Chemoselective Hydrosilylation of the α,β-Site Double Bond in α,β- and α,β,γ,δ-Unsaturated Ketones Catalyzed by Macrosteric Borane Promoted by Hexafluoro-2-propanol

Abstract

The B­(C6F5)3-catalyzed chemoselective hydrosilylation of α,β- and α,β,γ,δ-unsaturated ketones into the corresponding non-symmetric ketones in mild reaction conditions is developed. Nearly 55 substrates including those bearing reducible functional groups such as alkynyl, alkenyl, cyano, and aromatic heterocycles are chemoselectively hydrosilylated in good to excellent yields. Isotope-labeling studies revealed that hexafluoro-2-propanol also served as a hydrogen source in the process.

HFIP in Organic Synthesis

“…The frustrated Lewis pair combination of B­(C 6 F 5 ) 3 and Et 3 SiH in HFIP promoted the chemoselective hydrosilylation of α,β- and α,β,γ,δ-unsaturated ketones (Scheme ). HFIP provided the highest yield of the protic solvents screened (MeOH, EtOH, i -PrOH), while THF and toluene gave only trace amounts of product. A mixture of toluene and HFIP was required for select substrates that had limited solubility in HFIP alone.…”

Synthesis of Suffranidine B

“…Synthesis of vinylogous ketoaldehyde 20 commenced with a palladium-catalyzed reductive detriflation of known kojic acid-derived enol triflate 17. 34 Regioselective hydrogenation of the resulting 4-pyrone 18 was accomplished by borane-catalyzed hydrosilylation in HFIP solvent 35 to deliver enone 19 in 72% yield. Final TBS deprotection and oxidation of the resulting alcohol yielded key aldehyde 20 in a 76% yield over two steps (Scheme 3A).…”

Selective Synthesis of Ketones and Chiral Allylic Alcohols from the Addition of Arylboronic Acids to α,β-Unsaturated Aldehydes Mediated by a Transition Metal/Monophosphorus Ligand System

“…Similarly, Ir, Cu, Ni, Co, and Mn complexes were employed in the dehydrogenative cross-coupling of alcohols to afford aryl alky ketones. In addition, self-hydrogen transferring isomerization, selective epoxide opening/alkylation, allylation of β-chloro ketones, α-arylation of enolates, chemoselective reduction of α,β-unsaturated ketones, and acylative Suzuki coupling have also been used. On the other hand, the preparation of chiral allylic alcohols typically involves enantioselective hydroboration of α,β-unsaturated ketones and 1,2-additions to α,β-unsaturated aldehydes of, e.g., arylboronic acids and organometallic compounds of zinc, lithium, titanium, magnesium, or aluminum (Scheme b); other methods are detailed elsewhere …”