Halogenation Lewis acid catalysis

Juenge3 has now found that the reagent is effective for nuclear or side-chain halogenation of aromatic systems, the former occurring under ionic conditions, the latter under free-radical conditions. Thus benzene and naphthalene are chlorinated under Lewis acid catalysis 1 -chloronaphthalene can be prepared in 58% yield. Toluene under similar conditions gives a mixture of 2- and 4-chloro-toluene in 66% yield. In the presence of benzoyl peroxide, benzyl chloride is obtained in 44% yield. 

The rapid anation reaction observed in the presence of I3 implies that 12 should be a very efficient catalyst for the aquation of Co(CN)5r-3. Qualitative observations confirm this prediction, but a careful study of the catalyzed aquation has not yet been completed. A study of the catalysis by other halogen molecules and Lewis acids will also be undertaken in future work. 

Nitration and halogenation of naphthalene occur almost exclusively in the 1-position. Chlorination or bromination takes place so readily that a Lewis acid is not required for catalysis. 

The 1/BQ system alone or with Lewis acid additives was also employed for asymmetric catalysis in the synthesis of /3-amino acids235. During an elaboration of the tandem catalytic asymmetric chlorination/esterification process, Lectka and coworkers found that proton sponge 1 competes with ketenes in the reaction with halogenating agents, such... 

Vinylsilanes react with chloral in the presence of Lewis acids (Scheme 33), but this type of reaction is little used, probably because the products are allylic alcohols, which are apt to undergo ionization in the presence of Lewis acids to give allyl cations, and hence further reaction. Reactions employing nucleophilic catalysis, although free of this problem, are also limited, only anion-stabilized systems undergoing reaction (Scheme 34). On the other hand, there is less of a problem with 3-elimination of a halide ion, as there would be with most metals 3 to a halogen. ... 

Unfortunately, all of the above-described synthetic approaches towards HBC (1) suffer from serious drawbacks, such as harsh reaction conditions, a complicated experimental work-up, and low yields. Furthermore, under aluminum(III) chloride catalysis, dealkylation, migration of the alkyl substituents - or even chlorination of the aromatic system- occurs, which clearly limits the accessibility of functionalized HBC derivatives for further investigations and applications. In order to overcome these problems, the weaker Lewis acid iron(III) chloride in nitromethane was used instead of AICI3, and the reaction conditions were carefully optimized [55, 56]. In this way, access was obtained to a multitude of HBC derivatives 8 and 9 with diverse substitution patterns and symmetries bearing solubilizing alkyl chains and halogen substituents, starting from functionalized hexaphenylbenzenes. The sixfold symmetric hexaphenylbenzenes 10 were synthesized by the Co2(CO)g-catalyzed cyclotrimerization of substituted diphenyl-acetylenes 11 (Scheme 13.4a) [57], whereas the intramolecular Diels-Alder reaction... 

When halogen atoms are present in the epoxide such as in epichlorohydrin, 3,3,3-trichloropropylene oxide (TCPO) or 4,4,4-trichloro-l,2-butylene oxide (TCBO), or in the initiator, acid catalysts, e.g. boron trifluor-ide etherate, may be used (13-18). Vogt cind Davis (16) found that, if the concentration of catalyst/ini-tiator (polyol) complex is decreased with respect to TCPO in order to obtain higher molecular weight products, side reactions such as cyclization reactions become increasingly important. Boron trifluoride also promotes dimerization of alkylene oxides to dioxane or alkyl derivatives of dioxane as described by Fife and Roberts ( ). The use of acid catalysts, e.g. Lewis acids, promotes formation of a greater amount of terminal primary alcohol groups when compared to base catalysis of epoxides. 

In both homogeneous and heterogeneous catalysis, carbon monoxide activation involves first the coordinaiive interaction of carbon monoxide with a metal acceptor center. Carbon monoxide, being a weak donor base, does not react with a proton and produces only a vety weak interaction with a hard acid center such as BH3, With less hard Lewis centers, such as CuX, AgX, AuXj etc. (X - halogen), more or less stable carbon monoxide adducts can be isolated. A variety of modes of CO coordination in well characterized organometallic complexes is known. Scheme 1 contains some selected examples. 

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