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

Article abstract of DOI:10.1021/acs.joc.8b02675

Aromatic aldehydes and acetophenones undergo silylative pinacol-type reductive dimerization in their reaction with silylborane pinB-SiMe₂Ph (pin: pinacolato) catalyzed by supported Au nanoparticles on TiO₂. 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.

Full text of DOI:10.1021/acs.joc.8b02675

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Reaction of Aromatic Carbonyl Compounds with Silylborane
Catalyzed by Au Nanoparticles: Silylative Pinacol-
Type Reductive Dimerization via a Radical Pathway
Marios Kidonakis, Anisa Mullaj, and Manolis Stratakis
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02675 • Publication Date (Web): 30 Nov 2018
Downloaded from http://pubs.acs.org on December 1, 2018
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Reaction of Aromatic Carbonyl Compounds with Silylborane
Catalyzed by Au Nanoparticles: Silylative Pinacol-Type Reductive
Dimerization via a Radical Pathway
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Marios Kidonakis, Anisa Mullaj and Manolis Stratakis*
Department of Chemistry, University of Crete, Voutes 71003, Heraklion, Greece.
E-mail: stratakis@uoc.gr
Graphical Abstract for the Table of Contents
PhMe2Si-Bpin (1.5 eq)
O
PhMe2SiO
OSiMe2Ph
R
Ar
Au/TiO2 (1 mol%)
R
Ar
(meso+dl)
benzene, 70 oC, 3 h
yields 50-75%
Ar
R
(R = H or Me)
PhMe2Si-Bpin (1.5 eq)
Au/TiO2 (1 mol%)
PhMe2SiO
Ar
O
N
benzene, 70 oC, 3 h
TEMPO (1.2 equiv)
O
Ar
H
Ar = p-Tolyl
~95%
ABSTRACT: Aromatic aldehydes and acetophenones undergo silylative pinacol-type
reductive dimerization in their reaction with silylborane pinB-SiMe2Ph (pin: pinacolato)
catalyzed by supported Au nanoparticles on TiO2. 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.
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Silylboranes is a class of compounds whose chemistry has attracted a
considerable interest in recent years. The Si-B bond can be readily activated by a series
of catalysts, primarily transition metals, and undergoes addition to unsaturated
compounds.1 The activation of silylborane pinB-SiMe2Ph (pin: pinacolato) by supported
Au nanoparticles (Aun), and the subsequent addition of the Au-nanoparticle bonded
boron and silyl moieties (pinB-Aun-SiMe2Ph) on alkynes,2 allenes3 or strained cyclic
ethers4 has been recently documented by our group. As a continuation of our studies in
this field we examined the possible reaction between pinB-SiMe2Ph and carbonyl
compounds in the presence of Au nanoparticles. It is well known that silylboranes are
unreactive against carbonyl compounds,5 as also proved in our hands. In the presence of
a NHC-Cu complex, however, a catalytic C-Si bond forming reaction is taking place
(Scheme 1). Thus, nucleophilic addition of the silicon moiety from pinB-SiMe2Ph
occurs to aldehydes or imines forming -silyl alcohols6 or -silyl amines,7 respectively,
and to CO2 which undergoes reduction.8 Also, a metal-free catalytic enantioselective
1,2-silylation of aromatic aldehydes with pinB-SiMe2Ph was latter reported.9 Analogous
catalytic reactions between silylborane and -unsaturated carbonyl compounds occur
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OH
O
O
PhMe2Si-Bpin
R
SiMe2Ph
R
H
H
NHC-Cu catalyst
Known in literature
(after hydrolysis)
PhMe2SiO
OSiMe2Ph
H
Ar
PhMe2Si-Bpin
H
Ar
(meso+dl)
Ar
Au/TiO2 (catalyst)
Our work
Scheme 1. Catalyzed Reactions Between Aldehydes and Silylborane pinB-SiMe2Ph.
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via a 1,4-silyl addition.1a A common characteristic of these reactions, is that the silyl
part of pinB-SiMe2Ph behaves as a nucleophile, through the formation of a metal-
SiMe2Ph complex as a key intermediate in the proposed catalytic cycles.
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In this manuscript, we present our studies regarding the reaction of silylborane
pinB-SiMe2Ph with carbonyl compounds catalyzed by supported Au nanoparticles. In
the presence of 1.0 mol% Au/TiO2, a series of para- and meta-substituted aromatic
aldehydes or acetophenones (1 equiv) and pinB-SiMe2Ph (1.5 equiv) react smoothly in
o
dry benzene at 70 C forming after 3 h silylated pinacol-type dimerization products in
good isolated yields (Scheme 1 and Table 1). The dimeric products appear as an
approximately equimolar mixture of meso and dl diastereomers. The assignment of
unknown compounds as meso or dl was done by deprotection and characterization of
the resulting known diols. Typically, no side products derived from the carbonyl
compounds are seen. In case of using non-dried solvent, additional excess of silylborane
is required (2-3 equiv) to compensate its destruction, and at the same time minor
amounts of hydrosilylation products of the carbonyl compounds10 can be seen due to the
in situ formed HSiMe2Ph, which is essentially an intermediate product from the Au-
catalyzed hydrolysis of pinB-SiMe2Ph.2a Apart from dry benzene, 1,2-dichloroethane is
also a suitable solvent providing similar yields under the same reaction conditions. In
other solvents such as hexane, acetonitrile, ethyl acetate or tetrahydrofuran the yields
are below 20%, while several unidentified side products are also formed (Table S1).
Ortho-substituted benzaldehydes and bulky ketones such as benzophenone are
unreactive. The involvement of Au nanoparticles in this catalytic transformation is
without any doubt, as TiO2 itself (rutile or anatase forms) does not promote any reaction
between pinB-SiMe2Ph and the carbonyl compounds. In contrast to the aromatic
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