Multiple insertion of a silyl vinyl ether by (alpha-diimine)PdMe+ species.

This paper reports that (alpha-diimine)PdMe+ species (1) (alpha-diimine = (2,6-iPr2 C6H3)N CMeCMe N(2,6-iPr2 C6H3)) undergo multiple insertions of CH2 CHOSiPh3 (2), ultimately forming Pd allyl products. The reaction of (alpha-diimine)PdMeCl, [Li(Et2O)2.8][B(C6F5)4] (1 equiv), and 2 (8 equiv) in CH2Cl2 yields [(alpha-diimine)Pd{eta3-CH2CHCHCH(OSiPh3)CH2CH(OSiPh3)Me}][B(C6F5)4] (7-B(C6F5)4) in 83% NMR yield. 7 is formed as a 95/5 mixture of isomers (7a/b), which converts to an equilibrium 40/60 mixture at 23 degrees C. X-ray diffraction analysis established that the configuration of the kinetically favored isomer 7a is S,S,S (ent-R,R,R), where the descriptors refer to the configurations of the substituted allyl carbon and the side chain methine carbons, respectively. 7b has an R,S,S (ent-S,R,R) configuration and differs from 7a in that the Pd is coordinated to the opposite allyl enantioface. The reaction of (alpha-diimine)PdMeCl, Ag[SbF6] (1 equiv), and 2 (8 equiv) under the same conditions yields [(alpha-diimine)Pd{(eta3-CH2CHCHCH(OSiPh3)Me)}][SbF6] (8-SbF6) in 90-100% NMR yield. 8 is formed as a 90/10 mixture of isomers (8a/b) that differ in the configuration of the substituted allyl carbon and evolve to a 70/30 equilibrium mixture at 23 degrees C. These results are consistent with a mechanism involving generation of 1 and insertion of 2 to give (alpha-diimine)PdCH2CH(OSiPh3)Me+ (4). In the absence of 2, 4 undergoes beta-OSiPh3 elimination and allylic C-H activation to give (alpha-diimine)Pd(eta3-CH2CHCH2)+ (6) and Ph3SiOH. In the presence of excess 2, 4 undergoes a second insertion of 2 to form (alpha-diimine)Pd{CH2CH(OSiPh3)CH2CH(OSiPh3)Me+ (9), which can undergo beta-OSiPh3 elimination/allylic C-H activation to form 8, or insert a third equivalent of 2 ultimately leading to 7. Further chain growth is probably disfavored by steric crowding.