Alkyl− and Aryl−Oxygen Bond Activation in Solution by Rhodium(I), Palladium(II), and Nickel(II). Transition-Metal-Based Selectivity

Reaction of [RhCl(C 8 H 14 ) 2 ] 2 (C 8 H 14 = cyclooctene) with 2 equiv of the aryl methyl ether phosphine 1 in C 6 D 6 results in an unprecedented metal insertion into the strong sp 2 −sp 3 aryl−O bond. This remarkable reaction proceeds even at room temperature and occurs directly, with no intermediacy of C−H activation or insertion into the adjacent weaker ArO−CH 3 bond. Two new phenoxy complexes ( 8 and 9 ), which are analogous to the product of insertion into the ArO−CH 3 bond (had it taken place) were prepared and shown not to be intermediates in the Ar−OCH 3 bond cleavage process. Thus, aryl−O bond activation by the nucleophilic Rh(I) is kinetically preferred over activation of the alkyl−O bond. The phenoxy Rh(I)−η 1 -N 2 complex ( 8 ) is in equilibrium with the crystallographically characterized Rh(I)−μ-N 2 −Rh(I) dimer ( 12 ). Reaction of [RhCl(C 8 H 14 ) 2 ] 2 with 2 equiv of the aryl methyl ether phosphine 2, PPh 3, and excess HSiR 3 (R = OCH 2 CH 3, CH 2 CH 3 ) results also in selective metal insertion into the aryl−O bond and formation of (CH 3 O)SiR 3 . Thus, transfer of a OCH 3 group from carbon to silicon was accomplished, showing that hydrosilation of an unstrained aryl−O single bond by a primary silane is possible. The selectivity of C−O bond activation is markedly dependent on the transition-metal complex and the alkyl group involved, allowing direction of the C−O bond activation process at either the aryl−O or alkyl−O bond. Thus, contrary to the reactivity of the rhodium complex, reaction of NiI 2 or Pd(CF 3 CO 2 ) 2 with 1 equiv of 1 in ethanol or C 6 D 6 at elevated temperatures results in exclusive activation of the sp 3 −sp 3 ArO−CH 3 bond, while reaction of the analogous aryl ethyl ether 4 and Pd(CF 3 CO 2 ) 2 results in both sp 3 −sp 3 and sp 2 −sp 3 C−O bond activation. The resulting phenoxy Pd(II) complex ( 18 ) is fully characterized by X-ray analysis. Heating the latter under mild dihydrogen pressure results in hydrodeoxygenation to afford an aryl−Pd(II) complex ( 19 ).