Methanolysis Catalytic

It was found that La -catalysed methanolysis of hydroxy-p-nitrophenyl phosphate (HPNPP) (76) is a model for the RNA transestrification reaction (Scheme 16). The same authors proposed catalytic methanolysis promoted by La as a new method for controlled decomposition of paraoxon (74) (Scheme 17). They found that methanolysis of (74) promoted by La(OTf)3 (Tf=0S(0)2CF3) in a methanol medium is billion-fold accelerated. Investigation of the reaction of oximate ot-nucleophiles with diisopropylphosphoro-fluoridate (DFP) (77) and two model phosphonates (78) and (79) has been... 

One hurdle in the hydrolysis of AB, particularly, when working with concentrated solutions is the liberation of ammonia gas in small quantities which would hinder its use in fuel cell applications moreover the hydrolysis product of AB is not recyclable. A recent study [162] has shown that the problem can be circumvented by using methanol instead of water, as the catalytic methanolysis of ammonia borane by using transition metal chlorides (RuQs, RhClj, PdQ2, and C0Q2) as precursor yields hydrogen gas without ammonia, and the methanolysis product, ammonium tetramethoxyborate, is recyclable (Eq. (7.3)). 

The alkali-catalysed methanolysis of poly(2,2-bis(4-hydroxyphenyljpropane carbonate) (PC) in a mixture of methanol (MeOH) and toluene or dioxane was studied. The treatment of PC in meOH, with a catalytic amount of sodium hydroxide, yielded only 7% bisphenol A. Using a mixed solvent of MeOH and toluene completely depolymerised PC to give 96% free bisphenol A in solid form and dimethyl carbonate in solution. The eharaeteristies of the catalysis are discussed together with the pseudo-first rate kinetics of the depolymerisation. The reaetion eonditions were investigated to facilitate the reeyeling of PC plasties. 17 refs. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

The catalysis afforded by the La3 + system for the transesterifications of paraoxon in ethanol and methanol is quite spectacular relative to the background reactions that are assumed to be promoted by the lyoxide. The reaction rate constant of ethoxide with paraoxon in ethanol at 5.1 x 10-3 dm3 mol-1 s-133 is roughly a factor of two lower than the rate constant of methoxide with paraoxon in methanol (1.1 x 10 2dm3mol 1 s-1).17a However a solution 2mmoldm-3 in total [La3 + ], which contains 1 mmol dm-3 of Lal+, has a maximum rate constant of 7 x 10-4s-1 for decomposition of 1 in ethanol at pH of 7.3, and accelerates the rate of ethanolysis of paraoxon by a factor of 4.4 x 10n-fold relative to the ethoxide reaction at the same pH.34 By way of comparison, the acceleration afforded by a 1 mmol dm-3 solution of the La + dimer catalyzing the methanolysis of 1 at the maximal pH of 8.3 (kobs = 0.0175 s 1) is 109-fold greater than its background methoxide reaction. On this simple basis La2+ in ethanol appears to be catalytically superior to La2+ in methanol, but this stems almost exclusively from the pH values... 

While the acceleration afforded to the cyclization of 32 by La3 + in methanol is certainly spectacular, this is not a biologically relevant metal ion and its charge exceeds that of the natural metal ion Zn2+. Very recent investigations of Zn2+-catalysis of the methanolysis and ethanolysis of 32 indicated that there were indeed interesting catalytic effects, and that the situation in pure ethanol is quite different.85 Shown in Figs 15 and 16 are plots of the pseudo-first-order rate constant (kobs) for ethanolysis and methanolysis of HPNPP (32) as a function of [Zn2+]total when the [ OR]/[Zn2+] ratio is 0.5. This ratio was chosen to buffer the system at the half neutralization jjpH of 7 in ethanol8 and 9.5 in methanol at [Zn2+]to)ai = l-2 mM7... 

Catalytic reduction and methanolysis of the methylated capsular polysaccharide of Type III pneumococcus gives a mixture of methyl 2,3,6-trimethyl- and 2,4-dimethyl-a/3-D-glucopyranosides. The latter, which arises from glucuronic acid units in the polysaccharide, can be separated into crystalline a- and 6-isomers identical with synthetic specimens.84,88... 

Ideally, the Pd-OCH3+ or Pd-H+ species that initiate the catalytic cycle regenerate at the termination step of the chain propagation process. Chain transfer occurs via protonolysis or methanolysis. 

If the ligands dppomf and dppo maintain frans-coordination throughout the catalytic cycle, it will be interesting to establish how the insertions of CO and ethene and methanolysis to MP occur. 

Figure 15.8 a simple example is presented of a subsequent insertion of CO and methanolysis of the palladium acyl intermediate [14], This is not a very common reaction, because both the ligand requirements and the redox conditions for Wacker and carbonylation chemistry are not compatible. For insertion reactions one would use cis coordinating diphosphines or diimines, which makes the palladium centre more electron-rich and thus the nucleophilic attack in the Wacker part of the scheme will be slowed down. In addition, the oxidants present may lead to catalytic oxidation of carbon monoxide. 

The above hydrochloride is treated with thionyl chloride in liquid sulfur dioxide, to produce an amorphous chloride hydro chloride, which is converted to the nitrile with sodium cyanide in liquid hydrogen cyanide, Methanolysis then gives the ester of the nitrile. Alkaline hydrolysis of this last compound, followed by catalytic dehydrogenation in water using a deactivated Raney Nickle catalyst (see JOC, 13, 455 1948) gives dl-lysergic acid. 

Peterson and Scarrah 165) reported the transesterification of rapeseed oil by methanol in the presence of alkaline earth metal oxides and alkali metal carbonates at 333-336 K. They found that although MgO was not active for the transesterification reaction, CaO showed activity, which was enhanced by the addition of MgO. In contrast, Leclercq et al. 166) showed that the methanolysis of rapeseed oil could be carried out with MgO, although its activity depends strongly on the pretreatment temperature of this oxide. Thus, with MgO pre-treated at 823 K and a methanol to oil molar ratio of 75 at methanol reflux, a conversion of 37% with 97% selectivity to methyl esters was achieved after 1 h in a batch reactor. The authors 166) showed that the order of activity was Ba(OH)2 > MgO > NaCsX zeolite >MgAl mixed oxide. With the most active catalyst (Ba(OH)2), 81% oil conversion, with 97% selectivity to methyl esters after 1 h in a batch reactor was achieved. Gryglewicz 167) also showed that the transesterification of rapeseed oil with methanol could be catalyzed effectively by basic alkaline earth metal compounds such as calcium oxide, calcium methoxide, and barium hydroxide. Barium hydroxide was the most active catalyst, giving conversions of 75% after 30 min in a batch reactor. Calcium methoxide showed an intermediate activity, and CaO was the least active catalyst nevertheless, 95% conversion could be achieved after 2.5 h in a batch reactor. MgO and Ca(OH)2 showed no catalytic activity for rapeseed oil methanolysis. However, the transesterification reaction rate could be enhanced by the use of ultrasound as well as by introduction of an appropriate co-solvent such as THF to increase methanol solubility in the phase containing the rapeseed oil. 

A pure sample of 2 undergoes methanolysis under conditions identical to those of the catalytic experiment with a first-order rate constant of 7.7 x 10 min, in good agreement with the value obtained from the two-parameter treatment of the kinetics... 

Figure 5.2 Catalytic efficiency of 4-Ba in the methanolysis of aryl acetates calculated for a substrate-to-catalyst ratio of 10 1 (curve a) and 100 1 (curve b) versus ester reactivity (as measured by the log kbg values).

The catalytic efficiency, as conveniently measured by the ratio of the catalyzed process under steady state conditions to the rate of background methanolysis (Figure 5.2), is a function of the substrate-to-catalyst ratio and reaches maximum values in the reaction of pCPOAc. 

It is apparent that an increase in the substrate-to-catalyst ratio dramatically decreases the catalytic efficiency for the pNPOAc reactions, but affects to a much lower extent the POAc reaction. This is easily understood with reference to Table 5.3. Since in the reaction of pN POAc the rate-determining step is mainly deacetylation, an increase in ester concentration causes a proportional increase in the rate of background methanolysis, but hardly affects the rate of deacetylation, with the result that catalytic efficiency varies inversely to ester concentration. Conversely, the reaction of POAc approaches a situation in which acetylation of the catalyst is rate determining, which implies that both acetylation and background reactions increase on increasing ester concentration. 

Extrapolation of the above considerations to acetate esters less reactive than POAc, i.e., with leaving groups more basic than PO , leads to the prediction of lower catalytic efficiencies because the rate of acetylation of the catalyst is expected to decrease more rapidly than the rate of background methanolysis. In contrast, with esters of phenols more acidic than pNPOH, the catalytic reaction is predicted to be obscured by increased rates of background methanolysis. Thus, for one reason or... 

With the idea that crown ethers based on the p-tert-butylcalix[5]arene platform could provide an interesting extension of our catalytic studies, several calix[5]-crown-ethers were investigated as potential catalysts of ester methanolysis in the presence of a Ba + salt [22]. Of the various structures investigated, the calixcrown-5 derivative 7 gave the... 

Figure 5.4 Competing catalytic mechanisms of methanolysis of pNPOAc. Mechanism A thiol-mediated methanolysis via an acylation-deacylation cycle mechanism B direct delivery of complex-bound methoxide ion.

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