Stereoselective Oxidative Addition of Silanes and Hydrogen Halides to the Iridium(I) Cis Phosphine Complexes IrX(CO)(dppe) (X = Br, CN; dppe = 1,2-Bis(diphenylphosphino)ethane)

Article abstract of DOI:10.1021/ja00309a019

The oxidative addition of silanes, R_nCl_3-nSiH (n = 3, R = Et, Ph, OEt; n = 2, R = Me; n = 1, R = Me), to the Ir(I) cis phosphine complexes IrX(CO)(dppe) (X = Br, CN; dppe = 1,2-bis(diphenylphosphino)ethane) has been found to proceed stereoselectively under kinetic control.Of the four possible diastereomers that can form by concerted cis addition of the Si-H bond to the iridium(I) center, the one having hydride trans to CO and Si trans to P(dppe) is formed initially with >98percent stereoselectivity.For X = Br, this diastereomer is not the thermodynamically favored product.Isomerization of the initially formed silyl hydride product to the equilibrium mixture of diastereomers follows first-order kinetics for the triphenylsilyl derivative with k₁ = 0.015 min^-1.The rate of isomerization for the kinetic silyl hydride adducts decreases in the order Et3SiH > Ph3SiH > (OEt)3SiH > Me2ClSiH with the MeCl2SiH derivative not isomerizing even after prolonged heating.The most stable diastereomer for X = Br has hydride trans to Br and silyl trans to P(dppe).For X = CN, the kinetic isomer wiht H trans to CO and Si trans to P(dppe) is also the most stable isomer, although other isomers are observed to form after initial reaction.Secondary chemistry of the triethylsilyl hydride products for X = Br and CN is observed over longer reaction times leading to the formation of IrHX2(CO)(dppe) (X = Br), IrH2(SiEt3)(CO)(dppe), and Et3SiSiEt3.This secondary chemsitry is consistent with reductive elimination/oxidative addition sequences.The oxidative addition of HX to IrX'(CO)(dppe) also proceeds stereoselectively, giving the isomer with H trans to X' and X trans to P(dppe).This diastereomer results from cis addition in wich H-X approaches the square-planar Ir(I) complex with its axis parallel to X'-Ir-P.Thus, while R3SiH and HX both add to IrX(CO)(dppe) by a cis concerted mechanism, the diastereoselection for HX is opposite to that for R3SiH.It is proposed that this difference arises because the silane approach to the Ir(I) complex is nucleophilic while that of HX is electrophilic.