Alkyl C —H bonds

There are three sorts of C-H bonds in cyclohexene, and Table 5.3 gives an estimate of their relative strengths. Although a typical secondary alkyl C-H bond has a strength of about 400 kj/mol (96 kcal/mol) and a typical vinylic C-H bond has a strength of 445 kj/mol (106 kcal/mol), ail allylic C-H bond has a strength of only about 360 kj/mol (87 kcal/mol). An allylic radical is therefore more stable than a typical alkyl radical with the same substitution by about 40 kj/mol (9 kcal/mol). 

The characteristic tendency of hypovalent transition metals to interact in a side-on (T-shaped) manner with nearby alkyl C—H bonds has been designated as the agostic effect.45 Agostic M- C—H interactions can also be identified with general 3c/2e donor-acceptor interactions of 2c— lc type. Availability of suitable lc acceptor orbitals is a signature of hypovalent early transition metals. 

Other derivatives can effect such functionalizations, including iron, ruthenium, rhodium, and tungsten complexes. Excellent regioselectivities were observed in catalytic thermal processes involving unfunctionalized alkanes and primary alkyl C-H bonds were functionalized (Equation (20)).31... 

C-H. Since the alcohol O-H bond is stronger ( 104 kcal mol 1) than the alkyl C-H bond ( 96 kcal mol 1) (Lide, 1998-1999), one would expect that abstraction from the alkyl group would predominate, e.g., reaction (69a) would be faster than reaction (69b) ... 

This then was the first report of a compound in which alkyl C—H bond activation by a transition metal had occurred. In the solid state, this equilibrium is also in favor of the hydrido complex (V), and its crystal structure has recently been determined (15). It shows compound V to be a dimer (VI), the oxidative addition of the methyl group of a ligand on each ruthenium atom being to a second ruthenium atom. Presumably one reason why this occurs is because the product of intramolecular ring closure would contain a highly strained three-membered Ru—P—C ring (i.e., in monomer V). 

X-ray analysis shows distorted square planar coordination around Pd. This is the first example of alkyl C-H bond activation with a cationic diimine Pd(II) complex under such mild conditions. 

Singlet carbenes insert into alkyl C-H bonds randomly, with retention of configuration. Triplet carbenes insert into alkyl C-H bonds selectively, but not stereospecifically. 

Marder reported that the alkyl C-H bonds in benzylic compounds are selectively borylated with a rhodium catalyst. Aromatic borylation prodncts were only observed as minor by-products. Similar results were obtained by Miyaura with Pd/C (equation 28). ... 

Alkoxy radicals are electrophiles and they preferentially attack C-H bonds with high HOMO energies, for instance, the a-C-H bond of ethers and amines or the alkyl C-H bond of esters. In contrast, nucleophilic alkyl radicals abstract a hydrogen atom from the acyl group of esters, because this C-H bond has a lower LUMO energy. 

It must be pointed out that there are other complications regarding the addition of alkyl C-H bonds to a metal. The resulting metal alkyl complex could undergo either reductive elimination back to starting material (Section 7-3) or, if a p C-H bond is present, rapid... 

Since aryl halides are fairly cheap reagents, there has been less recent emphasis (see Section 3 below) on the development of aryl relative to that of alkyl C-H bond activation [13-17]. However, the manufacture of aryl halides is not an environmentally friendly process and thus the future of bulk aromatic synthesis may he in the direct activation of C-H bonds. For example, the formation of ben-zaldehyde from the insertion of CO into a C-H bond of benzene is a recent development in this area [17]. 

Relative BDEs have been used to estimate the relative radical stabilities. Therefore, the smaller BDE is a measure for the higher stability of the radicals that are formed by the homolytic dissociation of the bond. For example, BDEs of a series of substituted alkyl C—H bonds decrease in the order of the degree of substitution, CH3—H (104.8), CH3CH2—H (101.1),... 

Cu(TpMs)] catalyzes the addition of the iCHCC Et unit (derived from N2HCHCC>2Et) to benzene which yields a cycloheptatriene ring, in analogy with the Buchner reaction. When alkyl groups are linked to the aromatic rings, the selectivity of the reaction can be oriented toward insertion into an alkyl C-H bond by using [Cu(TpMs)].48 On the other hand, [Cu(Tp )] efficiently catalyzes the insertion of... 

In some cases it is possible to control the site of palladation, choosing either an sp or sp C-H bond for insertion. When A -thiobenzoylpyrrolidine is treated with Pd in methanol, cyclometalation takes place at the aryl C-H, directed by one of the lone pairs on S (equation 73). When the same reaction is carried out in HMPA, an alkyl C-H bond reacts. The reason for this change of reactivity with change of the solvent is not known. Certainly, different palladium complexes will be present in the two solvents. 

Calculated (B3LYP) enthalpies of addition and the effects of phosphine methylation (substitution of PMca for PH3) in reactions of some iridium complexes with C-H compounds (as well as with molecular hydrogen) are summarized in Table VI. 3 [46], It can be seen that the addition of aryl and especially acetylene C-H bonds is thermodynamically more favorable than the addition of simple alkyl C-H bonds. Addition of an aryl C-H bond has been found to be at least 16 kcal mol less exothermic than H2 addition. However, on the basis of the Bryndza-Bercaw relationship [47]... 

The room-temperature rate constant determined by Corse, Lll, and Saunders (239) for dlethylhydroxylamlne Is also extremely fast, and they conclude (239) that the reaction proceeds mainly via H-atom abstraction from the 0-H group, although by comparison with the other amines It would appear that at least part of the reaction pathway must involve H-atom abstraction from the secondary alkyl C-H bonds. 

Carboalkoxynitrenes generated from azidoformates undergo insertion reactions with saturated hydrocarbons (Scheme 6.29). Triplet nitrenes do not insert into alkyl C-H bonds. 

Recent Advances in the Selective Oxidation of Alkyl C-H Bonds Catalyzed by Iron Coordination Complexes... 

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