Palladium-catalyzed or -promoted reductive carbon-carbon coupling. Effects of phosphines and carbon ligands

Article abstract of DOI:10.1016/0022-328X(87)80048-7

The reaction of Cl2Pd(PEt3)2 with 2 equiv of (E)-t-BuCH=CHLi, t-BuC<*>CLi, or MeLi gives the corresponding R¹2Pd(PEt3)2 in a nearly quantitative yield without producing R¹R¹, where R¹=(E)-t-BuCH=CH, t-BuC<*>C, or Me.The reaction of PhLi or PhZnCl gives Ph2Pd(PEt3)2 and biphenyl in approximately 90 and 10percent yields, respectively.On the other hand, the corresponding reaction of Cl2Pd(PPh3)2 produces R¹R¹ in >95percent yield within 1 h in each case.Attempts to detect R¹2Pd(PPh3)2 have failed.Their formation as unstable intermediates may be inferred from those cases where PEt3, PPhMe2, and PPh2Me are used.The ease of formation of R¹R¹ is inversely proportional to the basicity of PR3, i.e., PEt3 < PPhMe2 < PPh2Me < PPh3.Although Me2Pd(PEt3)2 is cis, all the other R¹2Pd(PEt3)2 and R¹R²Pd(PEt3)2 prepared in this study are trans.All but one are stable for 3 h at 22 deg C. trans-<(E)-n-HexCH=CH>PhPd(PEt3)2, however, undergoes a slow reductive elimination to give (E)-n-HexCH=CHPh in 70percent within 12 h at room temperature.Biphenyl and <(E)-t-BuCH=CH>2 can be produced in high yields based on Pd via the corresponding R¹2Pd(PR3)2, where PR3 is PEt3, PPhMe2, or PPh2Me, through the use of 8 equiv. of R¹Li.Similar acceleration effects have also been observed in the reaction of R¹R²Pd(PR3)2 with an excess of R¹Li, R²I, or PR3.However, the reductive elimination reactions of (p-Tol)PhPd(PEt3)2 promoted by p-TolLi and PhI give significant amounts of homo-coupled products.These results can be explained in terms of various associative mechanisms that have previously been proposed.The reaction of aryl- or alkenyl-zinc chlorides with aryl or alkenyl iodides catalyzed by "Pd(PR3)2" can give the desired cross-coupled products in >/= 60percent yields.However, only "Pd(PPh3)2" leads to uniformly high cross/homo ratios.With PEt3, PPh2Me, and PhMe2, cross-homo scrambling occurs to considerable extents, although the reaction of t-BuC<*>CZnCl with PhI catalyzed by "Pd(PEt3)2" is exceptional.The high cross/homo ratio observed with PPh3 is consistent with some dissociative mechanisms but does not appear to be consistent with any associative mechanisms.