Elimination reactions from coordinated alkoxide ligands

This method, most frequently employing EtOH, is used widely because the reaction is easy to perform. Examples are found in reviews " and in " Table 1. /3-Hydrogen is eliminated from a coordinated alkoxide ligand ... 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

This hydrogen transfer is essentially an intramolecular acid-base reaction. The hydrogen of the coordinated alcohol function is acidified by coordination to the copper center whilst the hydrazine ligand possesses basic properties. The elimination of the hydrazine substituent is irreversible under these neutral conditions. Indeed, in the absence of excess base, DBADH2 is unable to displace the alkoxide ligand from the copper complex G. 

A convenient and widely used mettiod for the preparation of late transition metal hydride complexes involves the generation of coordinated alkoxide from base and an alcohol, followed by elimination of an aldehyde or ketone. In one case, the alkoxide intermediate has been observed, while use of the labeled alcohol in Equation 3.120 has confirmed that the a-hydrogen becomes the hydride ligand. This reaction has even been used to produce a Co(III) hydride from a Co(III) aquo complex (Equation 3.121). Only primary and secondary alcohols can be used. 

The mechanism of reversible (5-hydrogen elimination from square planar lr(l) alkoxide complexes with labile dative ligands, followed by associahve displacement of the coordinated ketone or aldehyde by incoming phosphine, which can be implied in TH reactions, was proposed by Hartwig and coworkers [36]. 

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