NICKEL-CATALYZED, CHLOROTRIALKYLSILANE-ASSISTED CONJUGATE ADDITION OF ALKENYLTRIBUTYLTIN REAGENTS TO ALPHA,BETA-UNSATURATED ALDEHYDES - EVIDENCE FOR A [1-((TRIALKYLSILYL)OXY)ALLYL]NICKEL(II) MECHANISM

Alpha,beta-unsaturated aldehydes R1CH=CR2CHO (R1 = H, Me, n-Pr; R2 = H, Me) react with ethenyl-, (1-ethoxyethenyl)-, (2-phenylethenyl)-, and 1-propenyltrialkyltin reagents and chlorotrialkylsilanes (Me3SiCl or t-BuMe2SiCl) in the presence of Ni(COD)2 (COD = 1,5-cyclooctadiene) or 1-((trialkylsilyl)oxy)allylnickel(II) chloride catalyst precursors to afford the corresponding trialkylsilyl (E)-enol ethers in 48-79% yield. High C(3)-regioselectivities (greater-than-or-equal-to 15:1 crude, greater-than-or-equal-to 50:1 purified) are observed when R1 = H; moderate C(3)-regioselectivities (2-12:1 crude, 2->50:1 purified) are observed when R1 = Me or n-Pr. High (E)-enol ether selectivities (5->19:1 crude, 10->50:1 purified) are observed in all cases save the addition of (1-ethoxyethenyl)tributyltin to 2-propenal, for which case a 2:1 E/Z ratio is observed. Stoichiometric model reaction and kinetic studies strongly support a Ni(0)/Ni(II) mechanism involving 1-((trialkylsilyl)oxy)allylnickel(II) intermediates and turnover-limiting alkenyl group transmetalation.