Reactivity Studies of a Zirconium Methylidene Complex: Group Transfer and Methylenation Reactions

The zirconium methylidene complex (PNP)­ZrCH2(OAr) (1; PNP = N­[2-PiPr2-4-methylphenyl]2–, Ar = 2,6-iPr2C6H3), prepared from photolysis of (PNP)­Zr­(CH3)2(OAr), can engage in incomplete and complete methylidene group transfer reactions with CO, CDCl3, cyclododecanone, and benzophenone. When CO was treated with 1, methylenation occurred via the putative metalloketene adduct [(PNP)­Zr­(OCCH2)­(OAr)], which ultimately afforded the C–C coupled enolate dimer [(PNP)­Zr­(OAr)]2(OCH2CCCH2O) (2). Addition of CDCl3 to 1 rapidly resulted in formation of (PNP)­ZrCl2(OAr) (3) along with liberation of d1-vinyl chloride, H2CCDCl. Complex 3 could be readily prepared independently from (PNP)­ZrCl3 and 1 equiv of NaOAr. Wittig reactivity was observed between 1 and cyclododecanone to afford methylenecyclododecane. Similarly, treating 1 with the ketone OCPh2 resulted in methylenation of the ortho position followed by tautomerization to produce the chelated alkyloxide complex (PNP)­Zr­(OAr)­[OCHPh­(C6H4)­CH2] (4). In addition to the isolation of 4, complex 1 also engages in a cross-metathesis reaction involving the methylidene and ketone oxygen of benzophenone, as well as in olefin metathesis involving the formed olefin H2CCPh2. Complexes 2–4 have been fully characterized, including solid-state structural analysis, and proposed mechanisms for the formation of species such as 2–4 are presented and discussed.