Synthesis and reactivity of phosphametallacycles: Sterically induced epimerizations and retrocycloadditions

Elimination of methane from Cp(2)Zr(PR*H)(Me) (R* = C6H2-2,4,6-t-Bu(3)) (4) in the presence of diphenylacetylene or phenylpropyne afforded the [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] cycloaddition reaction between (Cp(2)Zr=PR*)(PMe(3)) (1) and phenylacetylene yielded Cp(2)Zr(P(R*)C(H)=CPh) (6). Metallacycles 3 and 5 undergo facile [2 + 2] retrocycloaddition reactions; addition of 1 equiv of phenylpropyne to 3 resulted in an equilibrium mixture of 3 and 5. In contrast, 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. Attempts to synthesize a phosphametallacycle with less sterically hindered substituents on phosphorus by reaction of Cp(2)ZrMeCl, diphenylacetylene, and LiHPMes (Mes = C6H2-2,4,6-Me(3)) instead led to the formation of Cp(2)Zr(P(Mes)P(Mes)C(Ph)=CPh) (8). Probing the mechanism of formation of 8 by reaction of (Cp(2)ZrCl)(2)(mu-PMes) with Li(2)PMes in the 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). Phosphametallacyclobutene 3 reacts with tert-butyl isocyanide, acetone, cyclohexanone, benzonitrile, benzaldehyde and styrene oxide to give the insertion 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), and Cp(2)Zr(OCH(2)CHPhP(R*)C(Ph)=CPh) (17), respectively. Compounds 16 and 17 were also obtained by reaction of either benzaldehyde or styrene oxide with 13, 14, or 15, via [4 + 2] retrocycloadditions. Epimerization 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 calculations have been employed to address this issue.