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 [RhClL2](2) (L = cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6-((CH2PBu2)-Bu-t)(2)C6H3} (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) ArCH2-CH3 bond. The identity of complex Rh(Et)(2,6-((CH2PBu2)-Bu-t)(2)C6H3)Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh(eta(1)-N-2){2,6-((CH2PBu2)-Bu-t)(2)C6H3} (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 (Bu-t vs Ph), and of the alkyl moiety (Me vs Pt). A Rh(III) complex which is analogous to the product of insertion into the ArCH2-CH3 bond (had it taken place) was prepared and shown not to be an intermediate in the Ar-CH2CH3 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-CH2CH3 bond, followed by beta-H elimination, may be preferred over sp(2)-sp(3) C-C activation of an Ar-CH3 group.