Tl. Breen et Dw. Stephan, SYNTHESIS AND REACTIVITY OF PHOSPHAMETALLACYCLES - STERICALLY INDUCEDEPIMERIZATIONS AND RETROCYCLOADDITIONS, Organometallics, 15(26), 1996, pp. 5729-5737
Elimination of methane from Cp(2)Zr(PRH)(Me) (R* = C6H2-2,4,6-t-Bu(3)
) (4) in the presence of diphenylacetylene or phenylpropyne afforded t
he [2 + 2] cycloaddition products Cp(2)Zr(P(R)C(Ph)=CPh) (3) and Cp(2
)Zr(P(R)C(Me)=CPh) (5), respectively. Alternatively, the [2 + 2] cycl
oaddition reaction between (Cp(2)Zr=PR)(PMe(3)) (1) and phenylacetyle
ne yielded Cp(2)Zr(P(R)C(H)=CPh) (6). Metallacycles 3 and 5 undergo f
acile [2 + 2] retrocycloaddition reactions; addition of 1 equiv of phe
nylpropyne to 3 resulted in an equilibrium mixture of 3 and 5. In cont
rast, addition of phenylacetylene to 3 yielded Cp(2)Zr(C=CPh)(C(Ph)=C(
Ph)PHR) (7), the product of C-H activation of the terminal alkyne. At
tempts to synthesize a phosphametallacycle with less sterically hinder
ed substituents on phosphorus by reaction of Cp(2)ZrMeCl, diphenylacet
ylene, and LiHPMes (Mes = C6H2-2,4,6-Me(3)) instead led to the formati
on of Cp(2)Zr(P(Mes)P(Mes)C(Ph)=CPh) (8). Probing the mechanism of for
mation of 8 by reaction of (Cp(2)ZrCl)(2)(mu-PMes) with Li(2)PMes in t
he presence of diphenylacetylene afforded Cp(2)Zr(P(Mes)C(Ph)=CPh) 9.
However, reaction of 9 with H(2)PMes instead resulted in the formation
of the unstable compound Cp(2)Zr(C(Ph)=C(Ph)PMesH)(PMesH) (10). Phosp
hametallacyclobutene 3 reacts with tert-butyl isocyanide, acetone, cyc
lohexanone, benzonitrile, benzaldehyde and styrene oxide to give the i
nsertion products Cp(2)Zr(C(=N-t-Bu)P(R)C(Ph)=CPh) (12), Cp(2)Zr(OCMe
(2)P(R)C(Ph)=CPh) (13), Cp(2)Zr(O(c-CC5H10)P(R*)C(Ph)=CPh) (14), Cp(2
)Zr(N=C(Ph)P(R)C(Ph)=CPh) (15), Cp(2)Zr(OCHPhP(R*)C(Ph)=CPh) (16), an
d Cp(2)Zr(OCH(2)CHPhP(R)C(Ph)=CPh) (17), respectively. Compounds 16 a
nd 17 were also obtained by reaction of either benzaldehyde or styrene
oxide with 13, 14, or 15, via [4 + 2] retrocycloadditions. Epimerizat
ion at phosphorus has been identified in complexes 3, 5, 6, 8, 9, and
12-15, and has been attributed to steric congestion in these species.
Spectroscopic methods, X-ray crystallography, and molecular orbital ca
lculations have been employed to address this issue.