Rhodium-Catalyzed [2+2+2] Cycloadditions of Diynes with Morita-Baylis-Hillman Adducts: A Stereoselective Entry to Densely Functionalized Cyclohexadiene Scaffolds

Article abstract of DOI:10.1002/adsc.201600039

A rhodium-catalyzed asymmetric synthesis of 5,5-disubstituted cyclohexa-1,3-dienes has been achieved by [2+2+2] cycloaddition reactions between diynes and Morita-Baylis-Hillman (M-B-H) adducts as unsaturated substrates. Products containing two adjacent ch

Full text of DOI:10.1002/adsc.201600039

UPDATES
DOI: 10.1002/adsc.201600039
Rhodium-Catalyzed [2+2+2] Cycloadditions of Diynes with
Morita–Baylis–Hillman Adducts: A Stereoselective Entry to
Densely Functionalized Cyclohexadiene Scaffolds
Martí Fernµndez,a Magda Parera,a Teodor Parella,b Agustí Lledó,a Jean Le Bras,c
Jacques Muzart,c Anna Pla-Quintana,a,* and Anna Roglansa,*
a
Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus de
Montilivi, s/n, 17071 Girona, Catalonia, Spain
Fax: (+34)-972-41-8150; phone: (+34)-972-41-8275; e-mail: anna.plaq@udg.edu or anna.roglans@udg.edu
Servei de Ressonància Magn›tica Nuclear, Universitat Autònoma de Barcelona, 08193 Cerdanyola, Catalonia, Spain
Institut de Chimie MolØculaire de Reims, UMR 7312 CNRS – UniversitØ de Reims Champagne-Ardenne, B.P. 1039,
b
c
51687 Reims Cedex 2, France
Received: January 11, 2016; Revised: February 23, 2016; Published online: April 27, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600039.
Abstract: A rhodium-catalyzed asymmetric synthe-
sis of 5,5-disubstituted cyclohexa-1,3-dienes has
been achieved by [2+2+2] cycloaddition reactions
between diynes and Morita–Baylis–Hillman (M-B-
H) adducts as unsaturated substrates. Products con-
taining two adjacent chiral centres (quaternary and
tertiary, respectively) were obtained with complete
diastereoselectivity and high enantioselectivity (84–
97%) through a kinetic resolution of the M-B-H
adduct. Furthermore, these highly substituted cyclo-
hexadienes reacted with dienophiles to afford the
corresponding Diels–Alder cycloadducts in good
yields.
sion between the carbon substituents.[1] The transition
metal-catalyzed [2+2+2] cycloaddition reaction of un-
saturated motifs is a well-established and atom-eco-
nomical protocol for the construction of six-mem-
bered ring systems[2] and various substrates have been
used in this reaction, including alkenes.[3] In the pres-
ence of a chiral ligand, the Rh-catalyzed [2+2+2] cy-
cloaddition between diynes and 1,1-disubstituted al-
kenes provides an approach to the enantioselective
synthesis of cyclohexadiene derivatives (Scheme 1).[4]
Keywords: cycloaddition; diastereoselectivity; ki-
netic resolution; Morita–Baylis–Hillman adducts;
rhodium
Scheme 1. Transition metal-catalyzed [2+2+2] cycloaddition
reactions of diynes with 1,1-disubstituted olefins.
Morita–Baylis–Hillman (M-B-H) adducts are readi-
ly available, densely functionalized molecules contain-
Opposite enantiomers of chiral compounds often dis- ing at least three functional groups in proximity,
play disparate bioactivity profiles. In particular, the which can participate in a wide range of organic trans-
introduction of quaternary stereocentres in a molecule formations.[5] To the best of our knowledge, the partic-
constitutes an efficient strategy to complement the ipation of M-B-H adducts as alkene substrates in
chiral three-dimensional space of biological receptors. transition metal-catalyzed [2+2+2] cycloaddition re-
This approach has long been recognized by medicinal actions has not been reported previously. With this
chemists but is not straightforward from a synthetic transformation in mind, we hypothesized that the
point of view. In fact, all current marketed drugs of presence of an alcohol in the vicinity of the double
this type derive from natural products already con- bond would enhance stereodiscrimination through
taining the required quaternary stereocentres. Indeed, substrate chelation or anchimeric assistance.[6] In this
the formation of all-carbon quaternary stereocentres scenario, a highly diastereoselective process can be
represents one of the most difficult contemporary envisaged. In the event of using a chiral catalyst, a ki-
challenges in synthetic organic chemistry since the netic resolution process can be foreseen as well.[6c]
creation of such centres is hampered by steric repul- For this reason, we decided to study the [2+2+2] cy-
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Table 1. Optimization of the [2+2+2] cycloaddition between 1a and 2a.[a]
Entry
Ligand
Temp. [8C]
Time
Yield of 3a [%][b]
ee[c] of 3a [%]
1[d]
2
3
(rac)-BINAP
(rac)-BINAP
(R)-BINAP
(R)-BINAP
(R)-BINAP
(R)-H8-BINAP
(R)-SEGPHOS
(R)-DTBM-SEGPHOS
(S)-BINAP
84
22 h
0
–
–
80 (MW)
60 (MW)
40 (MW)
84
60 (MW)
60 (MW)
80 (MW)
60 (MW)
60 (MW)
10 min
20 min
40 min
4.5 h
20 min
20 min
40 min
20 min
10 min
27
50
0
55
47
35
0
93
–
73
95
72
–
4
5[e]
6
7
8
9[f]
10[g]
24
51
81
94
(R)-BINAP
[a]
Conditions: 1a (1 equiv.), 2a (1.5 equiv.).
Yield of the isolated product based on 1a.
Enantiomeric excess determined by chiral HPLC.
[b]
[c]
[d]
[e]
[f]
Homocoupling product 4a and isomerized M-B-H adduct 5 were obtained.
Slow addition of the diyne to a mixture of catalyst and 2a with a syringe pump for 4 h.
The enantiomer obtained in this case was the opposite to that obtained using (R)-BINAP.
Reaction run in ethanol.
[g]
cloaddition of symmetrical diynes with M-B-H ad- the two stereocentres, it was not possible to determine
ducts in order to obtain bicyclic cyclohexa-1,3-dienes their relative configuration by NMR spectroscopy.
with a tertiary/quaternary stereodiad (Table 1).
We then studied the stereoselectivity of this cyclo-
The feasibility of the cycloaddition was assessed addition using various chiral phosphines. To our de-
with N-tosyl-tethered non-terminal diyne 1a and M- light, the use of (R)-BINAP at 608C led to a better
B-H adduct 2a using 10 mol% of cationic rhodium yield of 3a with excellent enantioselectivity (entry 3,
complex [Rh(COD)2]BF4 with (rac)-BINAP in reflux- Table 1). A single X-ray crystallographic analysis of
ing dichloroethane (DCE) (Table 1). Only undesired 3a[7] revealed an (R)-configuration for the quaternary
homocoupling product 4a and isomerized M-B-H stereogenic centre and an (S)-configuration for the
adduct 5 (Figure 1) were obtained (entry 1, Table 1). tertiary alcohol stereogenic centre (Figure 2). Having
The effect of microwave as the heating source was started from racemic M-B-H adduct 2a, an enantioen-
then evaluated by running the reaction at 808C for riched cycloadduct was obtained, suggesting that a ki-
10 min. A 27% yield of 3a was obtained and the for- netic resolution process was operative. The recovered
mation of both 4a and 5 was reduced. Careful unreacted M-B-H adduct was submitted to chiral
1H NMR analysis of 3a showed the formation of only
one of the two possible diastereoisomers, demonstrat-
ing the high diastereoselectivity of the process. How-
À
ever, due to free rotation about the C C bond linking
Figure 1. Homocoupling product 4a and isomerization prod-
uct 5.
Figure 2. ORTEP diagram of cycloadduct (R,S)-3a.[7]
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HPLC analysis resulting in an ee of 17%, which corro-
borated our hypothesis.[8]
An attempt to reduce the temperature to 408C re-
sulted in a complete loss of reactivity (entry 4,
Table 1). We then evaluated the ee obtained under
conventional heating using slow addition of the diyne
1a to avoid its homodimerization (entry 5, Table 1).
The yield of 3a was similar but the enantioselectivity
was lower (73% instead of 93, entries 5 and 3,
Table 1). Different axially chiral biphosphines were
then tested in dichloroethane under microwave heat-
ing. Whereas (R)-H8-BINAP provided analogous
yields and ees (entry 6, Table 1), the use of both (R)-
SEGPHOS and especially (R)-DTBM-SEGPHOS
gave worse results (entries 7 and 8, Table 1). As
a result, (R)- and (S)-BINAP were chosen for the rest
of the study. (S)-BINAP promoted the reaction of the
other enantiomer of the M-B-H adduct to afford
(S,R)-3a with 81% enantiomeric excess; however, the
yield was lower than when (R)-BINAP was used
(compare entries 3 and 9, Table 1).
In all the above experiments, isomerization of 2a to
ketone 5 occurred consistently. A cationic rhodium
system with (R)-BINAP has recently been described[9]
in which a non-coordinating solvent such as dichloro-
methane (DCM) favours the isomerization of allylic
alcohols to saturated carbonyl compounds and so we
decided to evaluate the effect of the solvent. Using
EtOH, 3a was obtained in a 51% yield and with 94%
of ee and, importantly, no trace of the isomerized
compound was detected (entry 10, Table 1). Two con-
trol experiments were carried out to determine
whether or not the isomerization process was rhodi-
um-catalyzed. Treatment of M-B-H adduct 2a with
the Rh-catalytic system in DCE at 808C for three
Scheme 2. Kinetic resolution of M-B-H adduct 2a [Eq. (1)]
and cycloaddition reactions with enantioenriched (R)-2a
[Eq. (2)].
hours led to its quantitative conversion into 5. We (R)-2a with 1a under the optimized reaction condi-
then repeated the experiment in EtOH. The M-B-H tions (entry 10, Table 1) with either (R)- or (S)-
adduct was submitted to MW heating in EtOH BINAP [Eq. (2), Scheme 2]. As expected, no cycload-
(10 min at 608C). Starting material alone was recov- duct was isolated in the reaction with (R)-BINAP. On
ered, indicating that EtOH prevents isomerization. the other hand, (S)-BINAP efficiently promoted the
Additionally, in the reaction of entry 10 (Table 1), the cycloaddition of (R)-2a to (S,R)-3a, corroborating the
recovered M-B-H adduct was obtained in 43% ee, high enantiospecificity of the process.
confirming that the lower ee previously observed orig-
inates from the isomerization process.
Finally, the cycloaddition of 1a with 2a was moni-
tored over time in a reaction carried out under con-
The kinetic resolution process was then analyzed in ventional heating, which has a longer reaction time
greater detail. First, a [2+2+2] cycloaddition reaction than that run under microwave heating. This kinetic
was set up using an excess of diyne 1a [Eq. (1), experiment revealed a rapid consumption of (S)-2a to
Scheme 2]. We obtained (R,S)-3a in a 50% yield and form (R,S)-3a cycloadduct with a high enantiomeric
86% ee, together with an excess of diyne homocou- excess, which did not decrease over time. After ap-
pled compounds and (R)-2a in a 48% yield and an ex- proximately 2.5 h of reaction, the cycloadduct yield
cellent 98% ee The S-factor for the kinetic resolution and the ee of the M-B-H adduct reached a plateau in-
of 2a was determined to be 64. Therefore, the rhodi- dicating that (R)-2a does not react at a significant
um-catalyzed [2+2+2] cycloaddition reaction devel- rate under the reaction conditions (see the Supporting
oped appears to be a potentially efficient way to re- Information). This experiment constitutes further evi-
solve M-B-H adducts.
dence that the two enantiomers of the M-B-H adduct
Further insight into the resolution process was show different reactivities under asymmetric Rh/bi-
gained by treating the enantioenriched M-B-H adduct phosphine catalysis.
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In a final set of optimization experiments, the rela- again focused on the stereoselective synthesis of
tive amount of the M-B-H adduct 2a was modified. highly functionalized cyclohexadienes, proceeding to
Using either 3 or 4 equivalents of 2a, the yield of 3a evaluate the scope of the reaction by varying both the
was not improved (41% and 42%, respectively).[10] tethers of the diyne and the M-B-H adduct.
Therefore, we decided to use only 1.5 equivalents of
Diyne 1b containing a 4-nitrobenzenesulfonamide
the M-B-H adduct. In order to study the effect of the (NNs) instead of N-tosylsulfonamide was used since
hydroxy group of the M-B-H adduct in the cycloaddi- this group could be deprotected under milder reaction
tion process, acetyl- and Boc-protected M-B-H ad- conditions[11] (compounds 3b–3d, Scheme 3). Substitu-
ducts were tested using the optimized reaction condi- ents in the para-position of the phenyl ring of 2 with
tions. In neither case was cycloadduct 3a obtained, in- different electronic demand (OMe, Cl) gave the cor-
dicating that the free hydroxy group has a determining responding cycloadducts with good yields and high
role in the process.
enantiomeric excesses. In the case of cycloadduct 3b,
Having established the background for the kinetic we tested again the effect of increasing the M-B-H
resolution that operates in the described process, we adduct equivalents. With 2, 3 or 4 equiv. of 2a, the
Scheme 3. Scope of the process.
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yield was only improved to 43%, 45% and 56%, re-
spectively. Using the enantiomeric (S)-BINAP ligand,
(À)-3b, (À)-3c and (À)-3d (Scheme 3) were obtained
in high ees. We then changed the substituent in the
para position of the phenyl ring at the sulfonamide
tether. Fluoride and bromide substituted diynes led to
(+)-3e and (+)-3f with high enantiomeric excesses
(Scheme 3). Changing the para substituent to a bulkier
moiety such as a morpholine ring did not affect the
yield or the enantiomeric excess, affording compound
(+)-3g in a 49% yield with 94% ee. A diyne bearing
the 5-methyl-2-pyridinesulfonyl group provided
(+)-3h in a 30% yield with 97% ee. The presence of
a nitrogen atom in the molecule did not seem to
affect the process by poisoning the catalyst. However,
M-B-H adduct 2a failed to provide significant
amounts of cycloadducts 3i and 3j when reacted with
malonate and oxygen tethered diynes.
M-B-H adducts containing heteroaromatic rings
such as thiophene, furan, benzothiophene, and benzo-
furan were then synthesized and tested as more chal-
lenging substrates (Scheme 4). In all cases, the corre-
Scheme 5. Diels–Alder cycloaddition between 1,3-cyclohexa-
dienes 3b and 3c and dienophile 8.
sponding cycloadducts were obtained with high enan-
tioselectivities. Thus, a heteroatom in the M-B-H
adduct did not affect the process and did not poison
the rhodium catalyst. Finally, an effective reaction
was also obtained with a M-B-H adduct with a naph-
thalene group (Scheme 4). The relative and absolute
configurations of 3b–3h and 7a–7e were tentatively
assigned by analogy with (+)-3a.
We then exploited the 1,3-cyclohexadiene deriva-
tives generated by the [2+2+2] cycloaddition reaction
to access more densely functionalized scaffolds via
Diels–Alder reactions[12] (Scheme 5). A reactive nitro-
gen-containing heterodienophile, N-phenyltriazoline-
dione 8, was reacted with the unactivated and highly
substituted cycloadducts 3b and 3c in toluene at room
temperature, leading to 9b and 9c, respectively, in
high yields. 2D NMR experiments (see the Supporting
Information) of the Diels–Alder adducts revealed
a highly diastereoselective [4+2] cycloaddition occur-
ring to the opposite side of the ester group. In both
cases, traces of the stereoisomer resulting from addi-
tion of 8 to the same face of the ester group were also
observed (see the Supporting Information).
To summarize, we have developed a diastereo- and
enantioselective [2+2+2] cycloaddition reaction be-
tween diynes and Morita–Baylis–Hillman adducts
using a combination of [Rh(COD)2]BF4 and chiral
BINAP ligands. A kinetic resolution of the M-B-H
adduct was achieved generating an optically pure 1,3-
cyclohexadiene containing vicinal tertiary and quater-
nary carbon centres. Furthermore, Diels–Alder reac-
tions using these dienes as substrates occurred in
a highly diastereoselective fashion to provide densely
functionalized polycyclic scaffolds.
Scheme 4. [2++2+2] cycloaddition reactions between diyne
1b and Morita–Baylis–Hillman adducts 6.
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graphy, see: Transition-metal-mediated aromatic ring
construction, (Ed.: K. Tanaka), Wiley, Hoboken, 2013.
Experimental Section
[3] For selected references, see: a) T. Shibata, Y. Arai, Y.
Tahara, Org. Lett. 2005, 7, 4955; b) S. Brun, L. Garcia,
I. Gonzµlez, A. Torrent, A. Dachs, A. Pla-Quintana, T.
Parella, A. Roglans, Chem. Commun. 2008, 4339; c) A.
Geny, S. Gaudrel, F. Slowniski, M. Amatore, G. Chour-
aqui, M. Malacria, C. Aubert, V. Gandon, Adv. Synth.
Catal. 2009, 351, 271; d) A. Dachs, A. Pla-Quintana, T.
Parella, M. Solà, A. Roglans, Chem. Eur. J. 2011, 17,
14493; e) T. León, M. Parera, A. Roglans, A. Riera, X.
Verdaguer, Angew. Chem. 2012, 124, 7057; Angew.
Chem. Int. Ed. 2012, 51, 6951; f) K. Masutomi, N. Sa-
kiyama, K. Noguchi, K. Tanaka, Angew. Chem. 2012,
124, 13208; Angew. Chem. Int. Ed. 2012, 51, 13031.
[4] For selected references, see: a) K. Tsuchikama, Y.
Kuwata, T. Shibata, J. Am. Chem. Soc. 2006, 128,
13686; b) T. Shibata, A. Kawachi, M. Ogawa, Y.
Kuwata, K. Tsuchikama, K. Endo, Tetrahedron 2007,
63, 12853; c) K. Tanaka, M. Takahashi, H. Imase, T.
Osaka, K. Noguchi, M. Hirano, Tetrahedron 2008, 64,
6289; d) M. Kobayashi, T. Suda, K. Noguchi, K.
Tanaka, Angew. Chem. 2011, 123, 1702; Angew. Chem.
Int. Ed. 2011, 50, 1664.
General Procedure for the Rh-Catalyzed [2+2+2]
Cycloaddition Reactions
A stirred mixture of [Rh(COD)2]BF4 (4.1 mg, 0.01 mmol,
0.1 equiv.) and (R)-BINAP (6.3 mg, 0.01 mmol, 0.1 equiv.) in
dichloromethane (2 mL) was degassed under nitrogen. Hy-
drogen gas was introduced to the catalyst solution and the
mixture was stirred for 30 min. The resulting mixture was
concentrated to dryness. Ethanol (3 mL) was then added
and the solution was stirred under a nitrogen atmosphere.
The solution was introduced to a 10-mL screw-capped vial.
To this solution Morita–Baylis–Hillman adduct 2 or 6
(0.15 mmol, 1.5 equiv.) and diyne 1 (0.1 mmol, 1 equiv.)
were added under a nitrogen atmosphere. The vial was then
placed inside the cavity of the microwave synthesizer. The
reaction mixture was heated at 608C for 10 min (TLC moni-
toring). The solvent was then evaporated and the residue
was purified by column chromatography on silica gel to give
compounds 3 or 7.
[5] For selected reviews, see: a) D. Basavaiah, P. D. Rao,
R. S. Hyma, Tetrahedron 1996, 52, 8001; b) D. Basa-
vaiah, A. J. Rao, T. Satyanarayana, Chem. Rev. 2003,
103, 811; c) V. Singh, S. Batra, Tetrahedron 2008, 64,
4511; d) D. Basavaiah, B. S. Reddy, S. S. Badsara,
Chem. Rev. 2010, 110, 5447; e) T.-Y. Liu, M. Xie, Y.-C.
Chen, Chem. Soc. Rev. 2012, 41, 4101; f) D. Basavaiah,
G. Veeraraghavaiah, Chem. Soc. Rev. 2012, 41, 68.
[6] For selected references on the stereocontrolled reaction
of M-B-H adducts, see: a) T. Gendrineau, N. Demoulin,
L. Navarre, J.-P. Genet, S. Darses, Chem. Eur. J. 2009,
15, 4710; b) Y. Wang, X. Feng, H. Du, Org. Lett. 2011,
13, 4954. For a reference to the kinetic resolution of
tertiary propargylic alcohols by a [2+2+2] cycloaddi-
tion, see: c) K. Tanaka, T. Osaka, K. Noguchi, M.
Hirano, Org. Lett. 2007, 9, 1307.
[7] CCDC 1429936 (compound 3a) contains the supple-
mentary crystallographic data for this paper. These
data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[8] The low ee obtained can be justified on the grounds of
a competing enantiocomplementary isomerization pro-
cess of the M-B-H adduct. When the isomerization pro-
cess is suppressed a higher ee is obtained (vide infra).
[9] K. Ren, L. Zhang, B. Hu, M. Zhao, Y. Tu, X. Xie, T. Y.
Zhang, Z. Zhang, ChemCatChem 2013, 5, 1317.
Acknowledgements
Financial support from the Spanish Ministry of Education
and Science (MINECO) (Projects No.: CTQ2014-54306-P,
CTQ2012-32436 and a RyC contract to A. L.) and the DIUE
of the Generalitat de Catalunya (Project No.: 2014SGR931)
are acknowledged. Thanks are due to Generalitat de Catalu-
nya for predoctoral grants to M.F. and M.P.
References
[1] For selected reviews, see: a) E. J. Corey, A. Guzman-
Perez, Angew. Chem. 1998, 110, 2092; Angew. Chem.
Int. Ed. 1998, 37, 388; b) I. Denissova, L. Barriault, Tet-
rahedron 2003, 59, 10105; c) C. J. Douglas, L. E. Over-
man, Proc. Natl. Acad. Sci. USA 2004, 101, 5363;
d) B. M. Trost, C. Jiang, Synthesis 2006, 369; e) M.
Bella, T. Gasperi, Synthesis 2009, 1583; f) B. Wang,
Y. Q. Tu, Acc. Chem. Res. 2011, 44, 1207; g) Y. Liu, S. J.
Han, W.-B. Liu, B. M. Stoltz, Acc. Chem. Res. 2015, 48,
740.
[2] For selected and recent reviews, see: a) A. Thakur, J.
Louie, Acc. Chem. Res. 2015, 48, 2354; b) M. Amatore,
C, Aubert, Eur. J. Org. Chem. 2015, 265; c) Y. Satoh,
Y. Obora, Eur. J. Org. Chem. 2015, 5041; d) D. L. J.
Broere, E. Ruijter, Synthesis 2012, 2639; e) S. Okamo-
to, Heterocycles 2012, 85, 1579; f) Y. Shibata, K.
Tanaka, Synthesis 2012, 323; g) N. Weding, M. Hapke,
Chem. Soc. Rev. 2011, 40, 4525; h) R. Hua, M. V. A.
Abrenica, P. Wang, Curr. Org. Chem. 2011, 15, 712;
i) M. R. Shaaban, R. El-Sayed, A. H. M. Elwahy, Tetra-
hedron 2011, 67, 6095; j) G. Domínguez, J. PØrez-Cas-
tells, Chem. Soc. Rev. 2011, 40, 3430; k) A. Pla-Quinta-
na, A. Roglans, Molecules 2010, 15, 9230. l) For a mono-
[10] The reaction shows substrate inhibition when a high
excess of M-B-H adduct is used. Our interpretation is
that this inhibition is probably due to coordination of
multiple M-B-H adducts to the rhodium complex.
[11] T. Fukuyama, C.-K. Jow, M. Cheung, Tetrahedron Lett.
1995, 36, 6373.
[12] a) A. L. Jones, J. K. Snyder, Org. Lett. 2010, 12, 1592;
b) M. Shibuya, T. Sudoh, T. Kawamura, Y. Yamamoto,
Org. Biomol. Chem. 2015, 13, 5862.
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