Iridium‐Catalyzed, Diastereoselective Dehydrogenative Silylation of Terminal Alkenes with (TMSO)₂MeSiH

Abstract

Vinylsilanes and vinylboranes are versatile synthetic intermediates that can be constructed through catalytic functionalization of CÀH bonds with boron and silicon reagents. [1,2] The majority of borylations of alkenes to form vinylboronates as the major product require cyclic alkenes, vinylarenes, or specific substituted alkenes (vinyl ethers and allyltrimethylsilane); [3] therefore methods for the alternative dehydrogenative silylation of terminal alkenyl C À H bonds are desirable. Current methods for the preparation of vinylsilanes include the silyl-Heck reaction, [4] alkyne hydrosilylation, [5] direct dehydrogenative silylation of alkenes, [6] and manipulation of compounds with existing CÀSi bonds; [7] each of these methods suffers from a number of drawbacks, including the requirement of an excess of the alkene, limitation to vinylarenes, or the production of the more readily-accessible E vinylsilane as the major product.Recently, Lu and Falck reported the Z-selective silylation of terminal alkenes with Et 3 SiH in the presence of the iridium/di-tert-butylbipyridine catalyst developed by our group for the borylation of arenes.[8] However, the lack of electronegative atoms attached to the silicon atom prevents the products from being substrates for the Tamao oxidation or Hiyama-Denmark coupling reactions.[9] Unfortunately, silylation reactions often occur in lower yields with silanes, such as alkoxysilanes, bearing electronegative atoms than with trialkylsilanes. Because the alkoxysilyl group is electron withdrawing, the hydride is less hydridic.[10] In addition, the rates of side reactions, such as hydrosilylation, silane dehydrocoupling, and silane redistribution, are affected by the identity of the substituents on the silanes. [6b, 11] Therefore, reactions with a silane containing a silicon-heteroatom bond are unlikely to parallel directly the reactions of trialkylsilanes. We hypothesized that the dehydrogenative silylation could be made more practical by conducting the reactions with a tertiary hydrosilane containing bulky siloxy groups and that the catalysts we recently developed for the silylation of aliphatic CÀH bonds [2f] could make the dehydrogenative silylation faster than the redistribution reactions of a hydrosilane containing one or more electronegative groups.We report herein the dehydrogenative silylation of terminal alkenes with (TMSO) 2 MeSiH, a silane that is commercially available in bulk quantities, catalyzed by iridium complexes of 3,4,7,8-tetramethyl-1,10-phenanthroline (Me 4 Phen), together with subsequent cross-coupling and oxidation of the vinylsilane products. The reaction is highly selective for the Z vinylsilane. Isotope labeling suggests that the reaction occurs by insertion of the alkene, rather than direct CÀH activation, and the stereoselectivity of the process can be reversed by conducting the reaction with a hindered, chelating nitrogen ligand.To begin to develop the dehydrogenative silylation with a silane suitable for synthetic purposes, we surveyed the reactions of all...

Rhodium‐Catalyzed Dehydrogenative Borylation of Aliphatic Terminal Alkenes with Pinacolborane

“…It is similar to the one proposed by Hartwig et al for the iridium(I)-catalyzed dehydrogenative silylation using norbornene as the hydrogen acceptor. [19] Oxidative addition of the BÀHb ond of 2 onto rhodium(I) affords the boryl-(hydride)rhodium species A.S ubsequent insertion of the alkene 1 into the Rh À Bb ond of A takes place to give the alkyl-rhodium intermediate B.T he initial conformer undergoes rotation along the CÀCb ond axis to form the other conformer C.T hen, syn b-hydride elimination furnishes the E isomer of the alkenyl boronate 3.T he (dihydride)rhodium species D reacts with norbornene to generate an active rhodium(I) species together with norbornane. [17,20] The strained structure of norbornene enhances the reactivity toward D. [18] Therefore,h ydroboration of norbornene is preferred over the alkenyl pinacolboronate 3a.…”

“…A commercially available P, Nb identate ligand, iPr-Foxap, [15] exhibited ad ramatic effect to favor the formation of 3a. [19] Oxidative addition of the BÀHb ond of 2 onto rhodium(I) affords the boryl-(hydride)rhodium species A.S ubsequent insertion of the alkene 1 into the Rh À Bb ond of A takes place to give the alkyl-rhodium intermediate B.T he initial conformer undergoes rotation along the CÀCb ond axis to form the other conformer C.T hen, syn b-hydride elimination furnishes the E isomer of the alkenyl boronate 3.T he (dihydride)rhodium species D reacts with norbornene to generate an active rhodium(I) species together with norbornane. [17] The E/Z ratio of 3a was 85:15.…”

New Members of the Editorial Board and International Advisory Board of Angewandte Chemie

“…He moved to the University of Illinois at Urbana‐Champaign in 2006, and was made Henry Rapoport Professor of Chemistry at the UC Berkeley in 2011. Hartwig is interested in organic synthesis, organometallic synthesis, and mechanistic analysis of catalytic systems using transition‐metal complexes 16. Hartwig is also on the International Advisory Board of ChemCatChem…”

Neu im Kuratorium und im Internationalen Beirat der Angewandten Chemie

“…2006 wechselte er an die University of Illinois at Urbana‐Champaign, und 2011 wurde er Henry Rapoport Professor of Chemistry an der UC Berkeley. Ihn interessieren die organische Synthese, die Organometallsynthese und die mechanistische Analyse von Katalysesystemen, bei denen Übergangsmetallkomplexe verwendet werden 16. Hartwig ist außerdem im International Advisory Board von ChemCatChem…”