Rhodium-catalyzed phenylthiolation reaction of heteroaromatic compounds using α-(phenylthio)isobutyrophenone

Article abstract of DOI:10.1016/j.tetlet.2011.02.077

In the presence of catalytic amounts of RhH(PPh₃)₄ and 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-benzothiazoles, 1,3-benzoxazoles, and benzothiophene reacted with α-(phenylthio) isobutyrophenone giving 2-phenylthio derivatives. Reactive monocyclic heteroaromatics, 1-methyl-1,2,3,4-tetrazole and 2-cyanothiophene were also converted into the 5-phenylthio derivatives. The use of an appropriate phenylthio transfer reagent is crucial for the efficient catalyzed conversion of heteroaromatic C-H bonds into C-S bonds.

Full text of DOI:10.1016/j.tetlet.2011.02.077

Tetrahedron Letters 52 (2011) 2344–2347
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rhodium-catalyzed phenylthiolation reaction of heteroaromatic
compounds using a-(phenylthio)isobutyrophenone
Mieko Arisawa a, Fumihiko Toriyama b, Masahiko Yamaguchi a,c,
⇑
a Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan
b Process Technology Research Laboratories, Daiichi Sankyo Co., Ltd, Hiratsuka, Kanagawa 254-0014, Japan
c WPI Advanced Institute for Materials Research, Tohoku University, Aoba, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In the presence of catalytic amounts of RhH(PPh3)4 and 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-
Received 13 January 2011
Revised 14 February 2011
Accepted 18 February 2011
Available online 22 February 2011
benzothiazoles, 1,3-benzoxazoles, and benzothiophene reacted with
a-(phenylthio)isobutyrophenone
giving 2-phenylthio derivatives. Reactive monocyclic heteroaromatics, 1-methyl-1,2,3,4-tetrazole and
2-cyanothiophene were also converted into the 5-phenylthio derivatives. The use of an appropriate phen-
ylthio transfer reagent is crucial for the efficient catalyzed conversion of heteroaromatic C–H bonds into
C–S bonds.
Keywords:
Ó 2011 Published by Elsevier Ltd.
Phenylthiolation
Rhodium catalyst
Heteroaromatics
C–H activation
Equilibrium
Benzothiazole
O
O
O
O
O
Rh cat
Heteroaromatic compounds possessing organothio groups are
potentially useful as drugs and materials. In general, such com-
pounds are synthesized by the reaction of halogenated heteroaro-
matics and thiolate anions1 or the reaction of heteroaromatic
thiolates and organohalogen compounds.2,3 The direct conversion
of a heteroaromatic C–H bond into a C–S bond is attractive for such
synthesis, and electrophilic reactions are known for electron-rich
heteroaromatic compounds, typically indoles.4 Recently, base-
promoted reactions of 1,3-benzoxazoles, 1,3-benzothiazoles, and
related heteroarenes have been reported.5,6 The use of transition
metal catalysis for such transformation is interesting, because the
method has the advantage of not employing an acid or base and
can also exhibit different reactivities from conventional methods.
The fact that the catalyzed method, however, was not known
was ascribed to the lack of methodology and/or concepts to devel-
op the chemical process. The reaction should proceed via C–H
activation, activation of the sulfur reagent, and C–S bond forma-
tion. The series of effective activations results in low total activa-
tion energy of the reaction, which makes the overall reaction
reversible and under equilibrium. The relative thermodynamic
stability of substrates and products must then be considered.
Development of a methodology for controlling both kinetic and
SMe
SMe
p-NCC6H4
+
R
+
+
+
p-NCC6H4
R
R
SMe
X
X
1
2
X = SPh, Ph
O
R
O
O
Rh cat
Rh cat
Ph
Ph
SMe
+
Ph
Ph
Ph
Ph
3
4
N
S
O
O
N
SPh
+
S
SPh
5
6
7
8
Scheme 1.
thermodynamic properties of heteroaromatic C–H functionaliza-
tion is a challenge.
During our investigations on the development of transition-me-
tal-catalyzed synthetic methods for organosulfur compounds,7 we
reported reactions that converted the relatively acidic protons of
organic compounds into sulfides. The reaction of 1-alkynes and
disulfides gave 1-alkylthio-1-alkynes.8 The
reaction of
-phenylthio ketones9 and
ducted using -methylthio-p-cyanoacetophenone 1 as the methyl-
thio donor. In this reaction, the methylthio group was transferred
a
-methylthiolation
a
a
-phenyl ketones10 was con-
a
⇑
Corresponding author. Fax: +81 22 795 6811.
E-mail address: yama@mail.pharm.tohoku.ac.jp (M. Yamaguchi).
0040-4039/$ - see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2011.02.077

M. Arisawa et al. / Tetrahedron Letters 52 (2011) 2344–2347
2345
RhH(PPh3)4 (4 mol%)
dppe (8 mol%)
from 1 to the
tion of p-cyanoacetophenone 2. Unactivated ketones possessing
less acidic -protons were not methylthiolated by 1 but by 2-
a-position of the ketone with the concomitant forma-
N
S
O
O
N
S
SR
+
+
Ph
Ph
a
C6H5Cl, refl., 3 h
SR
5 (1 eq)
methylthio-1,2-diphenyl-1-ethanone 3, which was converted into
1,2-diphenyl-1-ethanone 4 during the reaction (Scheme 1).11 The
higher efficiency of 3 than of 1 in the reaction with unactivated
ketones could be ascribed to the stronger C–S bond of 3 than of 1,
which shifted the equilibrium in the desired direction. To extend
this method to the functionalization of heteroaromatic C–H bonds
with even less acidic protons, it was considered crucial to develop
an appropriate methylthio donor. Described in this study is the
8
23%
2%
R = Me (11)
R = Bn
R = C6H4OMe-p 52%
R = C6H4Me-p
R = C6H5 (7)
91%
92%
26%
12%
R = C6H4Cl-p
R = C6H4CF3-p
Scheme 4.
use of a-(phenylthio)isobutyrophenone 6, which effectively trans-
ferred the phenylthio group to heteroaromatic compounds such
as 1,3-benzothiazole 5 giving 2-phenylthio-1,3-benzothiazole 7
with the formation of isobutyrophenone 8 (Scheme 1).
On the basis of our previous observation that 3 is a better
methylthio donor than 1, the reaction of 1,3-benzothiazole 5 and
3 was examined; however, it gave no methylthiolated product.
RhH(PPh3)4 (4 mol%)
dppe (8 mol%)
N
S
O
O
N
S
SPh
+
+
Ph
Ph
X
6
C6H5Cl, refl., 3 h
X
SPh
6
8
(7)
X = H
92%
X = OMe 66%
X = Me
X = Cl
91%
94%
Then,
a-(methylthio)propiophenone 9, which was considered to
have a stronger C–S bond than 3 and, therefore, to be a stronger
methylthio donor was reacted with 6. When 6 (5 equiv) was trea-
ted with 9 (1 equiv) in the presence of RhH(PPh3)4 (4 mol %), dppe
(8 mol %), and dimethyl disulfide (12 mol %) in refluxing THF for
3 h, the desired 2-methylthio-1,3-benzothiazole 11 was obtained
X = CF3 92%
Scheme 5.
of either RhH(PPh3)4 or dppe. The reaction of 5 and 10 under the
same conditions gave 11 only in 23% yield, and a similar result
was obtained with the benzylthio derivative. The higher yield of
the phenylthio derivative 7 than of the methylthio 11 may be re-
lated to the dependence of the reaction on the relative C–S bond
energies of the substrate and product. A study of the p-substituent
effect at the arylthio moiety revealed phenyl or p-tolyl derivatives
to be more favorable than the p-methoxy, p-chloro, and p-trifluoro-
methyl. The reaction of several 6-substituted 1,3-benzothiazoles
was conducted under the same conditions, and the corresponding
2-phenylthio-1,3-benzothiazoles were obtained in high yields.
Exceptions were those with electron-withdrawing groups, 6-nitro-
and 6-methoxycarbonyl-1,3-benzoxazoles, which gave the product
in <5% yield (Scheme 5).
1,3-Benzoxazole 14 (3 equiv) and its 5- and 6-substituted deriv-
atives were also reacted with 6 in the presence of the rhodium cat-
alyst (Scheme 6). The 5- and 6-derivatives with methoxy, methyl,
hydrogen, and chloro groups gave the 2-phenylthiolated products
in high yields. However, the 6-trifluoromethyl derivative gave only
19% yield of the product, and 5-cyano and 5-methoxycarbonyl re-
sult in essentially no product. The significantly reduced reactivity
of the substrates with electron-withdrawing groups suggested an
important role of the basicity of 1-oxygen and/or 3-nitrogen in
the reaction rather than the acidity at the 2-position. The reaction
of 3-cyanobenzothiophene gave the 2-phenylthio derivative in 80%
yield (Scheme 7). Benzothiophene itself was inert, and the pres-
ence of the cyano activating group was essential for this
transformation.
in 13% yield based on 9 (Scheme 2).12 The use of
a-(methylthio)iso-
butyrophenone 10 improved the yield of 11 to 43%. The effect of
the aromatic substituent on isobutyrophenone was examined,
and the phenyl derivative gave a higher yield of 11 than of the
derivatives with electron-donating or electron-withdrawing
groups (Scheme 3). The occurrence of optimum yields with respect
to electronic properties suggested the equilibrium nature of this
reaction, in other words, the essential role of thermodynamics bal-
ances between substrates and products.
The reactivities of several
were compared. Notably, the yield of 2-organothiolated 1,3-benzo-
thiazole increased considerably with the use of -(phenylthio)iso-
a-(organothio)isobutyrophenones
a
butyrophenone 6. When 5 (1 equiv) was reacted with 6 (1 equiv) in
the presence of RhH(PPh3)4 (4 mol %), and dppe (8 mol %) in reflux-
ing chlorobenzene for 3 h, 2-phenylthio-1,3-benzothiazole 7 was
obtained in 92% yield, which was accompanied by 8 in 94% yield
(Scheme 4). It was confirmed that 7 was not formed in the absence
RhH(PPh3)4 (4 mol%)
dppe (8 mol%)
O
N
S
O
N
S
R1
R2
Me2S2 (12 mol%)
SMe
R1
+
+
Ph
Ph
THF, refl., 3 h
R2
SMe
5 (5 eq)
11
R1 = Ph, R2 = H
Not detected
3
9
R1 = Me, R2 = H
13%
43%
10 R1 = R2 = Me
Scheme 2.
RhH(PPh3)4 (4 mol%)
3
N
5
6
X
Y
N
O
X
dppe (8 mol%)
+
6
+
8
SPh
RhH(PPh3)4 (4 mol%)
2
dppe (8 mol%)
C6H5Cl, refl., 6 h
O
O
N
O
Y
N
S
1
Me2S2 (12 mol%)
SMe
+
+
3 eq
Ar
Ar
S
X = MeO, Y = H
X = Me, Y = H
X = Y = H (14)
X = Cl, Y = H
80%
79%
85%
86%
48%
THF, refl., 3h
SMe
5 (5 eq)
11
Ar = C6H4OMe-p
Ar = C6H4Me-p
Ar = C6H5
X = CF3, Y = H
21%
23%
43%
15%
10%
X = H, Y = MeO
X = Y = H (14)
X = H, Y = CF3
70%
85%
19%
Ar = C6H4Cl-p
Ar = C6H4CN-p
Scheme 3.
Scheme 6.

2346
M. Arisawa et al. / Tetrahedron Letters 52 (2011) 2344–2347
CN
O
O
O
3
CN
SPh
RhH(PPh3)4 (4 mol%)
dppe (8 mol%)
O
Rh cat
+
SPh
SPh
+
Ph
Ph
Ar
Ar
+
6
+
8
Ph
S
H
S
SMe
SMe
C6H5Cl, refl., 6 h
16
1
2
H
pKa +4.9
pKa 17.1
pKa 22.0
80%
Ar = p-NCC6H4
Scheme 7.
O
O
O
O
Rh cat
Ph
SMe
+
Ph
+
Ar
Ar
Ph
RhH(PPh3)4 (4 mol%)
dppe (8 mol%)
SMe
3
H
O
O
4
H
1
pKa +4.3
2
N
N
+
SPh
D
+
Ar = p-NCC6H4
pKa 22.0
pKa 17.7
Ph
Ph
O
12
C6H5Cl, refl., 3 h
O
SPh
13 (5 eq)
16%
14
17%
O
RhH(PPh3)4 (4 mol%)
dppe (8 mol%)
O
O
O
O
O
N
N
S
Rh cat
+
Ph
Ph
+
+
SPh
+
Ph
Ph
Ph
Ph
Ph
Ph
S
C6H5Cl, refl., 3h
H
SMe
SMe
13
pKa 24.4
3
SMe
4 H
SPh
D
9
p
K
a –6.7
6
8-d
5-d (99%d)
7 93%
pKa 17.7
99%
(74%d)
O
Scheme 8.
O
N
X
Rh cat
N
H
+
SMe
+
Ph
Ph
X
14
(X = O)
pKa 24.4
5 (X = S)
pKa 27.0
H
13
9
p
a
0
15 (X = O)
K
The reversibility of the methyl transfer reaction was confirmed
(Scheme 8). The treatment of 2-phenylthio-1,3-benzoxazole 12
and propiophenone 13 (5 equiv) with the catalyst in refluxing chlo-
pKa 24.4
11 (X = S)
pKa –2.6
robenzene for 3 h gave 14 (16%) and a-(phenylthio)propiophenone
(17%), which were accompanied by the recovery of 12 in 62% yield.
The treatment of 12 with isobutyrophenone gave a very low yield
of the product, which again showed the importance of the combi-
nation of a substrate and a methylthio donor. The metathesis of the
phenylthio C–S bond and heteroaromatic C–H bond was confirmed
by an experiment using deuterated 5-d.
1-Methyl-1,2,3,4-tetrazole underwent the reaction giving
1-methyl-5-phenylthio-1,2,3,4-tetrazole in 43% yield (Scheme 9).
2-Cyanothiophene was phenylthiolated giving 2-cyano-5-(phenyl-
thio)thiophene in 19% yield using 6, whereas thiophene itself
was inert under the same conditions; monocyclic 1,3-thiazole
and 1,3-oxazole were also inert. An activated heteroaromatic
system was required for this reaction, probably because of the
higher acidity of the protons.
O
N
S
O
Rh cat
N
H
+
SPh +
Ph
Ph
S
7
H
SPh
pKa –0.7
5
6
8
pKa 27.0
pKa 26.3
Scheme 10.
be overcome in the organothiolating reaction, because the relative
thermodynamic stabilities of reactant and product are governed by
the C–S bond energy as well as by the C–H bond energy. The reac-
tion of less acidic propiophenone 13 (pKa 24.415), which proceeded
with 1-methylthio-1,2-diphenyl-2-ethanone 3, was examined
next. The pKa of 1,2-diphenyl-2-ethanone 4 is 17.7, and
DpKa is –
Our study of the rhodium-catalyzed organothiolation reaction
was conducted by changing the substrates from acidic to less
6.7 for this reaction. As indicated in this work, 1,3-benzoxazole
1412 (pKa 24.413) and 1,3-benzothiazole 5 (pKa 27.013) were meth-
ylthiolated with 9 giving the 2-methylthio derivatives 15 and 11
acidic,
a-phenylthio ketones, a-phenyl ketones, unactivated ke-
tones, and 1,3-benzoxazoles/1,3-benzothiazoles. The idea was to
find an appropriate organothio donor for each substrate to control
with the formation of propiophenone 13; the
and –2.6 for these reactions, respectively. The yield increased in
the reaction of 5 using -(phenylthio)isobutyrophenone 6 giving
2-phenylthio-1,3-benzothiazole 7; pKa is –0.7 for this reaction.
DpKa values are 0
the equilibrium. Initially, the methylthiolation of
thio)acetophenone 16 was conducted with -methylthio-p-cyano-
acetophenone 1 with the formation of p-cyanoacetophenone 2
(Scheme 10). The pKa values of 16 and 2 are 17.113 and 22.014
respectively, which provided a pKa difference of pKa +4.9 be-
tween the reactant and the product. Compound 1 could also be
used for the reaction of -(phenyl)acetophenone 4. The pKa values
of 4 and 2 are 17.713 and 22.0, respectively, and
pKa +4.3. The
a-(phenyl-
a
a
D
The higher efficiency of 6 than that of 9 may be the result of a fine
adjustment in the balance between the reactants and the products
at equilibrium.
,
D
A notable aspect of this series of equilibrium reactions is that
product 3 obtained in the reaction of 1 and 4 was effectively em-
ployed in the reaction of less acidic 13. Analogously, 9 obtained
in the reaction of 3 and 13 was used in the reaction of the even less
acidic 5 and 14. It was also noted that 1 was not effective for the
reaction of 13 and 5; 3 was not effective for 5. This method of cor-
relating reactants and products can be useful for the control of
equilibrium reactions involving C–H activation.
a
D
results indicated that these magnitudes of pKa differences could
RhH(PPh3)4 (4 mol%)
N
N
N
N
N
dppe (8 mol%)
N
N
6
+
SPh
43%
+
8
8
N
C6H5Cl, refl., 3 h
Me
Me
Typical experimental procedures
RhH(PPh3)4 (4mol%)
dppe (8 mol%)
In a two-necked flask equipped with a reflux condenser were
placed RhH(PPh3)4 (4 mol %, 11.5 mg), dppe (8 mol %, 8.0 mg),
SPh
+
6
+
S
NC
S
C6H5Cl, refl., 3 h
NC
1,3-benzothiazole 5 (0.25 mmol, 27.3
butyrophenone (0.25 mmol, 64.0 mg) in chlorobenzene
(0.25 mL) under an argon atmosphere, and the solution was heated
lL), and (a-phenylthio)iso-
19%
6
Scheme 9.

M. Arisawa et al. / Tetrahedron Letters 52 (2011) 2344–2347
2347
Chem. Soc. Jpn. 2009, 82, 381; (f) Kotali, E.; Varvoglis, A. J. Chem. Res. 1989, 5,
142.
at reflux for 3 h. The mixture was purified by flash column chroma-
tography on silica gel giving 7 (55.8 mg, 92%) and isobutyrophe-
none 8 (34.3 mg, 93%) with the recovery of 5 (3.5 mg, 10%) and 6
(0.8 mg, 1%).
3. For transition-metal-catalyzed reactions of heteroarmatic thiols and
organohalogen compounds, see: Pd: (a) Schopfer, U.; Schlapbach, A.
Tetrahedron 2001, 57, 3069; (b) Barbiéri, R. S.; Bellato, C. R.; Dias, A. K. C.;
Massabni, A. C. Catal. Lett. 2006, 109, 171; Fe: (c) Correa, A.; Carril, M.; Bolm, C.
Angew. Chem., Int. Ed. 2008, 47, 2880; (d) Wu, W.-Y.; Wang, J.-C.; Tsai, F.-Y.
Green Chem. 2009, 11, 326; Cu: (e) Savarin, C.; Srogl, J.; Liebeskind, L. S. Org. Lett.
2002, 4, 4309; (f) Prasad, D. J. C.; Naidu, A. B.; Sekar, G. Tetrahedron Lett. 2009,
50, 1411; (g) Prasad, D. J. C.; Sekar, G. Synthesis 2010, 79; (h) Feng, Y. S.; Li, Y. Y.;
Tang, L.; Wu, W.; Xu, H. J. Tetrahedron Lett. 2010, 51, 2489; (i) Larsson, P.-F.;
Correa, A.; Carril, M.; Norrby, P.-O.; Bolm, C. Angew. Chem., Int. Ed. 2009, 48,
5691; (j) Wang, H.; Jiang, L.; Chen, T.; Li, Y. Eur. J. Org. Chem. 2010, 2324.; Fe-Cu:
(k) Ku, X.; Huang, H.; Jiang, H.; Liu, H. J. Comb. Chem. 2009, 11, 338.
4. For direct sulfenylation reaction of indoles with electrophilic sulfur reagents,
see: (a) Schlosser, K. M.; Krasutsky, A. P.; Hamilton, H. W.; Reed, J. E.; Sexton, K.
Org. Lett. 2004, 6, 819; (b) Campbell, J. A.; Broka, C. A.; Gong, L.; Walker, K. A.
M.; Wang, J.-H. Tetrahedron Lett. 2004, 45, 4073; (c) Yadav, J. S.; Reddy, B. V. S.;
Reddy, Y. J. Tetrahedron Lett. 2007, 48, 7034; Transition-metal-catalyzed
reactions (d) Ranken, P. F.; McKinnie, B. G. J. Org. Chem. 1989, 54, 2985; (e)
Maeda, Y.; Koyabu, M.; Nishimura, T.; Uemura, S. J. Org. Chem. 2004, 69, 7688;
(f) Yadav, J. S.; Reddy, B. V. S.; Reddy, Y. J.; Praneeth, K. Synthesis 2009, 1520; (g)
Fang, X.-L.; Tang, R.-Y.; Zhong, P.; Li, J.-H. Synthesis 2009, 4183.
5. Fukuzawa, S.-i.; Shimizu, E.; Atsuumi, Y.; Haga, M.; Ogata, K. Tetrahedron Lett.
2009, 50, 2374.
6. Popov, I.; Do, H.-Q.; Daugulis, O. J. Org. Chem. 2009, 74, 8309.
7. Review Arisawa, M.; Yamaguchi, M. J. Synth. Org. Chem., Jpn. 2007, 65, 1213;
Arisawa, M.; Yamaguchi, M. Pure Appl. Chem 2008, 80, 993.
8. Arisawa, M.; Fujimoto, K.; Morinaka, S.; Yamaguchi, M. J. Am. Chem. Soc. 2005,
127, 12226.
9. Arisawa, M.; Suwa, K.; Yamaguchi, M. Org. Lett. 2009, 11, 625.
10. Arisawa, M.; Toriyama, F.; Yamaguchi, M. Heteroat. Chem. 2011, 22, 18.
11. Arisawa, M.; Toriyama, F.; Yamaguchi, M. Tetrahedron 2011, 67, 2305.
12. When 1,3-benzoxazole 14 was reacted under the same conditions,
2-methylthio-1,3-benzoxazole 15 was obtained in 5% yield.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Re-
search (No. 21229001), the GCOE program, and the WPI Initiative
from JSPS. M.A. expresses her appreciation to the financial supports
from the Grant-in-Aid for Scientific Research (No. 22689001) and
to the Asahi Glass Foundation.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.02.077.
References and notes
1. (a) Brooker, L. G. S.; White, F. L.; Keyes, G. H.; Smyth, C. P.; Oesper, P. F. J. Am.
Chem. Soc. 1941, 63, 3192; (b) Rakowitz, D.; Hennig, B.; Nagano, M.; Steger, S.;
Costantino, L.; Matuszczack, B. Arch. Pharm. Chem. Life Sci. 2005, 338, 411; (c)
Degani, I.; Fochi, R.; Regondi, V. Synthesis 1983, 630; (d) Rodriguez, R.;
Chapelon, A.-S.; Ollivier, C.; Santelli, M. Tetrahedron Lett. 2009, 65, 7001; (e)
Lohmann, S.; Andrews, S. P.; Burke, B. J.; Smith, M. D.; Attfield, J. P.; Tanaka, H.;
Kaneko, K.; Ley, S. V. Synlett 2005, 1291.
2. (a) Bethge, L.; Jarikote, D. V.; Seitz, O. Bioorg. Med. Chem. 2008, 16, 114; (b)
Ghosh, A. K.; Zajc, B. Org. Lett. 2006, 8, 1553; (c) Hu, Y.; Chen, Z.-C.; Le, Z.-G.;
Zheng, Q.-G. Synth. Commun. 2004, 34, 2039. and references cited therein.; (d)
Shieh, W.-C.; Lozanov, M.; Loo, M.; Repic, O.; Blacklock, T. J. Tetrahedron Lett.
2003, 44, 4563; (e) Kuroda, K.; Maruyama, Y.; Hayashi, Y.; Mukaiyama, T. Bull.
13. Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
14. Bordwell, F. G.; Cornforth, F. J. J. Org. Chem. 1978, 43, 1763.
15. Bordwell, F. G.; Harrelson, J. A., Jr. Can. J. Chem. 1990, 68, 1714.