Lexa, Doris published the artcileOuter-sphere and inner-sphere processes in reductive elimination. Direct and indirect electrochemical reduction of vicinal dibromoalkanes, Application of 2,3-Dibromo-2,3-dimethylbutane, the publication is Journal of the American Chemical Society (1990), 112(17), 6162-77, database is CAplus.
The reduction of vicinal dibromo alkanes (OlX2) is investigated as an example of the dichotomy between outer-sphere and inner-sphere processes in reductive elimination. As a result from the anal. of the kinetic data, outer-sphere reagents such as carbon electrodes and aromatic anion radicals react with vicinal dibromo alkanes according to an electron-transfer mechanism in which the rate-determining step is a concerted electron-transfer bond-breaking reaction leading to the β-bromoalkyl radical. The latter is then reduced very rapidly, in a second step, most probably along another concerted electron-transfer bond-breaking pathway leading directly to the olefin in the heterogeneous case and through halogen atom expulsion in the homogeneous case. In the absence of steric constraints, the reduction goes entirely through the antiperiplanar conformer because the resulting β-bromoalkyl radical is then stabilized by delocalization of the unpaired electron over the C-C-X framework due to a favorable interaction between the pz orbital of the radical carbon and the σ* orbital of the C-Br bond. This interaction is enhanced by alkyl substitution at the reacting carbons, resulting in an approx. linear correlation between the reduction potential and the C-X bond energy of OlX2 on one hand and the vertical ionization potential of the olefin on the other. The stabilization energy is of the order of 0.2-0.3 eV for the anti conformers. It can also be taken as a measure of the rotation barrier around the C-C bond responsible for the loss of stereospecificity in the reduction This competes with the reduction of the two stable conformers of the OlX• radicals and for the expulsion of the halogen atom. There is a remarkably good agreement between the ensuing prediction of the E:Z olefin ratio that should be found upon reduction of threo and erythro OlX2 isomers by outer-sphere reagents such as aromatic anion radicals and the exptl. data. Although members of perfectly reversible redox couples, iron(I), iron(0), and cobalt(I) porphyrins offer typical examples of inner-sphere reagents in their reaction with vicinal dibromo alkanes. They indeed react much more rapidly than outer-sphere electron donors (aromatic anion radicals) of the same standard potential. On the basis of steric hindrance experiments, it was shown that they do not react according to an SN2 rate-determining step involving the transient formation of an organometallic species. Complete stereospecificity is obtained, showing that they react along a halonium transfer E2 elimination mechanism rather than by an E1 elimination or a halogen atom transfer mechanism. As shown on a quant. basis, this is related to the large driving force offered to halonium abstraction by the strong affinity of the iron(III) and cobalt(III) complexes toward halide ions. In regard to catalysis, the investigated systems provide typical examples showing the superiority of inner-sphere (chem.) catalysis over outer-sphere (redox) catalysis of electrochem. reactions. Not only is the catalytic efficiency much better since the rate constants of the key steps are larger, given the standard potential of the catalyst, but also selectivity is dramatically improved.
Journal of the American Chemical Society published new progress about 594-81-0. 594-81-0 belongs to bromides-buliding-blocks, auxiliary class Bromide,Aliphatic hydrocarbon chain, name is 2,3-Dibromo-2,3-dimethylbutane, and the molecular formula is C6H12Br2, Application of 2,3-Dibromo-2,3-dimethylbutane.
Referemce:
https://en.wikipedia.org/wiki/Bromide,
bromide – Wiktionary