One-electron versus two-electron mechanisms in the oxidative addition reactions of chloroalkanes to amido-bridged rhodium complexes

The compound syn-[[Rh(mu-NH{p-tolyl})(CNtBu)(2)}(2)] (1) oxidatively adds C-CI bonds of alkyl chlorides (RCI) and dichloromethane to each metal centre to give the cationic complexes syn-[{Rh(mu-NH{p-tolyl})(eta(1)-R)-(CNtBu)(2)}(2)(mu-Cl)]Cl and anti-[{Rh(mu-NH{p-tolyl})Cl(CNtBu)(2))(2)(mu-CH2)]. Reaction of 1 with the chiral alkyl chloride (-)-(S)-ClCH(Me)CO2Me (R*Cl) gave [{Rh(mu-NH{p-tolyl})(eta(1)-R*)(CNtBu)(2)}(2)(mu-Cl]Cl ([3]Cl) as an equirnolecular mixture of the meso form (R,S)-[3]Cl-C-s and one enantiomer of the chiral form [3]Cl-C-2. This reaction, which takes place in two steps, was modeled step-by-step by reacting the mixed-ligand complex syn-[(cod)Rh(mu-NH{p-tolyl})(2)Rh(CN-tBu)(2)] (4) with R*Cl, as a replica of the first step, to give [(cod)Rh([mu-NH{p-tolyl})(2)RhCl(eta(1)-R*)(CNtBu)(2)] (5) with racemization of the chiral carbon. Further treatment of 5 with CNtBu to give the intermediate [(CNtBu)(2)Rh(mu-NH-{p-tolyl})(2)RhCl(eta(1)-R*)(CNtBu)(2)], followed by reaction with R*Cl reproduced the regioselectivity of the second step to give (R,S)-[3]Cl-C-s and [3]Cl-C-2 in a 1:1 molar ratio. Support for an S(N)2 type of reaction with inversion of the configuration in the second step was obtained from a similar sequence of reactions of 4 with ClCH2CO2Me first, then with CNtBu, and finally with R*Cl to give [(CNtBu)(2)(eta(1)-CH2R)Rh-(mu-NH{p-tolyl})(2)( mu-Cl)Rh(eta(1)-R*)(CNtBu)(2)]Cl (R = CO2Me, [7]Cl) as a single enantiomer with the R configuration at the chiral carbon. The reactions of 1 with (+)-(S)-XCH2CH(CH3)CH2CH3 (X = Br, 1) gave the related complexes [{Rh(mu-NH[p-tolyl))-(eta(1)-CH2CH(CH3)CH2CH3)(CNtBu)(2)}(2)- (R-X)]X, probably by following an S,2 profile in both steps.