Oxidation activated C—H bonds

The widely used Moifatt-Pfltzner oxidation works with in situ formed adducts of dimethyl sulfoxide with dehydrating agents, e.g. DCC, AcjO, SO], P4O10, CCXTl] (K.E, Pfitzner, 1965 A.H. Fenselau, 1966 K.T. Joseph, 1967 J.G. Moffatt, 1971 D. Martin, 1971) or oxalyl dichloride (Swem oxidation M. Nakatsuka, 1990). A classical procedure is the Oppenauer oxidation with ketones and aluminum alkoxide catalysts (C. Djerassi, 1951 H. Lehmann, 1975). All of these reagents also oxidize secondary alcohols to ketones but do not attack C = C double bonds or activated C —H bonds. 

The addition of acetic acid (0.5 equiv. to the substrate) to the catalyst system led to increased activity (doubling of yield) by maintaining the selectivity with 1.2 equiv. H2O2 as terminal oxidant. Advantageously, the system is characterized by a certain tolerance towards functional groups such as amides, esters, ethers, and carbonates. An improvement in conversions and selectivities by a slow addition protocol was shown recently [102]. For the first time, a nonheme iron catalyst system is able to oxidize tertiary C-H bonds in a synthetic applicable and selective manner and therefore should allow for synthetic applications [103]. 

Spectroscopy of the PES for reactions of transition metal (M ) and metal oxide cations (MO ) is particularly interesting due to their rich and complex chemistry. Transition metal M+ can activate C—H bonds in hydrocarbons, including methane, and activate C—C bonds in alkanes [18-20] MO are excellent (and often selective) oxidants, capable of converting methane to methanol [21] and benzene to phenol [22-24]. Transition metal cations tend to be more reactive than the neutrals for two general reasons. First, most neutral transition metal atoms have a ground electronic state, and this... 

The reactivity of these oxidants towards organic substrates depends in a rough manner upon their redox potentials. Ag(II) and Co(III) attack unactivated and only slightly activated C-H bonds in cyclohexane, toluene and benzene and Ce(IV) perchlorate attacks saturated alcohols much faster than do Ce(lV) sulphate, V(V) or Mn(III). The last three are sluggish in action towards all but the active C-H and C-C bonds in polyfunctional compounds such as glycols and hydroxy-acids. They are, however, more reactive towards ketones than the two-equivalent reagents Cr(VI) and Mn(VIII) and in some cases oxidise them at a rate exceeding that of enolisation. 

These reactions demonstrate the Brflnsted base role of adsorbed oxygen perviously found on Ag(llO) and show further that more active transition metals which themselves activate C-H bonds catalytically oxidize via a two-step mechanism in which the surface intermediates are scavenged by adsorbed oxygen. 

In reference 44 the authors found that the [Cu2(p-0)2]2+ core with either the LlPr3 or LBn3 ligand system will activate C-H bonds for cleavage and oxidation, again having important implications for oxygenation catalysis. These authors provide a useful summary for the two core structures, which are replicated in Table 5.7. 

Synthetic Reactions via C-H Bond Activation Oxidation of C-H Bonds... 

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