Cycloisomerization of 1,6-dienes mediated by Lewis super acids without additives: Easy access to polysubstituted six-membered carbocycles

Article abstract of DOI:10.1002/anie.200602020

(Chemical Equation Presented) The tin salt Sn(NTf₂)₄ (Tf=trifluoromethanesulfonyl) is an efficient catalyst for the selective and ring-size-specific cycloisomerization of highly substituted 1,6-dienes to give six-membered-ring carboc

Full text of DOI:10.1002/anie.200602020

Angewandte
Chemie
Cyclohexane Derivatives
tallocene scandium hydride complexes.[14] These latter exam-
ples involve the use of highly sterically hindered catalysts in
association with alkylation or reducing agents. No simple
catalytic system has been yet reported for the cycloisomeri-
zation of 1,6-dienes to give highly substituted cyclohexane
carbocycles.
In the presence of Lewis acids,the intramolecular ene
reaction[15] of 1,6-dienes generally leads to five-membered-
ring compounds.[16] In a particular example,an alkenylpente-
nolide was transformed in the presence of pure sulfuric acid
into a cyclohexane ring,as a result of a Wagner–Meerwein
DOI: 10.1002/anie.200602020
Cycloisomerization of 1,6-Dienes Mediated by
Lewis Super Acids without Additives:Easy
Access to Polysubstituted Six-Membered
Carbocycles
Fanny Grau, Andreas Heumann,* and Elisabet Duæach*
transposition with the formation of
a spirocyclic ring
Dedicated to Professor Peter Vollhardt
on the occasion of his 60th birthday
system.[17]
We have recently reported on Lewis acid catalyzed
cycloisomerizations of nonactivated and differently substi-
tuted olefinic alcohols to give cyclic ethers.[18] We found that
the catalyst,tin(IV) triflate,Sn(OTf) 4,is involved in the
activation of both the hydroxy group and the double bond.
The following order of reactivity was observed: trisubstituted
ꢀ 1,1-disubstituted > 1,2-disubstituted > monosubstituted
olefins. It seemed to us that enhancing the electrophilicity of a
double bond with a strong Lewis acid might also favor
Alkyl-substituted carbocycles are a common structural com-
ponent in a number of naturally occurring and biologically
active molecules.[1] One of the challenging problems in the
synthesis of compounds containing five- and six-membered
rings is the efficient control of the ring size during a
cyclization or cycloaddition step. With innumerable applica-
tions,the Diels–Alder cycloaddition is the best established
reaction leading to six-membered rings exclusively.[2] The
advent of stereoselective transition-metal catalysts has stimu-
lated new ways to catalytic cyclization[3] as another important
approach for the construction of carbocyclic compounds.[4]
Though often highly selective,none of these reactions are
really ring-size specific.[5] The cycloisomerization of polyun-
saturated substrates has been studied extensively and is well
established as an efficient method for the synthesis of five-
membered carbocycles.[6,7] Though many metals have been
reported to achieve catalytic cycloisomerization reactions,[8]
only a few examples are known that lead to the selective
formation of six-membered rings. Cyclohexane derivatives
have been obtained from 1,6-dienes (e.g. olefinic indoles)
with palladium[9] and platinum[10] complexes without special
additives. However,other systems require catalyst combina-
tions such as bulky ethylenebis(tetrahydroindenyl)titanium
dichloride in association with nBuMgBr,[11] [Cp2ZrCl2]/meth-
À
intramolecular C C bond formation within dienes,in a
process comparable to the olefin activation by cationic late
transition metals.[19] However,in contrast to order of reac-
tivity towards late transition metals,highly substituted double
bonds were supposed to be much more reactive with Lewis
acids. We now report on our preliminary results on the
transformation of 1,6-dienes that are highly substituted at the
alkene termini in the presence of catalytic amounts of tin(IV)
bis(trifluoromethanesulfonyl)imide,Sn(NTf) 4,allowing the
selective formation of highly substituted six-membered-ring
carbocycles.
The active catalyst,Sn(NTf )4,and other metal bis(tri-
2
fluoromethanesulfonyl)imides and metal triflate salts were
prepared by using a simple electrochemical procedure.[20]
These metal salts can be considered as Lewis super acids
owing to the strong acidity of HNTf2 (pKa in water estimated
at 1.7).[21]
ylaluminoxane ([Zr]:[Al] = 0.1),[12] [Cp2 ZrMe2]/B(C6F5)3/
Treatment of diethyl diprenylmalonate (1a) in refluxing
1,2-dichloroethane with a 5 mol% Sn(NTf2)4 afforded the
gem-dimethyl cyclohexane derivative 1b in 60% yield
(Table 1). The structure of the new compound was easily
*
AlMe3 (Cp* = C5Me5),[13] methylaluminoxane and butylme-
1
deduced from the H and 13C NMR spectra.[22] Indeed,the
[*] F. Grau, Dr. E. Duæach
Laboratoire de Chimie
new carbon–carbon bond in cyclohexane derivative 1b arises
from cyclization of diene 1a occurring nonsymmetrically at
two different sites of the two identically substituted double
bonds: the first double bond undergoes a nucleophilic attack
by the second double bond with Markovnikov-type regio-
chemistry.
des MolØcules Bioactives et des Arômes
UniversitØ de Nice-Sophia Antipolis
CNRS, UMR 6001
Parc Valrose, 06108 Nice Cedex 2 (France)
Fax : (+33)492-076-151
E-mail: dunach@unice.fr
Dr. A. Heumann
UniversitØ Paul CØzanne
To examine the scope of this novel catalytic system,we
investigated the reactions of 1a using several SnXn derivatives
(X = Cl,OTf,NTf 2, n = 2 or 4; Table 1,entries 1–4). More
classical Lewis acids such as SnCl4 and SnCl2 led to low
conversion and low selectivity. The other metal triflates and
bistriflimidates tested (e.g. with WVI,Ni II,Sm III,Bi III,Table 1,
entries 5–8) were found to be inefficient. Only Sn(OTf)4 and
Sn(NTf2)4 showed interesting activities (Table 1,entries 1 and
4). Sn(NTf2)4 was by far the most efficient catalyst,since it
UMR CNRS 6180 “Chirotechnologies et Biocatalyse”
FacultØ Saint-JØrôme – Case A62
13397 Marseille Cedex 20 (France)
Fax : (+33)491-288-278
E-mail: andreas.heumann@univ-cezanne.fr
Homepage: http://www.unice.fr/lcmba/
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Effect of Lewis acid catalysts in the cycloisomerization of 1a to
give 1b.
phosphate 6a (entries 5 and 6) also afforded the correspond-
ing highly substituted cyclohexane structures 5b and 6b,
respectively,arising from the head-to-tail diene cycloisome-
rization.
The influence of the double-bond substitution upon
cycloisomerization was further examined with substrates
[25]
7a–12a (Table 2,entries 7–12).
The transformation of
Entry
Cat. (mol%)
t, T
Conv. [%]
Yield [%]
these substrates proceeded with satisfactory yields and
selectivities. The common structural feature of the cyclic
products is the presence of the gem-dimethyl group at the
1
2
3
4
5
6
7
8
Sn(OTf)4 (5)
SnCl4 (5)
SnCl2 (5)
Sn(NTf2)4 (2)
Ni(OTf)3 (5)
W(NTf2)6 (5)
Bi(OTf)3 (5)
Sm(OTf)3 (5)
71 h, 508C78
24 h, 408C25
28 h, 508C28
1.5 h, 508C100
24 h, 838C32
20 h, 838C43
7 h, 838C31
24 h, 838C
60
11
14
88
24
36
À
cycloalkane skeleton,indicating that the C C bond-forming
step always occurs at the more substituted site of the double
bond. It is also worth noting that the substitution pattern of
the second (less substituted) double bond influences the
cyclization in several ways: 1) When the second allyl group is
substituted at the terminal alkene carbon,the usual cyclo-
hexane structures are formed selectively (Table 2,entries 7–
10). 2) Substrates with a terminal (mono- or disubstituted)
double bond such as 9a and 11a (Table 2,entries 9 and 11)
yielded cycloheptene and cycloheptane derivatives 9b[26] and
11b,respectively. Whereas the gem-dimethyl cycloheptene
derivative 9b was obtained selectively,in the case of 11a the
reaction was less efficient: instead of a single carbocyclic
product,we could isolate only the bridged bicyclic lactone
derivative 11b in low yields (22%) as the result of a cascade
23
9
–
could be used in 2-mol% quantities under mild conditions.
The rapid and selective cycloisomerization of 1a led to 1b in
88% yield. To our knowledge,this is the first example of a
selective Lewis acid catalyzed cycloisomerization of a highly
(alkene-) substituted 1,6-diene.
A number of 1,6-dienes with different substitution pat-
terns at the double bonds as well as at the homoallylic carbon
were prepared from simple starting materials.[23] Table 2
summarizes some representative examples of this reaction
using a variety of differently substituted substrates 1a–13a.
The conditions were optimized for the different substrates by
using a catalytic amount of Sn(NTf2)4 (5 mol%). Two
solvents,nitromethane and dichloroethane,turned out to be
the best reaction media. The influence of the malonate unit
was first studied with diprenyl-substituted derivatives 1a–6a
(Table 2,entries 1–6). In comparing ethyl and methyl malo-
nates 1a and 2a,the reactions conducted under several
experimental conditions led to similar selectivities and
reactivities. Both cyclohexane derivatives 1b and 2b were
obtained selectively in about 90% yield under mild conditions
(from room temperature to 408C). Under more severe
reaction conditions,a cyclization involving one ester group
became competitive. Thus,in refluxing dichloroethane
(838C),the cycloisomerization of 2a led to the corresponding
cyclohexane derivative 2b in lower yield (58%),together
with a by-product (10% yield),which was identified as the d-
lactone 2c. This lactone by-product is the
À
À
of C C and C O couplings,3) With less substituted (less
nucleophilic) double bonds such as in 12a (Table 2,entry 12),
À
À
C C bond formation is disfavoured,and a C O coupling
reaction occurs between one of the double bonds and one of
the ester groups (as mentioned before) to give 12b in
selectively 70% yield.
Another point of interest was the competition between
À
olefinic and aromatic double bonds in the C C bond-forming
step. When a phenyl group is present as in 13a (Table 2,
entry 13),a double cyclization occurs very selectively afford-
ing tricyclic compound 13b in 78% yield. Each of the double
bonds of the diene reacts preferentially with an aromatic ring
À
by C H activation in
entry 13).[27–28]
a
catalytic process (Table 2,
To evaluate whether other strong acids are also effective
catalysts,the reaction of 1a was carried out in the presence of
HNTf2. The cyclization giving 1b occurred in 80% yield;[29a]
however,no conversion was observed with 3a.[29b] With other
olefins,the presence of this super acid led to partial or
complete polymerization. These results indicate that the
Lewis acid catalyst Sn(NTf2)4 has a different reactivity and is
more effective than the protic acid HNTf2 for the cyclo-
isomerization of highly substituted 1,6-dienes. We tested a
mixture of Sn(NTf2)4/HNTf2 (1:1,5 mol% each) for the
cyclization of 1a in a Brønsted acid assisted Lewis acid
catalyzed process,[30] but the results were not better than those
obtained with Sn(NTf2)4 alone.
result of an intramolecular cyclization
involving one ester unit and one of the
double bonds.[24] It is interesting to note
that this reaction between an alkene and
the ester moiety may become the domi-
nant reaction when less substituted double
bonds (with lower nucleophilicity) are
present in the diene (see Table 2,entry 12).
When one malonate ester group was replaced by a nitrile
(Table 2,entry 3) or an ethyl ketone (entry 4),the corre-
sponding cycloisomerization products were still obtained
selectively in yields of 70–74%. However,the malononitrile
analogue (with two nitrile groups) was unreactive. This may
indicate that the ester oxygen atom(s) play an important role
in the mechanism of the cycloisomerization. Sulfone 5a and
The possibility of a partial hydrolysis of Sn(NTf2)4 by
reaction with traces of water was also considered. Preliminary
kinetic studies carried out with 1a and added water in distinct
quantities. Whereas traces of water had almost no effect on
either the reaction rate or the yield,the reaction rate
decreased notably after the addition of greater amounts of
water (up to 2 equiv H2O with respect to the catalyst). At the
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Chemie
Table 2: Cycloisomerization of 1,6-dienes with Sn(NTf2)4 (5 mol%).
Entry
Substrate
t [h]
Solvent, T [8C]
Conversion [%]
Product (Yield [%])
4
2.5
Cl(CH2)2Cl, 40
CH3NO2, 25
100
100
1b (92)
1b (92)
1
1a
2
3
4
5
2a
3a
4a
5a
5
2
5
5
Cl(CH2)2Cl, 40
100
80
2b (88)
C
C
C
3NO2, 101H
3NO2, 25H
3NO2, 40H
3b (70), 1 isomer
90
4b,c (74), 2 diastereomers
5b (74), 1 isomer
100
6
7
6a
7a
5
3
C
C
3NO2, 101H
3NO2, 60H
59
6b (46), 1 isomer
7b (88), 1 isomer
100
8
9
8a
5
1
1
5
6
Cl(CH2)2Cl, 60
100
100
100
54
8b (70)
9a
C
C
C
C
3NO2, 101H
3NO2, 101H
3NO2, 101H
3NO2, 101H
9b (61)
10
11
12
10a
11a
12a
10b,c (61), 0.69:1
11b (22)
75
12b (70)
13
13a
130
CH3NO2, 101
100
13b (78)
same time,the amount of by-products increased considerably.
The importance of the acidity of the SnIV catalyst system was
further demonstrated after the addition of 2,6-di-tert-butyl-
pyridine as a highly hindered base (Sn(NTf2)4/base 1:1).
Under these conditions,the cyclization of 1a was completely
inhibited.
We do not know yet the exact mechanism of these
cycloisomerizations. However,all our results seem to point in
favor of a carbocationic-type mechanism. The 1,6-dienes with
the more substituted double bonds were the most efficient
and selective substrates in the cyclization reaction. The
product formation may be explained as the result of a first
coordination of the Lewis acid to the more electron-rich
double bond,followed by the coupling of the second less
electron-rich olefin at the more substituted end of the first
olefin (Scheme 1). The ester group present should also
participate in the coordination to the SnIV. Thus,the electro-
philic-type addition of the Lewis acid to the isobutenyl group
of 14 gives an intermediate zwitterion 14a,which is stabilized
À
by the second alkene group in 14b. The ring-closing step (C
C bond formation) is controlled by the stability of the
carbocation intermediates 14c and 14d,which depends on the
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7287

Communications
brine and dried over MgSO4. After filtration and evaporation of the
solvent,the products were purified by flash chromatography on silica
gel. The compounds 1b–13b were characterized by 1H and 13C NMR
spectroscopy,mass spectrometry,and high-resolution MS or elemen-
tal analysis. Compounds 3b, 5b, 6b,and 7b were obtained as single
diastereomers. Compounds 4b,c were obtained as a 1.2:1 mixture of
diastereomers; the configuration could not be determined by NMR
experiments. The cis configuration of 7b was established according to
the Karplus rules. The E configuration of 8b was established by
NOESY NMR experiments.
Received: May 22,2006
Published online: October 6,2006
Keywords: cyclization · dienes · Lewis acids ·
.
six-membered rings · tin
Scheme 1. Cycloisomerization of 1,6-dienes and competition between the
formation of six- and seven-membered rings (LA=Lewis acid).
[1] T. L. Ho, Carbocycle Construction in Terpene Synthesis,Wiley-
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[2] K. C. Nicolaou,S. A. Snyder,T. Montagnon,G. Vassilikogian-
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[5] A notable exception is the Pd-catalyzed cycloaddition: B. M.
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[6] a) B. M. Trost,M. J. Krische, Synlett 1998,1 – 16; b) B. Bogda-
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Chem. 2003, 1,215 – 236.
[8] For representative examples with transition metals,see a) Rh:
A. Bright,J. F. Malone,J. K. Nicholson,J. Powell,B. L. Shaw,
Chem. Commun. 1971,712 – 713; b) R. Grigg,J. F. Malone,
T. R. B. Mitchell,A. Ramasubbu,R. M. Scott, J. Chem. Soc.
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2004, 116,4155 – 4159; Angew. Chem. Int. Ed. 2004, 43,4063 –
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1998, 120,8007 – 8008; g) Ti: S. Okamoto,T. Livinghouse, J. Am.
Chem. Soc. 2000, 122,1223 – 1224.
substitution of the second olefin. Though in most reactions a
six-membered ring formed,cycloheptanes were obtained in
two cases. The ring-size specificity is explained by the
predominance of the more stable (tertiary) cationic carbon
center in 14c and 14d,respectively (Scheme 1).
This mechanistic proposition does not involve protic acids.
However,the role of protons (from residual water) in the
catalytic process cannot be completely excluded.[30] The
combination of Brønsted and Lewis acids has been reported
as a particularly useful tool in several efficient reactions in
asymmetric catalysis.[31] Nevertheless,for the moment we
have no experimental support for the direct interaction of
protons in association with Lewis acids in the cycloisomeriza-
tion of 1,6-dienes.
The enhancement of the electrophilic alkene activation in
dimerization reactions by highly charged transition-metal
complexes has been recently reported and is now well
established.[19] However,the use of Lewis acids for this
purpose is much less developed. The role of tin derivatives in
olefin polymerization has also been emphasized.[32] Catalyst
design is important since the action of strong Lewis acid
anions (OTfÀ,SbF À) combined with cationic palladium
5
complexes leads to quite uncontrolled isomerization process-
es in 1,6-dienes.[33]
[9] a) B. M. Trost,J. M. D. Fortunak, Organometallics 1982, 1,7 – 13;
b) A. F. Barrero,J. E. Oltra,M. Alvarez, Tetrahedron Lett. 1998,
39,1401 – 1404.
[10] a) W. D. Kerber,J. H. Koh,M. R. Gagnꢀ, Org. Lett. 2004, 6,
3013 – 3015; b) C. Liu,X. Han,X. Wang,R. A. Widenhoefer, J.
Am. Chem. Soc. 2004, 126,3700 – 3701.
[11] S. Okamoto,T. Livinghouse, Organometallics 2000, 19,1449 –
1451.
[12] S. Thiele,G. Erker, Chem. Ber. 1997, 130,201 – 207.
[13] K. H. Shaughnessy,R. M. Waymouth, J. Am. Chem. Soc. 1995,
117,5873 – 5874.
In conclusion,we have found that Sn(NTf 2)4 is a powerful
catalyst for the cycloisomerization of malonate-type highly
substituted 1,6-dienes. The present transformation provides
simple and selective procedure for the preparation of highly
functionalized six-membered-ring carbocyclic systems. More-
over,the development of new catalysts for diene cyclo-
isomerization broadens the field of atom-economic cycliza-
tion processes.[34]
[14] a) W. E. Piers,P. J. Shapiro,E. E. Bunel,J. E. Bercaw,
Synlett
Experimental Section
1990,74 – 84; b) also see P. W. Chum,S. E. Wilson, Tetrahedron
Lett. 1976,1257 – 1258.
[15] W. Oppolzer,V. Snieckus, Angew. Chem. 1978, 90,506 – 516;
Angew. Chem. Int. Ed. Engl. 1978, 17,476 – 486.
[16] a) L. F. Tietze,U. Beifuss,M. Ruther,A. Rꢁhlmann,J. Antel,
G. M. Sheldrick, Angew. Chem. 1988, 100,1200 – 1201; Angew.
Chem. Int. Ed. Engl. 1988, 27,1186 – 1187; b) For the conversion
General cyclization procedure: A mixture of the 1,6-diene (1 mmol)
and Sn(NTf2)4 (0.05 mmol) was stirred under nitrogen at the reported
temperature in distilled dichloroethane or nitomethane (5 mL). The
progress of the reaction was monitored by GC analysis. The reaction
products were hydrolyzed by addition of water (10 mL) and extracted
with ether (3 ” 10 mL). The combined organic phases washed with
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Angewandte
Chemie
of 1,7-dienes to give cyclohexanes, see Ref. [4]; c) For the
conversion of 1,5-dienes to give cyclohexanes, see T. Kato, S.
Kumazawa,Y. Kitahara, Synthesis 1972,573 – 575.
[17] V. Jꢂger,W. Kuhn,U. Schubert,
2583 – 2586.
Tetrahedron Lett. 1986, 27,
[18] L. Coulombel,I. Favier,E. Duꢃach,
2286 – 2288.
Chem. Commun. 2005,
[19] C. Hahn, Chem. Eur. J. 2004, 10,5888 – 5899.
[20] I. Favier,E. Duꢃach, Tetrahedron Lett. 2003, 44,2031 – 2032.
[21] A. Vij,R. L. Kirchmeier,J. M. Shreeve,R. D. Verma,
Chem. Rev. 1997, 158,413 – 432.
Coord.
[22] The presence of three nonequivalent CH3 groups,two at a
quaternary carbon (singlets at d = 0.82 and 0.85 ppm) and
another one in a vinylic position (d = 1.7 ppm) was easily
deduced from the 1H NMR spectrum. The two ester ethyl
groups and two vinylic protons (two broad singlets at d = 4.6 and
4.75 ppm) were also magnetically different. The 13C NMR
analysis revealed 17 carbon atoms compatible with the structure
of the pentasubstituted cyclohexane derivative 1b. Complete
1
NMR data for 1b: H NMR (500 MHz,CDCl ): d = 4.78 (1H,
3
brs),4.55 (1H,brs),4.12 (2H,brq, J = 7.2 Hz),4.09 (2H,q, J =
7.2 Hz),2.10 (1H,ddd, J = 13.9,6.3,2.8 Hz),2.03 (1H,dd,
J =
7.5,3.1 Hz),2.03 (1H,dd, J = 4.6,3.1 Hz),1.87 (1H,dd, J = 7.5,
4.6 Hz),1.86 (1H,ddd, J = 13.9,11.2,6.3 Hz),1.64 (3H,s),1.49
(1H,ddd, J = 11.4,11.2,6.3 Hz),1.27 (1H,ddd,
6.3 Hz),1.17 (3H,t, J = 7.1 Hz),1.14 (3H,t, J = 7.1 Hz),0.81
(3H,s),0.78 ppm (3H,s);
13C NMR (126 MHz,CDCl 3): d =
J = 11.4,2.8,
172.86,171.51,146.83,113.27,61.67,61.36,55.95,49.97,39.19,
33.44,32.77,31.13,27.46,24.44,20.44,14.45,14.41 ppm.
[23] See for example: R. Mook,P. M. Sher, Org. Synth. Coll. Vol.
VIII, 1993,381 – 386.
[24] The participation of an ester group in cycloisomerizations has
already been proposed: L. A. Goj,A. Cisneros,W. Yang,R. A.
Widenhoefer, J. Organomet. Chem. 2003, 687,498 – 507.
[25] As we could anticipate,the Sn(IV) catalyst system was
unselective for the cyclization of simple dialkyl diallylmalonates.
The reaction led preferentially to
a mixture of isomeric
unsaturated five-membered-ring compounds.
1
[26] NMR data for 9b : H NMR (500 MHz,CDCl ): d = 5.08 (1H,
3
s),4.11 (4H,q, J = 7.3 Hz),2.55 (2H,s),2.14 (2H,m),1.67 (3H,
s),1.48 (2H,m),1.18 (6H,t,
J = 7.4 Hz),0.93 ppm (6H,s);
13C NMR (126 MHz,CDCl 3): d = 172.38 (2C),136.44,130.28,
61.56 (2C),57.06,44.98,36.10,35.94,30.27 (2C),29.96,28.14,
14.39 ppm (2C); NOESY d = 5.08 ppm correlates with d =
0.93 ppm.
[27] For transition-metal-catalyzed intramolecular hydroarylation of
alkenes,see for example: S. W. Youn,S. J. Pastine,D. Sames,
Org. Lett. 2004, 6,581 – 584.
[28] These alkene arylation reactions will be developed in a separate
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 7289