Direct Observation of C−O Reductive Elimination from Palladium Aryl Alkoxide Complexes To Form Aryl Ethers

Abstract

Reaction of KOCH 2 CMe 3 with [( R )-Tol-BINAP]Pd( p -C 6 H 4 CN)Br formed [( R )-Tol-BINAP]Pd( p -C 6 H 4 CN)(OCH 2 CMe 3 ) ( 5 ) in quantitative yield ( 1 H NMR spectroscopy). Thermolysis of 5 in THF- d 8 at 47 °C led to C−O reductive elimination with formation of p -neopentoxybenzonitrile (85 ± 2%). A secondary P−C bond-cleavage process led to formation of 4,4‘-dimethylbiphenyl (16 ± 2%). Kinetic analysis of the decomposition of 5 at 47 °C in the presence of excess potassium neopentoxide established the two-term rate law, rate = k [ 5 ] + k‘ [ 5 ][KOCH 2 CMe 3 ], where k = 1.50 ± 0.07 × 10 -3 s -1 and k‘ = 6.2 ± 0.4 × 10 -3 s -1 M -1, consistent with reductive elimination via competing alkoxide-dependent and alkoxide-independent pathways. In addition, excess KOCH 2 CMe 3 exchanged rapidly with the palladium-bound alkoxide ligand of 5 at 47 °C according to the rate law: rate exchange = k ex [ 5 ][KOCH 2 CMe 3 ], where k ex = 1.0 ± 0.1 × 10 2 s -1 M -1 . Thermolysis of the related palladium p -cyanophenyl alkoxide complexes (P−P)Pd( p -C 6 H 4 CN)(OR) [P−P = ( S )-BINAP, R = CH 2 CMe 3; P−P = ( R )-Tol-BINAP, R = CHMe 2, CMe 3; P−P = dppf, R = CH 2 CMe 3, CMe 3 ] to aryl ether formation in 46−91% yield.