Catalytic enantioselective α-acylvinyl anion reactions of silyloxyallenes

Article abstract of DOI:10.1002/anie.200702818

(Chemical Equation Presented) Alternatives with silicon: The enantioselective Lewis acid catalyzed addition of racemic silyloxyallenes to aldehydes is reported. A {(salen)Cr^III} complex efficiently catalyzes the reaction of these α-acylvinyl an

Full text of DOI:10.1002/anie.200702818

Communications
DOI: 10.1002/anie.200702818
Asymmetric Catalysis
Catalytic Enantioselective a-Acylvinyl Anion Reactions of
Silyloxyallenes**
Troy E. Reynolds and Karl A. Scheidt*
The asymmetric and catalytic construction of carbon–carbon
bonds remains a considerable challenge and important goal in
organic chemistry.[1] This aim is especially relevant for the
synthesis of pharmaceutical agents and biologically active
natural products. A significant focus of our research program
is the development of new methods to facilitate nonobvious
access this type of reactivity.[5] Even with recent advances in
this area, intermolecular MBH reactions are typically
restricted to acrylates and unsubstituted vinyl ketones,
because the generation of the necessary enolate involves the
initial conjugate addition of a nucleophilic catalyst. Use of
other a-acylvinyl anion equivalents primarily involves the
trapping of allenolate intermediates accessed in situ, which
limits the control of stereoselection and reactivity.[6,7]
The value of this unusual bond construction and limita-
tions of existing approaches are compelling reasons to
investigate alternatives with potentially much broader utility.
In this vein, we recently reported the addition of silyloxy-
allenes to aldehydes under scandium(III)-catalyzed condi-
tions, giving a wide range of b-substituted a,b-unsaturated
carbinols in excellent yields with control over alkene geom-
etry.[8,9] The synthesis of silyloxyallenes from the correspond-
ing acylsilanes is based on the straightforward Kuwajima–
[2,3]
À
and unconventional C C bond disconnections.
Recently,
we have expanded our interests in these unusual strategies to
include the development of useful a-acylvinyl anion equiv-
alents. These anions are nontraditional nucleophiles that
provide access to highly valuable a,b-unsaturated carbonyl
compounds in a convergent fashion.[4] The Morita–Baylis–
Hillman (MBH, Scheme 1) reaction is the classic method to
Reich
rearrangement
of
hydoxypropargysilanes
(Scheme 2).[10] Herein, we describe the catalytic enantiose-
lective development of this unconventional a-acylvinyl anion
reaction.
Scheme 1.
Scheme 2. Silyloxyallenes from acylsilanes.
[*] T. E. Reynolds, Prof. K. A. Scheidt
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
Fax: (+1)847-467-2184
Given the synthetic potential of silyloxyallenes as a-
acylvinyl anions, we initiated work to develop asymmetric
processes using these unique nucleophiles. Initial studies
using enantioenriched silyloxyallenes as the chiral compo-
nents in combination with achiral Lewis acids were discon-
tinued owing to the lack of efficient transfer of stereochemical
information. However, after an extensive survey of potential
chiral Lewis acids and reaction conditions with racemic
silyloxyallene 1 and 2-chlorobenzaldehyde, we determined
that [(salen)Cr(SbF6)] (2, salen = N,N’-bis(3,5-di-tert-butylsa-
licylidene)-1,2-cyclohexandiamine) was an efficient catalyst
with excellent control over the alkene geometry and the new
stereogenic center (Z:E > 20:1, 94% ee).[11]
E-mail: scheidt@northwestern.edu
[**] Financial support for this work has been provided by the NIH/
NIGMS (GM073072). We thank Abbott Laboratories, Amgen, 3M,
andBoerhinger–Ingelheim for generous research support and
Wacker Chemical Corp., FMCLithium, andBASF for reagent
support. K.A.S. is a fellow of the A. P. Sloan Foundation. Funding for
the NU Analytical Services Laboratory has been furnishedin part by
the NSF (CHE-9871268). T.E.R. is a recipient of a 2007–2008 ACS
Division of Organic Chemistry fellowship sponsoredby Bristol-
Myers Squibb.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
7806
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Chemie
Table 2: Silyloxyallene Scope.[a]
On the basis of these optimized conditions, the scope of
this asymmetric a-acylvinyl anion addition was explored by
combining silyloxyallene 1 in CH2Cl2 at À208C with 10 mol%
2 and various aldehydes (Table 1). Aromatic aldehydes as
Table 1: Aldehyde scope.[a]
Entry
R1
R2
Yield[%] [b]
Z:E[c]
ee [%][d]
1
2
3
4
5
Me
Me
Me
Me
Et
nBu
SiMe3
tBu
(CH2)3OTBS
Ph
98
90
99
92
96
15:1
20:1
20:1
20:1
20:1
80 (12)
90 (13)
90 (14)
87 (15)
88 (16)
[a] 1.5 equiv silyloxyallene, 1 equiv aldehyde. [b] Yield of isolated product
after column chromatography. [c] Determinedby 1H NMR spectroscopy
(500 MHz). [d] Determined by chiral HPLC, Chiralcel OD column.
Entry
R
Yield[%] [b]
Z:E[c]
ee [%][d,e]
1
2
3
4
5
6
7
8
9
Ph
88
84
99
98
79
94
74
95
57
20:1
20:1
20:1
20:1
20:1
20:1
20:1
20:1
20:1
85 (3)
91 (4)
94 (5)
91 (6)
86 (7)[f]
88 (8)
84 (9)
61 (10)
34 (11)
applications in total synthesis; further exploration of addi-
tional silyloxyallene structures are ongoing.
1-naphthyl
2-Cl(C6H4)
2-Br(C6H4)
2-OTBS(C6H4)
4-Cl(C6H4)
4-Me(C6H4)
PhCH2CH2
cyclohexyl
After initially examining the scope of the transformation,
we explored the stereochemical aspects of the a-acylvinyl
anion addition. Since the reaction involves two chiral reagents
(racemic silyloxyallene and optically active catalyst 2), kinetic
resolution of the racemic allene during the reaction is a
distinct possibility. To explore this potential situation, two
equivalents of 1 and a single equivalent of 2-chlorobenzalde-
hyde were combined in the presence of chromium(III) Lewis
acid 2. After full consumption of the aldehyde (as determined
by thin layer chromatography), the reaction mixture was
filtered through a pad of silica and concentrated in vacuo. At
100% conversion to product, the analysis of the remaining
silyloxyallene (as determined by HPLC with a Chiralcel OD
column) indicated that there was no optical enrichment—the
allene starting material remains racemic during the course of
the reaction. From this result, one enantiomer of allene does
not appear to undergo preferential reaction with the chiral
aldehyde–Lewis acid complex. However, subjecting enan-
tioenriched silyloxyallene to CrIII catalyst 2 in CH2Cl2 at
À208C for 12 h results in racemization of the allene.[13] While
we currently cannot make conclusive statements regarding
the reaction rates of addition of each antipode of the
silyloxyallenes, this interesting observation provides future
opportunities to use racemic silyloxyallenes in enantioselec-
tive processes.
[a] 1.5 equiv 1, 1 equiv aldehyde. [b] Yield of isolated product after
column chromatography. [c] Determinedby 1H NMR spectroscopy
(500 MHz). [d] Determined by chiral HPLC, Chiracel OD column.
[e] The absolute configuration of 8 was determined by single crystal X-
ray diffraction and the configuration of the other compounds were
assignedby analogy; see the Supporting Information. [f] tert-butyl
methyl ether usedas solvent; TBS =tert-butyldimethylsilyl.
substrates provide excellent yields and enantioselectivities of
the carbinol products (Table 1, entries 1–7). Aliphatic alde-
hydes are competent electrophiles for the addition reaction,
but the current optimal CrIII Lewis acid affords secondary
alcohols with lower enantioselectivity (Table 1, entries 8 and
9). The reaction with cyclohexanecarboxaldehyde proceeds at
the slowest rate of all substrates examined to date and
provides a poor yield of the desired product.
In addition to the electrophilic component of this
reaction, various silyloxyallenes were also examined for
their nucleophilic abilities in the presence of the chiral
chromium(III) catalyst.[12] The achiral work with Sc(OTf)3
(OTf = O3SCF3) as the Lewis acid had demonstrated that an
advantage of this methodology was the capacity of the
silyloxyallene to incorporate a broad range of substituents
at the b-position of the a,b-unsaturated ketone product.
Gratifyingly, this aspect of the reaction remains true for the
new CrIII-catalyzed transformation. The alkyl-, trimethylsilyl-,
and tert-butyl-substituted silyloxyallenes add to 2-chlorobenz-
aldehyde in very high yields and selectivities (Table 2,
entries 1, 2, and 3). A protected alcohol can also be used,
giving excellent results as well (Table 2, entry 4). The use of 1-
ethyl silyloxyallene (Table 2, entry 5) also works well, with
88% ee, thereby indicating that the n-alkyl substitution in this
position is not problematic. The methyl and ethyl substituents
at the 1-position of these nucleophiles are significant for
Given the limited number of examples of silyloxyallenes
in Lewis acid promoted reactions, we probed their relative
reactivity.[14] For example, reaction of silyloxyallene
1
(1 equiv) and enolsilane 17 (1 equiv) in the presence of one
equivalent of benzaldehyde and 10 mol% 2 affords only 3
after desilylation (Scheme 3).[15] In a second experiment,
10 mol% Sc(OTf)3 as the Lewis acid with the same reactants
provides a 1.5:1 ratio of the aldol product (from addition of
17) and 3. Since related CrIII Lewis acids are known to
catalyze carbonyl ene reactions between enolsilanes and
aldehydes,[11] we currently favor this mechanistic pathway via
18.[16] In support of this contention, we can observe the
sensitive hetero-ene intermediate analogous to 18 from 2-
chlorobenzaldehyde and silyloxyallene 1 using a modified
procedure. These silyloxydienes are instable towards various
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over both the resulting alkene and the secondary alcohol
stereocenter. In the presence of the CrIII catalyst, silyloxy-
allenes undergo selective additions to aldehydes in the
presence of a standard enolsilane. The isolation of silyloxy-
dienes from the reaction before a hydrolysis step supports a
Lewis acid catalyzed carbonyl ene-type mechanism. Lastly,
the products from the reaction can be converted easily into
substituted indanones and chromenes with good transfer of
chirality. The development and applications of new reactions
involving these latent allenolates derived easily from acylsi-
lanes are in progress.[20]
Scheme 3. Competition experiment.
purification techniques, but after rapid chromatography, the
intermediate diene from entry 3of Table 1 can be isolated in
18% yield along with the hydrolyzed product 5 in 69% yield. Experimental Section
Aryl-substituted silyloxyallenes (R1 = Ph) do not afford
products, which also supports an ene-type mechanism.
Lastly, the above comparison of the two Lewis acid experi-
ments indicates that these allenes are similar in reactivity to
enolsilanes in a Mukaiyama reaction[17] but more reactive in a
carbonyl ene pathway.
Our preliminary studies on the synthetic utility of the
products from this reaction have produced two new cycliza-
tion reactions of the 2-substituted aryl compounds. The aryl
bromide addition product (91% ee, 6) can be converted to a
disubstituted indanone in 10 min with 0.5 mol% palladi-
um(II) in DMF with microwave heating. The subsequent
exposure of this diketone to methyl iodide in the presence of
potassium carbonate delivers indanone 19, which possess a
new quaternary center, in 70% ee and greater than 20:1 d.r.
(Scheme 4).[18] In a second approach, the exposure of 2-
2-Chlorobenzaldeyde (26 mL, 0.23mmol) was added to a 2-dram vial
equipped with a magnetic stir bar and 2 (19 mg, 0.023mmol). The vial
was cooled to À208C and silyloxyallene 1 (75 mg, 0.34 mmol) in
CH2Cl2 (250 mL) was added via syringe in one portion. Upon
consumption of aldehyde (24 h) as determined by TLC, the solution
was concentrated in vacuo. The resulting residue was dissolved in
THF (5 mL) and treated with 1m HCl (1 mL). After 30 min, the
solution was diluted with water (10 mL) and ether (20 mL). The
aqueous layer was discarded, and the ether layer was washed with
saturated NaHCO3 solution (10 mL) and brine (10 mL). The resulting
ether layer was dried over anhydrous Na2SO4, filtered, and concen-
trated to provide the unpurified carbinol. The residue was purified by
flash chromatography (15% EtOAc/hexanes) to afford 65 mg (99%)
of 5 as yellow oil. IR (film): n˜ = 3423, 3061, 2920, 1681, 1431, 1354,
1
1196, 1028, 755, 700 cmÀ1; H NMR (500 MHz, CDCl3): d = 7.64 (d,
J = 7.7 Hz, 1H), 7.39–7.27 (m, 6H), 7.20 (m, 2H), 6.79 (s, 1H), 5.88 (d,
J = 5.1 Hz, 1H), 3.54 (d, J = 5.3Hz, 1H), 1.89 ppm (s, 3H); 13C NMR
(125 MHz, CDCl3): d = 208.4, 142.9, 138.3, 135.6, 134.5, 132.7, 129.8,
129.3, 129.0, 128.9, 128.8, 127.4, 72.9, 31.4 ppm; LRMS (electrospray):
Exact mass calcd for C17H15O2Cl [M]+ 286.08. Found [MÀH] 285.4.
[a]D = + 22.2 degcm3 gÀ1 dmÀ1 (CH2Cl2, c = 1.0 gcmÀ3
, ee = 94%).
Enantiomeric ratio was measured by chiral HPLC (Chiralcel OD-
H, 5% IPA/Hexanes, Rt1 = 11.86, Rt2 = 12.41).
Received: June 26, 2007
Revised: July 20, 2007
Published online: September 11, 2007
Keywords: acylvinyl anions · asymmetric catalysis · chromium ·
.
silicon · synthetic methods
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[15] As observed by H NMR spectroscopy (500 MHz).
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