Copper-catalyzed enantioselective intramolecular alkene amination/intermolecular heck-type coupling cascade

Article abstract of DOI:10.1021/ja211272v

Enantioselective copper-catalyzed cyclization of γ- alkenylsulfonamides and a δ-alkenylsulfonamide in the presence of a range of vinyl arenes results in variously functionalized 2-substituted chiral nitrogen heterocycles via a formal alkene C-H functionalization process. Application of this reaction to the concise synthesis of a 5-HT₇ receptor antagonist is demonstrated.

Full text of DOI:10.1021/ja211272v

Communication
pubs.acs.org/JACS
Copper-Catalyzed Enantioselective Intramolecular Alkene
Amination/Intermolecular Heck-Type Coupling Cascade
Timothy W. Liwosz and Sherry R. Chemler*
Department of Chemistry, The State University of New York at Buffalo, Buffalo, New York 14260, United States
S
* Supporting Information
We have previously shown that γ-pentenylsulfonamides
undergo enantioselective Cu-catalyzed aminofunctionalizations,
for example, carboamination,7 aminooxygenation,8 and
diamination.9 These reactions involve carbon radical intermediates
as evidenced by radical trapping experiments and regio-
and stereoselectivity patterns of reactivity.10 The particular
aminofunctionalization observed is a function of substrate
structure and reaction components.11 The reaction disclosed
herein significantly extends our previously reported doubly
intramolecular carboamination methodology7 to an intermo-
lecular C−C bond-forming process by using aryl-substituted
alkenes as the otherwise unfunctionalized π-component.12 The
resulting allyl-functionalized chiral indoline, pyrrolidine, and
tetrahydroisoquinoline products (vide infra) should find
application in drug discovery and organic synthesis.
The intermolecular carboamination reaction was first
investigated with N-tosyl-2-allylaniline 1a using 1,1-diphenyl-
ethylene (DPE) as the aryl alkene coupling partner.13 At the
onset, we were uncertain if intermolecular radical addition to
DPE would out-compete an intramolecular carboamination
process, where the radical adds to the aryl ring of the tosyl
group, generating sultam 3.7a While the reaction promoted
by Cu(2-ethylhexanoate)2, a copper(II) source known to
promote the alkene amination step,11 provided a low yield of
2a (Table 1, entry 1), the catalytic version, using Cu(OTf)2
(20 mol%), the 2,2′-bipyridine ligand, and MnO2 (3 equiv) as
the stoichiometric oxidant provided a higher yield of 2a (Table 1,
entry 2). Encouraged by these results, we further employed
(R,R)-Ph-box as the ligand to enable an enantioselective
variant. To our delight, this process was feasible, and chiral
indoline 2a was formed in 62% yield and 73% ee (Table 1,
entry 3). We found that use of activated 4 Å molecular sieves
was beneficial to the yield and enantioselectivity (Table 1, entry 4).
While an increase in diphenylethylene equivalents and reaction
concentration did not further increase the yield of 2a, the
DPE stoichiometry could be reduced to 3 equiv (Table 1,
entries 5−7). The yield of 2a peaked around 75%, where the
remaining mass was predominantly sultam 3 (15%). We further
optimized the reaction by reducing reaction temperature and
time (Table 1, entry 8). The catalyst loading could also be
further reduced to 15 mol % Cu(OTf)2 for this substrate
(Table 1, entry 9).
ABSTRACT: Enantioselective copper-catalyzed cycliza-
tion of γ-alkenylsulfonamides and a δ-alkenylsulfonamide
in the presence of a range of vinyl arenes results in
variously functionalized 2-substituted chiral nitrogen
heterocycles via a formal alkene C−H functionalization
process. Application of this reaction to the concise synthesis
of a 5-HT7 receptor antagonist is demonstrated.
he C−H functionalization of alkenes is currently under
Tintensive study as a direct carbon−carbon bond-forming
method for the synthesis of higher substituted alkenes.1 The
commonly employed Heck reaction2 enables direct C−H to
C−C functionalization of alkenes with aryl and vinylhalides, but
is rarely performed3 with alkyl halide coupling partners due to
the propensity of the alkyl to undergo β-hydride elimination.
The Heck-type coupling with alkyl halides has experienced
some success with alternative methods that involve carbon
radical intermediates, generally of low functionality.3b,4 New
methods for the oxidative coupling of more functionalized alkyl
groups with alkenes, however, are still required. Along these
lines, the intermolecular oxidative Heck-type coupling of
β-amino alkyl reagents with alkenes is unprecedented.5 Herein
is reported the first intermolecular oxidative Heck-type coupling
with a β-aminoalkyl intermediate.6 Furthermore, rather than
starting from a preformed chiral β-amino alkyl halide, we
envisioned the alkene could intercept a chiral β-aminoalkyl
radical generated in situ from an enantioselective aminocupration
of a γ-aminoalkene followed by C−Cu(II) homolysis. Under
oxidizing reaction conditions, the resulting carbon radical
coupling intermediate could be oxidized to an alkene, thus
completing a net Heck-type reaction sequence (Scheme 1).
Scheme 1. Alkene Amination−Heck-Type Coupling Cascade
Following the optimized conditions (Table 1, entry 8), a
series of substituted N-sulfonyl-2-allylanilines underwent the
enantioselective alkene amination/Heck-type coupling reaction
Received: December 1, 2011
Published: January 17, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
that if the coupling partner is not highly reactive toward radical
addition, the alkyl radical intermediate could ring open and
close reversibly prior to coupling, resulting in erosion of
enantioselectivity. It is also noteworthy that N-tosyl-2-allylani-
line 1a gave the sultam 3 in low enantioselectivity in our doubly
intramolecular carboamination reaction,7a while the majority of
the intra-/intermolecular reactions in Table 2 provide indolines
with very good %ee. This again seems to indicate that if the
carbon radical addition is rapid, selectivity erosion via ring-
opening becomes less problematic.
Electron-deficient alkene coupling partners such as acryloni-
trile, methyl methacrylate, and 2(5H)-furanone either were
not reactive, underwent aza-Michael addition (acrylonitrile),
or produced what appeared to be polymers in reactions with
1a or 1e.
4-Pentenylsulfonamides 4 also underwent the coupling
reaction in good yield and moderate to excellent enantio-
selectivity (Table 3). These relatively less reactive substrates
required longer reaction time (24 h) and higher tempera-
ture (120 °C). It is instructive that both the carbon backbone
and the N-sulfonyl group have a significant effect on the
enantioselectivity. For example, the N-tosyl substrate 4a
provided the DPE coupling product 5a in 92% ee, while the
N-mesyl substrate 4b gave 5b in only 55% ee (Table 3, entries 1
and 2). The N-tosyl-2,2-diphenyl-4-pentene 4c reacted with the
highest enantioselectivity, 95% ee, while the N-mesyl-2,2-
diphenyl-4-pentene 4d provided the coupled product in 90% ee
(Table 3, entries 3 and 4). Clearly the 2,2-diphenyl substitution
has a significant affect on the enantioselectivity. An example of
a six-membered ring synthesis is shown in Table 3, entry 7,
where a chiral isoquinoline 7 is formed in 51% yield and 79% ee.
This substrate was less reactive and required a 48 h reaction
time. It is noteworthy that most of the coupling products are
crystalline, so their enantiopurity can in principle be increased
by recrystallization.
The absolute stereochemistry of 2a was established by X-ray
crystallography. The stereochemistry of 2b−2o, 4a−4f, and 6
was assigned by analogy to 2a and literature precedent.7,8
The proposed mechanism for this reaction is shown in Scheme 1.
Ligand exchange provides the reactive R2N-[Cu] complex.
Stereodetermining cis-aminocupration occurs via chairlike tran-
sition state A that places the N-substituent anti to the closest
bis(oxazoline) phenyl substituent. The resulting unstable
organocopper(II) species undergoes homolysis to provide Cu(I)
and the corresponding primary carbon radical. Radical addition to
the vinylarene and subsequent oxidation of the resulting benzylic
radical provides the chiral nitrogen heterocycle.
Table 1. Alkene Amination−Heck-Type Coupling
a
Optimization
b
DPE
time (h),
yield
c
entry
CuX2·ligand
Cu(eh)2
equiv temp (°C)
2a (3)
ee
de
,
1
2
3
4
5
5
5
5
5
8
3
3
3
24, 120
24, 120
24, 120
24, 120
24, 120
24, 120
24, 120
8, 105
51 (40)
65 (30)
62 (25)
75 (15)
76 (15)
75 (15)
74 (15)
75 (15)
75 (15)
e
e
Cu(OTf)2·Bipy
Cu(OTf)2·(R,R)-Ph-box
Cu(OTf)2·(R,R)-Ph-box
Cu(OTf)2·(R,R)-Ph-box
Cu(OTf)2·(R,R)-Ph-box
Cu(OTf)2·(R,R)-Ph-box
Cu(OTf)2·(R,R)-Ph-box
Cu(OTf)2·(R,R)-Ph-box
73
88
87
nd
89
92
91
ef
,
fg
5 ,
fg
6 ,
fg
7 ,
fg
8 ,
fgh
,
9 ,
8, 105
a
Reactions run with 1a (0.174 mmol), Cu(II) (0.0348 mmol), ligand
(0.0435 mmol), DPE (equiv), K2CO3 (0.174 mmol), and MnO2
b
(0.522 mmol) in CF3Ph (0.15 M) unless otherwise noted. Isolated
c
yield after flash chromatography on SiO2. Determined by chiral
d
e
HPLC analysis. Cu(eh)2 (3 equiv) used in this reaction. Reaction
f
run at 0.10 M with respect to 1a. Reaction carried out with activated
g
h
4 Å mol. sieves. Reaction run at 0.15 M with respect to 1a. Reaction
run with 15 mol% Cu(OTf)2 and 19 mol% (R,R)-Ph-box. L = ligand,
bipy = 2,2′-bipyridine, Cu(eh)2 = copper (2-ethylhexanoate)2, DPE = 1,
1-diphenylethylene. nd = not determined.
with vinylarenes. As illustrated in Table 2, the N-arylsulfonyl-
anilines gave products with relatively higher enantiomeric
excess than the N-mesyl- and N-trimethylsilylethylsulfonyl ana-
logues (compare Table 2, entries 1−4 to entries 5 and 6).
The yields and selectivities were relatively insensitive to the
nature of the 4-substitution on the aniline (F, Cl, MeO, see
Table 2, entries 7−9). Although diphenylethylene is generally
the most reactive coupling partner, other vinylarenes such as
styrene, α-methylstyrene, α-acetoxystyrene, α-pivaloxystyrene,
benzofuran, and 3-methylbenzofuran also yielded coupling
products. The coupling with styrene provided a mixture (4:1
E:Z) of alkene isomers (Table 2, entry 10). Ozonolysis of this
mixture followed by reductive workup with NaBH4 provided
one terminal alcohol product in 88% ee (see Supporting
Information for details). Coupling of 1a with α-methylstyrene
provided an inseparable 3:1 mixture of the internal and terminal
alkenes, 2k and 2l, respectively (Table 2, entry 11). Ozonolysis
of this mixture provided the respective aldehyde and ketone in
85% and 91% ee (see Supporting Information for details). The
β-amino aldehyde derived from 2k has a lower %ee, possibly
due to configurational instability (via reversible retro-Michael/
Michael addition). Coupling of 1a with α-acetoxystyrene4e
provided ketone 2m in 82% ee, albeit in only 20% yield
(Table 2, entry 12). The remainder of the material was N-acyl-
N-tosyl-2-allylaniline along with sultam 3. Changing to the
α-pivaloxystyrene coupling partner provides 2m in 50% yield
and 90% ee.
An alternative mechanism for addition to the vinylarene
could involve carbocupration of the organocopper intermediate
followed by β-hydride elimination.2e−g To differentiate between
carbon radical addition and carbocupration for this step, a
competition experiment was performed where indoline 1a
underwent the alkene amination/Heck-type cascade in the
presence of 1.5 equiv of DPE and 1.5 equiv of styrene (eq 1).
Coupling of N-mesyl-2-allylaniline 1e with benzofuran and
3-methylbenzofuran, respectively, provided the unique coupling
products 2n and 2o, albeit is relatively lower %ee (Table 2,
entries 14 and 15). In these reactions the N-tosyl substrate 1a
was less effective, and intramolecular carboamination product 3
predominated (not shown). A comparison of Table 2, entries 5,
14, and 15, also illustrates that the reactivity of the coupling
partner can affect the enantioselectivity. This seems to indicate
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a
a
Table 2. Indoline Synthesis Scope
Table 3. Pyrrolidine and Tetrahydroisoquinoline Synthesis
a
The same general reaction conditions as Table 1, entry 8, were used
b
except the reactions were run at 120 °C for 24 h. Isolated yields from
flash chromatography on SiO2. Enantiomeric excess determined by
chiral HPLC analysis. Not determined. The enantiomers could not be
separated by chiral HPLC. Reaction was run for 48 h.
c
d
e
ethylene, is a substrate for Cu-catalyzed Heck reactions.2e−g) This
competition reaction led to formation of a 4.2:1 mixture of 2a and
2j (eq 1). Thus, it appears the radical mechanism is more likely.
Scheme 1 shows the stereochemistry-determining step to be
the cis-aminocupration, yet in Table 2 we observe that the
nature of the vinyl arene can affect the enantioselectivity of
the reaction (vide supra). This could be indicating that the
organocopper intermediate or the carbon radical can undergo
some degree of ring-opening and unselective re-closing if the
vinylarene is not a very reactive radical acceptor.
An illustration of the utility of this reaction is shown in the
concise synthesis of a 5-HT7 receptor antagonist 9 (Scheme 2).14
Scheme 2. Concise Synthesis of a 5-HT7 Receptor
Antagonist
a
The same conditions as Table 1, entry 8, were used in these reactions.
These conditions were more general than Table 1, entry 9. Yields
b
c
and ee's are the average of two runs. Isolated yield. Determined
d
by chiral HPLC. N-Acylation was a competing side reaction.
e
f
Reaction run at 120 °C for 24 h. N-Mesyl-2-methylindoline, a
hydroamination product, was a minor component of the crude
mixture.
Synthesis of sulfonamide 4g was performed by SN2 displacement
on the commercially available 5-bromo-1-pentene with 3-methyl-
benzenesulfonamide. The enantioselective amination/Heck-type
cascade reaction provided pyrrolidine 5g in 66% yield and 85% ee.
The (4S,5R)-di-Ph-box ligand was used in this example because it
provided greater enantioselectivity than (S,S)-Ph-box. Oxidative
cleavage of the alkene and subsequent reductive amination provided
We reasoned that a radical mechanism should favor addition to
DPE over styrene since radicals generally add more rapidly to
DPE.13 Conversely, in analogy to the migratory insertion step in
the Pd-catalyzed Heck reaction, a carbocupration mechanism
should favor addition to styrene, which is less hindered than
DPE.2d (It is also noteworthy that styrene, but not diphenyl-
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Journal of the American Chemical Society
Communication
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the 5-HT7 receptor antagonist 9. This synthesis was accom-
plished in 4 steps from commercially available reagents without the
aid of protecting groups and is several steps shorter than its original
synthesis.14
In conclusion, we have developed an efficient and
enantioselective alkyl Heck-type coupling cascade for the
formation of functionalized chiral indolines, pyrrolidines
and an isoquinoline from the respective acyclic γ- and δ-
alkenylsulfonamides. The concise synthesis of a 5-HT7 receptor
antagonist was accomplished to demonstrate the utility of the
method. Sulfonamides and sultams are common moieties found
in bioactive compounds.14,15 This method accesses such targets
directly, enantioselectively and without the use of protecting
groups. Removal of the N-sulfonyl groups can further provide
valuable chiral amines. Further investigation into the hetero-
cycle and alkene scope and application in the total synthesis of
bioactive alkaloids is ongoing.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, spectroscopic data, and crystallog-
raphy data of 2a (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
schemler@buffalo.edu
■
(11) Chemler, S. R. J. Organomet. Chem. 2011, 696, 150.
(12) Selected Pd-catalyzed intermolecular carboamination/cycliza-
tion of alkenes with unfunctionalized arenes and functionalized alkenes
and arenes: (a) Rosewall, C. F.; Sibbald, P. A.; Liskin, D. V.; Michael,
F. E. J. Am. Chem. Soc. 2009, 131, 9488. (b) Mai, D. N.; Wolfe, J. P.
J. Am. Chem. Soc. 2010, 132, 12157. (c) Houlden, C. E.; Bailey, C. D.;
Ford, J. G.; Gagne, M. R.; Lloyd-Jones, G. C.; Booker-Milburn, K. I.
J. Am. Chem. Soc. 2008, 130, 10066. Au-catalyzed intermolecular
carboamination/cyclization of alkenes with functionalized arenes:
(d) Zhang, G.; Cui, L.; Wang, Y.; Zhang, L. J. Am. Chem. Soc. 2010,
132, 1474. (e) Brenzovich, W. E.; Benitez, D.; Lackner, A. D.;
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S. K.; Saunders, D. V.; Collin, L. L.; Hagan, J. J.; Riley, G. J.; Thomas,
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Institutes of Health (NIGMS 078383)
for support of this work. We thank William W. Brennessel and
the X-ray Crystallographic Facility at the University of
Rochester for the X-ray structure of 2a.
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