DOI: 10.1002/chem.201502942
Communication
&
Organoboranes
A General, Practical Triethylborane-Catalyzed Reduction of
Carbonyl Functions to Alcohols
Dongjie Peng, Mintao Zhang, and Zheng Huang*[a]
have also been developed. Seminal work by Piers showed that
Abstract: A combination of the abundant and low-cost
the strong Lewis acid B(C6F5)3 catalyzes ester hydrosilylation to
triethylborane and sodium alkoxide generates a highly ef-
form aldehydes with Ph3SiH as the reductant.[15] It is noted that
ficient catalyst for reduction of esters, as well as ketones
selective ester hydrosilylation is difficult to control because
and aldehydes, to alcohols using an inexpensive hydrosi-
silyl acetals are susceptible to over-reduction to silyl ethers
lane under mild conditions. The catalyst system exhibits
and alkanes. While olefins, internal alkynes, and halogens can
excellent chemoselectivity and a high level of functional
be tolerated, nitrile and alcohol functionalities are difficult. The
group tolerance. Mechanistic studies revealed a resting
mechanism involves novel activation of the SiÀH bond of hy-
state of sodium triethylalkoxylborate that is the product
drosilane by B(C6F5)3 through h1 coordination,[15b,16] which has
of the reaction of BEt3 with sodium alkoxide. This borate
been termed as ‘frustrated Lewis pair’ bond activation.[17]
species reacts with hydrosilane to form NaBEt3H, which
The development of low-cost and abundant non-transition-
rapidly reduces esters.
metal catalysts for ester reductions is of significant interest.
Triethylborane (BEt3) is manufactured on a tremendous scale in
industry and far less expensive than B(C6F5)3 (>100 times less).
The reduction of carbonyl compounds and carboxylic acid de-
rivatives to alcohols is one of the most fundamental transfor-
mations in organic synthesis.[1] Stoichiometric reactions using
organometallic hydrides, such as LiAlH4, NaBH4, and DIBAL–H
(diisobutylaluminium hydride), are prevalent on a laboratory-
scale.[2] However, these highly reactive pyrophoric agents are
air-, and moisture-sensitive, are not compatible with many
functional groups, and require exigent reaction conditions.
Moreover, the use of stoichiometric amounts of such reducing
agents creates a cost hurdle on bulk scale synthesis. Thus, at-
tention has been given to the development of catalytic meth-
ods for reduction of carbonyl functions. Transition-metal-cata-
lyzed hydrogenation of ketones and aldehydes has been well
established,[3] but hydrogenation of the less reactive carboxylic
esters is still a challenge.[4] Known examples require harsh reac-
tion conditions (high pressures and temperatures) and func-
tionalities such as olefin, alkyne, cyanide and nitro groups
often compete favorably over esters for hydrogenation.[5]
However, BEt3 is a weak Lewis acid, and hence itself cannot
catalyze ester hydrosilylations through silane activation as
B(C6F5)3 does. It is worth noting that BEt3 is a precursor to
NaBEt3H,[18] a powerful and selective reducing agent. On the
other hand, ester reductions with NaBEt3H would produce BEt3
and sodium alkoxides. We became intrigued with the possibili-
ty of the conversion of BEt3 to NaBEt3H using a suitable hy-
dride source under the conditions applied to NaBEt3H-mediat-
ed ester reductions (Scheme 1). Herein, we report a mild, gen-
eral, user-friendly, and high-yielding BEt3-catalyzed method for
reduction of esters, as well as aldehydes and ketones. The pro-
cedure uses polymethylhydrosiloxane (PMHS), an inexpensive
waste product of silicon industry, as the reducing agent.
Catalytic hydrosilylation is an attractive alternative to hydro-
genation for a mild and selective ester reduction.[6] Various
transition-metal catalyst systems with Ti,[7] Rh,[8] Ru,[9] Mo,[10]
Zn,[11] Mn,[12] and Fe[13] metals have been reported for ester hy-
drosilylations.[14] Nevertheless, most of the protocols require
either expensive reducing agents or high reaction tempera-
tures. Remarkably, catalysts based on earth-abundant boron
Scheme 1. Conversion of BEt3 to NaBEt3H under the conditions for NaBEt3H-
mediated ester reductions.
We commenced the study by investigating the reduction of
methyl phenylacetate (1a) with PMHS as a model reaction
(Table 1). As expected, in the presence of BEt3 (5 mol%), no re-
action took place between 1a and PMHS (3 equiv of SiÀH rela-
tive to 1a) in Et2O at room temperature (Table 1, entry 1). How-
ever, with the addition of NaOMe (5 mol%) the reaction oc-
curred, and 2-phenylethanol (2a) was obtained in 99% yield
(94% isolated yield) after hydrolysis with NaOH (aq) and MeOH
(Table 1, entry 2). Control experiments (1) without NaOMe and
BEt3, and (2) with NaOMe, but without BEt3 did not afford the
[a] Dr. D. Peng, M. Zhang, Prof. Dr. Z. Huang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
345 Lingling Road, Shanghai 200032 (China)
Fax: (+86)21-54925533
Supporting information for this article is available on the WWW under
Chem. Eur. J. 2015, 21, 14737 – 14741
14737
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim