Silver-catalyzed highly regioselective formal hydroboration of alkynes

Article abstract of DOI:10.1021/ol501465x

A silver(I)-N-heterocyclic carbene complex has proven to be a potent catalyst for formal hydroboration of alkynes, providing a variety of borylalkenes in regio- and stereoselective manners. Under the silver catalysis, allenes also undergo regioselective hydroboration to give borylalkenes.

Full text of DOI:10.1021/ol501465x

Letter
pubs.acs.org/OrgLett
Silver-Catalyzed Highly Regioselective Formal Hydroboration of
Alkynes
Hiroto Yoshida,*,†,‡ Ikuo Kageyuki,† and Ken Takaki†
†Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
‡JST, ACT-C, Higashi-Hiroshima 739-8527, Japan
S
* Supporting Information
ABSTRACT: A silver(I)−N-heterocyclic carbene complex has
proven to be a potent catalyst for formal hydroboration of alkynes,
providing a variety of borylalkenes in regio- and stereoselective
manners. Under the silver catalysis, allenes also undergo
regioselective hydroboration to give borylalkenes.
Table 1. Optimization of Reaction Conditions for Ag-
Catalyzed Hydroboration
ecause of the high synthetic significance of organoboron
Bcompounds, whose carbon−boron bonds are utilizable for
carbon−carbon bond formation and introduction of functional
groups, development of new methods for accessing them has
been a central subject in modern synthetic organic chemistry.1
Recently considerable attention has been riveted on unique
copper catalysis for carbon−boron bond-forming reactions2
with unsaturated carbon−carbon linkages,3,4 organic halides,5
etc., where a borylcopper(I) species6 arising from σ-bond
metathesis between a copper(I) complex and a diboron acts as
a key intermediate. In marked contrast to the great progress in
the copper-catalyzed borylation reactions, the catalytic use of
silver, which also composes group 11 elements, for synthesizing
organoboron compounds remains almost totally unexplored,
except for diborylation of alkenes with bis(catecholato)-
diboron.7 In view of the difference between copper and silver
in fundamental properties such as atomic radius, electro-
negativity, softness as a Lewis acid, etc., we envisaged that
borylation reactions of different mode and/or selectivity should
be feasible under silver catalysis.8 Herein we report on formal
hydroboration of unsaturated carbon−carbon triple and double
bonds with the aid of a silver(I) catalyst, demonstrating the
potential silver catalysis toward carbon−boron bond-forming
reactions.
a
entry
Ag
base
solvent
time (h) yield (%)
1
(Ph3P)3AgOAc
(IMes)AgCl
(SIMes)AgCl
(IPr)AgCl
KOtBu
KOtBu
KOtBu
KOtBu
KOAc
K2CO3
−
KOtBu
KOtBu
KOtBu
KOtBu
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
33
16
12
48
43
43
68
93
42
24
8
68
85
85
<1
<1
63
<1
<1
37
<1
76
2
3
4
5
(IMes)AgCl
(IMes)AgCl
(IMes)AgCl
−
(IMes)AgCl
(IMes)AgCl
(IMes)AgCl
6
7
8
9
10
DMF
b
c
11
MeOH
We initially investigated the reaction of 1-phenyl-1-propyne
(1a) with bis(pinacolato)diboron ((pin)B−B(pin)) in the
presence of MeOH using (Ph3P)3AgOAc9 (2 mol %) and
KOtBu (6 mol %) and found that (Z)-2-boryl-1-phenyl-1-
propene (2a), a formal hydroboration product, was formed
regio- and stereoselectively in 68% yield (entry 1, Table 1). The
use of a silver−mesityl-substituted N-heterocyclic carbene
(NHC) complex10 ((IMes)AgCl or (SIMes)AgCl) improved
the efficiency of the hydroboration to give an 85% yield of 2a
(entries 2 and 3), whereas the reaction did not proceed at all
with a sterically more demanding complex ((IPr)AgCl) (entry
4). A drop in basicity of an added base (KOAc and K2CO3) led
to decrease in yield (entries 5 and 6), and furthermore the
a
b
c
Determined by NMR. Room temperature. Isolated yield.
combined use of a silver catalyst and a base was found to be
indispensable for the reaction to proceed (entries 7 and 8).
Among solvents surveyed, MeOH was the best, and the
reaction smoothly occurred even at room temperature (entries
9−11).
The intriguing feature of the silver catalysis in the present
hydroboration is the higher regioselectivity as compared to that
Received: May 21, 2014
© XXXX American Chemical Society
A
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Letter
obtained with a copper catalyst:11 such aliphatic terminal
alkynes having n-hexyl (1b), isoamyl (1c), isobutyl (1d), or
phenethyl (1e) were readily converted into the respective linear
borylalkenes (2b−2e) with high β-selectivity (entries 1−4,
Table 2).12 The reaction could also be applied to cyclopentyl-
(1f), cyclohexyl- (1g), and tert-butylacetylene (1h) to provide
high yields of the products (2f−2h), irrespective of the steric
congestion around the triple bonds (entries 5−7). Only a
carbon−carbon triple bond of an enyne (1i) underwent the
hydroboration to afford boryldiene 2i (entry 8),13 and
furthermore high chemoselectivity of the present system has
been demonstrated by the reaction of functionalized alkynes
bearing a cyano (1j), halogen (1k and 1l), silyl ether (1m), or
hydroxy (1n) moiety, leaving these reactive groups intact
(entries 9−13). The high β-selectivity was also observed with a
propargylamine (1o) (entry 14), whereas the reaction of
propargyl ethers (1p and 1q), N-propargyltheobromine (1r), or
N-propargylphthalimide (1s) gave α-adducts to some extent
(entries 15−18). Although the hydroboration of 1b also took
place with (IPr)AgCl or (IMes)CuCl under the present
reaction conditions, the regioselectivities became somewhat
lower (entries 19 and 20). In contrast to the silver and the
copper catalysis toward the hydroboration, a gold complex,
(Ph3P)AuCl, did not promote the reaction at all (entry 21).
In addition to 1a, diphenylacetylene (1t) also underwent the
stereoselective hydroboration to afford (Z)-borylstilbene (2t)
in 95% yield (Scheme 1). The wide applicability of the silver
Table 2. Ag-Catalyzed Hydroboration of Terminal Alkynes
Scheme 1. Ag-Catalyzed Hydroboration of Other
Unsaturated C−C Bonds
catalysis toward the borylation reaction has been shown by
conversion of a carbon−carbon double bond: n-octyl- (3a),
cyclohexyl- (3b), and phenylallene (3c) were facilely trans-
formable into borylalkenes with high regio- and stereo-
selectivities, where boron and hydrogen were preferentially
attached to the less congested double bond to provide (Z)-4 as
the major products.3f,h,i Besides, treatment of 1,1-diphenylallene
(3d) led to the regioselective formation of 4d, and boryl
a
b
c
Isolated yield. Determined by NMR. (IPr)AgCl was used instead of
d
e
(IMes)AgCl. NMR yield. (IMes)CuCl was used instead of
(IMes)AgCl. (Ph3P)AuCl was used instead of (IMes)AgCl.
f
B
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Letter
conjugate addition to 3-methylcyclohexenone (3e) also took
place to give 4e in 67% yield.14
The present hydroboration may be triggered by formation of
a borylsilver(I) species15 (5) arising from σ-bond metathesis
between a silver(I) alkoxide and diboron as is the case in the
copper(I)-catalyzed borylation reactions (step A, Scheme 2).
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
■
Scheme 2. A Proposed Catalytic Cycle for Ag-Catalyzed
Hydroboration
*E-mail: yhiroto@hiroshima-u.ac.jp.
Notes
The authors declare no competing financial interest.
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