Carbonyl transesterification reactions

An analogue of the transesterification process has also been demonstrated, in which the diacetate of BPA is transesterified with dimethyl carbonate, producing polycarbonate and methyl acetate (33). Removal of the methyl acetate from the equihbrium drives the reaction to completion. Methanol carbonylation, transesterification using phenol to diphenyl carbonate, and polymerization using BPA is commercially viable. The GE plant is the first to produce polycarbonate via a solventiess and phosgene-free process. 

The transesterification reaction can be attributed to the perturbation of the ester carbonyl group in the [CDc/o5e< -substrate] complex. The possibility of this side reaction was predicted by earlier quantum-chemical calculations [18]. These results indicated that the reaction pocket in methyl pyruvate for the nucleophilic attack is situated between the two carbonyl groups, i.e. both carbonyl groups can be perturbed by a nucleophile provided both carbonyl groups have the right "directionality". However, the right "directionality" for both carbonyl groups can be obtained if they are in syn position. 

According to the Lewis theory, alkaline earth metal hydroxides are weaker bases than their oxides, the order of the strength of the basic sites being Ba(OH)2> SrO(OH)2 > Ca(OH)2 > Mg(OH)2. The hydroxides have been used recently as solid catalysts for organic transformations, such as the conjugate addition of methanol to a, S-unsaturated carbonyl compounds (12), cyanoethylation of alcohols (163,164), and transesterification reactions (166,167,171,172) which are described above. The extensive work of Sinisterra et al. (282) on the number and nature of sites and on the catalytic activity of the most basic alkali metal hydroxide, Ba(OH)2, is emphasized. It was found that commercial barium hydroxide octahydrate can be converted into... 

Many catalysts do not use metals in their pure reduced metallic forms. Anchored organometallic complexes are often analogs of homogenous catalysts fixed on a solid support. In particular, titanate complexes both in solution and in supported form have been found to be especially active in transesterifications of simple esters.It was proposed that titanates catalyze the transesterification reaction through a Lewis acid mechanism where the reactant ester and metal form a Lewis complex activating the carbonyl groups for a nucleophilic attack by the reactant alcohol. The tetrahedral intermediate that is formed breaks down into the product alcohol and an ester-metal Lewis... 

Sigman and Jorgenson490 have found that zinc(II) catalyzes the transesterification reaction between N- (/3-hydroxy ethyl) ethylenediamine and 4-nitrophenyl picolinate. This reaction involves a reactive mixed ligand complex (152) in which the zinc(II) ion perturbs the p/fa of the hydroxyethyl group of N-(/3-hydroxyethyl)ethylenediamine to provide a high effective concentration of the nucleophile. Intramolecular nudeophlic attack then occurs at the carbonyl group of p-nitrophenyl picolinate. This system provides a somewhat unique example of intramolecular... 

Transesterification or transamination are metal-directed reactions which are commonly encountered. We have discussed transamination processes in Section 5.5.2 and also in Chapter 3. The key step involves the attack of a co-ordinated alcohol or alkoxy group upon the carbonyl of a co-ordinated ester or amide. Many Lewis acidic metal ions have been shown to be effective catalysts for transesterification reactions for example, heating diethyl picolinate with copper(n) salts in methanol results in rapid and clean transesterification (Fig. 5-86). In the absence of the metal ion, the rate of reaction is vanishingly slow. 

Transesterification reactions of 2-alkoxycarbonylethyltin trichlorides and their adducts with neutral donors with alcohols proceed readily. This is attributed to the intramolecular Lewis catalysis by the electrophilic SnCl group owing to the coordination of the ester carbonyl group to the tin atom, C = O —Sn, which decreases the electronic density at the carbonyl carbon atom727,741 745 754. 

The above conclusion on the role of reactivity ratios on microstructure assumes the absence of rapid transesterification reactions between chains. Since such processes might also tend to randomize the microstructure, it seemed important to isolate the role of interchain transesterification. A unique experiment was designed in which a 13c labeled carbonyl in acetoxy benzoic acid monomer (B ) was reacted with the dimer of HBA-HNA. At 99% enrichment, the only resonances in the carbonyl region of the spectrum will arise from the enriched benzoic acid carbonyl. In the absence of any interchain transesterification the microstructure of the polymer would consist only of B -B dyads (see scheme 1). 

The alkoxide base and ester side chain should be matched. For example, the ethoxide ion should be used as a base for ethyl esters (R = Et in the scheme shown above). This ensures that if the ethoxide ion attacks the carbonyl group of an ethyl ester (in a transesterification reaction see Section 9.7), then an ethyl ester will be re-formed. 

The mechanism of this equilibrium is typical for base catalysed transesterification reactions, the alcoholate anion of the catalyst (for example sodium or potassium methoxide) attacks the carbonyl group of the ester bonds first (reactions 17.6). 

Hedrick and Waymouth also used 1,3-dimethylimidazolylidene as a nucleophile to activate esters for transesterification reactions [73]. Here, the NHC first adds into the carbonyl, eliminating l.Oequiv. of alcohol, and is then displaced by... 

Transesterification reactions could occur during anionic ROP of six-membered carbonates (Scheme 4.5). The intramolecular nucleophilic attack on carbonyl carbon atom (back-biting) leads to cyclic oligomers. The control of the polymerisation is rather poor, and bimodal distribution of molar masses is often observed (Matsuo et al., 1998b Pahovnik and Hadjichristidis, 2015). 

Reaction mechanisms for transesterification reactions were discussed and it was concluded that the 8-caprolactone ends 10 are more reactive than the lactide ends 9b, both in terms of electronic and steric effects. The carbonyl group in close proximity to the end of the lactide chain was considered to reduce the nu-cleophilicity of the terminal hydroxyl group and the alpha-methyl group was also considered to provide steric hindrance [117]. 

The usefulness of osmium derivatives in nucleic acid chemistry has prompted an investigation of the synthesis and reactions of osmium ligand complexes [e.g. (334)—(336)] of common nucleosides. Such dipyridylosmium esters underwent relatively rapid transesterification reactions with glycols. Other groups have reported n.m.r. studies on the complexes formed between nucleosides and transition-metal carbonyls e.g, [Rh(COa)Cl]2 and W(CO)e and on the ionic binding of Li+ and Cl ions to nucleosides (based on Li and C1 nuclear magnetic relaxation times). ... 

Dialkylamino)pyridines (Figure 1) behave either as weak bases or as nucleophiles via the sp -hybridized nitrogen atom. DMAP and 4-pyrrolidinopyridine (PPY), which are commercially available, are prototypes of such 4-aminopyridine derivatives. In molecular chemistry, these compounds have proven efficient in acylation transesterification reactions. Section 4.06.4 will discuss their use for the organocatalyzed ROP of carbonyl-containing heterocycles. 

The ROP of six-membered cyclic carbonates with nucleophilic initiators is a chain reaction in which, besides initiation and propagation reactions, transesterification reactions can also take place (Scheme 32).Intramolecular nucleophile (e.g., alkoxide) attacks on carbonyl carbon atom (backbiting) lead to cyclic oligomers, while intermolecular transesterifica-tion leads to a change of the macromolecule length with the consequence that, at equilibrium, the most probable distribution of the molecular weight is obtained (Scheme 32). 

Preparation of the key tropine is achieved by any one of several variations on the method first developed by Robinson, which involves reaction of a primary amine with dihydroxyacetone and glyoxal. Reduction of the carbonyl group in the product (86) followed by acylation affords the aminoester (88). Transesterification with ester aldehyde 89... 

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