Ketones cross-dehydrogenative

Efficient proeedures for the cross-dehydrogenative synthesis of aryl ketones have been introduced by complementing the radieal acylation with transition-metal-eatalyzed activation of aromatic C-H bonds. The first eross-dehydrogenative Pd-eatalyzed acylation of 2-phenylpyridines was developed independently by the groups of Chengf and Li. The latter demonstrated that various aliphatie, alicyclic and some aromatie aldehydes are smoothly converted in the presenee of 5 mol% of Pd(OAe)2 in combination with 1.5 equiv. of TBHP at 120 °C under an air atmosphere (Scheme 1.38). 

Based on the studies of Gottschalk and Neckers in 1985, the group of Lei developed a radical Cu-catalyzed cross-dehydrogenative olefination reaction that gives direct access to a,p-unsaturated ketones (Scheme 1.45). ... 

Activation by rhodium complexes has been used to achieve direct exchange of ketone methyl or aryl groups with an aryl group on ArB(OH)2, selective C(CO)-C bond cleavage on reaction of ketones with water, oxidative acylation between secondary benzamides and aryl aldehydes with subsequent intramolecular cyclization to 3-hydroxyisoindolin-l-ones, " cross dehydrogenative coupling to form xanthones from 2-aryloxybenzaldehydes, and activation of the aldehydic C-H bond to achieve hydroacylation of unactivated alkenes by salicylaldehyde derivatives and of vinylsilane by benzaldehyde. ... 

Quinones have been extensively used for aromatization reactions in addition to the dehydrogenation of steroidal ketones and lactones. Interestingly, whereas chloranil (29) and a number of other quinones oxidize steroidal 4-ene-3-ones (32) selectively to 4,6>dienones (34),DDQ (28) results only in the formation of the 1,4-dienone (36 Scheme 21).This divergent b vior is best exfdained by the intermediacy of the kinetic enolate (35) in the case of the higher potential DDQ, but of the thermodynamic enolate (33) in the case of the less reactive quinones.Addic conditions need to be avoided if the cross-conjugated ketone (36) is the desired inoduct since under these conditions the 3,5-dienol (33) becomes both the kinetic and the thermodynamic enol, resulting only in the formation of the linear di-enone." ... 

It is proposed that primarily (2-arnino)benzylalcohol is dehydrogenated to (2-amino)benzaldehyde by the Ru-catalyst, which is reduced to a hydrido-Ru-complex and re-oxidized by excess of methylene ketone in the catalytic cycle. Cross aldol... 

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