4204
J. Am. Chem. Soc. 1996, 118, 4204-4205
ppm in the 31P NMR. This signal was initially attributed to
Synthesis and Reactivity of
Phosphametallacyclobutenes: Sterically Induced
phosphametallacycle Cp2Zr(P(R*)CPhdCPh) 2, the product of
a [2 + 2] cycloaddition between ZrdPR* and PhCtCPh. A
more expeditious route to 2 parallels the previously reported8h
synthesis of 1. Spontaneous loss of methane from Cp2Zr-
(PR*H)Me to generate the reactive terminal phosphinidene
intermediate 3, followed by a cycloaddition reaction with
diphenylacetylene, provided an 87% yield of 2. Similarly, the
use of 1-phenylpropyne as a trapping agent gave a 68% yield
[4 + 2] Retrocycloadditions
Tricia L. Breen and Douglas W. Stephan*
Department of Chemistry and Biochemistry
UniVersity of Windsor, Windsor, ON, Canada N9B 3P4
ReceiVed October 16, 1995
of Cp2Zr(P(R*)CMedCPh) 4.
The cycloaddition of alkynes to metal carbene complexes is
an established reaction in organometallic chemistry,1 yet
analogous reactions with metal-heteratom multiple bonds are
much less common. The resulting metallacyclobutenes often
undergo further reaction and consequently have been proposed
as intermediates.2-5 Nonetheless, the synthesis and structural
characterization of stable aza-6, oxa-,7 and sulfametallacy-
clobutenes,7 prepared through [2 + 2] cycloadditions of alkynes
with the appropriate metal-ligand multiply bonded species, have
been dominated by the research of Bergman et al. Although
the preparation and structural studies of several terminal
phosphinidenes and arsinidenes have been reported in the
literature,8 the chemistry of these highly reactive species is just
beginning to emerge. Herein we describe the first examples of
phosphametallacyclobutenes, prepared by the [2 + 2] cycload-
dition of an alkyne with a terminal zirconium phosphinidene.
The subsequent chemistry of these metallacycles differs from
their imido counterparts since insertion of aldehyde or ketone,
as well as ring opening of epoxide into the Zr-P bond, affords
new phosphaoxametallacycles which undergo [4 + 2] retrocy-
cloadditions under steric duress.
1H and 13C NMR data9 indicate that complexes 2 and 4 are
best described as phosphametallacyclobutenes. For both com-
pounds only a single resonance is observed for the cyclopen-
tadienyl ligands, indicating that these groups are equivalent on
the NMR time scale and the metallacycles are either planar10
or rapidly undergoing inversion at phosphorus. However, these
alternatives could not be distinguished, as cooling solutions of
2 or 4 to -80 °C merely resulted in the observation of line
broadening.
The reversible formation of metallacycles 2 and 4 was
illustrated by the addition of trimethylphosphine, which rapidly
resulted in the quantitative conversion to 1 along with the release
of alkyne (eq 1). Similarly, addition of 1 equiv of 1-phenyl-
propyne to 2 resulted in alkyne exchange to give an equilibrium
mixture of 2 and 4 (Keq ) 1.1 at 25 °C). Comparable to related
oxa-,7a aza-,6b-d and sulfametallacyclobutenes,7b,c these reactions
are thought to occur through [2 + 2] retrocycloadditions to
generate transient phosphinidene intermediate 3, which then
reacts with either PMe3 or alkyne.
We have previously described the synthesis of the terminal
zirconium phosphinidene Cp2Zr(PR*)(PMe3) (R* ) C6H2-2,4,6-
t-Bu3) 1.8e,h Reaction of 1 with 1 equiv of diphenylacetylene
proceeded slowly. As the released trimethylphosphine was
successively removed from solution under vacuum, the forma-
tion of 2 was indicated by the observation of a singlet at 55
* Author to whom correspondence should be addressed. E-mail:
Acetone reacts with metallacycle 2 over a 10 min period to
(1) (a) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John
Wiley & Sons: New York, 1988. (b) ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon
Press: New York, 1982; Vol. 3. (c) AdVances in Metal-Organic Chemistry;
Liebeskind, L. S., Ed.; Jai Press Inc.: Greenwich, CT, 1989; Vol. 1. (d)
AdVances in Metal-Organic Chemistry; Liebeskind, L. S., Ed.; Jai Press
Inc.: Greenwich, CT, 1994; Vol. 3.
(2) (a) Glueck, D. S.; Hollander, F. J.; Bergman, R. G. J. Am. Chem.
Soc. 1989, 111, 2719. (b) Glueck, D. S.; Wu, J.; Hollander, F. J.; Bergman,
R. G. J. Am. Chem. Soc. 1991, 113, 2041.
(3) De Boer, E. J. M.; Dewith, J.; Orpen, A. G. J. Am. Chem. Soc. 1986,
108, 8271.
produce a 51% yield of the insertion product Cp2Zr(OCMe2P-
(R*)CPhdCPh) 5. In contrast to reactions of related aza-11 and
oxametallacyclobutenes7e where insertions occur exclusively at
the Zr-C bond, 1H, 13C, and 31P NMR spectra of 5 were
consistent with acetone insertion into the Zr-P bond. An X-ray
crystallographic study12 of 5 confirmed the formulation of the
phosphaoxametallacyclohexene, in which the geometry at
phosphorus is pyramidal and the supermesityl substituent adopts
an equatorial position relative to the ring. The inequivalence
of the cyclopentadienyl rings and methyl groups indicated by
the solid state structure is consistent with the 1H NMR spectrum
at -80 °C. However, the fluxional nature of this compound
(4) de With, J.; Horton, A. D. Organometallics 1993, 12, 1493.
(5) (a) Sharpless, K. B.; Teranishi, A. Y.; Blackvall, J.-E. J. Am. Chem.
Soc. 1977, 99, 3120. (b) Hentges, S. G.; Sharpless, K. B. J. Am. Chem.
Soc. 1980, 102, 4263.
(6) (a) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc.
1988, 110, 8729. (b) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J.
Am. Chem. Soc. 1992, 114, 1708. (c) Walsh, P. J.; Hollander, F. J.;
Bergman, R. G. Organometallics 1993, 12, 1305. (d) Baranger, A. M.;
Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 2753.
(7) (a) Carney, M. J.; Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J.
Am. Chem. Soc. 1989, 111, 8751. (b) Carney, M. J.; Walsh, P. J.; Bergman,
R. G. J. Am. Chem. Soc. 1990, 112, 6426. (c) Carney, M. J.; Walsh, P. J.;
Hollander, F. J.; Bergman, R. G. Organometallics 1992, 11, 761. (d) Polse,
J. L.; Anderson, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1995, 117, 5393.
Cp*2Zr(OCPhdCPh) has also been synthesized by the oxidation of Cp*2-
Zr(C2Ph2) with N2O: (e) Vaughan, G. A.; Hillhouse, G. L.; Rheingold, A.
L. J. Am. Chem. Soc. 1990, 112, 7994.
(8) (a) Hitchcock, P. B.; Lappert, M. F.; Leung, W. P. J. Chem. Soc.,
Chem. Commun. 1987, 1282. (b) Cowley, A. H.; Pellerin, B. J. Am. Chem.
Soc. 1990, 112, 6734. (c) Ho, J.; Stephan, D. W. Organometallics 1991,
10, 3001. (d) Hou, Z.; Stephan, D. W. J. Am. Chem. Soc. 1992, 114, 10088.
(e) Hou, Z.; Breen, T. L.; Stephan, D. W. Organometallics 1993, 12, 3158.
(f) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int. Ed.
Engl. 1993, 32, 756. (g) Bonanno, J. B.; Wolczanski, P. T.; Lobkovsky,
E. B. J. Am. Chem. Soc. 1994, 116, 11159. (h) Breen, T. L.; Stephan, D.
W. J. Am. Chem. Soc. 1995, 117, 11914.
(9) Full details of the 1H and 13C{1H} NMR spectra of compounds 2-6
are reported as supporting information. 31P{1H} NMR (25 °C, C6D6): δ
2, 55.3; 3, 70.1; 4, 36.1; 5, 13.9; 6, -2.4; 7a,b, 46.3 and 37.0.
(10) Planarity at phosphorus in a phosphametallacyclobutene formulation
seems unlikely, as orthogonality of the lone pair of electrons on phosphorus
and the LUMO of the metallocene fragment precludes the stabilization of
M-P multiple bonding.
(11) Hanna, T. A.; Baranger, A. M.; Walsh, P. J.; Bergman, R. G. J.
Am. Chem. Soc. 1995, 117, 3292.
(12) Mo KR radiation λ ) 0.71069 Å and a Rigaku AFC5-R diffracto-
meter were used to collect the data (4.5° < 2θ < 50°) for each compound.
5: Triclinic space group P1h (No. 2), a ) 13.26(1) Å, b ) 15.414(6) Å, c
) 11.085(8) Å; R ) 101.75(5)°, â ) 112.10(5)°, γ ) 94.96(5)°. 6:
Monoclinic space group Cc (No. 9), a ) 31.835(6) Å, b ) 16.316(4) Å, c
) 16.685(4) Å; â ) 95.86(2)°. 7: Monoclinic space group P21/c (No.
14), a ) 9.944(4) Å, c ) 41.12(2) Å; â ) 106.37(3)°. The solution was
obtained and refined employing the TEXSAN software from MSC.
Refinements (data I > 3σ(I), variables, R, Rw): 5, 2495, 208, 0.0799, 0.0668;
6, 1540, 248, 0.0777, 0.0634; 7, 2707, 229, 0.0953, 0.0761. Full details
are deposited as supporting information.
S0002-7863(95)03470-6 CCC: $12.00 © 1996 American Chemical Society