Gold-catalyzed intramolecular allylation of silyl alkynes induced by silane alcoholysis

Article abstract of DOI:10.1021/ja062560p

The activation of alkynyl allyl silanes with a cationic gold catalyst in the presence of alcohols provides vinyl silanes that contain the allyl group at the β-position and the alkoxysilyl group in cis-orientation. The bond reorganization process is most c

Full text of DOI:10.1021/ja062560p

Published on Web 07/28/2006
Gold-Catalyzed Intramolecular Allylation of Silyl Alkynes Induced by Silane
Alcoholysis
Sangho Park and Daesung Lee*
Department of Chemistry, UniVersity of Wisconsin, Madison, Wisconsin 53706
Received April 12, 2006; E-mail: dlee@chem.wisc.edu
Table 1. Optimization of Catalyst and Reaction Conditionsa
The pursuit of synthetic efficiency has promoted constant
development of new concepts and innovated synthetic arsenals.1
One of the most effective ways of achieving synthetic efficiency
is to implement tandem reactions2 into a synthetic sequence whereby
many bond-forming and -cleaving events can occur in one synthetic
yield
operation. Following from our interest in silyl ether-based metathesis
entry enyne
R
catalyst/conditions
(2+
3, %) 2:3c
chemistry,3 we envisioned a tandem reaction to form C-H, C-C,
and Si-O bonds in one step to generate alkenyl and alkynyl silyl
ethers of stereochemically defined vinyl silanes (eq 1).4 Overall,
this is a net addition of H-OR to the alkynyl allyl silanes
accompanied by an allyl transfer from silicon to carbon.5 To achieve
this tandem bond formation efficiently, we propose to use carbo-
philic metal catalysts instead of strong mineral acid catalysts to
promote the allyl transfer and the addition of H-OR to carbon
and silicon centers. Herein we report a stereoselective intramolecular
allylation of silyl alkynes to generate alkoxy vinyl silanes via a
gold-catalyzed alcoholysis of alkynyl allyl silanes.6
1
2
3
4
5
6
7
1a Me Ph3PAuCl/AgSbF6, rt, dry CH2Cl2
traces 2:1
d
d
Ph3PAuCl/AgSbF6, rt, wet CH2Cl2
93b
1:20
d
Ph3PAuCl/AgOTf rt, wet CH2Cl2
traces
traces
20
d
AuCl3 rt, wet CH2Cl2
PtCl2 90 °C, toluene
2:1
9:1
1b Ph Ph3PAuCl/AgSbF6, rt, wet CH2Cl2
traces
Ph3PAuCl/AgSbF6, rt, CH2Cl2 + iPrOH 75b
a Reactions with 1 mol % of catalyst for 5-10 h. b Isolated yields. c The
ratios were determined by H NMR. d Undistilled CH2Cl2.
1
Table 2. Intramolecular Allylation-Alcoholysis Catalyzed by
Golda,b
The general reactivity feature of allyl alkynyl silanes was
examined with 1-octynyl allyl dimethylsilane 1a by using several
metal catalysts under different conditions (Table 1). The reaction
of 1a with Ph3PAuCl/AgSbF6 in dry CH2Cl2 at room temperature
provided a trace amount of 2a and 3 (entry 1).7 However, under
otherwise identical conditions in undistilled CH2Cl2, desilylated
product 3 was obtained in 93% yield with small amount of 2a (entry
2). Reaction with Ph3PAuCl/AgOTf gave no conversion (entry 3),
whereas PtCl2 (toluene, 90 °C) gave products 2a and 3 in 20%
yield (entry 5).9 The extent of desilylation was reduced by replacing
the dimethyl silyl with a diphenyl silyl group in 1b, which, however,
was recovered unchanged (entry 6) under the conditions (Ph3PAuCl/
AgSbF6 in undistilled CH2Cl2) where 1a gave high conversion. On
a Reactions with 1 mol % of catalyst for 10 min. b Isolated yields are in
1
parentheses. c The ratios of major:minor were determined by H NMR.
provided 6 in 85% yield; however, the stereochemistry of the double
bond was scrambled, giving 1:1.7 mixture of Z/E-isomers (entry
2). This is, probably, the consequence of isomerization of the
initially formed Z-isomer catalyzed by either the gold catalyst or a
proton.10 Substrates 1e-g with a methyl, phenyl, and benzyloxy-
methyl substituent on the alkyne behave uneventfully, giving 7-10
in good yields and Z/E-selectivity (entries 3-6). On the other hand,
substrate 1h generated more 12 than the expected product 11 (81%,
12:11 ) 3:1). Presumably, this is the consequence of more favorable
activation of the allyl group by the gold catalyst due to the presence
of a sterically hindered tert-butyl group on the alkyne. Substrates
with an alkyl substituent on the allyl segment showed variable
reactivity depending on the position of the substituent compared
to that of the parent system.11 Thus, 1i bearing a crotyl group gave
excellent yields and Z/E-selectivity of 13 and 14 (entries 8 and 9),
whereas the reaction of methallyl-bearing substrate 1j provided a
1:5.5 mixture of 15 and the methallyl-displaced product 16 in 89%
overall yield (entry 10).
i
the other hand, the same reaction with added PrOH (1 equiv)
provided silyl ether 2b in good yield (75%) (entry 7). This clearly
indicates that the nucleophilic assistance is crucial for an efficient
transfer of an allyl group.
Having established optimized conditions for an intramolecular
allyl transfer assisted by oxygen-based nucleophiles, we next
examined the scope of this reaction by employing allyl silane 1c
and a variety of alcohols (Table 2). Treatment of 1c with Ph3PAuCl/
AgSbF6 in dry CH2Cl2 with 1°, 2°, and 3° alcohols gave good yield
of products 4a-i as inseparable mixtures of Z/E-isomers in the
range of 1:1.7 to 10:1 ratio. A salient feature of these reactions is
that the alkene and alkyne functionalities in the alcohol counterpart
do not interfere with the reaction.
To broaden the substrate scope, substituents on the alkyne and
allyl moieties were introduced (Table 3).8 Reaction of 1b with
4-penten-2-ol under the optimized conditions gave 5 in 71% yield
with a 10:1 Z/E ratio (entry 1). Substrate 1d with a terminal alkyne
From a mechanistic standpoint, we surmised that the activation
of the alkynyl moiety of 1 by a carbophilic catalyst would induce
9
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J. AM. CHEM. SOC. 2006, 128, 10664-10665
10.1021/ja062560p CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 3. Intramolecular Allylation-Alcoholysis Catalyzed by Golda
Scheme 2. Ring-Closing and Cross Metathesis of Silyl Ethers
In conclusion, we have developed a gold-catalyzed tandem
intramolecular allyl transfer reaction induced by an alcoholysis of
alkynyl allyl silanes, which generates alkoxy vinyl silanes17 in high
yield and Z/E-selectivity. Synthetic application of this tandem bond-
forming process will be reported in due course.
Acknowledgment. We thank NIH (RO1 CA106673) and the
Sloan Foundation for financial support of this work, as well as the
NSF and NIH for NMR and Mass Spectrometry instrumentation.
a Reactions with 1 mol % of catalyst for 10 min. b Isolated yields. c The
Z/E ratios were determined by 1H NMR. d The initially formed isomers
with the silyl and allyl groups in cis-orientation are defined as the Z-isomer.
Supporting Information Available: General experimental proce-
dures and characterization of all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 1. Gold-Catalyzed Reorganization of Alkynyl Allyl Silane
References
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