Exceptionally E‐ and β‐Selective NHC–Cu‐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

Abstract

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 (1.0 mol%) and Me(2)PhSi-B(pin), readily and inexpensively prepared in one vessel, are used to convert terminal alkynes to (E)-β-vinylsilanes efficiently (79-98% yield) and in >98% E and >98% β-selectivity. Vinylsilanes are converted to borosilanes with 5.0 mol% of a chiral NHC-Cu complex in 33-94% 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: C(1)-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.

Versatile Homoallylic Boronates by Chemo‐, SN2′‐, Diastereo‐ and Enantioselective Catalytic Sequence of Cu−H Addition to Vinyl‐B(pin)/Allylic Substitution

“…By providing an efficient, chemo-, S N 2’-, diastereo- and enantioselective method for accessing otherwise difficult-to-prepare and readily alterable homoallylic boronate compounds, we provide further evidence regarding the importance of sulfonate-containing chiral NHC ligands. These Cu-based complexes have formerly proven optimal in catalyzing enantioselective allylic substitution reactions [24] and conjugate addition processes [15,25] with C-based nucleophiles as well as Cu–B(pin) additions to alkenes [26] and allenes; [27] this is however the first time that a member of this catalyst class has emerged as the most effective for enantioselective Cu–H additions to an alkene.…”

Copper(I)‐Catalyzed Regioselective Addition of Nucleophilic Silicon Across Terminal and Internal Carbon–Carbon Triple Bonds

“…Loh and co‐workers had also elaborated a procedure for the addition across selected terminal alkynes in which the steric demand of a monodentate phosphine ligand secures high branched selectivity (Scheme , top) 14. Later, Hoveyda and co‐workers disclosed the corresponding linear‐selective addition, employing an N‐heterocyclic carbene as a supporting ligand (Scheme , bottom) 15. Aside from these reports, the related 1,4‐addition to acceptor‐substituted triple bonds was independently published by four laboratories this year (not shown) 16.…”

Copper(I)‐Catalyzed Highly Regio‐ and Stereoselective Hydrosilylation of Terminal Alkynes with Boryldisiloxane

“…Very recently, N-heterocyclic carbene (NHC)-copper(I) complexes were found to be more efficient catalytic systems for the above reaction, and they afforded b-(E)-vinylsilanes with high regio-and stereoselectivity (> 98 % E and b). [12] During the preparation of this manuscript, Calderone and Santos [13] and Lipshutz [14] successively reported the copper-catalyzed hydrosilylation of electron-deficient internal alkynes by using Me 2 PhSiB(pin) in water at room temperature to selectively afford b-silyl-a,b-unsaturated carbonyl compounds.…”

“…Subsequent insertion of the terminal alkyne into the CuÀSi bond of B selectively affords b-silylvinylcopper complex C in a cis manner. [13] Next, protonation [12,13] of C with MeOH provides desired hydrosilylation product D and the NHC-CuOMe complex to complete the catalytic cycle. According to the reaction mechanism, the regioselectivity is effectively controlled by the insertion process.…”

“…However, stoichiometric amounts of Fleming's silyl cuprate reagents (CuÀSi) by employing silyl-lithium (SiÀLi) [8] or silyl-stannane (SiÀSn) [9] reagents as the silicon sources are needed for this transformation. Recently, much attention has been directed toward the catalytic formation of functionalized vinylsilanes under copper catalysis, in which the key CuÀSi species arise from activation of disilane (SiÀSi) [10] or silylborane (SiÀB) reagents [11][12][13][14] (Scheme 1). The first copper(I)-catalyzed system for the hydrosilylation of alkynes conjugated to carbonyl groups was reported by Molander's group, who used diphenyltetramethyldisilane to obtain b-silyl-a,b-unsaturated carbonyl mixtures with different E/Z ratios in moderate yields.…”