Stable transition metal-enol complexes in the cycloheptatriene ligand system

Full text of DOI:10.1021/ja9617754

8178
J. Am. Chem. Soc. 1996, 118, 8178-8179
derived triene 311 to either (MeCN)3M(CO)3 or M(CO)6 afforded
complexes 4a-c9 in good yields, and fluoride-mediated cleavage
of the silyl ethers in the Cr(0) and W(0) species gave the
corresponding, stable enol complexes 5a,b9,10b in 85% and 84%
Stable Transition Metal-Enol Complexes in the
Cycloheptatriene Ligand System
James H. Rigby,* Noormohamed M. Niyaz, and
Priyantha Sugathapala
Department of Chemistry, Wayne State UniVersity
Detroit, Michigan 48202-3489
ReceiVed May 28, 1996
Complexation with transition metals is well-known to stabilize
otherwise labile organic moieties. For example, transient species
such as cyclobutadiene,1 the dienone tautomer of phenol,2
benzyne,3 and cyclopentadienone4 enjoy enhanced stability when
complexed to appropriate transition metals. Stabilization of the
enol form of the carbonyl keto-enol tautomeric system via
metal complexation has also received attention, recently. To
date, the bulk of these studies has dealt with structurally simple
η2-enol complexes prepared as isolable materials or as inter-
mediates in various reaction pathways.5,6 In contrast, stable
metal-enol complexes involving greater hapticity in structurally
more elaborate ligand systems appear to be quite rare.7
We now report on the preparation and reactions of several
stable, isolable group 6 metal-based enol complexes derived
from the cycloheptatriene ligand system. These species are
easily accessed by fluoride ion induced desilylation of the
corresponding [(trialkylsilyl)oxy]cycloheptatriene complexes. In
a typical example, treatment of complex 18 with anhydrous tetra-
n-butylammonium fluoride in THF afforded the stable enol
complex 29 in 90% yield. Support for the enol structure
yields, respectively. In contrast, no enol complex could be
identified when the molybendum-based complex 4c was treated
under identical conditions. This result is not particularly sur-
prising since second-row transition metal π-complexes are fre-
quently more labile than their first- and third-row counterparts.12
Although a number of metal-enol complexes have been pre-
pared recently, very few reports regarding the chemical reactivity
of these species (other than keto-enol tautomerism and H/D
exchange) have surfaced.7,13 As a consequence, few data are
currently available concerning the utility of most enol complexes
for subsequent derivatization of the hydroxyl function. It is
noteworthy then that many of the metal-enol complexes report-
ed in this document undergo smooth reaction to afford deriva-
tives without compromising the integrity of the metal complex.
For example, reaction of complex 2 with Ac2O/pyridine in
methylene chloride afforded the corresponding acetoxy complex
6 in 71% yield. Even more remarkable is the ease with which
assigned to the red-orange complex 2 (mp 87-9 °C) was
provided by infrared (3511 cm-1) data as well as D2O exchange
experiments.10a The complex also gave satisfactory combustion
analysis results.
Structurally more elaborate enol complexes can also be
prepared in the same fashion. Thus, exposure of the eucarvone-
(1) (a) Emerson, G. F.; Watts, L.; Pettit, R. J. Am. Chem. Soc. 1965, 87,
131. b) Criegee, R.; Schroeder, G. Justus Liebigs Ann. Chem. 1959, 623,
1.
(2) Birch, A. J.; Cross, P. E.; Lewis, J.; White, D. A.; Wild, S. B. J.
Chem. Soc. A 1968, 332.
(3) (a) Buchwald, S. L.; Nielsen, R. B. Chem. ReV. 1988 88, 1047. (b)
Buchwald, S. L.; King, S. M. J. Am. Chem. Soc. 1991 113, 258.
(4) Weiss, E.; Mere´nyi, R.; Hu¨bel, W. Chem. Ber. 1962, 95, 1170.
(5) For a review of metal-enol complexes, see: Milstein, D. In The
Chemistry of Enols; Rappoport, Z., Ed.; Wiley: Chichester, 1990; pp 691-
711.
(6) (a) Wakatsuki, Y.; Nozakura, S.; Murahashi, S. Bull. Chem. Soc.,
Jpn. 1969, 42, 273. (b) Thyret, H. Angew. Chem., Intl. Ed. Engl. 1972, 11,
520. (c) Hillis, J.; Tsutsui, M. J. Am. Chem. Soc. 1973, 95, 7907. (d) Francis,
J.; Tsutsui, M. Chem. Lett. 1973, 663. (e) Cotton, F. A.; Francis, J. N.;
Frenz, B. A.; Tsutsui, M. J. Am. Chem. Soc. 1973, 95, 2483. (f) Alper, H.;
Hachem, K. J. Org. Chem. 1980, 45, 2269. (g) Doney, J. F.; Bergman, R.
G.; Heathcock, C. H. J. Am. Chem. Soc. 1985, 107, 3724. (h) Chin, C. S.;
Park, J.; Lee, S. Y.; Kim, C. J. Organomet. Chem. 1988, 352, 379. (i)
O’Connor, J. M.; Uhrhammer, R.; Rheingold, A. L.; Roddick, D. M. J.
Am. Chem. Soc. 1991, 113, 4530. (j) Grotjahn, D. B.; Lo, H. C. Ibid. 1996,
118, 2097, and references cited therein.
(7) (a) DePuy, C. H.; Greene, R. N.; Schroer, T. E. J. Chem. Soc., Chem.
Commun. 1968, 1225. (b) Fornals, D.; Perica´s, M. A.; Serratosa, F.; Vinaixa,
J.; Font-Altaba, M.; Solans, X. J. Chem. Soc., Perkin Trans. 1 1987, 2749.
(8) 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.
the hindered triisopropylsilyl group can be installed onto enol
(9) This compound exhibited spectral (1H NMR, 13C NMR, IR) and
analytical (combustion analysis and/or HRMS) data consistent with those
of the assigned structure.
(11) Rigby, J. H.; Niyaz, N. M.; Short, K.; Heeg, M. J. J. Org. Chem.
1995, 60, 7720.
(10) (a) Complex 2: a broad singlet at 5.2 ppm is exchangeable with
D2O. b) Complex 5b: a broad singlet at 4.2 ppm is exchangeable with
D2O.
(12) (a) Atwood, J. D., Inorganic and Organometallic Reaction Mech-
anisms; Brooks/Cole: Monterey, CA, 1985; Chapter 4. (b) Basolo, F. Inorg.
Chim. Acta 1981, 50, 65.
S0002-7863(96)01775-1 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 34, 1996 8179
complex 2 employing conventional silylation conditions. Fur-
thermore, the racemic mixture of enol complexes (()-2 can be
acylated with (R)-(-)-R-methoxyphenylacetic acid in the pres-
ence of DCC in excellent yield and the resultant diastereomers
conveniently separated. Careful reductive cleavage of the ester
function in each of these diastereomers with DIBALH afforded
the enantiomeric enol complexes (+)-2 ([R]D ) +680°) and
(-)-2 ([R]D ) -686°) in enantiomerically pure form as deter-
mined by conversion into the corresponding Mosher esters.14
In an intriguing variation on this theme, direct derivatization
of [(trialkylsilyl)oxy]cycloheptatriene complexes can also be
achieved in good to excellent yields (eq 4). For example, the
knowledge no metal-stabilized enol complex has been success-
fully treated in this fashion.17 In the event, exposure of (()-6
to Amano PS-30 lipase under conditions similar to those
reported for organic substrates18 afforded an easily separable
mixture of enantiomerically-enriched (+)-6 and (-)-2 with
modest but encouraging efficiency. The experiment was taken
to approximately 30% conversion, and the extent of resolution
of enol complex 2 was established by comparison with
enantiomerically pure (-)-2 prepared previously. It is note-
worthy that this particular enzyme appeared not to accept
racemic 2 for the corresponding acylation process. Other lipases
(from Candida rugosa and Candida antartica lipase B (Novo
SP-435)) provided similar results. The ability to prepare
enantiomerically enriched metal-enol complexes through en-
zymatic resolution appears to be unprecedented, and its suc-
cessful implementation affords numerous opportunities for
utilizing these complexes in organic synthesis.
In summary, several transition metal-stabilized hydroxycy-
cloheptatrienes can be prepared and characterized. Furthermore,
many of these enol complexes can undergo derivatization and
biocatalytic resolution without significant demetalation. Ap-
plications for these stable metal-enol complexes in organic
synthesis will be reported in due time.
highly substituted silyloxy species 1015 afforded the correspond-
ing acetoxy complex 11 in excellent yield in the presence of
KF under anhydrous conditions. It is noteworthy that O-
alkylation prevails under these conditions when benzyl bromide
is employed as the electrophilic agent in reaction with complex
1. These desilylation results may afford some insight into the
stability of the corresponding metal-enolate complexes, which
are presumably involved as intermediates in these transforma-
tions.16
Biocatalytic resolution of these stable, metal-enol complexes
represents a fascinating and synthetically appealing objective.
Recent reports of enzymatic resolution of arene-chromium
tricarbonyl complexes have appeared, but to the best of our
Acknowledgment. We thank the National Institutes of Health
(Grant GM-30771) for their generous financial support of this research.
Supporting Information Available: Typical experimental proce-
dures and complete spectroscopic data for all new compounds (9 pages).
See any current masthead page for ordering and Internet access
instruction.
(13) (a) Clarkson, R.; Jones, E. R. H.; Wailes, P. C.; Whiting, M. C. J.
Am. Chem. Soc. 1956, 78 6206. (b) Sternberg, H. W.; Markby, R.; Wender,
I. Ibid. 1958, 80, 1009. (c) Cutler, A.; Raghu, S.; Rosenblum, M. J.
Organomet. Chem. 1974, 77, 381. (d) Harman, W. D.; Dobson, J. C.; Taube,
H. J. Am. Chem. Soc. 1989, 111, 3061.
(14) The absolute stereogenicity of these complexes was determined by
conversion into a derivative of known stereochemistry prepared previously
in our laboratory: (a) Rigby, J. H.; Sugathapala, P.; Heeg, M. J. J. Am.
Chem. Soc. 1995, 117, 8851. (b) Diagnostic 1H NMR data for (+)-2: δ
4.77 (dd, J ) 8.1, 7.5 Hz, 1H); 5.02 (d, J ) 8.7 Hz, 1H); 5.23 (br s, 1H,
exchangeable).
JA9617754
(17) (a) Uemura, M.; Nishcmura, H.; Yamada, S.; Hayashii, Y.;
Nakamura, K.; Ishihara, K.; Ohno, A. Tetrahedron: Asymmetry 1994, 5,
1673. (b) Baldoli, C.; Maiorana, S.; Carrea, G.; Riva, S. Ibid. 1993, 4, 767.
(c) Malezieux, B.; Jaouen, G.; Salau¨n, J.; Howell, J. A. S.; Palin, M. G.;
McArdle, P.; O’Gara, M.; Cunningham, D. Ibid. 1992, 3, 375. (d) Yamazaki,
Y.; Morohashi, N.; Hosono, K. Biotechnol. Lett. 1991, 13, 81. (e) Top, S.;
Jaouen, G.; Baldoli, C.; Del Buttero, P.; Maiorana, S. J. Organomet. Chem.
1991, 413, 125. (f) Yamazaki, Y.; Hosono, K. Agric. Biol. Chem. 1990,
54, 3357. (g) Yamazaki, Y.; Hosono, K. Tetrahedron Lett.1990, 31, 3895.
(18) Majeric, M.; Sunjic, V. Tetrahedron: Asymmetry 1996, 7, 815.
(15) Niyaz, N. M., Wayne State University. Unpublished results.
(16) (a) Noyori, R.; Nishida, I.; Sakata, J. J. Am. Chem. Soc. 1983, 105,
1598. (b) Kuwajima, I.; Nakamura, E.; Shimizu, M. Ibid. 1982, 104, 1025.