Copper(II)-catalyzed silylation of activated alkynes in water: Diastereodivergent access to E- or Z-β-silyl-α,β-unsaturated carbonyl and carboxyl compounds

Article abstract of DOI:10.1002/anie.201310695

Copper(II)-catalyzed silylation of substituted alkynylcarbonyl compounds was investigated. Through the activation of Me₂PhSiBpin in water at room temperature and open atmosphere, vinylsilanes conjugated to carbonyl groups are synthesized in high yield. A surprising diastereodivergent access to olefin geometry was discovered using a silyl conjugate addition strategy: aldehydes and ketones were Z selective while esters and amides were exclusively transformed into the E products. Dial a diastereomer: The title reaction proceeds through the activation of Me₂PhSiBpin in water at room temperature and open atmosphere to produce high yields of vinylsilanes conjugated to carbonyl groups. A surprising diastereodivergent access to olefin geometry was discovered using this silyl conjugate addition strategy: aldehydes were Z selective while esters and amides exclusively delivered the E-configured products.

Full text of DOI:10.1002/anie.201310695

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Communications
DOI: 10.1002/anie.201310695
Synthetic Methods
Copper(II)-Catalyzed Silylation of Activated Alkynes in Water:
Diastereodivergent Access to E- or Z-b-Silyl-a,b-Unsaturated
Carbonyl and Carboxyl Compounds**
Joseph A. Calderone and Webster L. Santos*
Abstract: Copper(II)-catalyzed silylation of substituted alky-
nylcarbonyl compounds was investigated. Through the activa-
tion of Me2PhSiBpin in water at room temperature and open
atmosphere, vinylsilanes conjugated to carbonyl groups are
synthesized in high yield. A surprising diastereodivergent
access to olefin geometry was discovered using a silyl conjugate
addition strategy: aldehydes and ketones were Z selective while
esters and amides were exclusively transformed into the
E products.
V
inylsilanes are versatile building blocks in organic synthesis
because of their utility in diverse types of chemical trans-
Scheme 1. Strategies toward carbonyl conjugated vinylsilanes.
À
formations such as C C bond formation in the Hiyama
coupling, oxidation to a carbonyl with the Tamao–Fleming
reaction, and alkene formation by protodesilylation.[1] Owing
to their stability, low toxicity, and affordability, methods for
the preparation of organosilicon compounds have received
increased attention.[2] Transition-metal-catalyzed regioselec-
tive hydrosilylation of alkynes provides the most straighfor-
ward and atom economical access to vinylsilanes.[3] Synthesis
of substituted vinylsilanes conjugated to carbonyl groups is
challenging because of the difficulty in controlling the regio-
(a versus b) and stereoselectivity (E versus Z) of products
(Scheme 1).[4] Platinum-[5] and palladium-catalyzed[6] hydro-
silylation of alkynyl carbonyl compounds affording vinyl-
silanes with excellent regioselectivity, where the silicon group
is transfered to the a position of the carbonyl, have been
developed.[7] In contrast, ruthenium-catalyzed hydrosilylation
methods generate the complementary b-silyl-a,b-unsaturated
carbonyl products with excellent Z selectivity.[8] Most
recently, Molander and co-workers reported a copper(I)-
catalyzed conjugate silylation method using disilane reagents
to yield b-silyl-a,b-unsaturated esters in varying yields and
modest E/Z selectivity.[9] Hence, there is a paucity of methods
capable of providing products with the necessary regioselec-
tivity towards b silylation and selectivity with regards to olefin
geometry. Furthermore, to the best of our knowledge, there
are no examples of amides or aldehydes which have been
employed as substrates in these reactions.
Given that a mild method with broad substrate scope,
functional-group tolerance, and high stereoselectivity is
currently lacking, and our interest in copper-catalyzed
boryl[10] and silyl[11] conjugate addition,[12] we sought to
develop a process to address these shortcomings. In partic-
ular, we employ commercially available Me2PhSiBpin[13] as
the silicon source, which was recently used by Loh and co-
workers in the synthesis of linear and branched vinylsilanes.[14]
Herein, we report a copper(II)-catalyzed conjugate addition
of substituted alkynyl carbonyls to generate the correspond-
ing b-silylated products. Surprisingly, the method furnishes
a substrate-controlled diastereodivergent access to stereoiso-
meric products: aldehydes selectively afford Z olefins
whereas esters and amides exclusively generate the E-
alkene geometry. The protocol offers several practical advan-
tages. Firstly, it utilizes a catalytic amount of redox-stable and
inexpensive transition metal, a copper(II) source, which is
resistant to oxidation. Copper(I) sources require special
handling which includes either Schlenk techniques or use of
a glovebox. Secondly, the reaction is insensitive to moisture
and provides a green chemistry process which utilizes water as
the solvent. Finally, the method is mild and proceeds when
open to the atmosphere at room temperature within 2–4 h.
We initiated our investigations by examining the appro-
À
priate base additive which can activate a B Si bond (Table 1).
We previously demonstrated that 4-picoline was sufficiently
effective in acting as a Brønsted base to activate a nucleophilic
water molecule towards Me2PhSiBpin and chemoselectively
transfer silicon to a,b-unsaturated carbonyl compounds in the
presence of catalytic amounts of copper(II).[11] An important
aspect of our studies is the generation of a water-stable,
[*] J. A. Calderone, Prof. Dr. W. L. Santos
Department of Chemistry, Virginia Tech
900 West Campus Drive, Blacksburg, VA 24061 (USA)
E-mail: santosw@vt.edu
Homepage: http://www.santosgroup.chem.vt.edu
À
nucleophilic Cu Si intermediate to initiate silyl transfer to the
[**] Financial support has been provided by the ACS Petroleum
Research Fund and NIH (NIGMS RO1 GM093834).
b-carbon atom. A screen of bases confirmed that most organic
and inorganic bases efficiently catalyzed the conversion
of 2-octyn-1-al (1b) into (E)- and (Z)-2b within 2 hours
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310695.
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Table 1: Screening of base additives and copper source.[a]
Table 2: Silyl conjugate addition to substituted acetylenic aldehydes.[a]
Entry
Base
Copper
source
Conv.
[%][b]
E/Z[c]
Entry Substrate
1
Product
Yield[b] [%]
2a 74[d]
E/Z[c]
1
2
3
4
5
6
7
8
4-picoline
pyridine
DBU
TEA
NaOtBu
NaOH
KOAc
none
4-picoline
4-picoline
4-picoline
4-picoline
CuSO4
CuSO4
CuSO4
CuSO4
CuSO4
CuSO4
CuSO4
CuSO4
none
97
98
>99
97
>99
>99
69
9:91
9:91
22:78
30:70
22:78
26:74
25:75
–
11:89
2
3
4
5
2b 97
2c 93
2d 95
2e 98
5:95
10:90
11:89
9:91
trace
9
n.r.
>99
>99
>99
–
10
11
12
Cu(OAc)2
Cu(BF4)2
CuCl2
11:89
10:90
10:90
[a] General procedure: Base (5 mol%), 2-octyn-1-al (1 equiv),
Me2PhSiBpin (1.5 equiv), and 1 mL of 0.325 mgmLÀ1 CuSO4 solution
1
were mixed at RT for 2 h. [b] Conversion values determined by H NMR
analysis of the crude reaction mixture. [c] Determined by H NMR
1
analysis of the crude reaction mixture. DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene, n.r.=no reaction.
6
2 f
86
12:88
7
8
2g 51[e]
19:81
2:98
(entries 1–7). However, 4-picoline and pyridine preferentially
afforded 2b in greater than 97% conversion with 9:91 E/Z
ratio. Surprisingly, the Z geometry of the double bond
observed is contrary to that of a copper(I)-catalyzed addition
to acetylenic esters recently reported by Iannazzo and
Molander.[15] The double-bond geometry was unambiguously
assigned by nuclear Overhauser effect (nOe) experiments
(see the Supporting Information for details). In addition,
exclusive formation of the b-silylated product was observed.
Control experiments suggest both amine and copper were
required for this reaction to proceed (entries 8–9). Finally,
investigations on copper reagents indicate a number of
copper(II) sources efficiently afford (Z)-2b in excellent
conversion and selectivity (entries 10–12).
With optimized reaction conditions established, we inves-
tigated the scope of the silyl conjugate addition reaction with
aldehydes (Table 2). Use of 1.1 equivalents of Me2PhSiBpin
afforded the product 2a in 74% yield with 11:89 E/Z
selectivity (entry 1). As the aldehyde starting material was
not fully consumed in the reaction, the equivalency of
Me2PhSiBpin was increased to 1.5 in subsequent reactions.
Thus, alkynyl aldehydes with alkyl substituents, ranging from
a short pentyl to a much longer dodecyl chain, on the b-
carbon atom were efficiently transformed into the corre-
sponding vinyl silanes (2b–e) in excellent yield (up to 98%)
and in up to 5:95 E/Z selectivity (entries 2–5). A more
sterically encumbered cyclohexyl group was also an efficient
substrate (entry 6), but a phenyl substituent suffered from
2h 60
9
2i
2j
84
90
14:86
8:92
10
[a] General procedure: 4-picoline (5 mol%), substrate (1.0 equiv),
Me2PhSiBpin (1.5 equiv), and 1 mL of 1.30 mgmLÀ1 CuSO4 solution are
mixed at RT for 2 h. Each reaction is an average of at least two
experiments. [b] Yield of isolated product. [c] Ratio determined by
1H NMR analysis and assignment by nOe experiments; see the
Supporting Information for details. [d] 1.1 equiv of Me2PhSiBpin was
used. [e] Competing 1,2-addition product was observed. Ratio of 1,4-
versus 1,2-addition product was 2:1.
yield despite possible competing d- and 1,2-addition (entry 9).
Finally, cyclopropylacrylaldehyde efficiently underwent the
reaction in excellent yield and selectivity, thus affording the b-
silylated product 2j (entry 10).
With an efficient method for accessing Z silylenals
developed, we directed our efforts towards investigating the
reaction outcome with esters (Table 3). Treatment of the
butynoate ethylester 3a with 1.1 equivalents of Me2PhSiBpin,
catalytic amounts of copper sulfate, and 4-picoline in water at
room temperature and open to the atmosphere, provided the
desired vinylsilane 4a in 95% yield (entry 1). To our delight,
a single isomer was exclusively formed and nOe studies (see
the Supporting Information for details) unambiguously
a
competing 1,2-addition to afford 2g in 51% yield
(entry 7).[16] To explore functional-group tolerance of the
reaction conditions, the propargyl amine 1h was converted
into the desired product in almost complete Z selectivity
(entry 8). Additionally, the a,b,g,d-unsaturated ynal 1i spe-
cifically reacted on the b-carbon atom to afford 2i in 84%
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assigned the alkene geometry as E, which is in sharp contrast
to the stereochemistry described for aldehydes. To determine
whether the switch in olefin geometry in the product was
general, substrates with varying substituents on the ester and
alkyne functionalities were investigated. Substrates with
increasingly larger alkyl substituents on the b-carbon atom
generated the corresponding products in excellent yields with
exclusive E selectivity (entries 2–5). Indeed, a hydrophobic
dodecyl chain on the b-carbon atom efficiently afforded the
vinyl silane (E)-4e in 96% yield. Even though hydrogen on
the b-carbon atom in ethyl propiolate generated 4 f in
a slightly lower yield (entry 6), aryl substituents were also
effective substrates. For example, a phenyl group resulted in
98% yield and 100% E selectivity (entry 7). Both electron-
donating and electron-withdrawing groups on the aryl ring
were tolerated and afforded the desired products in excellent
yields and selectivity (entries 8 and 9). The nature of the ester
functionality has negligible impact on the reaction yield
(compare entries 3 and 4 versus entries 10 and 11). Methyl,
ethyl, isobutyl, and phenyl esters underwent the silylation
reaction efficiently. Furthermore, no evidence of ester
hydrolysis was observed even with phenyl ester (entry 10).
To investigate the functional-group tolerance of the protocol,
substrates bearing heteroatoms were subjected to the reaction
conditions. For example, a propargyl ether with phenyl or
silicon protecting groups (TBS) were tolerated and efficiently
converted into the corresponding products 4l and 4m in 87
and 89% yield, respectively (entries 12 and 13). Interestingly,
coordinating groups were also good substrates and did not
affect the regio- and stereochemistry of the reaction. Tertiary
amine and pyridine moieties exclusively gave E products in
moderate to good yields (entries 14 and 15).
Table 3: Silyl conjugate addition to substituted acetylenic esters.[a]
Entry Substrate
1
Product
Yield[b] [%]
95
4a
4b
4c
4d
2
3
4
83
92
93
5
4e
4 f
4g
4h
4i
96
57
98
89
99
94
95
87
6
7
We were intrigued by the change in the stereochemical
outcome of the reaction based on the identity of the carbonyl.
A key difference between these substrates is the nature of the
electron-withdrawing ability of the carbonyl groups. A
proposed mechanism and origin of stereoselectivity observed
is shown in Scheme 2. A nucleophilic silylcopper species (5) is
8
9
À
generated by water-mediated activation of the Si B bond and
10
11
4j
subsequent transmetalation.[10d,11] In the case of a,b-acety-
lenic esters, silylcupration across the triple bond (6) occurs in
a syn fashion to afford the intermediate 7 (path a). Proto-
nation affords the (E)-b-silyl-a,b-unsaturated ester 8 as well
as turns over the copper(II) catalyst. In this particular case,
a formal 1,4-conjugate addition is disfavored because of
electron donation to the carbonyl group from the ester
moiety. In contrast, when more-electron-deficient aldehydes
are used, addition of 6 may proceed through two possible
pathways: 1) a syn addition in a similar path to that observed
with esters (path a), thus generating the E isomer as a minor
product, and 2) a competing 1,4-conjugate addition (path b)
which leads to the allenolate intermediate copper(II) complex
9. Because the substituents on the a- and d-positions of 9 are
orthogonal, protonation occurs on the site opposite the
sterically encumbered dimethylphenylsilyl group to provide
the Z isomer 10 as the major product. A similar mechanism
has been proposed with trimethylgermyl copper addition to
alkynyl esters, that is, protonation occurs from the side
opposite a bulky trimethylgermanium group.[17]
4k
4l
12
13
14
4m 89
4n
4o
50
15
74[c,d]
[a] General procedure: 4-picoline (5 mol%), substrate (1.0 equiv),
Me2PhSiBpin (1.1 equiv), and 1 mL of 1.30 mgmLÀ1 CuSO4 solution are
mixed at RT for 2 h. Each reaction is an average of at least two
experiments. [b] Yield of isolated product. [c] Reaction run for 5 h.
[d] Yield as determined by NMR spectroscopy; product and starting
material were chromatographically inseparable. TBS=tert-butyldime-
thylsilyl.
To test this hypothesis, carbonyl groups with better
electron-donating substituents, such a,b-alkynyl amides,
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Scheme 2. Proposed mechanism and origin of diastereoselectivity.
vigorously stirred together. The reaction was quenched with 1 mL of
were subjected to the reaction conditions. To our delight,
amides bearing either a butyl (11a) or phenyl (11b) moiety on
the alkyne provided the products 12a,b in excellent yields
with exclusive E configuration (Scheme 3). As predicted,
substrates with intermediate carbonyl electrophilicity (i.e.,
between aldehydes and esters), such as the a,b-unsaturated
ketones 11c,d, afforded a mixture of isomers but with a slight
perference for the Z alkene (12c,d).
hexanes as followed by TLC. The aqueous layer was extracted 3 ꢀ
with hexanes. The combined hexanes layer was back extracted 5 ꢀ
with water and then dried over sodium sulfate. Silica gel chromatog-
raphy was used to purify the product.
Received: December 9, 2013
Published online: February 14, 2014
Keywords: copper · diastereoselectivity ·
.
heterogeneous catalysis · silicon · water chemistry
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