Lewis acids, as catalysts

With Lewis acids as catalysts, compounds containing more than one alkoxy group on a carbon atom add across vinyl ether double bonds. Acetals give 3-alkoxyacetals since the products are also acetals, they can react further with excess vinyl ether to give oligomers (228—230). Orthoformic esters give diacetals of malonaldehyde (231). With Lewis acids and mercuric salts as catalysts, vinyl ethers add in similar fashion to give acetals of 3-butenal (232,233). 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

An enantioselective aldol reaction may also be achieved with non-chiral starting materials by employing an asymmetric Lewis acid as catalyst ... 

The use of catalysts for a Diels-Alder reaction is often not necessary, since in many cases the product is obtained in high yield in a reasonable reaction time. In order to increase the regioselectivity and stereoselectivity (e.g. to obtain a particular endo- or exo-product), Lewis acids as catalysts (e.g. TiCU, AICI3, BF3-etherate) have been successfully employed." The usefulness of strong Lewis acids as catalysts may however be limited, because they may also catalyze polymerization reactions of the reactants. Chiral Lewis acid catalysts are used for catalytic enantioselective Diels-Alder reactions. ... 

Phenolic esters (1) of aliphatic and aromatic carboxylic acids, when treated with a Lewis acid as catalyst, do undergo a rearrangement reaction to yield ortho- and para-acylphenols 2 and 4 respectively. This Fries rearrangement reaction is an important method for the synthesis of hydroxyaryl ketones. 

The stereogenic centers may be integral parts of the reactants, but chiral auxiliaries can also be used to impart facial diastereoselectivity and permit eventual isolation of enantiomerically enriched product. Alternatively, use of chiral Lewis acids as catalysts can also achieve facial selectivity. Although the general principles of control of the stereochemistry of aldol addition reactions have been well developed for simple molecules, the application of the principles to more complex molecules and the... 

Beller and coworkers [312] have recently published a very useful domino reaction using a Ti-based Lewis acid as catalyst to prepare indoles. The new procedure consists of a titanium-catalyzed amination of a chloroalkylalkyne to give an aryl hydra-... 

The use of silica-supported Lewis acids as catalysts for the Diels-Alder reactions of 2,5-dimethylfuran leads to fairly good yields of adducts [28]. Solid supports such as... 

Kalvoda149 studied the reaction of l-0-acetyl-2,3,5-tri-0-benzoyl-/3-D-ribofuranose (198) with various aromatic compounds in the presence of Lewis acids as catalysts, and prepared the corresponding D-ribofuranosylbenzenes, such as 199-201. It is of interest that an-omeric mixtures were sometimes formed under essentially uniform conditions, and it is not yet clear whether the a anomer arises from... 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

Lewis acids as catalysts (Diels-Alder reactions),... 

A number of reactions that are traditionally carried out using either Bronsted or Lewis acids as catalysts have been shown to take place in the presence of EG A formed either in situ or ex situ. The reactions do not involve charge-consuming conversions of the substrates. 

Friedeb Crafts reaction Anisole undergoes Friedel-Crafts reaction, i.e., the all rl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous alurntnlurn chloride (a Lewis acid) as catalyst. 

In summary, there are now several examples of phosphonium salt-based Lewis acids as catalysts known, which have shown a good catalytic activity. However, an asymmetric catalyzed reaction with an enantiopure phosphonium salt has not been reported yet. 

TABLE 1. Physical Properties of Poly(2,2,6,6-tetramethyl piperidine-4-vinyloxy-l-oxyl) Prepared by Cationic Polymerization Using Different Lewis Acids as Catalysts"... 

Mechanistic studies were performed for simple systems such as 1-thiabutadiene and ethylene481, although dimerization and cycloaddition of thiochalcones have received more attention, including several calculations482-484. Nevertheless, more synthetic utility possesses the uses of Lewis acids as catalysts in hetero-Diels-Alder reactions (equation 142)485,486 involving 1-thiabutadienes. 

Nevertheless, the use of chirally modified Lewis acids as catalysts for enantioselective aminoalkylation reactions proved to be an extraordinary fertile research area [3b-d, 16]. Meanwhile, numerous publications demonstrate their exceptional potential for the activation and chiral modification of Mannich reagents (generally imino compounds). In this way, not only HCN or its synthetic equivalents but also various other nucleophiles could be ami-noalkylated asymmetrically (e.g., trimethylsilyl enol ethers derived from esters or ketones, alkenes, allyltributylstannane, allyltrimethylsilanes, and ketones). This way efficient routes for the enantioselective synthesis of a variety of valuable synthetic building blocks were created (e.g., a-amino nitriles, a- or //-amino acid derivatives, homoallylic amines or //-amino ketones) [3b-d]. 

Although asymmetric conjugate additions have over the years been dominated by the application of chiral Lewis acids as catalysts [1, 2], more recently organo-catalysts have been added as efficient tools [3, 4]. 

The stereochemical outcome of the Diels-Alder reaction was rationalized from steric interactions existing in the two possible transition states (Scheme 43). The use of the Lewis acids as catalysts, which became associated with the sulfinyl oxygen thus increasing their effective size, improves the stereoselectivity of the reactions. 

Chiral Lewis acids as catalysts in hetero-Diels-Alder and in [2 + 2]-cycloaddition reactions leading to heterocycles 91S1. 

Chiral Lewis acids as catalysts in asymmetric ring opening of epoxides 91S1. 

Reaction of cyclopropane with bromine in the presence of a Lewis acid as catalyst (FeBrs, AICI3, AlBra) gives a mixture of ring-opened dibromides (1,1 1,2 1,3) (Eq. (35)) which are best accounted for by postulating the intermediacy of protonated cyclopropanes >, although... 

Water is the most frequently invoked cocatalyst in Fiiedel-Crafts reactions and in particular in cationic polymerisations promoted by Lewis acids. This is not only due to the almost inevitable presence of traces of water as an impurity in most re nts and solvents, but also to its higji efficiency compared to many other cocatalysts. Thus, when a polymerisation system employing a Lewis acid as catalyst is examined, it is Mways indispensable to check and assess the possible role of adventitious moisture as the main cocatalyst. 

Reactions in SCCO2 have also been used for the production of minor lipid components, such as tocopherols and sterol esters. The synthesis of D, L, ot-tocopherol in SCCO2 and nitrous oxide by condensation of trimethyUiydroquinone with iso-phytol in the presence of various Bronsted or Lewis acids as catalysts resulted in... 

Besides anionic nucleophiles (e. g. NJ), poor nucleophiles such as F can also be used in the ring-opening reactions in the presence of acids or Lewis acids as catalysts. For example, the reaction of methyl 2,3-anhydro-/3-D-ribopyranoside 26 with F does not proceed except when acidic KF HF is used to provide the nucleophile, yielding 3-deoxy-3-fluoroxylopyranoside 27 (O Scheme 13) [18]. 

A. Corma, H. Garcia, Lewis acids as catalysts in oxidation reactions From homogeneous to heterogeneous systems, Chem. Rev. 102 (2002) 3837. 

However, in the course of a recent mechanistic study of the electrophilic chlorination of toluene in the presence of a mixture of bis(4-chlorophenyl)selenide-Lewis acid as catalyst, Graham et al studied the decomposition of bis(4-chlorophenyl) (4-methylphenyl)selenonium chloride (36). They found that the ligand coupling pathway occured only in the presence of aluminum (O) chloride. Moreover, only chlorotoluene derivatives were obtained dichlorobenzene was not detected. The reductive elimination appears to be obviously mostly intramolecular. However, the important amount of o/t/io>chlorotoluene is not consistent with a single ligand coupling mechanism." ... 

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