Alkanes transition metal catalysis

Investigations which concern the mechanisms of skeletal rearrangements of saturated hydrocarbons induced by heterogeneous transition metal catalysis are of great interest for industrial applications, e.g. for petroleum reforming processes The developments in this field were reviewed recently by Hejtmanek, and by Maire and Garin , who focussed on the probable reaction mechanisms which include bond-shift and cyclic mechanisms for the skeletal isomerization of acyclic alkanes. Scheme 1 summarizes the... 

Xiao (1998) applied ab initio calculations to test the transition metal catalysis hypothesis. The key is to understand the kinetics and mechanisms of catalytic gas generation, promoted by transition metals through the activation of alkanes and the condensation of hydrogen and a-alkenes, as well as weather these reactions can proceed under realistic geological conditions. 

Opportunities for alternative raw materials applying homogeneous transition metal catalysis can be seen in synthesis gas chemistry, alkane chemistry, and CO2 chemistry. 

The activation of C-H bonds for direct C C bond formation reactions has the potential to become very important especially if it can be accomplished for sp C-H bonds, in methane or alkanes as these are the major feedstocks available. In addition, C-H bond activation of functionalized organic compounds for selective C-C bond formation has been and will continue to be a very important goal of organometallic catalysis. So far the use of transition metal complexes has led to interesting results which however are not yet industrially relevant. 

The study of mixed-ligand 0x0 derivatives is closely related to the use of these species in catalytic oxidation systems (including the epoxidation of aUcenes and the oxidation of alkanes. See Oxidation Catalysis by Transition Metal Complexes). In such complexes, the 0x0 group can be terminal, doubly bridging, or triply bridging. 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

Catalysts which contain reduced transition metal clusters besides acid sites are able to catalyze reactions that are not observed on catalysts exposing one type of site only. The reaction network is inadequately described by models which assume only additivity of catalytic functions and shuttling of intermediates between sites. There is strong evidence that metal clusters and Bronsted sites form metal-proton adducts. These act as "collapsed bifunctional sites" all alkane isomerization steps can take place on such sites during one single residence of the adsorbed molecule. At low temperature, adsorption in a mode reminiscent of a carbenium ion can suppress pure metal catalysis. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

In connection with the activation of saturated hydrocarbons via homogeneous catalysis, we have examined transition metal catalyzed reactions of various strained hydrocarbon systems that have unique steric and electronic properties. Strained carbon-to-carbon single bonds have considerable TT-bonding character. The chemistry of these substrates should be intermediate between well-documented transition metal chemistry of alkenes and rather unclarified alkane chemistry (1, 2, 3). Our attention has been focused particularly on the stereoselectivity, regioselectivity, and periselec-tivity of the Ni(0)-catalyzed reactions (4-14). 

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