Catalysts for cleavage

The second step involves coal activation. The relative ability of different media to split reactive crosslinkages of the coal is a crucial factor in obtaining conversion. The reactive crosslinks appear to be primarily ether bonds and aliphatic linkages, with suitably substituted neighboring aromatic centers (.5,6). Work in these laboratories has shown that ZnCl2 is an active catalyst for cleavage of these crosslinks (5,9). Addition of methanol may enhance this activity, whereas excessive solvent appears to dilute the catalyst. 

Oxidation of cobalt(ll) to cobalt(lll) by oxygen in the presence of N-hydroxyethylethylenediamine and carbon produces large amounts of ethylenediamine. Other products are formaldehyde, formic acid, and ammonia. The sum of the moles of ethylenediamine and ammonia produced is equal to the total number of moles of cobalt(ll) oxidized. A steady-state concentration of Co(ll)-Co(lll) is established in which the ratio Co(lll)/ Co(ll) = 1.207. Thus cobalt ion behaves as a true catalyst for cleavage of the N-hydroxyethyl-ethylenediamine. The total amount of cobalt(ll) oxidized per unit time, X, was calculated from the derived equation X = 3.8 + 7.0 k2 T — 3.8e-2-2k 1, where k2 = 0.65 hr.—1 The observed rate of formation of ethylenediamine plus ammonia also follows this equation. It is proposed that the cobalt ion serves as a center where a superoxide ion [derived from oxidation of cobalt-(II) by oxygen] and the ligand are brought together for reaction. 

Peters and Sweeny carried out cyclic voltammetry of nickel(ll) salen at a glassy carbon electrode in [BMlMJiBFJ. They found that nickel(ll) salen exhibits one-electron, quasi-reversible reduction to nickel(l) salen, and that the latter species serves as a catalyst for cleavage of carbon-halogen bonds in iodoethane and... 

Hampl, F., Mazac, J., Liska, F., Srogl, J., Kabrt, L., Suchanek, M. (1995). Quaternary heteroarenium aldoximes as catalysts for cleavage of phosphate esters. Collect. Czech. Chem. Commun. 60 883-93. 

The catalyst for cleavage of peptide deformylase was searched with a library of catalyst candidates synthesized by the Ugi reaction (Scheme 2) (134). In this multicomponent condensation reaction, the mixture of a carboxylic acid, an amine, an aldehyde, and an isocyanide produces an iV-acyl amino acid amide. The catalyst candidates, therefore, are iV-acylamino acid amides containing various polar and nonpolar pendants as well as the Co(III) complex of cyclen. The Co(III) complex of cyclen (135) was chosen as the proteolytic center in view of the results described in Section V.A. Cyclen with three secondary amines protected with ferf-butyloxycarbonyl (t-boc) groups was incorporated in either the carboxyl or the amine component of the Ugi reaction. Later, the t-boc groups were removed and Co(III) ion was inserted to the cyclen portion. 

From the three kinds of libraries of catalyst candidates used in the studies of catalysts for cleavage of peptide deformylase and angiotensins, the diastereo-meric mixture of (55)-CN and (T .S)-CN was found to have the activity to cleave melanin-concentrating hormone (150). Examination of the activity of (55)-CN synthesized separately by a stepwise synthetic route revealed that (5.S)-CN was the active isomer. Upon incubation with (5.S)-CN, the TV-terminal aspartate residue of melanin-concentrating hormone was converted to pyruvate residue by... 

Cleavage of aziridines. Copper(II) triflate is a good catalyst for cleavage of... 

Co(ll) salts are catalysts for cleavage of 1,2-glycols with O2 in aprotic, polar solvents the yields of aldehydes are good. The method is adaptable to preparation of acids without isolation of the intermediate aldehydes. 

Single-bond cleavage with molecular hydrogen is termed hydrogenolysis. Palladium is the best catalyst for this purpose, platinum is not useful. Desulfurizations are most efficiently per-formed with Raney nickel (with or without hydrogen G.R. Pettit, 1962 A or with alkali metals in liquid ammonia or amines. The scheme below summarizes some classes of compounds most susceptible to hydrogenolysis. 

Interestingly, the allylation of a stabilized carbon nucleophile has been found to be reversible. Complete isomerization of dimethyl methylmalonate, involving bis-allylic C—C bond cleavage, from a secondary carbon 38 to a primary carbon 39 was observed by treatment with a Pd catalyst for 24 h. The C—C bond cleavage of a monoaliylic system proceeds slowly[40]. 

One potential approach extends the idea of chemical amplification introduced in our preceding description of dry-film resists. In 1982, Ito and co-workers (37,38) recognized that if a photosensitizer producing an acidic product is photolyzed in a polymer matrix containing acid-labile groups, the acid will serve as a spatially localized catalyst for the formation or cleavage of chemical bonds. 

Acids are poor catalysts for ring cleavage of thiirane 1,1-dioxides but are good catalysts for reactions of thiirane 1-oxides with nucleophiles. These reactions of episulfoxides are believed to proceed by protonation of the oxygen atom (but see the NMR evidence cited above for 5-protonation in fluorosulfonic acid) and will be treated in the section on nucleophilic reactions. 

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

Tetrabutylammonium fluoride (TBAF) is usually used in the form of the trihydrate or as a solution in tetrahydrofuran (THF). The pure form is difficult to isolate, owing to decomposition to FFF, tributylamine, and but-l-ene [18, 19] on dehydration. It has been used for a variety of reactions, including as a catalyst for various reactions with silicon compounds [20, 21]. One of its main uses is in the cleavage of silyl ether protecting groups [22]. 

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... 

These alkoxo complexes are also capable of cleavage of the C—Si bond of TMSCF3. The gold(III) fluoroalkoxides m-[AuMe2(OR)(PPh3)] (R = CH2CF3, CF1(CF3)2) can act as catalysts for the Knoevenagel condensation reaction.1775... 

Pd is the most widely used catalyst for the cleavage of benzylic C—O bonds, and 10% Pd/C is the most popular form of Pd for hydrogenolysis of benzyl alcohols. This catalyst usually contains residual acids, which increase reaction rates. 

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