Intermolecular C-H oxidation

Liquid rare gases have been used as unfamiliar solvents which are well suited for the application of time-resolved infra-red spectroscopy to the direct measurement of an intermolecular C-H oxidative additive reaction of alkenes. ... 

Two molecules with comparable geometry in an asymmetric unit were found for 3,4-bis(4-fluorophenyl)-l,2,5-oxadiazole 2-oxide. The bond length of the dipolar N-O bond is 1.107 (7) A <2006AXEo4827>. In the molecule of 5-(6,7-dimethoxy-l,2,3,4-tetrahydroisoquinolin-2-yl)-4-phenyl-l,2,5-oxadiazole Ar-oxide, the six-membered heterocyclic ring has a flattened boat form. Intermolecular C-H- O hydrogen bonds link the molecules into dimers, which may be effective in the stabilization of the crystal structure <2006AXEo3130>. 

Intermolecular C-H insertion of diazoacetate (59) into the bisallylic position of cyclohexa-1,4-dienes (60) followed by DDQ oxidation of the substituted cyclohexa-diene gave the polysubstituted arenas (61) in medium to very good overall yields.50... 

Rhodium-mediated intermolecular C— H insertion is thought to proceed via oxidative addition of an intermediate rhodium carbene into the alkane C—H bond. Evidence that the rhodium and its ligands are directly associated with the product-determining transition state has been put forward by Callot, who ob-... 

DeBoef and co-workers have reported a similar reaction, wherein direct C-H to C-H indole-arene cross-coupling can be controlled through the use of a particular oxidant (Scheme 34) [53,54], The basis of their selectivity concept is the formation of different polyvalent clusters between the Pd(OAc)2 and the AgOAc or Cu(0 Ac)2 oxidants respectively, and the subsequent reactivity of these complex in the aryla-tion reaction. The same group also demonstrated the utility of an intermolecular C-H to C-H coupling reaction. 

Fig. 30 Highly chemoselective methods for intra- and intermolecular allylic C-H oxidation...

The theoretical calculations using density functional theory showed that the intermolecular C-H activation of alkanes by the complex CpIr(PMc3)(CH3) (described by Bergman, vide supra) is a lower-energy process and that both inter- and (nfronolecular C-H activation proceed only through an oxidative-addition mechanism (Scheme VI.8) [64] (compare Scheme VI.7). 

Third row unsaturated complexes are, in principle, the best candidates to activate methane, but this depends, a good deal, on the environment of the metal center and its oxidation state. The unfavorable thermodynamics of reaction (61) contrasts with the easy intermolecular C-H activation observed in reaction (67), for which (67) is estimated as — 70kJ moP by using data in Table 1 and 7 AjA (67) Rs —44kJmol ... 

Several examples of intermolecular C-H bond functionalization have appeared during the past decade. In addition to the oxidations reported above in Shilov-type systems, and the dehydrogenation of alkanes to make alkenes, catalytic systems have been developed to introduce functional groups into hydrocarbons. 

More recently, Yu and coworkers [9] described the paUadium(II)-catalyzed ortho alkylation of simple benzoic acids with either 1,2-dichloroethane or dibro-momethane (Scheme 19.5). The use of the less activated 1-chloropentane was also feasible, but furnished only a 26% yield. This reaction was shown to proceed through initial intermolecular C-H alkylation followed by intramolecular Sj 2-cyclization to give the corresponding benzolactone. As mentioned, the C-H alkylation step was proposed to occur through oxidative addition-reductive elimination or o-bond metathesis. 

Mechanisms of Direct Arylations A plausible mechanism for Pd -catalyzed intermo-lecular direct arylations with ArX (X=C1, Br, I) involves (i) oxidative addition of a Pd catalyst into an aryl halide, (ii) intermolecular C—H activation, and (iii) reductive elimination to release the aiylated prodnct and regenerate the catalyst (Scheme 24.2a) [1,5, 6,8]. 

Photolysis of [Ir(u5- neomenthyl)C5H4)(PMe3)(H)2l in cyclohexane or benzene leads to intermolecular C-H activation of solvoit molecules. Reaction of enantiopure [Ir(i) - neomenthyl -C5H4)(PMe3)(Me)(I)] with silver hexafluoroantimonate proceeds via a planar at iridium intermediate and a C-H oxidative addition. A p- elimination reaction completes the preparation of a hydrido-alkene cation which retains very high optical purity. ... 

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