cis-(R′2PC2H4PR′2)PdH (SnR3) complexes:: Trapped intermediates in the palladium-catalyzed hydrostannation of alkynes

The complexes (R'2PC2H4PR'(2))Pd(C2H4) (R' = Pr-i, Bu-t) react with R3SnH (R = Me, Bu-n) by displacement of the ethene ligand and oxidative addition of the Sn-H bond to generate the chelating phosphane stabilized cis Pd-II hydride stannyl complexes (R'2PC2H4PR'(2))PdH(SnR3) (R' = Pr-i (1), Bu-t (2)). Complex 1a (R' = Pr-i, R = Me), containing the smallest substituents, is only transiently formed but has been detected at -80 degrees C by NMR spectroscopy. It reacts further with Me3SnH, even at -120 degrees C, by eliminating hydrogen to give (d(i)ppe)Pd(SnMe3)(2) (3). In contrast, the isolated (d(i)ppe)PdH((SnBu3)-Bu-n) (Pb) is briefly stable at ambient temperature, whereas the sterically encumbered species (d(t)bpe)PdH(SnR3) (R = Me (2a), Bu-n (2b)) are stable well above 100 degrees C. The molecular structure of 2a has been determined by X-ray crystallography. Complex 2a reacts with 2 equiv of C2R "(2) (R " = CO2Me) to give (d(t)bpe)Pd(C2R "(2)) (4) and predominantly the corresponding (E)-vinylstannane (E)-(R ")(H)C=C(SnMe3)(R ") (E-5). Since 2a also catalyzes the hydrostannation of the alkyne, the cis Pd-II hydride stannyl complexes 1a,b and 2a,b represent trapped intermediates in the Pd-catalyzed hydrostannation of alkynes. The existence of the complexes also sheds light on the mechanism of the Pd-catalyzed degradation of R3SnH into Sn2R6 and H-2.