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.
In the title system, meso-stilbene dibromide gave trans-stilbene, while dl-stilbene dibromide gave both stilbenes, [trans] /[cis] 'v (83 f 2)/(17 f 2). The rate data at 59.4' were, for meso, AH* = 20.6 kcal/mol and AS* = -9 eu; for dl, AH* = 28.9 kcal/mol and AS* = 8 eu; k(meso)/k(dl) = 50. Stannous chloride proved to be an efficient scavenger in the dl reaction, for which rate data could not otherwise have been obtained. In methanol, lithium bromide is inert; in the aprotic solvent (DMF), it is an effective debrominating agent. A complete conformational analysis has been performed: the reactant free-energy difference in benzene at 80" has been measured, ( G~L -G,,,) = 0.78 kcal/mol; the transition state free-energy difference in DMF at 80" has been estimated, (G*dr -G*,eso) Our rates show the trend, anti debromination of meso >> sgn debromination of dl = anti debromination of dl >>>> syn debromination of meso, in accordance with the idea that "discrete" stereoelectronic and steric factors may make aligned or opposed contributions to the rates.4.6 kcal/mol.
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