Metal-free catalytic C-Si bond formation in an aqueous medium. enantioselective NHC-catalyzed silyl conjugate additions to cyclic and acyclic α,β-unsaturated carbonyls

Article abstract of DOI:10.1021/ja203031a

A metal-free method for enantioselective conjugate addition of a dimethylphenylsilyl group to α,β-unsaturated carbonyls is reported. Transformations are catalyzed by a chiral N-heterocyclic carbene (NHC), performed in an aqueous solution (3:1 mixture of water and tetrahydrofuran) and are operationally simpler to perform than the NHC-Cu-catalyzed variant. The chiral catalyst is generated from an enantiomerically pure imidazolinium salt (prepared in three steps) and a common organic amine base (dbu). NHC-catalyzed processes proceed with 5.0-12.5 mol % catalyst loading at 22 °C within 1-12 h, affording the desired β-silyl carbonyls in 85:15 to >98:2 enantiomeric ratio and in 50% to >98% yield. Cyclic enones or lactones and acyclic α,β-unsaturated ketones, esters, and aldehydes can be used as substrates.

Full text of DOI:10.1021/ja203031a

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Metal-Free Catalytic CÀSi Bond Formation in an Aqueous Medium.
Enantioselective NHC-Catalyzed Silyl Conjugate Additions to Cyclic
and Acyclic r,β-Unsaturated Carbonyls
Jeannette M. O’Brien and Amir H. Hoveyda*
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
S Supporting Information
b
Scheme 1. NHC-Catalyzed Silyl Conjugate Addition to
Unsaturated Carbonyls
ABSTRACT: A metal-free method for enantioselective con-
jugate addition of a dimethylphenylsilyl group to R,β-unsatu-
rated carbonyls is reported. Transformations are catalyzed by a
chiral N-heterocyclic carbene (NHC), performed in an aqueous
solution (3:1 mixture of water and tetrahydrofuran) and are
operationally simpler to perform than the NHCÀCu-catalyzed
variant. The chiral catalyst is generated from an enantiomerically
pure imidazolinium salt (prepared in three steps) and a common
organic amine base (dbu). NHC-catalyzed processes proceed
with 5.0À12.5 mol % catalyst loading at 22 °C within 1À12 h,
affording the desired β-silyl carbonyls in 85:15 to >98:2
enantiomeric ratio and in 50% to >98% yield. Cyclic enones
or lactones and acyclic R,β-unsaturated ketones, esters, and
aldehydes can be used as substrates.
Scheme 2. Facile Formation of an NHCÀBÀSi Complexa
e recently discovered that achiral N-heterocyclic carbenes
W
(NHCs), in the absence of a metal salt, activate the BÀB
bond of bis(pinacolato)diboron [B2(pin)2] and catalyze efficient
(nonenantioselective) boronate conjugate additions to R,β-un-
saturated carbonyls.1 A related set of transformations corre-
sponds to NHC-catalyzed silyl conjugate addition (SCA)
reactions with commercially available dimethylphenylsilylpina-
colatoboron [Me2PhSiÀB(pin), 1].2 Notwithstanding the value
of CÀSi bond forming processes in chemical synthesis,3 the basis
for pursuing silyl conjugate additions4 was twofold: First, we
were interested in determining if, as outlined in Scheme 1, the
unsymmetrical silylboronate can be activated to cause selective trans-
fer of the PhMe2Si unit to a polarized olefin (vs (pin)BÀC bond
formation). Second, we were keen on investigating whether a chiral
NHC can sufficiently influence the reaction course to render such
metal-free catalytic processes highly enantioselective.5 Herein, we
report enantioselective SCA reactions involving cyclic and acyclic
enones and R,β-unsaturated esters as well as various enals. Transfor-
mations are performed in the presence of 1 and promoted by 5.0À
12.5 mol % of an NHC catalyst, which is prepared and used in situ
from a readily accessible chiral imidazolinium salt and a common
organic base. NHC-catalyzed processes are carried out at ambient
temperature in a 3:1 mixture of water and tetrahydrofuran (thf).6 The
desired β-silylcarbonyls are isolated in up to >98% yield and >98:2
enantiomeric ratio (er). The method complements the related metal-
catalyzed variants4,7,8 and, to the best of our knowledge, represents the
first examples of CÀSi bond forming reactions that do not require an
organometallic complex as the catalyst.
a Mes = 2,4,6-(Me)3C6H2.
and activate the BÀSi bond. From the outset, we judged that a
practical advantage of the metal-free protocol would be the
procedural simplicity with which it can be performed. Thus,
we decided to establish whether a relatively robust amine might
be used to generate the requisite NHC instead of one of the
moisture sensitive metal alkoxides employed with the Cu-cata-
lyzed process (e.g., NaOMe or NaOt-Bu).4a Such considerations
led us to determine that, as shown in Scheme 2, treatment of
commercially available imidazolinium salt 2 with 1.05 equiv of
1,8-diazabicycloundec-7-ene (dbu) followed by the addition of 1
leads to complete consumption of the silylboron reagent within
5 min. Complexation was monitored by the appearance of a new
signal at δ 8.02 ppm in the 11B NMR spectrum, likely corresponding
to 1•NHC complex 3; the signal for 1, also a singlet, appears at
δ 33.38 ppm.9 Similar to the reaction of B2(pin)2,1 such a significant
shift is likely due to the associated electronic reorganization caused
by the conversion of the sp2-hybridized boron atom in 1 to one that
We first investigated the efficiency with which an NHC can
associate with 1, presumably at the more Lewis acidic boron site,
Received: April 6, 2011
Published: April 23, 2011
r
2011 American Chemical Society
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Table 1. Initial Studies on Catalytic Reaction of Cyclo-
hexenone and Dimethylphenylsilylpinacolatoboron 1a
Table 2. Evaluation of Various Chiral N-Heterocyclic
Carbenesa
a Performed under N2 atmosphere; with 15 mol % dbu for entries 1À6
and 20 mol % dbu for entries 7À9. b Conversion values determined by
analysis of 400 MHz 1H NMR spectra of unpurified mixtures.
is sp3-hybridized (3).10 Treatment of 1 with dbu, under otherwise
identical conditions, does not lead to a new entity.
Next, we evaluated the ability of the carbene derived from
imidazolinium salt 5 to promote silyl conjugate addition to
cyclohexenone (Table 1); the selection of this particular NHC
precursor was based on its structural similarity to the chiral
variants developed in these laboratories4a,11 (versus unsaturated
imidazolium salts). As illustrated in entry 1, with 5.0 mol % 5 and
15 mol % dbu, a minimal amount of the desired β-silylcarbonyl
4 is generated (<10%). To facilitate SCA, the effect of methanol
as an additive was examined; we surmised that such modification
might facilitate formation of the charged intermediates likely
involved in the catalytic process. The presence of the alcohol
additive indeed results in a more efficient silyl conjugate addition
(22À34%) without any CÀB bond formation (<2%, 1H NMR
analysis; entries 2À3, Table 1). In light of the positive effect of
MeOH, we chose to examine the influence of a more polar
medium, leading us to the finding that when the catalytic SCA is
performed in an equal mixture of thf and water, there is 95%
conversion to the desired 4 within three hours at 22 °C (entry 4,
Table 1); when pure water is used as solvent, SCA is complete
(>98% conv, entry 5).12 In the absence of 5, under otherwise
identical conditions to entry 4, there is <2% conversion to the
desired 4. As the representative data in entry 6 of Table 1 point
out, the addition of MeOH to the aqueous solution does not
improve efficiency (vs entry 3). Moreover, an aryl and an alkyl
phosphine or a phosphine oxide, in spite of higher catalyst
loadings (10 mol %), either do not promote any reaction or
are far less effective catalysts (entries 7À10, Table 1).
a Performed under N2 atmosphere. b Conversion values determined
by analysis of 400 MHz 1H NMR spectra of unpurified mixtures.
c Enantiomeric ratio values determined by HPLC analysis ((2%); see
the Supporting Information for details. Mes = 2,4,6-Me3C6H2.
more efficient (>98% conv) and the desired product is formed in
82:18 er. Increasing the size of the symmetrical NÀAr moiety of the
catalyst (i.e., 8bÀc, entries 4À5) leads to diminution in efficiency
and/or enantioselectivity. When C2-symmetric imidazolinium salt
9a is used (entry 6), lower conversion but similarly high er values are
attained (compared to 8a, entry 3). Further catalyst screening led us
to establish that incorporation of a m-Me unit within the NÀAr
moieties furnishes an NHC catalyst that affords β-silylketone 4
efficiently (>98% conv) and in 98:2 er. The low selectivity observed
with 10 underlines the significance of the C2-symmetric structure of
the optimal 9b in this application.
A range of cyclic enones, including cyclopentenones as well as
seven- and eight-membered ring R,β-unsaturated ketones, un-
dergoes enantioselective NHC-catalyzed SCA to afford the β-silyl
ketones in 50% to >98% yield and in 90:10 to >98:2 er (entries
1À6 of Table 3). There is complete consumption of the starting
materials in the transformations shown in entries 1À4 (Table 3).
However, catalytic additions that furnish sterically demanding
cyclopentenone 14 and bicyclic cycloheptenone 15 proceed less
readily (83% and 60% conv in 3 h, respectively); longer reaction
times do not result in further conversion, suggesting that the
NHC catalyst is perhaps undergoing competitive decomposi-
tion.14 As the example in entry 7 of Table 3 indicates, unsaturated
cyclic esters can be used to access the derived β-silyl lactones (16 in
71% yield and 85:15 er); as was also observed in studies regarding
the NHCÀCu-catalyzed version of this process, such additions are
less efficient and enantioselective than with the corresponding cyclic
enones (compare entries 2 and 7, Table 3).4a
With conditions for effective NHC-catalyzed SCA outlined, we
focused on identifying an effective chiral carbene for the enantio-
selective version of the process. Cyclohexenone continued to serve
as the model substrate; transformations were performed at 22 °C in
a solution that largely consists of water (3:1 H2O/thf)13 and were
analyzed after 3 h. Bidentate imidazolinium salts 6 and 7, used as
ligands in various metal-catalyzed enantioselective transforma-
tions,4a,11 promote CÀSi bond formation with moderate efficiency,
but 4 is obtained in negligible enantiomeric purity (entries 1À2,
Table 2). With 8a, which, similar to 6 and 7, is C1-symmetric, but
lacks the hydroxyl or the sulfonate unit (entry 3, Table 2), SCA is
Acyclic R,β-unsaturated ketones can be used in NHC-cata-
lyzed SCA (Scheme 3). Generally, reactions proceed less readily
than the cyclic unsaturated carbonyls and require a slightly higher
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Table 3. NHC-Catalyzed Enantioselective Silyl Conjugate
Addition (SCA) Reaction with Cyclic r,β-Unsaturated
Carbonylsa
Scheme 3. NHC-Catalyzed Enantioselective Silyl Conjugate
Addition (SCA) Reactions with Acyclic r,β-Unsaturated
Carbonylsa
a Reactions performed in 3:1 H2O/thf under the conditions shown in
Table 1 and with imidazolinium salt 9b as the catalyst precursor.
b Conversion values determined by analysis of 400 MHz 1H NMR
spectra of unpurified mixtures. c Yields of isolated products after
purificaton ((5%). d Enantiomeric ratio values determined by HPLC
analysis ((2%); see the Supporting Information for details.
a All reactions performed under identical conditions as those shown for
synthesis of 18, except for 24 and 25 (12.5 and 10 mol % 9b, and 37.5 and
30 mol % dbu, respectively). For determination of conversions and er values,
see Table 3. Yields relate to isolated and purified products ((5%).
catalyst loading for achieving complete conversion.15 Formation
of β-silyl methyl ketones bearing an aryl substituent (cf. 18À20)
is relatively more facile and enantioselective than those that carry
an alkyl unit (cf. 22À23). NHC-catalyzed SCA reactions invol-
ving chalcone (cf. 24) and an acyclic ester (cf. 25) are highly
enantioselective but less efficient than the corresponding methyl
ketones. Diminished efficiency and substantially lower enantios-
electivity are observed with a corresponding cis isomer: when
Z-17 is used, 18 is generated in nearly racemic form (54:46 vs
94:6 er). Considering that cyclic β-silylketones are obtained in
high enantiomeric purity (entries 1À4, Table 3), the aforemen-
tioned variation in selectivity is striking. Furthermore, in contrast to
substrates bearing a phenyl or an electron-rich p-methoxyphenyl
substituent (cf. 18 and 19), there is <2% conversion to 21, which
contains the electron-deficient p-trifluoromethylphenyl moiety. The
above trends might shed light on the inner workings of the NHC-
catalyzed process and are the subject of ongoing investigations.
A particularly noteworthy type of NHC-catalyzed SCA in-
volves R,β-unsaturated aldehydes;8f three cases are illustrated in
Scheme 4. β-Silyl aldehydes are formed in similarly high levels of
enantiomeric purity as observed with acyclic ketones. What renders
the transformations in Scheme 4 intriguing is that unsaturated
aldehydes have been utilized extensively in NHC-catalyzed coupling
reactions, which proceed via Breslow-type intermediates,16 generated
by the addition of the carbene to the relatively uncongested carbonyl.
That the SCA products, represented by 27À29, can be isolated in
66À85% yields implies that the silylboron reagent competes effec-
tively with the aldehyde site for reaction with the nucleophilic catalyst.
Initial studies indicate that NHC-catalyzed silyl 1,2-additions (vs 1,4-)
to aldehydes are feasible;17 thus, the transformations shown in
Scheme 4 illustrate notable chemoselectivity in favor of conjugate
addition. It is also worthy of note that the NHC-catalyzed SCAs with
unsaturated aldehydes require higher catalyst loadings compared to
the corresponding methyl ketones (e.g., reaction of 17 vs 26); the
Scheme 4. NHC-Catalyzed Enantioselective Silyl Conjugate
Addition (SCA) Reactions with r,β-Unsaturated Aldehydesa
a All reactions performed under identical conditions as those shown for
synthesis of 27.
reason for such differences in reactivity is a topic that is being probed
as part of our mechanistic explorations.
The metal-free protocol offers a complementary catalytic
approach to the SCA promoted by chiral metal-based complexes,
where the more costly Pd or Rh salts might be needed.8aÀe With
the chiral NHCÀCu-catalyzed system,4a lower catalyst loadings
are sufficient (1.0À2.0 mol % vs 5.0À12.5 mol %), at times higher
enantioselectivities are observed with acyclic substrates, and elec-
tron-deficient β-silylketone 21 (Scheme 3) can be accessed. On the
other hand, the NHC-catalyzed transformations are more attractive
as there is no need for the use of an air- and moisture-sensitive
Cu salt (CuCl), and dbu can be employed in lieu of the more
sensitive metal alkoxides. The above factors and the availability of
the requisite chiral imidazolinium salt, which can be prepared by
a three-step operation, together with the demonstrated utility
of β-silyl carbonyls,3,18 render the present strategy of value.
The enantioselectivities observed indicate that an NHC
catalyst is intimately involved in the formation of the CÀSi bond
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(cf. Scheme 1) and that the role of the heterocyclic carbene is not
limited to activation of the silylboron reagent. Design and
development of catalytic reactions that involve NHC-activation
of other B-based reagents may thus be feasible; investigations
along these lines are in progress. Moreover, study of the mech-
anism of the metal-free transformations, including identification
of the origins of enantioselectivity, the reactivity differences
among various substrate classes, and the critical role of water,
is underway.
146, 87. (d) Ito, H.; Ishizuka, T.; Tateiwa, J.-i.; Sonoda, M.; Hosomi, A.
J. Am. Chem. Soc. 1998, 120, 11196. (e) Ogoshi, S.; Tomiyasu, S.; Morita,
M.; Kurosawa, H. J. Am. Chem. Soc. 2002, 124, 11598. (f) Clark, C. T.;
Lake, J. F.; Scheidt, K. A. J. Am. Chem. Soc. 2004, 126, 84.
(8) For catalytic enantioselective silyl conjugate addition methods
with metal-containing catalysts, see: (a) Hayashi, T.; Matsumoto, Y.; Ito,
Y. J. Am. Chem. Soc. 1988, 110, 5579. (b) Matsumoto, Y.; Hayashi, T.;
Ito, Y. Tetrahedron 1994, 50, 335. (c) Walter, C.; Auer, G.; Oestreich, M.
Angew. Chem., Int. Ed. 2006, 45, 5675. (d) Walter, C.; Oestreich, M.
Angew. Chem., Int. Ed. 2008, 47, 3818. (e) Walter, C.; Fr€ohlich, R.;
Oestreich, M. Tetrahedron 2009, 65, 5513.(f) Ibrahem, I.; Santoro, S.;
Himo, F.; Cꢀordova, A. Adv. Synth. Catal. 2011, 353, 245. The last four
studies involve the use of 1 as a reagent.
’ ASSOCIATED CONTENT
S
(9) See the Supporting Information for details.
Supporting Information. Experimental procedures and
b
(10) For an in-depth analysis of the concept of Lewis base activation
and its use in chemical synthesis, see: Denmark, S. E.; Beutner, G. L.
Angew. Chem., Int. Ed. 2008, 47, 1560.
spectral, analytical data for all reaction products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) For example, see: (a) Van Veldhuizen, J. J.; Campbell, J. E.;
Giudici, R. E.; Hoveyda, A. H. J. Am. Chem. Soc. 2005, 127, 6877.
(b) Gillingham, D. G.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2007,
46, 3860. (c) Brown, M. K.; Hoveyda, A. H. J. Am. Chem. Soc. 2008,
130, 12904. (d) Lee, Y.; Hoveyda, A. H. J. Am. Chem. Soc. 2009,
131, 3160. (e) O’Brien, J. M.; Lee, K-s.; Hoveyda, A. H. J. Am. Chem.
Soc. 2010, 132, 10630. (f) Dabrowski, J. A.; Gao, F.; Hoveyda, A. H.
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(12) Control experiments indicate that the more facile β-silylketone
formation is not due to involvement of the corresponding silyl hydride
that might be generated through reaction of 1 with water.
(13) Subsequent optimization studies indicated that use of 3:1
H2O/thf, versus an equal mixture, as used in the studies summarized
in Table 1, affords improved efficiency and enantioselectivity. For
example, reaction in 100% water results in 96% conversion but affords
4 in 75:25 er in the presence of the NHC catalyst derived from 9b.
Solutions appear as emulsions while stirring continues but become
biphasic while standing.
(14) Initial mechanistic studies indicate that indeed NHCs undergo
decomposition under the reaction conditions and the resulting products
promote nonselective conjugate additions albeit at a slower rate (vs
NHC-catalyzed). Details of these investigations will be disclosed in the
full account of this work.
(15) The reason for the higher catalyst loadings might partly be
because, with slower SCA reactions, decomposition of the imidazolinium
salt/NHC through reaction with water becomes more competitive. Initial
mechanistic studies indicate that the derived products promote reactions
with little or no enantioselectivity. Details will be disclosed in the full
account of this work. For detailed studies on hydrolysis of imidazolium
salts, see: Hollꢀoczki, O.; Terleczky, P.; Szieberth, D.; Mourgas, G.; Gudat,
D.; Nyulꢀaszi, L. J. Am. Chem. Soc. 2011, 133, 780.
’ AUTHOR INFORMATION
Corresponding Author
amir.hoveyda@bc.edu
’ ACKNOWLEDGMENT
Financial support was provided by the NIH (GM-57212) and
the NSF (CHE-0715138). We are grateful to Maitane Fernandez
for valuable experimental assistance and to Dr. Kang-sang Lee,
Hao Wu, Dr. Adil R. Zhugralin, and Dr. Simon J. Meek for helpful
discussions. Mass spectrometry facilities at Boston College are
supported by the NSF (DBI-0619576).
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