99355-15-4Relevant articles and documents
Oxidative addition mechanisms of a four-coordinate rhodium(I) macrocycle
Collman, James P.,Brauman, John I.,Madonik, Alex M.
, p. 310 - 322 (2008/10/08)
Oxidative addition of a wide range of alkyl halides (RX) to the four-coordinate Rh(I) macrocyclic complex 5b yields six-coordinate Rh(III)-alkyl adducts. The reactions exhibit second-order kinetics (first order in substrate and Rh(I) complex) for all substrates except hindered alkyl iodides, and the reactivity trends (X = I > Br > OTs > Cl; R = methyl > primary > secondary > tertiary [no reaction]) are consistent with a nucleophilic, SN2-like mechanism. The observed activation parameters and rate variation with solvent polarity also support this interpretation. Complex 5b is the most reactive neutral nucleophile which has been isolated (k = 2000 M-1 s-1 for iodomethane at 25°C in tetrahydrofuran); it exhibits unusually high sensitivity to steric hindrance and leaving-group polarizability, characteristics it shares with other metal-centered nucleophiles. The shift to a one-electron mechanism in reactions with hindered iodides (2,2-dimethyl-1-iodopropane, 3,3-dimethyl-1-iodobutane, and 2-iodopropane) is signaled by their departure from second-order kinetics and by their unexpectedly high rates; even 1-iodoadamantane forms an adduct. Furthermore, scrambling of stereochemistry is observed in the addition of erythro-3,3-dimethyl-1-iodobutane-1,2-d2. While a radical chain mechanism is implicated by these kinetic results, any free radical intermediates must be very short-lived, as no rearrangement was detected in the reaction of 6-iodo-1-heptene. It seems likely that electron transfer from the Rh(I) reagent leads to the formation of radical pairs, most of which collapse to product without escaping the solvent cage.
