Anaerobic oxidation of secondary alcohols

Scheme 184 Catalytic anaerobic oxidation of secondary alcohols [16].

TABLE 18.4 Anaerobic Oxidation of Secondary Alcohols Under MW Irradiation Catalyzed by (NHC)-Pd" ... 

Scheme 18.6 Proposed mechanism for anaerobic oxidation of secondary alcohols with aryl halides (Ar = aryl X = 1, Cl, Br) [17],...

Landers B, Berini C, Wang C, Navano O. (N-heterocycHc carbene)-Pd-catalyzed anaerobic oxidation of secondary alcohols and domino oxidation—arylation reactions. J Org Chem. 2011 76 1390-1397. 

N-Heterocyclic carbene)-Pd (NHC-Pd) has been used as a precatalyst for the anaerobic oxidation of secondary alcohols. It was reported that the use of this complex allows for a drastic reduction in the reaction times and catalyst loading, when compared to the unsaturated counterpart. This catalytic system was compatible with the use of microwave dielectric heating. Domino Pd-catalyzed oxidation-arylation reactions of secondary alcohols have also been carried out (Landers et al., 2011). 

An efficient procedure of anaerobic MW-assisted oxidation of secondary alcohols (Scheme 18.4) in the presence of N-heterocyclic carbene palladium (NHC)-Pd catalysts (Scheme 18.5) with 2,4-dichlorotoluene or other aryl halides as oxidants was reported recently [16]. 

As it can be seen (Table 18.4), the TON values are comparable to those obtained for metal-catalyzed peroxidative oxidation of secondary alcohols discussed above. However, in the case of the anaerobic oxidation, the reaction time is shorter and hence the TOP values (up to 11.2 x 10, Table 18.4, run 1) [16] are superior to those for the peroxidative oxidation (Tables 18.1-18.3) [10a, 11-14]. 

An air-stable, well-defined (NHC)-Ni complex is another effective 3d-metal catalyst for the mild anaerobic catalytic oxidation of secondary alcohols with such functionahties as ether, tertiary amine, and alkene, using nonanhydrous, -degassed 2,4-dichlorotoluene as both oxidant and sol-... 

The (SIPr)Pd(cinnamyl)Cl (SlPr= l,3-bis(2,6-diisopropyl phenyl)-4,5-dihydro imidazol-2-ylidene) catalysed anaerobic oxidation of aryl/alkyl, aryl/aryl, and alkyl/ alkyl secondary alcohols to the corresponding ketones in high yields has been achieved using aryl chlorides in toluene with f-BuONa. Domino Pd-catalysed synthesis of a-arylated ketones from secondary aryl alcohols in very good yields was also reported. " ... 

Chaudhuri et al. (216) reported that the dinuclear bis(phenoxyl)dicopper(II) species [Cu2(Ls )2]Cl2 (Fig- 30) reacts under anaerobic conditions in dry THF in a stoichiometric fashion with primary and secondary alcohols (ethanol, benzyl alcohol, 2-propanol, diphenylcarbinol, and 2-butanol) with formation of two different products, namely, aldehydes (or ketones) and 1,2-diols (and/or other oxidative C— C coupling products). 

The electrocatalytic oxidation of several secondary and primary alcohols has been also described, in keeping with the original work by Masui and coworkers it resorts to HPI or to X-substituted HPIs as electron carriers . The tetrafluoroaryl-substituted HPI was the most efficient among these catalysts. Secondary alcohols gave carbonyl compounds primary alcohols gave the corresponding aldehyde exclusively under anaerobic conditions, whereas a mixture of aldehyde plus carboxylic acid was formed in the presence of 2 . 

Although a suitable acceptor for the transfer dehydrogenation of benzylic alcohols has not yet been found, under the present conditions the low conversion of benzylic alcohols is only an apparent drawback. Indeed, it has a positive side as it allows us to fine-tune the system s selectivity. This makes the catalytic system unique among all the others known, operating both under aerobic and anaerobic conditions, that preferentially oxidize benzylic alcohols with respect to nonacti-vated secondary ones. 

In 2016, Xu and coworker reported another TM-fee aerobic Al-alkylation method for anilines, some heteroarylamines, and sulfinamides with alcohols (Scheme 49) [209]. As described in their work, these amines are not effective substrates under anaerobic catalyst-free autocatalyzed conditions, so an oxidant has to be employed to initiate the reaction. Differing from Yao and Zhao s protocol by adding O2 to an argon atmosphere [208], they found that a catalytic amount of air is active enough to initiate the reaction. In addition to primary alcohols, secondary alcohols can also be used as the aUcyl source. In mechanistic studies, the authors observed that primary and secondary alcohols underwent different initiation processes, i.e., primary alcohols require the presence of the amine to facilitate the aerobic oxidation to... 

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