Shilov-type system

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... 

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. 

One final report of alkane activation has been reported by Moiseev. The mechanism of the reaction was not investigated, but this system might be classified as an electrophilic activation of methane, either of the Shilov type or of the concerted four-center type (Fig. lc) where X=triflate. Reaction of methane with cobalt(III)triflate in triflic acid solution leads to the formation of methyltriflate in nearly stoichiometric quantities (90% based on Co) (Eq. 18). Carbon dioxide was also observed, but not quantified. Addition of 02 led to catalysis (four turnovers) [79]. 

The Shilov Pt(II) system for methane functionalization has been studied using computations. For example, using MCl2(H20)2 (M = Pt or Pd) systems as models for Shilov-type reactions, the overall catalytic cycle was studied using a combination of... 

Theoretical studies are still sparse on catalytic systems of the Shilov type. It is not yet clear why Pt and Pd are so much more effective than other elements. 

Quantitative predictions about the heats of formation of hydrides of intermetallic compounds have been discussed by Shilov etal. (1989) they studied reversible and irreversible transformations in intermetallic compound-hydrogen systems and observed that four basic types of PIT diagrams exist for these systems. 

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]. 

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. 

The first alkane reactions involving organometallic intermediates were investigated by Shilov. The best current understanding of the pathway involved is shown in equation la. The metal M appears to bind the C—H bond to form a side-on a complex of a type discussed in more detail below. In the Shilov systems, this species loses a proton (equation la) to give a metal alkyl which is usually unstable and goes on to give a variety of catalytic reactions. Occasionally, however, the metal alkyl has been observed directly. 

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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