Photochemical generation of six- and five-membered cyclic vinyl cations

Article abstract of DOI:10.1021/jo0518957

The photochemical solvolyses of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate (1) and cyclopent-1-enyl(phenyl)iodonium tetrafluoroborate (2) in methanol yield vinylic ethers and vinylic cycloalkenyliodobenzenes and cycloalkenylbenzene, which are the trapping products of the geometrically destabilized C₆-ring and C₅-ring vinyl cation with the solvent and with the leaving group iodobenzene. Iodonium salt 2 also yields an allylic ether and allylic cyclopentenyliodobenzenes and cyclopentenyl-benzene, which are the trapping products of the C₅-ring allylic cation produced from the C₅-ring vinyl cation by a hydride shift in a typical carbocationic rearrangement.

Full text of DOI:10.1021/jo0518957

Photochemical Generation of Six- and Five-Membered Cyclic Vinyl
Cations
†
†
†
‡
#
Micha Slegt, Roel Gronheid, Dennis van der Vlugt, Masahito Ochiai, Tadashi Okuyama,
§
†
,†
Han Zuilhof, Hermen S. Overkleeft, and Gerrit Lodder*
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden,
The Netherlands, Faculty of Pharmaceutical Sciences, UniVersity of Tokushima, Shomachi, Tokushima
7
70-8505, Japan, Graduate School of Material Science, Himeji Institute of Technology, UniVersity of
Hyogo, Kohto, Kamigori, Hyogo 678-1297, Japan, and Laboratory of Organic Chemistry,
Wageningen UniVersity, Dreijenplein 8, 6703 HG Wageningen, The Netherlands
lodder@chem.leidenuniV.nl
ReceiVed September 9, 2005
The photochemical solvolyses of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate (1) and
cyclopent-1-enyl(phenyl)iodonium tetrafluoroborate (2) in methanol yield vinylic ethers and vinylic
cycloalkenyliodobenzenes and cycloalkenylbenzene, which are the trapping products of the geometrically
destabilized C
Iodonium salt 2 also yields an allylic ether and allylic cyclopentenyliodobenzenes and cyclopentenyl-
benzene, which are the trapping products of the C -ring allylic cation produced from the C -ring vinyl
cation by a hydride shift in a typical carbocationic rearrangement.
6 5
-ring and C -ring vinyl cation with the solvent and with the leaving group iodobenzene.
5
5
Introduction
the prop-1-enyl cation with the cyclohex-1-enyl cation and the
cylopent-1-enyl cation shows that decreases in the angle from
The reactivity of vinyl cations, one of the most unstable types
1
79° via 156° to 141° are accompanied by decreases in the
1
of intermediates known to organic chemists, has been the
5
relative stabilities of 17.0 and 27.3 kcal/mol, respectively. The
cyclopent-1-enyl cation may well be the ultimate in geometrical
destabilization of simple cyclic vinyl cations: calculations
indicate that while this cation exists as a classical vinyl cation,
subject of extensive scrutiny in the past decades.1 This research
,2
has culminated in the determination of the crystal structure of
a tamed, i.e., highly stabilized, vinyl cation. At the other end
of the reactivity spectrum, destabilized vinyl cations are found.
Destabilization may be caused by electronic effects of electron-
withdrawing groups at the R- or â-position, or by geometric
effects if the vinyl cation is part of a cyclic system.
Vinyl cations are sp-hybridized, and possess a linear orienta-
tion at the positive carbon atom. A theoretical comparison of
3
6
the cyclobut-1-enyl cation is a bridged nonclassical ion. The
solvolysis products of cyclobut-1-enyl nonaflate can indeed be
interpreted as stemming from such a nonclassical intermediate.7
Several research groups have sought to generate and trap
cyclic vinyl cations in thermal solvolysis reactions. But, whereas
4
1
-methylprop-1-enyl triflate readily solvolyzes via an SN1
mechanism in trifluoroethanol, cyclohex-1-enyl triflate does so
very slowly, and cyclopent-1-enyl triflate does not exhibit
†
Leiden University.
University of Tokushima.
University of Hyogo.
Wageningen University.
‡
#
§
(5) Mayr, H.; Schneider, R.; Wilhelm, D.; Schleyer, P. v. R. J. Org.
Chem. 1981, 46, 5336-5340.
(6) (a) Saunders, M.; Laidig, K. E.; Wiberg, K. B.; Schleyer, P. v. R. J.
Am. Chem. Soc. 1988, 110, 7652-7659. (b) Koch, W.; Liu, B.; DeFrees,
D. J. J. Am. Chem. Soc. 1988, 110, 7325-7328.
(7) (a) Subramanian, L. R.; Hanack, M. Angew. Chem. 1972, 84, 714-
715. (b) Subramanian, K.; Hanack, M. Tetrahedron Lett. 1973, 3365-3368.
(c) Subramanian, L. R.; Hanack, M. J. Org. Chem. 1977, 42, 174-175. (d)
Hanack, M.; Carnahan, E. J.; Krowczynski, A.; Schoberth, W.; Subramanian,
L. R.; Subramanian, K. J. Am. Chem. Soc. 1979, 101, 100-108. (e) Auchter,
G.; Hanack, M. Chem. Ber. 1982, 115, 3402-3413. (f) Hanack, M.; Auchter,
G. J. Am. Chem. Soc. 1985, 107, 5238-5245.
(
1) Dicoordinated Carbocations; Rappoport, Z., Stang, P. J., Eds.; John
Wiley & Sons: Chichester, UK, 1997.
(
T.; Lodder, G. In AdVances in Physical Organic Chemistry; Tidwell, T. T.,
Richard, J. P., Eds.; Elsevier Science Ltd.: Amsterdam, The Netherlands,
2) (a) Okuyama, T. Acc. Chem. Res. 2002, 35, 12-18. (b) Okuyama,
2
4
1
002; pp 1-56.
3) M u¨ ller, T.; Juhasz, M.; Reed, C. A. Angew. Chem., Int. Ed. 2004,
3, 1543-1546.
4) (a) van Alem, K.; Lodder, G.; Zuilhof, H. J. Phys. Chem. A 2002,
06, 10681-10690. (b) van Alem, K.; Belder, G.; Lodder, G.; Zuilhof, H.
(
(
J. Org. Chem. 2005, 70, 179-190 and references therein.
1
0.1021/jo0518957 CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/21/2006
J. Org. Chem. 2006, 71, 2227-2235
2227

Slegt et al.
SCHEME 1. Product Formation upon Photolysis of 1 in Methanol
8
TABLE 1. Product Compositiona after 90 min of Irradiation of 1
unimolecular dissociation. Due to the extremely good leaving
b
at λexc ) 254 nm (ca. 10% conversion )
group ability of neutral iodobenzene, 4-tert-butylcyclohex-1-
enyl(phenyl)iodonium tetrafluoroborate solvolyzes much faster
than cyclohex-1-enyl triflate, but cyclopent-1-enyl(phenyl)-
iodonium tetrafluoroborate still displays no reactivity.
Employing photolysis, it has been possible to generate a score
of vinyl cations from their vinyl halide precursors. Recent
photochemical studies have shown that pseudohalides, such as
vinyl(phenyl)iodonium salts, are an even better source of vinyl
cations. We surmised that the latter method is powerful enough
to generate geometrically destabilized vinyl cations.
Here we report the results of the photolysis of 4-tert-
butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate (1) and
cyclopent-1-enyl(phenyl)iodonium tetrafluoroborate (2) in metha-
nol. Our findings unambiguously demonstrate that not only the
-tert-butylcyclohex-1-enyl cation (I1a) but also the cyclopent-
-enyl cation (I2a) is accessible through photochemistry.
3
4a
4b
5
6
7
8
9a
9b 10a
10b-d
3.9 11.7c 16.8 5.2 80 1.3 2.9 1.5d 3.0 1.5/0.8/1.2
20
9
,10
a
As percentages of converted starting material. b 1.7% 3 and 8.2% 7
c
(
GC yields relative to internal standard). Secondary photoproduct (see text).
1
1
d Secondary thermal product (see text).
leaving groups 3 and 7, two reductive dehalogenation products,
12
5
and 8, are produced by replacement of the phenyliodonium
group by a hydrogen atom. Along with the nucleophilic vinylic
substitution product, vinyl ether 9a, four different products of
Friedel-Crafts aromatic vinylation are formed, 10a and 10b-
d. Anisole (6) results from a nucleophilic aromatic substitution
reaction. Product 4a is the formal Markovnikov addition product
of methanol to the vinylic double bond of 4-tert-butyl-1-
iodocyclohexene (3). The products are grouped along the
proposed mechanisms for their formation (Scheme 2). In
4
1
(8) (a) Pfeifer, W. D.; Bahn, C. A.; Schleyer, P. v. R.; Bocher, S.;
Harding, C. E.; Hummel, K.; Hanack, M.; Stang, P. J. J. Am. Chem. Soc.
1
971, 93, 1513-1516. (b) Hanack, M.; Bentz, H.; M a¨ rkl, R.; Subramanian,
L. R. Liebigs Ann. Chem. 1978, 1894-1904. (c) Hanack, M.; M a¨ rkl, R.;
Martinez, A. G. Chem. Ber. 1982, 115, 772-782.
(9) Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J. Am. Chem. Soc.
1
995, 117, 3360-3367.
(
10) For other attempts to prepare the cyclopent-1-enyl cation see:
Martinez-Alvarez, R.; Sanchez-Vazquez, A.; Hanack, M.; Subramanian, L.
R. J. Phys. Org. Chem. 1996, 227-233 and references therein.
(11) (a) Lodder, G. In Dicoordinated Carbocations; Rappoport, Z., Stang,
P. J., Eds.; John Wiley & Sons: Chichester, UK, 1997; pp 377-431. (b)
Kropp, P. J. In CRC Handbook of Organic Photochemistry and Photobi-
ology, 2nd ed.; Horspool, W. M., Lenci, F., Eds.; CRC Press LLC: Boca
Raton, FL, 2003; Chapter 1. (c) Kitamura, T. In CRC Handbook of Organic
Photochemistry and Photobiology, 2nd ed.; Horspool, W. M., Lenci, F.,
Eds.; CRC Press LLC: Boca Raton, FL, 2003; Chapter 11.
Results and Discussion
(
12) (a) Gronheid, R.; Lodder, G.; Ochiai, M.; Sueda, T.; Okuyama, T.
Formation of Photoproducts. Irradiation of 1 in methanol,
at λexc ) 254 nm, yields the product mixture depicted in Scheme
J. Am. Chem. Soc. 2001, 123, 8760-8765. (b) Gronheid, R.; Lodder, G.;
Okuyama, T. J. Org Chem. 2002, 67, 693-702. (c) Fujita, M.; Furutani,
M.; Okuyama, T. Tetrahedron Lett. 2002, 43, 8579-8581.
1
, in the composition recorded in Table 1. Next to the two
2228 J. Org. Chem., Vol. 71, No. 6, 2006

Generation of 6- and 5-Membered Cyclic Vinyl Cations
SCHEME 2. Mechanism of Formation of Photoproducts from 1 in Methanol
molecule 1 two bonds are prone to photolysis, the phenyl-
iodonium (A) bond and the vinyl-iodonium (B) bond. Both
bonds are subject to homolytic (A1, B1) and heterolytic (A2,
B2) cleavage.
B1 superfluous, which is unlikely in view of the observation of
radical cation I7 as transient in the picosecond flash photolysis
of diphenyliodonium salts.
1
4
The results obtained in the photolysis of 1 encouraged us to
photolyze the cyclopent-1-enyl compound 2 in methanol at λexc
) 254 nm (Scheme 3, Table 2). Most rewarding is the formation
of two nucleophilic substitution products, the vinylic ether
1-methoxycyclopent-1-ene (14a) (and its hydrolysis product
cyclopentanone (14b)), and the allylic ether 3-methoxycyclo-
pent-1-ene (16). Also four Friedel-Crafts vinylation products,
cyclopent-1-enylbenzene 15a and cyclopent-1-enyliodobenzenes
15b-d, and four allylation products, cyclopent-2-enylbenzene
17a and cyclopent-2-enyliodobenzenes 17b-d, are produced.
Again, both carbon-iodine bonds (C and D) are photolabile
and proposed to be subject to homolytic and heterolytic
cleavage. Homolysis of the C bond (Route C1) will form the
radical cation of 1-iodocyclopentene and the phenyl radical I4.
The products of these intermediates are 1-iodocyclopentene (11),
Homolytic cleavage of the A bond (Route A1) yields the
4-tert-butyl-1-iodocyclohexene radical cation I3 and the phenyl
radical I4. The radical cation I3 can acquire an electron from
its environment to form 4-tert-butyl-1-iodocyclohexene (3) or
undergo addition of methanol with Markovnikov orientation,
followed by abstraction of a hydrogen atom (vide infra, Scheme
5
), to form 4-tert-butyl-1-iodo-1-methoxycyclohexane (4a).
Benzene (5) is formed after hydrogen atom abstraction from
the solvent by I4. Upon heterolytic cleavage of bond A (Route
A2), 3 is formed, alongside phenyl cation I5. In methanol this
elusive reactive intermediate will be trapped immediately,
forming anisole (6).
Photohomolysis of bond B (Route B1) will yield the 4-tert-
butylcyclohex-1-enyl radical I6 and the iodobenzene radical
cation I7. Radical I6 will abstract a hydrogen atom from the
solvent and form 4-tert-butylcyclohex-1-ene (8). Radical cation
I7 forms iodobenzene (7) by acquiring an electron. Upon
heterolysis of the B bond (Route B2), leaving group 7 and vinyl
cation I1a are formed. The vinyl ether 4-tert-butyl-1-methoxy-
cyclohexene (9a) is the trapping product of I1a with methanol.
Vinyl ether 9a is found to hydrolyze under the experimental
conditions or upon workup, in part, to 4-tert-butylcyclohexanone
1
-iodo-1-methoxycyclopentane (12a) (vide infra), and 5. The
heterolytic cleavage of the C bond (Route C2) yields the leaving
group 11 and the phenyl cation I5, which is trapped by methanol
to yield anisole (6). Homolytic cleavage of the D bond (Route
D1) yields the radical cation of iodobenzene I7 and the
cyclopent-1-enyl radical. These intermediates will produce 7,
after electron transfer, and cyclopentene (13), after hydrogen
atom abstraction, respectively.
Crucial in this report is the photoheterolysis of bond D (Route
D2) as depicted in Scheme 4. This cleavage generates 7 and
vinyl cation I2a. In the product mixture the trapping product of
I2a and methanol, the vinyl ether 14a (+14b), is accompanied
by the allylic ether 16, which is the trapping product of the
(9b). The 4-tert-butylcyclohex-1-enyliodobenzenes 10b (ortho),
1
0c (meta), and 10d (para) are products of the Friedel-Crafts
reaction of cation I1a with 7. Electrophilic aromatic substitution
on the ipso position of 7 results in the formation of 4-tert-
butylcyclohex-1-enylbenzene (10a). In principle, product 10a
may also have been formed by attack of the phenyl cation I5
on the leaving group 3 (Route A2) or by recombination of the
radical pairs I3 and I4 (Route A1) or I6 and I7 (Route B1).
Alternative routes toward the formation of the radical pairs may
be photoheterolysis to 3 and I5 (route A2) and to I1a and 7
(13) Gronheid, R.; Zuilhof, H.; Hellings, M. G.; Cornelisse, J.; Lodder,
G. J. Org. Chem. 2003, 68, 3205-3215.
(14) (a) DeVoe, R. J.; Sahyun, M. R. V.; Serpone, N.; Sharma, D. K.
Can. J. Chem. 1987, 65, 2342-2349. (b) Klemm, E.; Riesenberg, E.;
Graness, A. Z. Chem. 1983, 23, 222. (c) Pappas, S. P.; Pappas, B. C.;
Gatechair, L. R.; Schnabel, W. J. Polym. Sci., Polym. Chem. Ed. 1984, 22,
69-76.
(
Route B2) followed by electron transfer within the ion-
13
molecule pairs. These alternatives would make routes A1 and
J. Org. Chem, Vol. 71, No. 6, 2006 2229

Slegt et al.
SCHEME 3. Product Formation upon Photolysis of 2 in Methanol
a
b
TABLE 2. Product Composition after 90 min of Irradiation of 2 at λexc ) 254 nm (ca. 15% conversion )
11
12a
12b
5
6
7
13
14a
14b
15a
15b-d
16
17a
17b-d
11c
61d
19
62
8.9
e
0.8f
3.8
8.8/4.7 /2.3
g
2.5
5.6
e
38
6.7
a
As percentages of converted starting material. 5.8% 11 and 9.3% 7 (GC yields relative to internal standard). c Secondary photoproduct (see text). d In
b
part secondary photoproduct of 7. These products could only be identified on GC-MS. Secondary thermal product (see text). g The GC peak of 15c
e
f
somewhat overlaps with that of the internal standard)
SCHEME 4. Mechanism of Formation of Photoproducts 14-17 from 2 in Methanol
allylic cation I2b. Further, next to the products of the electrophilic
aromatic substitution reaction of 7 and I2a (15a (ipso), 15b
from their 1-iodo-1-methoxycycloalkane precursors 4a (see
Scheme 5) and 12a, respectively.
(
ortho), 15c (meta), 15d (para)), also products reasoned to stem
Quantum Chemical Studies. To assess the nature and
plausibility of the possible cationic intermediates in the pho-
from the trapping of I2b by 7, the cyclopent-2-enyliodobenzenes
1
1
5
7a,b-d, are observed.
All reactions were checked to be photochemical of origin. A
dark” thermal reaction of 1 in methanol, at 25 °C for 3 h,
tolysis of 1 and 2, CBS-Q calculations were carried out on
the structures and stabilities of the parent six- and five-
membered cyclic vinyl and allyl cations I1a and I1b (R ) H)
and I2a and I2b (see Table 3, Figure 1). Such calculations have
been shown to reproduce the relative stabilities of cations within
experimental errors for a wide range of cationic species.16 Also,
some related species (cf. Scheme 6) were studied. The cyclohex-
“
yielded no appreciable amount of product. Thermal solvolysis
of 2 in methanol shielded from light at 25 °C yielded no products
at all after 3 h. Study of the composition of the photoproduct
mixtures of 1 and 2 as a function of the time of irradiation shows
that 4-tert-butyl-1,1-dimethoxycyclohexane (4b) and 1,1-
dimethoxycyclopentane (12b) (presented in parentheses in
Schemes 1 and 3) are secondary products, apparently produced
1
-enyl cation I1a is found to be less stable than the cyclohex-
(15) Ochterski, J. W.; Peterson, G. A.; Montgomery, J. A. J. Chem. Phys.
1996, 104, 2598-2619.
2230 J. Org. Chem., Vol. 71, No. 6, 2006

Generation of 6- and 5-Membered Cyclic Vinyl Cations
SCHEME 5. Mechanism of Formation of Photoproducts 4a and 4b
unreactive in 50% methanol at 130 °C for 17 days,17 the
unimolecular solvolysis of cyclohex-1-enyl triflate in deuterated
acetic acid (130 °C, for 4 weeks) does yield a mixture of
cyclohex-1-enyl acetate and its hydrolysis product cyclohex-
TABLE 3. Relative Stabilities of the Cyclic Vinylic Cations and
Isomers
8
anone. In ethanol/water, cyclohexanone is the sole product. This
is also the case for the corresponding cyclohex-1-enyl nonaflate,
which solvolyzes a factor of 2 faster than the triflate (ethanol/
1
8
water 150 °C, 2 days). In the thermal solvolysis of 1 in
6
methanol (25 °C, for 9 days), which occurs about 10 times
faster than that of the triflate/nonaflate, the formation of 10b-
d, next to 9a and 9b, provides additional evidence for the
generation of the cyclic vinyl cation, these compounds being
9
the trapping products of I1a by iodobenzene 7. The photosol-
volysis of 1 in methanol takes only hours at 20 °C, using a
standard light source. The difference in reaction conditions
needed to prepare the C6-ring vinyl cation attests to the
tremendous effect of photoexcitation upon leaving group
1
1,12
abilities.
The cylohex-1-enyl cation (I , R ) H) has also been
2
-enyl cation I1b by 27.8 kcal/mol. In the cyclopentenyl cations
1
a
the vinyl cation is 47.8 kcal/mol less stable than the allyl cation.
Vinyl cation I2a is a planar species (Cs symmetry) that
corresponds to a real minimum at the potential energy surface
at both the B3LYP/6-311G(2d,2p) and MP2/6-311G(2d,2p)
levels of theory. While the positive charge, obtained with NBO
analysis, is highest at the formally positively charged vinylic
C
over the rest of the molecule. Of specific interest is that the
charges of the CH and CH2 moieties next to this vinylic C
atom are less positive (+0.093 in both cases) than at the
remaining CH2 moieties (+0.209 for the CH2 next to the first
CH2 and +0.175 for the CH2 next to the CH moiety). The strain
in this molecule is significant. This is shown in two ways: first,
proposed as the product-forming intermediate in the photolysis
of 1-iodocyclohexene (3, R ) H) in methanol.19 In this
photoreaction, at almost complete conversion, the only product,
next to cyclohexene (8, R ) H), is 1,1-dimethoxycyclohexane
(4b, R ) H), which according to the authors was apparently a
secondary product arising from acid-catalyzed addition of
methanol to the nucleophilic trapping product 1-methoxycy-
+
atom (+0.431), the remainder is delocalized significantly
1
9
clohexene (9a, R ) H). In the photolysis of 1, reported here,
4-tert-butyl-1,1-dimethoxycyclohexane (4b) is proposed to be
a secondary photoproduct, formed at the expense of 1-iodo-1-
methoxycyclohexane 4a. The contradictory results prompted us
to study the origin of 4a and 4b in more detail.
+
Irradiation of a solution of vinyl iodide 3 (R ) tert-butyl) in
methanol in the presence of 9,10-dicyanoanthracene at λexc)
+
the CH-C -CH2 angle is 149.9°, which is significantly
different from the 180° observed for an unstrained vinyl cation;
second, the lengths of specifically the CH2-CH2 bonds are
larger than in unstrained species: 1.611 Å for the C-C bond
350 nm (well outside the UV absorption band of 3) produced
4a (Scheme 5). Under these electron-transfer photosensitization
conditions alkenes are known to give radical cations. Here the
3
+
farthest away from the C atom, and 1.642 Å for the other CH2-
radical cation I is generated, which adds to methanol and yields
2
0
CH2 bond with a concomitantly low bond order of 0.822.
4a after a hydrogen atom transfer. Compound 3 even yields
4a, next to 8, in the absence of the sensitizer and upon irradiation
in methanol at λexc) 248 nm (Scheme 5). This means that radical
3
For the six-membered 1-cyclohexenyl cation (I1a, R ) H),
+
similar observations are made. In this species the CH-C -
CH2 angle is 156.3°, which indicates a more relaxed structure.
cation I is also produced by a direct photoinduced ejection of
+
21
The vinylic C atom bears a charge of +0.450 while the
an electron from the substrate into the solvent. No vinyl ether
remainder of the charge is delocalized over the rest of the ring.
The C6-Ring Vinyl Cation. The parent cyclohex-1-enyl
cation (I1a, R ) H) has already been postulated as an
intermediate in thermal solvolyses several times. The major
indication for its generation is the formation of a vinyl ether
accompanied by or completely converted to its hydrolysis
product. Whereas cyclohex-1-enyl tosylate and brosylate are
9a (or its hydrolysis product 9b) is formed. Only after prolonged
(
17) (a) Peterson, P. E.; Indelicato, J. M. J. Am. Chem. Soc. 1968, 90,
6
9
515-6516. (b) Peterson, P. E.; Indelicato, J. M. J. Am. Chem. Soc. 1969,
1, 6194-6195.
(18) Subramanian, L. R.; Hanack, M. Chem. Ber. 1972, 105, 1465-
1
470.
(
19) (a) McNeely, S. A.; Kropp, P. J. J. Am. Chem. Soc. 1976, 98, 4319-
4
320. (b) Kropp, P. J.; McNeely, S. A.; Davis, R. D. J. Am. Chem. Soc.
1
983, 105, 6907-6915.
(
16) (a) van Alem, K.; Lodder, G.; Zuilhof, H. J. Phys. Chem. A 2000,
04, 2780-2787. (b) van Alem, K.; Sudholter, E. J. R.; Zuilhof, H. J. Phys.
Chem. A 1998, 102, 10861-10868. (c) Reference 4a.
(20) (a) Yasuda, M.; Pac, C.; Sakurai, H. Bull. Chem. Soc. Jpn. 1980,
1
53, 502-507. (b) Nixdorf, A.; Gr u¨ tzmacher, H.-F. Chem. Eur. J. 2001, 7,
1248-1257.
J. Org. Chem, Vol. 71, No. 6, 2006 2231

Slegt et al.
FIGURE 1. The B3LYP/6-311G(2d,2p)-calculated structures of the vinyl cations I2a and I1a (R ) H).
irradiation of 3 are the reaction products cyclohexene 8 and
,1-dimethoxycyclohexane 4b, indicating that 4b is a secondary
photoproduct of 4a. 1-Iodo-1-methoxycycloalkanes are known
to be photolabile and give the corresponding 1,1-dimethoxyal-
kanes as photoproduct.22
In theory, as observed in the photochemistry of 2, the C6-
ring vinylic cation I1a can convert, by a hydride shift, to the
allylic cation I1b. However, neither in the thermal solvolysis of
cyclohex-1-enyl triflate, cyclohex-1-enyl nonaflate, and iodo-
nium salts nor upon irradiation of 1 are allylic products observed.
The CBS-Q calculated stabilities (see Table 3) of I1a and I1b
show that the production of allyl cation I1b from I1a is
exothermic by 27.8 kcal/mol, but considerably less so than for
the five-membered ring where 47.8 kcal/mol is gained in
converting the vinyl cation I2a into the allyl cation I2b. A 1,2-
hydride shift does occur in a benzannellated form of I1a, the
1
In the photolysis of 1, the formation of enol ether 9a as
primary photoproduct signals the generation of I1a and its
subsequent trapping by methanol. Vinyl cation I1a also reacts
with iodobenzene (7) yielding 10b-d. These products are found
in an o:m:p ratio of 43:22:34. This ratio is different from the
9
ratio found in the thermal solvolysis of 1 (o:m:p ) 87:6:9).
2
7
Another difference between the thermal and photochemical
reaction of 1 is that only the light-induced reaction yields ipso
alkylation product 10a, in a 1:1.2 ratio relative to the Friedel-
1,2-dihydronaphthyl vinyl cation, where the driving force is
28
57 kcal/mol. Apparently, for I1a the driving force for a hydride
shift is not large enough for that shift to compete with reaction
of the vinyl cation with the solvent or the leaving group.
Crafts products 10b-d. Such ipso substitution is frequently
observed in iodonium salt photolysis.1
2b,23
Also, the relative
The C -Ring Vinyl Cation. In the irradiation of 2 in
5
importance of the two product-forming pathways from I1a,
reaction with the solvent and with the leaving group, is
significantly different in the thermal and photochemical metha-
methanol the heterolytic cleavage of bond D yields cation I .
2a
This conclusion is based on the formation of vinyl ether 14a,
next to its hydrolysis product 14b, as well as on the formation
of 15a, and of three isomers of cyclopent-1-enyliodobenzene
(15b-d), the trapping products of I2a with iodobenzene. The
conclusion is strongly supported by the formation of allylic ether
16 and the allylic, aromatic trapping products of I2b (17a,b-
d). These products imply that upon generation of I , it is
nolysis of 1. Upon thermolysis the ratio of products 9 and
9
1
1
0b-d is 6:1, and upon photolysis the ratio of products 9 and
0 is 1:1.5. Clearly, the photogenerated C6-ring vinyl cation
differs from the thermally generated species in its regioselec-
tivity toward iodobenzene and in its chemoselectivity toward
the nucleophiles methanol and iodobenzene. The differences
may be due to the formation of an aliphatic vinyl cation of high
2a
converted through a hydride shift to the allylic cation I , which
2b
is trapped by methanol or iodobenzene. Hydride shifts are typical
of carbocation behavior and therefore products from I2b provide
direct evidence that a cation, i.e., I , is generated photochemi-
19,24
energy (a “hot” vinyl cation) in the photochemical reaction.
We speculate that the species generated in the photolysis is the
2a
triplet state of I1a, produced upon heterolytic cleavage of the
cally in methanol. The driving force for the 5-ring vinyl cation
I2a to undergo a hydride shift is much larger (see Table 3) than
that for the 6-ring vinyl cation I , allowing the conversion of
I2a to allyl cation I2b to compete with the trapping by the solvent
or the leaving group. The ratio between vinyl and allyl cation-
derived products is about 2.5:1.
+
C-I bond of the triplet excited state of 1. This triplet cation
reacts (in part) faster with nucleophiles than it deactivates, by
1
a
25
spin-inversion, to its singlet ground state. As in the case of
2
6
triplet aryl cations, triplet vinyl cations are expected to prefer
reaction with π-nucleophiles, such as iodobenzene, over reaction
with n-nucleophiles, such as methanol.
The trapping of the C -ring vinyl cation I by the leaving
5
2a
group 7 yields 15b-d in an o:m:p ratio of about 56:30:14. The
C5-ring vinyl cation reacts less abundantly at the ipso position
than at the combined o:m:p positions compared to the C6 ring
ion: a ratio of 1:4 between these two pathways is found. For
the C5-ring vinyl cation, the ratio between solvent trapped
products versus leaving group trapped products is 1:25, com-
(
21) Photoionization in solution in the absence of electron acceptors has
previously been proposed, for instance in the photocyanation of aromatics:
Cornelisse, J.; Lodder, G.; Havinga, E. ReV. Chem. Intermed. 1979, 89,
31-265.
2
(
(
22) Kropp, P. J.; Pienta, N. J. J. Org. Chem. 1983, 48, 2084-2090.
23) (a) Dektar, J. L.; Hacker, N. P. J. Org. Chem. 1990, 55, 639-647.
(
2
(
b) Dektar, J. L.; Leff, D. V.; Hacker, N. P. J. Org. Chem. 1991, 56, 2280-
282. (c) Dektar, J. L.; Hacker, N. P. J. Org. Chem. 1991, 56, 1838-1844.
d) Kampmeier, J. A.; Nalli, T. W. J. Org. Chem. 1994, 59, 1381. (e) Bi,
(26) (a) Milanesi, F.; Fagnoni, M.; Albini, A. Chem. Commun. 2003, 2,
216-217. Milanesi, S.; Fagnoni, M.; Albini, A. J. Org. Chem. 2005, 70,
603-610. (b) Fagnoni, M.; Mella, M.; Albini, A. Org. Lett. 1999, 1, 1299-
1301. Guizzardi, B.; Mella, M.; Fagnoni, M.; Freccero, M.; Albini, A. J.
Org. Chem. 2001, 66, 6353-6363. (c) Protti, S.; Fagnoni, M.; Mella, M.;
Albini, A. J. Org. Chem. 2004, 69, 3465-3473.
(27) Kitamura, T.; Muta, T.; Tahara, T.; Kobayashi, S.; Taniguchi, H.
Chem. Lett. 1986, 759-762.
(28) van Alem, K. J. Effects of R-Substituents on Carbocations and
Radicals, Thesis, Leiden University, 2001; pp 43-62.
Y.; Neckers, D. C. Macromolecules 1994, 27, 3683-3693. (f) Kitamura,
T. In CRC Handbook of Organic Photochemistry and Photobiology, 2nd
ed.; Horspool, W. M., Lenci, F., Eds.; CRC Press LLC: Boca Raton, FL,
2
003; Chapter 110.
24) Sonowane, H. R.; Nanjundiah, B. S.; Rajput, S. I. Indian J. Chem.
984, 23b, 331-335.
25) B3LYP/6-311G(2d,2p) calculations show that triplet I1a and triplet
(
1
(
I2a are less stable than their ground-state singlet counterparts by 34 and 24
kcal/mol, respectively.
2232 J. Org. Chem., Vol. 71, No. 6, 2006

Generation of 6- and 5-Membered Cyclic Vinyl Cations
SCHEME 6. Possible Mechanisms of Conversion of I2a into I2b
pared to the 1:1.5 ratio for the C6-ring vinyl cation. This
difference in preference for the reaction with iodobenzene (7)
over methanol probably reflects the higher reactivity of the C5-
ring species. As discussed for the C6-ring vinyl cation, this
species may be (in part) the triplet state of I2a.25
ring, the difficulty in creating the initial overlap may signifi-
cantly add to an already high activation energy.
Second, a 1,3-H shift across the double bond, also yielding
allylic cation I . The activation energy for such a 1,3-H shift
2
b
is calculated to be 11.4 kcal/mol in the linear prop-1-enyl cation,
considerably less than the activation energy for the correspond-
The formation of C5-ring vinyl cation I2a-derived products
also occurs in the irradiation of a derivative of 2 with a 4-tolyl
instead of a phenyl ligand, cyclopent-1-enyl(4-methylphenyl)-
iodonium tetrafluoroborate, in methanol (λexc) 254 nm). The
photolysis of the tolyl salt proceeds 1.6 times more efficiently
than the photoreaction of the phenyl salt. The product formation
resembles that of the photolysis of 2. The formation of vinyl
ether 14a through the presence of 14b is confirmed in this
experiment and also allyl ether 16 was found. The ratio between
vinyl- and allyl cation-derived products is about 1.6:1. Vinyl
cation I2a reacts with the leaving group 4-iodotoluene to yield
the ipso product 4-cyclopent-1-enyltoluene, and the Friedel-
Crafts products 3-(cyclopent-1-enyl)-4-iodotoluene and 2-(cy-
clopent-1-enyl)-4-iodotoluene. The latter two are produced in
a 3:1 ratio, which resembles the ratio obtained in the thermal
31
ing 1,2-H shift. This kinetic preference for a 1,3- over a 1,2-H
shift for linear vinyl cations is probably even more pronounced
for the ring vinyl cation, because of the higher conformational
3
2
freedom of C-3 compared to C-5.
3
3
Third, a trans-annular hydride shift from C-4 to C-1
producing the homoallylic cation I2c, which in turn may undergo
a 1,2 H shift to yield allylic cation I2b. Several reports on the
generation and reactivity of the cyclopent-3-enyl cation I2c have
3
4
appeared. When generated in a nonnucleophilic environment,
the homoallylic cation I2c yields allylic products, in a more
nucleophilic reaction medium a mixture of homoallylic and
allylic products is formed. Thus I2c is easily converted, through
a 1,2 H shift, in an allyl cation in competition with trapping by
the solvent. In our experimental setup, however, employing the
nucleophilic solvent methanol, the trapping product of cation
I2c should have been present, if this mechanism is operative.
This is not the case, no 4-methoxycyclopentene is produced.
Therefore this sequence of reaction steps is unlikely.
9
solvolysis of the 4-tolyl derivative of 1 (76:24). The Friedel-
Crafts products are formed in a 3.3-fold excess over the ipso
products. The C5-ring vinyl cation again shows a large prefer-
ence for reaction with 4-iodotoluene over methanol. An exact
ratio could not be determined because the GC peak of 14b is
obscured by that of toluene.
Fourth, proton loss from C-5 of I forming the highly strained
2a
3
5,36
cyclic allene I ,
2
e
which upon reprotonation may yield the
For the mechanism of conversion of the C5-ring vinylic cation
I2a into the allylic cation I2b, four possibilities come to mind
allyl cation I2b or the vinyl cation I2a, depending on the site of
protonation. Proton loss from C-2 of I2a, yielding cyclopentyne,
is less likely because CBS-Q calculations show (data Table 3)
that cyclopentyne (I2f) is 14 kcal/mol less stable than cyclopenta-
1,2-diene (I2e). It is doubtful whether the deprotonation of I2a
to I2e is thermodynamically feasible, the more so because only
(Scheme 6). First, a 1,2-H shift across the single bond toward
the carbon-carbon double bond. As already discussed, the
resulting allylic cation is far more stable than the vinylic cation
(data in Table 3), which thermodynamically allows this reaction.
However, the activation energy for such a 1,2-H shift has both
experimentally and computationally been shown to be consider-
(32) Quantum chemical transition state calculations to support this idea
were not successful.
able (18-20 kcal/mol) in linear systems.2
9,30,31
Allylic stabiliza-
(
33) Hargrove, R. J.; Stang, P. J. Tetrahedron 1976, 32, 37-41.
tion as in the product cannot be achieved in the bent transition
state because of the lack of overlap between the newly created
empty orbital and the double bond. In the rigid cyclopentenyl
(
34) (a) Saunders, M.; Berger, R. J. Am. Chem. Soc. 1972, 94, 4049-
4050. (b) Lambert, J. B.; Finzel, R. J. Am. Chem. Soc. 1983, 105, 1954-
1
958. (c) Schleyer, P. v. R.; Bentley, T. W.; Koch, W.; Kos, A. J.; Schwarz,
H. J. Am. Chem. Soc. 1987, 109, 6953-6957. (d) Bouchoux, G.; Salpin,
J.-Y. Chem. Phys. Lett. 2002, 366, 510-519. (e) Brunelle, P.; Sorensen, T.
S.; Taeschler, C. J. Phys. Org. Chem. 2003, 16, 654-568.
(35) (a) Angus, R. O.; Schmidt, M. W.; Johnson, R. P. J. Am. Chem.
Soc. 1985, 107, 532-537. (b) Johnson, R. P. Chem. ReV. 1989, 89, 1111-
1124. (c) Algi, F.; O¨ zen, R.; Balci, M. Tetrahedron Lett. 2002, 43, 3129-
3131.
(29) Fairley, D. A.; Milligan, D. B.; Wheadon, L. M.; Freeman, C. G.;
Maclagan, R. G. A. R.; McEwan, M. J. Int. J. Mass. Spectrom. 1999, 185/
1
86/187, 253-261.
30) Okuyama, T.; Yamataka, H.; Ochiai, M. Bull. Chem. Soc. Jpn. 1999,
2, 2761-2769.
31) McAllister, M.; Tidwell, T. T.; Peterson, M. R.; Csizmadia, I. G. J.
Org. Chem. 1991, 56, 575-580.
(
7
(
(36) Fujita, M.; Kim, W. H.; Fujiwara, K.; Okuyama, T. J. Org. Chem.
2005, 70, 480-488.
J. Org. Chem, Vol. 71, No. 6, 2006 2233

Slegt et al.
methanol or iodobenzene is available as base in the solvent
cage.1 However, if a vinyl cation of higher energy is involved
15a, and their iodine substituted counterparts, further substanti-
ates this conclusion.
2b
or if it is the triplet state of I2a that is involved, the propensity
for proton loss may be enhanced.1
9,37
Anyhow, the preferred
Experimental Section
reaction for structurally unbiased linear allenes is reprotonation
at the terminal allene carbons atoms,38 here yielding I2a and
Materials. The iodonium salts 4-tert-butylcyclohex-1-enyl-
(phenyl)iodonium tetrafluoroborate (1), cyclopent-1-enyl(phenyl)-
not I2b. This reaction sequence is not productive toward allylic
ether formation and is therefore disregarded.
Summarizing, we propose that I2b is produced from I2a by a
,3-H shift. As far as we know, this mode of rearrangement is
iodonium tetrafluoroborate (2), and cyclopent-1-enyl(4-methylphenyl)-
iodonium tetrafluoroborate were synthesized as described in refs 9
and 43. Methanol (HPLC grade) was checked to be UV transparent
and purged with Argon prior to use. n-Hexane and n-hexadecane
were used as received as were all available reference compounds
1
39
unprecedented in vinyl cation chemistry.
(5, 6, 7, 9b, 13, 14b, and 16). Nonavailable reference compounds
The data presented here constitute the first unambiguous case
for the direct formation of C5-ring vinyl cation I2a.40 Attempts
were synthesized following literature procedures with use of
commercially available starting materials.
to thermally generate vinyl cation I2a from cyclopent-1-enyl
triflate or nonaflate did yield cyclopentanone, but in both cases
Photochemistry. All reaction mixtures were purged with Argon
prior to irradiation. The solutions are irradiated in a quartz reaction
tube that was sealed with a rubber septum (to allow sampling) and
placed in a merry-go-round apparatus. A Hanau TNN-15/32 low-
pressure mercury lamp placed in a water-cooled quartz tube is used
to supply light with a main emission at λ ) 254 nm.
In a typical experiment 10 mL of a 5 mM solution of the
iodonium salt, containing 10 µL n-hexadecane as internal standard,
was irradiated. At appropriate time intervals 50 µL samples were
taken, using a syringe piercing through the septum. In kinetic runs,
samples are taken every 5 min. The samples were injected in a test
tube containing ∼1 mL demineralized water and 100 µL of
n-hexane or diethyl ether. Extraction was ensured by shaking the
stoppered test tube. After settling of the layers, the organic layer
was removed with a rinsed syringe and analyzed on GC and GC-
MS. At the end of the irradiation, the remainder of the reaction
mixture was reduced in volume by purging with nitrogen and, if
necessary, redissolved in 1 mL of diethyl ether. This sample is also
analyzed on GC and GC-MS. Experiments were carried out in
triplicate.
the reaction occurred through S-O bond fission rather than
vinyl-O bond fission.8
b,41
I2a has been proposed as an inter-
mediate in the photolysis of 1-iodocyclopentene in methanol at
25 °C, which yielded 1,1-dimethoxycyclopentane (12b) next
to cyclopentene (13). It may, however, well be that 12b, in
analogy with 4b, is a radical cation-derived rather than a vinyl
cation-derived product. The nonformation of vinyl ether 14a
-
19
(
and hence no cyclopentanone 14b), or allyl ether 16, is in line
with this idea.
Relative Photoefficiencies. Comparison of the rates of
formation of the leaving groups from 1 and 2 (at low conversion)
showed that photolysis of 2 is 1.4 times more efficient than
that of 1. Thus 2 is more photolabile than 1, while 2 is far less
thermolabile than 1. Presumably, the cyclopentenyl ligand
contributes less electron density to the carbon-iodine bond in
the reactive excited state than the cyclohexenyl moiety, which
lowers the photostability.
In the irradiation of 1, almost 5 times more iodobenzene (7)
is cleaved off (B-bond fission) than vinyl iodide 3 (A-bond
fission); in the photoreaction of 2 only 1.8 times more 7 than
vinyl iodide 11 is produced. These efficiencies reflect the
instability of the intermediates that are generated in these light-
induced reactions, the phenyl cation, the cyclohex-1-enyl cation,
and the cyclopent-1-enyl cation. This is in agreement with a
report about thienyl(aryl)iodonium salts that the cationic iodine
center allows π electron communication between the two ligands
via its d-orbitals.42 There is a substantial effect of one ligand at
the iodonium center on the bond to the other ligand.
The λexc ) 248 nm irradiations of 3 (R ) tert-butyl) are carried
out in a different setup. A high-pressure Hg/Xe arc, from which
the IR output was removed by a water filter, was used as the
irradiation source. The light beam was guided through a 77250
model Oriel monochromator to select the desired wavelength, and
aimed at a 3 mL quartz cell, equipped with a glass stopper with
Teflon septum. Per experiment 3 mL of 5 mM solutions of 3 and
1
mM n-hexadecane were used. The λexc ) 350 nm 9,10-
dicyanoanthracene-sensitized irradiation of 3 is carried out in a
Rayonet Photochemical Reactor, RPR 200, equipped with 350 nm
lamps, which was placed in a cool room (4 °C). Reaction mixtures
of 6 mM sensitizer and 50 mM starting material (in methanol, 10
mL) in Pyrex were used.
In summary, we have demonstrated that not only the C6-ring
vinyl cation, but also the C5-ring vinyl cation can be generated
photochemically, under mild reaction conditions. Compelling
evidence for the formation of the C5-ring vinyl cation I2a is the
production, next to the direct trapping products 14a (and 14b),
of the allylic ether 16, which is the trapping product of the
hydride shifted intermediate I2b. The formation of 17a next to
Products. All reaction products were characterized by compari-
son of their retention times on analytical GC and of their mass
spectra (by GC-MS) with those of authentic samples, using
equipment described in ref 12b. In many cases the product mixture
was co-injected with the alleged product. Benzene (5), anisole (6),
iodobenzene (7), 4-tert-butylcyclohexanone (9b), cyclopentene (13),
cyclopentanone (14b), and 3-methoxycyclopentene (16) are com-
mercially available. Most other products were synthesized according
1
9b
to literature procedures: 4-tert-butyl-1-iodocyclohexene (3),
44
(
37) Huck, L. A.; Wan, P. Org. Lett 2004, 6, 1797-1799.
4-tert-butyl-1,1-dimethoxycyclohexane (4b), 4-tert-butylcyclo-
(38) (a) Fornarini, S.; Speranza, M.; Attina, M.; Cacace, F.; Giacomello,
45
46,47
44
hexene (8), 4-tert-butylcyclohex-1-enylbenzene (10a),
1-io-
P. J. Am. Chem. Soc. 1984, 106, 2498-2501. (b) Cramer, P.; Tidwell, T.
T. J. Org. Chem. 1981, 46, 2683-2686.
19b,48
docyclopentene (11),
1,1-dimethoxycyclopentane (12b), 1-meth-
(
39) (a) Newman, M. S.; Beard, C. D. J. Am. Chem. Soc. 1970, 92, 7564-
7
5
567. (b) Shchegolev, A. A.; Kanishchev, M. I. Russ. Chem. ReV. 1981,
(43) Ochiai, M.; Shu, T.; Nagaoka, T.; Kitagawa, Y. J. Org. Chem. 1997,
62, 2130-2138.
(44) Eliel, E. L.; Badding, V. G.; Rerick, M. N. J. Am. Chem. Soc. 1962,
84, 2371-2377.
(45) Stork, G.; White, W. N. J. Am. Chem. Soc. 1956, 78, 4604-4608.
(46) (a) Garbisch, E. W. J. Org. Chem. 1961, 26, 4165-4166. (b) Larock,
R. C.; Baker, B. E. Tetrahedron Lett. 1988, 29, 905-908.
(47) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020-
4028.
0, 553-564.
(40) Indirect formation has been reported (via π-participation or by using
release of ring strain as the driving force): (a) Mayr, H.; Seitz, B.;
Halberstadt-Kausch, I. K. J. Org. Chem. 1981, 46, 1041-1044. (b)
Reference 8c. (c) Reference 19.
(
41) Hanack, M. Angew. Chem. 1978, 90, 346-359.
(42) Bykowski, D.; McDonald, R.; Hinkle, R. J.; Tykwinski, R. R. J.
Org. Chem. 2002, 67, 2798-2804.
2234 J. Org. Chem., Vol. 71, No. 6, 2006

Generation of 6- and 5-Membered Cyclic Vinyl Cations
oxycyclopentene (14a),49 cyclopent-1-enylbenzene (15a),50 and
3
alongside 15a and 17a, are not present in the photoproduct mixture
of 2.
-phenylcyclopentene (17a).50 Four products, 4-tert-butyl-1-meth-
oxycyclohexene (9a) and the isomeric 4-tert-butylcyclohex-1-
enyliodobenzenes 10b-d, are assigned by comparison of the
Quantum Chemical Calculations. The computations were
5
2
performed with the Gaussian 03 program, version B3. All
optimizations on singlet and triplet states of I and I2a were
9
thermal solvolysis products of 1 with the products of the photolysis.
a
The isomeric cyclopent-1-enyliodobenzenes (15b-d) are tentatively
assigned just as the cyclopent-2-enyliodobenzene isomers (17b-
d) are. The MS patterns of the allylic products 17b-d differ from
those of the vinylic compounds 15b-d, which do resemble the
patterns obtained for the vinylic products 10b-d. In the irradiation
of 1 two products with a molecular mass of the diastereoisomers
performed with B3LYP/6-311G(2d,2p) computations to properly
account for ring strain effects. Relative thermodynamics on the
compounds under study (Table 3) were obtained by using the
CBS-Q model chemistry method.15
Acknowledgment. The authors would like to thank Professor
Jan Cornelisse of Leiden University for fruitful discussions.
4a are formed. The occurrence of these products has been certified
by comparing their retention times and mass spectra with the
products of the (9,10-dicyanoanthracene sensitized) photoreaction
of 4-tert-butyl-1-iodocyclohexene in methanol yielding 4a. The
formation of just one product 12a, identified by its mass spectrum,
in the photolysis of 2 is in line with these findings. Finally, two
reference compounds were synthesized that turned out to be no
JO0518957
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Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
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Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
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50
4
-methoxycyclopentene51 and 4-phenylcyclopentene, synthesized
(
(
48) Pross, A.; Sternhell, S. Aust. J. Chem. 1970, 23, 989-1003.
49) (a) Heiszwolf, G. J.; Kloosterziel, H. Recl. TraV. Chim. Pays-Bas
1
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(
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1
5
959, 81, 5833. (b) Fleming, I.; Thomas, E. J. Tetrahedron 1972, 28, 4989-
001.
J. Org. Chem, Vol. 71, No. 6, 2006 2235