Rhodium-catalyzed regio-, diastereo-, and enantioselective [2+2+2] cycloaddition of 1,6-enynes with acrylamides

Article abstract of DOI:10.1002/anie.201206122

Transition-metal-catalyzed intermolecular [2+2+2] cycloadditions of a,w-diynes or enynes with unsaturated compounds are valuable methods for the synthesis of complex bicyclic molecules in a single step.[1] For example, the transitionmetal- catalyzed [2+2+2] cycloaddition of 1,6-enynes with alkynes enables the facile preparation of densely substituted annulated cyclohexadienes.[2] In 2005, the groups of Evans and Shibata developed asymmetric variants of this reaction that furnish annulated cyclohexadienes with one stereogenic center by using cationic rhodium(I)/chiral bisphosphine complexes as catalysts (Scheme 1).[3, 4] Additionally, it has been reported that the cationic rhodium(I)/chiral bisphosphine complexes catalyze the asymmetric [2+2+2] cycloaddition of 1,6-diynes with electron-deficient alkenes, to also afford annulated cyclohexadienes with one stereogenic center (Scheme 1).[5] However, the transition-metal-catalyzed [2+2+2] cycloadditions involving two alkene units have been largely limited to the intramolecular reactions of dienynes.[6] Only two examples of the transition-metalcatalyzed intermolecular [2+2+2] cycloaddition involving two alkene units have been reported to date.[7, 8] In 1999, Montgomery and co-workers reported the nickel-catalyzed [2+2+2] cycloaddition of 1,6-enynes with enones.[7] In 2010, Ogoshi et al. reported the nickel-catalyzed [2+2+2] cycloaddition of two enones with alkynes.[8] However, these reactions are limited to enone derivatives and their asymmetric variants have not been realized (Scheme 1). On the other hand, our research group has demonstrated that acrylamide derivatives are highly reactive substrates in cationic rhodium(I)/bisphosphine-catalyzed carbon-carbon bond-forming reactions.[9] Herein, we have achieved the unprecedented catalytic asymmetric [2+2+2] cycloaddition of 1,6-enynes with alkenes by using acrylamides as alkenes and a cationic rhodium(I)/(R)-H8-binap complex as the catalyst.

Full text of DOI:10.1002/anie.201206122

Angewandte
Chemie
DOI: 10.1002/anie.201206122
Asymmetric Catalysis
Rhodium-Catalyzed Regio-, Diastereo-, and Enantioselective [2+2+2]
Cycloaddition of 1,6-Enynes with Acrylamides**
Koji Masutomi, Norifumi Sakiyama, Keiichi Noguchi, and Ken Tanaka*
Transition-metal-catalyzed intermolecular [2+2+2] cycload-
ditions of a,w-diynes or enynes with unsaturated compounds
are valuable methods for the synthesis of complex bicyclic
molecules in a single step.[1] For example, the transition-
metal-catalyzed [2+2+2] cycloaddition of 1,6-enynes with
alkynes enables the facile preparation of densely substituted
annulated cyclohexadienes.[2] In 2005, the groups of Evans
and Shibata developed asymmetric variants of this reaction
that furnish annulated cyclohexadienes with one stereogenic
center by using cationic rhodium(I)/chiral bisphosphine
complexes as catalysts (Scheme 1).[3,4] Additionally, it has
been reported that the cationic rhodium(I)/chiral bisphos-
phine complexes catalyze the asymmetric [2+2+2] cyclo-
addition of 1,6-diynes with electron-deficient alkenes, to also
afford annulated cyclohexadienes with one stereogenic center
(Scheme 1).[5] However, the transition-metal-catalyzed
[2+2+2] cycloadditions involving two alkene units have
been largely limited to the intramolecular reactions of
dienynes.[6] Only two examples of the transition-metal-
catalyzed intermolecular [2+2+2] cycloaddition involving
two alkene units have been reported to date.[7,8] In 1999,
Montgomery and co-workers reported the nickel-catalyzed
[2+2+2] cycloaddition of 1,6-enynes with enones.[7] In 2010,
Ogoshi et al. reported the nickel-catalyzed [2+2+2] cyclo-
addition of two enones with alkynes.[8] However, these
reactions are limited to enone derivatives and their asym-
metric variants have not been realized (Scheme 1). On the
other hand, our research group has demonstrated that
acrylamide derivatives are highly reactive substrates in
cationic rhodium(I)/bisphosphine-catalyzed carbon–carbon
bond-forming reactions.[9] Herein, we have achieved the
unprecedented catalytic asymmetric [2+2+2] cycloaddition
of 1,6-enynes with alkenes by using acrylamides as alkenes
and a cationic rhodium(I)/(R)-H8-binap complex as the
catalyst.
We first examined the reaction of 1,6-enyne 1a, possessing
a disubstituted alkene moiety, and N,N-dimethylacrylamide
(2a) at room temperature in the presence of the cationic
rhodium(I)/chiral bisphosphine catalysts (Table 1, entries 1–
6). Pleasingly, the use of biaryl bisphosphine ligands furnished
the desired annulated cyclohexene 3aa with complete regio-
and diastereoselectivity and high enantioselectivity together
with diene 4aa as a minor product (Table 1, entries 1–3).
However, the use of nonbiaryl bisphosphine ligands failed to
furnish both 3aa and 4aa (Table 1, entries 4–6). H8-binap,
possessing the largest dihedral angle of the tested ligands,
showed the highest yield of 3aa as well as the highest
enantioselectivity (Table 1, entry 3). Sterically more-demand-
ing xyl-H8-binap was also tested, and the yield of 3aa
decreased significantly (Table 1, entry 7). As the catalytic
activity of the rhodium/H8-binap catalyst was very high, the
reaction could even be carried out in the presence of 5 or
3 mol% of the catalyst without erosion of the product yield
and ee value (Table 1, entries 8 and 9).
Scheme 1. Transition-metal-catalyzed asymmetric [2+2+2] cycloaddi-
tions involving alkene units.
[*] K. Masutomi, N. Sakiyama, Prof. Dr. K. Tanaka
Department of Applied Chemistry, Graduate School of Engineering
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
E-mail: tanaka-k@cc.tuat.ac.jp
Homepage: http://www.tuat.ac.jp/~tanaka-k/
Prof. Dr. K. Noguchi
Instrumentation Analysis Center, Tokyo University of Agriculture
and Technology (Japan)
We tested the generality of the reaction with regard to
both cycloaddition partners by using the cationic rhodium(I)/
(R)-H8-binap catalyst at room temperature (Scheme 2).
N,N-Dialkyl- (2a–c), N-methyl-N-phenyl- (2d), and N,N-
diphenyl- (2e) acrylamides and Weinreb amide 2 f reacted
with 1,6-enyne 1a to give the corresponding annulated
cyclohexenes in high yields with high ee values,[10] although
sterically demanding amide 2c and Weinreb amide 2 f
required high catalyst loadings. With respect to 1,6-enynes,
not only tosylamide- (1a) but also malonate- (1b) and
oxygen- (1c) linked 1,6-enynes, possessing the disubstituted
alkene moiety, reacted with 2d to give the corresponding
Prof. Dr. K. Tanaka
JST, ACT-C, 4-1-8 Honcho, Kawaguchi
Saitama, 332-0012 (Japan)
[**] This work was supported partly by a Grant-in-Aid for Scientific
Research (No. 20675002) from MEXT and JST, ACT-C (Japan). We
are grateful to Takasago International Corporation for the gift of
segphos, H8-binap, and xyl-H8-binap, and Umicore for generous
support in supplying rhodium complexes.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206122.
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Table 1: Optimization of reaction conditions for rhodium-catalyzed
asymmetric [2+2+2] cycloaddition of 1a with 2a.[a]
Entry Ligand
Catalyst 3aa/Yield [%] Yield of 4aa [%] [b]
(mol%) (% ee)[b]
1
2
3
(R)-segphos
10
10
10
(ꢀ)-3aa/63
(>99)
5
14
8
(R)-binap
(ꢀ)-3aa/82
(98)
(R)-H8-binap
(ꢀ)-3aa/85
(>99)
4
(S,S)-diop
(S,S)-chiraphos
(R,R)-Me-duphos 10
(S)-xyl-H8-binap
(R)-H8-binap
(R)-H8-binap
10
10
0
0
0
0
0
0
3
5[c]
6[c]
7
10
5
(+)-3aa/33
(>99)
8[d]
9[e]
(ꢀ)-3aa/86
9
(>99)
3
(ꢀ)-3aa/85
(>99)
11
[a] [Rh(cod)2]BF4 (0.010 mmol), ligand (0.010 mmol), 1a (0.10 mmol),
2a (0.11 mmol), and CH2Cl2 (1.5 mL) were used. [b] Yield of the isolated
product. [c] [Rh(nbd)2]BF4 was used. [d] [Rh(cod)2]BF4 (0.010 mmol),
ligand (0.010 mmol), 1a (0.20 mmol), 2a (0.22 mmol), and CH2Cl2
(1.5 mL) were used. [e] [Rh(cod)2]BF4 (0.0060 mmol), ligand
(0.0060 mmol), 1a (0.20 mmol), 2a (0.22 mmol), and CH2Cl2 (1.5 mL)
were used. cod=cyclooctadiene, nbd=norbornadiene, Ts=p-toluene-
sulfonyl.
Scheme 2. Rhodium-catalyzed asymmetric [2+2+2] cycloaddition of
1,6-enynes 1a–j with acrylamides 2a-f. Reaction conditions:
[Rh(cod)2]BF4 (0.0060–0.020 mmol), (R)-H8-binap (0.0060–
0.020 mmol), 1 (0.20 mmol), 2 (0.22–0.60 mmol), and CH2Cl2 (1.5 mL)
were used. Cited yields are of isolated products.
annulated cyclohexenes in good yields with high ee values,
although elevated temperature and/or high catalyst loading
were required. Various substituents, such as methyl (1a,b),
pentyl (1c), phenyl (1d), and hydrogen (1e) could be
incorporated at the alkyne terminus. Not only methyl (1a–
e) but also ethyl (1 f) and phenyl (1g) substitutions at the
alkene moiety were tolerated. Furthermore, 1,6-enynes 1h–j,
possessing the monosubstituted alkene moiety, reacted with
2a and 2d–f to give the corresponding cyclohexenes in good
yields with high ee values.
observed. In the reaction of 1,6-enyne 1a, possessing the
disubstituted alkene moiety, with sterically less-demanding
and more-coordinative N,N-dimethylacrylamide (2a), cyclo-
hexene 3aa was obtained in high yield and diene 4aa was
obtained in low yield (Table 2, entry 1). In the reaction of 1a
As shown in Table 2, interesting substituent effects on the
yields and ee values of cyclohexenes 3 and dienes 4 were
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Table 2: Effects of substituents on the yields and ee values of cyclo-
hexenes 3, dienes 4, and dienes 5.[a]
Entry
1
2
3/Yield [%][b] 4/Yield [%][b] 5/Yield [%][b]
(R1)
(R2)
(% ee)
(% ee)
1
2
3
4
5
1a
2a
(ꢀ)-3aa/86
4aa/9
5aa/0
5ae/0
(Me) (NMe2) (>99)
1a
(Me) (NPh2)
1h
(H)
1h
(H)
1a
2e
(ꢀ)-3ae/97
4ae/0
(>99)
2a
(ꢀ)-3ha/51
(+)-4ha/41
(68)
4he/0
5ha/0
5he/0
5ag/16[c]
(NMe2) (97)
2e
(NPh2)
2g
(ꢀ)-3he/77
(95)
(ꢀ)-3ag/11
(99)[c]
4ag/10[c,d]
(Me) (OMe)
[a] [Rh(cod)2]BF4 (0.010 mmol), (R)-H8-binap (0.010 mmol),
Scheme 3. A possible mechanism for the reaction of enyne 1 with
1 (0.20 mmol), 2 (0.22 mmol), and CH2Cl2 (1.5 mL) were used. [b] Yield
of the isolated product. [c] Isolated as a mixture of 3ag/(E)-4ag or (Z)-
4ag/5ag. Yields were determined by 1H NMR spectroscopy and
analytically pure compounds were isolated by GPC. [d] E/Z=32:68.
alkene 2.
the formation of A, the ee value of 4 should be as high as that
of 3.[12] The observed markedly lower ee value of 4ha than of
3ha might suggest that the less-enantioselective latter path-
way (1!F!D!E!4) may be involved, at least in part, in
the formation of diene 4.[13] The observed substituent effects
on the yields of 3 and 4 (Table 2) might be explained as
follows. With respect to acrylamides, the formation of D from
A would be more difficult with sterically more-demanding
amide 2e than sterically less-demanding amide 2a as a result
of the steric interaction between the carbamoyl group and the
ligand. The formation of D from F would be more likely with
more-coordinative amide 2a than less-coordinative amide 2e.
These explanations are consistent with the formation of
dienes 4 from 2a but not from 2e. With respect to 1,6-enynes,
insertion of the sterically less-demanding monosubstituted
alkene moiety of 1h into F leading to D would be more facile
than that of the sterically more-demanding disubstituted
alkene moiety of 1a. This explanation is consistent with the
markedly higher yield of 4ha than of 4aa.
N,3,4-trisubstituted octahydroisoindoles show pharma-
ceutical activity.[14] These compounds were prepared through
the Diels–Alder reaction with subsequent alkene hydrogena-
tion and carbonyl reduction (Scheme 4).[14] As the Diels–
Alder reaction was employed as a key step, 2- and 7-positions
were limited to trisubstituted carbons.
Reductive and oxidative transformations of the present
[2+2+2] cycloaddition products enable the asymmetric syn-
thesis of novel chiral octahydroisoindole derivatives, includ-
ing those possessing a tetrasubstituted carbon at the 2- or 7-
position (Scheme 5 and 6). Hydrogenation of 3eb, possessing
a trisubstituted alkene moiety, afforded N,4,7-trisubstituted
octahydroisoindole 7eb, possessing a tetrasubstituted carbon
at the 7-position, in high yield with perfect diastereoselectiv-
ity. Hydrogenation of 3ie, possessing the tetrasubstituted
and sterically more-demanding and less-coordinative N,N-
diphenylacrylamide (2e), cyclohexene 3ae was obtained in
higher yield than 3aa and diene 4ae was not obtained at all
(Table 2, entry 2). On the other hand, in the reactions of 1,6-
enyne 1h, possessing the monosubstituted alkene moiety,
with 2a and 2e (Table 2, entries 3 and 4), cyclohexenes 3ha
and 3he were obtained in lower yields than 3aa and 3ae, and
diene 4ha was obtained in significantly higher yield than 4aa.
Interestingly, the ee value of 4ha was markedly lower than
3ha (Table 2, entry 3). Importantly, regioisomeric diene 5ag
was generated, along with cyclohexene 3ag, in the reaction of
1,6-enyne 1a and acrylate 2g (Table 2, entry 5).
A possible mechanism for the formation of 3, 4, and 5 is
shown in Scheme 3. Enyne 1 reacts with rhodium to generate
rhodacyclopentene A. Regioselective insertion of alkene 2
into A generates rhodacycle B. Reductive elimination affords
cyclohexene 3. Chelation of the amide carbonyl oxygen to
rhodium would suppress b-hydride elimination, thus resulting
in formation of diene 5 via rhodium hydride C. However,
diene 5ag was generated in the reaction of 1a and acrylate 2g
presumably owing to weak chelation of the ester carbonyl
oxygen to rhodium. On the other hand, insertion of alkene 2
into A could also occur with opposite regioselectivity to
generate rhodacycle D. b-Hydride elimination to generate
rhodium hydride E is more favorable than reductive elimi-
nation to generate cyclohexene 6 and then subsequent
reductive elimination affords diene 4. Alternatively, the
alkyne moiety of enyne 1 reacts with alkene 2 and rhodium
to generate rhodacyclopentene F.[11] Insertion of the alkene
moiety of 1 into F generates D. However, as the enantio-
determining step in the former pathway (1!A!D!E!4) is
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[2+2+2] cycloaddition reactions involving two alkene units is
underway in our laboratory.
Received: July 31, 2012
Published online: November 19, 2012
Scheme 4. Synthesis of pharmaceutically active N,3,4-trisubstituted
octahydroisoindoles.
Keywords: asymmetric catalysis · cycloaddition · cyclohexenes ·
enynes · rhodium
.
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Scheme 6. Oxidative transformations of chiral annulated cyclohexenes
3.
alkene moiety, also proceeded to give N,3,4-trisubstituted
octahydroisoindole 7ie, but the hydrogenation of 3aa did not
proceed at all. The oxidation of 3eb, 3aa, and 3ie with
mCPBA (m-chloroperoxybenzoic acid) proceeded smoothly
to give the corresponding epoxides 8 and 8’ in high yields with
moderate diastereoselectivities. The epoxide 8ie could be
reduced to the corresponding alcohol 9ie, possessing a tetra-
substituted carbon at the 2-position, in moderate yield. The
relative and absolute configurations of octahydroisoindoles
(ꢀ)-7eb and (ꢀ)-8aa were unambiguously determined to be
(3aS,5S,7aR) and (1aS,2S,4aS,7aS), respectively, by the anom-
alous dispersion methods.[15]
In conclusion, the unprecedented regio-, diastereo-, and
enantioselective [2+2+2] cycloaddition of 1,6-enynes with
alkenes to give annulated cyclohexenes was achieved by using
acrylamides as alkenes and a cationic rhodium(I)/(R)-H8-
binap complex as a catalyst. In the present rhodium catalysis,
regioselective insertion of the acrylamide into a rhodacyclo-
pentene intermediate and the coordination of the carbonyl
group of the acrylamide to the cationic rhodium would
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[9b – d].
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[13] Similarly, the reaction of 1j and 2a furnished the corresponding
cyclohexene 3ja with a high ee value and the corresponding
diene 4ja with a low ee value, although 4ja could not been
isolated in
a pure form by silica gel chromatography.
[10] The reactions of 1,6-enynes 1a and 1h with crotonamide 2h and
methacrylamide 2i were also examined, and the desired [2+2+2]
cycloaddition products were not obtained at all instead the
corresponding homo-[2+2+2] cycloaddition products of 1a and
1h were generated.
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[15] CCDC 894213 ((ꢀ)-7eb) and 894214 ((ꢀ)-8aa) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[11] A similar regioselective formation of rhodacyclopentene G,
possessing the alkoxycarbonyl group at the sp3- hybridized
carbon atom adjacent to rhodium, was proposed in the cationic
rhodium(I)/(R)-H8-binap-catalyzed dimerization of acrylates
with internal alkynes, see: Y. Shibata, M. Hirano, K. Tanaka,
Org. Lett. 2008, 10, 2829.
Angew. Chem. Int. Ed. 2012, 51, 13031 –13035
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13035