Phosphines reaction with alkyl halides, kinetics

Pertinent to this review is the study by Henderson and Buckler (130) on the kinetics of the reactions of variously substituted phosphines with alkyl halides. These authors point out that while attack by phosphine on an alkyl halide is almost certainly SN2, it does not follow that reaction proceeds by attack from the rear with inversion. For nucleophiles capable of expanding their valence shells, including esters of phosphorous... 

Oxidative addition reactions of dihydrogen , iodine ", alkyl halides and Hg(CN)2 to carbonyl, olefin or phosphine substituted derivatives of rhodium(I) and iridium(I) have been described. In order to determine the effect on the rate of the reaction, the kinetics of the oxidative addition of Hg(CN)2 to Rh(dik)(P(OPh)3)2 has been studied . A second-order rate law coupled to large negative values of the activation entropy suggest an associative mechanism which probably proceeds via a cyclic three-centred transition state (equation 58). Analogous results were obtained with Ir(dik)(cod) . ... 

Most 8 2 reactions are faster in aprotic solvents and slower in protic solvents. Water tends to promote ionization. In pro-tic media, particularly aqueous media, ionization of tertiary halides occurs to give a carbocation intermediate (more slowly with secondary halides) 9,10,11,12,49, 55,69, 70, 77,86. Carbocation intermediates can be trapped by nucleophiles in what is known at an 8 1 reaction. An 8 1 reaction proceeds by ionization to a planar carbocation containing an sp hybridized carbon, follows first-order kinetics, and proceeds with racem-ization of a chiral center. Carbocations are subject to rearrangement to a more stable cation via 1,2 hydrogen or alkyl shifts 24, 25, 26, 27, 29, 50, 65, 67, 76, 77, 78, 79, 80, 81, 83, 84, 85,87,88, 89. A variety of nucleophiles can be used in the substitution reactions, including halides, alkoxides, amines, phosphines, azides, cyanide, acetylides, and enolate anions 2,3,13,14,36,62,63,88, 89,105. 

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