J. Am. Chem. Soc. 1996, 118, 8949-8950
Scheme 1
8949
Novel Migrating Group in 1,2-Anionotropic
Reactions: Cobalt Complexation Facilitates
1,2-Shift of Alkynyl Groups
Tetsuya Nagasawa, Kimiko Taya, Mitsuru Kitamura, and
Keisuke Suzuki*,†
Department of Chemistry, Keio UniVersity
Hiyoshi, Yokohama 223, Japan
ReceiVed March 28, 1996
It is well documented that an alkyne-Co complex strongly
stabilizes a cationic charge at its R-position,1a a phenomenon
reminiscent of cyclopropylcarbinyl stabilization.1b The effect
is attributed to the metallacyclopropyl structure of the complex
isolobal to tetrahedrane,2 where participation of the C-Co bond-
(s) with the adjacent vacant orbital enables the effective charge
delocalization onto the cluster (see canonical forms A and A′).1a,2
By analogy, we focused our attention on the behaVior toward
the â-positiVe charge, with which a cyclopropyl group is known
to undergo neighboring group participation, ultimately undergo-
ing a 1,2-shift without ring opening as shown in B.3 The
corresponding behavior of the Co complex is unknown; does
the alkyne-Co complex behave similarly as shown in C? In
this communication, we report an affirmative answer to this
question: a Co-complexed alkynyl group is an excellent
migrator in 1,2-anionotropic reactions.4 The finding enables
the 1,2-shift of alkynyl groups (eq 1), otherwise an unfavorable
process,5,6 to be used to access various stereodefined alkyne-
containing building blocks of potentially broad synthetic utility.
Table 1. Me3Al-Promoted 1,2-Rearrangement of Complexes 3
R
run
product
T/°Ca
yield/%
1
2
3
4
n-Bu-
(3a)
(3b)
(3c)
(3d)
4a
4b
4c
4d
20
0
-20
20
78
84
80
58
Me3Si-
MeOCH2-
H2CdCMe-
a At this temperature, each reaction proceeds smoothly, thereby
completing within ca. 2 h.
sponding Co complex 3a underwent clean 1,2-shift to give the
ketone 4a in 61% yield.8 Oxidative decomplexation gave
unstable R-hexynyl ketone 2a in 99% yield.8
When Me3Al was used as a promoter, the reaction proceeded
smoothly at lower temperature, giving the ketone 4a in high
yield (Table 1, run 1).8,9 Notably, these Lewis acidic conditions
did not induce any Nicholas-type alkylation, even though the
expulsion of the hydroxyl in 3a could generate a highly
stabilized cationic species.1a As can be seen in other runs, the
migratory aptitudes of the complexes vary depending on the
alkynyl substituent. A silyl or a methoxymethyl group enhances
the migratory aptitude,10 while a propenyl group makes the
complex less prone to migrate (run 4).
The migratory aptitude of these complexes proved to be far
larger than we expected and, in fact, exceeds that of alkyl (see
eq 4) or even aryl groups as shown in eq 2.11 Competition
with a phenyl group (see the reaction of 5a) gave 6a as the
exclusive product.8,12,13 Surprisingly, the migratory aptitude of
Scheme 1 shows the preliminary experiment that gave the
initial promising result of our study. As reported by Wender
et al.,6a the alkynyl chlorohydrin 1a7 remained unchanged when
subjected to the conditions for alkoxide-induced 1,2-shift
(EtMgBr/benzene, 55 °C, 2 h). In sharp contrast, the corre-
(5) The poor migratory aptitude of an alkynyl group is attributed to the
deficiency of its π-electrons to participate to the neighboring cationic species
in terms of electronic (the sp hybridization) and/or spatial (the linearity)
factors. Computational studies showed that the activation energy of the
ethynyl shift is larger than, for example, that of the vinyl migration by
10.5 kcal/mol. Nakamura, K.; Osamura, Y. J. Am. Chem. Soc. 1993, 115,
9112.
(6) (a) Wender, P. A.; Holt, D. A.; Sieburth, S. M. J. Am. Chem. Soc.
1983, 105, 3348. (b) Suzuki, K.; Ohkuma, T.; Miyazawa, M.; Tsuchihashi,
G. Tetrahedron Lett. 1986, 27, 373. (c) Shoenen, F. J.; Porco, J. A.;
Schreiber, S. L.; VanDuyne, G. D.; Clardy, J. Tetrahedron Lett. 1989, 30,
3765.
(7) For preparation of the chlorohydrins, see supporting information.
(8) All new compounds were fully characterized by spectroscopic means
(1H and 13C NMR, IR), high-resolution MS, and/or combustion analysis
(see supporting information).
(9) The parent chlorohydrin 1a did not undergo 1,2-shift even after 2 h
reflux in CH2Cl2.
(10) Note that the reactions proceed at lower temperatures in runs 2 and
3. Origin of the substituent effect is under investigation. At present, we
assume that the migratory aptitude is enhanced by a substituent that stabilizes
a positive charge that would develop at the migrating cluster during the
1,2-shift by analogy with the R-cation stabilization (see A and A′).
(11) For organoaluminum-promoted 1,2-shift of aryl groups in â-mesy-
loxy alcohols, see: (a) Suzuki, K.; Katayama, E.; Tsuchihashi, G.
Tetrahedron Lett. 1983, 24, 4997. For a classical example under solvolytic
conditions, see: (b) Winstein, S.; Lindegren, C. R.; Marshall, H.; Ingraham,
L. L. J. Am. Chem. Soc. 1953, 75, 147.
† Present address: Department of Chemistry, Tokyo Institute of Technol-
ogy, Meguro-ku, Tokyo 152, Japan.
(1) (a) For a review, see: Nicholas, K. M. Acc. Chem. Res. 1987, 20,
207. (b) Olah, G. A.; Reddy, V. P.; Prakash, G. K. Chem. ReV. 1992, 92,
69 and references cited therein.
(2) (a) Hoffman, D. M.; Hoffmann, R.; Fisel, C. R. J. Am. Chem. Soc.
1982, 104, 3858 and references cited therein. (b) Schreiber, S. L.; Sammakia,
T.; Crowe, W. E. J. Am. Chem. Soc. 1986, 116, 5505. (c) Schreiber, S. L.;
Klimas, M. T.; Sammakia, T. J. Am. Chem. Soc. 1987, 109, 5749.
(3) For the 1,2-shift of cyclopropyl groups, see: (a) Shono, T.; Fujita,
K.; Kumai, S.; Watanabe, T.; Nishiguchi, I. Tetrahedron Lett. 1972, 3249.
(b) Shimazaki, M.; Hara, H.; Suzuki, K. Tetrahedron Lett. 1989, 30, 5443.
(4) For reviews on 1,2-anionotropic reactions, see: (a) Saunders, M.;
Chandrasekhar, J.; Schleyer, P. v. R. In Rearrangement in Ground and Exci-
ted States; Mayo, P. de, Ed.; Academic Press: New York, 1980; Vol. 1, p
1. (b) Shubin, V. G. In Topics in Current Chemistry; Rees, C., Ed.; Springer:
Berlin, 1984; Vol. 116-117, p 267. (c) Collins, C. J.; Eastham, J. F. The
Chemistry of the Carbonyl Group (The Chemistry of Functional Groups);
Patai, S., Ed.; Wiley: New York, 1966; Chapter 15, p 761. (d) Barto´k, M.;
Molna´r, AÄ . The Chemistry of Functional Groups, Supplement E; Patai, S.,
Ed.; Wiley: New York, 1980; Chapter 16, p 721. For reviews on pinacol
rearrangement, see: (e) Rickborn, B. In ComprehensiVe Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 3, Chapter
3.2, p 721. (f) Suzuki, K. J. Synth. Org. Chem., Jpn. 1988, 46, 365.
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