Synthesis of Organothioacrylonitriles and Organoselenoacrylonitriles by Reaction of 1-Halo-1-chalcogenoalkenes with CuCN

Functionalized alkenyl chalcogenides (S, Se and Te) have great potential in organic synthesis, since they are valuable intermediates for the selective preparation of several organic compounds.1-7 The synthesis of vinylic chalcogenides has therefore attracted the attention of several research groups, and many novel methods for their preparation have been proposed in the last years.1-7 We described practical methodologies for the preparation of vinylic chalcogenides based on Wittig and Wittig-Horner reactions.8-10 Studies on the synthesis of α-phenylseleno and α-phenyltelluro acrylonitriles and the chemical reactivity of α-phenylseleno acrylonitriles in reactions with DIBAL-H, amines and in a Diels-Alder-type reaction were also described.10 Among the functionalized vinylic chalcogenides, those containing electron withdrawing groups, like a cyano group at the sp2 carbon (α-chalcogeno-α,β-unsaturated nitriles), are of great interest since they combine the chemical reactivity of the vinyl chalcogenide and the vinyl nitrile.1 Vinylic sulfides α-substituted by strong electron withdrawing groups have been synthetically used as potent Michael acceptors,11,12 in a variety of cycloaddition reactions13-18 and in studies as precursors of extended enolates.19,20 The selenium analogs have been used as dienophiles in DielsAlder reactions,21,22 as Michael acceptors23-26 and in the synthesis of 2,3-dihydroselenophenes and butadienes.27 α-Cyano substituted vinylic sulfides have been prepared by a few different routes.28,29 Methods for the preparation of α-phenylselenoacrylonitriles are also just a few10,30 and the synthesis of the corresponding α-phenyltelluroacrylonitriles was recently described by us.10 α-Phenylselenoacrylonitriles have been prepared by the addition of benzeneselanyl chloride or bromide to cyano olefins leading to α-seleno adducts, which are subjected to in situ dehydrohalogenation.27,30,31 Recently, Chinese authors described the preparation of several α-phenylseleno-α,β-unsaturated nitriles in moderate to good yields via α-phenylselanyl cyanomethylene triphenylarsorane.32,33 However, this method is restrict to aromatic aldehydes and no mention regarding the stereochemistry of the products was made. Besides these drawbacks, this method suffers from the high toxicity of arsenium compounds. More recently, Perin et al.34 described the synthesis of α-phenylselenoacrylonitriles and α-phenylseleno-α,βunsaturated esters by Knoevenagel reaction under solventfree conditions.


Introduction
][10] Studies on the synthesis of α-phenylseleno and α-phenyltelluro acrylonitriles and the chemical reactivity of α-phenylseleno acrylonitriles in reactions with DIBAL-H, amines and in a Diels-Alder-type reaction were also described. 10Among the functionalized vinylic chalcogenides, those containing electron withdrawing groups, like a cyano group at the sp 2 carbon (α-chalcogeno-α,β-unsaturated nitriles), are of great interest since they combine the chemical reactivity of the vinyl chalcogenide and the vinyl nitrile. 1 Vinylic sulfides α-substituted by strong electron withdrawing groups have been synthetically used as potent Michael acceptors, 11,12 in a variety of cycloaddition reactions [13][14][15][16][17][18] and in studies as precursors of extended enolates. 19,20The selenium analogs have been used as dienophiles in Diels-Alder reactions, 21,22 as Michael acceptors [23][24][25][26] and in the synthesis of 2,3-dihydroselenophenes and butadienes. 27-Cyano substituted vinylic sulfides have been prepared by a few different routes. 28,29Methods for the preparation of α-phenylselenoacrylonitriles are also just a few 10,30 and the synthesis of the corresponding α-phenyltelluroacrylonitriles was recently described by us. 10 α-Phenylselenoacrylonitriles have been prepared by the addition of benzeneselanyl chloride or bromide to cyano olefins leading to α-seleno adducts, which are subjected to in situ dehydrohalogenation. 27,30,31 Recently, Chinese authors described the preparation of several α-phenylseleno-α,β-unsaturated nitriles in moderate to good yields via α-phenylselanyl cyanomethylene triphenylarsorane. 32,33However, this method is restrict to aromatic aldehydes and no mention regarding the stereochemistry of the products was made.Besides these drawbacks, this method suffers from the high toxicity of arsenium compounds.More recently, Perin et al. 34 described the synthesis of α-phenylselenoacrylonitriles and α-phenylseleno-α,βunsaturated esters by Knoevenagel reaction under solventfree conditions.

Results and Discussion
Due to our continuous interest directed toward the development of new methods for the synthesis of functionalized vinylic chalcogenides, we would like to report herein a new, simple and efficient synthesis of α-organothioacrylonitriles and α-organoselenoacrylonitriles by the vinylic substitution of α-halo vinylic chalcogenides with CuCN in N-methylpyrrolidone, according to Scheme 1.
The α-halo vinylic chalcogenides 1a-o were easily prepared stereoselectively by the addition of the hydrogen halide (generated in situ by reaction of trimethylsilyl halide with anhydrous methanol) to the acetylenic chalcogenides, 35 usually obtained as a single isomer.The 1-halo-1-selenium alkenes could also be prepared by addition of hydrogen halides to acetylenic selenides. 36ur initial studies were made toward to the determination of optimum conditions to perform the reaction.Thus, we chose the (E)-1-bromo-1-phenylselenohexene (1c) as starting material to establish the best conditions for the vinylic substitution reaction.The 1bromo-1-phenylselenohexene (1.0 equiv.) was treated with CuCN (3.0 equiv.)as a source of cyanide anion in different solvents, such as DMSO, THF, NMP and mixtures of these solvents with HMPA under heating.The experiment carried out in NMP at 90 o C for 24 h was the only one to produce the desired α-phenylselenoacrylonitrile (40% yield after column chromatography).The use of HMPA as co-solvent with NMP did not increase the yield.The effect of the temperature in this reaction is noteworthy on the yield.We observed that satisfactory yield (60%) could only be obtained when the reaction was performed at 130 o C (oil bath temperature) for 24 h in NMP.The stoichiometry of CuCN was also studied, and the best results were achieved by using 3.0 equivalents of this reagent.Experiments carried out in presence of 1.0 and 2.0 equiv. of CuCN resulted in lower yields of desired products (10% and 43%, respectively) and a little amount of the starting material was recovered unchanged.At higher amounts of CuCN, no improvement on the yield was observed.Next step was a detailed study on the effect of the halogen atom on the α-halo vinylic chalcogenides and better yields were obtained using the iodo compared to the bromo derivatives, as can be observed in Table 1.The chloro derivatives showed to be inert under the conditions used by us.Thus, the optimum conditions for substitution reaction according to Scheme 1 were established to be the use of CuCN (3.0 mmol) in N-methylpyrrolidone (5 mL) and the 1-halo vinylic chalcogenide (1.0 mmol) at 130 o C for 24 h.A full study was performed with several different substrates, including sulfur and selenium derivatives substituted by alkyl and aryl groups.As can be seen on the Table 1, good yields of the products were obtained in most of the cases.
The method described here exhibits good generality to organoseleno-and organothioacrylonitriles and it is successful with aromatic and aliphatic derivatives.Although most experiments were performed on a 1.0 mmol scale, these reactions can also be performed successfully on higher scales with comparable yields.
Analysis of the 1 H NMR and 13 C NMR spectra showed that all α-organochalcogenoacrylonitrile compounds presented data in full agreement with their assigned structures.Concerning the stereochemistry of the obtained olefins, we usually observed the formation of Z/E mixture of isomers, even starting from pure (E)-1-halo vinylic chalcogenide.This is probably a result of the high temperature (130 o C) necessary to the reaction to occur.
For most examples, a nearly 1:1 E/Z ratio was observed, usually with a very small preference for one isomer.In a few examples (entries 7, 8, 11-15, Table 1), a higher preference for one isomer could be observed, but we could not establish a general pattern for this preference.The relative stereochemistry and the diastereomeric ratio of the obtained compounds 2a-h were assigned by comparing the chemical shifts with known aromatic acrylonitriles (2cg). 10The E and Z attributed stereochemistry was based on an X-ray structural analysis of one pure isomer and also from DIBAL-H reduction of the nitrile to the corresponding aldehyde, followed by a NOESY correlation study between the vinylic H and the aldehydic H.In the case of aliphatic acrylonitriles (2a, 2b and 2h), evidence for the determination of stereochemistry of the products was obtained by comparing 1 H and coupled and decoupled 13 C NMR spectra in an enriched mixture.The major isomer in aliphatic series can be easily determined by integration of the triplets (from the coupling of vinylic H with the vicinal CH 2 ) of the mixture.The determination of stereochemistry of the major isomer can be made, by analyzing of the nitrile carbon peaks in the 13 C NMR spectra.For example, for compound 2b, in the decoupled spectra the carbon signals for the isomers are at 115.5 and 116.9 ppm.In the hydrogen-coupled 13 C NMR spectra, the singlets split into doublets, with 3 J C-H couplings of 14.1 Hz and 7.1 Hz, typical of E (vinylic H and cyano anti) and Z (vinylic H and cyano sin) 3 J C-H coupling constants, 37 respectively.Correlation of the most intense carbon in 13 C NMR with most intense triplet in 1 H NMR allows the determination of the upfield triplet as from the E isomer (for 2b in 6.77 ppm) and the downfield triplet as from the Z isomer (for 2b in 6.80 ppm).The mixture of isomers could not be separated by column chromatography, since they have nearly the same Rf values.

Conclusions
In summary, we have developed a new, simple and efficient methodology for the synthesis of α-organoselenoand α-organothioacrylonitriles by the vinylic substitution of a halogen atom by a cyano group.The products were obtained as isomers mixtures in moderate to good yields.

General
All 1 H and 13 C NMR spectra were recorded at 200 MHz and 50 MHz, respectively, on a Bruker DPX 200 instrument, using CDCl 3 as solvent.Chemical shifts (δ) are expressed in parts per million (ppm) downfield from tetramethylsilane or CHCl 3 as internal standard, and J values are given in Hz.Mass spectra (EI) were obtained at 70 eV with a Hewlett Packard EM/CG HP-5988A spectrometer.Infra-red spectra were acquired on a Perkin-Elmer 1310 Spectrometer and the elemental analyses were performed with a Vario EL Elementar Analysis System.Merck's silica gel (230-400 mesh) was used for flash chromatography.All reactions were performed in flamedried glassware under a positive pressure of argon.Air and moisture sensitive reagents and solvents were transferred via syringe, and were introduced into reaction vessels through a rubber septum.NMP was distilled over argon before use.

General procedure for the synthesis of α-phenylseleno and α-phenylthio acrylonitriles (2a-h)
To a solution of the α-halo vinylic chalcogenide 1 (1.0 mmol) in NMP (5 mL) was added CuCN (0.26 g, 3.0 mmol) and the flask was immersed in an oil bath previously heated at 130 ºC.The reaction mixture was heated at this temperature for 24 h, cooled to room temperature, water was added and extracted with ethyl acetate (3 × 25 mL).The organic phase was washed with aqueous saturated solution of NH 4 Cl (2 × 50 mL), dried over MgSO 4 and the solvent removed in vacuo.The residue was purified by column chromatography (SiO 2 ) using hexane/ethyl acetate (99:1) as eluent.

Table 1 .
Vinylic substitution of 1-halo vinylic chalcogenides according to Scheme 1 a Isolated yields of products after column chromatography.b diastereomeric ratio determined by 1 H NMR (200 MHz) from the crude reaction mixture.Scheme 1.