Carbon−Carbon Bond Activation by Rhodium(I) in Solution. Comparison of sp2−sp3 vs sp3−sp3 C−C, C−H vs C−C, and Ar−CH3 vs Ar−CH2CH3 Activation

Reaction of [RhClL 2 ] 2 (L = cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 } ( 1 ) in toluene results in a selective metal insertion into the strong Ar−Et bond. This reaction proceeds with no intermediacy of activation of the weaker sp 3 −sp 3 ArCH 2 −CH 3 bond. The identity of complex Rh(Et){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 }Cl ( 3 ) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh( η 1 -N 2 ){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 } ( 7 ) with EtI. It is possible to direct the bond activation process toward the benzylic C−H bonds of the aryl−alkyl group by choice of the Rh(I) precursor, of the substituents on the phosphorus atoms ( t Bu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to the product of insertion into the ArCH 2 −CH 3 bond (had it taken place) was prepared and shown not to be an intermediate in the Ar−CH 2 CH 3 bond activation process. Thus, aryl−C activation by Rh(I) is kinetically preferred over activation of the alkyl−C bond in this system. Moreover, cleavage of an Ar−CH 2 CH 3 bond, followed by β-H elimination, may be preferred over sp 2 −sp 3 C−C activation of an Ar−CH 3 group.