Hetero-metal alkoxides

As emphasized in the opening paragraph of this section, heteroalkoxoniobates and -tantalates are less stable than those of aluminates and zirconates this aspect is also reflected in the chemistry of their homo-alkoxides. The chemistry of hetero-metal alkoxides of those two metals is dominated in most cases (particularly with primary alcohols) by their tendencies for disproportionation as well as formation of oxo-derivatives. Two review articles on these heterometal alkoxides reflect these aspects prominently, even questioning the validity of some earlier findings. 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

Heterocyclic structures analogous to the intermediate complex result from azinium derivatives and amines, hydroxide or alkoxides, or Grignard reagents from quinazoline and oigahometaJlics, cyanide, bisulfite, etc. from various heterocycles with amide ion, metal hydrides, or lithium alkyls from J>T-acylazinium compounds and cyanide ion (Reissert compounds) many other examples are known. Factors favorable to nucleophilic addition rather than substitution reactions have been discussed by Albert, who has studied examples of easy covalent hydration of hetero-cycles. 

Metal alkoxides (homo as well as hetero) undergo facile and reversible alcoholysis and transesterification reactions that can be represented by the following equations ... 

In keeping with our general theme of presenting similarities between hetero-and homometal alkoxides, it may be pointed out that ester elimination was found to be a side reaction in the preparation of a metal carboxylate from the reaction between the alkoxide of a metal and a carboxylic acid leading to an oxo-car-boxylate and sometimes to an oxo-alkoxide at the intermediate stage(s) [cf. (6, 145)] ... 

Veith M, Mathur S, Mathur C, Huch V. Synthesis, reactivity and structure of Hf-containmg homo and hetero-(bi- and tri-) metallic alkoxides based on edge-and face-sharing bioctahedral alkoxometalate ligands. J Chem Soc Dalton Trans 1997 2101-8. 

C. Andrews, J.G. MacLellan, R.E. Mulvey and P.J. Nichols, New homo- and hetero-alkali metal alkoxide cages Crystal structures of [MejN(CH2)20Li]g and [ Me3N(CH3)30 12Li3KJ0, Dalton Trans., vol. 8, pp. 1651-1655, 2002. 

Given the isoelectronic relationship between [CR] and [NO] and the ubiquity of this latter ligand in the coordination chemistry of later transition metals, the scarcity of mononuclear alkylidyne complexes of metals from groups 8-10 is surprising [1-4]. Isolated examples have been reported for iron [5], cobalt [6], ruthenium [4,7], osmium [4,8-9] and iridium [10]. Most of the examples known employ routes with extensive precedent in early transition metal systems, i.e., either electrophilic attack at the p-atom of a hetero carbonyl (CS [5], CTe [4], or C=CH2 [10]) or the Lewis-acid assisted abstraction of an alkoxide group from a carbene precursor [5] (Scheme 1). The one approach which is, too date, peculiar to group 8 metals involves reduction of a divalent dichlorocarbene complex by lithium aryls [4]. The limitation of this procedure to ruthenium and osmium is presumably not a feature of these metals but rather a result of the present lack of synthetic routes to suitable dihalocarbene precursor complexes of earlier metals. 

Synthesis (by metathetic reactions) of a large number of hetero-tri and tetrametallic alkoxides of a number of metals has been reported since 1985 (Section 2.3 and Table 3.4(c)) mainly from the research school of Mehrotra " and the... 

It might be appropriate to conclude this brief account of heterometallic alkoxide systems by mentioning [(Bu 0)8Li4K4], prepared by Clegg et alP in their continuing efforts to develop hetero-s-block metal chemistiy. 

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