Exceptionally E- and β-Selective NHCCu-Catalyzed Proto-Silyl Additions to Terminal Alkynes and Site- and Enantioselective Proto-Boryl Additions to the Resulting Vinylsilanes: Synthesis of Enantiomerically Enriched Vicinal and Geminal Borosilanes

An exceptionally site- and E-selective catalytic method for preparation of Si-containing alkenes through protosilylation of terminal alkynes is presented. Furthermore, the vinylsilanes obtained are used as substrates to generate vicinal or geminal borosilanes by another catalytic process; such products are derived from enantioselective protoborations of the Si-substituted alkenes. All transformations are catalyzed by N-heterocyclic carbene (NHC) copper complexes. Specifically, a commercially available imidazolinium salt, cheap CuCl (1.0mol%) and Me2PhSiB(pin), readily and inexpensively prepared in one vessel, are used to convert terminal alkynes to (E)--vinylsilanes efficiently (7998% yield) and in >98% E and >98% -selectivity. Vinylsilanes are converted to borosilanes with 5.0mol% of a chiral NHCCu complex in 3394% yield and up to 98.5:1.5 enantiomeric ratio (e.r.). Alkyl-substituted substrates afford vicinal borosilanes exclusively; aryl- and heteroaryl-substituted alkenes deliver the geminal isomers preferentially. Different classes of chiral NHCs give rise to high enantioselectivities in the two sets of transformations: C1-symmetric monodentate Cu complexes are most suitable for reactions of alkyl-containing vinylsilanes and bidentate sulfonate-bridged variants furnish the highest e.r. for substrates with an aryl substituent. Working models that account for the observed trends in selectivity are provided. Utility is demonstrated through application towards a formal enantioselective total synthesis of naturally occurring antibacterial agent bruguierol A.