Ruthenacyclobutanes side-bound

Despite the wealth of information provided by mechanistic studies, the preferred orientation of the alkylidene/olefin and ruthenacyclobutane complexes relative to neutral ligand (L) remained unclear. As of the first edition of this book in 2003, evidence had begun to be accumulated for both the side-bound (17/18) and... 

In side-bound ruthenacyclobutane 7a, the metallacycle is positioned directly underneath the N-aryl substituent of the NHC. It is this close proximity to the N-aryl... 

Grubbs and Houk reported the computed energy barriers for the coupling of ethene with 59, which indicated a clear preference for the formation of the side-bound ruthenacyclobutane intermediate (Figure 4.5). The preference for the side-bound mechanism was attributed to a combination of steric and electronic factors. Since the alkylidene adopts a horizontal conformation in the bottom-bound transition states, steric repulsion of the alkylidene and... 

Piers et al. were able to characterize a 14-electron ruthenacyclobutane obtained from the reaction of [RuCl2(lMesH2)(CHPCy3 ) B(QF5)4 ] with ethylene [109]. In view of these data and the results reported by Snapper et al. [110], a (flattened) intermediary ruthenacyclobutane trans to the phosphane or N-heterocyclic carbene ligand appeared to be the most abundant structure. Recent studies conducted by Grubbs et al. supported these data [111], although side-bound stmctures have also been reported by the same group [112]. 

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