An efficient catalytic method for fulvene synthesis

Article abstract of DOI:10.1016/j.tet.2011.09.036

The effects of the nature and amount of base, substrate structure, amount of added water and solvent on the condensation of carbonyl compounds with cyclopentadiene in the presence of secondary amines were investigated. Based on these studies, a new efficient and green synthesis of fulvenes was developed.

Full text of DOI:10.1016/j.tet.2011.09.036

Tetrahedron 67 (2011) 8607e8614
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
An efficient catalytic method for fulvene synthesis
,
Necdet Cos¸ kun b, Ihsan Erden a
*
a San Francisco State University, Department of Chemistry and Biochemistry, 1600 Holloway Avenue, San Francisco, CA 94132, USA
b Uludag University, Department of Chemistry, 16059 Bursa, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 August 2011
Received in revised form 10 September 2011
Accepted 12 September 2011
Available online 17 September 2011
The effects of the nature and amount of base, substrate structure, amount of added water and solvent on
the condensation of carbonyl compounds with cyclopentadiene in the presence of secondary amines
were investigated. Based on these studies, a new efficient and green synthesis of fulvenes was developed.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Fulvenes
LFER studies
Green synthesis
Catalysis
Cyclopentadiene
1. Introduction
two-step method appears to be the method of choice for the syn-
thesis of 6-methylfulvene, a substrate most frequently used in or-
Fulvenes constitute an important class of cross-conjugated
olefins with unique electronic, spectroscopic and chemical prop-
erties.1 Though the first fulvene synthesis was reported by Thiele in
1900,2 considerable effort has been directed toward synthetic im-
provements,3,4 since Thiele’s original method based on the con-
densation of aldehydes and ketones with cyclopentadiene in the
presence of alkoxides gives low yields with considerable amounts
of resinous byproducts, presumably due to competing aldol
condensations.
Neuenschwander developed an alternative synthesis of fulvenes
that are not readily accessible by the Thiele procedure.5,6 Alkylation
of sodium cyclopentadienide with 1-chloroalkyl acetates followed
by elimination of acetic acid from acetoxyalkyl cyclopentadienes
with base gives fulvenes in moderate to low yields. However, this
method allows the synthesis and isolation of volatile fulvenes, such
as the unsubstituted parent fulvene, 6-vinylfulvene and 6-
ethynylfulvene since no aqueous work-up is necessary. This three-
step method requires the synthesis of 1-chloroalkyl acetates and is
also less suited for the synthesis of 6,6-dialkyl or diarylfulvenes.
Hafner’s synthesis7 of 6-monosubstituted fulvenes is based on
alkyl or aryl lithium reagents onto 6-dimethylaminofulvene; since
the pyrrolidine variant (Little and Stone, vide infra) with lower MW
aldehydes, such as ethanal and propanal is unsatisfactory, Hafner’s
ganometallic chemistry for fulvene-based metal complexes as well
as materials chemistry.8e12 In this full report we also describe an
exceptionally simple and practical synthesis of the aforementioned
6-methyl and 6-ethylfulvenes that allows their isolation in pure
form without extensive work-up and chromatography (separation
of these volatile fulvenes from chromatography solvents without
significant losses is extremely difficult).
A method for the synthesis of volatile and sensitive fulvenes,
such as the parent fulvene, 6-methyl and 6-ethylfulvene employing
basic ion exchange resin has been reported, however, the authors
were not able to isolate the aforementioned fulvenes from the re-
action mixtures and their formation was inferred from the UV ab-
sorption of possible fulvene contents.13,14
Stone and Little reported a procedure whereby excess pyrroli-
dine was used as base with methanol as solvent and achieved good
yields in the case of aliphatic aldehydes and ketones and aromatic
aldehydes.15 The Stone and Little variant has beenwidely used in the
last 25 years (over 275 citations), and deservedly attained the status
as ‘the method of choice for fulvene synthesis’ from simple alde-
hydes and non-aromatic ketones. Yet it has some limitations mostly
brought on by the fact that (a) at least 1.5 equiv of the highly toxic
and malodorous pyrrolidine and excess cyclopentadiene need to be
used. Some of the other limitations include (b) conjugate addition of
pyrrolidine onto the enone when a,b-unsaturated ketones or alde-
hydes are used although this side reaction may be overcome in some
instances by using diethylamine16 (c) sterically hindered ketones, in
particular aromatic ketones cannot be used; (d) sterically hindered
* Corresponding author. Tel.: þ1 415 338 1627; fax: þ1 415 338 2384; e-mail
address: ierden@sfsu.edu (I. Erden).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.09.036

8608
N. Cos¸kun, I. Erden / Tetrahedron 67 (2011) 8607e8614
enones as well as b-aryl enones undergo Michael attack by the
cyclopentadienide ion.17
The only method utilizing catalytic amount of amine base was
reported by Freiesleben in 1963.3 The author claimed having ach-
ieved near quantitative yields with catalytic HNEt2 (or any other
base, such as methyl-and phenylhydrazine) without solvent.
Among the systems that gave exceptionally high yields were also
Scheme 1. Synthesis of fulvene derivatives.
aromatic ketones, a,b-unsaturated carbonyl compounds, including
methyl vinyl ketone. In the two reports by Freiesleben, there was no
spectral characterization of the products. These claims were later
refuted by a careful study by Neuenschwanderet al.5 who found
that the best yield that was achieved by the Swiss group under
Freiesleben’s conditions was 25% for 6,6-dimethylfulvene (instead
of 99.8%), while in other cases the yields were 0e10%. Even methyl
vinyl ketone was reported by Freiesleben in a patent to give the
corresponding fulvene, however, Griesbeck found that this reaction
leads to a DielseAlder adduct rather than the fulvene.17
In 1995 we reported a survey of fulvene syntheses and imple-
mented existing condensation methods (either Thiele or Little and
Stone) in the syntheses of functionalized fulvenes as well as several
6-vinylfulvenes and explored the scope and limitations of each
method.18 More recently we disclosed a convenient and high yield
synthesis of 6-methyl-6-vinylfulvene by a three-step protocol in an
overall yield of 67% (previously available in only 0.5% yield by the
Thiele procedure).19
In the course of our studies on the synthesis of new function-
alized fulvenes, their implementation in organic synthesis and the
chemistry of the endoperoxide derived from fulvenes20e24 we
needed an improved and environmentally friendly method for
fulvene synthesis. Herein we report new efficient procedures for
the preparation of a wide variety of fulvenes using catalytic pyr-
rolidine in MeOH/H2O that not only give high yields but also min-
imize product losses during extractive work-up and avoid the use of
excess cyclopentadiene whose dimer presents challenging sepa-
ration problems from the desired fulvenes. Under our conditions,
even low molecular-weight fulvenes, such as 6-methyl and 6-
ethylfulvene are prepared and isolated in high yields and for
which the Little procedure proves unsatisfactory. In this report the
scope and limitations of our catalytic variant are also discussed and
alternative routes suggested. The optimization of the method is
based on some mechanistic studies involving effect of base, solvent
and substituent effects.
A series of aromatic aldehydes (Table 1, entries aej, method A)
were subjected to condensation with cyclopentadiene at room
temperature under the conditions mentioned above and the initial
rates were determined at the 0.83rd h. The correlation of the initial
rates (Rx) with Hammett substituent constants
s provided a linear
plot with
r¼ꢀ2.19 (Fig. 1). As the plot clearly indicates, electron-
donating groups accelerate the reaction while electron-withdraw-
ing substituents decelerate it. In the cases of strong electron
withdrawing groups, such as p-, and m-NO2 groups the reactions do
not proceed at all. The observed reactivity order of the aldehyde
series was surprising since results from a previous study revealed
that the reactivity order with nitrogen nucleophiles in THF is op-
posite to the one observed here.25 Therefore, we followed the
equilibria for hemiacetal formations of aromatic aldehydes 2a,b,
gei in MeOD at room temperature in order to understand this re-
versal of reactivity (Table 3). The data revealed that in the cases of
aromatic aldehydes with strong electron-withdrawing groups
hemiacetal formation equilibria are shifted toward right, which
Table 1
Method development study on effects of NEt3 and molecular sieves on pyrrolidine-
catalyzed fulvene formation rates
Entry
R
R1
Method A:a % 0.83 h
(after completion)
Method B:b % 0.83 h
(after completion)
a
b
c
d
e
f
g
h
i
H
H
H
H
H
H
H
H
H
H
Me
Me
Ph
H
4-MeC6H4
4-MeOC6H4
4-Me2NC6H4
4-HOC6H4
3-MeOC6H4
3,4-(MeO)2C6H3
C6H5
4-ClC6H4
4-NO2C6H4
3-NO2C6H4
Me
47(98)
70(97)
50(100)
71(93)
99(94)
67(97)
f
9(85)
f
35(96)
79(99)
50(95)
18(96)
39c(30)h
45c(31)
86 (89)
(82)e
47(98)
23(96)
6(96)
025
0
j
k
l
69
4-MeC6H4
Ph
i-Propyl
0d
m
n
o
p
t
u
w
x
0d
0
2. Results and discussion
2.1. Effect of base
35
55
64(88)
80(90)
77(85)
R¼R1¼cyclobutyl
R¼R1¼cyclopentyl
51
Me
Me
H
(CH2)2CO2H
(CH2)3CO2H
2-BrC6H4
74g
87g
The use of cyclopentadiene in excess causes the formation of
dicyclopentadiene, which is troublesome in the purification pro-
cess as well. Moreover, we found that the use of excess pyrrolidine
prevents the crystallization of solid fulvenes in the aromatic al-
dehyde series. Thus, we decided to test whether less than stoi-
chiometric amounts of pyrrolidine might be enough for the
formation of the iminium ion intermediate and the use of NEt3
would help the condensation/dehydration step in acceptable
yields and rates. The reaction of 1a was first conducted only in the
presence of triethylamine, however no fulvene formation was
observed after 3 h stirring at room temperature. Pyrrolidine
(10 mol %) was then added and the reaction was followed by 1H
NMR; after 1 h the conversion was only 15%. However, performing
the reaction in the presence of pyrrolidine and molecular sieves
accelerated the reaction significantly (47% after 1 h). The reaction
was quantitative after stirring overnight at room temperature.
Moreover, the isolation of the product consisted of only vacuum
filtration of the crude mixture and evaporation of the solvent to
give pure fulvene (Scheme 1).
(50)
(45)
H
4-CF3C6H4
a
The reactions completed within 24 h except entry d where the conversion was
88%.
b
The reactions completed within 5 h except in the case of 1 h, which was stirred
overnight for completion.
c
The reactions were performed in acetonitrile with 15 mol % pyrrolidine and
0.3 g/mmol molecular sieves and the data are for the third h.
d
Addition of 10 mol % of DBU did not result in fulvene formation after 24 h
stirring at room temperature.
e
Method B with 3 equiv of added pyrrolidine as catalyst; reaction time 48 h. The
yield of the product when no MS was used is 79%.
f
Reactions not performed; method A: 5 mmol carbonyl compound, 2.5 equiv
ꢀ
cyclopentadiene (CP), 1.5 g 3 A molecular sieves, 10 mol % pyrrolidine, 1.5 equiv
NEt3, 5 mL MeOH; method B: same as A, no NEt3, 1.2 equiv CP.
g
Method A without molecular sieves with 20 mol % of added pyrrolidine as
catalyst; isolated yield after overnight stirring and regular aqueous work-up was
performed (see Experimental section).
h
The reaction according to method B gave 10% fulvene. The main product
cyclopenta-1,3-dien-1-yl(4-nitrophenyl)methanol and cyclopenta-1,4-dien-1-yl(4-
nitrophenyl)methanol (1:1) was isolated in 40% yield after 18 h stirring at room
temperature.

N. Cos¸kun, I. Erden / Tetrahedron 67 (2011) 8607e8614
Table 2 (continued)
8609
Table 2
Method C:a optimal conditions (% isolated yields)
Entry
Carbonyl
Fulvene
(%)
25
Entry
a
Carbonyl
Fulvene
(%)
98
w
x
30
b
c
97
98
a
Method C: same as B, no molecular sieves, 5 mL MeOH/H2O (4:1).
b
To a solution of 1o (20 mmol) and CP (10.0 mmol), in MeOH/H2O (10 mL 7/3)
pyrrolidine (10 mol %,) was added, and the reaction mixture stirred for 2.5 h at 15 ꢁC.
The mixture was poured into an ice-cold mixture of brine solution (10 mL) containing
AcOH (1 mmol). The upper oil phase was separated and dried over molecular sieves.
c
To a solution of 2s (5 mmol) and CP (10 mmol) in 5 mL of methanol/water (4:1),
pyrrolidine (10 mmol, 2 equiv) was added, and the mixture stirred at room tem-
perature overnight. The mixture was subjected to flash chromatography on silica gel
to give 3s in 44% yield.
eb
95
98
d
Due to anticipated greater water solubility of product, extractive work-up was
used.
e
Pyrrolidine (3 equiv) was used: reaction time 24 h.
f
gb
97
86
hb
kb
77
11
le
nb
90
90
ob
CH3CHO
pb
qb
85
92
rb
95
44
sb,c
vb,d
60
Fig. 1. Hammett plot of initial rate log(RX) of fulvene 3a,b,feh (top graph) and hemiacetal
formation equilibrium constants (log KX) of 4a,b, gei versus
s constants (The correlation
equations are log(RX)¼ꢀ2.19
s
ꢀlog(RX¼H); r2¼0.99 and log KX¼2.16
s
ꢀlog KX¼H; r2¼0.98,
respectively).26

8610
N. Cos¸kun, I. Erden / Tetrahedron 67 (2011) 8607e8614
Table 4
explains the reversed reactivity order in the fulvene formations.
The Hammett type plot (Fig. 1) of log K versus constants produced
constants with the same magnitude as for the fulvene formations
Determination of rate law for the pyrrolidine (5 mol %)-catalyzed fulvene formation
between 2b and 1 in MeOH
s
r
2b (mol Lꢀ1
)
1 (mol Lꢀ1
)
Initial ratesb
a
but with opposite sign (Scheme 2).
1.0
1.0
2.0
1.0
2.0
1.0
2.12
3.53
4.59
Table 3
Substituent effect on the hemiacetal formation equilibria of aromatic aldehydes with
MeOD
a
Initial concentration.
2
s
log Ka
b
The initial rates (mol Lꢀ1 hꢀ1) were determined at the 10th min of the reactions
at room temperature.
a
b
g
h
i
ꢀ0.17
ꢀ0.27
0
0.23
0.78
ꢀ1.510
ꢀ1.440
ꢀ1.227
ꢀ0.989
ꢀ0.288
ꢀ1.510
ꢀ1.996
ꢀ1.061
ꢀ0.630
0.477
ꢀ1.495
ꢀ1.880
ꢀ0.974
ꢀ0.568
0.477
If this conclusion is correct, the reaction without NEt3 should be
much faster because the presence of base may shift the equilibrium
from E to C by deprotonating E and/or engaging cyclopentadiene in
a hydrogen bond (for solvent effect on the catalyst elimination, see
Scheme 4).
a
K¼[4]/[2]; K for 1 h, 3.5 h and 24 h.
Scheme 2. Hemiacetal formation equilibria of aromatic aldehydes.
The correlations discussed above impelled us totake a closer look
at the pyrrolidine-catalyzed fulvene formation reaction mechanism.
The Hammett correlations performed confirm the assumption
that the rate-determining step of the reaction must involve the
formation of a positively charged species. The question was
whether it is solely the iminium salt C or the CeN bond heterolysis
in the transformation of D to 3. The other possibility was that both
steps may affect the reaction rate. Our observations with other CeH
acids and the aromatic aldehyde series where the addition products
are detectable clearly revealed that the addition step of the reaction
is faster in the cases of electron-withdrawing substituents. How-
ever the substituent effect is reversed for the elimination step of the
reaction. The same effect of strong electron-withdrawing sub-
stituents was observed under the conditions of methods B and C.
The competing formations of cyclopenta-1,3-dien-1-yl(4-aryl)
methanol and cyclopenta-1,4-dien-1-yl(4-aryl)methanols 5 were
monitored by TLC as well as 1H NMR. To exclude the possibility of
formation of 4 by water addition to the corresponding fulvenes, 3x
was stirred in MeOD in the presence of 20% water and 50 mol %
pyrrolidine. No hydration was observed after 24 h. This implies that
in the cases of strong electron-withdrawing groups amine dis-
placement competes with amine elimination (Scheme 3). Similarly,
the reaction of 4-nitrobenzaldehyde with cyclopentadiene in the
presence of only NEt3 in MeOH-d4 did not give compounds 4i after
24 h stirring at room temperature. The experiments summarized in
Table 4 support a second order reaction with rate¼k[2]a[1]b where
a and b are ca. 1.1 and 0.73, respectively. In the present case, the
observed substituent effect actually confirms the fact that the rate-
limiting step for the overall fulvene formation is the elimination
reaction of D to give 3 and the catalyst (Table 4).
Scheme 4. Solvent role in catalytic cycle.
To understand the role of NEt3 on the reaction rate we set up
a series of experiments reacting the carbonyl compounds with
slight excesses of cyclopentadiene in the presence of only 10 mol %
pyrrolidine and molecular sieves (Table 1, method B). To our sur-
prise the absence of NEt3 enhanced the fulvene formation rates.
Again the correlation of the initial rates for the aromatic aldehyde
series with Hammett
s constants provided a linear plot with slope
(r
) with a lower absolute value (Fig. 2). The comparison of the
Scheme 3. Equilibria in pyrrolidine-catalyzed condensation of carbonyl compounds
with cyclopentadiene.
Fig. 2. Hammett plot of initial rate according to method B log(RX) of fulvene 3a,b, gei
versus
s
constants. The correlation equation is log(RX)¼ꢀ1.34
s
ꢀlog(RX¼H) r2¼0.97.27

N. Cos¸kun, I. Erden / Tetrahedron 67 (2011) 8607e8614
8611
reaction susceptibility constants implies that the presence of excess
amine decelerates the reaction. This may arise from the direct in-
teraction of intermediate D with the amine or an effect on the
iminium salt formation. Therefore the use of molecular sieves
makes an important difference in the method with a base. In the
absence of NEt3 the rate-limiting step is probably the elimination
reaction of D to 3. If so, the use of molecular sieves should not have
an effect on the reaction rate (Table 5).
MeOH (Table 6). The result was dramatic: water did not lower the
rate. Rather, dramatic rate acceleration was observed at 20% water
concentration in MeOH. This clearly proves that the rate-
determining step in the reactions without additional base involves
the elimination step. However, it seems that this is not the case
when stericallyencumbered ketones and aldehydes are used (Tables
1 and 2, entries l,w) where the presence of water decelerates the
reactions (e.g., the conversions according to method C are 11 and
25% while according to B they are 82 and 50%, respectively, for en-
tries l and w). This is most probably due to the fact that the iminium
ion formation equilibrium is being shifted to the left.
Table 5
Effect of molecular sieves amount and the pyrrolidine concentration on the reaction
rate
Entrya
MS (g/mmol)
Cat. (mol %)
Solvent
% Conversioni
Table 6
Effect of water concentration on the reaction rate in the absence of NEt3
1
2
3
4
5
6
7
8
0
10 catb
10 catb
10 catb
10 catb
10 catb
10 catb
10 catb
5 catb
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
CH3CN
DMSO
Ether
80
74
78
85
84
84
84
54
11
0
100
100
100
100
100
100
100
84
Cata (mol %)
% Conversion (0.25 h)
0.1
0.2
0.3
0.4
0.5
0.7
0
0
0
0
0
0
0
0
0
0
0
0
H2O (concn %)
0
2.5
5
10
20
2.5
10
10
10
10
10
35b
35
37
39
62
81
100
58
0
200
9
1 catb
28
0
0
10
11
12
13
14
15
16
17
18
19
0.1 catb
0.01 catb
40 catc
80 catc
10 catb
10 catb
10 catb
10 catb
10 catb,d
10 catb,d
a
Pyrrolidine.
b
0
0
Product 3a crystallized in the reaction medium in all cases except the last entry
0.5e
at the 0.5 h and the reactions were completed when checked at 4.5 h by 1H NMR.
5f
23
19
0
55
48
0
0
1
89g
88g
0
In addition to the accelerating effect discussed above, the use of
MeOH/H2O proved to be of crucial importance in achieving excel-
lent yields in amine-catalyzed synthesis of fulvenes derived from
unbranched aliphatic aldehydes by suppressing substrate de-
composition due to aldol condensation.
THF
Ether
THF
0
0
0
0
4h
4h
1
a
All entries are for the reactions of 2b with cyclopentadiene in 1:1.2 ratios at
room temperature.
b
Catalyst pyrrolidine.
Catalyst DEA.
MeOH (200 mol %).
The yield after 24 h is 39.
66% after 24 h.
100% after 24 h.
20% after 24 h.
Conversion at 0.5 h, 1.7 h and 5 h, respectively.
c
2.3. Effect of carbonyl structure and limitations of the amine
base promoted fulvene syntheses
d
e
f
The structure of the carbonyl component appears to dictate the
success of our amine catalyzed variants of fulvene syntheses. With
aldehydes and sterically unencumbered aliphatic ketones pyrroli-
dine catalysis (method C) appears to be the ideal method. As the
steric hindrance increases in the ketone, the use of larger quantities
of pyrrolidine (transition to LittleeStone procedure) becomes
necessary. As a test substrate for a sterically hindered ketone we
used 2-methylcyclohexanone (Table 1, entry s). Whereas methods A
and B failed to affect fulvene formation, finally the use of 2 equiv of
pyrrolidine (as compared to 5 equiv in Little’s procedure) gave the
corresponding fulvene in 44% isolated yield. Benzophenone is
unreactive under amine base catalysis as well as stoichiometric (or
excess base) conditions. However, the use of excess (e.g., 3 equiv)
pyrrolidine provided the synthesis of 6-methyl-6-aryl fulvene from
the corresponding acetophenone in good yield according to
method B and also according to Little’s conditions, providing that
the reaction times are prolonged (Table 1, entry l). In the cases of
diaryl ketones the classical Thiele procedure or a variant thereof
with alkoxide base is the method of choice. In the case of keto acids
(entries t and u) as well as phenolic substrates (e.g., Table 1, entry d)
the best results were obtained with method A (without molecular
sieves), still avoiding the use of excess pyrrolidine.
Another important outcome of our studies is the fact that our
catalytic method is ideally suited for the synthesis and isolation of
fulvenes derived from unbranched aldehydes, in particular those
derived from acetaldehyde and propionaldehyde.29 The stoichio-
metric (or multiple equivalents) pyrrolidine method using un-
branched aldehydes fails to give satisfactory results is presumably
due to the fact that excess pyrrolidine in methanol solution results
in considerable enamine formation, which induces intermolecular
reactions (i.e., aldol condensations) thus rendering the method
g
h
i
Diethylamine was also tested for its catalytic activity (entries 12
and 13) and found that it is less effective than pyrrolidine. However,
in spite of the higher concentrations required in some cases, es-
pecially in the synthesis of more volatile fulvenes, diethylamine
could be preferred due to its lower boiling point.
2.2. Solvent effects
Finally, we thought that a study of the solvent effect on the re-
action rate would shed some light on the mechanism of the reaction.
The solvent series was chosen among those having polarity above
and below that of MeOH (entries 1 and 14e17). The reaction rate in
acetonitrile was only 1/4th of the rate in MeOH though the polarity
of the former is very close to that of MeOH. The rate in much more
polar DMSO was again similar to that in acetonitrile. In aprotic sol-
vents as ether and THF no reaction was detected. However, addition
of small amounts of MeOH to the latter mixtures provided the for-
mation of 3b (Table 5, entries 18e19). This clearly proves the cata-
lytic activity of protic solvents in the fulvene formation reactions.
Having determined the importance of the solvent to be necessarily
protic we have postulated a role for it as depicted in Scheme 4. In-
termediate D is in equilibrium with E, which probably undergoes
concerted or near concerted elimination to 3 and the catalyst.28
If this postulate is correct, the presence of the more acidic and
sterically less demanding water should accelerate the reaction. To
prove this we reacted 2a with cyclopentadiene in water containing

8612
N. Cos¸kun, I. Erden / Tetrahedron 67 (2011) 8607e8614
unsuitable for these systems. We found that this difficulty can be
ameliorated by working in a methanol/H2O mixture. It is therefore
not surprising to note that particularly 6-alkylfulvenes with short
alkyl chains (C1 and 2) that are popular starting materials used in
organometallic chemistry are still prepared by the three-step Haf-
ner procedure7 by way of alkyllithium addition to 6-N,N-dime-
thylaminofulvene instead of the Little procedure. Our catalytic
method now offers an excellent direct alternative to Hafner’s
multistep procedure for the synthesis of the aforementioned 6-
alkylfulvenes.
prepared in this study are known compounds, their high resolution
NMR spectra are included in the Supplementary data since they are
not available in literature in several cases. The initial rates of the
reactions used in Hammett30 correlations were measured at 23
(ꢂ1) ꢁC. The change of fulvene or hemiacetal concentrations within
the specified periods of time were monitored by 1H NMR by in-
tegrating the appropriate signals of the products and the starting
carbonyl compounds.
4.2. Synthesis of (2-ylidene)cyclopenta-1,3-dienes 3. Method
A. General procedure
3. Conclusions
To a solution of aldehyde or ketone (5 mmol) and cyclo-
pentadiene (12.5 mmol, 0.8262 g, 1.025 mL) in reagent grade
methanol (5 mL), molecular sieve (3 A, 1.5 g) was added; to this
solution pyrrolidine (10 mol %, 0.0355 g) in NEt3 (7.24 mmol,
0.733 g) was added dropwise within 10 min. The mixture was
stirred under nitrogen overnight. The mixture was filtered through
sintered glass funnel and the solvent evaporated under vacuum to
produce the corresponding fulvene in quantitative yield.
A systematic investigation of the initial rates of sec-amine cat-
alyzed fulvene formations with or without additional NEt3 and
correlations with substituent constants allowed elucidating the
mechanism of these reactions. Use of excess secondary amine as
well as additional NEt3 suppressed the reaction rates. In the pres-
ence of added NEt3, the use of molecular sieves as water capturing
agent was shown to be necessary. Our studies have established the
following facts: (a) protic solvents, such as MeOH and water ac-
celerate the reaction of cyclopentadiene with carbonyl compounds;
(b) the presence of tertiary amines retards reaction rates; (c)
electron-donating substituents on the phenyl ring accelerate rates.
Based on our mechanistic studies a mechanism involving the in-
teraction of cyclopentadiene with the preformed iminium salt ap-
pears reasonable. Based on the studies on the effects of base, MS
and solvent we have developed an efficient, catalytic method for
the synthesis of a variety of fulvenes. For crystalline products
method C is exceptionally simple and practical. The use of MeOH/
H2O provides the formation of products directly isolable in excel-
lent purity and nearly quantitative yields by vacuum filtration. For
liquid fulvenes the procedure is almost as simple. The isolation of
the products involves the addition of a specified amount of brine
solution containing an equivalent of AcOH at 0 ꢁC resulting in the
separation of the organic phase containing practically pure (>95%
by 1H NMR) fulvene that can be pipetted out. Our amine-catalyzed
procedure for fulvene synthesis also boasts the added advantage of
providing a green alternative to the existing base-promoted
methods. Moreover, our alternative methods A and B may be
employed in certain cases with success when a phenolic OH or
carboxylic acid functionality is present without sacrificing the
catalytic use of pyrrolidine. Thus, a wider range of fulvenes, in-
cluding 6-alkylfulvenes have now become accessible by our cata-
lytic method.
ꢀ
4.3. Synthesis of (2-ylidene)cyclopenta-1,3-dienes 3. Method
B. General procedure
To a solution of aldehyde or ketone (5 mmol) and cyclo-
pentadiene (6.0 mmol, 0.3966 g, 0.492 mL) in reagent grade
ꢀ
methanol (5 mL) molecular sieve (1.5 g, 3 A) was added, followed by
dropwise addition of pyrrolidine (0.0355 g, 10 mol %). The mixture
was filtered through sintered glass funnel and the solvent evapo-
rated under vacuum to yield the corresponding fulvene in quanti-
tative yield.
4.4. Synthesis of (2-ylidene)cyclopenta-1,3-dienes 3. Method
C. General procedure
To a solution of aldehyde or ketone (5 mmol) and cyclo-
pentadiene (0.3966 g, 6.0 mmol/0.492 mL for the crystalline ful-
venes or 0.4 mL/5 mmol for the aliphatic substrates) in MeOH/H2O
(5 mL 4/1) pyrrolidine (0.0355 g, 10 mol %) was added. The formed
crystals were filtered, washed with small amounts of cold MeOH/
H2O mixture and dried in air. For the isolation of oily products the
reaction mixture was transferred to an ice-cold mixture of brine
solution and 10 mol % AcOH in a narrow graduated cylinder. The
organic phase pure was pipetted out. Drying over molecular sieves
produced practically pure (>95%) fulvene.
4. Experimental section
4.1. General
4.4.1. 1-(Cyclopenta-2,4-dienylidenemethyl)-4-methylbenzene
(3a). (Method C). Orange crystals, mp 60e61 ꢁC. Lit.31 mp 70.5 ꢁC
1H NMR (500 MHz, CDCl3):
d
7.54 (d, J¼8.1 Hz, 2H), 7.26 (d, J¼8.0 Hz,
2H), 7.23 (s, 1H), 6.76 (d, J¼5.3 Hz, 1H), 6.71e6.68 (m, 1H),
Melting points were determined in open class capillary tubes
and are uncorrected. 1H and 13C NMR spectra were recorded on
300 MHz and 500 MHz NMR spectrometers, using CDCl3 as solvent
and TMS as internal standard, unless specified otherwise. Most
column chromatographic separations were carried out on a flash
6.55e6.52 (m, 1H), 6.36 (dt, J¼5.1, 1.7 Hz, 1H), 2.43 (s, 3H). 13C NMR
(126 MHz, CDCl3):
d 145.2, 140.2, 139.2, 135.9, 134.8, 131.5, 131.1,
130.2, 128.0, 120.9, 22.1.
4.4.2. 1-(Cyclopenta-2,4-dienylidenemethyl)-4-methoxybenzene
chromatography system using 40e60
mm silica gel columns using
(3b). (Method C). Orange crystals, mp 68e70 ꢁC. 1H NMR
ethyl acetate/n-hexane solvent mixtures. For preparative TLC,
Merck silica gel (grade 60 PF254) was used. All reactions were
conducted under an atmosphere of dry nitrogen or argon. Non-
deuterated solvents were dried and distilled prior to use. Fresh
commercial samples of ethanal (free of trimer) and propanal and 2-
methylpropanal were used for fulvene synthesis. Pyrrolidine is an
exceptionally foul-smelling and toxic compound and should be
handled with care in a well-ventilated hood. Fulvenes are oxygen-
and heat-sensitive compounds; all reactions should be carried out
under a nitrogen or argon atmosphere. Though all fulvenes
(500 MHz, CDCl3):
d
7.61 (d, J¼8.6 Hz, 2H), 7.19 (s, 1H), 6.97 (d,
J¼8.8 Hz, 2H), 6.76 (d, J¼5.3 Hz, 1H), 6.69 (ddd, J¼5.2, 3.5, 1.6 Hz,
1H), 6.53e6.49 (m, 1H), 6.35 (dt, J¼5.1, 1.8 Hz, 1H), 3.88 (s, 3H). 13C
NMR (126 MHz, CDCl3):
d 161.4, 144.0, 139.0, 135.7, 133.2, 130.52,
130.3, 128.1, 120.6, 115.0, 56.1.
4.4.3. 4-(Cyclopenta-2,4-dienylidenemethyl)-N,N-dimethylbenzen-
amine (3c). (Method C). Orange crystals, mp 104e105 ꢁC. Lit.31 mp
106.5 ꢁC 1H NMR (500 MHz, CDCl3):
d
7.62 (d, J¼8.7 Hz, 2H), 7.18 (s,
1H), 6.85 (d, J¼5.2 Hz, 1H), 6.74 (d, J¼9.0 Hz, 2H), 6.69 (ddd, J¼5.2,

N. Cos¸kun, I. Erden / Tetrahedron 67 (2011) 8607e8614
8613
3.6, 1.6 Hz, 1H), 6.52e6.45 (m, 1H), 6.37 (dt, J¼5.0, 1.8 Hz, 1H), 6.37
2.3 Hz, 2H), 2.25 (s, 6H). 13C NMR (126 MHz, CDCl3):
131.4, 121.3, 23.8.
d
150.8, 143.3,
(dt, J¼5.0, 1.8 Hz, 1H), 3.07 (s, 6H). 13C NMR (126 MHz, CDCl3):
d
151.8, 141.5, 140.7, 134.4, 133.6, 128.9, 128.3, 125.4, 120.2, 112.7,
40.8.
4.4.12. 1-(1-(Cyclopenta-2,4-dien-1-ylidene)ethyl)-4-methylbenzene
(3l). Prepared according to method B by using 3 equiv of pyrroli-
4.4.4. 4-(Cyclopenta-2,4-dienylidenemethyl)phenol (3d). (Method
dine. Reddish oil. 1H NMR (300 MHz, CDCl3):
d
7.31 (2H, d, J¼8 Hz),
A). Solid, does not melt below 260 ꢁC 1H NMR (300 MHz, CDCl3):
7.21 (2H, d, J¼8 Hz), 6.67e6.62 (1H, m), 6.57 (1H, d, J¼5 Hz), 6.49
d
7.54 (d, J¼8.6 Hz, 2H), 7.16 (s, 1H), 6.88 (d, J¼8.6 Hz, 2H), 6.72 (d,
(1H, d, J¼5 Hz), 6.24e6.20 (1H, m), 2.54 (3H, s), 2.40 (3H, s).13C NMR
J¼5.2 Hz,1H), 6.67 (dd, J¼5.2,1.6 Hz,1H), 6.50 (d, J¼4.9 Hz,1H), 6.33
(75 MHz, CDCl3): d 150.0, 143.0, 139.0, 138.2, 131.5, 131.2, 129.2,
(dd, J¼3.3, 1.7 Hz, 1H). 13C NMR (75 MHz, CDCl3):
d
156.6, 143.2,
128.5, 123.7, 121.0, 22.5, 21.2.
138.1, 134.9, 132.6, 129.8, 129.6, 127.3, 119.8, 115.7. ESI-HRMS cal-
culated for C12H11Oþ (MþHþ) 171.0804, found 171.0895.
4.4.13. 5-(2-Methylpropylidene)cyclopenta-1,3-diene (3n). (Method
C). Yellow oil. 1H NMR (300 MHz, CDCl3):
d
6.54 (t, J¼3.6 Hz, 2H),
4.4.5. 1-(Cyclopenta-2,4-dienylidenemethyl)-3-methoxybenzene
6.48 (d, J¼5.0 Hz, 1H), 6.26 (d, J¼10.1 Hz, 1H), 6.23e6.19 (m, 1H),
(3e). (Method C). 1H NMR (300 MHz, CDCl3):
d
7.34 (t, J¼7.9 Hz,1H),
3.11e2.94 (m, 1H), 1.15 (d, J¼6.6 Hz, 6H). 13C NMR (75 MHz, CDCl3)
7.19 (d, J¼8.7 Hz, 2H), 7.13 (s, 1H), 6.93 (dd, J¼8.2, 2.1 Hz, 1H), 6.69
d 149.7, 143.4, 132.8, 130.6, 125.8, 119.1, 30.3, 23.0.
(dt, J¼5.3, 3.5 Hz, 2H), 6.53 (d, J¼5.0 Hz, 1H), 6.34 (dd, J¼3.4, 1.7 Hz,
1H), 3.86 (s, 3H). 13C NMR (75 MHz, CDCl3):
d
159.6, 145.3, 138.0,
4.4.14. 5-Ethylidenecyclopenta-1,3-diene (3o). (Method C). Yellow
135.4, 130.9, 129.6, 127.1, 123.2, 120.3, 115.56, 114.9, 55.2.
oil. 1H NMR (300 MHz, CDCl3):
d
6.61 (t, J¼1.6 Hz, 2H), 6.58e6.48
(m, 2H), 6.26 (dt, J¼5.1, 1.7 Hz, 1H), 2.21 (d, J¼7.3 Hz, 3H). 13C
4.4.6. 4-(Cyclopenta-2,4-dienylidenemethyl)-1,2-dimethoxybenzene
NMR (75 MHz, CDCl3):
16.7.
d 146.9, 137.8, 132.9, 130.6, 125.4, 118.8,
(3f). (Method C). Orange crystals, mp 72e73 ꢁC. Lit.32 mp 72 ꢁC 1H
NMR (500 MHz, CDCl3):
d 7.23e7.19 (m, 2H), 7.18 (s, 1H), 6.93 (d,
J¼8.2 Hz, 1H), 6.75 (d, J¼5.3 Hz, 1H), 6.70 (ddd, J¼5.2, 3.5, 1.6 Hz,
4.4.15. 5-Propylidenecyclopenta-1,3-diene (3p). (Method C). Yellow
oil. Yield 85%. 1H NMR (300 MHz, CDCl3):
1H), 6.51 (dd, J¼3.7, 1.4 Hz, 1H), 6.35 (dt, J¼5.1, 1.8 Hz, 1H), 3.95 (s,
d
6.55 (dd, J¼4.7, 3.5 Hz,
3H), 3.95 (s, 3H). 13C NMR (126 MHz, CDCl3):
d
151.1, 149.8, 144.2,
2H), 6.45 (dd, J¼18.2, 6.5 Hz, 2H), 6.23 (dd, J¼3.5, 1.6 Hz, 1H), 2.57
139.2, 135.8, 130.6, 130.5, 128.2, 125.5, 120.5, 113.9, 111.9, 56.7, 56.6.
(p, J¼7.6 Hz, 2H), 1.17 (t, J¼7.5 Hz, 3H). 13C NMR (75 MHz, CDCl3):
d
145.2, 144.6, 132.9, 130.7, 125.6, 118.9, 24.3, 14.0.
4.4.7. 1-(Cyclopenta-2,4-dienylidenemethyl)benzene (3g). (Method
C). 1H NMR (300 MHz, CDCl3):
d
7.61 (d, J¼6.9 Hz, 2H), 7.51e7.32 (m,
4.4.16. 5-Cyclobutylidenecyclopenta-1,3-diene (3q). (Method C).
Yellow oil. 1H NMR (300 MHz, CDCl3):
6.45 (d, J¼5.2 Hz, 2H),
3H), 7.24 (s, 1H), 6.76e6.64 (m, 2H), 6.54 (d, J¼5.1 Hz, 1H), 6.35 (dd,
d
J¼3.4, 1.7 Hz, 1H). 13C NMR (75 MHz, CDCl3):
d
145.2, 138.1, 136.7,
6.31 (dd, J¼4.4, 2.0 Hz, 2H), 3.14 (t, J¼7.8 Hz, 4H), 2.18 (p,
135.4, 130.8, 130.6, 128.9, 128.6, 127.1, 120.3.
J¼7.9 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d 160.3, 138.7, 130.2,
119.8, 32.2, 16.9.
4.4.8. 1-Chloro-4-(cyclopenta-2,4-dienylidenemethyl)benzene
(3h). (Method C). Red crystals, mp 43e45 ꢁC. 1H NMR (300 MHz,
4.4.17. 5-Cyclopentylidenecyclopenta-1,3-diene (3r). (Method C).
CDCl3):
d
7.52 (d, J¼8.5 Hz, 2H), 7.40 (d, J¼8.5 Hz, 2H), 7.16 (s, 1H),
Yellow oil. 1H NMR (300 MHz, CDCl3):
d
6.44 (dt, J¼6.2, 2.9 Hz, 4H),
6.68 (dt, J¼13.4, 3.3 Hz, 2H), 6.54 (d, J¼4.8 Hz, 1H), 6.33 (dd, J¼3.6,
2.84 (t, J¼7.2 Hz, 4H), 1.91e1.73 (m, 4H). 13C NMR (75 MHz, CDCl3):
1.6 Hz, 1H). 13C NMR (75 MHz, CDCl3):
135.1, 131.7, 131.1, 128.9, 127.1, 119.9.
d
145.6, 136.4, 135.8, 135.1,
d 162.5, 138.3, 130.0, 121.3, 33.0, 26.0.
4.4.18. 1-(Cyclopenta-2,4-dien-1-ylidene)-2-methylcyclohexane
4.4.9. 1-(Cyclopenta-2,4-dien-1-ylidenemethyl)-4-nitrobenzene
(3i). To a solution of 4-nitrobenzaldehyde (5 mmol) and cyclo-
pentadiene (6.0 mmol, 0.3966 g, 0.492 mL) in reagent grade
(3s). (Method C). Yellow oil. 1H NMR (300 MHz, CDCl3):
d
6.70e6.45 (m, 4H), 3.48e3.32 (m,1H), 2.94 (d, J¼14.0 Hz,1H), 2.49
(td, J¼13.7, 4.9 Hz, 1H), 2.06e1.93 (m, 1H), 1.86e1.43 (m, 5H), 1.29
(d, J¼7.2 Hz, 3H). 13C NMR (75 MHz, CDCl3):
d 162.4, 139.2, 130.6,
ꢀ
acetonitrile (5 mL) molecular sieve (1.5 g, 3 A) was added, fol-
lowed by dropwise addition of pyrrolidine (0.0532 g, 15 mol %).
The mixture was stirred overnight and filtered through sintered
glass funnel and the solvent evaporated under vacuum. The
mixture was separated by flash chromatography using silica
packed column and hexane/ethyl acetate as eluent. For further
purification preparative TLC was used. Reddish oil. 1H NMR
120.1, 119.5, 35.1, 33.7, 28.9, 28.4, 20.5, 19.5.
4.4.19. 4-(Cyclopenta-2,4-dien-1-ylidene)pentanoic acid (3t). (Method
A without MS). Yellow solid mp 65e67 ꢁC. Lit.12 mp 66e67 ꢁC. 1H
NMR (300 MHz, CDCl3):
d
10.9 (br s, 1H), 6.52 (d, J¼5.7 Hz, 4H),
2.95e2.82 (m, 2H), 2.62 (dd, J¼8.8, 7.0 Hz, 2H), 2.22 (s, 3H). 13C NMR
(500 MHz, CDCl3):
d
8.27 (1H, d, J¼9 Hz), 7.70 (1H, d, J¼9 Hz),
(75 MHz, CDCl3):
31.4, 20.5.
d 178.9, 149.6, 143.4, 131.5, 131.3, 120.7, 120.1, 33.4,
7.20 (1H, s), 6.81e6.63 (1H, m), 6.57 (1H, dd, J¼5, 2 Hz),
6.38e6.26 (1H, m). 13C NMR (126 MHz, CDCl3):
d 148.6, 147.8,
143.3, 137.5, 134.6, 132.9, 131.2, 127.2, 124.1, 120.1.
4.4.20. 5-(Cyclopenta-2,4-dien-1-ylidene)hexanoic acid (3u). (Method
A without MS). Yellow oil, which solidifies on standing in the fridge.
4.4.10. 1-(Cyclopenta-2,4-dien-1-ylidenemethyl)-3-nitrobenzene
1H NMR (300 MHz, CDCl3):
d
10.01 (br s, 1H), 6.58e6.41 (m, 4H), 2.60
(3j). Prepared and purified as 3i. Reddish oil. 1H NMR (300 MHz,
(t, J¼7.7 Hz, 2H), 2.41 (t, J¼7.4 Hz, 2H), 2.22 (s, 3H), 1.93 (p, J¼7.5 Hz,
CDCl3):
d
8.42 (1H, s), 8.21 (1H, dd, J¼8, 1 Hz), 7.86 (1H, d, J¼8 Hz),
2H). 13C NMR (75 MHz, CDCl3):
d 179.5,151.6, 143.2, 131.1, 131.0,120.6,
7.60 (1H, t, J¼8 Hz), 7.20 (1H, s), 6.76e6.67 (1H, m), 6.61 (1H, d,
120.2, 35.7, 33.35, 23.6, 20.6. ESI-HRMS calculated for C11H15O2þ
J¼5 Hz), 6.56 (1H, d, J¼5 Hz), 6.36e6.28 (1H, m). 13C NMR (75 MHz,
(MþHþ) 179.1067, found 179.1086.
CDCl3):
d 148.3, 147.4, 138.2, 136.9, 135.9, 134.2, 132.2, 129.5, 126.9,
124.8, 123.2, 119.6.
4.4.21. 2-(Cyclopenta-2,4-dienylidene)propan-1-ol (3v). (Method C).
Yellow oil, which slowly solidifies in the fridge. Mp 52e53 ꢁC.
4.4.11. 5-(Propan-2-ylidene)cyclopenta-1,3-diene (3k). (Method C).
Lit.33 mp 54e54.5 ꢁC 1H NMR (300 MHz, CDCl3):
d
6.52 (dd, J¼6.2,
1H NMR (500 MHz, CDCl3):
d
6.58e6.56 (m, 2H), 6.52 (dd, J¼4.0,
4.0 Hz, 4H), 4.54 (s, 2H), 2.28 (s, 3H), 1.91 (br s, 1H). 13C NMR

8614
N. Cos¸kun, I. Erden / Tetrahedron 67 (2011) 8607e8614
Day, J. H. Chem. Rev. 1963, 53, 167e189; (d) Zeller, K. P. Pentafulvenes In.
(75 MHz, CDCl3):
18.3.
d 149.0, 142.5, 132.3, 131.7, 121.2, 119.5, 64.6,
Methoden der Organischen Chemie; 1985; Vo1.5/2c, pp 504e684; (e) Neu-
enschwander, M. Fulvenes In The Chemistry of Double-Bonded Functional Groups;
Patai, S., Ed.; John: New York, NY, 1989; pp 1131e1268.
2. (a) Thiele, J. Chem. Ber. 1900, 33, 666e673; (b) Thiele, J. Justus Liebigs Ann. Chem.
1906, 348, 1e15.
4.4.22. 1-Bromo-2-(cyclopenta-2,4-dien-1-ylidenemethyl)benzene
(3w). Reddish oil. 1H NMR (300 MHz, CDCl3):
d
7.62 (2H, ddd, J¼14,
3. (a) Freiesleben, W. Angew. Chem., Int. Ed. Eng. 1963, 2, 396e397; (b) Freiesleben,
W. U.S. Patent 3,192,275, 1965.
8, 1 Hz), 7.41 (1H, s), 7.38e7.30 (1H, m), 7.22 (1H, td, J¼8, 2 Hz), 6.67
4. (a) Kerber, R. C.; Linde, H. G., Jr. J. Org. Chem. 1966, 31, 4321e4322; (b) Olah, G.
A.; Prakash, G. K. S.; Liang, G. J. Org. Chem. 1977, 42, 661e666; (c) Hanack, M.;
Eggensperger, H. Justus Liebigs Ann. Chem. 1963, 663, 31e45.
5. Neuenschwander, M.; Kyburz, R.; Iseli, R. Chimia 1970, 24, 342e344.
6. Kyburz, R.; Schaltegger, H.; Neuenschwander, M. Helv. Chim. Acta 1971, 54,
1037e1046.
(1H, ddd, J¼5, 3, 2 Hz), 6.57 (1H, d, J¼5 Hz), 6.49 (1H, d, J¼5 Hz),
6.43e6.36 (1H, m) 13C NMR (75 MHz, CDCl3):
d 146.4, 136.4, 135.5,
132.9, 132.8, 131.8, 129.9, 127.2, 126.4, 120.5. ESI-HRMS calculated
for C12H10Brþ (MþHþ) 232.9960, not observed.
7. Hafner, K.; Sturm, E. Angew. Chem., Int. Ed. Engl. 1964, 3, 749.
8. Macomber, D. W.; Spink, W. C.; Rausch, M. D. J. Organomet. Chem. 1983, 250,
311e318.
9. Alves, N. P.; Maia, A. d. S.; de Oliveira, W. J. Alloys Compd. 2000, 303e304,
178e181 and references cited therein.
10. Greger, I.; Kehr, G.; Frohlich, R.; Erker, G. Organometallics 2010, 29, 3210e3221.
11. Boul, P. J.; Reutenauer, P.; Lehn, J.-M. Org. Lett. 2005, 7, 15e18.
12. Reutenauer, P.; Buhler, E.; Boul, P. J.; Candau, S. J.; Lehn, J.-M. Chem.dEur. J.
2009, 15, 1893e1900.
4.4.23. 1-(Cyclopenta-2,4-dien-1-ylidenemethyl)-4-(trifluoromethyl)
benzene (3x). Red crystals mp 60e62 ꢁC. 1H NMR (300 MHz,
CDCl3):
d 7.67 (4H, s), 7.21 (1H, s), 6.73e6.66 (1H, m), 6.61 (1H, d,
J¼5 Hz), 6.56 (1H, d, J¼5 Hz), 6.33 (1H, dd, J¼4, 2 Hz) 13C NMR
(75 MHz, CDCl3):
d 147.0, 140.0, 136.4, 135.6, 131.9, 130.5, 126.9,
125.42 (q, J¼3.75 Hz), 120.0.
13. McCain, G. H. J. Org. Chem. 1958, 23, 632e633.
14. McCain, G. H. U.S. Patent 3,051,765, 1962.
4.4.24. Cyclopenta-1,3-dien-1-yl(4-nitrophenyl)methanol compound
15. Stone, K. J.; Little, R. D. J. Org. Chem. 1984, 49, 1849e1853.
16. Little, R. D. Private communication.
with
cyclopenta-1,4-dien-1-yl(4-nitrophenyl)methanol
(1:1)
(4i). Yellowish oil. 1H NMR (300 MHz, CDCl3):
d
8.15 (2H, d, J¼9 Hz),
17. Griesbeck, A. G. J. Org. Chem. 1989, 107, 5308e5309.
18. Erden, I.; Xu, F.-P.; Sadoun, A.; Smith, W.; Sheff, G.; Ossun, M. J. Org. Chem. 1995,
60, 813e820.
7.54 (2H, dd, J¼8, 7 Hz), 6.49e6.20(3H, m), 5.73 (1H, d, J¼12 Hz),
3.01 (1H, t, J¼3 Hz), 3.11e2.59 (2H, m) 13C NMR (75 MHz, CDCl3):
€
19. Erden, I.; Gartner, C. Tetrahedron Lett. 2009, 50, 2381e2383.
d
150.6, 149.9, 149.0, 148.0, 147.0, 147.0, 135.3, 133.8, 131.6, 130.9,
20. Erden, I.; Amputch, M. Tetrahedron Lett. 1987, 28, 3779e3782.
130.0, 128.9, 127.0, 126.8, 123.5, 72.0, 71.4, 41.3, 40.0. ESI-HRMS
21. Erden, I.; Drummond, J.; Alstad, R.; Xu, F. Tetrahedron Lett. 1993, 34, 1255e1258.
22. Erden, I.; Drummond, J.; Alstad, R.; Xu, F. J. Org. Chem. 1993, 58, 3611e3612.
23. Erden, I.; Xu, F.; Cao, W. Angew. Chem., Int. Ed. 1997, 36, 1516e1518.
calculated for C12H10Brþ (MþHþ) 232.9960, not observed.
€
€
24. Erden, I.; Ocal, N.; Song, J.; Gleason, C.; Gartner, C. Tetrahedron 2006, 62,
10676e10682.
Acknowledgements
25. The powerful electron-withdrawing NO2 group, as expected, prevented fulvene
formation, instead, aldehyde 2i rapidly converted to a polymeric material. To
explore the mechanism of this unusual conversion we reacted equimolar
amounts of 2i with cyclopentadiene in methanol at room temperature. The 1H
NMR spectrum of the mixture revealed the formation of the hemiacetal of 2i in
23% yield within 15 min. The addition of NEt3 to the mixture only led to the
polymerization of 2i; cyclopentadiene remained unchanged.
26. Cos¸ kun, N.; Aksoy, C. Tetrahedron Lett. 2009, 50, 3008e3012.
27. The 4-NMe2 group somewhat deviates from linearity presumably due to direct
interaction with the reaction center. In the case of 2d the reaction rate is lowered
because of the engagement of the catalyst with the acidic phenol function.
28. Patil, M. P.; Sunoj, R. B. J. Org. Chem. 2007, 72, 8202e8215.
29. In our hands, Little’s procedure did not produce 6-methylfulvene from ethanal
while with propionaldehyde under the same conditions a complex mixture
was obtained containing 6-ethylfulvene in 17% along with dicyclopentadiene,
as determined by integration of the relevant signals in the crude 1H NMR
spectrum.
30. (a) Jaffe, H. H. Chem. Rev. 1953, 53, 191e261; (b) Hansch, C.; Leo, A.; Taft, R. W.
Chem. Rev. 1991, 91, 165e195; (c) March, J. Advanced Organic Chemistry Reactions
Mechanism and Structure, 3rd ed.; Wiley: 1985; p 244.
This work was supported by funds from the National Institutes
of Health, (Grant No. 1SC1GM082340). Dr. Coskun acknowledges
a BIDEP-2219 fellowship from the Turkish Scientific and Techno-
_
logical Research Council (TUBITAK). We also acknowledge funding
from the National Science Foundation (Grants No. DBI-0521342 and
DUE-9451624) for the purchase of 500 MHz and 300 MHz NMR
spectrometers.
Supplementary data
1H and 13C NMR spectra of compounds 3aeh, k, nes and v.
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2011.09.036.
References and notes
31. Kresze, G.; Goetz, H. Chem. Ber. 1957, 90, 2161e2176.
€
32. Ocal, N.; Bagdatli, E.; Arslan, M. Turk. J. Chem. 2005, 29, 7e16.
33. Knight, D. B.; Burnette, J. C.; Hall, R. W.; Lambeth, G. H. J. Chem. Eng. Data 1980,
25, 184e186.
1. For reviews, see: (a) Yates, P. Advanced Alicyclic Chemistry; Academic: New York,
NY, 1968; Vol. 2; p 59e184; (b) Bergman, E. D. Chem. Rev. 1968, 68, 41e84; (c)