Mechanisms of 1,1-reductive elimination from palladium

Abstract

The 1,1-reductive elimination of ethane from three cis-bis(phosphine)dimethylpalladium complexes, L/sub 2/Pd(CH/sub 3/)/sub 2/ (L = PPh/sub 3/,PPh/sub 2/CH/sub 3/; L/sub 2/ = Ph/sub 2/PCH/sub 2/CH/sub 2/PPh/sub 2/), and three trans analogues (L = PPh/sub 3/, PPh/sub 2/CH/sub 3/; L/sub 2/ = 2,11-bis(diphenylphosphinomethyl)benzo(c)phenanthrene (TRANSPHOS) was carried out. The three cis complexes underwent reductive elimination in the presence of coordinating solvents (Me/sub 2/SO, DMF, THF). The trans complexes which could isomerize to cis (L = PPh/sub 3/,PPh/sub 2/CH/sub 3/) did so in polar solvents and then underwent reductive elimination. (TRANSPHOS)dimethylpalladium would not undergo eductive elimination of ethane, even at 100/sup 0/C in Me/sub 2/SO. The eliminations from the cis isomers were intramolecular as determined by the lack of crossover with the perdeuteriomethylpalladium analogue and displayed first-order kinetics (k = 1.04 x 10/sup -3/s/sup -1/, L = PPh/sub 3/, 60/sup 0/C; K = (6.5 to 9.5) X 10/sup -5/s/sup -1/, L = PPh/sub 2/CH/sub 3/, 60/sup 0/C; k = 4.78 x 10/sup -7/s/sup -1/, L/sub 2/ = Ph/sub 2/PCH/sub 2/CH/sub 2/PPh/sub 2/, 80/sup 0/C). The presence of diphenylacetylene in the reaction mixture traps the palladium(0) product as the bis(diphenylmethylphosphine)(diphenylacetylene)palladium complex. Although (TRANSPHOS)dimethylpalladium would not undergo a 1,1-reductive elimination of ethane, the addition ofmore » CD/sub 3/I to a Me/sub 2/SO solution of this complex at 25/sup 0/C rapidly produced CD/sub 3/-CH/sub 3/, implicating a transient palladium(IV) intermediate. 5 figures, 4 tables.« less