In Lewis acids

Drago and co-workers have correlated a large body of enthalpies of adduct formation in Lewis acid-base systems, including some solvents as reactants, with this four-parameter equation ... 

With the success in Lewis acid-catalyzed thiol conjugate addition reactions mentioned above, we further tried to apply the J ,J -DBFOX/Ph-nickel(II) aqua complex catalyst to the catalyzed asymmetric conjugate addition reactions of hydroxyl-amines [88, 89]. However, after some preliminary examinations, we found that... 

The number of theoretical investigations of hetero-Diels-Alder reaction is very limited. The few papers dealing with this class of reactions have shown that the influence of the Lewis acid on the reaction course can to a high extent be compared to those found the carbo-Diels-Alder reactions. At the present stage of investigations, however, more work is needed if we are to understand the influence and control of selectivity in Lewis acid-catalyzed hetero-Diels-Alder reaction - we are probably at the beginning of a new era in this field. 

In Lewis acid catalyzed carbonyl additions of allylsilanes and -stannanes, syn diastereoselectiv-ity predominates, irrespective of the double-bond configuration, indicating that open-chain transition states are involved. 

Complete trans stereoselectivity is observed in Lewis acid induced alkylation reactions of 4-acetoxyazetidinones with allenylstannanes110, propargylsilanes111, allylstannanes112, and allylsilanes113. 

G. Majetich, K. Hull, D. Lowery, C. Ringold,. 1. Defauw in Selectivities in Lewis Acid-Promoted Reactions, D. Schinzer, Ed., Kluwer, Dordrecht 1989. 

These values show that from the two possible alternatives of ion formation that one is preferred, which leads to the formation of an anion with the largest number of F-, after which, of Cl-ligands. It is remarkable that ionization in Lewis acid mixtures is favoured versus that in pure Lewis acids in all cases. This could be the reason why the polymerization conversion increases when using Lewis acid mixtures as initiators. However, it should be pointed out here that the quantum chemical reaction energies employed are only then comparable with each other, when they are valid for the same process used for modelling the reactions. 

Studies in Lewis acid and LiCi04 (or nafion-H) catalyzed ionic Diels-Alder reactions of chiral and achiral olefinic acetals respectively [96]... 

Fringuelli F., Piermatti O., Pizzo F., Vaccaro L. Recent Advances in Lewis-Acid Catalyzed Diels-Alder Reactions in Aqneons Media Eur. J. Org. Chem. 2001... 

Nishiyama H., Motoyama Y. Other Transition Metal Reagents Chiral Transition-Metal Lewis Acid Catalysis for Asymmetric Organic Synthesis in Lewis Acid Reagents 1999 225, Ed Yamamoto H., Pb. Oxford Univ. Press, Oxford Keywords asymmetric Diels-Alder reactions, chiral transition metal Lewis-acid catalysis, asymmetric synthesis... 

In this case, the diminution in Lewis acidity caused by the introduction of the two, less-electronegative, sulfur ligands prevents any reaction with the dithiol enzyme, regardless of the nature of the alkyl group. 

The highly ordered cyclic TS of the D-A reaction permits design of diastereo-or enantioselective reactions. (See Section 2.4 of Part A to review the principles of diastereoselectivity and enantioselectivity.) One way to achieve this is to install a chiral auxiliary.80 The cycloaddition proceeds to give two diastereomeric products that can be separated and purified. Because of the lower temperature required and the greater stereoselectivity observed in Lewis acid-catalyzed reactions, the best diastereoselectivity is observed in catalyzed reactions. Several chiral auxiliaries that are capable of high levels of diastereoselectivity have been developed. Chiral esters and amides of acrylic acid are particularly useful because the auxiliary can be recovered by hydrolysis of the purified adduct to give the enantiomerically pure carboxylic acid. Early examples involved acryloyl esters of chiral alcohols, including lactates and mandelates. Esters of the lactone of 2,4-dihydroxy-3,3-dimethylbutanoic acid (pantolactone) have also proven useful. 

The first carbonyl complex of gold, [AuCl(CO)], was prepared in 192 52080,2081 and since then only a few more derivatives have been obtained. The [AuBr(CO)] derivative was prepared later and is unstable in the solid state.2082,2083 The reductive carbonylation of Au(S03F)3 in fluorosulfonic acid leads via [Au(CO)2]+ (solvent) to solid [Au(S03F)(C0)] (Scheme 30).2084 [Au(CO)2]+ salts are produced in strongly ionizing protic acids or in Lewis acids such as SbF5. 

Schinzer, D., Ed. Selectivities in Lewis Acid Promoted Reactions 1989, Kluwer Academic Dordrecht, Netherlands. 

Luder, W. F., and Zuffanti, S. (1946). The Electronic Theory of Acids and Bases. Wiley, New York. A small book that is a classic in Lewis acid-base chemistry. Also available as a reprint volume from Dover. 

Also, manganese added to cobalt on activated carbon catalysts resulted in a decrease in bulk carbide formation during reduction and a decrease in the subsequent deactivation rate.84 Magnesium and yttrium added to the support in alumina-supported cobalt catalysts showed a lower extent of carburization. This was explained by a decrease in Lewis acidity of the alumina surface in the presence of these ions.87... 

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

A seven-membered ring is formed in the cyclization of 195 (equation 95)105. The homologue 196 affords the fused cyclooctane 197, together with the cis- and trans-decalinones 198 (equation 96)106. Six-, seven- and eight-membered rings are produced in Lewis acid-catalysed reactions of various cyclohexenones with side-chains terminating in allylic trimethylsilyl groups (equations 97 - 99)107. 

K. Manabe, S. Kobayashi, Effeds of Metal Cations in Lewis Acid-Surfadant-Combined Catalyst-Mediated Aldol Readions in Water Synlett 1999,547-548. 

In the Re(V) and W(IV) aqua oxo complexes, comparison of both the complex formation of the [MO(OH2)(CN)4], by NCS ions and the water exchange (k iq) shows a relative increase in reactivity of approximately 3 orders of magnitude (Table II), which is in direct agreement with the previously (1, 2, 50) concluded dissociative mechanism. The increase in Lewis acidity of the Re(V) center compared to that of W(IV) is expected to result in a much less reactive system in a dissociative activated mode. 

Maruoka, K. Synthetic Utility of Bulky Aluminum Reagents as Lewis Acid Receptors. In Lewis Acid Reagents-, Yamamoto, H., Ed. Oxford University Press Oxford, 1999 pp 5-29. 

Cycloalkenes such as cyclohexene, 1-methylcyclohexene, cyclopentene, and nor-bornene are hydrosilylated with triethylsilane in the presence of aluminum chloride catalyst in methylene chloride at 0 °C or below to afford the corresponding hydrosilylated (triethylsilyl)cycloalkanes in 65-82% yields [Eq. (23)]. The reaction of 1-methylcyclohexene with triethylsilane at —20 °C occurs regio- and stereoselectively to give c/i-l-triethylsilyl-2-methylcyclohexane via a tra x-hydrosilylation pathway. Cycloalkenes having an alkyl group at the double-bonded carbon are more reactive than non-substituted compounds in Lewis acid-catalyzed hydrosilylations. ... 

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