Dioxygen terminal oxidant

In the first reports by Ishii and coworkers , catalytic amounts of both HPI and Co(II)acetylacetonate, Co(acac)2, were employed for the oxidation of alkanes in AcOH at 100 °C, dioxygen being the terminal oxidant. The appeal of this procedure for the oxidative transformation of simple hydrocarbons into carbonyl derivatives is clear. Cycloalkanes were converted into a mixture of cyclic ketones plus open-chain a, )-dicarboxylic acids (Table 11), while linear alkanes yielded the corresponding alcohols plus ketones in significant amounts (40-80%), and alkylbenzenes could be oxidized in almost quantitative yields . 

Palladium-catalyzed, Wacker-type oxidative cycHzation of alkenes represents an attractive strategy for the synthesis of heterocycles [139]. Early examples of these reactions typically employed stoichiometric Pd and, later, cocat-alytic palladium/copper [140-142]. In the late 1970s, Hegedus and coworkers demonstrated that Pd-catalyzed methods could be used to prepare nitrogen heterocyles from unprotected 2-allylanilines and tosyl-protected amino olefins with BQ as the terminal oxidant (Eqs. 23-24) [143,144]. Concurrently, Hosokawa and Murahashi reported that the cyclization of allylphenol substrates can be accomplished by using a palladium catalyst with dioxygen as the sole stoichiometric reoxidant (Eq. 25) [145]. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

From economical and environmental points of view, air or dioxygen are the most attractive cooxidants. Several groups have addressed this problem without success [18-23], but in recent years Krief and Colaux-Castillo and the Bel-ler group have independently developed cooxidant systems with dioxygen or air as the terminal oxidants. While Krief and Colaux-Castillo used selenoxides as cocatalysts, the Beller group was able to achieve oxidation of osmium(VI) to osmium(VIII) directly by dioxygen or air without the addition of cocatalysts. 

Over the past 25 years, biomimetic model systems have been extensively studied and a wide variety of interesting oxidation processes such as the epoxidation of olefins, the hydroxylation of aromatics and alkanes, the oxidation of alcohols to ketones, etc., have been accomplished some of these are also known in enantioselective versions with spectacular ee s. The vast majority of these transformations were obtained using monooxygen donors such as those mentioned above as primary oxidants. The complexity of the catalysts and the practical impossibility to use dioxygen as the terminal oxidant have so far prevented the use of such systems for large industrial applications, but some small applications in the synthesis of chiral intermediates for pharmaceuticals and agrochemicals, are finding their way to market. 

C. Dobler, G. M. Mehltretter, U. Sundermeier, M. Beller, Osmium-catalyzed dihydroxylation of olefins using dioxygen or air as the terminal oxidant, ]. Am. Chem. Soc. 122 (2000) 10289. 

The results of catalytic epoxidation of various olefins, using f-BuOOH as the terminal oxidant and Mn(Me2EBC)Cl2 as the catalyst, are summarized in Table 3.4. The color of the reaction mixture turns to purple upon addition of f-BuOOH and the ultraviolet-visible spectrophotometry shows that the manganese is present predominantly in the tetravalent state, and no dioxygen evolution was observed. 

Nevertheless, direct use of dioxygen as a terminal oxidant would be very attractive for both economic and ecologic reasons. For example, easy and safe procedures for the partial oxidation of methane according to Reactions 3 and 4 would offer tremendous possibilities for the on-site conversion of CH4, stemming from biological or geological resources, into more valuable feedstocks. 

A key in the use of dioxygen as a terminal oxidant in catalyzed oxidations lies in equations 8-10, namely the separation of catalyst (polyoxometalate) reduction - substrate oxidation, equation 8, from the reduced catalyst reoxidation by O2, equation 9. In part, as the reduced forms of the polyoxometalates are usually low in reactivity and very stable under turnover conditions, equations 8 and 9 can be separated from one another in time and/or in space. As radicals and other reactive species that can initiate radical chain oxidation by O2 (autoxidation), the dominant mode of organic oxidation by this oxidant, are generated in equation 8, autoxidation can be avoided by separating equations 8 and 9. This fact has been appreciated by other groups working in this area. We turn now to another aspect of the chemistry in equations 8-10 that is subtle but has considerable potential consequences for the metal-catalyzed or facilitated C>2-based oxidations and that is the nature of the O2 reoxidation step, equation 9. 

The vanadium-substituted polyoxometalates are applied to chemical pulps as stoichiometric oxidants, much as elemental chlorine is currently used. The reduced polyoxometalate bleaching solutions are regenerated in a second step by treatment with chlorine-free terminal oxidants, preferably air or dioxygen. By exclusion of dioxygen during the bleaching step, exposure of the pulp to non-selective species such as hydroxyl radicals is avoided entirely. 

In contrast to the case where aniline is used as the nucleophile, the benzamide reaction can be improved by utilizing dioxygen in the reaction mixture since 11 is resistent to autoxidation. Under aerobic conditions the nitrobenzene radical anion is readily trapped by O2 generating superoxide and nitrobenzene (Figure 10) (11). This reaction pathway inhibits the formation of azoxybenzene by diverting the electron transfer cascade and ultimately utilizing dioxygen as the terminal oxidant. Thus, under aerobic reaction conditions 12 is the only observed reaction product. 

Homogeneous (liquid phase) catalytic oxidations with dioxygen, hydrogen peroxide and other peroxidic reagents constitute an important area of organic synthesis on both laboratory and industrial scale. When dioxygen is employed as terminal oxidant (i.e. the oxidant which appears in the overall stoichiometric equation of the reaction), of special interest is the way in which 0 enters the catalytic cycle,... 

Metalloporphyrins applied alone show poor performance as oxygenation catalysts for saturated hydrocarbons with dioxygen as terminal oxidant. There are two ways in which efficient catalytic oxidation systems can be obtained on the basis of metalloporphyrins ... 

The popularity of catechol and its derivatives as model substrates for catalytic oxygenations with dioxygen as terminal oxidant is connected with the discovery of the first oxygenase enzyme by Hayaishi and Hashimoto in 1950 [1]. That enzyme, involved in tryptophan... 

Enantioselective oxidative esterification, thioesterification, and amidation of aldehydes have been achieved using a combined organocatalyst system of a chiral NHC and riboflavin, with dioxygen as terminal oxidant." ... 

Aerobic oxidation of alkanes is also possible, using dioxygen as the terminal oxidant. In these cases, Ru-porphyrin and RuCla systems have been shown to oxidize cyclohexane to cyclohexanone in the presence of acetaldehyde, with a fairly high turnover number (TON = 14,100 moles/(mole catalyst-h)). The mechanism for alkane oxidation remains largely unexplored but is suspected to be similar to the oxo-transfer mechanism that governs epoxidation of alkenes (44). 

Su and coworkers established a Pd-catalyzed method for decarboxylative Mizoroki-Heck coupling, in which 1.2 equiv. of p-benzoquinone is used in place of the Ag(l) salt. This method met with some success only with electron-rich (hetero)aromatic carboxylic acids [33]. Subsequently, the same authors reported that the Pd catalyst itself can induce decarboxylative Mizoroki-Heck coupling of aromatic carboxylic acids when dioxygen is used as the terminal oxidant completely replacing the Ag salt [34]. Depending on the structure of the acids, two different Pd catalysts were required for the Mizoroki-Heck coupling to occur Pd(OAc)j worked efficiently for electron-rich aromatic carboxylic acids, while the Pd(OAc)2/SIPr system (SIPr l,3-bis(2,6-diisopropylphenyl), 5-dihydroiniidazol-2-ylidene) enabled the use of electron-deficient substituents (Scheme 22.24) [34]. 

Fu, Z., Huang, S., Su, W., Hong, M. (2010). Pd-catalyzed dearboxylative Heck coupling with dioxygen as the terminal oxidant. Organic Letters, 12, 4992—4995. 

Epoxidation of Alkenes using Dioxygen as Terminal Oxidant... 

By combining heteropolyanions (polyoxometalates POMs for convenience) and selected cations in acetonitrile, more than 150 combinations were assayed for their catalytic activity towards selective CEES oxidation to CEESO by dioxygen under ambient (room temperature and atmospheric pressure) conditions. The main criteria in choosing POMs were their ability to undergo reversible redox transformations and to catalyze homogeneous oxidations either by peroxides or other terminal oxidants. The cations chosen included redox-active transition metal ions or cations conventionally used as the counterions in POMs. In control experiments the chloride, nitrate or perchlorate salts of the same transition metal ions were also examined. The list of these catalytic systems and some selected results were recently published. ... 

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