Borane-Catalyzed Reductive α-Silylation of Conjugated Esters and Amides Leaving Carbonyl Groups Intact

Article abstract of DOI:10.1002/anie.201508669

Described herein is the development of the B(C₆F₅)₃-catalyzed hydrosilylation of α,β-unsaturated esters and amides to afford synthetically valuable α-silyl carbonyl products. The α-silylation occurs chemoselectively, thus leaving the labile carbonyl groups intact. The reaction features a broad scope of both acyclic and cyclic substrates, and the synthetic utility of the obtained α-silyl carbonyl products is also demonstrated. Mechanistic studies revealed two operative steps: fast 1,4-hydrosilylation of conjugated carbonyls and then slow silyl group migration of a silyl ether intermediate.

Full text of DOI:10.1002/anie.201508669

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International Edition: DOI: 10.1002/anie.201508669
German Edition: DOI: 10.1002/ange.201508669
Hydrosilylation
Borane-Catalyzed Reductive a-Silylation of Conjugated Esters and
Amides Leaving Carbonyl Groups Intact
Youngchan Kim and Sukbok Chang*
Abstract: Described herein is the development of the B(C6F5)3-
catalyzed hydrosilylation of a,b-unsaturated esters and amides
to afford synthetically valuable a-silyl carbonyl products. The
a-silylation occurs chemoselectively, thus leaving the labile
carbonyl groups intact. The reaction features a broad scope of
both acyclic and cyclic substrates, and the synthetic utility of the
obtained a-silyl carbonyl products is also demonstrated.
Mechanistic studies revealed two operative steps: fast 1,4-
hydrosilylation of conjugated carbonyls and then slow silyl
group migration of a silyl ether intermediate.
H
ydrosilylation of unsaturated bonds with silanes is
a straightforward approach to organosilicon compounds,
which are widely utilized in organic synthesis as well as
medicinal, polymer, and materials chemistry.[1] This method
Scheme 1. Selective hydrosilylation of a,b-unsaturated carbonyls.
À
takes advantage of the labile Si H bond of silanes, bonds
which can be activated by either Lewis acids,[2] radical
mediators,[3] or transition-metal species.[4] Alkenes and
alkynes are representative unsaturated hydrocarbon sub-
strates which react with silanes and lead to the corresponding
alkyl- or alkenylsilicon products.[5] Hydrosilylation of unsa-
turated polar bonds, such as carbonyls, imines, nitriles, or
heteroaromatics has been shown to yield heteroatom-con-
taining silyl compounds.[6] The a,b-unsaturated carbonyl
compounds containing these two types of functional groups
connected, that is an olefin and carbonyl, can bring out an
interesting selectivity issue in the hydrosilylation reaction.
Indeed, reaction of a,b-unsaturated carbonyls with silanes
may lead to four types of silylated molecules: O-silyl enol
ether (1,4-addition adduct), O-silyl allyl ether (1,2-), a-C-silyl
carbonyl, and b-C-silyl carbonyl products (Scheme 1a).
Although there have been efforts made to achieve chemo-
selective hydrosilylation of a,b-unsaturated esters, only a few
examples of a-C-silylation are known, and transition-metal
catalysts such as Ni, Rh, Pd, or Pt were employed with limited
scope and unsatisfactory selectivity.[7] Moreover, no report
has been published on the chemoselective hydrosilylation of
a,b-unsaturated amides, to the best of our knowledge.
Piers et al. reported that tris(pentafluorophenyl)borane
B(C6F5)3 is an efficient catalyst for the hydrosilylation of aryl
aldehydes, ketones, and esters.[8] Its applicability to additional
types of important substrates such as imines, olefins, carbox-
ylic acids, or nitriles was elegantly demonstrated by several
groups.[9] Mechanistic studies illuminated the activation mode
of silanes by Lewis-acidic boranes to disclose that a key
borane-silane adduct reacts with Lewis-basic substrates.[10]
Based on the mechanistic consideration that B(C6F5)3-cata-
=
lyzed hydrosilylation of polar C X bonds furnishes exclu-
À
À
sively a H CX [Si] connectivity, reaction of a,b-unsaturated
carbonyls is predicted to give O-silylated products, rather
than C-silylated ones under the borane catalyst system
(Scheme 1a).[11]
Herein, we report the first example of B(C6F5)3-catalyzed
hydrosilylation of a,b-unsaturated esters and amides to form
3
À
a valuable a-C(sp ) Si bond, thus leaving the labile carbonyl
groups intact (Scheme 1b).[12] Mechanistic studies revealed
that the reaction proceeds by two stepwise processes: fast 1,4-
hydrosilylation of conjugated carbonyls and a subsequent
slow silyl-group migration of a silyl ketene acetal intermedi-
ate in the case of ester substrates. The substrate scope is broad
and includes both acyclic and cyclic conjugated esters and
amides to afford synthetically versatile a-silyl carbonyl
compounds.[13]
[*] Y. Kim, Prof. Dr. S. Chang
Department of Chemistry
Korea Advanced Institute of Science and Technology (KAIST)
Deajeon 305-701 (Korea)
and
Center for Catalytic Hydrocarbon Functionalizations
Institute for Basic Science (IBS)
Deajeon 305-701 (Korea)
At the beginning of our study, we wondered if any notable
selectivity could be attained from the treatment of ethyl
cinnamate (1b) with triethylsilane (1.0 equiv) in the presence
of the B(C6F5)3 catalyst [Eq. (1)]. To our surprise, an a-silyl
ester (2a) was obtained, with the carbonyl group left intact,
after 12 hours at room temperature, albeit in moderate
yield.[14] To get some preliminary insight into this interesting
chemoselectivity, the reaction progress was briefly monitored
E-mail: sbchang@kaist.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508669.
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Table 1: Screening of silanes in the silylative reduction.[a]
Entry
Silane
Et3SiH
t
3
2
1
Yield [%][b]
Yield [%][b]
by H NMR spectroscopy. This analysis revealed that a silyl
ketene acetal intermediate was formed immediately at 258C
and that the intermediate was observed to convert gradually
into 2a at the same temperature. This result was noteworthy
for several reasons: 1) the formation of a new C(sp ) Si bond
was regio- and chemoselective, thus leaving a labile ester
group intact, 2) the B(C6F5)3-catalyzed silyl-group migration
from a silyl ketene acetal intermediate is unprecedented, and
3) a synthetically valuable a-silyl ester was produced.
1
2
5 min
12 h
80
41
6
48
3
À
3
4
EtMe2SiH
BnMe2SiH
5 min
12 h
67
19
32
79
5
6
5 min
12 h
83
18
14
78
Although there are two relevant transformations known,
our present result is distinct from such precedents. The groups
of Piers and Cantat independently reported that B(C6F5)3-
catalyzed hydrosilylation of enones and enamides leads to O-
silyl ethers, but without giving a-C-silyl carbonyls, which can
be formed by silyl group migration (Scheme 2a).[11] In
7
MePh2SiH
5 min
18
79
8
Me2PhSiH
5 min
12
82
[a] 1b (0.20 mmol), silane (0.22 mmol), and catalyst (5 mol%) in CDCl3
(0.5 mL) at 258C. [b] Determined by 1H NMR analysis of the crude
reaction mixture (internal standard: mesitylene).
Table 2: Substrate scope of a,b-unsaturated esters.[a]
Scheme 2. Relevant previous studies. Tf =trifluoromethanesulfonyl.
addition, although the groups of Yamamoto and Fujiwara
independently showed that either lanthanide metals or
aluminum can facilitate the silyl group migration, the silyl
ketene acetals need to be prepared from the corresponding
alkyl esters using a strong base, such as LDA (Scheme 2b).[15]
Moreover, the scope of this approach was narrow and no
mechanistic details were reported.
The above initial observation prompted us to examine the
factors that control the selectivity in the present hydro-
silylation of conjugated carbonyls. The influence of silanes on
the reaction efficiency was first examined. The initial 1,4-
hydrosilylation step to form a silyl ketene acetal intermediate
(3) was observed to be fast, largely irrespective of silanes
tested, while the subsequent silyl group migration was highly
dependent upon the silanes (Table 1, and see also the
Supporting Information for details). On the basis of the
screening experiments, the reactivity trend of silanes exam-
ined in the present transformation of ethyl cinnamate (1b)
into a-silyl esters can be summarized as follows: Me2PhSiH ꢀ
MePh2SiH @ BnMe2SiH ꢀ EtMe2SiH > Et3SiH.
[a] 1 (0.20 mmol), dimethylphenylsilane (0.22 mmol), and catalyst in
CHCl3 (0.5 mL) at 258C for 0.5 h. Yields are those of the isolated
products. [b] Diastereomeric ratio determined by 1H NMR analysis of the
crude reaction mixture. [c] For 12 h at 258C.
corresponding a-silyl esters in reaction with dimethylphenyl-
silane (Table 2). The alkoxy moiety of the substrates turned
out to be flexible in use, and methyl (4a), ethyl (4b), isopropyl
(4c), cyclohexyl (4d), and benzyl (4e) cinnamates were all
smoothly reacted by the action of the B(C6F5)3 catalyst
(5 mol%) in a short period of time (30 min, 258C).[16] The
With the above optimized reaction conditions and mech-
anistic insights in hand, the generality of the present a-
silylative conversion was next investigated. A wide range of
a,b-unsaturated esters were successfully transformed into the
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reaction conditions were compatible with various functional
groups, including halides (4 f and 4g), phenyloxy (4h), and
silyl ether (4i). In addition, the procedure was readily applied
to alkyl-substituted a,b-unsaturated esters to furnish the
desired a-silyl alkyl carboxylate esters in high yields (4j and
4k). A substrate bearing a remote double bond underwent the
desired chemoselective hydrosilylation in good yield without
reacting at the isolated olefin (4l). When conjugated esters
having a chiral alkoxy group were subjected to the reaction
conditions, only a moderate level of diastereoselectivity was
observed (4m and 4n), although the product yields were still
satisfactory.
amides bearing various labile groups (6g–j). Moreover, the
same reaction conditions were also adaptable to the a-
silylation of alkyl enamides (6k and 6l). Also, an isolated
double bond was completely tolerated (6m)
We were pleased to see that the present procedure could
be applicable to the a-silylation of conjugated lactones and
lactams with similar efficiency (Table 4). a-Silyl-g-butyrolac-
Table 4: a-Silylation of lactones and lactams.[a]
Subsequently, our attention turned to an important type
of substrate: enamides (Table 3). We were pleased to see that
Table 3: Substrate scope of enamides.[a]
[a] 7 (0.20 mmol), dimethylphenylsilane (0.22 mmol), and catalyst in
CHCl3 (0.5 mL) at 258C for 0.5 h. Yields are those of isolated products.
[b] Diastereomeric ratio determined by 1H NMR analysis of the crude
reaction mixture. [c] For 12 h at 258C.
tones were obtained from 2-furanones in excellent yields after
30 minutes (8a–c). In cases where substituents are present in
lactones, the reaction proceeded with moderate diastereose-
lectivity under the present reaction conditions. In addition, 1-
methyl-2(1H)-quinolinone was readily converted into its a-
silyl lactam in acceptable yield (8d).
The present reaction procedure was conveniently per-
formed on large scale, and a,b-unsaturated esters and amides
were transformed into their corresponding a-silyl carbonyls
adducts in gram quantities (Scheme 3; 4j and 6b). Moreover,
the silyl group was readily oxidized, according to a known
method,[17] to the a-hydroxy amide 9 in good yield.[18] A post-
transformation of the obtained a-silyl ester was also success-
fully demonstrated in the facile reduction of the a-silyl ester
4j into 1,2-silylalcohol 10, which has a versatile synthetic
utility.[19]
To shed light on the details of the reaction pathway,
a series of mechanistic studies were designed (Scheme 4).
When a separately prepared silyl ketene acetal (3a)[20] was
[a] 5 (0.20 mmol), dimethylphenylsilane (0.22 mmol), and catalyst in
CHCl3 (0.5 mL) at 258C for 12 h. Yields are those of isolated products.
[b] For 48 h at 258C. Thermal ellipsoids shown at 50% probability.[24]
the same reaction conditions were also successfully applied to
enamide substrates to afford a-silyl amides in high to
moderate yields, although the reaction was slower than a,b-
unsaturated esters. Alternation of the N,N-dialkylamido
group did not change the reaction efficiency much, as
demonstrated in the reactions of N,N-dimethyl (6a), N,N-
diethyl (6b), N,N-diisopropyl (6c), and N,N-dicyclohexyl (6d)
amides. The structure of one a-silyl amide product (6a) was
confirmed by its solid-state X-ray crystallographic analysis. In
addition, pyrrolidinyl and piperidinyl enamides were readily
hydrosilylated to give the corresponding a-silyl amides (6e
and 6 f). Functional-group tolerance was observed to be
satisfactory as shown in the reaction of a,b-unsaturated
Scheme 3. Gram-scale reaction with subsequent transformations.
DIBAL-H=diisobutylaluminum hydride.
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in cases of conjugated esters. Subsequently, an intermolecular
silyl-group migration of this intermediate yields a-silyl
carbonyl products, and is assumed to be facilitated by the
B(C6F5)3 catalyst, presumably either by silylium ion (R3Si+)
extrusion or through a pentacoordinate silicon cation inter-
mediate.
In summary, we have developed the B(C6F5)3-catalyzed
silylative reduction of a,b-unsaturated esters and amides to
afford synthetically valuable a-silyl carbonyls. Especially
significant is the regio- and chemoselective introduction of
3
À
a C(sp ) Si bond with the labile carbonyl groups intact.
Mechanistic investigations revealed that the reaction takes
place by distinct cascade steps mediated by the borane
catalyst: fast 1,4-hydrosilylation of a,b-unsaturated carbonyls
and a subsequent rate-limiting silyl-group migration of an O-
silyl ether intermediate.
Acknowledgements
Scheme 4. Mechanistic investigations.
This research was supported by the Institute for Basic Science
(IBS-R010-D1). We thank Dr. Jaesung Kwak, Dr. Seewon
Joung, Dr. Jeung Gon Kim, and Mr. Yoonsu Park for helpful
discussions and Mr. Jinseong Gwak for the XRD analysis.
subjected to the reaction conditions, the silyl group migration
occurred smoothly only in the presence of the B(C6F5)3
catalyst, whereas no conversion was observed without the
borane species (Scheme 4a). This result indicates that the
borane catalyst is essential in the second stage of the silyl
group migration, as well as in the initial 1,4-hydrosilylation
step.
When an equimolar mixture of two silyl ketene acetals (3a
and 3b) was treated with the B(C6F5)3 catalyst, four scram-
bled a-silyl ester compounds (4b, 2c, 2b, and 4j) were
obtained with the same ratio within 5 minutes at room
temperature (Scheme 4b). This complete crossover outcome
implies that the silyl group migration more likely occurs in an
intermolecular manner rather than intramolecularly.
Keywords: boron · chemoselectivity · conjugation ·
hydrosilylation · reaction mechanisms
How to cite: Angew. Chem. Int. Ed. 2016, 55, 218–222
Angew. Chem. 2016, 128, 226–230
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Received: September 16, 2015
Published online: November 9, 2015
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