A Room Temperature Direct Metal Insertion into a Nonstrained Carbon−Carbon Bond in Solution. C−C vs C−H Bond Activation

The diphosphine 1,3-bis[(di- tert -butylphosphino)methyl]-2,4,6-trimethylbenzene ( 1a ) upon reacting with the rhodium and iridium olefin complexes M 2 (olefin) 4 Cl 2 (M = Rh, Ir) undergoes rapid, selective metal insertion into the strong unstrained aryl−methyl bond under very mild conditions (room temperature), yielding ClM(CH 3 )[C 6 H(CH 3 ) 2 (CH 2 P( t -Bu) 2 ) 2 ] (M = Rh ( 4a ), Ir ( 7a )). The carbon−carbon bond activation is competitive with a parallel C−H activation process, which results in formation of complexes ClMH(L)[CH 2 C 6 H(CH 3 ) 2 (CH 2 P( t -Bu) 2 ) 2 ] (M = Rh ( 3a ), Ir ( 6a ); L = cyclooctene in the case of 6a and is absent in 3a ). Complexes 3a and 6a undergo facile C−H reductive elimination (at room temperature ( 3a ) or upon moderate heating ( 6a )), followed by C−C oxidative addition, resulting in clean formation of 4a and 7a, respectively. The C−C bond activation products are stable under the reaction conditions, demonstrating that this process is the thermodynamically favorable one. X-ray single-crystal analysis of 4a demonstrates that the rhodium atom is located in the center of a square pyramid, with the methyl group occupying the position trans to the vacant coordination site. Direct kinetic comparison of the C−C and C−H activation processes shows that in contrast to theoretical calculations metal insertion into the carbon−carbon bond in this system is not only thermodynamically but also kinetically preferred over the competing insertion into the carbon−hydrogen bond. When the ligand 1,3-bis[(di- tert -butylphosphino)methyl]-2,4,6-trimethyl-5-methoxybenzene ( 1b ), bearing the strong electron-donating methoxy group in the position trans to the Ar−CH 3 bond to be cleaved, was used instead of 1a, no effect on the reaction rate or on the ratio between the C−H and C−C activation products was observed. Our observations indicate that the C−C oxidative addition proceeds via a three-centered mechanism involving a nonpolar transition state, similar to the one proposed for C−H activation of hydrocarbons. An η 2 -arene complex is not involved in the C−C activation process.