Active bonds, metal alkoxide

The most effective, and commercially applied, method to produce polylactide is via the ring-opening polymerization of lactide. This process is initiated by metal complexes and proposed to occur via a coordination-insertion mechanism, as illustrated in Fig. 2. The most common initiators for this polymerization are Lewis acidic metal alkoxide or amide complexes. Key initiator criteria are sufficient Lewis acidity to enable binding and activation of the lactide unit and a labile metal alkoxide (or amide) bond so as to enable efficient insertion. 

The putative mechanism involves coordination and activation of the lactide by the metal complex (1, Fig. 2). The lactide, once activated, is subsequently attacked by the metal alkoxide group (another way to view this is that lactide inserts into the metal alkoxide bond) (2, Fig. 2). The putative intermediate then undergoes ring opening of the lactide, by an acyl bond cleavage, and a new metal alkoxide bond is... 

The mode of lactone ring opening depends on the kind of catalyst. It is characteristic that -lactone polymerisation with a catalyst containing a metal alkoxide active bond (Mt-X X = OR) involves C(0)-0 bond scission in the coordinating monomer (via the metal orthocarbonate species) with regeneration of the metal alkoxide active bond [scheme (7)] [87]. On the other hand, the application of a catalyst with a metal carboxylate active bond [Mt-X X = 0C(0)R] for -lactone polymerisation results in Cp — O bond scission in the coordinating monomer with regeneration of the metal carboxylate active bond [scheme (8)] [88-90],... 

Most coordination catalysts have been reported to be formed in binary or ternary component systems consisting of an alkylmetal compound and a protic compound. Catalysts formed in such systems contain associated multinuclear species with a metal (Mt)-heteroatom (X) active bond ( >Mt X Mt—X > or — Mt—X—Mt—X— Mt = Al, Zn, Cd and X = 0, S, N most frequently) or non-associated mononuclear species with an Mt X active bond (Mt = Al, Zn and X = C1, O, S most frequently). Metal alkyls, such as triethylaluminium, diethylzinc and diethylcadmium, without pretreatment with protic compounds, have also been reported as coordination polymerisation catalysts. In such a case, the metal heteroatom bond active in the propagation step is formed by the reaction of the metal-carbon bond with the coordinating monomer. Some coordination catalysts, such as those with metal alkoxide or phenoxide moieties, can be prepared in other ways, without using metal alkyls. There are also catalysts consisting of a metal alkoxide or related compound and a Lewis acid [1]. 

Allyl silanes will also attack carbonyl compounds when they are activated by coordination of the carbonyl oxygen atom to a Lewis acid. The Lewis acid, usually a metal halide such as TiCLj or ZnCl2, activates the carbonyl compound by forming an oxonium ion with a metal-oxygen bond. The allyl silane attacks in the usual way and the (3-silyl cation is desilylated with the halide ion. Hydrolysis of the metal alkoxide gives a homoallylic alcohol. 

The effect of the substitution on the phenyl ring can be illustrated by considering two parallel effects (1) the steric obstacle created by both the chloride and the methyl groups, which hinder the approaching of an aldehyde to the metal-alkoxide bond when disposed in the ortho position and (2) the electrostatic interaction between the metal and the chloride, which may facilitate the approach of the aldehyde to the metal center, and hence the activity (Fig. 1). 

The activated complexes, which form, have steric configurations that allow minimum amounts of non-bonded interactions. Other mechanisms were proposed since. For instance, Furukawa et al. [344] concluded that metal alkyl compounds must become metal alkoxides through reactions with the aldehydes ... 

Oxidative additions to form late-metal alkoxides by C-0 bond cleavage are also known, but are less common than oxidative additions of 0-H bonds. Graziani reported insertions of palladium(O) complexes into C-0 bonds in activated epoxides (Equation 4.73). The resulting metallaoxetanes were stabilized by the cyano... 

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