The methylene-transfer reaction: Synthetic and mechanistic aspects of a unique C-C coupling and C-C bond activation sequence

Oxidative addition of aryl iodides ArI (Ar = (a) C6H5, (b) C6H4CF3, (c) C6H 3(CF3)(2), (d) C6H4CH3, (e) C6H4OCH3), to the PCP-type complex Rh(PPh3)[CH2 C6H(CH3)(2)(CH2PPh2)2] (1), yields the complexes Rh(Ar)[CH2C6H(CH3)(2)(CH2P Ph2)(2)](I) (2a-e). Compounds 2a-e undergo intramolecular methylene transfe r from the bis-chelating ligand to the incoming aryl under mild conditions (room temperature) giving Rh(CH2- Ar)[C6H(CH3)(2)(CH2PPh2)(2)](I) (3a-e). T he methylene transfer, which is a unique sequence of sp(2)-sp(3) C-C bond r eductive elimination and sp(2)-sp(3) C-C bond activation, was investigated kinetically (reaction 2a --> 3a), yielding the activation parameters Delta H-double dagger = 17 +/- 3 kcal/mol, Delta S-double dagger = -23 +/- 4 eu. The rate-determining step of this reaction is the C-C reductive elimination rather than the C-C activation step. X-ray structural analysis of 2a and 3 b demonstrates that the Rh atom is located in the center of a square pyrami d with the aryl (2a) and the benzyl (3b) trans to the vacant coordination s ite. Reaction of the complex Rh(CH2C6H4CF3) [C6H3(CH2PPh2)(2)](Br) (7c) wit h carbon nucleophiles (MeLi, PhLi, BzMgCl) leads to a competitive sp(2)-sp( 3) and sp(3)-sp(3) C-C coupling, resulting in migration of a methylene or b enzylidene into the bis-chelating ring and formation of the corresponding o rganic products. sp2--sp3 C_C coupling was shown to be kinetically preferre d over the sp(3)-sp(3) one, and the more electron-rich the benzyl ligand, t he better the migratory aptitude observed. X-ray structural analysis of two benzyl migration products, complexes Rh(PPh3)[CH(C6H4CF3)C6H3(CH2PPh2)(2)] (11) and Rh(PPh3)[CH(C6H5)C6H(CH3)(2)(CH2PPh2)(2)] (16), demonstrates that the rhodium atom is located in the center of a square planar arrangement w here the PPh3 ligand occupies the position trans to the methyne carbon of t he benzylidene bridge. The methylene and benzylidene migration reaction is an important transformation for the regeneration of the methylene-donating moiety in the methylene-transfer process.