In Diels-Alder and 1,3-dipolar cycloaddition reactions, Lewis acids lower the LUMO energy of the dienophile or dipolarphile, respectively, making it more reactive toward the diene or the dipole. Among the types of reactions that can be catalyzed by Lewis acids , those with carbonyl -containing substrates have received the greatest amount of attention. The first major discovery in this area was in , when Yates and Eaton reported the significant acceleration of the Diels-Alder reaction by AlCl 3 when maleic anhydride is the dienophile.
In addition to rate acceleration, Lewis acid-catalyzed reactions sometimes exhibit enhanced stereoselectivity, which stimulated the development of stereoinduction models. The models have their roots in knowledge of the structures of Lewis acid-carbonyl complexes which, through decades of research in theoretical calculations , NMR spectroscopy, and X-ray crystallography , were fairly firmly established in the early s: .
The Mukaiyama aldol reaction and the Sakurai reaction refer to the addition of silyl enol ethers and allylsilanes to carbonyl compounds, respectively. Only under Lewis acid catalysis do these reactions occur under synthetically useful conditions. Acyclic transition states are believed to be operating in both reactions for either 1,2- or 1,4- addition, and steric factors control stereoselectivity. This is in contrast with the rigid Zimmerman-Traxler cyclic transition state that has been widely accepted for the aldol reaction with lithium, boron, and titanium enolates. As a consequence, the double bond geometry in the silyl enol ether or allylsilane does not translate well into product stereochemistry.
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A model for the Sakurai 1,2-addition, proposed by Kumada, is presented in the scheme below;  the syn diastereomer is predominant when the E silane is used, and also slightly favored when the Z silane is used. A similar analysis by Heathcock  explains the fact that, with simple substrates, there is essentially no diastereoselectivity for the intermolecular Mukaiyama aldol reaction. The Lewis acid catalyst plays a role in stereoselectivity when the aldehyde can chelate onto the metal center and form a rigid cyclic intermediate.
The stereochemical outcome is then consistent with approach of the nucleophile anti to the more bulky substituent on the ring.
The enhancement in rate is often dramatic, and regioselectivity towards ortho- or para-like products is often improved, as shown in the reaction between isoprene and methyl acrylate. The catalyzed Diels-Alder reaction is believed to be concerted. The carbonyl-ene reaction is almost always catalyzed by Lewis acids in synthetic applications. In a seminal review in the early s, Mikami and colleagues  proposed a late, chair-like transition state, which could rationalize many observed stereochemical results, including the role of steric bulk in diastereoselectivity: .
Citing that methyl gloxylate chelates tin Lewis acids but not aluminum ones, they invoked an early, envelope-like transition state and rationalized the divergent stereochemical outcome of the ene reaction between E butene and methyl glyoxylate.
Lewis-acid catalyzed carbonyl addition reactions are routinely used to form carbon—carbon bonds in natural product synthesis. The Diels-Alder reaction catalyzed or promoted by Lewis acids is a powerful and widely used method in natural product synthesis to attain scaffold complexity in a single step with stereochemical control. In Friedel—Crafts alkylation, a Lewis acid — usually a simple metal halide salt — promotes heterolytic cleavage of a carbon—halogen bond in an alkyl halide and generates a carbocation , which undergoes electrophilic aromatic substitution.
Selectivities in Lewis acid promoted reactions - Semantic Scholar
Although vastly useful in synthesis, the reaction often suffers from side reactions that arise from carbocation rearrangement, alkyl migration, and over-alkylation. Similarly, in Friedel-Crafts acylation, a Lewis acid assists in the generation of an acylium ion from an acid chloride or occasionally acid anhydride.
Although the acylium ion is often assumed to be the active intermediate,  there is evidence that the protonated acylium dication is the active electrophile that undergoes subsequent electrophilic aromatic substitution. The nitrile-based acylation is particularly useful because it allows direct ortho-acylation of aniline without protecting the amine group.
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- Selectivities in Lewis acid promoted reactions.
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Guided by this mechanism, and equipped with knowledge that gallium trihalides are among the strongest Lewis acids,  process chemists at Merck were able to develop highly efficient conditions for this condition towards a drug candidate. Asymmetric catalysis by Lewis acids rely on catalysts with chiral ligands coordinated to the metal center. Over the years, a small number of chiral ligand scaffolds have stood out as having "privileged" catalytic properties suitable for a wide range of applications, often of unrelated mechanisms.
Current research efforts in asymmetric Lewis acid catalysis mostly utilize or modify those ligands rather than create new scaffolds de novo. The "privileged" scaffolds share a few common features, including chemical stability and relative ease of elaboration. The majority of the scaffolds are multidentate. Most of them also have high scaffold rigidity within the ligand. Organic synthesis will play a major role in this book connected with some fundamental mechanistic work on allylsilane and -tin chemistry.
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Selectivities in Lewis acid promoted reactions
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