. The calculated band gap for the polymer is 2.1 eV. a value in excellent agreement with the long wavelength absorption in the UV spectrum of the substituted polymer (Fig. 3). The bonding-antibonding electronic patterns appearing on the polymer electronic bands have been analyzed and compared to those obtained for the electronic levels of naphthalene and anthracene. It appears that the HOMO of the polymer arises from an antibonding interaction between the HOMOS of naphthalene and anthracene while the LUMO of the polymer comes from the bonding interaction between the LUMO of naphthalene and LUMO + 1 of anthracene.

The properties of the polymer are currently under investiga- tion with a view to its application in the areas mentioned in the introduction. Received. Junc I . 1994 [ L 6987 I € ]

Germail \ersion A n g ~ v . C / w n . 1994. fM. 27x1

COMMUNICATIONS [ I ] A . - D Schluter. A h . M c i t w 1991. 3. 282 291. [2] S. Kivelsoii, 0. L Chapman. P h n . K c v . B 1983. 18, 72.36 7243. [3] L. Yu. M. Chen. I.. R. Dalton. C'/irm. M a r c r . 1990, 2. 649-659. [4] J. .I Bredas. R. Silbcy. Coiiliqrirrd Pol~ni i~rs , Tlic ,Vow/ S(,ioicr cmr i l i ~ h n o l o g i ~

of Hi,ahli Condiir /in,? and .Vonlinrar Opric cdls A < r i v e lWor~~riul,s. Kluwcr. Dor- drccht. 1991

151 M. Schwoercr. P / I ~ Bl. 1994. 5 0 . 52-58. [h] First example A -D. Schluter. M. Ldller. V. Eiikclmanii, ,Vrirnrc, 1994. 36K.

831 -834. [7] H. Schirnier. A.-D. Schluter. V. Eiikelinann, C/lein. Brr. 1993. 126. 343-2546, [8] M. Liiffler. A.-D. Schliiter. SI-ii/c,ri 1994. 75-78. [9] See Tor example M. Ballauff. A n p m . Clwni. 1989. 101. 261 -276. Anjicu . Cllmi.

[lo] Single cr\,stals of' 7 were wown from toluene. 6565 uniuue reflcctions were 1/11, E d /+,a/. 1989, 28- 253-267.

L -

collected on a n Enraf-Nonius Turbo-CAD4 difTractonieter with Ni-filtered Cu,, radiation from ii rotating-anode generator. (/. = 1.542, " 1 ' 2 0 scan mode. Y,,,,, = I20 . nominal resolution ;.;2 sin Om,, = 0.89 A. Y scan absorption cor- redion. T= 260 K ) . The data &ere corrected for radiation damagc (12% reduction in intensity of three relerence reflections) Space group triclinic. P I . Z = 2 with two toluene per formula unit in the asymmetric unit. N = 12.390(4). h = 16.164(6). = lX.96(2) A, 2 = 87 67(5). p = 83.30(5). ;' = 68.66(3) , V = 3514(4) A'. The structure was determined by direct methods using pro- gram SIR92 and refined with SHELX76 and SHELX93 on the basis of 3643 rellections with \(,I t 3o- ( l f J ) . H atoms bonded to the aroin:itic <-atoms were placed in their calculated positions. TWJ atoms in one of the aliphatic cliains. C45 and C49 are tnofold disordered with 0 . 5 occupancy. The cocrystallired toluene inolecules are fully ordered. The relatively high crystallographic R

mciated with radiation damagc of the crystal. with only 5 5 % of the data above . 3z ( l~ , l ) . and wi th the atom disorder. The extended aromatic system is x ~ i r t n ~ 1 1 1 ~ planar: with iio atom deviatiiig more than 0.105 A from the weighted least-squues plane [I I ] Further details of the crystal btructure investigation may be obtained from the ~achinforiiiationczentruin Karlsruhe. D-76344 Eggenstein-Leopoldshafen on quoting the depository number CSD-5X461. M. .I. S . Dc\+ar. E. G Zoebisch. E. F. Heal?. J. J. P. Stewart. J. ,4iJI. C'lien?. So( . 1985. 107. 3902 3909. G. Nicolas, P. Duriind. J C % i w i . P1ii.i. 1979. 70. 2020-2021 J. M. Andre. L. A. Burkc. G. Delhalle. G. Nicolas. P Durand. h i . J . Q w i i r I n i i C3rni. 1979. 13. 783 2x7: J:L. Bredas. K. R. Chance. R. Silbey. G. Nicolas. P. Durand. J. Chm Plij.5. 1981, 73. 255 267.

Dimesityldioxirane-A Dioxirane Stable in the Solid State** Andreas Kirschfeld, Sengodagounder M u t h u s a m y , and Wolf ram Sander * Dcdzcated to P r o f i ~ ~ o l - Cllrzstopli Ruchard 017 the o ( c u i o r i of his 65th hirthdu,

Since the discovery by Murray and Jeyaraman"] in 1985 that dimethyldioxirane (1) can be synthesized and is stable in ace- tone. 1 has become an important and powerful oxidant in or- ganic chemistry.['- 5 1 The only other dioxirane that has found synthetic application is methyl (trifluoromet1iyl)dioxirane (2),

introduced by Curci et aLL6] Dioxi- ranes 1 , 2, and a few other alkyl-

H3C$ 1 R = C H 3 substituted derivatives were syn- thesized by the oxidation of 2 R = C F 3

the corresponding ketones with Caroate,['I a method originally developed by Edwards and Curci.['. "I These dioxiranes are moderately stable in solution at

R

[*] Prof'. Dr. W. Sander. Dipl.-Chem. A. Kirschfeld. Dr. S. Muthusamy Lchrstuhl fur Organische Chemie I1 der Ruhr-UnivcrsitHt Universitdtsstresse 150. D-44780 Bochuin ( F R G ) Telefax' Int. code +(234)7094-353

[**I This work was financially supported by the Deutsche Forschungsgeinein- schaft, the Buiidesministerium f i r Forschung und Technologie (EUROTRAC 07EU761 6) . and the Fonds der Chemischen Industric. S. M. thanks the Alexander von Humboldt Foundatioii for a postdoctoral fellowship and A. K. the Land Niedcrsachsen for a poctgraduate fellowship.

room temperature. but cannot be isolated as pure compounds. Two stable perfluorinated dioxiranes seem to have been synthe- sized by Talbott and Thompson by fluorination of the lithium salts of ketone hydrates.["] Recently Russo and Des- MarteauI' reported the synthesis of difluorodioxirane from FCOOF and CsF!CIF. This gaseous dioxirane is stable a t room temperature for several days.

The most general route to dioxiranes is the oxidation of diazo compounds either directly with singlet oxygen["] or by first photochemically generating the carbenes followed by a thermal reaction with triplet Both routes lead to the highly labile carbonyl 0-oxides iis primary products, which upon irra- diation into their strong UVNis absorptions (370-550 nm) re- arrange to dioxiranes. These intense absorptions enable the identification of the carbonyl oxides by time resolved spec- troscopy (laser flash photolysis, LFP) in solution at room tem- perature if the lifetime is of the order of p s to ms.~12~'4-' ' l or by matrix isolation spectroscopy at cryogenic temperatures,[' 31 Although carbene oxidation has been used to generate a variety of dioxiranes. this method has not yet been applied to synthesis on a preparative scale. because the solvent, the conditions of photolysis. and the temperature have to be carefully controlled to avoid carbene reactions (such as dimerization and insertion into the solvent) or secondary reactions of the dioxiranes (re- arrangement to esters or reduction to ketones).

Dimesitylcarbene 3 exhibits a lifetime in solution at room temperature which is two orders of magnitude larger than that ofdiphenylcarbene (200 vs. 1.7 ps),['*] but the reactivity of these two carbenes towards 0, is roughly equal.['"] Thus, although the sterically demanding substituents prevent most bimolecular chemistry, the small 0, molecule is able to approach the cdrbene center almost unhindered. In an LFP study Scaiano et al. were able to generate carbonyl oxide 5 as a transient species by laser photolysis of oxygen-containing solutions of diazomethane 4. In acetonitrile 5 shows an intense absorption band at 390 nm

C=N, hv -

hv' t-

and a first-order decay of its intensity several orders of magni- tude slower than other carbonyl oxides.['g1 This makes 3 an ideal candidate for the study of the oxidation of carbenes in solution.

Here we describe the first synthesis of a dioxirane isolable at room temperature--both in solution and in pure form-by the