Addition of alkanes

Alkyl radicals can be generated from tetramethylammonium phenyltrialkylborate salts (used as electron donors) in the PET reaction with excited benzophenone in the presence of a 10-fold excess of an electron-poor olefin in an acetonitrile/benzene (1 5) solution [8]. The advantages of this method are that more than one equivalent of alkyl radicals is obtained per molecule of the borate derivative, and that part of the benzophenone is usually recovered after the reaction [8]. 

The generation of alkyl radicals by direct and selective activation under mild conditions of C—H bonds in alkanes is clearly desirable, and still considered as the 

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G. Carbon on the Other Side 15-18 Addition of Alkanes Hydro-alkyl-addition... 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

The oxidative addition of alkane C-H bonds to Pt(II) has also been observed in these TpRa -based platinum systems. As shown in Scheme 19, methide abstraction from the anionic Pt(II) complex (K2-TpMe2)PtMe2 by the Lewis acid B(C6F5)3 resulted in C-H oxidative addition of the hydrocarbon solvent (88). When this was done in pentane solution, the pentyl(hydrido)platinum(IV) complex E (R = pentyl) was observed as a... 

The next step, the oxidative addition of alkane to form the Ir(V) intermediate is quite sensitive to the methods and presents a challenging problem to determine which method gives the most accurate answer. The calculated activation (AE ) and reaction (AE°) energies show... 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

Another effective way of staying clear of the thermodynamic barriers of C-H activation/substitution is the use of the c-bond metathesis reaction as the crucial elementary step. This mechanism avoids intermediacy of reactive metal species that undergo oxidative additions of alkanes, but instead the alkyl intermediate does a o-bond metathesis reaction with a new substrate molecule. Figure 19.13 illustrates the basic sequence [20],... 

The value of = 1 X 10 s for the first-order rate constant for collapse of an ion pair between Me-4 and pentaflourobenzoate ion is larger than the second-order rate constant rcoo = 5x10 M s reported for the bimolecular addition of alkane carboxylates to Me-4. This second-order rate constant is limited by the rate constant for formation of an ion pair between Me-4 and a carboxylate ion. The larger barrier to encounter-limited reactions of carboxylate ions compared with the diffusion-limited reactions of anions such as azide ion, = 5 X 10 represents the barrier to desolvation of nucleophile that must precede formation of an ion pair between Me-4 and a carboxylate ion (Scheme 13). ... 

In this section we shall deal only with the reverse reaction to dealkylation by cracking (Sect. 2.5), that is with additions of alkanes and aromatic compounds to the carbon—carbon double bond. The former reaction is described only in a single paper [339] the formation of 2,2,4-trimethyl-pentane from isobutane and isobutene... 

Nanoparticles of metals such as Au, Ag, Pd and Pt embedded in exfoliated sheets of aminociays of the type R8Si8Mgs0lfi-(OH)4, where R = CH2CH2NH2 are entirely water soluble. These sheets of the composite come to the organic-aqueous interface on addition of alkane thiols to the aqueous layer. 

The oxidative addition of alkanes to Rh1 and Ir1 species CpML is very facile. In the case of the tris(pyrazolyl)borate complex Tp Rh(CO)2 [Tp = HB(2,4-Me2pyr)3], the lifetimes of the intermediates were determined by ultrafast time-resolved infrared spectroscopy. In this case de-chelation of one of the pyrazolyl arms was found to precede the C—H oxidative addition step. The proposed intermediates for R—H addition and their lifetimes are shown in Fig. 21-1.127... 

The addition of alkanes to Ir111 species such as [CpIr(Me)L]+ can proceed via two pathways, oxidative addition (A) or or-bond metathesis akin to that in lanthanide complexes (B). 

Robert G. Bergman Wdham A. G. Graham Oxidative addition of alkanes by iridium complexes... 

Patricia L. Watson Tobin J. Marks Oxidative addition of alkanes by lanthanide and actinide compounds, respectively... 

Photolysis of Cp M(CO)2, M = Rh or Ir, and CpIr(CO)2 in methane matrices afforded the first direct observation of C-H activation from a photointermediate.Similarly, photolysis of Cp fr(CO)(H)2 in methane matrices yielded Cp Ir(CO)(H)(CH3). The rate of oxidative addition of alkanes to a CpRh(CO) photointermediate in the gas phase has been measured. 

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