Shilov-type oxidation

Shilov-type chemistry has been extended to complex organic synthesis by Sames, who finds that amino acids such as valine can be functionalized at their terminal H3 groups by conversion to -CH2OH (equation 4). The preferred catalyst is the usual Pt(II) but the oxidant is not Pt(TV) but the much more convenient Cu(II). An aqueous medium is used and functional group protection is not needed, but 130° is required for catalytic turnover. 

A more promising way would use a porphyrin producing hydrogen directly in solution using a Shilov-type system (see Scheme 6.5.2). The oxidative part, which should give oxygen from water as in plant photosynthesis, would then be... 

CHs-Halide bond formation is a side reaction in the Shilov methane oxidation process (Scheme 24) [64]. Mechanistic analysis of several catalytic steps by Bercaw and coworkers showed that the formation of the carbon-chlorine bond takes place in parallel to the formation of methanol, often being the major reaction pathway [65]. The reaction most likely involves a nucleophilic attack of the chloride-anion at the coordinated methyl group of the Pt(IV) intermediate [66]. Thus, the overall mechanism is closely related to the organic SN2-type reaction. Further support for such a mechanism operating in Pt(lV) systems came from the Goldberg group which reported the competitive CH3-I and CH3-CH3 reductive elimination reactions in platinum phosphine complexes (Scheme 25) [67, 68]. 

Several examples of intermolecular C-H bond functionalization have appeared during the past decade. In addition to the oxidations reported above in Shilov-type systems, and the dehydrogenation of alkanes to make alkenes, catalytic systems have been developed to introduce functional groups into hydrocarbons. 

Fundamental notions and Shilov s classification of conjugated oxidation reaction types. 

A later variant involved incorporation of an oxidant, Pt(IV), which led to formation of functionalized species, RX, from alkane, RH. In the typical chloride-rich Shilov systems, X is commonly Cl and OH. The Pt(IV) oxidant is reduced to Pt(II) during the reaction, but it has proved hard to replace this expensive oxidant by a cheaper one while retaining activity. A remarkable system of this type discovered by Periana [109] uses cone. H2SO4 as both oxidant and solvent and a Pt(II) 2,2 -bipyrimidine complex as catalyst with the result that CH3OSO3H, a methanol derivative, is formed from methane. 

As Shilov proposed [11], C—H bond activation by metal species can be classified into three categories based on their mechanisms. The first type is called organo-metallic activation where the reaction takes place in the first coordinatimi sphere of metal center and leads to the formation of M-C a-bond. This mechanism includes various derivatives, such as oxidative additimi, a-bond metathesis, 1, 2 addition, and electrophilic substitution, as shown in Fig. 1. Interestingly, recent theoretical studies also revealed that there exist a few alternatives to these classic types, namely, metal-assisted a-bond metathesis [18], oxidatively added... 

In the second type of process the metal acts as a carbenoid and inserts into the C—H bond, a process generally termed oxidative addition in organometallic chemistry (equation 1 b). This reaction is believed to go via the same sort of alkane complex as in the Shilov system, but, instead of losing a proton, it goes instead to an alkylmetal hydride. This may be stable, in which case it is observed as the final product, or it may react further. 

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