Transesterification mechanism

The fact, that almost all basic functions in caprolactam or polyamide medium occur in the form of amidic anions [-CO-N-]- arose the idea that these particular anions are directly responsible for the polymerization reaction. In analogy to the transesterification mechanism an intermediate addition complex might be formed by combining the amidic group with the respective anion... 

A large variety of metal ions catalyze the cleavage of RNA through the transesterification mechanism, and some of them can also catalyze hydrolysis of cyclic phosphates (e.g. Pb +, Zn +, and Cd +) and nucleoside phosphomonoesters (e.g. Pb +, La +, and Th +). The most efficient cleavage of RNA in aqueous solutions by free metal ions is achieved by the action of rare metal ions (e.g. Eu +, La +, and Tb +), Pb +, and Zn +. Zn + is only about 4% as efficient as Pb2+,327 other catalytically active metal ions (e.g. AP+, Cd +, Mn +, Cu +, Co +, Ni +, or Mg +) are 1-2 orders of magnitude less efficient than Zn +. As a rule, RNA cleavage has a maximum rate at pH values around the pKaS of the first metal-bound H2O (see Table 1), that is, when the hydroxo-metal species still bear positive charge. ... 

Mizuuchi K, Adzuma K. Inversion of the phosphate chirality at the target site of Mu DNA strand transfer evidence for a one-step transesterification mechanism. Cell 1991 66 129-140. 

In commercial practice today reactions of the second type frequently utilize an excess of glycol, increasing the rate of esterification. Later stages of such polycondensations, as in the preparation of polyethylene terephthalate, take place by a transesterification mechanism with liberation of glycol. The direct esterification reaction may be catalysed by a second molecule of the carboxylic acid (self-catalysis) or by an independent acidic catalyst (catalysed esterification). 

The addition of a higher molecular weight acid such as capric acid, is beneficial, and under these conditions high yields of propionates are obtained, presumably via a transesterification mechanism. Using this enzymatic technique it is possible to carry out relatively large scale preparative syntheses of natural flavor-active esters, using natural raw materials as substrates. 

Wei et al. [98-101] investigated the transesterification mechanism and reaction rate in blends of PC and an LCP (PHB/PET60/40). More recently, Guo [109-111] suggested experimentally that transesterification was a result of the compatibility, instead of a prerequisite of compatibility, e.g., transesterification was determined by the initial compatibility of polymer blends. This is an academic argument, but the presence of transesterification at least favors the compatibilization in in situ composites. Further investigation of the transester-iflcation in LCP blends is needed in the coming years. 

The transesterification mechanism occurs when the carbonyl carbon of the starting ester (RCOOR ) undergoes nucleophilic attack by the incoming alkoxide (R O ) to give a tetrahedral intermediate, which either reverts to the starting material, or proceeds to the transesterified product (RCOOR ). This is not an attractable occurrence as it increases the polydispersity and reduces the molecular weight, which affects the mechanical properties of the material. 

Monoglyceride Methanol FIGURE 12.2 Scheme for stepwise transesterification mechanism. 

Cheirsilp B, H-Kittikun A, Limkatanyu S. 2008. Impact of transesterification mechanisms on the kinetic modeling of biodiesel production by immobilized lipase. Biochem Eng J 42 261-269. 

A proposed mechanism of PET glycolysis would be (1) glycol diffusion into die polymer (2) swelling of the polymer, which increases the rate of diffusion and (3) reaction (transesterification) of the glycol hydroxy group at an ester group in die polyester chain.24 Since it is a transesterification process, metal acetate salts are effective catalysts. 

Transesterification occurs by mechanisms that are identical with those of ester hydrolysis—except that ROH replaces HOH—that is, by the acyl-oxygen fission mechanisms. When alkyl fission takes place, the products are the acid and the ether. 

The effect of incorporating p-hydroxybenzoic acid (I) into the structures of various unsaturated polyesters synthesised from polyethylene terephthalate (PET) waste depolymerised by glycolysis at three different diethylene glycol (DEG) ratios with Mn acetate as transesterification catalyst, was studied. Copolyesters of PET modified using various I mole ratios showed excellent mechanical and chemical properties because of their liquid crystalline behaviour. The oligoesters obtained from the twelve modified unsaturated polyesters (MUP) were reacted with I and maleic anhydride, with variation of the I ratio with a view to determining the effect on mechanical... 

Cyclization of substituted phenylacetylene sequences afforded functionalized macrocycles that were amenable to subsequent manipulation. For example, transesterification of 42 with octanol in the presence of 18-crown-6 ether and potassium carbonate gave the corresponding ester in 85% yield (Scheme 13). The ester functionalities could be reduced by DIBALH to give the hydroxymethyl-substituted macrocycle (43) in 61 % yield. The low yield of this particular transformation is attributed to mechanical losses during purification, due to the highly polar nature of the product. Macrocycle 43 could then be treated with alkyl bromides to give a group of benzyl ether derivatized PAMs. 

This work has been extended to transesterification with secondary alcohols [23], and of phosphonate esters [24], Movassaghi and co-workers have demonstrated that NHCs effectively catalyse the amidation of esters with amino alcohols, although an alternative mechanism involving the NHC acting as a Brpnsted base, resulting in nucleophilic activation of the alcohol for an initial transesterification event, followed by rapid O- to iV-acyl transfer, has been proposed [25, 26],... 

Scheme 14-1. General in-line monoanionic mechanism of phosphodiester cleavage transesterification catalyzed by hairpin ribozyme the first proton transfer (PT1), the nucleophilic attack (Nu), and the exocyclic cleavage (Cl) steps are shown, and the Oip and O2p pathways are indicated by blue and red colored hydrogens, respectively. For the uncatalyzed model reaction in solution, the Ojp and O2p pathways are energetically equivalent...

Figure 14-7. Snapshots of the active site structures near the transition state of (top) the nucleophilic attack and (bottom) the exocyclic cleavage for the in-line monoanionic O2p mechanism of cleavage transesterification in the hairpin ribozyme. The yellow and red colored cartoon is for the substrate and ribozyme strands, respectively, and water molecules interacting with non-bridging oxygens and O5/ are shown...

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

Reinhoudt at al. have recently reported [48] the preparation of a calix[4]arene functionalized with two Zn(II) centers 25, which is highly efficient on transesterification of HPNP. The dimer complex is reported to be 50 times more active (in 50 (v/v)% acetonitrile-water at pH = 7.4 and I = 298 K) than the corresponding monomer (26) which is itself 6 times more active than 27, implying the contribution of the calix[4]arene moiety in the mechanism. The saturation kinetic experiments showed high association constant for the catalyst-substrate complex (K -... 

Scheme 2 A proposed mechanism for Lal+ ( OCH3)2 catalyzed transesterification of carboxylate esters with good and poor leaving groups.

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