Methanolysis reaction

However, it is clear that in terms of the second-order rate constants the methanolysis reaction is more effective by about 25-fold at the maximum pH values for the La2+-catalyzed reactions having the same amount of catalyst. For example kf2... 

Table 11 Acid dissociation constants for the phenols in water and methanol, as well as second-order rate constants for the various methanolysis reactions of phosphonates 22a-e promoted by methoxide, La3 + and 9 Zn2 + ( OCH3)...

Scheme 6 A proposed mechanism for the concerted La + ( OCH3)2-catalyzed methanolysis reaction of phosphate triesters with XAr leaving groups.

Scheme 7 Generalized mechanism for the methanolysis reaction of 32, HPNPP proceeding via intramolecular cylcization to form a 5-membered cyclic phosphate which undergoes subsequent opening via attach of methoxide.

Given that the autoprotolysis constants of ethanol and methanol are 10 191 and 10-16-77, respectively, the background kQbs values for the lyoxide-catalyzed ethanolysis and methanolysis reactions of paraoxon at pH 7.3 (ethanol) and 8.3 (methanol) are 8.1 x 10-15 and 3.5 x 10 us 1, respectively... 

The final step in the formation of the polyol is the reverse of the methanolysis step, however, the alcohol used is not limited to glycerin. Almost any alcohol can be used as the initiator in what might better be termed an oligomerization. Like the methanolysis, this reaction is an equilibrium and can be catalyzed by both acids and bases. The primary difference is that the methanolysis reaction is generally driven by the phase separation of the glycerin, while the polymerization is driven by removal of methanol (Figure 5). 

These authors used 6 1 and 30 1 alcohol-to-oil molar ratios for both methanol and butanol. As expected, a pseudo first-order reaction was found at large excess of alcohol for both alcohols. At low excess alcohol, however, the butanolysis reaction (30°C) was second-order, but the methanolysis reaction (40°C) was reported to be a combination of a second-order consecutive reaction and a fourth-order shunt reaction. The shunt reaction, in which three methanol molecules simultaneously attack a TG molecule, was adopted to better fit the kinetic data. However, such a reaction is highly unlikely. Nureddini et al. later found that the inclusion of a shunt mechanism was not necessary to fit the kinetic data of the transesterification reaction, and Boocock et al showed that the shunt reaction assumption came as a misinterpretation of the observed kinetics. At low temperatures (20 0°C) the multiphase methanolysis reaction... 

The gross structural features, presence of a tetramic acid and E-decenoyl side chain, could be inferred from NMR studies. Methanolysis (HCl/MeOH) of 47 and pentane extraction of the quenched reaction mixture gave two compounds that were determined to be the methyl esters of decenoic acid and N-(2-decenoyl)leucine. The nature of the 3-acyl tetramic acid was deduced from the identification of 48 and 49 in the aqueous portion of the methanolysis reaction mixture following treatment with trifluoroacetic acid anhydride. The unusual C-C bond fragmentation under acidic conditions, and the structure of the antibiotic was confirmed by synthesis of racemic 47 [86]. The configuration at the lone chiral centre was established as R by chiral GC. The carbon NMR spectrum of 47 indicated an equilibrium between three tautomers in which the A2-pyrrolin-4-one form is preferred (60%) and the two internal tautomers (50, 51) make equal contributions (20% each). 

Reaction data for phenyldimethylethoxysilane. Acid catalyzed methanolysis reaction at 25 C... 

Table 6. Rate constants for the acid catalyzed methanolysis reactions at 25 C. Data are used to prepare a Hammett plot ...

Some highly selective metal ion-promoted reactions have been observed with phenyl phos-phatosulfate (70).263 In both aminolysis and methanolysis reactions it was found that Mg11 promoted P—O bond fission, while Fein promoted S—O bond fission (Scheme 15). The selectivity is almost 100% in each case. 

The hydrosilylation/methanolysis reaction sequence. One of the most convenient methods is to use a sequence of hydrosilylation/methanolysis reactions as shown in equation 4 for the formation of 39. This direct synthetic pathway was reported in cases of the alkyl family of precursors 1290. This sequence is also used for the preparation of dendrimers and arborols121,123. Alternatively, hydrosilylation with HSi(OEt)3 can be performed advantageously in the case of dendrimers125,126 but it is of more interest in the case of carbonate precursors 26111. Indeed, hydrosilylation of double bonds can be achieved selectively in the presence of a carbonyl group with hexachloroplatinic acid or a rhenium catalyst130. 

Shimada and Bomscheuer (2002) developed two stepwise methanolysis reaction systems with immobilized Candida antarctica lipase. System one, the first-step reaction, was conducted in the presence of 1/3 molar equivalent of methanol for the stoichiometric amount, and the second-step reaction was performed by adding 2/3 molar equivalent of methanol. In system two, the... 

The metallo-siloxane 3a bears the possibility of a regiospecific exchange reaction as the electrophilicity of the a-Si-atoms is decreased due to the electron donating influence of the transition metal fragment. For this reason, the methanolysis reaction of 5a with three equivalents of MeOH involves the y-silicon-atom to yield preferentially the metallo-trimethoxydisiloxane 6, which can be converted into the pentamethoxydisiloxane 7 by using methanol as solvent. 

The methanolysis reaction of the ferrio-siloxane 13 in the presence of NEta shows clearly that the... 

The methanolysis reaction of 15 in MeOH yields the tetramethoxy-substituted (bis)metallo-tetrasiloxane 16 as a brown oil, showing good solubility in n-pentane, diethylether and benzene. 

Mechanistic Studies. - The mechanism of the reaction of tetra-zole-activated phosphoramidites with alcohols has been studied. A series of diethyl azolyl phosphoramidites (85) was prepared from diethyl phosphorochloridite and fully characterized, and the same compounds shown to be formed from the phosphoramidite (86) and azole. The degree of formation of (85) from (86) increases with the acidity of the azole, and the proposed mechanism is a fast protonation of (86), followed by a slow, reversible formation of (85) and a fast reaction of (85) with alcohols. Another study was concerned with the influence of amine hydrochlorides on the rate of methanolysis of the phosphoramidites (87) or (88), or tris(diethylamino)phosphine.The chloride content was measured to be 10-20 mM in doubly distilled samples which explains that "uncatalysed alcoholysis is possible. Intensive purification, including treatment with butyllithium and distillation from sodium, brought the chloride content down to 0.1-1 mM. The methanolysis reaction, in methanol as the solvent, was found to be first-order in catalyst concentration. An aJb initio calculation on N- and P-protonated aminophosphine (89) gave similar proton affinities for N and P this contrasts with earlier MNDO calculations which had ff-protonated species as the most stable. The M-protonated compound had an electronic structure reminiscent of a phosphenium ion-ammonia complex. 

Fig. 2 Effect of enzyme amount on rapeseed oil methanolysis. Reaction conditions Novozym 435 based on oil weight metha-nol/oil molar ratio 3 1 reaction temperature 40 °C...

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