Application of a new chiral phosphepine to the catalytic asymmetric synthesis of highly functionalized cyclopentenes that bear an array of heteroatom-substituted quaternary stereocenters

Article abstract of DOI:10.1021/ja2049012

Through the design and synthesis of a new chiral phosphepine, the first catalytic asymmetric method for the [3 + 2] cycloaddition of allenes with olefins has been developed that generates cyclopentenes that bear nitrogen-, phosphorus-, oxygen-, and sulfur

Full text of DOI:10.1021/ja2049012

ARTICLE
pubs.acs.org/JACS
Application of a New Chiral Phosphepine to the Catalytic Asymmetric
Synthesis of Highly Functionalized Cyclopentenes That Bear an Array
of Heteroatom-Substituted Quaternary Stereocenters
Yuji Fujiwara and Gregory C. Fu*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S Supporting Information
b
ABSTRACT: Through the design and synthesis of a new chiral phosphepine,
the first catalytic asymmetric method for the [3 + 2] cycloaddition of allenes with
olefins has been developed that generates cyclopentenes that bear nitrogen-,
phosphorus-, oxygen-, and sulfur-substituted quaternary stereocenters. A wide
array of racemic γ-substituted allenes can be employed in this stereoconvergent
process, which occurs with good enantioselectivity, diastereoselectivity, regio-
selectivity, and yield. Mechanistic studies, including a unique observation of a
(modest) kinetic resolution of a racemic allene, are consistent with addition of the phosphepine to the allene being the turnover-
limiting step of the catalytic cycle.
’ INTRODUCTION
substantially enhance the utility of these cyclopentene-forming
reactions. In this report, we establish that this objective can be
achieved with the aid of a new chiral phosphine catalyst (1)
(eq 2).
Interest in the use of tertiary phosphines as nucleophilic catalysts,1
particularly for enantioselective processes,2 has increased sub-
stantially in recent years. One especially powerful method,
initially reported by Lu, is a formal [3 + 2] cycloaddition reaction
between electron-poor allenes (or alkynes) and olefins,3 which
generates cyclopentenes that are useful as end points or as
intermediates for further functionalization.4 Zhang described
the first asymmetric variant of this process catalyzed by a chiral
phosphine,5 and we6 and others7 have subsequently expanded
the scope of this valuable transformation (eq 1).
’ RESULTS AND DISCUSSION
The most well-studied examples of phosphine-catalyzed [3 + 2]
cycloadditions of electron-poor allenes/alkynes with heteroatom-
substituted olefins involve nitrogen substituents.8 Consequently,
as the starting point for our effort to develop the first such catalytic
enantioselective annulations to generate heteroatom-bearing
quaternary stereocenters, we chose to examine the reaction of
a 5-methylene hydantoin8c,e (Table 1; product C can be trans-
formed into a bioactive analogue of hydantocidin8e).
We surveyed a range of phosphines that have been useful
in asymmetric catalysis,12 including nucleophilic catalysis. Several
bidentate phosphines, such as DIPAMP,7g Ph-BPE,13 and
TangPhos,14 were ineffective, affording little or no ee (Table 1,
The presence of a heteroatom, rather than a hydrogen or a
carbon, substituent on the olefin (A in eq 1) could be expected to
have a significant impact on both reactivity and stereoselectivity
in these [3 + 2] cycloadditions. Indeed, to the best of our knowledge,
there have been few reports of the use of heteroatom-substituted
olefins as partners in Lu annulations,8,9 and there have been
no descriptions of catalytic asymmetric processes that generate
heteroatom-substituted quaternary stereocenters.10 Nevertheless,
the ability to accomplish bond constructions of this type with
good enantioselectivity11 through the use of such olefins would
Received: May 27, 2011
Published: July 18, 2011
r
2011 American Chemical Society
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Table 1. Enantioselective [3 + 2] Cycloadditions with a
Heteroatom-Substituted Olefin: Effect of the Choice of
Phosphine Catalysta
entry
chiral catalyst
ee (%)b
C:D
yield (%)c
1
2
3
4
5
6
7
(S,S)-DIPAMP
(S,S)-Ph-BPE
(S,S0,R,R0)-TangPhos
(S)-2
<2
<2
1.0:1
1.0:1
0.8:1
0.8:1
1.1:1
1.2:1
1.3:1
59
66
51
40
35
88
92
Figure 1. X-ray crystal structure of phosphepine (R)-1 (for simplicity, a
solvent molecule (CH2Cl2) has been omitted).
11
ꢀ51
59
(S)-3
Table 2. Catalytic Enantioselective [3 + 2] Cycloadditions of
(S)-4
76
γ-Substituted Allenes with a Nitrogen-Substituted Olefina
(S)-1
97
a All data are the average of two experiments. b Enantiomeric excess of C.
A negative value signifies that the R enantiomer was formed predomi-
nantly. c The yield was determined by HPLC analysis with the aid of an
internal standard.
entries 1ꢀ3), whereas phosphepines 2ꢀ415 provided somewhat
promising enantioselectivity and yield (entries 4ꢀ6).
The addition of substituents to the 3,30 positions of the 1,10-
binaphthyl framework has proved to be a valuable approach to
enhancing the enantioselectivity afforded by an array of chiral
catalysts.16 However, this strategy has not yet been exploited
in the context of asymmetric nucleophilic catalysis by chiral
phosphepines. We were therefore pleased to determine that
replacement of the 3,30 hydrogens of phosphepine 4 with phenyl
groups leads to improved ee (Table 1, entry 7 vs entry 6).17,18
An examination of the X-ray crystal structure of this new phos-
phepine catalyst reveals how the 3,30 phenyl substituents extend
the chiral environment of the binaphthyl unit in the direction of
the nucleophilic phosphorus (Figure 1).
a All data are the average of two experiments. For the determination of
structure, including stereochemistry, see the Supporting Information.
b Ratio of regioisomers (determined by1H NMR analysis of the unpur-
c
ified reaction mixture); the ratio of diastereomers is g20:1. Yield of
purified product (rr = 9 to >20:1). d Catalyst loading: 10%.
(two stereocenters rather than one) diversity of the products
generated through this powerful cycloaddition process.
We have established that the conditions described in entry 7 of
Table 1 can in fact be applied directly to [3 + 2] annulations of a
wide array of γ-substituted racemic allenes with a 5-methylene
hydantoin, furnishing the desired highly functionalized cyclo-
pentenes, which bear adjacent quaternary and tertiary stereo-
centers, in good stereoselectivity, regioselectivity, and yield
(Table 2).21 With γ substituents that range in steric demand
from Me to CH2cyclopentyl, the cycloadditions proceed cleanly
with excellent enantioselectivity in the presence of 5% of new
phosphepine 1 (entries 1ꢀ3); in the case of a bulky i-Pr group, an
increased catalyst loading (10%) is required in order to obtain a
All studies to date of nucleophile-catalyzed enantioselective
cycloadditions of allenes with olefins have focused primarily on
the use of allenes that cannot undergo isomerization to 1,3-
dienes (i.e., R = H in eq 1; the 1,3-dienes are unreactive toward
cyclopentene formation).19,20 On the other hand, the ability
to employ γ-substituted allenes as reaction partners would
markedly increase the structural as well as the stereochemical
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Table 3. Catalytic Enantioselective [3 + 2] Cycloadditions of
Table 5. Catalytic Enantioselective [3 + 2] Cycloadditions of
γ-Substituted Allenes with a Phthalimide-Substituted Olefina
γ-Substituted Allenes with an Oxygen-Substituted Olefina
a All data are the average of two experiments. For the determination of
structure, including stereochemistry, see the Supporting Information.
b Determined by 1H NMR analysis of the unpurified reaction mixture;
for each cycloaddition, the ratio of regioisomers is g20:1. c Yield of
purified product (dr g12: 1).
a All data are the average of two experiments. For the determination of
structure, including stereochemistry, see the Supporting Information.
b Yield of purified product; for each cycloaddition, the ratio of regio-
isomers and diastereomers is g20:1 (determined by1H NMR analysis of
the unpurified reaction mixture).
Indeed, we have established that not only 5-methylene hydantoin
but also other pnictogen-bearing olefins are suitable cycloaddi-
tion partners in the presence of phosphepine catalyst 1 (Tables 3
and 4).22 Thus, an array of annulations of racemic γ-substituted
allenes with nitrogen- and phosphorus-substituted olefins pro-
ceed in excellent ee, regioselectivity, and diastereoselectivity
(>20:1). We are not aware of a prior example of the use of a
phosphorus-substituted alkene in such [3 + 2] cycloadditions
(including non-asymmetric processes).
Table 4. Catalytic Enantioselective [3 + 2] Cycloadditions of
γ-Substituted Allenes with a Phosphorus-Substituted Olefina
We have explored the possibility that the olefin coupling
partner can bear other families of heteroatoms, e.g., chalcogens.
In fact, C2-symmetric phosphepine 1 catalyzes [3 + 2] cyclo-
additions of both oxygen- and sulfur-substituted olefins with
γ-substituted allenes to generate highly functionalized cyclo-
pentenes with good-to-excellent stereoselectivity, regioselectivity
(g20:1), and yield (Tables 5 and 6). To the best of our knowledge,
oxygen-substituted alkenes have not previously been employed
in annulations of this type.
These catalytic asymmetric cycloadditions of heteroatom-
substituted olefins are not limited to couplings with allenoates.
For example, allenamides are also suitable partners (eq 3).23 As
far as we are aware, allenamides have not been reported by others
to serve as substrates in phosphine-catalyzed [3 + 2] reactions
with olefins.
a All data are the average of two experiments. For the determination of
structure, including stereochemistry, see the Supporting Information.
b Yield of purified product; for each cycloaddition, the ratio of regio-
isomers and diastereomers is g20:1 (determined by 1H NMR analysis of
the unpurified reaction mixture).
high yield (92%), and the cyclopentene is formed with moderate
ee (77%) but excellent regioselectivity (50:1; entry 4). Func-
tionalized γ substituents are compatible with the reaction con-
ditions, including a phthalimide-containing group (entries 5ꢀ7).
These are stereoconvergent processes wherein both enantiomers
of the racemic allene are being converted by the chiral catalyst
into the same stereoisomer of the product with good selectivity.
With the first method in hand for catalytic enantioselective
[3 + 2] cycloadditions of allenes with a heteroatom-substituted
olefin to produce quaternary stereocenters, we turned our attention
to determining the scope of this process with respect to other
substituents, with the goal of providing a versatile approach.
The products of these catalytic asymmetric [3 + 2] cycloaddi-
tions can be converted into other potentially useful compounds.
For example, the phthalimide can be deprotected to reveal a quaternary
amino-acid ester (eq 4),24 the thioether can be unmasked to a
tertiary thiol (eq 5), and the double bond of the cyclopentene can
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Table 6. Catalytic Enantioselective [3 + 2] Cycloadditions of
at partial conversion, we observe modest enantiomeric enrich-
ment of the unreacted allene (30% ee at 93% conversion); to the
best of our knowledge, this is the first example of a kinetic
resolution in a phosphine-catalyzed allene/olefin [3 + 2] cyclo-
addition. Finally, we do not detect a nonlinear effect: the ee of the
product correlates linearly with the ee of the catalyst. All of these
data can be reconciled with a mechanism in which the rate-
determining step of the catalytic cycle is the addition of the
phosphine to the allene.
γ-Substituted Allenes with a Sulfur-Substituted Olefina
’ CONCLUSIONS
Although there have been numerous reports of phosphine-
catalyzed [3 + 2] cycloadditions of allenes with olefins, a variety
of noteworthy challenges had not been addressed. For example,
with respect to non-asymmetric reactions, no processes had been
described that employ phosphorus- or oxygen-substituted olefins
to provide heteroatom-substituted quaternary centers; with
regard to enantioselective transformations, there had been no
success with any heteroatom-bearing olefin to produce a quaternary
carbon, and there had been only limited progress (R = Me in
eq 1) with γ-substituted allenes. By synthesizing and applying a
new chiral phosphepine (1), we have developed a versatile
method that furnishes cyclopentenes with an array of hetero-
atom-substituted (N, P, O, and S) quaternary stereocenters
in good ee, diastereoselectivity, regioselectivity, and yield. We
have used catalyst 1 in stereoconvergent reactions of racemic
γ-substituted allenes, which afford products with greater struc-
tural and stereochemical diversity as compared with unsubsti-
tuted allenes. Furthermore, for the first time, we have employed
allenamides in phosphine-catalyzed [3 + 2] cyclodditions. The
highly functionalized cyclopentenes that are generated in these
catalytic asymmetric annulations are poised for transformation
into a variety of other target molecules. Mechanistic studies,
including a unique observation of a (modest) kinetic resolution
of the racemic allene, are consistent with addition of phosphepine 1
to the allene being the turnover-limiting step of the catalytic cycle.
This is the first report in which modification of the 3,30 positions of
the binaphthyl unit of a phosphepine has been exploited in the
context of asymmetric nucleophilic catalysis, and we anticipate that
an array of other processes may benefit from this approach.
a All data are the average of two experiments. The absolute stereochem-
istry is tentatively assigned by analogy with Table 5. b Determined by 1H
NMR analysis of the unpurified reaction mixture; for each cycloaddition,
the ratio of regioisomers is g20:1. c Yield of purified product (dr g 8: 1).
be functionalized with high diastereoselectivity to afford a cyclo-
pentane that bears four contiguous stereocenters (two tertiary
and two quaternary; eq 6).
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
compound characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Detailed mechanistic experiments have not yet been described
for catalytic asymmetric [3 + 2] cycloadditions of allenes with
olefins.25 For the process catalyzed by phosphepine 1, specifi-
cally, the reaction illustrated in eq 7, the rate is dependent upon
the concentration of the allene and the catalyst, but not the olefin.
During the cycloaddition, the predominant phosphorus-containing
species (>90% according to 31P NMR spectroscopy) is the phos-
phepine itself, not a covalent adduct (e.g., B in eq 1). Furthermore,
Corresponding Author
gcf@mit.edu
’ ACKNOWLEDGMENT
Support has been provided by the National Institutes of
Health (National Institute of General Medical Sciences, Grant
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R01-GM57034) and Dainippon Sumitomo Pharma Co., Ltd.
(fellowship for Y.F.). We thank Dr. Jonathan E. Wilson for a
preliminary study.
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(21) Notes: (a) Under our standard reaction conditions: if the
temperature of the reaction is lowered, there is a substantial decrease
in rate and only a small improvement in ee; the cycloaddition proceeds
more slowly in the presence of protic additives such as pivalic acid or
phenol; on a gram scale, the reaction illustrated in entry 3 of Table 2
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