Bridge methylene ligands

Insertion of carbon monoxide into the bridging-methylene ligands was found to be up to 100 times faster if, instead of the bridging carbonyl, a bridging halide ion is present [59,60]. The osmium methylene complex 10 was found to add carbon monoxide rapidly affording the ketene complex 11 (reaction 8.30) [61]. 

Morrison, E.D., GeofFroy, G.L. and Rheingold, A.L. (1985) Halide-promoted insertion of CO into bridging-methylene ligands in triosmium clusters. Journal of the American Chemical Society, 107, 254—255. 

Salvadori et al. [62] tested the same strategy but derived the bis(oxazohne) ligands in such a way that they minimized the steric hindrance at the bridging methylene carbon (structure 53 in Scheme 25). The polymer was used affording enantiomeric excesses superior to 90% and was reused at least five times with almost no loss in enantioselectivity or activity. 

The reaction of methylene bis(dialkyldithiocarbamates) with [Au(C6F5)(tht)j leads to the complexes [ Au(C6F5) 2 CH2(S2CNR2)2 ] [43] (R = Me, Et, n-Bu, Bz). The NMR resonances of the bridging methylene groups in the complexes are observed at lower field than that in the free ligand and some modification of the C=S stretching is observed. 

In the context of Lck SH2 domain ligands it was demonstrated that the introduction of an additional hydroxyl group onto the bridging methylene group, yielding the chmF 16 [4-(carboxyhydroxymethyl)phenylalanine] (Scheme 2), improved the potency threefold when compared to the corresponding cmF analogue [119]. 

A notable double insertion of methylacetylene into the methylene bridge has been observed with the classical -y-lactone-type cobalt complexes 1. The heterocyclic methylene ligand (CRR ) originally attached to both metal centers is displaced by the new alkylidene ligand ju.-C(CH3)— C(H)=C(CH3)—C(H)=CRR (116, 122) (Scheme 28a other ligands omitted for clarity). This example also points to the possibility of methylene homologation in the coordination core of a catalyst (see Section IX). 

The rapid development of the chemistry of transition metal complexes containing terminal carbene (A) or carbyne (B) ligands (7) has been followed more recently by much research centered on bridged methylene compounds (C) (2). The importance of /t-methylidyne complexes, whether in recently established binuclear examples (D), the well-known trinuclear derivatives (E), or the unusual complexes (F), has also become apparent. All are based on one-carbon (C,) fragments, and considerable interest is centered on their possible significance as models for intermediates in surface-catalyzed reactions between carbon monoxide and hydrogen (Fischer -Tropsch reactions) and related processes. These topics have been extensively ... 

Fig. 29 Extension of the face bridging, C3-symmetric ligand 44 that forms M4L4 tetrahedron 47. Introduction of rigid phenyl bridges in ligand 50 preserves the necessary rigidity to form tetrahedron 51 [154], whereas introduction of a flexible methylene bridge results in the formation of mononuclear M1L1 complex 52 [156]...

Pettit [44] has stressed the importance of bridge melhylene ligands un transition metal surfaces- Similarities in product formation with CO/H] and surest a common mechanism for both processes, with methylene possibly being formed by carbide hydrogenation in Fischer-Tropsch reactions. 

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