Catalysis microwave-mediated

Microwave processing of zeolites and their application in the catalysis of synthetic organic reactions has recently been excellently reviewed by Cundy [23] and other authors [24], The microwave synthesis of zeolites and mesoporous materials was surveyed, with emphasis on those aspects which differ from conventional thermal methods. The observed rate enhancement of microwave-mediated organic synthesis... 

Gong, Y.D., Sohn, H.-Y. and Kurth, M.J., Microwave-mediated intramolecular carbanilide cyclization to hydantoins employing barium hydroxide catalysis, J. Org. Chem., 1998, 63, 4854-4856. 

Chromans and isochromans can be obtained by the reductive Pd-catalysed Heck cyclisation of 4-(2-bromophenoxy)-2-methylbut-l-ene and 3-(2-bromobenzyloxy)-2-methylpropene respectively <07TL2307>. Bromopropyl derivatives of 3,5-dihydroxy-iodobenzene and of 3,5-dihydroxymethyliodobenzene undergo sequential intramolecular alkylations and an intermolecular Heck coupling on treatment with an acrylate under microwave irradiation and Pd-catalysis mediated by norbornene. The products are 5-alkenyl derivatives of pyrano[3,2-g]benzo-2/7-1 -pyrans and pyrano[4,3-/]bcnzo-l77-2-pyrans (Scheme 13) <07JOC775>. 

Bismuth(lll) salts such as BiCls, BiBrj, Bi(OCOR)3, and Bi (OTf), [166] have been widely used as Lewis acid catalysts to mediate C-C bond formation. Bi (OTf) 3, Bi2O3, and BiCl, catalyze Friedel-Crafts acylation with acyl chlorides or acid anhydrides [167]. Both electron-rich and electron-deficient arenes are acylated in high yields under catalysis by Bi(OTf)3 (Scheme 14.82). Under microwave irradiation the catalytic activity of BiX3 (X = C1, OTf) in the acylation of aromatic ethers is enhanced [168]. The N-acyl group of p-substituted anilides migrates to the ortho position of the aromatic nucleus under BiCls catalysis [169]. Treatment of 2,3-dichloroanisole with the ethyl glyoxylate polymer in the presence of a catalytic amount of Bi(OTf)3 affords an a,a-diarylacetic acid ester quantitatively (Scheme 14.83) [170]. 

Microwave reactions have been successfully demonstrated for many different organic reactions including metal-mediated catalysis, cyclo-additions, heterocyclic chemistry, rearrangements, electrophilic and nucleophilic substitutions, and reduction. Many reactions work well in water, adding to the techniques green credentials [27]. 

Extension of the enamine-mediated carbonyl a-amination strategy to the generation of quaternary stereogenic centers at the a-position of a-branched aldehydes under catalysis by prohne 1 [8, 9], pyrrolidine tetrazole 3 [10, 11], or L-azetidin-2-carboxylic acid 4 [8] has also been explored (Table 11.1). The observed enantio-selectivities ranged from essentially none to >99%. Derivatives of 2-phenylpropanal gave better enantioselectivities than a,a-dialkyl substituted aldehydes. Erase and coworkers [11] employed microwave irradiation to accelerate the rate of proline-catalyzed amination, and found that yields as well as enantioselectivity can be somewhat improved with shorter reaction times. It appears that the pyrrolidine tetrazol 3 was a more effective catalyst than L-proline 1 for the amination of 2-phenylpropanal derivatives [10,11]. Subsequent reduction of adducts and cyclization could be carried out to afford the respective a-amino alcohols or the A-amino-oxazolidinones. 

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