Carbon-hydrogen bonds activations

Finally, the change in selectivity for the methane/pentane couple for the two different substrates (18% for hexane, 56% for cyclohexane) can be explained as follows in the case of cyclohexane, the Ci to C5 products are formed through the second carbon-carbon bond cleavage via the hexyl surface intermediate D whereas in the case of hexane, the initial carbon-hydrogen bond activation step can lead to any of three alkyl surface intermediates (D, E, and F) before arriving at the key metallacychc intermediates... 

G and H. This suggests that the isomerization of the surface alkyl fragments inter-converting D, E, and F, is slow with respect to the second carbon-hydrogen bond activation step and subsequent carbon-carbon bond cleavage. 

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

Alaimo, P.J.. Arndtsen, B.A. and Bergman. R.G. (2000) Alkylation of iridium via tandem carbon-hydrogen bond activation/decarbonylation of aldehydes. Access to complexes with tertiary and highly hindered metal-carbon bonds. OrganometaUics, 19 (11), 2130-2143. 

Carbon-hydrogen bond activation activated alkyl groups... 

Arndtsen, B.A., Bergman, R.G., Mobley, T.A. and Peterson, T.A. (1995) Selective intermolecular carbon—hydrogen bond activation by synthetic metal complexes in homogeneous solution. Acc. Chem. Res., 28, 154. 

Redistribution of electron density in CT complexes results in a modification of the chemical properties of coordinated arenes, and this effect is widely used in organometallic catalysis [2]. To demonstrate the relationship between charge transfer in arene complexes and their reactivity, we focus our attention on carbon-hydrogen bond activation, nucleophilic/ electrophilic umpolung, and the donor/acceptor properties of arenes in a wide variety of organometallic reactions. 

Charge-Transfer Activation of Coordinated Arenes 452 Carbon-Hydrogen Bond Activation 453 Nucleophilic/ Electrophilic Umpolung 455... 

Arene ruthenium and osmium complexes play an increasingly important role in organometallic chemistry. They appear to be good starting materials for access to reactive arene metal hydrides or 16-electron metal(O) intermediates that have been used recently for carbon-hydrogen bond activation. Various methods of access to cyclopentadienyl, borane, and carborane arene ruthenium and osmium complexes have been reported. 

The possibility of coordination of a two-electron ligand, in addition to arene, to the ruthenium or osmium atom provides a route to mixed metal or cluster compounds. Cocondensation of arene with ruthenium or osmium vapors has recently allowed access to new types of arene metal complexes and clusters. In addition, arene ruthenium and osmium appear to be useful and specific catalyst precursors, apart from classic hydrogenation, for carbon-hydrogen bond activation and activation of alkynes such compounds may become valuable reagents for organic syntheses. 

W. D. Jones, and F. J. Feher, Alkane Carbon-Hydrogen Bond Activation by Homogeneous Rhodium(I) Compounds, Organometallics 2, 562-563 (1983). 

Shilov chemistry, developed from 1970, employs [Pt(II)CLt] salts to oxidize alkanes RH to ROH or RCl with modest efficiency. Pt(IV) is an efficient (but economically impractical) primary oxidant that makes the process catalytic. This discovery strongly contributed to the continuing activity in CH activation. Periana developed a related and much more efficient system for methane oxidation to methanol using 2,2 -bipyrimidine ligands and sulfuric acid as solvent. In this case, the sulfuric acid is the primary oxidant and the methanol formed is protected by being converted in situ to MeOSOsH, an ester that strongly resists further oxidation. This area is more fully described under the entry Alkane Carbon-Hydrogen Bond Activation. 

With the development of powerful methods for molecular orbital calculations (e.g. DFT) (see Molecular Orbital Theory), several computational studies of the C-H oxidative addition (see Alkane Carbon-Hydrogen Bond Activation) process have been undertaken. One has used CpRh(PH3) to model the reactive intermediate Cp Rh(PMe3) proposed for the reaction shown in equation (13). The results... 

---