Cyclometallation strategy

To date, three distinct strategies have been utilized to impose a preference for i yn-metal-lacycles in Ru-based olefin metathesis catalysts, resulting in three families of Z-selective catalysts. Promising initial reactivity has been observed with both thiophenolate- and dithiolate-based catalysts and both frameworks offer many opportunities for further tuning of activity and Z-selectivity. The cyclometallated catalysts have been further developed and have demonstrated high activity and Z-selectivity for a wide variety of substrates. However, in all cases, further improvements will be necessary to achieve Z-selective metathesis across the broad substrate scope demonstrated by previous generations of Ru-based catalysts. 

Recent studies have focussed on the generation of metaUation-resistant aryloxide ligation. The strategy adopted has involved the introduction of meto-substituents onto the 2,6-diphenylphenoxide nucleus. The meta-substituents restrict rotation of the orthophenyl ring into a position for CH-bond activation. In the case of 3,5-di-tert-butyl-2,6-diphenylphenoxide, cyclometallation by tantalum alkylidene functional groups is... 

The metals thorium and uranium (as with the chemistry in general of these elements) dominate actinide aryloxide chemistry. Synthetic strategies normally focus on the halides reacting with group 1 metal aryloxides or reaction of metal amides with phenols. An important piece of early work was the demonstrated interconversion of eight-coordinate [UMe4(dmpe)2] and [M(OPh)4(dmpe)2] (M = Th, U Table 6.20) by addition of phenol or MeLi to each substrate respectively. The reaction of the cyclometallated... 

The selectivity of C-H activation is a key issue in developing practical C-H functionalization strategies, as most relevant substrates will have multiple C-H bonds that are available for activation. As part of our ongoing collaboration with the Davies group, a series of competition experiments were devised to probe the relative reactivity of C(sp )-H bonds in iV-alkylimines, H-Li s, and phenylpyridine, H-Lg [64]. These substrates all undergo directed C-H activation in the presence of [Cp MCl2]2 and NaOAc to give well-defined cyclometallated species as their chloride adducts (Fig. 7). Stoichiometric reactions with H-L3 in... 

Heteroatom-directed cyclometallation forms the basis of C-H functionalization strategies for heterocycle synthesis. An example is the Rh-catalysed oxidative coupling of 3-phenylpyrazoles (1) with 4-octyne (2) to give pyrazoloisoquinolines (3) [66]. The proposed catalytic cycle for this process is shown in Fig. 11 and was supported by DFT calculations (BP86) using both a simple model system [Model 1 3-phenylpyrazole reacting with HC=CH at CpRh(OAc)2] and the full system used experimentally [Model 2 3-phenyl-5-methylpyrazole reacting with 4-octyne at... 

This strategy has been reported for the synthesis of several 1,4,5-trisubstituted triazoles [144] Interestingly, the electrophile is introduced at the beginning of the reaction. Regioselectivity of the reaction, opposite to the one observed using lithiated, magnesiated, or zincated alkynes, suggests that the mechanism is indeed a [3+2] copper-assisted cycloaddition that proceeds via a 5-cuprotriazole, and not a cyclometalation that would have deliver 4-substituted triazoles (Scheme 58)... 

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