Catalytic Alkane Activation

Mixed-metal catalysts based on 002(00)9 have proved effective In the synthesis of N acyl amlnoaclds, starting with either allylic alcohols, oxiranes (eqn.15) or trifluoropropene. The oxidative carbonylatlon of organomercury compounds Is subject to solvent effects. The Pd catalysed carbonylative cross-coupling of aryl Iodides with triIsobutylaluminium gives secondary benzyl alcohols a variety of functional groups are tolerated.Carbamate esters 

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The process of a-elimination from methyl groups bound to metals may be a key step in catalytic alkane activation, and hydride addition to metal-bound methylene has been suggested as the chain propagation step in the Fischer-Tropsch reaction.The relationship between symmetrical, 17, and unsymmetrical bridging methyl groups, 18, and methylene hydrides, 19 and 20, can be addressed by isotopic perturbation of degeneracy studies and other perturbations of nmr spectra by the introduction of isotopes. 

Briffaud, T., Larpent, C. and Patia H. (1990) Catalytic Alkane Activations in Reverse Microemulsions Containing Iron Salts and Hydrogen Peroxide, J. Chem. Soc., Chem. Commun., 1193-1194. 

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... 

Until now, for most of the systems described here it has been accepted that alkane activation occurred through oxidative addition to the 14-electron intermediate complexes. Yet, Belli and Jensen [26] showed, for the first time, evidence for an alternative reaction path for the catalytic dehydrogenation of COA with complex [lrClH2(P Pr3)2] (22) which invoked an Ir(V) species. Catalytic and labeling experiments led these authors to propose an active mechanism (Scheme 13.12), on the basis of which they concluded that the dehydrogenation of COA by compound 22 did not involve an intermediate 14-electron complex [17-21], but rather the association of COA to an intermediate alkyl-hydride complex (Scheme 13.12). 

The most relevant catalytic reactions approached by SOMC are olefin polymerization (and depolymerization), alkane activation (including a new reaction, discovered thanks to SOMC-alkane metathesis), alkene metathesis and epoxidation. All these reactions are discussed in this chapter. 

Systematic examination of the catalytic properties of dimeric complexes was initiated shortly after the identification of dinuclear iron sites in metalloenzymes. The first report of a reactive dimeric system came from Tabushi et al. in 1980, who examined the catalytic chemistry of [Fe3+(salen)]20, 1 (salen is N,N -(salicylaldehydo)-l,2-ethylenediamine) (12). They reported interesting stereoselectivity in the oxidation of unsaturated hydrocarbons with molecular oxygen in the presence of mercaptoethanol or ascorbic acid and pyridine as a solvent ([l]<<[alkane]<<[2-mercaptoethanol]). With adamantane as substrate, they observed the formation of a mixture of (1- and 2-) adamantols and adamantanone (Table I) (12). Both the relative reactivity between tertiary and secondary carbons (maximum value is 1.05) and final yield ( 12 turnovers per 12 hr) were dependent on the quantity of added 2-mercaptoethanol. Because autoxidation of adamantane gave a ratio of 3°/2° carbon oxidation of 0.18-0.42, the authors proposed two coexisting processes autooxidation and alkane activation. 

Hydride complexes have been important precursors in the study of Alkane Activation. For example, alkanes can be catalytically dehydrogenated by ReH7(PR3)2 or [hH2(OH2)2(PR3)2] or (PCP)hH2 thermally or photo-chemically. Cyclooctane is the best snbstrate, presumably because it has the least unfavorable heat of dehydrogenation of all common alkanes (equation 27). 

The final remark of this sketchy section should be made cautiously. It seems that surprisingly, light alkane activation, although being heralded as a major area of future development, appears to benefit only from moderate "push" and moderate "pull". Catalytic combustion and the complete oxidation of pollutants seem areas with much more activity presently. But the "pull" by environmental concerns is strong in those cases. 

There are many reviews that cover various aspects of oxidation. These include ones on alkane activation,166 catalytic selective oxidation,167 metal complexes of dioxygen,168 metal-catalyzed oxidation,169 biomimetic oxidations,170 oxidation with peroxides,171 catalytic oxidations with peroxides,172 catalytic oxidations with oxygen,173 oxidations with dioxiranes,174 and oxidation of pollutants.175... 

Acetic acid and other organic acids are thermodynamically unstable at sedimentary conditions and will eventually decarboxylate to CO2 and alkanes (24). Experimental studies of acetic acid decarboxylation show that the rate is extremely sensitive to temperature and the types of catalytic surfaces available (Table II 25-261. Extrapolated rate constants for acetic acid decarboxylation at 100 C differ by more than 14 orders of magnitude between experiments conducted in stainless steel and catalytically less active titanium (Table II 26). Inherent (uncatalyzed) decarboxylation rates are similar for acetic acid and acetate (26). However, in catalytic environments their rates of decarboxylation differ markedly (25-261. and therefore a pronounced pH effect on total decarboxylation rate is observed. 

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