Metal alkoxides bond activation

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... 

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]. 

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). 

Additives that coordinate to Al perturb the polymerization kinetics in toluene. For instance, THF and cyclohexanone compete e-CL for coordination to aluminum (18), which decreases the propagation rate and may even inhibit polymerization. In contrast, kinetics is faster upon addition of a Lewis base, such as 4-picoline (19), because the coordination of this Lewis base onto the Al atom polarizes the metal-alkoxide bond and facilitates the monomer insertion. Although the reactivity of the active sites is increased, the extent of transesterification reactions is reduced, more likely for steric reasons. 

The first step of the coordination-insertion mechanism (I) consists of the coordination of the monomer to the Lewis-acidic metal center (Fig. 3.7). The monomer subsequently inserts into one of the aluminum-alkoxide bonds via nucleophilic addition of the alkoxy group on the carbonyl carbon (11) followed by ring opening via acyl-oxygen cleavage (1) hydrolysis of the active metal-alkoxide bond leads to the formation of a hydroxyl end group, while the second chain end is capped with an isopropyl ester, as indicated by NMR characterization of the resulting polymers [48]. 

Both Sn(Oct)2 and Al(Oi-Pr)3 have been extensively studied in terms of activity, polymerization control and mechanism [8, 9]. According to experimental and theoretical data, the polymerization proceeds via a three-step coordination-insertion mechanism (Scheme 10.2). With Sn(Oct)2, the key alkoxide complex is generated in situ upon reaction with the exogenous alcohol. The nature of the ester chain-end is intimately related to the initiating alkoxide, and it is classically determined experimentally by H NMR and/or mass spectrometry, using electrospray ionization (ESI) or matrix-assisted laser desorption ionization time-of-flight (MALDl-ToF) techniques. When all of the monomer has been consumed, the active metal-alkoxide bond is hydrolyzed and a hydroxyl end-group is Uberated. 

Recently, a new argument has been added on the basis of quantum chemical calculations [70] which has shown that the attack of an acid on the hydroxyl group activates the hydrogenes on for elimination whereas the loss of hydrogen from the hydroxyl or its substitution by a metal ion (corresponding to the formation of the surface alkoxide II) activates the Ca—H bond for dehydrogenation. 

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 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],... 

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 polymerization undergoes a coordination-insertion mechanism. The initiation step involves nucleophilic attack of the active group, such as a hydride, alkyl, amide or alkox-ide group, on the carbonyl carbon atom of a lactide or lactone to form a new lanthanide alkoxide species via acyl-oxygen cleavage. The continued monomer coordination and insertion into the active metal-alkoxo bond formed completes the propagation step as shown in Figure 8.50. 

In this section, we will highlight the development in the use of metal alkox-ides for the synthesis of new and interesting organometallic compounds, many of these are either inaccessible or difficult to synthesize by common synthetic procedures. We will not discuss (a) the chemistry of organometallic compounds containing alkoxides as supporting ligands, for which excellent reviews by Chisholm and co-workers (154, 513, 514) are available and (b) intramolecular cyclometalation (i.e., C—H bond activation) reactions of metal aryloxides due to the availability of an excellent account of this topic in a review article by Rothwell (515). Furthermore, a brief mention of the use of a related metal derivative (i.e., metal aryloxide) will be made merely for comparison. 

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