Regio- and Stereoselective Copper(II)-Catalyzed Hydrosilylation of Activated Allenes in Water: Access to Vinylsilanes

Article abstract of DOI:10.1021/acs.orglett.6b00981

By using catalytic amounts of copper(II), 4-picoline, and dimethylphenylsilylpinacol borane, a series of allenoates were silylated on the β carbon in good to excellent yields and high (E)-selectivity. The mild and efficient silylation method is conducted in water under atmospheric conditions to afford vinylsilanes.

Full text of DOI:10.1021/acs.orglett.6b00981

Letter
pubs.acs.org/OrgLett
Regio- and Stereoselective Copper(II)-Catalyzed Hydrosilylation of
Activated Allenes in Water: Access to Vinylsilanes
Srinath Pashikanti, Joseph A. Calderone, Matthew K. Nguyen, Christopher D. Sibley,
and Webster L. Santos*
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States
S
* Supporting Information
ABSTRACT: By using catalytic amounts of copper(II), 4-picoline,
and dimethylphenylsilylpinacol borane, a series of allenoates were
silylated on the β carbon in good to excellent yields and high (E)-
selectivity. The mild and efficient silylation method is conducted in
water under atmospheric conditions to afford vinylsilanes.
inylsilanes are an important class of synthetic building
γ carbon (Scheme 1). Whereas addition reactions on the α
1
Vblocks in organic synthesis. The carbon−silicon bond has carbon are scarce, a Lewis base such as phosphine-promoted
unique properties that allow it to be stable under harsh reaction
conditions and render it reactive under various conditions as in
the case of Tamao−Fleming oxidation or Hiyama cross-
coupling.2 Consequently, interest in the development of new
methods for their preparation is emerging. Alkyne3 hydro-
silylation is an approach heavily exploited to synthesize
vinylsilanes, and transition metal catalysts such as platinum,4
ruthenium,5 rhodium,6 palladium,7 iron,8 gold,9 cobalt,10
nickel,11 and copper12 play a prominent role in these
transformations. An attractive alternative strategy that utilizes
allenyl substrates can provide highly elaborated vinylsilane
products. However, silylation reactions involving allenes are
rare and underexplored. For example, Montgomery and co-
workers demonstrated the regioselective allene hydrosilylation
catalyzed by Ni and Pd N-heterocyclic carbene complexes.13
Scheme 1. Possible Silylation Products
15
Lewis acid14 and stoichiometric Co2(CO)8 mediated hydro-
γ-20 and β-additions21 of various nucleophiles to allenoates is
well precedented. Based on our experience with the
regioselective transfer of Bpin to the β carbon of allenoates
utilizing a preactivated sp2−sp3 hybridized diboron reagent
(pinacolatodiisopropylaminato diboron) and Cu(I) catalyst,22
we investigated the development of β-silylation of substituted
allenoates. Herein, we describe our efforts toward transitioning
this method in water and open to air using an air-stable, earth
abundant Cu(II) source.
We initiated our studies using conditions previously
determined by silylation of α,β-unsaturated carbonyl sub-
strates.19 Treatment of commercially available allenoate, ethyl
buta-2,3-dienoate (2a), with pinBSiMe2Ph in the presence of a
catalytic amount of Cu(II) and 4-picoline using water as the
solvent at room temperature and open to the atmosphere
resulted in >99% conversion to vinylsilane 3a as determined by
GC-MS analysis of the crude reaction mixture (Table 1, entry
1). The reaction was facile and reached completion within 30
min. Substitution of 4-picoline with pyridine afforded a similar
silylation has been reported for a limited number of phenyl-
and sugar allenes, respectively. More occurs on the more
substituted terminal allenes.16 However, this method is
restricted to terminal allenes, as 1,3-disubstituted substrates
were unreactive. The limited number of synthetic methods to
access these important molecules highlights the urgent need for
a general synthetic method for the silylation of allenes.
Nonetheless, elegant copper-catalyzed carbosilylations of
substituted allenes have been reported.17
Given our interest in silylation reactions utilizing Suginome’s
silylboron reagent [dimethylphenylsilylpinacolborane (pinBSi-
Me2Ph)]18 and development of sustainable reaction con-
ditions,19 we sought to develop a protocol for the silylation
of substituted allenoates. Water is an excellent solvent choice
because it is nontoxic, nonflammable, and environmentally
benign. When combined with an inexpensive, abundant, and
nontoxic transition metal such as copper used in a catalytic
fashion, an environmentally and user-friendly protocol could be
achieved. However, control of the regio- and stereoselectivity of
the reaction can be challenging, as up to eight potential
products are possible resulting from silyl addition to the α, β, or
Received: April 5, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00981
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of Reaction Conditions
Table 2. Silyl Addition to Substituted Allenoates
b
entry
base
mol % Cu
prod:SM 3a:2a
1
2
3
4
5
6
7
8
4-picoline
pyridine
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0
>99:1
>99:1
8:1
triethylamine
c
DBU
11:1
45:1
2.4:1
benzylamine
diethylamine
none
d
NR
d
4-picoline
NR
a
General procedure: base (5 mol %), ethyl buta-2,3-dienoate 2a (1
equiv), pinBSiPhMe2 (1.1 equiv), and 1.3 mg/mL of CuSO4 solution
b
were mixed at rt for 30 min. Determined by GC-MS of crude
c
d
material. 1,8-Diazabicyclo[5.4.0]undec-7-ene. NR = no reaction.
conversion (entry 2); however, a screen of other bases such as
triethylamine, DBU, benzylamine, and diethylamine resulted in
a significant decrease in product conversion (entries 3−6). As
expected, removal of 4-picoline or the copper catalyst afforded
no reaction, suggesting the key role of these reagents in the
reaction (entries 7−8).
With the optimized conditions in hand (5 mol % 4-picoline,
1 mol % CuSO4, 1.1 equiv pinBSiPhMe2), we investigated the
substrate scope of the developed reaction (Table 2). The
reaction of model substrate 2a resulted in 76% isolated yield of
product 3a with exclusive addition of the silicon group on the β
position (entry 1). Changing the ester functional group to
either a benzyl or phenyl afforded the desired products 3b and
3c in excellent yields (entries 2−3). However, allylallenoate 2d
diminished the yield; when propargylallenoate 2e was used
instead of 2d, only trace amounts of product were observed
(entries 4−5). The homologation of the propargyl group in 2f
similarly afforded the product in trace amounts. The inefficient
conversion of terminal alkynes may result from the insertion of
copper into the C−H bond, severely inhibiting the silylation
reaction. To test this hypothesis, we synthesized an alkyne
without an acidic hydrogen such as 2-butynyl ester 2g. The
reaction of 2g under the same reaction conditions produced the
silylated 3g in 60% yield. Further, this result demonstrate the
chemoselectivity of the reaction as silicon was added to the β
carbon in the presence of alkyne substituent. To explore the
substrate scope further, we investigated the effect of
substitutions on the α carbon of allenes (2h−i). A small alkyl
group such as a methyl (3h) or a more functionally diverse
moiety such as an ester (3i) resulted in very good yields
(entries 8−9). Finally, we investigated the reactivity of 1,3-
disubstituted allenoates, substrates bearing additional groups on
the γ carbon. Methyl substituted allenes bearing a benzyl (2j)
or ortho-nitrobenzyl (2k) ester yielded the corresponding
products in 95% and 51% yields, respectively. The lower yield
in 3k may arise from the instability of the photoreactive ortho-
nitrobenzyl group. In both cases, the (E)-olefin geometry was
preferred (∼9:1). When a much larger phenyl ring was placed
on the γ carbon, high (E)-selectivity and yields were observed
regardless of the ester functional group (entries 12−14).
A plausible mechanism and rationale for the stereoselectivity
of the reaction is shown in Scheme 2. The boron−silicon bond
in 1 is activated by a base-mediated nucleophilic water molecule
a
b
Reaction conditions identical to Table 1, entry 1. Isolated yield.
c
Isolated yields averaged from two or more experiments. Stereose-
lectivity was determined by 1H NMR of the crude material and
confirmed by NOE of the isolated product.
binding to boron.19a In the presence of a copper(II) source,23
transmetalation proceeds to generate a nucleophilic silyl-copper
intermediate (4). 3,4-Addition of 4 to the allenoate
B
DOI: 10.1021/acs.orglett.6b00981
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b00981.
■
S
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Detailed experimental procedures and full character-
ization of products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: santosw@vt.edu.
Notes
The authors declare no competing financial interest.
(19) (a) Calderone, J. A.; Santos, W. L. Org. Lett. 2012, 14, 2090.
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ACKNOWLEDGMENTS
We acknowledge financial support by the National Science
Foundation (CHE-1414458).
■
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