Direct observation of C-O reductive elimination from palladium aryl alkoxide complexes to form aryl ethers
Reaction of KOCH2CMe3 with [(R)-Tol-BINAP]Pd(p-C6H4CN)Br formed [(R)-Tol-BINAP]Pd(p-C6H4-CN)(OCH2CMe3) (5) in quantitative yield (1H NMR spectroscopy). Thermolysis of 5 in THF-d8 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 + k'[KOCH2-CMe3], where k = 1.50 ± 0.07 x 10-3 s-1 and k' = 6.2 ± 0.4 x 10-3 s-1 M-1, consistent with reductive elimination via competing alkoxide-dependent and alkoxide-independent pathways. In addition, excess KOCH2CMe3 exchanged rapidly with the palladium-bound alkoxide ligand of 5 at 47 °C according to the rate law: rate exchange = k(ex)[KOCH2CMe3], where k(ex) = 1.0 ± 0.1 x 102 s-1 M-1. Thermolysis of the related palladium p-cyanophenyl alkoxide complexes (P- P)Pd(p-C6H4CN)(OR) [P-P = (S)-BINAP, R = CH2CMe3; P-P = (R)-Tol-BINAP, R = CHMe2, CMe3; P-P = dppf, R = CH2CMe3, CMe3] and (dppf)Pd[o- C6H4(CH2)2C(Me)2O] led to aryl ether formation in 46-91% yield.
Widenhoefer, RA; Zhong, HA; Buchwald, SL
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