Stereoselectivity in the debromination of the stilbene dibromides with stannous chloride in dimethylformamide

Abstract

The debromination of meso-or ¿/-stilbene dibromide by stannous chloride in dimethylformamide is a second-order reaction. The rate data at 59.4°are as follows: for meso, k = 3.73 X 10-4 M-1 sec-1, AH* = 15.5 kcal/mol, 5* = -28 eu; for dl, k = 3.36 X 10-s M-1 sec-1, AH* = 19.3 kcal/mol, AS* = -21 eu.The meso gives only zra/zs-stilbene, while the dl gives ca. 95% trans-and 5% c/s-stilbene over the range 60-100°. A concerted E2 process seems appropriate for meso, but this is difficult to verify. The formation of a tin(III) species in a bromine atom (one electron) transfer is proposed as the slow step for dl; this reaction is completed by another one-electron step involving the radical intermediates (7). A conformational analysis and product patterns of this system are consistent with the proposed mechanisms.(1) (a) Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. This paper was presented in part at the 154th Meeting of the American Chemical Society, Chicago, 111., Sept 1967, V 17. (b) Taken from the Ph.D. thesis (1967) of W. K. K. (c) Author to whom inquiries should be addressed.(2) W. K. Kwok and S. I.

Stereoselective 1,3-debromination reactions

“…In a preliminary study of 1,2-debromination reactions, we found the stereospecificity of the well-known anti elimination product to be comparable to that obtained from the best of the previously described procedures (17). For example, while many reducing reagents give stereospecific trans-alkene formation on debromination of meso-1,2-dibromides, in the difficult cis-alkene cases, such as the formation of cis-stilbene from (+)-I ,2-dibromo-l,2-diphenylethane, the resulting stereospecificity is often poor or nonexistent (18,19). However, use of the PPN+-Cr(CO),NO-metallate (20) at -78°C gave a 19: 1 cisltransstilbene mixture.…”

Elimination and substitution in the reactions of vicinal dihalides and oxyhalides trimethylstanoylsodium. Effects of solvent and of ion aggregation on course and stereochemistry

“…be initiated by nucleophilic attack by the trimethylstannyl anion on the bromine, as represented in eq 11. The in- (11) termediate carbanion 8 would undergo inversion rapidly relative to loss of methoxide ion and to reaction with trimethylbromostannane, resulting in nonspecific overall reactions. A series of experiments designed to test this mechanism by trapping any carbanion formed with tertbutyl alcohol was carried out with the results depicted in Figure 4.…”