Styrenes base-catalyzed dimerization

The various methods of generating o-quinone methides,4-5 including the thermal or (Lewis) acid-catalyzed elimination of a phenol Mannich base,149 150-160-161163 the thermal or (Lewis) acid-catalyzed dehydration of an o-hydroxybenzyl alcohol (ether),147-149-151-153-156-157-162-163-165-168 171-175-178-183 the thermal 1,5-hydride shift of an o-hydroxy styrene,171-173 175 178-183 the thermal dissociation of the corresponding spirochromane dimer,158 163-164,166 oxidation of substituted o-alkylphenols,152-170 and the thermal or photochemical-promoted cheletropic extrusion154-155 159 of carbon monoxide, carbon dioxide, or sulfur dioxide (Scheme 7-III), as well as their subsequent in situ participation in regiospecific, intermolecular [4 + 2] cycloadditions with simple olefins and acetylenes,147 149-151 152 153159 162-164... 

A chiral anellated 1,2-azaphosphole (30) was prepared from the condensation of the Schiff base 28 with MePBr2 in the presence of base followed by reduction of the resulting salt 29 with Na in THF (Scheme 9) [38], Reduction of 29 with Mg gave a dimer found useful in the Ni-catalyzed asymmetric hydrovinylation of styrene [39, 40],... 

The copper-catalyzed cyclopropanation of alkenes with diazoalkanes is a particularly important synthetic reaction (277). The reaction of styrene and ethyl diazoacetate catalyzed by bis[/V-(7 )- or (5)-a-phenyl-ethylsalicylaldiminato]Cu(II), reported in 1966, gives the cyclopropane adducts in less than 10% ee and was the first example of transition metal-catalyzed enantioselective reaction of prochiral compounds in homogeneous phase (Scheme 90) (272). Later systematic screening of the chiral Schiff base-Cu catalysts resulted in the innovative synthesis of a series of important cyclopropane derivatives such as chrysanthemic acid, which was produced in greater than 90% ee (Scheme 90) (273). The catalyst precursor has a dimeric Cu(II) structure, but the actual catalyst is in the Cu(I) oxidation state (274). (S)-2,2-Dimethylcyclopropanecar-boxylic acid thus formed is now used for commercial synthesis of ci-lastatin, an excellent inhibitor of dehydropeptidase-I that increases the in vivo stability of the caibapenem antibiotic imipenem (Sumitomo Chemical Co. and Merck Sharp Dohme Co.). Attempted enantioselective cyclopropanation using 1,1-diphenylethylene and ethyl diazoacetate has met with limited success (211b). A related Schiff base ligand achieved the best result, 66% optical yield, in the reaction of 1,1-diphenylethylene and ethyl diazoacetate (275). 

Zhou and collaborators devised a procedure for the synthesis of 2-substituted quinazolines via an iridium-catalyzed hydrogen transfer (Scheme 32) (13RSCA334). Treatment of 2-aminobenzylamines 69 with aryl and alkyl aldehydes and styrene as the hydrogen acceptor in the presence of (pentamethylcyclopentadienyl)iridium (III) chloride dimer in xylene under refluxing conditions afforded the desired products in moderate yields. A base, for example, potassium hydroxide, had to be added when employing benzyl alcohol instead of benzaldehyde to furnish 2-phenylquin-azoline (70) in 61% yield (Scheme 32). 

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