Rearrangements and Stereomutations of Metallacycles Derived from Allenes and Imidozirconium Complexes

Abstract

The mechanisms of the rearrangements and stereoinversion of azametallacyclobutenes generated via [2+2] cycloaddition of allenes and imidozirconium complexes have been studied. Metallacycles derived from allenes bearing β-hydrogen atoms racemize at room temperature by reversible β-hydride elimination, a process which is also responsible for their eventual conversion to monoazadiene complexes. Metallacycles derived from diarylallenes racemize by reversible thermal bond homolysis at 95 °C; racemization of these metallacycles is also catalyzed by mild oxidants.

Synthesis and carbonylation of oxotitanocyclobutanes derived from allene cycloaddition to a monomeric titanocene oxide

“…These observations are consistent with similar isomerization processes reported for azametallocyclobutanes derived from cycloaddition of disubstituted aryl allenes with zirconocene imido complexes [35,36]. On the basis of kinetic and computational studies [37], E-Z isomerization via retrocycloaddition was eliminated and a pathway involving homolysis of the Zr-C bond was favored. Because the bond dissociation energies between titanocene and zirconocene alkyl bonds are similar ((g 5 -C 5 Me 5 ) 2 TiMe 2 BDE = 64.7 kcal/mol; (g 5 -C 5 Me 5 ) 2 ZrMe 2 BDE = 67.9 kcal/mol) a similar mechanism for isomer interconversion is likely operative in the present system.…”

Recent advances in the chemistry of heterometallacycles of group 4 metallocenes

“…44 A similar ring opening reaction with N-tosyl-2phenylaziridine gave rise to a diazazirconacyclopentane complex. 43 This type of saturated metallacyclic structure is also accessible by simple salt elimination from the dilithiated amine ligand and a zirconium chloro species as was shown by Jordan et al for an ansa-zirconocene complex. 45 Five-membered metallacycles containing exclusively Si atoms and the metal centre were extensively studied by Marschner and co-workers.…”

“…The latter species readily rearrange and undergo ring expansion to yield the corresponding isomeric five-membered zirconocene monoazadiene (MAD) complexes (Scheme 9). 43 Scheme 6 Formation of four-membered heterometallacycles by cycloaddition reactions with t BuCRP (py = pyridine).…”

Three-Membered-Ring-Based Molecular Architectures

“…404 In a recent study, 1,3-dicyclopropylpropadiene (562) was used as a probe for radical intermediates in the isomerization of azametallacyclobutanes 578 formed by the reaction of 1,3-disubstituted allenes with imidozirconium complexes 576 (Scheme 96). 405 Indeed, an (E)-zirconacycloheptene 579 instead of the usual azazirconacyclobutane 578 could be isolated and characterized by X-ray crystal structure analysis. This method can be used to obtain enantioenriched allenes 577 by employing enantioenriched zirconium complexes 576 and subsequent liberation of the isomerized 577 by treatment of the azazirconacyclobutanes 578 with allene.…”

“…This method can be used to obtain enantioenriched allenes 577 by employing enantioenriched zirconium complexes 576 and subsequent liberation of the isomerized 577 by treatment of the azazirconacyclobutanes 578 with allene. 405 Reactions of propargylic alcohols with chlorodiarylphosphines in the presence of organic bases yield phosphorylpropadienes. 406 Dibromocarbene addition to the phosphoryl- propadiene 581, prepared along this route, regioselectively furnished the adduct 582, which could be subjected to the Doering-Moore-Skattebøl rearrangement to afford butatriene 583 (Scheme 97).…”