Alkanes, activation functionalization

Class II dependence for the activation of a chemical bond as a function of surface metal atom coordinative unsaturation is typically found for chemical bonds of a character, such as the CH or C-C bond in an alkane. Activation of such bonds usually occurs atop of a metal atom. The transition-state configuration for methane on a Ru surface illustrates this (Figure 1.13). 

Synthetic organic chemistry applications employing alkane C-H functionalizations are now well established. For example, alkanes can be oxidized to alkyl halides and alcohols by the Shilov system employing electrophilic platinum salts. Much of the Pt(ll)/Pt(rv) alkane activation chemistry discussed earlier has been based on Shilov chemistry. The mechanism has been investigated and is thought to involve the formation of a platinum(ll) alkyl complex, possibly via a (T-complex. The Pt(ll) complex is oxidized to Pt(iv) by electron transfer, and nucleophilic attack on the Pt(iv) intermediate yields the alkyl chloride or alcohol as well as regenerates the Pt(n) catalyst. This process is catalytic in Pt(ll), although a stoichiometric Pt(rv) oxidant is often required (Scheme 6).27,27l 2711... 

In the preceding discussion, we have covered recent discoveries which shed light on the first two steps of Pt(II) catalyzed alkane functionalization (Section III Scheme 4) alkane activation by Pt(II) and oxidation of the resulting platinum(II) alkyl. The final step is the release of the... 

Figure 1. Catalytic scheme for alkane oxy-functionalization based on the C-H activation reaction.

This may, in part, be due to this reaction being the only industrial large scale functionalization of an alkane currently in operation, and knowledge gleaned from vanadium phosphate systems can provide valuable information that can be applied to different alkane activation reactions. To this end a number of the publications have focused and continue to focus on a fundamental understanding of the catalyst active surface and the active site. In recent years, transient techniques such as the use of TAP reactors and in situ characterization studies have advanced the under-... 

Jones WD (1989) Alkane activation by cyclopentadienyl complexes of rhodium, iridium and related species. In Hill CL (ed) Activation and functionalization of alkanes. 

The lack of alkane C-C activation also arises from the fact that two relatively weak M-C bonds are formed in the process [Eq. (6.105)]. However, in strained molecules such as small-ring cycloalkanes (cyclopropanes, cyclobutanes), relief of strain is an additional favorable factor. Furthermore, C-C activation can be rendered thermodynamically more favorable when an extra driving force is available, such as formation of an aromatic structure or by utilizing an activating functionality. 

C. L. Hill, B. C. Schardt, Alkane activation and functionalization under mild conditions by a homogeneous manganese(lll) porphyrin-iodosylbenzene oxidizing system, /. Am. Chem. Soc. 102 (1980) 6374. 

A key issue for synthetic chemists is the direct and selective functionalization of alkanes under mild conditions. A major problem in C-H bond activation by molecular catalysis is the lack of a suitable reaction medium, because most organic solvents are not inert under alkane activation conditions and therefore prevent the desired reactions. In this context, dense carbon dioxide seems to be a promising reaction medium as it is miscible with organics, including organometallics, and potentially stable under alkane activation conditions. Indeed, methane carbonylation and alkane dehydrogenation by molecular catalysis have been reported using dense carbon dioxide as the reaction medium (Scheme 67). " ... 

Notable during latter 1988 and 1989 was the publication of Activation and Functionalization of Alkanes, which devotes much attention to homogeneous and heterogeneous organometallic C—H bond activation in its 372 pages. Also during the reporting period, two entire journal issues were dedicated to relevant topics Alkane Activation and Functionalization [New J. Chem., 13, number 11 (1989)]... 

Among the earliest reports of alkane activation by a transition metal complex were the articles by Shilov in which Pt(n) served as a catalyst for methane oxidation and Pt(iv) served as a stoichiometric oxidant." The mechanism of C-H activation was termed electrophilic, as the cationic metal was postulated to interact with the electrons of the C-H bond which then lost a proton, forming a metal-carbon bond without a change in oxidation state. Oxidation of the complex by two electrons was then followed by nucleophilic attack at carbon, giving a functionalized hydrocarbon (Scheme 1). 

Pt(IV) in Alkane Activation and Functionalization by Ligand-Substituted Complexes 43... 

W. D. Jones, in Activation and Functionalization of Alkanes, C. L. Hill, Ed., Wiley, New York, 1988, pp. 111-149. Alkane Activation Processes by Cyclopentadienyl Complexes of Rhodium, Iridium and Related Species. 

Alkali metal polyhydrogen fluorides as halogen exchange media addition reaction, 240-242 concept, 237-238 previous studies, 238-239 regeneration reaction, 241-248 substitution reaction, 240-242 thermodynamics, 237-239 Alkane(s), functionalization strategies, 366 Alkane activation, 366-367 Alkenyl cadmium reagents, synthesis, 298-300... 

The useful comparison between the Wacker process and alkane CH activation is that all the coordination steps identified in the Wacker reaction (activation, functionalization and reoxidation) have parallels in catalytic, alkane CH activation and functionalization systems that operate with electrophilic catalysts. Thus, the coordination of the double bond of the olefin to electrophilic Pd(II) followed by cleavage by nucleophilic attack of water can be compared to CH activation of CH4 by an electrophilic substitution (ES) pathway. 

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