A remaining major challenge in the asymmetric addition of silicon nucleophiles to typical prochiral acceptors, the enantioselective 1,2-addition to aldimines, is addressed. Activation of the SiB bond in the silicon pronucleophile by a copper(I) alkoxide with McQuade's chiral six-membered N-heterocyclic carbene as a supporting ligand releases the silicon nucleophile, which adds to various aldimines with high levels of enantioselectivity. The new method provides a catalytic asymmetric access to α-silylated amines.

An enantio- and regioselective allylic silylation of linear allylic phosphates that makes use of catalytically generated cuprate-type silicon nucleophiles is reported. The method relies on soft bis(triorganosilyl) zincs as silicon pronucleophiles that are prepared in situ from the corresponding hard lithium reagents by transmetalation with ZnCl2 . With a preformed chiral N-heterocyclic carbene-copper(I) complex as catalyst, exceedingly high enantiomeric excesses are achieved. The new method is superior to existing ones using a silicon-boron reagent as the source of the silicon nucleophile.

No abstract

A full account of our work on enantioselective silylation of typical prochiral acceptors employing chiral six-membered NHC-copper(I) complexes introduced by McQuade and co-workers is presented. With these precatalysts, asymmetric branched-selective substitution of allylic phosphates and 1,2-addition to imines had become possible for the first time. The successful application of these catalysts in two fundamentally different reactions raised the question whether these are a privileged ligand motif for catalytic asymmetric silyl transfer. To assess their generality, these were utilized in the related 1,2-addition to aldehydes and in conjugate addition to representative α,β-unsaturated acceptors, but with limited success in both cases. This study also includes an optimization of the allylic silylation, now overcoming the limited scope of the previous protocol. The scope of the imine addition is extended to heteroaryl-substituted aldimines.The formation of silicon nucleophiles by activation of the silicon-boron interelement linkage through transmetalation with transition metal-oxygen bonds has added an invaluable tool to the arsenal of catalytic carbon-silicon bond-forming processes. 1,2 Among the transformations on the basis of this concept, catalytic asymmetric variants are particularly desirable, but had remained underdeveloped just until recently. Studies from our laboratory revealed that a chiral diphosphine-[Rh I -OH] complex activates Suginome's silyl boronic ester, Me 2 PhSiBpin (pin = pinacolato), 3 and subsequently transfers the rhodium(I)-bound silyl anion to α,β-unsaturated cyclic carbonyl/carboxyl as well as Z-configured acyclic carboxyl compounds in highly enantioselective fashion. 4 Regrettably, we had not been able to apply this catalytic system to reactions other than conjugate addition. By changing the transition metal from rhodium(I) to copper(I), Hoveyda and co-workers elaborated a related, broadly applicable 1,4-addition catalyzed by an in situ-generated chiral NHC-[Cu I -Ot-Bu] complex (Scheme 1, upper left). 5-7 Parallel to this development, our group demonstrated that transmetalation with [Cu I -OR] complexes is not limited to conjugate addition. The straightforward combination of CuCN/NaOMe without any phosphine or NHC as supporting ligand releases a highly reactive silicon-based cuprate from the silicon-boron pronucleophile that cleanly adds to allylic 8 and propargylic substrates 9 by selective S N 2′ displacement as well as to imines 10 and aldehydes 11 by 1,2-addition. The latter was later rendered enantioselective by Riant and co-workers using an unprecedented copper(I) bifluoride complex with DTBM-Segphos (L2) as chiral ligand and an excess of MeOH (Scheme 1, lower left). 12 Both aryl-and alkylsubstituted aldehydes were converted into the corresponding α-hydroxysilanes with high enantiomeric excesses and in good chemical yields. During our attempts to perform the branched-selective copper(I)-catalyzed allylic substitution enantioselectively, we became aware of the chiral six-membered NH...

No abstract