Metal-alkoxide insertion mechanism

This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

Two different mechanisms have been proposed for the ROP of (di)lactones depending on the nature of the organometalhc derivatives. Metal halides, oxides, and carboxylates would act as Lewis acid catalysts in an ROP actually initiated with a hydroxyl-containing compound, such as water, alcohol, or co-hydroxy acid the later would result more hkely from the in-situ hydrolysis of the (di)lac-tone [11]. Polymerization is assumed to proceed through an insertion mechanism, the details of which depends on the metal compound (Scheme la). The most frequently encountered Lewis acid catalyst is undoubtedly the stannous 2-ethylhexanoate, currently referred to as stannous octoate (Sn(Oct)2). On the other hand, when metal alkoxides containing free p-, d-, or f- orbitals of a favo-... 

Later, Kricheldorf and coworkers extended the concept of the aluminum alkox-ide-initiated ROP of lactones to a set of other metal alkoxides such as tin(lV) [23-25], titanium, and zirconium alkoxides. As a rule, the polymerization takes place according to the same coordination-insertion mechanism shown in Fig. 12. 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

Thus, in hydrogen-transfer reactions, most of the catalysts do prefer the outer-sphere mechanism instead of the MPV or the insertion mechanisms. For instance, the high stability of the intermediate formed, alkoxide in the case of carbonyl hydrogenation, is a major drawback for the inner-sphere mechanism. Nevertheless, in some particular cases, the inner-sphere mechanism may be competitive with the outer-sphere one. In these cases, some requirements must be accomplished, such as the high lability of one of the metal ligands in order to allow easily the substrate coordination or the formation of not very stable intermediates. 

B) Complexation of the alkene to the metal followed by its insertion between the metal-oxygen bond according to an intramolecular 1,3-dipolar mechanism, forming a five-membered pseudocyclic peroxometallacycle which decomposes to give the epoxide and the metal alkoxide.121,162,193,634... 

Metal-catalyzed reactions of C02 and epoxides that give polycarbonates and/or carbonates have been extensively investigated as a potentially effective C02 fixation (Beckman, 1999 Inoue, 1987). The possible reaction mechanism is illustrated in Figure 3.8 (Darensbourg et al., 1999). The repetition of the reaction sequence in which C02 inserts into a metal-alkoxide bond, followed by ring-opening of the epoxide with the metal carbonate forms the alternating copolymer. In 1969, this copolymerization was first reported by Inoue and Tsuruta who used a Zn catalyst derived from... 

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... 

Fig. 2 Coordination- insertion mechanism M metal, OR alkoxide group, L ligand(s)...

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. 

An alternative mechanism for oxygen transfer was proposed by Mimoun [21-25]. In this mechanism (eq. (11)) initial coordination of the olefin to the metal is followed by its rate-limiting insertion into the metal-oxygen bond giving a pseudocyclic dioxometallocyclopentane (Structure 3). The latter decomposes to the epoxide and the metal alkoxide. 

The generally accepted mechanism for the metal-catalyzed ROP of lactide is a coordination-insertion mechanism (Figure 2), which was proposed by Dittrich and Schulz. " In this mechanism, the lactide is activated after coordination to a metal center through the carbonyl oxygen. Then an initiator, such as an alkoxide, attacks the... 

Figure 2 Coordination-insertion mechanism for the polymerization of lactide by a metal complex with an alkoxide initiator...

The metal-insertion ROP owes its name to the propagation mechanism [Fig. 21.2]. After coordination of the metal-alkoxide with the carbonyl group of the lactone, the addition of the nucleophilic alkoxides takes place onto the electrophilic ester bond. Subsequently, an elimination reaction occurs via acyl-oxygen scission. The novel alkoxide will act as the newly generated propagating species. 

Acrylonitrile polymerizes also by anionic mechanism. There are many reports in the literature of polymerizations initiated bv various bases. These are alkali metal alkoxides, butvl-lithium, metal ketyls, "solutions of alkali metals in ethers, sodiummalonic esters, and others. The propagation reaction is quite sensitive to termination by proton donors. This requires the use of aprotic solvents. The products, however, are often insoluble in such solvents. In addition, there is a tendency for the polymer to be yellow. This is due to some propagation taking place by 1,4 and 3,4 insertion in addition to the 1,2 placement 1... 

Two mechanisms for the hydroesterification of alkenes have been considered. One pathway—the "alkoxide cycle"— begins with the insertion of CO into a metal alkoxide (Scheme 17.18) and the other—"the hydride cycle"— begins with the insertion of an alkene into a metal hydride (Scheme 17.19). The relative importance of the different pathways depends on the identity of the dative ligand. However, hydroesterification of ethylene with the bis(di-ferf-butylphosphinomethyl)benzene ligand is now generally accepted to occur through a palladium hydride. The mechanism of the hydroesterification of alkynes is less established, but is likely to occur by a sequence that shares some steps with the mechanism for the hydroesterifcation of alkenes. 

The alkoxide pathway occurs by initial insertion of CO into a palladium alkoxide, followed by insertion of the alkene into the bond between the metal and the alkoxycarbonyl group to form a paUadium-alkyl complex (Scheme 17.18). Protonation of this metal alkyl by alcohol would form the free organic product and regenerate the paUadium alkoxide. This mechanism has now been ruled out for the reactions of ethylene to form methyl propanoate. Although each of these steps has precedent, the absence of reduction products from the alkoxide argues against this pathway. Moreover, the alkyl generated from insertion of ethylene into the palladium-alkoxycarbonyl complex (Scheme 17.18) is chelated to the metal, and metha-nolysis of this species is slower than the steps of the alternative hydride mechanism. ... 

Other challenges not only in metal alkoxide catalysis but in catalytic processes generally are development of catalytic protocols which on one hand could work in solvent free condition or in green solvent such as water or liquid CO, and on the other, could recover without loss of its activity. Supporting metal alkoxide onto the inorganic solids [47,49, 50] especially magnetic ones [38] can effectively solve the later one. Reported results (Tables 7.1-7.4) are also showed that in most cases solvent free conditions made products with better properties especially in the catalytic polymerization processes. In these processes, coordinative solvents drastically reduced the quality of product because they competed with monomer to coordinate to the metal core. This step is the basic process in coordination-insertion mechanism in ROP reactions [4, 11, 31, 54]. 

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