Cite this: RSC Advances, 2013, 3,17260

New cycloimmonium ylide ligands and theirpalladium(II) affinities3

Received 18th April 2013,Accepted 9th July 2013

DOI: 10.1039/c3ra41911h

www.rsc.org/advances

Rodica Postolachi,a Ramona Danac,*a Niklaas J. Buurma,c Aurel Pui,a Mihaela Balan,b

Sergiu Shovab and Calin Deleanub

Ten new stable 4-(49-pyridyl)pyridinium disubstituted monoylides were synthesized by the reaction of 4-

(49-pyridyl)pyridinium monosubstituted ylides with electrophiles as aromatic isocyanates and isothiocya-

nates. The facile synthesis and high stability of the new ylides were attributed not only to the

delocalization of both negative and positive charges, but also to the intramolecular hydrogen bond. This

bond was proved to be present both in solution (by NMR) and solid phase (by X-ray crystallography). The

computational studies using density functional theory calculations (DFT) suggest, as well, an important

charge delocalization in gas-phase structures for simplified model disubstituted cycloimmonium ylides. The

palladium complexation for two of the new ylides was studied using NMR titrations and quantified using

UV-visible spectroscopy titrations.

Introduction

Cycloimmonium ylides are zwitterionic compounds in which acarbanion is covalently bonded to a positively chargednitrogen atom belonging to an azaheterocycle.1 The majorapplication of cycloimmonium ylides is as 1,3-dipolar inter-mediates in heterocyclic syntheses of new classes of azaheter-ocyclic compounds by various cycloaddition reactions.1–3 Thepresence of a negative charge at the ylidic carbanion center isthe source of the nucleophile behavior typical of these ylides4

but also the origin of their ability to behave as mono- orbidentate ligands for metals.5 There are only a few examples inthe literature of metal complexes with monosubstitutedcycloimmonium ylides (particularly with Pd(II) and Pt(II)).6

Because of the presence of additional donor atoms, viz. oxygenatoms in keto- or ester-stabilized ylides, or nitrogen atoms incyano-stabilized ylides, the bonding of the ylide to the metalthrough the carbanion is not necessarily the observed bondingmode in all cases.7 Contrary to the situation for monosub-stituted cycloimmonium ylides, no metal complexation stu-dies on disubstituted cycloimmonium ylides were found in theliterature.

Stable cycloimmonium ylides are obtained only when twoelectron-withdrawing groups are introduced on the carba-nion.4 Disubstituted cycloimmonium ylides have great poten-tial for use as analytical reagents8 and semiconductingmaterials,9 and some of them have shown biological activity.10

Cycloimmonium ylides are also used in the synthesis of ylidicpolymers by interphase-transfer polycondensation reactions.11

Because of their unique reactivity, (electronic) structure,stability, and associated spectroscopic properties cycloimmo-nium ylides are interesting subjects for computationalstudies12 and experimental studies of their electro-opticalmolecular parameters (for example, the rotational barriers forthe ylidic bond, the values of atomic orbital coefficientscorresponding to the frontier HOMO–LUMO orbitals, theformal charges on the nitrogen and the carbon ylidicatoms).1d,8b,13 In particular, in UV-visible spectroscopy, thecycloimmonium ylides showed a linear correlation betweenthe energy corresponding to the maximum of their absorptionband in the visible spectrum in binary solvent water-ethanoland the empirical solvent polarities defined by Kosower. Thesecorrelations suggest a common origin of the absorbances inthe visible region of the spectrum of pyridinium ylides, viz.intramolecular charge transfer.13c

To date, only few cycloimmonium ylides have beensubjected to structure elucidation using X-ray crystallography,all being disubstituted carbanions.1d,13a,13m,14 From theircrystal structures, a general classification has been acceptedthat distinguishes between two extreme situations dependingon the spatial distribution of the ylidic carbon with respect tothe aromatic ring, viz. the near planar and the non-planarcycloimmonium ylides.1d,13a,13m

aAlexandru Ioan Cuza University of Iasi Chemistry Department, 14 Carol I, Iasi

700506, Romania. E-mail: rdanac@uaic.ro; Fax: +40232201313; Tel: +40232201342bPetru Poni Institute of Macromolecular Chemistry, 41A Grigore Ghica Voda Alley,

Iasi 700487, Romania. E-mail: shova@icmpp.ro; Fax: +40232211299;

Tel: +40232421230cPhysical Organic Chemistry Centre, School of Chemistry, Cardiff University, Main

Building Park Place, Cardiff CF10 3AT, UK. E-mail: Buurma@Cardiff.ac.uk

3 Electronic supplementary information (ESI) available. CCDC 916649. For ESIand crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra41911h

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As part of our research in the field of 4-(49-pyridyl)pyridi-nium ylides structure and reactivity,3b,3e,f we synthesized aseries of new disubstituted cycloimmonium ylides using anadaptation of the ‘‘salt method’’15 in order to investigate theirstructural, electronic, biological, and complexation properties.We here report the synthesis of ten new disubstituted 4-(49-pyridyl)pyridinium methylides and their palladium(II)complexes. We report the first X-ray structure for a disub-stituted 4-(49-pyridyl)pyridinium ylide as well as the firstexample of the use of disubstituted cycloimmonium ylides asligands for metal complexation. The structures of the ligandsand their Pd affinities are discussed on the basis of NMR andUV-visible spectra.

Results and discussion

Synthesis

For the synthesis of the new pyridinium ylides derived from4,49-bipyridine, the salt method of Krohnke15 was used. Thus,monobromides 1(a–j)16 were dissolved in water and treatedwith an aqueous 0.2 N NaOH solution, resulting in theformation of the dark red monosubstituted ylides 2(a–j) as finesolid particles. The obtained suspension was centrifuged andthe ylides were separated and dried before use in the next stepin which the monosubstituted cycloimmonium ylides reactwith carbamoylating agents (aromatic isocyanates and iso-thiocyanates) in dry DMF (Scheme 1).

This reaction provided the 1-disubstituted 4-(49-pyridyl)pyridinium methylides 3(a–j) with two electron-withdrawinggroups on the carbanion in good yields (49–63%). Thestructures of ylides 3(a–j) were confirmed by elementalanalysis and spectroscopic data.

Spectroscopy

The disubstituted ylides are crystalline compounds with highlydelocalized positive and negative charges. The UV-visible

spectra of freshly prepared solutions of these compounds inethanol show intense absorption bands in the UV range (253–257 nm, 303–376 nm) and less intense bands in the visiblerange (482–507 nm). The UV bands are p A p* type, showingan important delocalization of p electrons, while the electronicbands in the visible region are likely associated with anintramolecular charge transfer from the carbanion to theheterocycle n A p*, decreasing in intensity to disappearancewhen the compounds are titrated with acid solution.1d In caseof the thioamide-substituted ylides, the visible bands areabsent (3h, 3j) or are very weak (3f, 3g, 3i), therefore theintramolecular charge transfer is very weak or does not occurin these compounds.

The IR spectra of the ligands show the expected character-istic bands. Bands corresponding to N–H stretches appear inthe range of 3125–3105 cm21. The band corresponding to NHbending group appears for all ligands around 1625 cm21. Forcompounds 3a–e this band is broad, due to the overlappingwith the CLO amide stretch. The thiocarbonyl CLS absorptionis not as strong as the absorption due to the carbonyl group.This difference is as expected since the sulphur atom is lesselectronegative than the oxygen atom and, therefore, the CLSgroup is less polar than the CLO group. In the case ofcompounds where the thiocarbonyl group is directly attachedto a nitrogen atom, i.e. N–CLS, the stretching the CLS stretch isstrongly coupled to the C–N, as a consequence of thiscoupling, several bands may, at least partly, be associatedwith CLS stretching vibration. Hence thioamides, thioureas,thiosemicarbazones, thiazoles and dithio-oxamides have threeabsorption bands in the region 1570–1395 cm21, 1420–1260cm21 and 1140–940 cm21, which are in part due to the CLSstretching vibration.17 The bands in the range of 1489–1507cm21 were attributed to the strong polarized CLOket stretches.

In addition, the carbonyl CLO and the N–H groups are likelystrongly polarized due to the delocalization of the negativecharge and this delocalization is strengthened by thepotentially important contribution of an intramolecularly

Scheme 1 Synthesis of 1-disubstituted 4-(49-pyridyl)pyridinium methylides 3(a–j).

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hydrogen-bonded species (Scheme 1). Key IR wavenumberswere confirmed by DFT studies (vide infra).

The intramolecular hydrogen bond is evidenced as well inthe 1H-NMR spectra of compounds 3, e.g. by the signals of the(thio)amide protons appearing at very low magnetic fields(12.33–13.29 ppm for compounds 3a–e and 14.07–15.09 ppmfor compounds 3f–j). The H2 and C2 signals appear moreshielded than the H29 and C29 signals. The crystal structure(vide infra) indicates that the p system on the pyridinium ringis at an almost 90u angle to the p system involving the ylidiccarbanion, which suggests that no delocalization of thenegative charge onto the pyridinium ring can take place.Similarly, the fact that only one signal is observed for the orthoand meta carbons and protons in the NMR spectra indicatesthat the pyridinium ring rotates fast on the NMR timescale. Ifthere were strong conjugation between the carbanion and thepyridinium ring, we would not expect rapid rotation. The NMRspectra are thus in line with minimal delocalisation of thenegative charge onto the pyridinium ring. DFT calculationsconfirm minimal density of the HOMO and HOMO-1molecular orbitals on the pyridinium ring (vide infra) althoughthe molecular orbitals are sensitive to the extent of hydrogenbonding. The shielding of the H29 and C29 nuclei is thereforeattributed to through space shielding rather than electrondelocalization. The ylidic carbons C7 themselves were identi-fied from the HMBC spectra through their coupling with theH2 protons, and appear at 114.3–116.0 ppm for compounds3a–e and at 124.1–125.3 ppm for compounds 3f–j, beinginfluenced by the nature of the substituents (amide orthioamide).

In addition, the mass spectra of two of the synthesizedylides (3d and 3g) show peaks corresponding to MH+ at m/z =518 for ylide 3g and peaks corresponding to MNa+ at m/z = 496for ylide 3d and m/z = 540 for ylide 3g.

X-ray crystallography

The structure of compound 3g was confirmed by X-raydiffraction crystallography (Fig. 1).

The unit cell contains two discrete molecules (denoted as Aand B), which exhibit quite similar geometric parameters. Asan example, Fig. 1 shows the molecular structure, togetherwith the atomic numbering scheme, of component A. It isnoteworthy that the molecular conformation is stabilized byan intramolecular hydrogen bond between the NH group asdonor and oxygen atom O1 as acceptor. The geometriccharacteristics of the H-bonds in the crystal structure aresummarized in Table 1. The ylide carbon atom (C8) and thenitrogen aromatic atom (N2) atoms both exhibit a planar sp2-hybridized state as confirmed by the sum of the bond anglesaround the atoms C8 and N2 which equal 359.8u and 359.7u,358.9u and 359.4u for components A and B, respectively. Thevalue for the C8-N2 interatomic distance for both crystal-lographic components is 1.46(1) Å, which is in a goodagreement with C8-N2 interatomic distances reported earlierfor similar ylidic systems1d (Table 1 in the SupportingInformation3). The carbanion atom deviates by 0.06(1) Årelative to the plane described by the pyridinium ring, whilethe angle between the pyridinium ring plane and the planeinvolving atoms C9-C8-C7 is 86.2(7)u for molecule A and74.7(4)u for molecule B. These data clearly show that ylide 3gbelongs to the class of nonplanar disubstituted cycloimmo-nium ylides.

The main crystal structure motif of compound 3g can becharacterized as a two-dimensional network, as shown inFig. 2. The construction of these layers occurs via a system ofN–H…O, C–H…O, C–H…N and C–H…S hydrogen bonds which,according to the classification system by Desiraju andSteiner,18 have been divided into strong and weak hydrogenbonds based on H…A and D–A distances as well as D–H…Abond angles (Table 1). The further consolidation of thesupramolecular architecture occurs through p–p stackinginteraction, which is evidenced by the short centroid-to-centroid distances between centrosymmetrically related aro-matic rings at 3.918 Å (Fig. 2).

Computational studies

Using the crystal structure for 3g as a starting point, gas-phasestructures for simplified model disubstituted cycloimmoniumylides (Scheme 2) were optimized using density functionaltheory calculations (DFT). The simplified model compoundswere selected to reduce conformational flexibility in non-essential parts of the molecule as well as the total number ofelectrons in the model compounds, while keeping the essenceof the electronic structure intact. For the thioamide-containingmodel compound, a hydrogen-bonded conformation (3q-S)was optimized, while for the amide-containing model com-pound both a hydrogen-bonded (3q-O) and a non-hydrogenbonded conformation (3q-O-noHB) were optimized.

Intramolecularly hydrogen-bonded 3q-S and 3q-O both showa flat central six-membered hydrogen-bonded ring involvingthe formal carbanion with the fluorobenzene ring connectedthrough the (thio)amide also essentially in the same plane.The pyridinium ring and the fluorobenzene ring connectedthrough the ketone are rotated out of the plane of the six-membered hydrogen-bonded ring. This is in agreement withthe structure from X-ray crystallography. In the optimizedconformation for 3q-O-noHB, on the other hand, the six atomsFig. 1 X-Ray crystal structure for compound 3g.

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are in distinctly non-planar relative orientations. Nevertheless,the ylidic carbanion remains sp2 hybridized as evidenced bythe sum over the bond angles around the anion of 360.0u (withno angle more than 2.5u different from 120.0u). Our computa-tional results confirm the classification of the intramolecularhydrogen bond as strong (vide supra): the N–H bond becomeslonger by 0.016 Å upon hydrogen-bond formation, 3q-O ispredicted to be more stable than 3q-O-noHB by 19 kcal mol21

(in the gas phase), the H…A distance is predicted to be 1.78 Åand the D–H…A angle is 142.2u.

The computational studies suggest efficient charge deloca-lization in disubstituted cycloimmonium ylides, as evidencedby low Lowdin charges on the ylidic C and N (Table 2); thecharges on the ylidic C and N are in fact not the most negativeand most positive Lowdin charges on the ylides.

Similarly, the distribution of the HOMO-1, HOMO andLUMO orbitals (Fig. 3) does not suggest significant chargelocalization on the ylidic C and N, as expected.

Fig. 3 shows the sensitivity of the molecular orbitals to theformation of the intramolecular hydrogen bond, with theHOMO-1 and the HOMO in 3q-O and 3q-O-noHB reversingorder. In addition, the pyridinium ring becomes more co-planar with the carbanion in 3q-O-noHB which results in asmall contribution of the orbitals on the pyridinium group tothe HOMO and vice versa for the LUMO.

Vibrational frequencies were also calculated and thecorresponding wavenumbers were corrected using a uniformscaling factor of 0.97 (Table 3).

The theoretical results obtained by DFT calculations(Table 3) are in agreement with experimental N–H stretchesfor the conformations involving an intramolecular hydrogen

Table 1 H-bonds parameters in compounds 3g

D–H…A

Distance, Å

Angle D–H…A, deg Symmetry code classificationaD–H H…A D…A

N1A–H…O1A 0.86 1.81 2.57(1) 146.3 x, y, z StrongN1B–H…O1B 0.86 1.88 2.566(9) 135.9 x, y, z StrongN1B–H…O1B 0.86 2.65 3.22(1) 124.5 2x, 1 2 y, 1 2 z WeakC20B–H…S1B 0.93 2.74 3.55(1) 145.0 x, 3/2 2 y, K + z WeakC21A–H…N3B 0.93 2.49 3.41(1) 168.0 x, 3/2 2 y, 2K + z WeakC17A–H…O1B 0.93 2.47 3.38(1) 165.7 1 2 x, 1 2 y, 1 2 z Weak

a Hydrogen bond classification according to Desiraju and Steiner.18

Fig. 2 The fragment of 2D supramolecular network in the crystal structure of compound 3g.

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bond. The calculated amide and ketone stretches are also inreasonable agreement with the experimentally determinedwavenumbers.

Palladium complexation of cycloimmonium ylides

The interaction of the new cycloimmonium ylides with PdII

was studied both qualitatively and quantitatively (vide infra).

NMR titrations

Complexation of new 4-(49-pyridyl)pyridinium disubstitutedylide ligands with PdII was clear from a titrating 3d withdichlorobis(acetonitrile)palladium(II) in CDCl3 as monitoredusing 1H NMR spectroscopy (Fig. 4).

Fig. 4 shows a decrease of the height of the peakscorresponding to the ligand protons and their eventualdisappearance, which is attributed to the saturation of ligandbinding sites with PdII and concomitant precipitation.

UV-visible titrations

To quantify the affinity of our ligands for PdII, we carried outUV-visible titrations using disubstituted cycloimmoniumylides 3d and 3g. We titrated the ligands 3d and 3g withdichloro bisacetonitrile palladium salt, Pd(CH3CN)2Cl2, indichloromethane (DCM) and in acetonitrile (ACN) (Fig. 5 and6).

Fig. 5 and 6 clearly show changes in the absorptionspectrum for 3d upon the addition of Pd(CH3CN)2Cl2 and

the fact that both increases and decreases are observedindicates that the decreases do not correspond to simpledilution of 3d. All UV-visible bands of the ligand are shifted,but the strongest shifted of 45 nm is observed for the bandoriginally at 485 nm. As the palladium concentration wasincreased, the color of the solution changed from orange toviolet purple (Fig. 1 in the Supporting Information3). Theoverlay spectra show several reasonable isosbestic points,although we note that the titration of 3d in DCM shows a‘‘drifting’’ isosbestic point near 500 nm, suggesting theinvolvement of more than two species in equilibrium.Despite the lack of sharpness in the isosbestic point, weanalyzed both data sets in terms of a multiple independentbinding sites model in order obtain an indicative quantifica-tion of the binding affinity and binding stoichiometry. InDCM, binding of palladium(II) with 3d (–HCLO) is strong withan equilibrium constant K . 1 6 108 M21 and one-to-onebinding event (best fit stoichiometry is 1 : 0.9). In ACN, thebinding affinity is lower; K is of the order of 1 6 106 M21 andthe binding stoichiometry is one-to-one ligand to palladium(II)salt (best fit stoichiometry is 1 : 1.4).

The titration was repeated with thioamide 3g (Fig. 7 and 8).Fig. 7 and 8 show trends similar to those in Fig. 5 and 6,

indicating that complexes with palladium are again beingformed during the titration. The isosbestic points areconsiderably sharper, suggesting the dominance of twospecies in the interaction. Although clear changes in the UV-visible absorption spectra are observed for titration of 3g withPd(CH3CN)2Cl2 in dichloromethane, saturation is not reachedup to 0.3 mM Pd(CH3CN)2Cl2 suggesting an equilibriumconstant for complexation K of less than 3 6 103 M21. Analysisof the titration data for 3g with Pd(CH3CN)2Cl2 in acetonitrilein terms of the multiple independent binding sites modelyielded an equilibrium constant of (2.0 ¡ 0.6) 6 106 M21 for aone-to-one binding process (best fit stoichiometry is 1 : 0.7).

Scheme 2 Simplified model compounds 3q used in DFT studies.

Table 2 Lowdin charges on simplified disubstituted cycloimmonium ylides

Ylide Charge on ylidic C Charge on ylidic N

3q-S 20.20 +0.213q-O 20.22 +0.223q-O-noHB 20.22 +0.21

Fig. 3 HOMO-1, HOMO and LUMO for 3q-S (top, left to right); HOMO-1, HOMOand LUMO for 3q-O (middle, left to right); HOMO-1, HOMO and LUMO for 3q-O-noHB (bottom, left to right), all plotted at contour values of 0.05.

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Conclusions

Ten new disubstituted 4-(49-pyridyl) pyridinium methylideshave been synthesized and characterized. A suitable singlecrystal for XRD analysis has been obtained for compound 3g,its molecular structure being the first example in the series ofdisubstituted 4-(49-pyridyl)pyridinium methylides. Two of theligands, viz. 3d and 3g, were tested as ligands for palladium(II).Ylides 3d and 3g appear to interact with PdII in a one-to-onefashion and binding is generally relatively strong with theexception of 3g interacting with PdII in DCM.

Experimental section

Melting points were recorded on an A. Kruss Optronic MeltingPoint Meter KSPI and are uncorrected. Proton and carbonnuclear magnetic resonance (dH, dC) spectra were recorded ona Bruker Avance 400 DRX (400 MHz) or a DRX-500 Bruker (500MHz). All chemical shifts are quoted on the d-scale in ppm.Coupling constants are given in Hz. IR spectra were recordedon a FTIR Shimadzu or Jasco 660 plus FTIR spectrophot-ometer. Low resolution mass spectra were recorded on aBruker microTOF-Q II Electrospray Ionization MassSpectrometer, ESI3 in the positive mode; m/z values are

Table 3 Calculated and experimental wavenumbers for characteristic vibrations in cm21

Vibration Calculated 3q-S Experimental 3q-S Calculated 3q-O Experimental 3q-O Calculated 3q-O-noHB

N–H stretch 3068 3118–3019 3230 3125–3025 3505C–H stretch 3183–3082 3188–3079 3165–3064(thio)amide carbonyl stretch — — 1640 1628–1622 1692ketone carbonyl stretch 1521 1507–1490 1534 1501–1493 1621

Fig. 4 Normalised 1H NMR spectra for 1.37 mM 3d in the absence of[Pd(CH3CN)2Cl2] and in the presence of 0.53 mM, 0.995 mM and 1.76 mM of[Pd(CH3CN)2Cl2] (bottom to top spectra) in CDCl3 followed by 1H NMRspectroscopy in CDCl3.

Fig. 5 UV-visible absorbance spectra for 0.0212 mM of 3d in the presence of 1.06 6 1023–0.042 mM of Pd(CH3CN)2Cl2 (left) and absorbance at 346 nm (circles) and570 nm (squares) as a function of Pd(CH3CN)2Cl2 concentration all in DCM at 25 uC. Solid lines correspond to the multiple independent binding sites model fit to theexperimental data (fit to solid data points only) (right).

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reported in Daltons. Thin layer chromatography (TLC) wascarried out on Merck silica gel 60F254 plates. Columnchromatography was carried out on silica gel (Roth 60, 0.04–0.063 mm). Visualisation of the plates was achieved using a UVlamp (lmax = 254 or 365 nm). UV-visible spectrophotometricmeasurements were carried out using a Jasco V-630 Bio,temperature-controlled through a Jasco EHC-716 Peltiersystem.

All commercially available products were used withoutfurther purification unless otherwise specified. Water waspurified using an ELGA option-R 7BP water purifier.

NMR titrations

A 1.96 mM solution of ligand 3d in CDCl3 was titrated byadding 50 mL, 100 mL, 200 mL aliquots of a 7.96 mM solution ofdichlorobis(acetonitrile)palladium(II) in CDCl3 directly in anNMR tube. Spectra were normalized using the peak for CHCl3

as a reference.

UV-visible titrations

Stock solutions of 3d in DCM and in ACN were 0.5 mM. Thestock solution concentrations of 3g were 0.5 mM in DCM and0.71 mM in ACN. The Pd(CH3CN)2Cl2 stock solutions were 0.5mM in DCM and 0.71 mM in ACN. Titrations involved aconcentration of 3d in the cuvette of 0.0212 mM in DCM and

Fig. 6 UV-visible absorbance spectra for 0.0108 mM of 3d in the presence of 1.06 6 1023–0.042 mM of Pd(CH3CN)2Cl2 (left) and absorbance at 525 nm (squares) as afunction of Pd(CH3CN)2Cl2 concentration all in ACN at 25 uC. Solid lines correspond to the multiple independent binding sites model fit to the experimental data(right).

Fig. 7 UV-visible absorbance spectra for 0.0212 mM of 3g in the presence of 1.06 6 1023–0.042 mM of Pd(CH3CN)2Cl2 (left) and absorbance at 350 nm (circles) and400 nm (squares) as a function of Pd(CH3CN)2Cl2 concentration all in DCM at 25 uC. Solid lines correspond to the multiple independent binding sites model fit to theexperimental data (right).

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0.0108 mM in ACN and a concentration of 3g in the cuvette of0.0212 mM in DCM and 0.0154 mM in ACN. Palladium(II)concentrations were varied in the range 1.06 6 1023 mM to0.042 mM, i.e. up to an excess of Pd(CH3CN)2Cl2.

X-ray crystallography

Crystallographic measurements were carried out with anOxford-Diffraction XCALIBUR E CCD diffractometer generat-ing graphite-monochromated Mo-Ka radiation. The crystalswere placed 40 mm from the CCD detector. The unit celldetermination and data integration were carried out using the

CrysAlis package of Oxford Diffraction.19 The structure wassolved by direct methods using SHELXS-9718 and refined byfull-matrix least-squares on Fo

2 with SHELXL-97.20 All H atomsattached to carbon were introduced in idealized positions (dCH

= 0.96 Å) using the riding model with their isotropicdisplacement parameters fixed at 120% of their riding atom.Positional parameters of the H attached to N atom wereobtained from difference Fourier syntheses and verified by thegeometric parameters of the corresponding hydrogen bonds.The main crystallographic data together with refinementdetails are summarized in Table 4.3

Computational studies

DFT calculations were carried out using GAMESS21 (version of11 August 2011), employing the B3LYP1 (VWN1 in B3LYP)functional and the 6–31(p,d,f) basis set for all atoms.Structures were optimized to (local) minima without con-straints. The gradient convergence criterion (OPTTOL) was setto the default value of 0.0001. Successful optimization to a(local) minimum was confirmed for all reported structures by afrequency calculation showing no negative frequencies.Calculated frequencies are uncorrected. Images were createdusing MacMolPlt.22

General procedure for synthesis of ylides 2(a–j)

1 mmol 4-(49-pyridyl)-pyridinium salt (1) was dissolved in 20mL water and 4 mL aqueous 0.2 N NaOH were added understirring. The color of the solution turns immediately to darkred. After 15 min of stirring the suspension is centrifuged,decanted and the obtained solid washed with water till the pHis neutral. The colored powder is dried and used in the nextstep without any purification.

Fig. 8 UV-visible absorbance spectra for 0.0154 mM of 3g in the presence of 1.54 6 1023–0.0568 mM of Pd(CH3CN)2Cl2 (left) and absorbance at 350 nm (circles) and400 nm (squares) as a function of Pd(CH3CN)2Cl2 concentration all in ACN at 25 uC. Solid lines correspond to the multiple independent binding sites model fit to theexperimental data (right).

Table 4 Crystallographic data, details of data collection and structure refine-ment parameters for 3g

Empirical formula C26H20BrN3O2SFormula weight 518.42 g mol21

T/K 293Crystal system MonoclinicSpace group P21/ca/Å 16.711(5)b/Å 21.455(5)c/Å 12.873(5)a (u) 90.00b (u) 99.700(5)c (u) 90.00V/Å3 4549(2)Z 8Dcalc/mg mm23 1.514m/mm21 1.929Crystal size/mm 0.1 6 0.05 6 0.05hmin, hmax (u) 5.74 to 52Reflections collected 16 069Independent reflections 8521[Rint = 0.1456]Data/restraints/parameters 8521/66/597GOFc 0.873R1

a (I . 2s(I) 0.0881wR2

b (all data) 0.1712Largest diff. peak/hole/e Å23 0.40/20.47

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General procedure for synthesis of ylides 3(a–j)

The dried ylide obtained from the procedure above wasdissolved in 4 ml dimethylformamide, then 1.1 mmol ofarylisocyanate or arylisothiocyanate was added. The resultingsolution is stirred under nitrogen for 5–6 h at 25 uC. If theproduct precipitated from solution, it was separated byfiltration, if not, water was added until the product separatedfrom solution. After filtration the product was washed with anMeOH : H2O (1 : 1) mixture. If necessary the product wasrecrystallized from MeOH : H2O (3 : 1, v/v).

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-chlorophenylamino)-1,3-dioxo-3-p-tolylpropan-2-ide (3a). Red crystals. Yield 60%, mp233–235 uC. IR (KBr, n (cm21)): 1628, 1533, 1493, 1411(CLOamide, CLOket, NH, C–N). UV-VIS (nm): 254, 303, 495. 1H-NMR (CDCl3, 400 MHz, d (ppm)): 12.56 (1H, s, NH), 8.84 (2H,d, H29, J = 6.0 Hz), 8.54 (2H, d, H2, J = 6.8 Hz), 7.74 (2H, d, H3, J= 6.8 Hz), 7.63 (2H, d, Hphenyl (ortoNH), J = 8.8 Hz), 7.49 (2H, d,H39, J = 6.0 Hz), 7.23 (2H, d, Hphenyl(metaNH), J = 8.8 Hz), 7.10(2H, d, H10, J = 7.6 Hz), 6.99 (2H, d, H-11, J = 7.6 Hz), 2.26 (3H,s, CH3). 13C-RMN (CDCl3, 100 MHz, d (ppm)): 178.7 (1C,CLOketone), 163.5 (1C, CLOamide), 151.4 (2C, C29), 150.6 (1C, C4),149.1 (2C, C2), 141.4 (1C, C49), 139.2 (1C, C9), 138.6 (1C,CaromaticNH), 137.11(1C, C12), 129.2 (2C, C11), 128.7 (2C,Cphenyl(metaNH)), 127.2 (2C, C10), 127.1 (1C, CCl), 123.1 (2C,C3), 121.1 (4C, Cphenyl(ortoNH), C39), 115.3 (1C, C7), 21.3 (1C,CH3). Anal. Calcd for C26H20ClN3O2: C 70.67; H 4.56; N 9.51.Found: C 70.55; H 4.48; N 9.58.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-chlorophenyl)-3-(4-nitro-phenylamino)-1,3-dioxopropan-2-ide (3b). Yellow crystals.Yield 63%, mp 257–259 uC. IR (KBr, n (cm21)): 1629, 1592,1500, 1389 (CLOamide, CLOket, NH, C–N), 1500, 1331 (NO2). UV-VIS (nm): 253, 361, 471. 1H-RMN (DMSO-d6, 400 MHz, d

(ppm)): 13.19 (1H, s, NH), 9.03 (2H, d, H2, J = 6.8 Hz), 8.82 (2H,d, H29, J = 6.0 Hz), 8.41 (2H, d, H3, J = 6.8 Hz), 8.18 (2H, d,Hphenyl(metaNH), J = 9.2 Hz), 8.00 (2H, d, H39, J = 6.0 Hz), 7.85(2H, d, Hphenyl(ortoNH), J = 9.2 Hz), 7.25 (4H, aq, H10, H11). 13C-RMN (DMSO-d6, 100 MHz, d (ppm)): 177.2 (1C, CLOketone),163.2 (1C, CLOamide), 151.6 (C, C4), 150.9 (2C, C29), 150.2 (2C,C2), 146.8 (1C, C–NO2), 140.8 (1C, C–NH), 140.6 (1C, C49), 139.2(1C, C12), 133.2 (1C, C9), 128.2 (2C, C11), 128.1 (2C, C10), 125.2(2C, Cphenyl(metaNH)), 124.0 (2C, C3), 121.8 (2C, C39), 118.2 (2C,Cphenyl(ortoNH)), 114.3 (1C, C7). Anal. Calcd for C25H17ClN4O4:C 63.50; H 3.62; N 11.85. Found: C 63.70; H 3.49; N 11.96.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-bromophenyl)-1,3-dioxo-3-(phenylamino)propan-2-ide (3c). Red-orange crystals. Yield59%, mp 227–229 uC. IR (KBr, n (cm21)): 1627, 1540, 1497,1397 (CLOamide, CLOket, N–H, C–N). UV-VIS (nm): 257, 303,489. 1H-NMR (CDCl3, 400 MHz, d (ppm)): 12.33 (1H, s, NH),8.88 (2H, d, H29, J = 6.0 Hz), 8.56 (2H, d, H2, J = 6.8 Hz), 7.81(2H, d, H3, J = 6.8 Hz), 7.69 (2H, d, Hphenyl (orto NH), J = 7.6 Hz),7.54 (2H, d, H39, J = 6.0 Hz), 7.34 (2H, d, H11, J = 8.4 Hz), 7.30(2H, t, Hphenyl (meta NH), J = 7.6 Hz), 7.13 (2H, d, H10, J = 8.4Hz), 7.03 (1H, t, Hphenyl (para NH), J = 7.6 Hz). 13C-RMN (CDCl3,100 MHz, d (ppm)): 176.9 (1C, CLOketone), 163.5 (1C, CLOamide),151.4 (2C, C29), 151.1 (1C, C4), 149.1 (2C, C2), 141.1 (1C, C49),139.7(1C, C12), 138.8 (1C, C9), 131.8 (2C, C11), 128.9 (1C,CaromaticNH), 128.8 (2C, C10), 123.3 (2C, C3), 128.9 (2C,Cphenyl(meta NH)), 121.1 (1C, C39), 120.1 (2C, Cphenyl(orto

NH)), 119.9 (1C, Cphenyl(para NH)), 114.3 (1C, C7). Anal. Calcdfor C25H18BrN3O2: C 63.57; H 3.84; N 8.90. Found: C 63.71; H3.70; N 9.01.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-methoxyphenyl)-3-(naphthalen-1-ylamino)-1,3-dioxopropan-2-ide (3d). Orangecrystals. Yield 49%, mp 233–235 uC, IR (KBr, n (cm21)): 1624,1540, 1496, 1390 (CLOamide, CLOket, NH, C–N), 1243, 1025 (C–O–C). UV-VIS (nm): 253, 331, 507. MS (ESI+): 496 (M + Na+); 1H-NMR-(CDCl3, 400 MHz, d (ppm)): 13.29 (1H, s, NH), 8.77 (2H,d, H29, J = 6.0 Hz), 8.63 (2H, d, H2, J = 6.8 Hz), 8.50 (2H, at,naphtyl, J = 6.8 Hz), 7.83 (1H, d, naphtyl, J = 7.6 Hz), 7.69 (2H,d, H3, J = 6.8 Hz), 7.36–7.59 (4H, m, naphtyl), 7.37 (2H, d, H39, J= 6.0 Hz), 7.23 (2H, d, H10, J = 8.8 Hz), 6.71 (2H, d, H11, J = 8.8Hz), 3.73 (3H, s, OCH3). 13C-RMN (CDCl3, 100 MHz, d (ppm)):178.4 (1C, CLOketone), 163.9 (1C, CLOamide), 160.0 (1C, C12),150.3 (C, C4), 151.3 (2C, C29), 149.2 (2C, C2), 141.5 (1C, C49),132.5 (1C, C9), 134.0 (1C, Cnaphtyl), 135.4 (1C, Cnaphtyl), 129.0(2C, C10), 128.4 (1C, CHnaphtyl), 126.1 (1C, CHnaphtyl), 126.0 (1C,CHnaphtyl), 125.9(1C, Cnaphtyl), 125.7 (1C, CHnaphtyl), 123.0 (2C,C3), 122.6 (1C, CHnaphtyl), 121.9 (1C, CHnaphtyl), 121.0 (2C, C39),116.5 (1C, CHnaphtyl), 115.8 (1C, C7), 113.9 (2C, C11), 55.3 (1C,OCH3). Anal. Calcd for C30H23N3O3: C 76.09; H 4.90; N 8.87.Found: C 75.92; H 4.78; N 8.95.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(naphthalen-1-ylamino)-1,3-dioxo-3-p-tolylpropan-2-ide (3e). Orange crystals. Yield 56%,mp 232–233 uC, IR (KBr, n (cm21)): 1622, 1543, 1500, 1388(CLOamide, CLOket, NH, C–N). UV-VIS (nm): 254, 329, 497. 1H-NMR-(CDCl3, 400 MHz, d (ppm)): 13.27 (1H, s, NH), 8.77 (2H,d, H29, J = 4.8 Hz), 8.62 (2H, d, H2, J = 5.6 Hz), 8.50 (2H, ad,naphtyl, J = 8.0 Hz), 7.83 (1H, d, naphtyl, J = 8.0 Hz), 7.69 (2H,d, H3, J = 5.6 Hz), 7.41–7.59 (4H, m, naphtyl), 7.38 (2H, d, H39, J= 4.8 Hz), 7.18 (2H, d, H10, J = 8.0 Hz), 7.01 (2H, d, H11, J = 8.0Hz), 2.26 (3H, s, CH3). 13C-RMN (CDCl3, 100 MHz, d (ppm)):178.8 (1C, CLOketone), 163.8 (1C, CLOamide), 150.5 (C, C4), 151.3(2C, C29), 149.2 (2C, C2), 141.4 (1C, C49), 139.2 (1C, C12), 137.3(1C, C9), 135.6 (1C, Cnaphtyl), 134.1 (1C, Cnapthyl), 129.3 (2C,C11), 128.4 (1C, CHnapthyl), 127.3 (2C, C10), 126.1 (1C, CHnapthyl),126.0 (1C, CHnapthyl), 125.8 (1C, Cnaphtyl), 125.7 (1C, CHnapthyl),123.0 (2C, C3), 122.7 (1C, CHnapthyl), 121.9(1C, CHnapthyl), 121.0(2C, C39), 116.6 (1C, CHnapthyl), 116.0 (1C, C7), 21.3 (1C, CH3).Anal. Calcd for C30H23N3O2: C 78.75; H 5.07; N 9.18. Found: C75.82; H 4.98; N 9.15.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-bromophenyl)-3-(4-methox-yphenylamino)-1-oxo-3-thioxopropan-2-ide (3f). Red crystals.Yield 55%, mp 175–176 uC. IR (KBr, n (cm21)): 1628, 1600,1507, 1478, 1393 (CLO, N–H, C–N), 1236, 1017 (C–O–C). UV-VIS(nm): 253, 334, 482. 1H-NMR (CDCl3, 400 MHz, d (ppm)): 14.07(1H, s, NH), 8.86 (2H, d, H29, J = 6.0 Hz), 8.66 (2H, d, H2, J = 6.8Hz), 7.81 (2H, d, H3, J = 6.8 Hz), 7.65 (2H, d, Hphenyl(orto-NH), J= 8.8 Hz), 7.56 (2H, d, H39, J = 6.0 Hz), 7.31 (2H, d, H11, J = 8.0Hz), 7.08 (2H, d, H10, J = 8.0 Hz), 6.90 (2H, d, Hphenyl(meta-NH),J = 8.8 Hz), 3.81 (3H, s, OCH3). 13C-NMR (CDCl3, 100 MHz, d

(ppm)): 185.0 (1C, CLS), 175.8 (1C, CLO), 157.1 (1C, C–OMe),152.9 (C, C4), 151.4 (2C, C29), 150.9 (2C, C2), 141.1 (1C, C49),139.5 (1C, C9), 133.7 (1C, C–NH), 131.7 (2C, C11), 128.3 (2C,C10), 126.0 (2C, Cphenyl(orto-NH)), 124.1(1C, C7), 124.0(1C, C–Br), 123.1 (2C, C3), 121.2 (2C, C39), 113.7 (2C, Cphenyl(meta-NH)),

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55.4 (1C, OCH3). Anal. Calcd for C26H20BrN3O2S: C 60.24; H3.89; N 8.11. Found: C 59.99; H 3.77; N 8.15.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-bromophenylamino)-3-(4-methoxyphenyl)-3-oxo-1-thioxopropan-2-ide (3g). Red-orangecrystals. Yield 53%, mp 170–171 uC. IR (KBr, n (cm21)): 1627,1598, 1489, 1389 (CLO, N–H, C–N). UV-VIS (nm): 253, 341, 491.MS (ESI+): 518 (M + H+); 540 (M + Na+). 1H-NMR (CDCl3, 400MHz, d (ppm)): 14.46 (1H, s, NH), 8.86 (2H, d, H29, J = 6.0 Hz),8.67 (2H, d, H2, J = 6.8 Hz), 7.81 (2H, d, H3, J = 6.8 Hz), 7.75(2H, d, Hphenyl(ortoNH), J = 8.8 Hz), 7.56 (2H, d, H39, J = 6.0 Hz),7.45 (2H, d, Hphenyl(metaNH), J = 8.8 Hz), 7.12 (2H, d, H10, J =8.8 Hz), 6.68 (2H, d, H11, J = 8.8 Hz), 3.73 (3H, s, OCH3).13C-NMR (CDCl3, 100 MHz, d (ppm)): 184.5 (1C, CLS), 177.5 (1C,CLO), 160.1 (1C, C12), 151.4 (2C, C29), 151.0 (2C, C2), 150.7 (1C,C4), 141.5 (1C, C49), 139.9 (1C, C–NH), 132.7 (1C, C9), 131.4 (2C,Cphenyl(ortoNH)), 128.4 (2C, C10), 125.7 (2C, Cphenyl(metaNH)),123.0 (2C, C3), 124.7(1C, C7), 121.2 (2C, C39), 117.5 (1C, C–Br),113.7 (2C, C11), 55.2 (1C, OCH3). Anal. Calcd forC26H20BrN3O2S: C 60.24; H 3.89; N 8.11. Found: C 60.05; H3.76; N 8.19.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-methoxyphenylamino)-3-oxo-1-thioxo-3-p-tolylpropan-2-ide (3h). Orange crystals. Yield57%, mp 187–188 uC. IR (KBr, n (cm21)): 1628, 1507, 1478,1386 (CLO, NH, C–N), 1236, 1019 (C–O–C). UV-VIS (nm): 253,329. 1H-NMR (CDCl3, 400 MHz, d (ppm)): 14.17 (1H, s, NH),8.84 (2H, d, H29, J = 6.0 Hz), 8.68 (2H, d, H2, J = 6.8 Hz), 7.78(2H, d, H3, J = 6.8 Hz), 7.66 (2H, d, Hphenyl(ortoNH), J = 8.8 Hz),7.54 (2H, d, H39, J = 6.0 Hz), 7.08 (2H, d, H10, J = 8.0 Hz), 6.96(2H, d, H11, J = 8.0 Hz), 6.90 (2H, d, Hphenyl(metaNH), J = 8.8Hz), 3.81 (3H, s, OCH3), 2.24 (3H, s, CH3). 13C-RMN (CDCl3,100 MHz, d (ppm)): 184.7 (1C, CLS), 177.5 (1C, CLO), 157.1 (1C,CaromaticOMe), 152.2 (C, C4), 151.4 (2C, C29), 150.9 (2C, C2),141.4 (1C, C49), 138.9 (1C, C12), 137.8 (1C, C9), 133.8 (1C,CaromaticNH), 129.1 (2C, C11), 126.7 (2C, C10), 126.1 (2C,Cphenyl(metaNH)), 124.2 (1C, C7), 122.8 (2C, C3), 121.2 (2C,C39), 113.7 (2C, Cphenyl(ortoNH)), 55.4 (1C, OCH3), 21.3 (1C,CH3). Anal. Calcd for C27H23N3O2S: C 71.50; H 5.11; N 9.26.Found: C 70.35; H 5.02; N 9.33.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-bromophenylamino)-3-(4-chlorophenyl)-3-oxo-1-thioxopropan-2-ide (3i). Orange crystals.Yield 59%, mp 180–182 uC. IR (KBr, n (cm21)): 1622, 1502,1485, 1367 (CLO, N–H, C–N). UV-VIS (nm): 256, 355, 490. 1H-RMN (CDCl3, 400 MHz, d (ppm)): 14.32 (1H, s, NH), 8.88 (2H,d, H29, J = 6.0 Hz), 8.64 (2H, d, H2, J = 6.8 Hz), 7.84 (2H, d, H3, J= 6.8 Hz), 7.74 (2H, d, Hphenyl(ortoNH), J = 8.8 Hz), 7.58 (2H, d,H39, J = 6.0 Hz), 7.46 (2H, d, Hphenyl(metaNH), J = 8.8 Hz), 7.15(4H, aq, H10, H11). 13C-NMR (CDCl3, 100 MHz, d (ppm)): 184.8(1C, CLS), 176.1 (1C, CLO), 153.0 (C, C4), 151.5 (2C, C29), 150.9(2C, C2), 141.4 (1C, C49), 139.6 (1C, CaromaticNH), 138.8 (1C,C12), 135.1 (1C, C9), 131.5 (2C, Cphenyl(ortoNH)), 128.8 (2C, C11),128.0 (2C, C10), 125.7 (2C, Cphenyl(metaNH)), 124.6 (1C, C7),123.2 (2C, C3), 121.3 (2C, C39), 117.9 (1C, C–Br). Anal. Calcd forC25H17BrClN3OS: C 57.43; H 3.28; N 8.04. Found: C 57.31; H3.15; N 8.17.

2-(4,49-Bipyridin-1-ium-1-yl)-1-(4-chlorophenyl)-3-(4-nitro-phenylamino)-1-oxo-3-thioxopropan-2-ide (3j). Yellow crystals.Yield 53%, mp 183–185 uC. IR (KBr, n (cm21)): 1629, 1591,1500, 1389 (CLO, N–H, C–N). 1500, 1331 (NO2). UV-VIS (nm):

256, 376. 1H-RMN (DMSO-d6, 400 MHz, d (ppm)): 15.09 (1H, s,NH), 9.18 (2H, d, H2, J = 6.4 Hz), 8.83 (2H, d, H29, J = 6.0 Hz),8.45 (2H, d, H3, J = 6.8 Hz), 8.28 (2H, d, Hphenyl(metaNH), J = 9.2Hz), 8.03 (2H, d, H39, J = 6.0 Hz), 7.86 (2H, d, Hphenyl(ortoNH), J= 9.2 Hz), 7.28 (4H, aq, H10, H11). 13C-NMR (DMSO-d6, 100MHz, d (ppm)): 184.2 (1C, CLS), 177.1 (1C, CLO), 151.6 (C, C4),151.6 (2C, C2), 150.9 (2C, C29), 147.1 (1C, C–NO2), 142.1 (1C, C–NH), 140.5 (1C, C49), 139.2 (1C, C12), 133.2 (1C, C9), 128.2 (2C,C11), 128.1 (2C, C10), 125.3 (1C, C7), 124.5 (2C,Cphenyl(metaNH)), 124.0 (2C, C3), 122.1 (2C, Cphenyl(ortoNH)),121.3 (2C, C39). Anal. Calcd for C25H17ClN4O3S: C 61.41; H 3.50;N 11.46. Found: C 61.31; H 3.35; N 11.47.

Acknowledgements

This research was financially supported by European RegionalDevelopment Fund, Sectoral Operational Programme‘‘Increase of Economic Competitiveness’’, Priority Axis 2(SOP IEC-A2-O2.1.2-2009-2, ID 570, COD SMIS-CSNR: 12473,Contract 129/2010-POLISILMET).

Notes and references

1 (a) I. Zugravescu and M. Petrovanu, N-Ylid Chemistry,McGraw-Hill, London, 1976; (b) A. Ed. Padwa, 1,3-DipolarCycloaddition Chemistry, Vol. 2, Chapters 12 and 13, Wiley,New York, 1984; (c) D. Klamann and H. Hagen, In Houben-Weyl, Organische Stickstoff-Verbindungen mit einer C,N-Doppelbindung, vol. E14b, Teil 1, pp. 1–160, Thieme,Stuttgart, New York, 1991; (d) Y. Karzazi andG. Surpateanu, Heterocycles, 1999, 51, 863.

2 (a) R. N. Butler, A. G. Coyne, P. McArdle, D. Cunninghamand L. A. Burke, J. Chem. Soc., Perkin Trans. 1, 2001, 12,1391; (b) W. Sliwa, Curr. Org. Chem., 2003, 7, 995; (c) P.C. Iuhas, F. Georgescu, E. Georgescu, C. Draghici and M.T. Caproiu, Rev. Roum. Chim., 2001, 46, 1145 and 893; (d)M. J. Minguez, J. J. Vaquero, J. Alvares-Builla, O. Castanoand J. L. Andres, J. Org. Chem., 1999, 64, 7788; (e) I. Druta,R. Dinica, E. Bacu and I. Humelnicu, Tetrahedron, 1998, 54,10811; (f) I. Druta, M. Andrei and P. Aburel, Tetrahedron,1998, 54, 2107; (g) O. Tsuge, S. Kanemasa and S. Takenaka,Bull. Chem. Soc. Jpn., 1985, 58, 3137; (h) I. Mangalagiu,M. Caprosu, G. Mangalagiu, G. Zbancioc and M. Petrovanu,ARKIVOC, 2002, 73; (i) I. Mangalagiu, G. Mangalagiu,C. Deleanu, G. Drochioiu and M. Petrovanu, Tetrahedron,2003, 59, 111; (j) G. Mangalagiu, I. Mangalagiu, R. Olariuand M. Petrovanu, Synthesis, 2000, 2047; (k) Y. S. Jung andJ. Y. Jaung, Dyes Pigm., 2005, 65, 205; (l) C. Moldoveanu,G. Mangalagiu, G. Zbancioc, G. Drochioiu, M. Caprosu andI. Mangalagiu, ARKIVOC, 2005, 7; (m) N. C. Lungu,A. Depret, F. Delattre, G. G. Surpateanu, F. Cazier,P. Woisel, P. Shirali and Gh. Surpateanu, J. FluorineChem., 2005, 126, 385; (n) Gh. Surpateanu, P. I. Dron,D. Landy, S. Fourmentin and M. Bria, Tetrahedron, 2008,64, 721; (o) F. Dumitrascu, M. R. Caira, E. Georgescu,F. Georgescu, C. Draghici and M. M. Popa, Heteroat. Chem.,2011, 22, 723; (p) F. Dumitrascu, M. Vasilescu, C. Draghici,M. T. Caproiu, L. Barbu and D. G. Dumitrescu, ARKIVOC,

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