Reaction of Aromatic Carbonyl Compounds with Silylborane Catalyzed by Au Nanoparticles: Silylative Pinacol-type Reductive Dimerization via a Radical Pathway

Abstract

Aromatic aldehydes and acetophenones undergo silylative pinacol-type reductive dimerization in their reaction with silylborane pinB-SiMe 2 Ph (pin: pinacolato) catalyzed by supported Au nanoparticles on TiO 2 . It is proposed that after initial activation of silylborane by Au nanoparticles and addition to the carbonyl functionality of an aromatic aldehyde or ketone, an aryl silyloxy radical is generated from the collapse of the intermediate adduct, which then dimerizes through a chain process. The silyloxy radical was almost quantitatively trapped in the presence of TEMPO.

Au Nanoparticle‐Catalyzed Silaboration of Aryl‐Substituted Cyclopropyl Aldehydes Forming Rearranged β‐Boronate Silyl Enol Ethers

“…Although, aliphatic aldehydes are unreactive against pinB‐SiMe 2 Ph, the phenyl‐substituted cyclopropyl aldehyde trans ‐ 1 reacted smoothly with 1.5 equiv. of pinB‐SiMe 2 Ph in the presence of Au/TiO 2 (1 mol‐%), within 1 h at 70 °C, in anhydrous benzene as solvent.…”

“…Our group was the first to report a few years ago the smooth activation of silylborane pinB‐SiMe 2 Ph (pin: pinacolato) on Au nanoparticles supported on titania (Au/TiO 2 ), and its subsequent cis‐1,2‐addition to alkynes or allenes, and on the σ C–O bond of strained cyclic ether . More recently we have presented its Au/TiO 2 ‐catalyzed reaction with aromatic aldehydes and acetophenones which results to a C–C bond forming pathway, instead . The activation of pinB‐SiMe 2 Ph on Au/TiO 2 most probably involves the chemisorption of the interelement Si‐B linkage on the electron deficient low‐coordinated Au atoms, primarily at the corners of the Au nanoparticle, and then follows the delivery of the two chemisorbed partners on the proximal physisorbed substrate .…”

“…The activation of pinB‐SiMe 2 Ph on Au/TiO 2 most probably involves the chemisorption of the interelement Si‐B linkage on the electron deficient low‐coordinated Au atoms, primarily at the corners of the Au nanoparticle, and then follows the delivery of the two chemisorbed partners on the proximal physisorbed substrate . While pinB‐SiMe 2 Ph adds to alkynes, allenes, epoxides and oxetanes,, in the case of carbonyl compounds, a silylative pinacol‐type reductive dimerization pathway is exclusively observed (Scheme ). It was clearly established that dimerization occurs through a radical‐chain process, as the postulated intermediate α‐silyloxy radical that eventually dimerizes was almost quantitatively captured in the presence of the free radical TEMPO.…”

“…Reaction of silylborane pinB‐SiMe 2 Ph with aromatic carbonyl compounds catalyzed by Au/TiO 2 , and the proposed intermediate α‐silyloxy radical …”

β-Borylation of conjugated carbonyl compounds with silylborane or bis(pinacolato)diboron catalyzed by Au nanoparticles

“…3 Since then, a range of applications appeared using this silylborane and Au/TiO 2 as the catalyst. Thus, we have additionally shown (Scheme 1) the regioselective silaboration of allenes, 4 the regioselective ring opening of epoxides and oxetanes yielding βor γ-silyloxy boronates respectively, 5 the pinacol-type reductive dimerization of aryl substituted carbonyl compounds promoted by the silylborane, 6 and more recently, the silaboration of 2-aryl-substituted cyclopropyl aldehydes, in which β-boronate silyl enol ethers are formed via ring-opening rearrangement of an intermediate α-silyloxy cyclopropyl radical. 7…”