N-heterocyclic carbene catalyzed reaction of cinnamils leading to the formation of 2,3,8-triaryl vinyl fulvenes: An uncommon transformation

Article abstract of DOI:10.1021/ol4030748

An unexpected transformation of 1,6-diarylhexa-1,5-diene-3,4-diones (cinnamils) to 2,3,8-triaryl vinyl fulvenes via N-Heterocyclic Carbene (NHC) catalysis is reported. Mechanistic as well as synthetic novelty is the hallmark of this reaction.

Full text of DOI:10.1021/ol4030748

ORGANIC
LETTERS
2013
Vol. 15, No. 24
6230–6233
N‑Heterocyclic Carbene Catalyzed
Reaction of Cinnamils Leading to the
Formation of 2,3,8-Triaryl Vinyl Fulvenes:
An Uncommon Transformation§
C. R. Sinu,† Eringathodi Suresh,‡ and Vijay Nair*,†
Organic Chemistry Section National Institute for Interdisciplinary Science and
Technology (CSIR-NIIST), Trivandrum - 695 019, India, and Analytical Department
and Centralized Instrument Facility CSIR-CSMCRI, Bhavnagar - 364 002, India
vijaynair_2001@yahoo.com
Received October 25, 2013
ABSTRACT
An unexpected transformation of 1,6-diarylhexa-1,5-diene-3,4-diones (cinnamils) to 2,3,8-triaryl vinyl fulvenes via N-Heterocyclic Carbene (NHC)
catalysis is reported. Mechanistic as well as synthetic novelty is the hallmark of this reaction.
In the general context of organocatalysis,1 N-heterocyc-
lic carbenes (NHCs) occupy a prominent position. NHC
catalysis2 originated as early as 1958 with Breslow’s
demonstration3 that thiazolylidene was involved in the
thiazolium catalyzed benzoin condensation.4 However,
with the exception of its application in benzoin conden-
sation5 and the Stetter reaction,6 NHC catalysis received
little attention for a long time. A significant change in
this situation occurred in 2004, when Glorius7 and Bode8
§ Dedicated with best wishes to Prof. Gilbert Stork.
† Organic Chemistry Section National Institute for Interdisciplinary
Science and Technology (CSIR-NIIST).
‡ Analytical Department and Centralized Instrument Facility CSIR-CSMCRI.
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r
10.1021/ol4030748
Published on Web 12/02/2013
2013 American Chemical Society

independently showed that NHC can generate homoeno-
lates from enals. Since this discovery, homoenolate annula-
tions to a wide range of carbonyl compounds9,10 and other
electrophiles11ꢀ16 have been reported by several research
groups including our own. Other NHC-catalyzed reactions
of importance include transesterification17 and cooperative
catalysis;18 chemistry of R,β-unsaturated acyl azolium ions
may also be mentioned in this context.19 The focal theme
of our work has been homoenolate annulation to enones,9
1,2-diones, and related compounds.2g
reaction, 1,6-diphenylhexa-1,5-diene-3,4-dione (cinnamil)
2 was exposed to p-methoxy cinnamaldehyde in the pre-
sence of 1,3-dimesityl imidazole carbene (IMes). A facile
reaction occurred; the product isolated, however, was not
the expected lactone 4, but the vinyl fulvene 2a (Scheme 1).
Scheme 1. Background to the Reaction
The present work has its origin in the observation that
homoenolate annulation of cyclic and acyclic 1,2-diones
afforded γ-lactones.12a,20 To explore the scope of this
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J. R.; Bode, J. W. J. Am. Chem. Soc. 2006, 128, 8418. (c) He, M.; Bode, J. W.
J. Am. Chem. Soc. 2008, 130, 418. (d) Rommel, M.; Fukuzumi, T.; Bode, J.
J. Am. Chem. Soc. 2008, 130, 17266. (e) Nair, V.; Varghese, V.; Babu, B. P.;
Sinu, C. R.; Suresh, E. Org. Biomol. Chem. 2010, 8, 761.
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Jiao, N. Chem.;Eur. J. 2012, 18, 9198.
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E. Org. Lett. 2009, 11, 5570. (b) Maji, B.; Ji, L.; Wang, S.; Vedachalam, S.;
Ganguly, R.; Liu, X.-W. Angew. Chem., Int. Ed. 2012, 51, 8276. (c) White,
N.; DiRocco, D.; Rovis, T. J. Am. Chem. Soc. 2013, 135, 8504.
Evidently, the enal was not a participant in the reaction;
the product 2a was formed by the interaction of two
molecules of cinnamil under the influence of NHC. Natu-
rally, the serendipitous formationof a vinyl fulvene and the
intriguing mechanistic aspects of the reaction fueled our
interest to pursue this uncommon reaction.
(14) (a) Chan, A.; Scheidt, K. A. J. Am. Chem. Soc. 2007, 129, 5334.
(b) Chan, A.; Scheidt, K. A. J. Am. Chem. Soc. 2008, 130, 2740.
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Chem. Soc. 2008, 130, 2416. (c) Yang, L.; Tan, B.; Wang, F.; Zhong, G.
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Zhang, W.; Kato, A.; Yu, C.-Y. Org. Biomol. Chem. 2013, 11, 4622.
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Takagi, K.; Suzuki, M. Org. Lett. 2011, 13, 3722. (c) Biju, A. T.;
Padmanaban, M.; Wurz, N.; Glorius, F. Angew. Chem., Int. Ed. 2011,
50, 8412. (d) Yao, W.; Bian, M.; Wang, G.; Ma, C. Synthesis 2011, 12,
1998. (e) Sinu, C. R.; Padmaja, D. V. M; Ranjini, U. P.; Seetha Lakshmi,
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X.; Miao, G.; Yao, W.; Ma, C. J. Org. Chem. 2013, 78, 6223. (g) Chen,
X.-Y.; Sun, L.-H.; Ye, S. Chem.;Eur. J. 2013, 19, 4441.
A systematic investigation of the reaction was initiated
(Scheme 2) by stirring a solution of 1,6-bis(p-tolyl)-1,5-
hexadiene-3,4-dione 5 and carbene, generated from IMes
HCl and DBU in THF. Conventional workup of the
reaction mixture afforded 5a and a trace of an isomeric
product (later identified as the o-terphenyl derivative 5b)
along with 4-methylcinnamic acid.
3
Scheme 2. NHC-Catalyzed Transformation of Cinnamil
€
(17) (a) Connor, E. F.; Nyce, G. W.; Myers, M.; Mock, A.; Hedrick,
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Nolan, S. P. Org. Lett. 2002, 4, 3583. (c) Nyce, G. W.; Lamboy, J. A.;
Connor, E. F.; Waymouth, R. M.; Edrick, J. L. Org. Lett. 2002, 4, 3587.
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Chem. 2004, 69, 209. (e) Chow, K. Y. K.; Bode, J. W. J. Am. Chem. Soc.
2004, 126, 8126. (f) Reynolds, N. T.; de Alaniz, J. R.; Rovis, T. J. Am.
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Angew. Chem., Int. Ed. 2007, 46, 3107. (l) Samanta, R. C.; Sarkar, S. D.;
The product 5a was characterized by standard spectro-
scopic and analytical methods. In the proton NMR spec-
trum, singlets due to three sets of methyl protons at δ 2.34,
2.35, and 2.37 ppm confirmed the presence of three tolyl
groups. The final proof for the structures was obtained from
single crystal X-ray determination of 8a and 5b (Figure 1).
The reaction was optimized with 1,6-bis(p-tolyl)-1,5-
hexadiene-3,4-dione 5 by varying the catalyst, base, solvent,
and temperature. The results are summarized in Table 1.
The scope of the reaction was examined under the opti-
mized conditions, and the results are summarized in Table 2.
In subsequent experiments it was found that a higher
amount of o-terphenyl compound could be obtained by
varying the reaction conditions from acetonitrileꢀsodium
hydride to tolueneꢀpotassium carbonate (Scheme 3).
€
Frohlich, R.; Grimme, S.; Studer, A. Chem. Sci. 2013, 4, 2177.
(18) (a) Raup, D. E. A.; Cardinal-David, B.; Holte, D.; Scheidt, K. A.
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Chem. Soc. 2011, 133, 12466. (c) De Vreese, R.; D’hooghe, M. Beilstein
J. Org. Chem. 2012, 8, 398. (d) Cohen, D. T.; Scheidt, K. A. Chem. Sci.
2012, 3, 53.
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1617. (b) Mahatthananchai, J.; Zheng, P.; Bode, J. W. Angew. Chem., Int. Ed.
2011, 50, 1673. (c) Ryan, S. J.; Candish, L.; Lupton, D. W. J. Am. Chem. Soc.
2011, 133, 4694. (d) Biswas, A.; De, S.; Tebben, L.; Studer, A. Chem.
Commun. 2012, 48, 5190. (e) Kravina, A. G.; Mahatthananchai, J.; Bode,
J. W. Angew. Chem., Int. Ed. 2012, 51, 9433. (f) Lyngvi, E.; Bode, J. W.;
Schoenebeck, F. Chem. Sci. 2012, 3, 2346. (g) Mahatthananchai, J.; Kaeo-
bamrung, J.; Bode, J. W. ACS Catal. 2012, 2, 494. (h) Samanta, R. C.; Maji,
€
€
B.; De Sarkar, S.; Bergander, K.; Frohlich, R.; Muck-Lichtenfeld, C.; Mayr,
H.; Studer, A. Angew. Chem., Int. Ed. 2012, 51, 5234. (i) Douglas, J.;
Churchill, G.; Smith, A. Synthesis 2012, 44, 2295. (j) Candish, L.; Forsyth,
C. M.; Lupton, D. W. Angew. Chem., Int. Ed. 2013, 52, 9149.
(20) Nair, V.; Vellalath, S.; Poonoth, M.; Suresh, E.; Viji, S. Synthesis
2007, 2007, 3195.
Org. Lett., Vol. 15, No. 24, 2013
6231

Table 1. Optimization of Reaction Conditions
Figure 1. Left: ORTEP diagram of 8a. Right: ORTEP diagram
of 5b.
time
(h)
yield
(%)b
entry
1
catalyst
conditionsa
Although the mechanistic underpinnings of the uncommon
reaction reported herein are not known, a rationalization for
the formation of the vinyl fulvene and o-terphenyl derivative
may be as follows (Scheme 4).
3a
DBU (0.5 equiv), THF,
0 °Cꢀrt
12
16
2
3
4
3a
3a
3b
DBU (0.5 equiv), DMF, rt
K2CO3 (1 equiv), CH3CN, rt
K2CO3 (1 equiv), CH3CN,
0 °Cꢀrt
5
64
50
ꢀ
24
24
It is conceivable that the addition of IMes to one of the
carbonyls of cinnamil will be followed by the formation of
the epoxy derivative A1, by a process analogous to the one
reported previously.21 Clearly A1 is set up for a [3,3]
sigmatropic rearrangement to afford the oxepine deriva-
tive C. Isomerization of C followed by intramolecular
elimination/displacement of IMes can lead to transient
bicyclic β-lactone E, and the latter can afford the diaryl
cyclopentadiene F by a retro [2 þ 2] process. Parenthetically
it may be added that such a retro [2 þ 2] process leading to the
formation of cyclopentene is supported experimentally10b
and by computational studies.22 The anion of F (G) pre-
sumably can undergo addition to a second molecule of
cinnamil and subsequently afford the transient epoxide I.
Fragmentation of I followed by protonation of the inter-
mediate Jwill lead to K. The latter is set up to deliver the vinyl
fulvene or o-terphenyl derivative by a concerted or stepwise
process. In the concerted process, a base can eliminate
cinnamate from K to afford the vinyl fulvene, whereas
elimination of cinnamate by anchimeric assistance from the
cyclopentadienyl moiety will lead to cyclohexadienyl cation
L and subsequently the o-terphenyl derivative. Alternatively
K can undergo ionization to deliver a cation (not shown),
which on proton loss can afford the fulvene, whereas it can
undergo ring expansion to afford the cyclohexadienyl cation
L. In this context, it is interesting to note that fulvene to
5
3c
3d
3a
3a
3a
3a
K2CO3 (1 equiv), CH3CN,
0 °Cꢀrt
24
24
24
24
10
20
35
19
40
44
56
25
6
K2CO3 (1 equiv), CH3CN,
0 °Cꢀrt
7
K2CO3 (1 equiv), DCM,
0 °Cꢀrt
8
K2CO3 (1 equiv), toluene,
0 °Cꢀrt
9
DBU (0.5 equiv), DCM,
0 °Cꢀrt
10
KOtBu (1 equiv), CH3CN,
rt
11
12
3a
3a
NaH (2 equiv), CH3CN, rt
K2CO3 (1 equiv), hexane,
0 °Cꢀrt
2
69
16
24
13
14
ꢀ
ꢀ
K2CO3 (1 equiv), CH3CN,
0 °Cꢀrt
24
24
ꢀ
ꢀ
DBU (1 equiv), CH3CN,
0 °Cꢀrt
a Detailed optimization studies are given in the Supporting Informa-
tion. b Overall yield of a and b.
benzene rearrangement has been observed previously by
irradiation23 or by gas phase pyrolysis.24 The reaction has
been rationalized by invoking a diradical intermediate.25
Recently Diederich observed a novel thermal pentafulvene
to benzene rearrangement in which an anionic “ring-walk”
mechanism has been suggested.26
To shed some light on the mechanism of the reaction a
crossover experiment was conducted between 1,6-bis(p-tolyl)-
1,5-hexadiene-3,4-dione and 1,6-bis(p-methoxyphenyl)-1,5-
hexadiene-3,4-dione and the formation of hybrid products
(21) Pillai, A. N.; Suresh, C. H.; Nair, V. Chem.;Eur. J. 2008, 14,
5851.
ꢀ
ꢀ
(22) (a) Domingo, L.; Zaragoza, R.; Arno, M. Org. Biomol. Chem.
2010, 8, 4884. (b) Verma, P.; Patni, P. A.; Sunoj, R. B. J. Org. Chem.
2011, 76, 5606.
(23) (a) Jano, I.; Mori, Y. Chem. Phys. Lett. 1968, 2, 185. (b) Ward,
H. R.; Wishnok, J. S. J. Am. Chem. Soc. 1968, 90, 5353. (c) Kent, J. E.;
Harman, P. J.; O’Dwyer, M. F. J. Phys. Chem. 1981, 85, 2726.
(24) (a) Henry, T. J.; Bergman, R. G. J. Am. Chem. Soc. 1972, 94, 5103.
(b) Hankinson, B.; Heaney, H.; Price, A. P.; Sharma, R. P. J. Chem. Soc.,
Perkin Trans. 1 1973, 2569. (c) Kopinke, F.-D.; Ondruschka, B.;
Zimmermann, G.; Dermietzel, J. J. Anal. Appl. Pyrolysis 1988, 13, 259.
(d) Zimmermann, G.; Remmler, M.; Ondruschka, B.; Kopinke, F.-D.;
Olk, B. Chem. Ber 1988, 121, 1855.
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(27) Details of the crossover experiment are given in the Supporting
Information.
6232
Org. Lett., Vol. 15, No. 24, 2013

Table 2. Scope of the Reaction
Scheme 4. Proposed Mechanism
entry
R
products
yield (%)a,b
1
2
3
4
5
6
7
8
9
phenyl
2a, 2b
74(89:11)c
69(91:09)
61(87:13)
55(91:09)
81(96:04)
40(90:10)
64(94:06)c
46(87:13)
61(90:10)
4-methylphenyl
4-methoxyphenyl
4-bromophenyl
4-chlorophenyl
3-chlorophenyl
4-fluorophenyl
furyl
5a, 5b
6a, 6b
7a, 7b
8a, 8b
9a, 9b
10a, 10b
11a, 11b
12a, 12b
thienyl
a Isolated yield. b Ratio of a:b is given in brackets. c UVꢀvis spectra
are given in the Supporting Information.
Scheme 3. Terphenyl Formation
was confirmed by mass spectroscopy.27 Arguably, these
results provide indirect support for the formation of 2,3-
diaryl cyclopentadiene and other intermediates invoked in
the mechanistic postulate.
As a unique class of trienes, pentafulvenes have evoked
great interest from both theoretical and synthetic perspec-
tives.28,29 In particular, their propensity to undergo multi-
ple modes of cycloadditions has been liberally exploited in
synthesis.30 In contrast, pentafulvenes with multiple sub-
stitution and conjugation have not received much atten-
tion presumably due to the difficulty in accessing them.
It may also be mentioned that o-terphenyls are known
to have interesting photophysical properties.31 However,
they are obtained by multistep synthesis32 that cannot
be easily adapted to the terphenyl derivatives reported
herein.
In conclusion, we have come across an uncommon
cascade reaction involving two molecules of 1,2-diones
that affords vinyl fulvenes and o-terphenyl derivatives.
The experimental conditions are simple and the products
are obtained in good yields, thus adding preparative
value to the reaction. In addition to its synthetic utility,
the intriguing mechanistic steps of the process described
here are also noteworthy. Further work aimed at ex-
ploring the scope and selectivity of the reaction will be
undertaken.
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Chem. A 2012, 116, 5008and references therein.
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Acknowledgment. We thank the Department of Science
and Technology (DST) for a Raja Ramanna fellowship for
V.N. and the Council of Scientific and Industrial Research
(CSIR) and the University Grants Commission (UGC)
New Delhi for financial assistance.
Supporting Information Available. General experimental
procedure, compound characterization data, and UVꢀvis
spectra of selected compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
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The authors declare no competing financial interest.
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