Skeleton-type catalysts

E ure 9.10 Correlation between specific activity for the ORR at 0.9 V and the d-band center for different Pt-skin and Pt-skeleton type catalysts. Note how PtjCo is atop both curves. ... 

For industrial hydrogenation of vegetable and animal oils in Russia a Raney type nickel was prepared by Bag and co-workers (64). Preparation of detergents from hydrogenated fats has been reported (11). Reviews of these so-called skeleton catalysts were published by Russian investigators, for instance, by Lel chuk and co-workers (197). These catalysts have also been discussed with reference to hydrocarbon synthesis from water gas (148). Lel chuk (197) states that Raney nickel is more drastic for water gas synthesis than are the skeleton nickel catalysts prepared by Bag, and that Bag s copper-nickel skeleton catalysts approach nickel in their activity. Destructive hydrogenation under mild conditions was said to be possible with Bag s skeleton catalyst as described by Lel chuk. 

Newer types of the dinuclear vanadium(IV) complex catalysts 84 have been developed. The abovementioned dinuclear vanadium complexes possess a VO V linkage whereas the ESR study on the catalyst 84 revealed no V—O—V linkage. The sense of enantioselection by the catalyst 84 of the (R,5,5)-structure is opposite to that of the binuclear complex 78a of the same (R,5,5)-structure. These results suggested two active sites attached to the binaphthyl skeleton in the catalyst 84 performed the dual activation of 2-naphthols in the oxidative couphng to achieve high enantioselectivity ... 

Enamine nucleophiles react readily with soft conjugated electrophiles, such as a, 3-unsaturated carbonyl, nitro, and sulfonyl compounds [20-22], Both aldehydes and ketones can be used as donors (Schemes 27 and 28). These Michael-type reactions are highly useful for the construction of carbon skeletons and often the yields are very high. The problem, however, is the enantioselectivity of the process. Unlike the aldol and Mannich reactions, where even simple proline catalyst can effectively direct the addition to the C = O or C = N bond by its carboxylic acid moiety, in conjugate additions the charge develops further away from the catalyst (Scheme 26) ... 

In the Park-Jew group s systematic investigation, two types of catalyst - the 1,3-phenyl- and 2,7-naphthyl-based dimeric ammonium salts - were selected as an efficient skeleton of chiral PTCs for the catalytic asymmetric phase-transfer alkylation... 

Later, Arcadi showed that /J-keto-imines react with alkynes intramolecularly to give pyrroles. The intermolecular animation with aniliaes was later developed by Hayashi and Tanaka using a cationic Au(I) catalyst to form imines (equation 27). More recently, Arcadi etal. developed an intramolecular version for the cyclization of o-alkynylanilines to form indoles (equation 28) and Li reported a double intra- and intermolecular hydroamination to obtain A-vinylindoles. " O-Substituted hydroxylamines can also undergo this type of transformation to dihydroisoxazole derivatives. " " Tandem sequences that involve a first alkyne-hydroamination step with anilines have been recently developed " " and are similar to the previonsly discnssed additions with phenols that access isoflavone skeletons. 

Two general reactions of this type are shown. There appears to be no report on stereoselective cyclizations of type a . Although transformation b has often been employed, e.g., for the construction of the chromanone skeleton 1, no stereoselective variants (simple diastereoselec-tion or enantioselection) appear to have been reported either. Since base catalysis has usually been applied, asymmetric induction by use of a chiral base catalyst appears feasible. However, this assumption still awaits experimental proof. 

However, the following are examples of direct electrophilic alkylation of imidazole. A two-step synthesis of marine natural product ageladine A 271 started from the commercially available histamine 269 and the pyrrole-2-carbox-aldehyde 268. The entire skeleton of the alkaloid was built in one step via a Pictet-Spengler type condensation. The reaction was accelerated by Lewis acid scandium triflate, although the reaction did proceed without a catalyst but at a slower pace (Scheme 68) <20060L4083>. 

The fundamental carbonium ion-type reactions of olefins— including double bond and carbon skeleton isomerization, polymerization, isotopic exchange, and hydrogen transfer—have been reviewed earlier (62). The importance of a thorough understanding of the nature of olefin transformations over zeolite catalysts cannot be underestimated. Probably the most important and frequently recurring pattern is the transfer or redistribution of hydrogen that is observed with olefins over acidic crystalline aluminosilicate catalysts. 

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