Stereospecific Rhodium-Catalyzed Allylic Substitution with Alkenyl Cyanohydrin Pronucleophiles: Construction of Acyclic Quaternary Substituted α,β-Unsaturated Ketones

Article abstract of DOI:10.1021/jacs.5b10270

A highly regio- and stereospecific rhodium-catalyzed allylic alkylation of tertiary allylic carbonates with alkenyl cyanohydrin pronucleophiles is described. This protocol offers a fundamentally novel approach toward the synthesis of acyclic quaternary-substituted α,β-unsaturated ketones and thereby provides a new cross-coupling strategy for target directed synthesis. A particularly attractive feature with this process is the ability to directly couple di-, tri- and tetrasubstituted alkenyl cyanohydrin pronucleophiles to prepare the corresponding α,β-unsaturated ketone derivatives in a highly selective manner. Additionally, the chemoselective 1,4-reduction of the enone products provides rapid access to acyclic enantiomerically enriched α,α′-dialkyl-substituted ketones, which are challenging motifs to prepare using conventional enolate alkylation.

Full text of DOI:10.1021/jacs.5b10270

Communication
pubs.acs.org/JACS
Stereospecific Rhodium-Catalyzed Allylic Substitution with Alkenyl
Cyanohydrin Pronucleophiles: Construction of Acyclic Quaternary
Substituted α,β-Unsaturated Ketones
Ben W. H. Turnbull,† Samuel Oliver,‡ and P. Andrew Evans*,†
†Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada
‡Department of Chemistry, The University of Liverpool, Crown Street, Liverpool, L69 7ZD, United Kingdom
S
* Supporting Information
Scheme 1. Factors Affecting the Development of the
Rhodium-Catalyzed Allylic Alkylation with Alkenyl
Cyanohydrin Pronucleophiles
ABSTRACT: A highly regio- and stereospecific rhodium-
catalyzed allylic alkylation of tertiary allylic carbonates with
alkenyl cyanohydrin pronucleophiles is described. This
protocol offers a fundamentally novel approach toward the
synthesis of acyclic quaternary-substituted α,β-unsaturated
ketones and thereby provides a new cross-coupling
strategy for target directed synthesis. A particularly
attractive feature with this process is the ability to directly
couple di-, tri, and tetrasubstituted alkenyl cyanohydrin
pronucleophiles to prepare the corresponding α,β-
unsaturated ketone derivatives in a highly selective
manner. Additionally, the chemoselective 1,4-reduction
of the enone products provides rapid access to acyclic
enantiomerically enriched α,α′-dialkyl-substituted ketones,
which are challenging motifs to prepare using conventional
enolate alkylation.
he asymmetric construction of α-quaternary-substituted
Tcarbonyl compounds represents a vibrant area of
investigation in organic synthesis due to the ubiquity of this
motif in bioactive natural products, pharmaceuticals and various
synthetic intermediates.1,2 Although the venerable enolate
alkylation remains an important carbon−carbon bond forming
reaction, problems associated with regioselective enolate
formation with unsymmetrical ketones, polyalkylation and the
necessity to employ simple electrophiles have significantly
hampered the development of asymmetric variants.3,4 For
instance, while the catalytic asymmetric alkylation,5 arylation6,7
and vinylation8 of prochiral enolates have been described, they
are generally optimal with cyclic substrates that circumvent the
need to control the enolate geometry. Furthermore, while the
enantioselective alkylation9 of a mixture of acyclic enolate
stereoisomers using a chiral chromium salen catalyst provides
access to acyclic α-quaternary-substituted methyl ketones, this
process is primarily restricted to simple electrophiles. Con-
sequently, we anticipated the ability to directly utilize an acyl
nucleophile would circumvent the formation of an enolate and
thereby offer unparalleled scope and versatility, particularly with
respect to the electrophile.10−14
protected aryl cyanohydrin, which is readily prepared from the
corresponding aldehyde,18 provides a “masked” acyl anion
equivalent that undergoes in situ deprotection with fluoride ion
to furnish the corresponding quaternary-substituted acyclic aryl
ketones via a classical double-inversion process. Although this
work delineated important proof-of-principle, the aromatic
ketone products are of relatively limited synthetic utility.
Hence, we envisaged that the ability to utilize alkenyl
cyanohydrin pronucleophiles would significantly expand the
scope of this novel protocol and thereby access α,β-unsaturated
ketones bearing an α-quaternary stereogenic center (Scheme
1B). The products produced from this process would be difficult
to obtain via conventional enolate alkylation and are of particular
importance as synthetic intermediates due to the presence of two
functional groups that exhibit bimodal reactivity. A key feature of
this strategy is the ability to control the reactivity of the lithiated
alkenyl cyanohydrin, which can either undergo the desired
alkylation at the α-position to function as an acyl anion
To this end, we recently described the highly regio- and
stereospecific rhodium-catalyzed allylic substitution reaction of
chiral nonracemic tertiary allylic alcohol derivatives with an acyl
anion equivalent (Scheme 1A).15−17 The tert-butyldimethylsilyl-
Received: September 30, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b10270
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
equivalent or react through the γ-position as a homoenolate
equivalent (Scheme 1C). Herein, we now describe the first highly
regio- and stereospecific rhodium-catalyzed allylic substitution of
tertiary allylic carbonates with alkenyl cyanohydrin pronucleo-
philes to facilitate the construction of acyclic α-quaternary-
substituted α,β-unsaturated ketones.
Table 2 summarizes the application of the optimized reaction
conditions (Table 1, entry 5) to a number of alkyl-substituted
Table 2. Scope of the Rhodium-Catalyzed Allylic Substitution
a b c
, ,
with Alkyl-Substituted Alkenyl Cyanohydrins (Method B)
Preliminary studies focused on the γ-alkyl-substituted alkenyl
cyanohydrin 2a (X = Ph(CH2)2), since previous studies
demonstrated that the anion of the unsubstituted derivative (X
= H) is unstable under the reaction conditions.17a Treatment of
the tertiary allylic carbonate 1a with the lithium salt of 2a, in the
presence of triphenyl phosphite-modified [RhCl(COD)]2 at
−10 °C for ca. 16 h, followed by treatment with tetra-n-
butylammonium fluoride (TBAF) at −40 °C19 furnished the
quaternary-substituted α,β-unsaturated ketone 3a in 66% yield
and with excellent regioselectivity (Table 1, entry 1). Gratify-
Table 1. Optimization of the Rhodium-Catalyzed Allylic
Substitution using Alkenyl Cyanohydrin 2 (R = Ph(CH2)2)
ab
,
c
d
entry
phosphite L =
Nu method 3:4 (b/l) yield of 3 (%)
1
2
3
4
5
6
7
P(OPh)3
P(OMe)3
2a
2a
2a
2a
2a
2b
2b
A
A
A
A
B
A
B
≥19:1
≥19:1
≥19:1
≥19:1
≥19:1
≥19:1
≥19:1
66
67
67
68
83
75
75
P(OCH2CF3)3
P(O-2,4-ditBuC6H3)3
P(OPh)3
P(OPh)3
P(OPh)3
a
All reactions were performed on a 0.5 mmol reaction scale using 2.5
mol % [Rh(COD)Cl]2 and 10 mol % ligand (L) in 5 mL THF at −10
b
°C for ca. 16 h, followed by the addition of TBAF at −40 °C. Method
a
All reactions were performed on a 0.5 mmol reaction scale using 2.5
mol % [Rh(COD)Cl]2, 10 mol % P(OPh)3, 2.0 equiv 2, and 2.8 equiv
LiHMDS in THF (5 mL) at −10 °C for ca. 16 h, followed by the
A: 1.3 equiv 2, 1.8 equiv LiHMDS, and 4.0 equiv TBAF were
employed; Method B: 2.0 equiv 2, 2.8 equiv LiHMDS, and 6.0 equiv
c
TBAF were employed. Regioselectivity was determined by 500 MHz
b
1H NMR analysis of the crude reaction mixtures prior to deprotection
of the cyanohydrin adducts, which are obtained as a 1:1 mixture of
addition of 6.0 equiv TBAF at −40 °C. Regioselectivity was
determined by 500 MHz H NMR analysis of the isolated ketones
3. Isolated yields. 79% yield (b/l ≥ 19:1, E/Z ≥ 19:1) using (Z)-2a
(E/Z = 1:7).
1
c
d
d
diastereomers. Isolated yields of the desired regioisomer 3.
ingly, there was no evidence for nucleophilic attack at the γ-
position of the alkenyl cyanohydrin 2a. Although the stereo-
electronics of the phosphite ligand generally play a critical role in
the efficiency and selectivity of the rhodium-catalyzed allylic
substitution reaction, screening various phosphites provided only
modest improvement in the yield (entries 2−4). Hence, we
elected to continue our studies using the readily available
triphenyl phosphite as the optimum ligand. Additional studies
demonstrated that increasing the stoichiometry of the
cyanohydrin pronucleophile provides the desired acyclic ketone
3a in excellent yield and with ≥19:1 regioselectivity (entry 5).
Interestingly, the aryl-substituted alkenyl cyanohydrin 2b (X =
Ph) afforded the desired ketone 3b in good yield and excellent
regioselectivity regardless of stoichiometry (entry 6 vs entry 7).
The difference in reaction stoichiometry was ascribed to the
greater kinetic acidity of the α-proton in 2b, which is supported
by deuterium quenching experiments that demonstrate a higher
concentration of the reactive nucleophile.15
alkenyl cyanohydrins and tertiary allylic carbonates. In the first
instance, the study focused on the pronucleophile component.
To this end, the reaction is tolerant of 1,2-disubstituted alkenyl
cyanohydrins bearing both long and short alkyl chains,
generating the α,β-unsaturated ketones in excellent yields and
with exceptional regioselectivity (entries 1−2). Furthermore, the
tert-butyl derivative and a cyanohydrin containing a potentially
reactive 1,3-diene proceed with excellent selectivity (entries 3−
4). Additionally, trisubstituted alkenyl cyanohydrins bearing a
gem-dimethyl or cyclohexene motif also provide competent
substrates (entries 5−6). Nevertheless, a particularly attractive
feature of the current work is the ability to employ differentially
tri- and tetra-substituted alkenyl cyanohydrins, which furnish the
α-quaternary-substituted ketones with complete retention of
geometrical selectivity in the double bond (entries 7−8).20
Remarkably, the cis-1,2-disubstituted cyanohydrin (Z)-2a (X/Z
= H, Y = Ph(CH2)2) undergoes complete isomerization under
B
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Journal of the American Chemical Society
Communication
the reaction conditions to furnish the trans-substituted enone 3a
in comparable yield. This unexpected observation most likely
arises from olefin equilibration triggered by the relief of A1,3-
strain between the phenethyl substituent and the large tert-
butyldimethylsilyl group.21 It is interesting to note that despite
this isomerization, alkylation at the γ-position is not evident.
Nevertheless, the scope with respect to the tertiary allylic
carbonate provides significant flexibility with this approach. For
instance, while both short and long chain alkyl substituents are
well tolerated (entries 9−10), branched and functionalized side-
chains provide the acyclic ketones in good yields and excellent
regioselectivity (entries 11−15). This is particularly noteworthy
due to the problems associated with the introduction of these
groups via conventional alkylation reactions, i.e., isopropyl and
hydroxyethyl groups.
electron-deficient olefin, which would be difficult to access via
conventional enolate alkylation reactions.
In order to further highlight the scope of this transformation,
we elected to examine the stereospecific rhodium-catalyzed
allylic alkylation of the chiral nonracemic tertiary allylic alcohols
5a−c, readily prepared following the three-step protocol
described by Aggarwal et al. (Scheme 2).22 In accord with our
Scheme 2. Stereospecific Allylic Substitution of Chiral
Nonracemic Allylic Alcohols 5a−c
Table 3 outlines the examination of a range of aryl-substituted
alkenyl cyanohydrins to further illustrate the scope of the
Table 3. Scope of the Rhodium-Catalyzed Allylic Substitution
with Aryl-Substituted Alkenyl Cyanohydrins (Method A) (R =
Ph(CH2)2)
previous studies, the use of the electron-poor tris(2,2,2-
trifluoroethyl)-phosphite ligand proved optimal, affording the
acyclic ketones 6a−c with excellent levels of regiocontrol (b/l ≥
19:1) and conservation of enantiomeric excess (≥92% cee).23
Additionally, the preparation of a compound of known absolute
configuration provided confirmation that the alkylation proceeds
with overall retention, which illustrates the cyanohydrin is
behaving as a soft nucleophile in this process.24
a b c
, ,
The asymmetric synthesis of α,α′-dialkyl acyclic ketones is
particularly challenging for conventional enolate alkylation
reactions, due to the problems associated with regioselective
enolate formation from an unsymmetrical ketone. Hence, we
envisaged an alternate approach to these compounds outlined in
Scheme 3, which combines the stereospecific allylic alkylation
Scheme 3. Asymmetric Synthesis of α,α′-Dialkyl Ketone 9 via
Stereospecific Rhodium-Catalyzed Allylic Substitution/
Chemoselective 1,4-Reduction (X = CH2CH2OBn)
a
All reactions were performed on a 0.5 mmol reaction scale using 2.5
mol % [Rh(COD)Cl]2, 10 mol % P(OPh)3, 1.3 equiv 2, and 1.8 equiv
LiHMDS in THF (5 mL) at −10 °C for ca. 16 h, followed by the
b
addition of 4.0 equiv TBAF at −40 °C. Regioselectivity was
1
determined by 500 MHz H NMR analysis of the isolated ketones
c
d
3. Isolated yields. 73% yield (b/l ≥ 19:1) on a 10 mmol (2.3 g) scale
using 0.5 mol % [Rh(COD)Cl]2 and 2 mol % P(OPh)3.
reaction using slightly modified reaction conditions (Table 1,
entry 6). In addition to the unsubstituted phenyl substituent
(entry 1), the reaction is tolerant of a number of electron-
deficient and electron-rich aryl groups (entries 2−5). Gratify-
ingly, a number of alkenyl cyanohydrins bearing heterocyclic
substituents also undergo the alkylation in good yield and with
comparable selectivity for the branched regioisomer (entries 6−
9). Finally, the ability to conduct the reaction on gram-scale using
1 mol % of the rhodium catalyst demonstrates the viability of this
methodology for target-directed synthesis. Overall, this work
provides access to an array of acyclic quaternary-substituted α,β-
unsaturated ketones bearing both an electron-rich and an
with a chemoselective 1,4-reduction. In this context, the
rhodium-catalyzed allylic alkylation of the linalool-derived allylic
carbonate 7 (99:1 er) furnished the α,β-unsaturated ketone 8 in
good yield and excellent regio- and stereospecificity (b/l ≥ 19:1,
93% cee). Treatment of the acyclic ketone 8 with Stryker’s
reagent25 affords the α,α′-dialkyl acyclic ketone 9 in excellent
yield to further illustrate the potential utility of this approach.
In conclusion, we have developed the regio- and stereospecific
rhodium-catalyzed allylic substitution of chiral nonracemic allylic
carbonates with trialkylsilyl-protected alkenyl cyanohydrins,
C
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Journal of the American Chemical Society
Communication
(b) Mukherjee, S.; List, B. J. Am. Chem. Soc. 2007, 129, 11336. (c) Trost,
B. M.; Xu, J.; Schmidt, T. J. Am. Chem. Soc. 2009, 131, 18343.
(6) For a recent review on the transition metal-catalyzed enolate
arylation reaction, see: Johansson, C. C. C.; Colacot, T. J. Angew. Chem.,
Int. Ed. 2010, 49, 676.
(7) For recent examples of asymmetric transition metal-catalyzed
enolate arylation reactions, see: (a) Åhman, J.; Wolfe, J. P.; Troutman,
M. V.; Palucki, M.; Buchwald, S. L. J. Am. Chem. Soc. 1998, 120, 1918.
(b) Chen, G.; Kwong, F. Y.; Chan, H. O.; Yu, W.-Y.; Chan, A. S. C.
Chem. Commun. 2006, 1413. (c) Liao, X.; Weng, Z.; Hartwig, J. F. J. Am.
Chem. Soc. 2008, 130, 195.
(8) For recent examples of asymmetric transition metal-catalyzed
enolate vinylation reactions, see: (a) Chieffi, A.; Kamikawa, K.; Åhman,
J.; Fox, J. M.; Buchwald, S. L. Org. Lett. 2001, 3, 1897. (b) Huang, J.;
Bunel, E.; Faul, M. M. Org. Lett. 2007, 9, 4343.
(9) Doyle, A. G.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2007, 46, 3701.
(10) For an alternate approach involving asymmetric reductive acyl
cross-coupling, see: Cherney, A. H.; Kadunce, N. T.; Reisman, S. E. J.
Am. Chem. Soc. 2013, 135, 7442.
(11) For an example of the palladium-catalyzed allylic alkylation with a
cyanohydrin pronucleophile, see: Tsuji, J.; Shimizu, I.; Minami, I.;
Ohashi, Y.; Sugiura, T.; Takahashi, K. J. Org. Chem. 1985, 50, 1523.
(12) For related palladium-catalyzed allylic substitution reactions with
acyl silanes and stannanes, see: (a) Obora, Y.; Ogawa, Y.; Imai, Y.;
Kawamura, T.; Tsuji, Y. J. Am. Chem. Soc. 2001, 123, 10489. (b) Obora,
Y.; Nakanishi, M.; Tokunaga, M.; Tsuji, Y. J. Org. Chem. 2002, 67, 5835.
(13) For three component palladium-catalyzed allylic substitution
reactions with carbon monoxide, see: (a) Tamaru, Y.; Yasui, K.;
Takanabe, H.; Tanaka, S.; Fugami, K. Angew. Chem., Int. Ed. Engl. 1992,
31, 645. (b) Sakurai, H.; Tanabe, K.; Narasaka, K. Chem. Lett. 1999, 309.
(14) For related metal-catalyzed allylic substitution reactions with acyl
anion equivalents, see: (a) Trost, B. M.; Dirat, O.; Dudash, J., Jr.;
which permits the construction of acyclic quaternary-substituted
α,β-unsaturated ketones bearing both an electron-rich and
electron-deficient olefin. Additionally, the use of branched and
functionalized alkyl substituents in the allylic alcohol moiety
provides access to products that would be challenging for
conventional enolate alkylation reactions. The ability to
construct di-, tri-, and tetrasubstituted enones, with complete
transposition of E-geometrical selectivity, further demonstrates
the broad scope of this methodology. Finally, the two-step
sequence involving the allylic alkylation and chemoselective 1,4-
reduction to give a α,α′-dialkyl acyclic ketone highlights the
synthetic potential of this process. Hence, given the utility of the
acyclic quaternary-substituted α,β-unsaturated ketone products,
we anticipate that this approach will permit the construction of
this important and challenging motif in target-directed synthetic
applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b10270.
Experimental procedures, spectral data, and copies of
spectra for all new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
*Andrew.Evans@chem.queensu.ca
■
Notes
Hembre, E. J. Angew. Chem., Int. Ed. 2001, 40, 3658. (b) Forster, S.;
̈
The authors declare no competing financial interest.
Tverskoy, O.; Helmchen, G. Synlett 2008, 2008, 2803. (c) Trost, B. M.;
Osipov, M.; Kaib, P. S. J.; Sorum, M. T. Org. Lett. 2011, 13, 3222.
(d) Ahire, M. M.; Mhaske, S. B. Angew. Chem., Int. Ed. 2014, 53, 7038.
(e) Breitler, S.; Carreira, E. M. J. Am. Chem. Soc. 2015, 137, 5296.
(15) Evans, P. A.; Leahy, D. K. In Modern Rhodium-Catalyzed Organic
Reactions; Evans, P. A., Ed.; Wiley-VCH: Weinheim, Germany, 2005;
Ch. 10, p 191−214.
ACKNOWLEDGMENTS
■
We sincerely thank the National Sciences and Engineering
Research Council (NSERC) for a Discovery grant and Queen’s
University for generous financial support. NSERC is also
thanked for supporting a Tier 1 Canada Research Chair
(PAE). We also acknowledge the Royal Society for a Wolfson
Research Merit Award (PAE) and the EPSRC and AstraZeneca
(Alderley Park) for a Ph.D. studentship (S.O.). We also offer our
gratitude to Dr. Paul Kemmitt (AZ) for his support and helpful
discussions and to the EPSRC National Mass Spectrometry
Service Centre (Swansea, U.K.) for high-resolution MS.
(16) For mechanistic studies, see: Evans, P. A.; Nelson, J. D. J. Am.
Chem. Soc. 1998, 120, 5581.
(17) (a) Evans, P. A.; Oliver, S.; Chae, J. J. Am. Chem. Soc. 2012, 134,
19314. (b) Evans, P. A.; Oliver, S. Org. Lett. 2013, 15, 5626.
(18) Kurono, N.; Yamaguchi, M.; Suzuki, K.; Ohkuma, T. J. Org. Chem.
2005, 70, 6530.
(19) The fluoride ion deprotection was conducted at low temperature
to suppress competitive Michael addition of the liberated cyanide to the
α,β-unsaturated ketone.
(20) In all cases the alkylation proceeds with complete retention of the
(E)-cyanohydrin stereochemistry (E/Z ≥ 19:1).
(21) Although the 1,2-disubstituted cyanohydrin (Z)-2a underwent
complete isomerization under the reaction conditions, a mixture of E/Z
isomers (1:1) of the tri- and tetra-substituted alkenyl cyanohydrins 2h/
2i provided the ketones 3h/3i as a mixture of isomers (E/Z = 3:1 and
4:1, respectively), albeit favoring the trans derivative.
(22) (a) Stymeist, J. L.; Bagutski, V.; French, R. M.; Aggarwal, V. K.
Nature 2008, 456, 778. (b) Bagutski, V.; French, R. M.; Aggarwal, V. K.
Angew. Chem., Int. Ed. 2010, 49, 5142.
(23) The term conservation of enantiomeric excess (cee) = (ee of
product/ee of starting material) × 100. Evans, P. A.; Robinson, J. E.;
Nelson, J. D. J. Am. Chem. Soc. 1999, 121, 6761.
(24) The absolute configuration of the major enantiomer in the
alkylation was confirmed by the preparation of the serotonin antagonist
LY426965, see the SI for details.
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DOI: 10.1021/jacs.5b10270
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