Oxidation of gem-borylsilylalkylcoppers to acylsilanes with air

Article abstract of DOI:10.1021/ol0600988

1-Boryl-1-silylalkylcoppers react with molecular oxygen in the presence of pyridine to afford acylsilanes efficiently. The one-pot process consists of two reactions: alkylation of 1-boryl-1-chloro-silylmethyllithium with Grignard reagents in the presence of copper(I) cyanide and aerobic oxidation of the alkylcopper species. This procedure enables us to access the divergent synthesis of acylsilanes.

Full text of DOI:10.1021/ol0600988

ORGANIC
LETTERS
2006
Vol. 8, No. 6
1185-1187
Oxidation of gem-Borylsilylalkylcoppers
to Acylsilanes with Air
Junichi Kondo,† Hiroshi Shinokubo,*,‡ and Koichiro Oshima*,†
Department of Material Chemistry, Graduate School of Engineering,
Kyoto UniVersity, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, and
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8502, Japan
oshima@orgrxn.mbox.media.kyoto-u.ac.jp; hshino@kuchem.kyoto-u.ac.jp
Received January 13, 2006
ABSTRACT
1-Boryl-1-silylalkylcoppers react with molecular oxygen in the presence of pyridine to afford acylsilanes efficiently. The one-pot process
consists of two reactions: alkylation of 1-boryl-1-chloro-silylmethyllithium with Grignard reagents in the presence of copper(I) cyanide and
aerobic oxidation of the alkylcopper species. This procedure enables us to access the divergent synthesis of acylsilanes.
Nowadays, organoboron compounds have fairly wide utilities
and are indispensable not only for synthetic chemistry but
also for modern science ranging from material science to
life science.1 Among a number of boron-based reagents,
R-borylcarbanions, which are gem-dimetallic species, have
been proven to be considerably useful synthetic intermediates
to participate in Wittig-type olefination with carbonyl
compounds to afford substituted alkenes.1,2 In addition, this
type of boron reagent is also known to react with dioxygen
to give rise to carbonyl compounds.3 These reagents are more
effective with the assistance of their vacant orbital and small
atom size in comparison to the silicon analogues. On the
other hand, it was reported that the olefination reactions of
carbonyl compounds with trimetallic species, R-boryl-R-
silylcarbanions, provided borylalkenes via faster â-oxygen
elimination of silicon than boron.4
peroxy species. In addition, we illustrate a facile preparative
method for the synthesis of gem-borylsilylalkylcoppers via
1,2-migration on ate-type copper carbenoids.
We have previously reported that the reaction of 1,1-
disilylalkylcoppers with air furnishes acylsilanes in both
protic and aprotic conditions (Scheme 1).6 The initial step
Scheme 1. Synthesis of Acylsilanes from Disilylalkylcoppers
via Aerobic Oxidation
Here, we wish to report the synthesis of acylsilanes5 by
aerobic oxidation of 1-boryl-1-silylalkylcoppers through
selective elimination of the boryl group from intermediary
is alkylative metalation of the lithium carbenoid with RMgX
and a stoichiometric amount of CuCN to afford the 1,1-
disilylalkylcopper, which is then converted to a peroxide
† Graduate School of Engineering, Kyoto University.
‡ Graduate School of Science, Kyoto University.
(1) (a) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic
Press: London, 1988. (b) ComprehensiVe Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995;
Vol. 11.
(2) (a) Matteson, D. S.; Moody, R. J.; Jesthi, P. K. J. Am. Chem. Soc.
1975, 97, 5608. (b) Mendoza, A.; Matteson, D. S. J. Org. Chem. 1979, 44,
1352. (c) ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon: Oxford, 1991; Vol. 1, p 689.
(4) (a) Matteson, D. S.; Majumdar, D. Organometallics 1983, 2, 230.
(b) Garad, M. V.; Pelter, A.; Singaram, B.; Wilson, J. W. Tetrahedron Lett.
1983, 24, 637.
(5) For reviews on the synthesis and utility of acylsilanes, see: (a) Ricci,
A.; Degl’Innocenti, A. Synthesis 1989, 647. (b) Page, P. C. B.; McKenzie,
M. J.; Klair, S. S.; Rosenthal, S. Acylsilanes. In The Chemistry of Organic
Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley-Interscience:
New York, 1998; Vol. 2, Chapter 27, p 1599. (c) Page, P. C. B.; McKenzie,
M. J. Product Subclass 25: Acylsilanes. In Science of Synthesis; Fleming,
I., Ed.; Georg Thieme Verlag: Stuttgart; New York, 2002; Vol. 4, p 513.
(3) Nakamura, M.; Hara, K.; Hatakeyama, T.; Nakamura, E. Org. Lett.
2001, 3, 3137.
10.1021/ol0600988 CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/17/2006

Scheme 2. Synthesis of Boryldichloromethylsilanes
Scheme 4. Aerobic Oxidation of Alkylcoppers 4a in Aprotic
Conditions
species upon the reaction with dioxygen. After this event,
one of the silyl groups eliminates in the peroxide intermediate
to yield silanol and acylsilane. We then contemplated
replacement of one of the silyl groups with a boryl group
with one question in our mind: which migrates, the silyl or
boryl group?
placement of silicon with boron gives the copper species
sufficient reactivity.
Next, we examined the aerobic oxidation of alkylcoppers
4aa and 4ab. The previous oxidation conditions with NH4-
Cl aq afforded disappointing results due to hydrolysis of the
copper species 4. Aerobic oxidation of 4aa in aprotic
conditions with the presence of 4.0 equiv of pyridine
proceeded smoothly to yield acylsilane 7aa in moderate yield,
along with the formation of silanol 8a.10 On the other hand,
4ab was converted into benzoylsilane 7ab in 86% yield
without the formation of silanol (Scheme 4).
Treatment of dichloromethylsilane 1a7 with 1.0 equiv of
n-BuLi in THF at -78 °C afforded silyldichloromethyl-
lithium via deprotonation, which reacted with methoxy-
(pinacolato)borane to provide the desired boryldichlorometh-
ylsilane 2a in excellent yield (Scheme 2). A series of boron-
bearing dichloromethanes can be synthesized in a similar
way as stable solids at room temperature under air.8
The addition of n-BuLi to a solution of boryldichloro-
methylsilane 2a in THF at -78 °C provided borylchlorosi-
lylmethyllithium 3a as a yellow solution quantitatively
through chlorine-lithium exchange. Fortunately, a byproduct
via 1,2-migration of the butyl group on the boron center was
not detected.9 Organolithium 3a was sequentially treated with
1.1 equiv of CuCN‚2LiCl and n-BuMgBr at -78 °C, and
the mixture was warmed gradually to 0 °C. After quenching
with dilute HCl aq under argon, we obtained the butylated
product 5aa in 81% yield. The use of PhMgBr also afforded
the desired product 5ab in 85% yield. An addition of methyl
iodide instead of HCl aq to trap the intermediary copper
species 4a furnished the desired products 6aa and 6ab,
respectively, in good yields (Scheme 3). In sharp contrast
Hence, we attempted the synthesis of aroylsilanes with
various aryl Grignard reagents. Several features of the
reaction are noteworthy. As shown in Table 1, mesityl
Table 1. Synthesis of Acylsilanes via Aerobic Oxidation of
1-Boryl-1-silylalkylcoppersa
Scheme 3. Preparation of 1-Boryl-1-silylalkylcoppers and
Their Electrophilic Trapping
a Conditions: CuCN‚2LiCl (1.1 equiv), RMgBr (1.1 equiv), and pyridine
(4.0 equiv) were employed. Si ) Ph2MeSi, PhMe2Si, t-BuMe2Si. b 2-
Thienyllithium was prepared by an addition of n-BuLi (1.1 equiv) to
thiophene (1.1 equiv) in THF.
to 1,1-disilylalkylcoppers which cannot be utilized as nu-
cleophiles in a carbon-carbon bond-forming process, re-
Grignard reagent can be efficiently employed despite its low
nucleophilicity due to the steric hindrance by ortho-disub-
(6) Inoue, A.; Kondo, J.; Shinokubo, H.; Oshima, K. J. Am. Chem. Soc.
2001, 123, 11109.
(7) Dichloromethylsilanes 1a-c are easily prepared according to the
reported procedure. (a) Bacquet, C.; Masure, D.; Normant, J. F. Bull. Soc.
Chim. Fr. 1975, 1797. (b) Shinokubo, H.; Miura, K.; Oshima, K.; Utimoto,
K. Tetrahedron 1996, 52, 503.
(8) Hiyama et al. previously reported the synthesis of 2b through gem-
dimetalation of lithium carbenoids with silylboranes. See: Shimizu, M.;
Kurahashi, T.; Kitagawa, H.; Shimono, K.; Hiyama, T. J. Organomet. Chem.
2003, 686, 286.
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Org. Lett., Vol. 8, No. 6, 2006

stitution (entry 2). The premixed reagent with 2-thienyl-
lithium and magnesium bromide provided 2-thiophenecar-
bonylsilane 7ad in good yield (entry 3), whereas 2-thienyl-
lithium itself did not work at all, affording no trace of 7ad.
This reaction procedure can be applied to incorporation of
silylcarbonyl groups into tolan derivatives as shown in entries
4 and 5. In contrast to our previous method where we
observed a decrease in yields due to hydrolysis of benzylic
copper species, the present protocol with pyridine does not
provide hydrolysis product 5. To our delight, similar copper
reagents with other silyl groups such as PhMe2Si and
t-BuMe2Si can be prepared from 2b and 2c in the same way,
and their oxidative conversion to acylsilanes worked well
without formation of silanols (entries 6-9). While our
previous protocol needed silyl groups with at least one aryl
group to enhance mobility, introduction of the boryl group
enables the synthesis of acyltrialkylsilanes with a variety of
silyl groups.
In conclusion, we have achieved the selective conversion
of 1-boryl-1-silylalkylcoppers with molecular oxygen to the
corresponding acylsilanes. In addition, we have developed
a simple and efficient method to prepare various gem-
borylsilylalkylcopper reagents via alkylation reaction of boryl
silyl carbenoids with various Grignard reagents and copper
cyanide. This one-pot process using air as the oxidant enables
the divergent synthesis of acylsilanes.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research and COE Research from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Government of Japan. J.K. acknowledge JSPS for
financial support.
(9) For reviews on the reactions of R-halo boronic esters, see: (a)
Matteson, D. S. Chem. ReV. 1989, 89, 1535. (b) Matteson, D. S. J.
Organomet. Chem. 1999, 581, 51.
(10) Silanol 8a may result from competing migration of the silyl group.
However, we could detect neither the presumed product, acylborane from
â-elimination of silyl group, nor its hydrolyzed products.
Supporting Information Available: Detailed experi-
mental procedures. This material is available free of charge
via the Internet at http://pubs.acs.org.
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