Rates and Equilibria of the Michael-Type Addition of Benzenethiol to 2-Cyclopenten-1-ones

Article abstract of DOI:10.1021/jo9906882

The triethylamine-catalyzed addition reactions of benzenethiol to 2-cyclopenten-1-one and its 2-and 3-methyl derivatives have been found to be appreciably reversible in chloroform solution. Rates and equilibria have been carefully measured at 25°C in order to assess the negative influence on addition exerted by methyl groups substituted on the carbon-carbon double bond. 2-Methyl-2-cyclopenten-1-one has been found to react with benzenethiol under kinetic control to give the cis adduct as the sole detectable product in a highly stereoselective anti addition process. However, on prolonged reaction times the system slowly evolved toward a new state of equilibrium in which the more stable trans adduct, derived from a syn addition mode, was the predominant isomer.

Full text of DOI:10.1021/jo9906882

8
122
J . Org. Chem. 1999, 64, 8122-8126
Ra tes a n d Equ ilibr ia of th e Mich a el-Typ e Ad d ition of
Ben zen eth iol to 2-Cyclop en ten -1-on es
†
,‡
†
,†
Valeria van Axel Castelli, Fernando Bernardi,* Antonella Dalla Cort, Luigi Mandolini,*
‡
†
Ivan Rossi, and Luca Schiaffino
Dipartimento di Chimica and Centro CNR Meccanismi di Reazione, Universit a` La Sapienza,
Box 34, Roma 62, 00185 Roma, Italy, and Dipartimento di Chimica “G. Ciamician”,
Universit a` di Bologna, via Selmi, 2, 40126 Bologna, Italy
Received April 23, 1999
The triethylamine-catalyzed addition reactions of benzenethiol to 2-cyclopenten-1-one and its 2-
and 3-methyl derivatives have been found to be appreciably reversible in chloroform solution. Rates
and equilibria have been carefully measured at 25 °C in order to assess the negative influence on
addition exerted by methyl groups substituted on the carbon-carbon double bond. 2-Methyl-2-
cyclopenten-1-one has been found to react with benzenethiol under kinetic control to give the cis
adduct as the sole detectable product in a highly stereoselective anti addition process. However,
on prolonged reaction times the system slowly evolved toward a new state of equilibrium in which
the more stable trans adduct, derived from a syn addition mode, was the predominant isomer.
In tr od u ction
The Michael addition of thiols to activated olefins has
3
The Et N-catalyzed addition of benzenethiol to 2-cy-
clopenten-1-one (1) in chloroform solution, yielding quan-
titatively (3-phenylthio)cyclopentanone (4), was our tar-
1
been known for more than 50 years. Its importance both
8
in biochemical processes2 and in synthesis3 is well
get reaction in a recent work aimed at developing supra-
molecular catalysts based on the salophen uranyl unit
8. Early attempts at extending our studies to the methyl
derivatives 2 and 3 were frustrated by very low conver-
sions into the corresponding addition products. Multiple
substitution of methyl (alkyl) groups on the enone double
bond is known to decrease yields of addition products in
recognized. This class of reactions has been the object of
4
5
a number of kinetic and stereochemical investigations,
6
as well as of many theoretical studies. However there
are still many unexplored features in particular concern-
ing the influence of substituents at the double bond on
7
addition rates and equilibria, and a real systematization
9
general, yet the strongly adverse effects of one methyl
of reaction mechanism and structural effects in Michael-
type additions of thiols is lacking.
group on yields in our reaction system caused much sur-
prise. A closer examination revealed that a major reason
for these low yields was thermodynamic in nature, but
to the best of our knowledge no equilibrium data for the
addition of thiols to enones were available for comparison.
†
Universit a` La Sapienza. Fax: Int. code + 06490421. E-mail:
lmandolini@uniroma1.it.
‡
Universit a` di Bologna. Fax: Int. code + 0512099456. E-mail:
nando@ciam.unibo.it.
(
(
1) Hurd, C.D.; Gershbein, L. L. J . Am. Chem. Soc. 1947, 69, 2328.
2) (a) Fluharty, A. L. In The Chemistry of the Thiol Group, part 2;
Patai, S., Ed.; Wiley: New York, 1974; p 589. (b) J ocelyn, P. C. In
Biochemistry of the SH Group; Academic Press: London, 1972; p 68.
(
c) Ivanetich, K. M.; Santi, D. V. Proc. Nucleic Acid Res. Mol. Biol.
992, 42, 127. (d) Talalay, P.; de Long, M. J .; Prochaska, H. J . Proc.
Natl. Acad. Sci. U.S.A. 1988, 85, 8261.
3) (a) Wynberg, H. In Topics in Stereochemistry; Eliel, E. L., Wilen,
1
(
S. H., Allinger, N. L., Eds.; Wiley: New York, 1986; Vol. 16, p 87. (b)
Emori, E.; Arai, T.; Sasai, H.; Shibasaki, M. J . Am. Chem. Soc. 1998,
1
20, 4043 and references therein.
4) (a) Dmuchovsky, B.; Vineyard, B. D.; Zienty, F. B. J . Am. Chem.
(
Soc. 1964, 86, 2874. (b) Dmuchovsky, B.; Zienty, F. B.; Vredenburgh,
W. A. J . Org. Chem. 1966, 31, 865. (c) De Maria, P.; Fini, A. J . Chem.
Soc. B 1971, 2335. (d) Hiemstra, H.; Wynberg, H. J . Am. Chem. Soc.
1
1
981, 103, 417. (e) Fan, E.; Vicent, C.; Hamilton, A. D. New J . Chem.
997, 21, 81.
(5) (a) Truce, W. E.; Levy, A. J . J . Am. Chem. Soc. 1961, 83, 4641.
(b) Truce, W. E.; Levy, A. J . J . Org. Chem. 1963, 28, 679. (c)
Abramovitch, R. A.; Rogic, M. M.; Singer, S. S.; Venkateswaran, N. J .
Org. Chem. 1972, 37, 3577. (d) Mohrig, J . R.; Fu, S. S.; King, R. W.;
Warnet, R.; Gustafson, G. J . Am. Chem. Soc. 1990, 112, 3665. (e)
Miyata, O.; Shinada, T.; Ninomiya, I.; Naito, T.; Date, T.; Okamura,
K., Inagaki, S. J . Org. Chem. 1991, 56, 6556.
(6) (a) Osman, R.; Namboodiri, K.; Weinstein, H.; Rabinowitz, J . R.
J . Am. Chem. Soc. 1988, 110, 1701. (b) Bayly, C. I.; Grein, F. Can. J .
Chem. 1989, 67, 2173. (c) Carrol, M. T.; Cheeseman, J . R.; Osman, R.;
Weinstein, H. J . Phys. Chem. 1989, 93, 5120. (d) Wong, S. S.; Paddon-
Row: M. N.; Li, Y.; Houk, K. N. J . Am. Chem. Soc. 1990, 112, 8679.
(e) Kamimura, A.; Sasatani, H.; Hashimoto, T.; Korwai, T.; Hori, K.;
(8) van Axel Castelli, V.; Dalla Cort, A.; Mandolini, L.; Reinhoudt,
D. N. J . Am. Chem. Soc. 1998, 120, 12688.
(9) (a) Duval, D.; G e´ ribaldi, S. In The Chemistry of the Enones, part
1; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; p 385. (b)
Yamaguchi, M.; Shiraishi, T.; Hirama, M. J . Org. Chem. 1996, 61, 3520.
Ono, N. J . Org. Chem. 1990, 55, 5900. (f) Pardo, L.; Osman, R.;
Weinstein, H.; Rabinowitz, J . R. J . Am. Chem. Soc. 1993, 115, 8263.
(
g) Thomas, B. E., IV; Kollman, P. A. J . Org. Chem. 1995, 60, 8375.
(7) Bernasconi, C. F. Tetrahedron 1989, 45, 4017.
1
0.1021/jo9906882 CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/14/1999

Addition of Benzenethiol to 2-Cyclopenten-1-ones
J . Org. Chem., Vol. 64, No. 22, 1999 8123
Ta ble 1. Ad d ition of Ben zen eth iol (0.100 M) to
-Cyclop en ten -1-on e (0.100M) in th e P r esen ce of Et3N in
a
2
Ch lor ofor m a t 25.0 °C
k1b
k-1c
entry [Et3N], mM [E]e, mM (L mol s-1
2
-2
)
(L mol s-1
)
-1
-
-
-
-
4
4
4
4
1
2
3
4
1.08
2.13
2.15
4.30
5
4
3
4
1.70
1.70
1.64
1.53
4.4 × 10
4.4 × 10
4.2 × 10
3.9 × 10
-
4
mean:
1.64 ( 0.04 (4.2 ( 0.2) × 10
a
In the absence of base catalyst no reaction (<5%) was observed
after 10 days. b Errors are on the order of 3-5%. Calculated as
c
3
-1
k1/K, with K ) (3.9 ( 0.2) × 10
M . Errors are in the order of
8
-10%.
F igu r e 1. Time-concentration profile for the addition of
benzenethiol to 1 (Table 1, entry 4). The points are experi-
mental, and the curve is calculated from eq 5. The dashed line
represents the calculated equilibrium concentration of 1.
As a first part of a systematic investigation of the basic
mechanistic features of the nucleophilic addition of
thiols to enones, we now report on a quantitative study
of rates and equilibria of the Et
benzenethiol to enones 1-3 in chloroform solution at
5 °C.
3
N-catalyzed addition of
2
Resu lts a n d Discu ssion
The proposed mechanism4a,d of the base-catalyzed
F igu r e 2. Kinetic profiles for the retrograde reaction of 4.62
mM 4 in the presence of 4.3 mM Et N (4) and in the presence
3
addition in apolar aprotic solvents involves the rate-
limiting reaction of the activated olefin with a weak,
rapidly formed complex between thiol and tertiary base
B (eq 1), for the nature of which the question is open as
to whether the structure of a simple hydrogen-bonded
complex (PhSH‚‚‚B) or a hydrogen-bonded ion pair salt
of 4.3 mM Et N plus 1.0 mM 8 (b). The curves are calculated
from eq 5. The rate constant of the metal catalyzed reaction
is given in ref 8.
3
catalyst, revealed the unexpected occurrence of small, but
clearly detectable amounts of unreacted enone at infinite
time that were suggestive for the existence of the
equilibrium of eq 3, where A represents the adduct, with
-
+
(
PhS ‚‚‚HB ) is more appropriate. In any case, since the
concentration of the complex is negligibly low, clean
3
-1
third-order kinetics were observed4
a,d
K values in the range of (3-5) × 10 M
.
(eq 2), first-order
in the enone (E), thiol (T), and base.
K
T + E y
\
z A
(3)
A definite confirmation of the appreciable reversibility
of the reaction, together with a more precise determina-
tion of the equilibrium constant was obtained from
experiments in which the equilibrium was approached
from the adduct side. A diluite solution of 4 (4.62 mM)
Rates and equilibria of thiol addition were conveniently
investigated by comparing the intensities of appropriate
H NMR signals of the enone and addition product to that
of an internal standard. No extra peaks attributable to
byproducts or reaction intermediates were observed. All
3
experiments were carried out in CDCl at 25.0 °C. For
3
was treated with Et N (4.3 mM). As shown in Figure 2
the liberation of the corresponding enone 1 was signifi-
cantly accelerated by the addition of 1.0 mM metal
catalyst 8. In the absence of metal catalyst the system
was still far from equilibrium after 44 h. The K value of
1
3
-1
3
.9 ( 0.2 × 10
M
calculated from the equilibrium
the sake of simplicity, we shall consider the results of
each enone in turn.
enone concentration of 0.97 mM obtained in the presence
of 8 compares very well with the range of K values
estimated from the data in Table 1. Once the equilibrium
constant was precisely known, treatment of rate data for
the reaction system of eq 4 was straightforward. For this
2
-Cyclop en ten -1-on e (1). In the presence of excess
benzenethiol, 1 was transformed smoothly and quanti-
8
tatively into 4 at room temperature. However, in a series
of kinetic experiments aimed at determining the order
with respect to base catalyst, the initial concentration of
both reactants was kept constant and equal to 0.100 M.
A typical time-concentration profile is shown in Figure
10
system the standard integrated equation is given in eq
5
, where the quantity 1/τ is defined in eq 6 and the
(10) (a) Bernasconi, C. F. In Relaxation Kinetics; Academic Press:
1
. The results are collected in Table 1. These experiments,
New York, 1976; p 77. (b) Bernasconi, C. F. In Techniques of Chemistry;
while showing a clean first-order dependence on base
Bernasconi, C. F., Ed.; Wiley: New York, 1986; Vol. 6, p 429.

8
124 J . Org. Chem., Vol. 64, No. 22, 1999
van Axel Castelli et al.
(7)
k1[B][T]
E yk-1[B]z A
[
E] - [E]e
)
exp[-(k [B][T] + k-1[B])t]
(8)
1
[E] - [E]e
o
An independent determination of K was obtained from
an experiment carried out in the presence of metal
catalyst. A solution containing 3 (0.103 M), benzenethiol
3
(0.487 M), Et N (4.9 mM), and 8 (2.0 mM) was monitored
as a function of time. Relative intensities of the signals
of the enone reactant and addition product reached
constant values after about 30 h and remained constant
over an extended period of time (120 h). The measured
equilibrium concentrations [3]
e
) 0.073 M and [7]
e
)
0
.030 M were translated into a K value of 0.91 ( 0.04
-
1
F igu r e 3. Time-concentration profile for the addition of
benzenethiol to 3. The points are experimental, and the curve
is calculated from eq 8. The dashed line represents the
calculated equilibrium concentration of 3.
M
, which compares well with the values determined
above.
2
-Meth yl-2-cyclop en ten -1-on e (2). The procedure
adopted for analysis of the reaction system involving 2
followed exactly the same protocol as for 3. In a first
experiment, a mixture of 2 (0.100 M), benzenethiol (1.01
subscripts o and e refer to initial and equilibrium
concentrations, respectively. Equation 5 contains k
the only unknown quantity. The k values listed in Table
were obtained by a conventional curve-fitting proce-
dure. The corresponding k-1 values were calculated as
1
as
3
M), and Et N (4.9 mM) was monitored at selected times
1
over a period of about 200 min (Figure 4). The gradual
decrease in intensity of the multiplet at δ 1.73-1.71 due
to the methyl group of 2 was accompanied by the buildup
of a doublet centered at δ 1.13, J ) 7.2 Hz, and a
multiplet at δ 4.00-4.07. These two signals, whose
intensities were consistently in a 3:1 ratio, were at-
1
k
1
/K.
k1[B]
T + E yk-1[B]z A
(4)
3
tributed to the methyl group and to the hydrogen on C ,
respectively, of an addition product. The latter was
[
E] - [E]e
exp(-t[B]/τ)
E] - [E]e 1 + k τ([E] - [E] )[1 - exp(-t[B]/τ)]
assigned the structure of the cis isomer 5 (vide infra).
This finding was in keeping with stereochemical studies
showing that addition of sulfur nucleophiles to activated
olefins are in general stereoselective with anti addition
of the proton.
5
)
[
o
1
o
e
(5)
Analysis of the time-concentration profile of Figure
1
τ
k1
)
k ([E] + [T] ) +
(6)
4, carried out as above on the basis of eq 8, afforded a
1
e
e
K
-4 -1
value of 0.044 M for [E]
quantity k
values were computed: K ) 1.28 ( 0.11 M , k ) (3.5 (
e
and one of 3.5 × 10
s
for the
1
[B][T] + k-1[B], from which the following
-
1
3
-Meth yl-2-cyclop en ten -1-on e (3). Transformation
1
-
2
2
-2 -1
-2
of 3 into the corresponding addition product 7 causes a
change of the resonance of the methyl group from a
narrow and unresolved multiplet at δ 2.05-2.06 to a
sharp singlet at δ 1.44. Since very low reaction rates and
conversions into 7 were observed in this case, a high
concentration of benzenethiol was used to maximize both
rate and yield of addition product. A solution of 3 (0.100
0.2) × 10 L mol
s
, and k-1 ) (2.7 ( 0.4) × 10
L
-
1
-1
mol . In a second experiment, we obtained K ) 1.31
s
-
1
-2
2
-2 -1
( 0.07 M , k ) (3.4 ( 0.2) × 10 L mol
1
s , and k-1
-
2
-1 -1
) (2.6 ( 0.3) × 10 L mol
s .
An additional experiment in which benzenethiol (0.325
M) and 2 (0.094 M) were reacted in the presence of 4.9
mM Et N and 1.0 mM 8 gave the results shown in Figure
3
M), benzenethiol (1.01 M), and Et
monitored at selected times over a period of 16 h (Figure
). In the presence of excess benzenethiol, the system
reduces to the simple case of eq 7, first order in both
directions, for which a convenient form of the integrated
rate equation11 is given by eq 8. A nonlinear least-squares
treatment of analytical data according to eq 8 afforded a
3
N (4.9 mM) was
5. The disappearance of the enone reactant was ac-
companied by the formation of adduct 5 as the sole
3
product only in the early stages of the reaction. However,
1
after about 2 h the H NMR spectra revealed the presence
of appreciable amounts of a second product, whose
buildup was complete after 140-160 h. Typical signals
of this product are a sharp doublet at δ 1.12, J ) 7.2 Hz,
and a complex multiplet at δ 3.10-3.20, with relative
intensities of 3:1. The multiplet at δ 3.10-3.20 was
assigned to the methine proton R to the sulfur of the trans
adduct 6, based on the well-known shielding effect of a
methyl substituent on cis and vicinal hydrogens in five-
-
5
best fit value of 0.052 M for [E]
e
and one of 3.5 × 10
[B][T] + k-1[B]. From the above
values, equilibrium and rate parameters were easily
-
1
s
for the quantity k
1
-
1
computed as K ) 0.88 ( 0.08 M , k
1
) (3.3 ( 0.2) ×
, and k-1 ) (3.8 ( 0.6) × 10 L mol
. An additional experiment gave K ) 0.90 ( 0.05 M-1,
-
3
2
-2 -1
-3
-1
1
s
0
L mol
s
-
1
12
membered rings. In the other isomer, the methine
-
3
2
-2 -1
k
×
1
) (3.5 ( 0.1) × 10 L mol
10 L mol
s
, and k-1 ) (3.9 ( 0.4)
proton R to the sulfur resonates at much lower fields,
consistent with the assigned structure 5, where the
methyl group is trans to the vicinal hydrogen.
-
3
-1 -1
s .
(
11) Hammett, L. P. Physical Organic Chemistry; McGraw-Hill:
New York, 1970; p 69.
(12) Lipnick, R. L. J . Am. Chem. Soc. 1974, 96, 2941.

Addition of Benzenethiol to 2-Cyclopenten-1-ones
J . Org. Chem., Vol. 64, No. 22, 1999 8125
Ta ble 2. Ad d ition of Ben zen eth iol to En on es 1-3 in th e
P r esen ce of Et3N in Ch lor ofor m a t 25.0 °C. Ra te a n d
Equ ilibr iu m P a r a m eter s
substrate k1 (L mol s-1
2
-2
)
k-1 (L mol-1 s-1
)
K (L mol-1
)
-
4
3
1
2
3
1.64 ( 0.04
(4.2 ( 0.2) × 10
(2.6 ( 0.2) × 10
(3.8 ( 0.3) × 10
(3.9 ( 0.2) × 10
(1.30 ( 0.05)
(0.90 ( 0.03)
a
-2
-2
-3
(3.4 ( 0.2) × 10
-
3
(3.4 ( 0.1) × 10
a
The data refer to the formation of the cis isomer (5). For the
process 5 h 6 the equilibrium constant is 6.1/1.3 ) 4.7.
composition of a system under kinetic control differs
markedly from that under thermodynamic control. A
similar situation has been reported for addition of ben-
zenethiolate ion to 4-tert-butyl-1-cyanocyclohexene in
ethanol solution.5
c
F igu r e 4. Time-concentration profile for the addition of
benzenethiol to 2. The points are experimental, and the curve
is calculated from eq 8. The dashed line represents the
calculated equilibrium concentration of 2.
Su m m a r y a n d Con clu d in g Rem a r k s
Rate and equilibrium constants for addition of ben-
zenethiol to enones 1-3 are collected in Table 2. It is
common knowledge that Michael additions are reversible
reactions.13 Yet we could trace in the literature very few
1
4
investigations of equilibria for this kind of reactions.
Thus, the data in Table 2 not only provide for the first
time an assessment of the influence of methyl substitu-
ents on rates and equilibria of nucleophilic addition to
the double bond but also the first equilibrium data for
addition of a sulfur nucleophile.
Given the well-known ability of methyl groups to
stabilize a double bond by acting as electron donors to
the π system, there seems to be little doubt that a major
reason for the adverse influence of methyl substituents
on both addition rates and equilibria is electronic in
nature. However, steric effects are also likely to play a
definite role in determining the low equilibrium constants
for addition of benzenethiol to 2 and 3 because there is a
substantial increase in bond eclipsing in the addition
products. The steric situation in the rate-determining
transition states leading to the enolate intermediates is
clearly quite different for that found in the addition
products. Consequently, failure of a rate-equilibrium
relationship15 was easily anticipated and actually con-
F igu r e 5. Time-concentration profiles for the addition of
benzenethiol to 2. 9, 2; 1, 5; O, 6. The lines are guides for
visual comparison.
Sch em e 1
firmed by the observation that a plot of log k
1
against
log K (not shown here) was decidedly nonlinear.
The very low K values measured for thiol additions to
and 3 have important implications from a preparative
2
standpoint. Whereas in the case of the parent compound
a slight excess of benzenethiol is sufficient to drive
1
With the exception of the initial transient phase, all
of the time-concentration data of the species involved
in equilibrium T + 2 h 5 are consistent with a K value
addition to completion at synthetically useful reactant
concentrations, benzenethiol concentrations in the order
of several moles per liter would be required to convert
significant fractions of 2 and 3 into the corresponding
adducts.
-1
of 1.30 ( 0.10 M . This is not only in excellent agreement
with the estimate from the previous experiments but also
shows that the above equilibrium is attained rapidly and
maintained throughout the entire time-course of reaction,
although the system is evolving toward a new state of
equilibrium. From the final composition of the system, a
We finally note that the reaction of benzenethiol with
enone 2 is highly stereoselective with anti addition of the
proton under kinetic control. This result is in keeping
with literature reports on the stereochemistry of addition
-
1
K value of 6.1 ( 0.3 M was calculated for the equilib-
rium T + 2 h 6.
of heteronucleophiles both to cyclic5 and open-chain
a-c
5d,e
activated olefins.
To sum up, the less stable cis isomer 5 is formed about
2
orders of magnitude more rapidly than the more stable
(
13) (a) March, J . Advanced Organic Chemistry, 4th ed.; Wiley: New
trans adduct 6 because of the anti addition mode of the
incoming proton. It seems likely that the overall process
can be conveniently described by Scheme 1, in which a
key role is played by a common enolate-ammonium ion-
pair intermediate. Thus, the system under study provides
an additional example of a situation in which the
York, 1992; p 796. (b) Bergmann, E. D.; Ginsburg, D.; Pappo, R. Org.
React. 1959, 10, 179.
(14) (a) Chamberlain, P.; Whitham, G. H. J . Chem. Soc. B 1969,
131. (b) Chamberlain, P.; Whitham, G. H. J . Chem. Soc., Perkin Trans.
1972, 130.
1
2
(15) Hammett, L. P. Physical Organic Chemistry; McGraw-Hill:
New York, 1970; p 349.

8
126 J . Org. Chem., Vol. 64, No. 22, 1999
van Axel Castelli et al.
available from a previous investigation.8 Chloroform-d was
stored over 4 Å molecular sieves for at least 24 h. (3-
Phenylthio)cyclopentanone was prepared according to the
literature.16
To gain a deeper insight into the mechanism of Michael
additions of sulfur nucleophiles, we are carrying out a
thorough computational study of the base-catalyzed thiol
addition to 2-cyclopenten-1-ones based on density func-
tional theory. The results of this investigation will be
published in due time.
Ra te a n d Equ ilibr iu m Mea su r em en ts. NMR tubes were
dried in an oven at 130 °C for at least 24 h and then stored in
a desiccator. All sample manipulations were carried out under
an argon athmosphere. Calculated amounts of triphenyl-
methane (internal standard) and of all of the reactants except
triethylamine were introduced into an NMR tube, and a
spectrum (either at 200 or 300 MHz) was recordered at time
zero. Then a known amount of triethylamine was added, and
spectra were recorded at selected time intervals. Nonlinear
least-squares fit of experimental data to eq 5 or, whenever
appropriate, to eq 8 gave the third-order rate constants. Errors
were calculated as ( 2σ (95% confidence limit).
Exp er im en ta l Section
Ma ter ia ls. Thiophenol was distilled under reduced pressure
before use. Triethylamine was distilled over sodium metal.
-Cyclopenten-1-one, 2-methyl-2-cyclopenten-1-one, and 3-meth-
yl-2-cyclopenten-1-one were reagent-grade commercial samples
and used as received. The salophen uranyl catalyst was
2
(16) Bakuzis, P.; Bakuzis M. L. F. J . Org. Chem. 1981, 46, 235.
J O9906882