Photochemical promotion of the intramolecular pauson - Khand Reaction. a new experimental protocol for cobalt-catalyzed [2 + 2 + 1] cycloadditions

Full text of DOI:10.1021/ja9535975

J. Am. Chem. Soc. 1996, 118, 2285-2286
2285
Table 1. Comparison of Reaction Conditions for the
Photopromoted Pauson-Khand Reaction
Photochemical Promotion of the Intramolecular
Pauson-Khand Reaction. A New Experimental
Protocol for Cobalt-Catalyzed [2 + 2 + 1]
Cycloadditions
Brian L. Pagenkopf and Tom Livinghouse*,1
Department of Chemistry and Biochemistry
Montana State UniVersity, Bozeman, Montana 59717
ReceiVed October 26, 1995
Metal-mediated cocyclization reactions are becoming increas-
ingly popular as tools for selective organic synthesis.2 Since
its disclosure in 1973, the Pauson-Khand reaction, which
represents a formal [2 + 2 + 1] cycloaddition involving an
alkyne, alkene, and carbon monoxide, has become regarded as
one of the most convergent methods for the synthesis of
cyclopentenones.3 A major disincentive to the large-scale use
of the classical Pauson-Khand reaction rests in its requirement
for stoichiometric quantities of Co2(CO)8.4 Very recently, Jeong
and collaborators reported that selected 1,6-enynes could be
converted to the corresponding cyclopentenones in the presence
of CO (at 4-5 atm pressure), 3-5 mol % Co2(CO)8 and 10-
20 mol % (PhO)3P at 120 °C.5 Although highly significant as
one of a very limited number of successful accounts involving
truly catalytic quantities of Co2(CO)8,6 the relatively strenuous
conditions of the Jeong procedure may discourage its widespread
adoption in many laboratory settings. In this communication,
we report that high-intensity Visible light effectiVely promotes
catalytic Pauson-Khand reactions at 50-55 °C and 1 atm of
CO pressure. It has previously been disclosed that tertiary
amine N-oxides and DMSO markedly accelerate stoichiometric
Pauson-Khand reactions.7 These reagents have been proposed
to function via the oxidative removal of CO, thereby providing
an empty coordination site for alkene complexation. For many
years it has been known that various metal carbonyl complexes
undergo photoinduced CO dissociation.8 It is therefore most
a All yields given in this table correspond to isolated, chromato-
graphically purified product.
surprising that this simple expedient has not been reported in
connection with facilitating catalytic Pauson-Khand reactions.9
Our first attempts at photopromotion using 10 mol % Co2-
(CO)8 under 1 atm of CO were not satisfying in that only low
levels (<30%) of substrate conversion were realized. We
10
ultimately discovered that the purity of Co2(CO)8 and the
choice of an appropriate light source11 were both critically
important for successful catalytic reactions. In addition, reaction
temperatures in the range of 50 °C were found necessary for
reasonable rates of product formation. Among the several light
sources examined, the Q-Beam MAX MILLION 106 candle-
power spotlight distributed by Brinkmann, Inc.,11,12 proved
extraordinary for photoinitiation. A summary of various
procedures that were utilized for the carbonylative cyclization
of enyne 1a to the corresponding cyclopentenone 1b under 1
atm of CO appears in Table 1.13
Of the various solvents that were examined, only 1,2-
dimethoxyethane (1,2-DME), ethyl acetate, and diglyme permit-
ted efficient conversion of 1a to 1b (in 95%, 86%, and 69%
isolated yields, respectively). Hexane, THF, and CH3CN were
(1) Fellow of the Alexander von Humboldt Foundation 1993-1995.
(2) Lautens, M.; Klute, W.; Tam, W. Chem. ReV., in press.
(3) (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.; Foreman,
M. I. J. Chem. Soc., Perkin Trans. 1 1973, 977. (b) Pauson, P. L.
Tetrahedron 1985, 41, 5855. (c) Schore, N. E. Chem. ReV. 1988, 88, 1081.
(d) Shore, N. E. Org. React. (N.Y.) 1991, 41, 1. (e) Schore, N. E. In
ComprehensiVe Organic Synthesis; Trost, B. M., Ed.; Pergamon: Oxford,
1991; Vol. 5, p 1037. (f) Schore, N. E. In ComprehensiVe Organometallic
Chemistry II; Hegedus, L. S.; Elsevier: Oxford, 1995; Vol. 12, p 703.
(4) (a) Smit, W. A.; Simonyan, S. O.; Tarasov, V. A.; Mikaelian, G. S.;
Gybin, A. S.; Ibragimov, I. I.; Caple, R.; Froen, D.; Kreager, A. Synthesis
1989, 472 and references therein. (b) Krafft, M. E.; Scott, I. L.; Romero,
R. H.; Feibelmann, S.; Van Pelt, C. E. J. Am. Chem. Soc. 1993, 115, 7199.
For examples involving the use of a substoichiometric amount of Co2(CO)8,
see: (c) Billington, D. C. Tetrahedron Lett. 1983, 24, 2905. (d) Magnus,
P.; Principe, L. M.; Slater, M. J. J. Org. Chem. 1987, 52, 1483. (e)
Billington, D. C.; Kerr, W. J.; Pauson, P. L.; Farnocchi, C. F. J. Organomet.
Chem. 1988, 356, 213. (f) MacWhorter, S. E.; Sampath, V.; Olmstead, M.
M.; Schore, N. E. J. Org. Chem. 1988, 53, 203. (g) A batch-catalytic
protocol with W(CO)5THF has been reported by Hoye and Suriano: Hoye,
T. R.; Suriano, J. A. J. Am. Chem. Soc. 1993, 115, 1154.
(9) Several instances of photochemically driven stoichiometric Pauson-
Khand reactions which proceed in low yield have been reported: Brown,
S. W.; Pauson, P. L. J. Chem. Soc., Perkin Trans. 1 1990, 1205.
(10) Impure samples of commercial Co2(CO)8 must be rigorously purified
by recrystallization from degassed HPLC grade hexane or room temperature
sublimation at 50 mTorr immediately prior to use. For the majority of the
cyclizations described herein, a freshly opened sample of Co2(CO)8 (Strem
Chemical Co., Inc.) which was stored in a Vacuum Atmospheres drybox
was utilized.
(11) Barton, D. H. R.; Jaczberenyi, J. Cs.; Tang, D. Tetrahedron Lett.
1993, 34, 3381.
(12) Purchased at K-Mart, Inc., 1126 N. 7th Ave., Bozeman, MT.
(13) Representative procedure: A solution of enyne 1a (119 mg, 0.5
mmol) and dicobalt octacarbonyl (8.5 mg, 0.025 mmol, 5 mol %) in
degassed 1,2-DME (5 mL) was magnetically stirred at room temperature
under an atmosphere of CO. After 30 min, the reaction mixture was
irradiated with a Q-beam MAX MILLION spotlight. During the course of
irradiation, the position of the lamp was adjusted such that the internal
reaction temperature was maintained between 50 and 55 °C. After 4 h,
irradiation was discontinued and the cooled reaction mixture was diluted
with EtOAc (15 mL), washed with brine (2 × 2 mL), dried (Na2SO4),
filtered, and concentrated in vacuo. The residual material was purified by
chromatography on silica gel (30-50% EtOAc/hexane gradient for elution)
to afford 127 mg (95%) of enone 1b as a slightly yellow oil. For preparative
scale, longer irradiation times are required. Accordingly, a solution of enyne
1a (4.77 g, 20.0 mmol) and dicobalt octacarbonyl (274 mg, 0.8 mmol, 4
mol %) in degassed 1,2-DME (200 mL) was magnetically stirred at room
temperature under an atmosphere of CO for 30 min. The resulting solution
was then irradiated for 16 h at 50-55 °C. The reaction mixture was
concentrated in vacuo, and the resulting oil was purified by chromatography
on silica gel (10-50% EtOAc/hexane gradient for elution) to afford 4.41
g (83%) of enone 1b as a yellow oil.
(5) Jeong, N.; Hwang, S. H.; Lee, Y.; Chung, Y. K. J. Am. Chem. Soc.
1994, 116, 3159.
(6) Rautenstrauch, V.; Megard, P.; Conesa, J.; Kuster, W. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 1413. More recently, the catalytic conversion of
enynes into bicyclic cyclopentenones employing the early transition metal
complex Cp2Ti(PMe3)2 had been reported: Berk, S. C.; Grossman, R. B.;
Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 8593.
(7) (a) Shambayati, S.; Crowe, W. E.; Schreiber, S. L. Tetrahedron Lett.
1990, 31, 5289. (b) Jeong, N.; Chung, Y. K.; Lee, B. Y.; Lee, S. H.; Yoo,
S.-e. Synlett 1991, 204. (c) Chung, Y. K.; Lee, B. Y.; Jeong, N.; Hudecek,
M.; Pauson, P. L. Organometallics 1993, 12, 220.
(8) For recent applications of organometallic photochemistry in synthesis,
see: (a) Rigby, J. H.; Ateeq, H. S.; Charles, N. R.; Cuisiat, S. V.; Ferguson,
M. D.; Henshilwood, J. A.; Krueger, A. C.; Ogbu, C. O.; Short, K. M.;
Heeg, M. J. J. Am. Chem. Soc. 1993, 115, 1382. (b) Hegedus, L. S.; de
Weck, G.; D’Andrea, S. J. Am. Chem. Soc. 1988, 110, 2122.
0002-7863/96/1518-2285$12.00/0 © 1996 American Chemical Society

2286 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Communications to the Editor
Table 2. Catalytic Pauson-Khand Photocyclizations under 1 atm of COa
a Unless otherwise note, all reactions were conducted using 5 mol % Co2(CO)8 and a substrate concentration of 0.1 M in 1,2-DME under
irradiation for 12 h at 50-55 °C. b All yields given in this table correspond to isolated, chromatographically purified products. c Irradiation time )
4 h. d Ratio of diastereomers ) 1.1:1.0. e Reaction performed with 10 mol % Co2(CO)8. f Reaction performed with 12.5 mol % Co2(CO)8.
all determined to be inferior. In general, carbonylative cycliza-
tion of representative enynes (0.1 M in DME) could be effected
by simply stirring in the presence of 5 mol % Co2(CO)8 under
1 atm of CO pressure at 50-55 °C with “Q-beam” irradiation
for 12 h.13 A series of catalytic enyne-CO cocyclizations
involving a wide range of substrates is shown in Table 2.
Several of the results presented in Table 2 are worthy of
comment. The successful conversion of 6a to 6b demonstrates
the utility of the new photochemical procedure for the cycliza-
tion of 1,7-enynes. The efficient cyclization of 8a shows the
compatiblity of the prescribed reaction conditions with relatively
labile hydroxyl moieties. The selectiVe cyclization of 5a to 5b
is most significant. Cyclization of 5a under Jeong’s reaction
conditions (120 °C) gives rise to 5b (58%) admixed with 19%
of the elimination product 5c.5 Interestingly, carbonylative
photocyclization of 9a furnished 9b to the complete exclusion
of the isomeric 1H-cyclopenta[c]furan-5-one. The latter result
is presumably a manifestation of the Thorpe-Ingold effect.14
In conclusion, we have developed an eminently practical
photochemically driven Pauson-Khand reaction that proceeds
catalytically under mild reaction conditions (50-55 °C) and 1
atm of CO pressure. The influence of achiral and chiral ligands
on the course of light-induced carbonylative enyne cyclizations
is currently under investigation. The results of these studies
will be disclosed in future accounts from these laboratories.
Acknowledgment. Generous financial support for this research by
grants from the National Institutes of Health and the National Science
Foundation is gratefully acknowledged.
Supporting Information Available: Listings of 1H and 13C NMR,
DEPT, IR, and HRMS or elemental composition data for all new
compounds (11 pages). This material is contained in many libraries on
microfiche, immediately follows this article in the microfilm version
of the journal, can be ordered from the ACS, and can be downloaded
from the Internet; see any current masthead page for ordering
information and Internet access instructions.
(14) Jung, M. E.; Gervay, J. J. Am. Chem. Soc. 1991, 113, 224.
JA9535975