J. Org. Chem. zyxwvut 1993,58, zyxwvu 4685-4690 A Variable Mechanism for the Nucleophilic Vinylic Substitutions in a Series of gem-Dihalogenated Alkenes by a Bidentate Sulfur Nucleophile: An Experimental and AM1 Theoretical Study Yves Gimbert? Alec Moradpour,*J Georges Dive,'J Dominique Dehareng? and Khaled Lahlilf Laboratoire de Physique des Solides (UA2 zyxwvut of CNRS), Uniuersitk de Paris-Sud, F-91405 Orsay, France, and Centre d'lnghierie des Protkines, Institut de Chimie, B-4000 Sart-Tilman, Likge, Belgium Received March 18, 1993 The nucleophilic substitutions of a series of gem-dihalogenated alkenes 3,5,7,8, and 9 (RS)zC=CXz (X zyxwvutsrqp = C1, F) with 1,2-benzenedithiolate b have been studied. Depending on the structures of the R groups (alkyl, saturated and unsaturated cycloalkyls, aromatic ring), the course of the reactions and the structures of the yielded products are modified. In the frame of the addition-elimination-type mechanism for these nucleophilic substitutions, the energy contents of the anionic intermediates, resulting from the additions of the nucleophile b to the unsaturated centers, is calculated at the AM1 method level. For the compounds 5, 7, 8, and 9, the calculated energies nicely corroborate the experimental results. For 3, anionic intermediates are no longer found by calculation, and a synchronous single-step substitution is strongly suggested. The nucleophilic vinylic substitutions of gem-dihalo- genated substrates, such as a, by a bidentate sulfur nucleophile, such as 1,2-benzenedithiolate b, can be considered as a general synthetic route to unsymmetrical tetrathia-substituted ethenes c (eq 1); if the R substituents RS s a b C X = halogen are, in addition, parts of a 1,3-dithiole ring, this process could then be a new entry to the preparation of unsym- metrical tetrathiafulvalenes (TTF), for which very few syntheses are available.' Several studies have been published along this line,Z4 but the course of all the reported reactions have not followed this simple picture (eq 1). In fact, depending on the nature of R, dramatic differences in the structures of the products have been found (Scheme I). The first example of this process (i.e., the substitution of tetrachloroethene by b) is the old Hurtley-Smiles synthesis of dibenzo-TI'F (2);z the last step of this synthesis is depicted in reaction 2 (Scheme I). This process has been more recently reinvestigated: with the study being aimed a t its modification for the synthesis of the monoben- zo-TTF (4) (reaction 3, Scheme I). However, the authors had not detected the expected compound 4. The sub- stitution of 3 by b leads surprisingly again to 2 through the intermediate formation of the gem-dichloroalkene l.3 On the other hand, starting from the alkyl-substituted substrate 5, we have found4 the expected substitutions leading, this time, to 6 and similar compounds. Scheme I 3 4 4686 As part of our interest in the synthesis of new donors: we needed a new access to unsymmetrical TTF's. We decided to reexamine these nucleophilic vinylic substi- tutions, in order to investigate the scope of the process and to rationalize the results summarized above which were, at first sight, inconsistent. Using a broader series of gem-dihalogenated alkenes, we have investigated such substitutions both experimentally and theoretically using the semiempirica16 AM1 method? Few theoretical studies have been devoted to the nucleophilic substitutions at vinylic centers;E a very complete analysis of such reactions has been carried out with model systems, on the ab initio computational 1evel.O Semiempirical (MNDO) calculations have been conducted to investigate the mechanism of the parent nucleophilic aromatic substitution,1° but not such studies are found for the nucleophilic vinylic substitutions. We would like to report here the unexpected results of our experimental and theoretical AM1 approach. They involve a new insight into the course of this series of t Universite de Paris-Sud. 8 Institut de Chimie. (1) Krief, A. Tetrahedron 1986, 42, 1210. See also: Lerstrup, K.; (2) Hurtley, W. R. H.; Smiles, S. J. Chem. Soc. 1926,2263. (3) Mizuno, M.; Cava, M. P. J. Org. Chem. 1978,43,416. (4) Gimbert, Y.; Moradpour, A. TetrahedronLett. 1991,32,4897 and Johannsen, I.; Jorgensen, M. Synth. Metals 1988,27, B9. the present work. (5) Gimbert, Y.; Moradpour, A.; Bittner, S. Tetrahedron Lett. 1990, (6) Dewar, M. S.; Jie, C. Acc. Chem. Res. 1992,25, 537. (7) Dewar, M. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. P. J. Am. (8) Shainyan, B. A. Rws. Chem. Rev. 1986,55, 511. (9) Cohen, D.; Bar, R.; Shaik, S. S. J. Am. Chem. SOC. 1986,108,231. (10) Dotterer, S. K.; Harris, R. L. J. Org. Chem. 1988,53, 777. 31, 1007. Chem. SOC. 1985,107,3902. 0022-3263/93/1958-4685$04.00/0 0 1993 American Chemical Society