Phase-transfer catalysts, heterocyclic

In an extension of atom-transfer radical reactions to heterocyclic systems, Byers has introduced a novel methodology for the addition of electron-deficient radicals to unprotected pyrroles and indoles in a stannane-fi ee, non-oxidative process <99TL2677>. For exanqrle, photochemical reaction of pyrrole (33) with etl l iodoacetate (34) in presence of thiosulfiite as an iodine reductant, phase transfer catalyst and propylene oxide led to high yields of the 2-alkylated pyrrole 35 <99TL2677>. 

In spite of the fact that cesium fluoride is very expensive and needs care in handling (very hygroscopic ), it is the reagent of choice for several reactions where halogens and fluorosulfonate groups have to be replaced by fluorine in aromatic and heterocyclic compounds. Cesium fluoride is more reactive than potassium fluoride and also possesses greater solubility. Often a phase-transfer catalyst is added to aid and enhance the reaction rate and yield of the products. 

Quaternary ammonium salts of heterocyclic compounds have been used in liquid-liquid phase-transfer syntheses. When these compounds are achiral, they show a behavior very similar to that of other quaternary ammonium salts. For example, 2-dialkylamino-l-alkylpyridinium tetrafluoroborates have been used by Tanaka and Mukayama282 in the alkylation of active methylene compounds PhCH2CN, PhCH(Et)CN, and PhCH(Me)COPh. However, comparative studies of the efficiency of the catalysts show that alkylpyridinium bromides283 or N-alkyl-Af-benzyl-piperidinium chloride284 have a smaller catalytic activity compared to tetraalkylammonium halides. McIntosh285 has described the preparation of azapropellane salts 186 as potential chiral phase transfer catalysts. 

Heterocyclic amines have also been used as phase transfer catalysts. However, because these amines quaternize easily, the question is whether the operative catalyst is the tertiary amine or the quaternary ammonium salt formed in situ Furukawa et al.286 have shown that a methyl 2-pyridyl sulfoxide may be used as a phase transfer catalyst and promote substitution reactions between lithium chloride or sodium cyanide and benzyl bromide. According to the authors, the catalyst behaves as a cation complexer and not as a quaternary ammonium salt formed in situ by a Menschutkin reaction. 

A variety of open-chain complexers, with repeating ethylenoxy or ethyl-enamino units [-(-OCH2CH2-)-n or -(-NRCH2CH2 - ] have been used as phase transfer catalysts. The following compounds contain heterocyclic subunits. 

Hassanaly, P, Applications of Phase Transfer Catalysts in Heterocyclic Chemistry, 36, 175. 

V-Acyl-1,2-dihydroisoquinolines, another example of nonaromatic nitrogen heterocycles, readily undergo addition of dichlorocarbene generated by the chloroform/base/phase-transfer catalyst method to give tricyclic products and 13. " ... 

The structure of the products formed in the reaction of chloroform, using base/phase-transfer catalyst conditions, with some nitrogen heterocycles may depend on the type of A -substituent, e.g. formation of and... 

Jew and Park have also utilized the dimerization effect, as observed in the development of Sharpless asymmetric dihydroxylation, where ligands with two independent cinchona alkaloid units attached to heterocyclic spacers led to a considerable increase in both the enantioselectivity and scope of the substrates, to design dimeric and trimeric cinchona alkaloid-derived phase-transfer catalysts 12 [12] and 13 [13]. These authors investigated the ideal aromatic spacer for optimal dimeric catalysts, and found that the catalyst 14 with a 2,7-bis(bromomethyl) naphthalene spacer and two cinchona alkaloid units exhibited remarkable catalytic and chiral efficiency (Scheme 11.3) [14]. 

A hydrazidoyl hatide (reviews [1437, 1753]) reacts with some C—C double bonds in the presence of TEA or a phase transfer catalyst [3086]. The reactive double bond of thiete sulphone (84.6) adds on diazoalkanes under mild conditions to form the fused pyrazole (84.7) but diazomethane gives mostly isomer (84.8) [2174]. Addition of diazomethanes to the unsubstituted CH sCH group of a 1,4-quinone occurs under mild conditions to give versatile intermediates [2693, 2700]. The chemistry of heterocyclic quinones has been reviewed [2947, 3650]. 

Recent development and application of chiral phase-transfer catalysts including heterocycles as catalysts 07CRV5656. 

There are also many uses for nonenzymatic polymeric catalysts. For instance, polymer-bound crown ethers, cryptates, and channel compounds behave as polymeric phase-transfer catalysts. The catalytic activity is based on selective complex formation. An example is the use of polystyrene-attached oxygen heterocycles [18]-crown-6 or a cryptand[222] to catalyze replacements of bromine in n-octyl bromide by an iodine or by a cyanide groups... 

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