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Involving Lewis Acids

The homoconjugation of SbFs with its anion leads to several possible complex secies. Thus, dilute solutions of this Lewis acid in hydrogen fluoride contain mostly Sb F, , but as the concentration is raised Sb3FJ g and Sb iFi t are also formed SbFs [Pg.106]

Many other metal fluorides can homo- and heteroconjugate with related anions. Thus, boron fluoride gives readily the complex anions B2F7 and and [Pg.106]

Kramer and coworkers recently showed that AlBr3 forms 2 1 salts with f-BuCl and f-BuBr in halogenated hydrocarbons at low temperatures. These ionogenic interactions were followed by C-NMR and clearly proved that homoconjugated anions of the type Al2BrgX accompanied the r-butyl cation. [Pg.106]


Many important biochemical reactions involve Lewis acid Lewis base chemistry Carbon dioxide is rapidly converted to hydrogen carbonate ion m the presence of the enzyme carbonic anhydrase... [Pg.46]

Some D-A reactions are catalyzed by high concentrations of LiC104 in ether,33 a catalysis that involves Lewis acid complexation of Li+ with the dienophile.34... [Pg.485]

Enantioselective D-A reactions of acrolein are also catalyzed by 3-(2-hydroxyphenyl) derivatives of BINOL in the presence of an aromatic boronic acid. The optimum boronic acid is 3,5-di-(trifluoromethyl)benzeneboronic acid, with which more than 95% e.e. can be achieved. The TS is believed to involve Lewis acid complexation of the boronic acid at the carbonyl oxygen and hydrogen bonding with the hydroxy substituent. In this TS tt-tt interactions between the dienophile and the hydroxybiphenyl substituent can also help to align the dienophile.114... [Pg.511]

Scheme 10.2 gives some examples of ene and carbonyl-ene reactions. Entries 1 and 2 are thermal ene reactions. Entries 3 to 7 are intermolecular ene and carbonyl-ene reactions involving Lewis acid catalysts. Entry 3 is interesting in that it exhibits a significant preference for the terminal double bond. Entry 4 demonstrates the reactivity of methyl propynoate as an enophile. Nonterminal alkenes tend to give cyclobutenes with this reagent combination. The reaction in Entry 5 uses an acetal as the reactant, with an oxonium ion being the electrophilic intermediate. [Pg.877]

Scheme 11.2 shows some representative halogenation reactions. Entries 1 and 2 involve Lewis acid-catalyzed chlorination. Entry 3 is an acid-catalyzed chlorination using NCS as the reagent. Entry 4 shows a high-yield chlorination of acetanilide by i-butyl hypochlorite. This seems to be an especially facile reaction, since anisole is not chlorinated under these conditions, and may involve the A-chloroamide as an intermediate. Entry 5 describes a large-scale chlorination done with NCS. The product was used for the synthesis of sulamserod, a drug candidate. [Pg.1010]

SiC>2N2 were clearly assigned. Since NH substitution for O in zeolite Y occurs preferably near A1 [70-72], a substitution mechanism may be surmised which involves Lewis acidic framework aluminum which reacts with the Lewis base NH3. [Pg.195]

The catalytic asymmetric aldol reaction has been applied to the LASC system, which uses copper bis(-dodecyl sulfate) (4b) instead of CufOTf. 1261 An example is shown in Eq. 6. In this case, a Bronsted add, such as lauric add, is necessary to obtain a good yield and enantioseledivity. This example is the first one involving Lewis acid-catalyzed asymmetric aldol reactions in water without using organic solvents. Although the yield and the selectivity are still not yet optimized, it should be noted that this appredable enantioselectivity has been attained at ambient temperature in water. [Pg.10]

Other recent work which attempts to relate structural parameters with selectivity in synthetically useful reactions involves Lewis-acid catalysed... [Pg.130]

Ethynylcarbazole was apparently formed by sodamide treatment of 9-dichlorovinylcarbazole (97) and zinc reduction of 97 gave 98. Where no reaction occurred with the Z isomer, 99 gave the dimer 100 with boron trifluoride the process presumably involves Lewis acid-catalyzed alkylation of one double bond by another double bond, complexed to boron trifluoride, followed by intramolecular Friedel-Crafts type alkylation at the carbazole 1-position. ... [Pg.118]

Gay and Liang also found large changes in 13C chemical shifts when amines interacted with Bronsted acid sites, and smaller ones when the interaction involved Lewis acid sites. They suggested therefore that fractional coverage of the surface of the amine could be calculated from the magnitude of the 13C chemical shift, based on wo assumptions (1) that chemical shifts are average values from protonated and unprotonated amine molecules and (2) that the shift for a protonated molecule is the same as for the acid solution of the amine. Experiments showed that both assumptions were well founded. It follows that the fraction, /B, of amine molecules bonded to the Bronsted sites can be derived from the equation... [Pg.321]

A 1,2-hydride shift has been invoked399 to account for the formation of p-methoxyphenylbutyraldehyde derivatives (337) during the treatment of />methoxy-benzyl-protected allylic alcohols (336) with zeolites. A similar C-glycosidation procedure involving Lewis acid-catalysed anomeric oxygen to carbon rearrangement of tetrahydropyranyl ether derivatives has been reported400 (see Scheme 82). It has been... [Pg.550]

In most of the examples of superelectrophilic reactions involving Lewis acids, they are conducted using an excess of the Lewis acid. This is in accord with electrophilic solvation by the Lewis acid, i.e. activation of the electrophile requires interaction with two or more equivalents of Lewis acid. As an example, superelectrophilic nitration can be accomplished with NO2CI and at least three equivalents of AICI3 (eq 23).46 This powerful nitrating reagent involves a superelectrophilic complexed nitronium ion (33). [Pg.90]

Another method for conversion of indoles to tryptophans involves Lewis acid-catalyzed conjugate addition to Af-(diphenylmethylene)dehydroalanine. This method was used to prepare 2-alkyl-, 2-bromo-, and 2-methylthiotryptophans <95S370> CO2CH3... [Pg.111]

A tactic for the synthesis of C3 imines has been developed by the Chatani group that involves Lewis acid-promoted insertion of isocyanides into indole C-H <07OL3351>. In a... [Pg.139]

Hydroxyindoles may be accessed using the Nenitzescu reaction, as illustrated by preparation of the indole 381 from the enamine 382 and 1,4-benzoquinone (Equation 107) <1996JOC9055>. Additional applications of this strategy encompass syntheses of 1 -alkyl-5 -hydroxynaltrindole derivatives <2005JME635>, and lO-hydroxy-5,6-dihydroindolo[2,l- ]isoquinolines <2001JOC4457>. An alternative approach to 5-hydroxyindole derivatives involves Lewis acid-mediated reactions of benzoquinone monoimines with enol ethers <1997TL6135>. [Pg.315]

Hydrogen halides have been frequently used as cocatalysts in cationic polymerisations involving Lewis acids. These systems will be discussed in Chap. IV. [Pg.52]

Another variation on the reaction involves Lewis acid exchanges with the BusSn moiety to form transient allylmetal species which then add to the aldehyde through a cyclic transition state (Eq. 3) [4]. These additions proceed under mild conditions owing to the strong affinity of the electron-deficient metal of the allylic MX , reagent for the carbonyl oxygen. [Pg.453]

In contrast to the situation found with acid catalyzed reactions, the role of the zeolite is less well defined for base catalyzed reactions. This results fiom the fact that all "basic" zeolites contain alkali cations that act as (weak) Lewis acid sites. Thus, most of the chemistry described in this chapter involves Lewis acid and base sites. It should be stressed that for all acid ase catalyzed reactions both sites are involved in the reaction sequence. In many of the acid catalyzed reactions die importance of the acid sites dominates so drastically that attention is paid only to the acidic function [119]. [Pg.381]

Araki, Y, Watanabe, K, Kuan, F H, Itoh, K, Kobayashi, N, Ishido, Y, Synthetic studies by the use of fluorinated intermediates. Part I. A novel procedure for C-glycosylation involving Lewis acid-catalyzed coupling-reactions of glycosyl fluorides with enol trimethylsilyl ethers, Carbohydr. Res., Ill, C5-C9, 1984. [Pg.358]

The mechanism for the reaction must involve Lewis acid complexation of the epoxide oxygen atom, cation formation, and migration of C02Et. This last point may surprise you but inspection of the structure of the product shows that the C02Et group is indeed bonded to the other carbon atom. [Pg.327]

A great majority of the catalytic aldol processes that have been developed over the last two decades involve Lewis acids derived from complexes of titanium, boron, tin, and, more recently, copper as well as silver. A recent, exciting area of rapid development for aldehyde addition reactions is represented by the catalytic aldol methods that utilize soft-metal and lanthanide coordination complexes which mediate addition reactions through metalloenolate intermediates. [Pg.243]

In contrast the mechanism of reaction involving Lewis acid catalysts continues to provide intense interest. These studies still seem to be carried out at two distinct levels. Reports appear of polymerizations initiated by very high concentrations of reagents such as PCla and POCla " and it is difficult to assess... [Pg.232]

Figure 2.6 Two point binding of N acylhydrazones involving Lewis acid and chiral ligand(s). Figure 2.6 Two point binding of N acylhydrazones involving Lewis acid and chiral ligand(s).

See other pages where Involving Lewis Acids is mentioned: [Pg.102]    [Pg.285]    [Pg.232]    [Pg.98]    [Pg.284]    [Pg.49]    [Pg.464]    [Pg.75]    [Pg.71]    [Pg.102]    [Pg.27]    [Pg.956]    [Pg.138]    [Pg.163]    [Pg.52]    [Pg.177]    [Pg.509]    [Pg.105]    [Pg.254]    [Pg.157]    [Pg.31]   


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Bridging coordination involving M Lewis acid interactions

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