Highly tunable anionic tethered N-heterocyclic carbene of Pd(II) complexes for asymmetric allylic alkylation reaction

Abstract

The versatility of the anionic tethered NHC ligand system for the Pd-catalyzed allylic asymmetric alkylation (AAA) reaction was investigated. A well defined anionic amidate/NHC–Pd(II) complex catalyzed the AAA reaction of 1,3-diphenylprop-3-en-1-yl acetate (5) with NaCH(CO2Me)2. Similarly, the AAA reaction occurred by employing an NHC–Pd(II) complex that was generated in situ. Screening of a wide variety of NHC ligand precursors revealed that the combination of [Pd(allyl)Cl]2 and the azolium salt 9, containing a 3-pentyl group at N(3) position in the NHC ring, efficiently promoted the AAA reaction to afford the corresponding alkylated product 7 with 67% ee. Furthermore, it was found that the Pd/NHC ratio was an important factor; the AAA reaction with a Pd/NHC ratio of 1:1 took place smoothly, whereas utilization of a Pd/NHC ratio of 1:2 resulted in low conversion of the substrates probably by the formation of the catalytically inert bis(NHC)–Pd(II) complex. In fact, an independent experiment showed that the bis(NHC)–Pd(II) complex was formed by treating [Pd(allyl)Cl]2 with two equivalents of NHC–Ag complex. Further investigations on the catalytic AAA reaction revealed significant improvements in both the product yield (99%) and the enantioselectivity (81% ee) when cyclopentyl methyl ether was used as a solvent. A plausible reaction intermediate was discussed.