83947-32-4Relevant articles and documents
Palladium-catalyzed or -promoted reductive carbon-carbon coupling. Effects of phosphines and carbon ligands
Negishi, Ei-ichi,Takahashi, Tamotsu,Akiyoshi, Kazunari
, p. 181 - 194 (2007/10/02)
The reaction of Cl2Pd(PEt3)2 with 2 equiv of (E)-t-BuCH=CHLi, t-BuCCLi, or MeLi gives the corresponding R12Pd(PEt3)2 in a nearly quantitative yield without producing R1R1, where R1=(E)-t-BuCH=CH, t-BuCC, 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 12Pd(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-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 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-BuCCZnCl 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.
