Catalysts, general addition effect

Enantioselective Catalysis of the Aldol Addition Reaction. There are also several catalysts that can effect enantioselective aldol addition. The reactions generally involve enolate equivalents, such as silyl enol ethers, that are unreactive toward the carbonyl component alone, but can react when activated by a Lewis acid. The tryptophan-based oxazaborolidinone 15 has proven to be a useful catalyst.148... 

Only a few detailed studies of the reaction mechanism of the homogeneous hydrogenation of imines have been published until now. A generalization seems to be very difficult for two reasons. First, rather different catalyst types are effective and probably act by different mechanisms. Second, the effect of certain additives (especially iodide or iodine and acid/base) is often decisive for ee and rate, but a promoter in one case can be a deactivator in another case. 

Great care must be taken not to generalize these effects, as the addition of primary diamines to ruthenium bisphosphine complexes generates a very active catalyst for ketone hydrogenation after the addition of base (see Chapter 32). 

Enantioselective addition of CjH zZn to aldehydes.1 Addition of diethylzinc to either aromatic or aliphatic aldehydes catalyzed by 1 (6 mole %) results in (S)-secondary alcohols in generally 90-95% ee. Although several chiral amino alcohols are known to effect enantioselective addition of R2Zn to aromatic aldehydes, this one is the first catalyst to be effective for aliphatic aldehydes. The dibutylamino group of 1 is essential for the high enantioselectivity the dimethylamino analog of 1, (lS,2R)-N-methylephedrine, effects this addition in only about 60% ee. 

In intramolecular cyclopropanation, Doyle s catalysts (159) show outstanding capabilities for enantiocontrol in the cyclization of allyl and homoallyl diazoesters to bicyclic y-and <5-lactones, respectively (equations 137 and 138)198 205. The data also reveal that intramolecular cyclopropanation of Z-alkenes is generally more enantioselective than that of E-alkenes in bicyclic y-lactone formation198. Both Rh(II)-MEPY enantiomers are available and, through their use, enantiomeric products are accessible. In a few selected cases, the Pfaltz catalyst 156 also results in high-level enandoselectivity in intramolecular cyclopropanation (equation 139)194. On the other hand, the Aratani catalyst is less effective than the Doyle catalyst (159) or Pfaltz catalyst (156) in asymmetric intramolecular cyclo-propanations201. In addition, the bis-oxazoline-derived copper catalyst 157b shows lower enantioselectivity in the intramolecular cyclopropanation of allyl diazomalonate (equation 140)206. 

Michael addition to a., -enoates- Aluminum ion-exchanged montmorillonite (Al-Mont1) is a very effective catalyst for Michael addition of silyl ketene acetals or silyl enol ethers to a,(3-enoates. In fact this heterogeneous catalyst is more effective than Lewis acids, which are generally required in a stoichiometric amount. It also facilitates Michael addition to a,p-enones.2 Examples ... 

Since the formulation of his concepts in 1939 Kobozev has carried out many investigations in the attempt to verify their general validity. In the decomposition of H202 and the oxidation of Na2SCh, the specific effect (173) of small amounts of iron (0.0005 to 40%) added to copper on carbon and the converse in which small amounts of copper were added to iron on carbon were studied. The activity of these catalysts was very effectively promoted by these additives, the extent of promotion being proportional to the concentration of the additives. The catalytic synthesis of ammonia by iron supported on carbon or asbestos was also studied. The results of this study and similar studies of catalytic... 

Highly propylene selective catalysts have been developed to meet this challenge. These catalysts, referred to as FCC olefin additives , are generally used in admixture with more traditional FCC catalysts. These additives are based on the MFI (H-ZSM-5) zeolite and the effect of the additive level, which, combined with higher temperatures increases C3 (Figure 5.21) and C4 (Figure 5.22) olefinicity. For instance, at a temperature of 566 °C and 32% Olefin additive, the propylene yield can reach 15% of the feed. 

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

As for heterogeneous catalysts, the addition of hydrogen is catalyzed by a large variety of materials, but synthetically useful procedures generally employ nickel or the platinum metals. In the latter case, the best results are obtained if the metal is finely divided over the surface of an inert support. Many materials can be used as catalyst supports, however, carbon or alumina are suitable for the majority of reactions. Calcium and barium carbonate or sulfate are also frequently used if less reactive catalysts are desired. The influence of the support is generally small compared to the effect of the metal3. The choice of metal is especially important when stereoselectivity is desired because different metals can catalyze the formation of different diastereomers upon hydrogenation. 

In general, addition of weak acids increases the ee but the presence of water is detrimental. The modified catalyst has higher activity and a lower activation energy than unmodified Ni [10]. It is not clear yet, however, whether the enhanced rate is because of higher dispersion of the modified (corroded) Ni particles or because of a ligand acceleration effect. Note that Ni is thermodynamically unstable under ambient conditions in the presence of oxygen, a feature which complicates not only the application but also the reliable characterization of Ni catalysts. 

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