Aldehydes aerobic dehydrogenation

A novel and efficient method for the synthesis of polysubstituted quinolines catalyzed by CuBr through aerobic oxidative dehydrogenative annulation of anilines and aldehydes by C-H functionalization, C-C formation, and cleavage was reported recently. Various substituted quinolines could be synthesized in good yields under the standard conditions. It is noteworthy that air was used in this procedure as the oxidant and it makes the procedure very practical and economical [158] (Scheme 8.88). 

An efficient CuS04/Cul-catalyzed aerobic intramolecular dehydrogenative cyclization reaction of A-methyl-Af-phenylhydrazones to cinnolines has been developed by Ge and coworkers through sequential C(sp )-H oxidation, cyclization, and aromatization processes (Scheme 8.107). This transformation is the first example of copper-catalyzed coupling reactions of hydrazones through a C(sp )-H bond functionalization pathway. This transformation starts with the oxidation ofiV-methyl-Af-phenylhydrazones into aldehyde intermediate through the activation of C(sp )-H under CuSO /O, catalytic system [181]. 

For direct aerobic a, -dehydrogenation of aldehydes and ketones with a new Pd(TFA)2/4,5-diazafluorenone catalyst, the turnover-limiting step is a-C-H cleavage. Means of oxidative cleavage of C-C bonds in aldehydes by O2, cyclic ketones " by nicotinium dichromate, and a-hydroxy ketones by O2 have been discussed. 

Intermolecular direct C(3)-alkenylation of indoles using palladium(II) as catalyst and oxygen as the oxidant has been achieved. The reaction shows complete regio- and stereo-selectivity all products are ii-isomers at the C(3)-position, and no Z-isomers or 2-substituted products are detected. Aerobic a, -dehydrogenation of aldehydes and ketones is catalysed by Pd(TFA)2/4,5-diazafluorenone. The cleavage of a-C-H bond of the ketone has been identified as the turnover-limiting step of the catalytic mechanism. ... 

Additional work has localized the oxidative activity in the particulate fraction of liver homogenate. The optimum pH is about 7, and maximal activity can be achieved in an aerobic atmosphere with the addition of TPNH. The need for oxygen and the reduced pyridine nucleotide is strong indication that hydroxylases are implicated. The probable first step in the oxidation is the formation of the primary alcohol. Beyond this point the further oxidation of the alcohol undoubtedly proceeds by routine enzymic hydration and dehydrogenations to the carboxylic acids. Single carbon fragments are then liberated as COg no aldehydic or acidic intermediates have as yet been isolated. 

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