Enantiocontrol in Tandem Allylic Sulfonium Ylide Generation and [2,3] Sigmatropic Rearrangement Catalyzed by Chiral Dirhodium(II) Complexes

Tandem sulfonium ylide formation and [2,3] sigmatropic rearrangement sequence from allylic sulfides and diazoacetates has been effected by using dirhodium(II) tetrakis[3(S)-phthalimido-2-piperidinonate], Rh 2 (S-PTPI) 4 , as a chiral catalyst, displaying enantioselectivities of up to 58%.It has been demonstrated that the [2,3] sigmatropic rearrangement proceeds through chiral, nonracemic free sulfonium ylides dissociated from the dirhodium(II) complex.Allylic sulfonium ylides readily undergo a [2,3] sigmatropic rearrangement which features a carboncarbon bond formation with a high degree of stereocontrol. 1 Considering that catalytic generation and [2,3] sigmatropic rearrangement of sulfonium ylides from allylic sulfides and a variety of metal carbenes have recently enjoyed widespread attention as an alternative to the process via deprotonation or desilylation of the corresponding sulfonium salts, 2 the development of an enantioselective version of this sequence mediated by chiral catalysts should be a significant addition to the field of asymmetric synthesis. 3mura and co-workers were the first to demonstrate asymmetric induction (up to 20% ee) in the reaction of (E)-cinnamyl phenyl sulfide (1a) and ethyl diazoacetate (2a) employing bis(oxazoline)-Cu(I) complex (3a) or Rh 2 (5S-MEPY) 4 as a catalyst. 4Recently, the bis(oxazoline)-Cu(I)-catalyzed process has been systematically investigated by McMillen and co-workers, in which a significant enhancement of up to 52% ee can be attained by the combinational use of the sterically demanding allyl 2,6dimethyphenyl sulfide and bis(oxazoline)-Cu(I) complex (3b). 5Katsuki and co-workers have reported that chiral salen-Co(III)-complex (4) is an efficient catalyst for the reaction of 1a with tert-butyl diazoacetate (2b) to display the highest degree of enantioselectivity (64% ee) known for this type of reactions. 6They have also demonstrated that the [2,3] sigmatropic rearrangement proceeds through chiral, nonracemic free sulfonium ylides rather than through the Co-salen-associated ylides.In this † Dedicated to Professor Shô Itô on the occasion of his 77th birthday.