Aliphatic C—H bonds

Insertion by arylnitrenes into aliphatic C-H bonds, which is usually an unfavored process, is now generally thought to involve the triplet state. Two obvious mechanisms are conceivable. Classical insertion of an arylnitrene into a C-H bond should involve a singlet species  

One can also envisage the same product being formed via the triplet nitrene, by an initial hydrogen abstraction followed by radical dimerization  

Hall and his coworkers found C-H insertion by phenylnitrene to be a low-yield reaction. Thermolysis of phenyl azide in n-pentane yielded aniline (30%) and only 10% of a mixture of 7V-pentylanilines. Evidence for triplet intermediacy derives from a study of the thermolysis of phenyl azide in a 

The extremely electrophilic tetrafluoropyridylnitrene inserts into C-H bonds much more readily than does phenylnitrene, and again evidence for the triplet nitrene was presented. In other experiments the same workers discounted the possibility of a hydride abstraction mechanism. Intramolecular C-H insertions are known to proceed in high yield through the triplet nitrene (see Section III.l)  

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Consider first that the atom of R bonded to H or D is carbon. An aliphatic C-H bond has a typical stretching frequency of 8.7 X 1013 s 1 or 2900 cm-1. Substitution into Eq. (9-96) gives... 

Fig. 13. Calculated 2H solid echo spectra for log-Gaussian distributions of correlation times of different widths. Note the differences of the line shapes for fully relaxed and partially relaxed spectra. The centre of the distribution of correlation times is given as a normalized exchange rate a0 = 1/3tc. For deuterons in aliphatic C—H bonds the conversion factor is approximately 4.10s sec-1...

Production of considerable amounts of cyclohexanol and cyclohexanone as well as benzaldehyde and benzoic acid in the oxidation of benzyl cyclohexyl ether shows the primary radical to be CgHjCHOCeHjj. Abstraction from aliphatic C-H bonds cannot occur in the case of diphenyl ether which is oxidised rapidly, and removal of a 7t-electron is likely. 

Treatment of [IrCl(CO)2(/ -toluidene)] with azine phosphines of type Z, -PPh2CH2C( Bu) =N-N=C(Q)R, Q = H, Me, R = an organic group, activates aryl, heterocyclic, alkenyl, or aliphatic C—H bonds to give cyclometalated Ir111 hydrides.339... 

The chemical reactions of sulphonyl nitrenes include hydrogen abstraction, insertion into aliphatic C—H bonds, aromatic substitution , addition to olefinic double bonds, trapping reactions with suitable nucleophiles, and Wolff-type rearrangement. Hydrogen-abstraction from saturated carbon atoms is usually considered to be a reaction typical of triplet... 

Sulphonyl nitrene-metal complexes also undergo insertion into aliphatic C—H bonds as witnessed by the insertion into dioxan on treatment with chloramine-T and copper 45> and into cyclohexane with di-chloramine-T and zinc 44> and into cyclohexene with benzenesulphonyl azide and copper 34> and with ferrocenylsulphonyl azide 25>. 

Another remarkable property of iodorhodium(III) porphyrins is their ability to decompose excess diazo compound, thereby initiating carbene transfer reactions 398). This observation led to the use of iodorhodium(III) me.vo-tetraarylporphyrins as cyclopropanation catalysts with enhanced syn anti selectivity (see Sect. 2.2.3) s7, i°o) as wep as catalysts for carbenoid insertion into aliphatic C—H bonds, whereby an unusually high affinity for primary C—H bonds was achieved (see Sect. 6.1)287). These selectivities, unapproached by any other transition metal catalyst,... 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

Figure 8.34 Strong, sharp absorption at 1700 cm 1 indicating a carbonyl group. No other significant patterns except the C-H pattern on the low side of 3000 cm . It is an aliphatic aldehyde or ketone. Figure 8.35 A benzene ring is indicated because of the band on the high side of 3000 cm-1 and the series of weak peaks between 1700 and 2000 cm . Aliphatic C-H bonds are also indicated (absorption bands on the low side of 3000 cm-1). Possibly ethylbenzene, or a similar structure.

Although most of the reported gas-phase experiments do not investigate the temporal evolution of alcohol clusters explicitly, the frequency-domain spectral information can nevertheless be translated into the time domain, making use of some elementary and robust relationships between spectral and dynamical features [289]. According to this, the 10-fs period of the hydrogen-bonded O—H oscillator is modulated and damped by a series of other phenomena. Energy flow into doorway states is certainly slower than for aliphatic C—H bonds [290] but on a time scale of a few picoseconds, energy will nevertheless have... 

One of the most fascinating transformations of free carbenes, generated for instance by photolysis of diazoalkanes or by a-elimination, is their insertion into aliphatic C-H bonds. This ability of carbenes is not only of theoretical interest, but also a unique tool for the synthesis of highly strained compounds such as, e.g., bicyclo[l. 1.0]butanes. 

Fig. 3.36. Possible mechanism of the insertion of nucleophilic carbene complexes into aliphatic C-H bonds.

Electrophilic carbene complexes can react with amines, alcohols or thiols to yield the products of a formal X-H bond insertion (X N, O, S). Unlike the insertion of carbene complexes into aliphatic C-H bonds, insertion into X-H bonds can proceed via intermediate formation of ylides (Figure 4.7). 

Since then, these ligands and their complexes have become increasingly important due to the fact that they exhibit the same characterishcs of robustness and thermal stability and, in most of cases, an enhanced reactivity compared to their phosphine counterparts. Thus, phosphinite PCP pincer complexes have been used intensely during the most recent era of catalytic processes and reactions involving the activation of aliphatic C—H bonds [40]. 

The transfer constant for f-butylbenzene is low, since there are no benzylic C—H bonds present. Primary halides such as n-butyl chloride and bromide behave similar to aliphatics with low transfer constants, corresponding to a combination of either aliphatic C—H bond breakage or the low stability of a primary alkyl radical on abstraction of Cl or Br. The iodide,... 

This ruthenium catalyst efficiently transforms cis-3-en-l-yne d3-9 into cyclopenta-dienyl species d3-ll selected examples are depicted in Table 6.1. The cyclization works not only for cis-3-en-l-ynes bearing a benzylic hydrogen but also for those bearing aliphatic C—H bonds. Table 6.1 also manifests additional use of this cyclization that 5-siloxyl-3-en-l-ynes were transformed into cydopentenone derivatives using the same mthenium catalyst. 

Like many singlet carbenes, nucleogenic, arc generated and chemically generated C atoms react with aliphatic C—H bonds by insertion. In the simplest case, reaction of chemically generated C atoms with methane yields ethylene and acetylene. When a mixture of CH4 and CD4 is used, product analysis indicates that the acetylene results from H abstraction followed by dimerization of the CH, while the ethylene results from C—H insertion followed by H migration in the carbene (Eq. 15). It seems probable that CH is formed in all reactions of carbon with hydrocarbons as acetylene is invariably produced in these reactions. 

T. Ruhland, K. Andersen and H. Pedersen, Selenium-linking strategy for traceless solid-phase synthesis Direct loading, aliphatic C-H bond formation upon cleavage and reaction monitoring by gradient MAS NMR spectroscopy, J. Org. Chem., 1998, 63, 9204-9211. 

They were used for the calculation of the activation energies for isomerization of several peroxyl radicals. Peroxyl radical isomerization involving the formation of a six-membered activated complex is energetically more favorable the activation energy of a thermally neutral reaction Ee is 53.2 kJ mol-1. For the seven-membered transition state, the Ee0 value (54.8 kJ mol-1) is slightly higher. The calculated hrc parameter for the six-membered transition state (13.23 (kJ mol-1)172) is close to the bre value (13.62 (kJ mol-1)172) for the transition state of the bimolecular H atom abstraction from the aliphatic C—H bond by the peroxyl radical. Therefore, the kinetic parameters for isomerization are close to those for bimolecular H-atom abstraction by the peroxyl radical. This allows the estimation of the kinetic parameters for peroxyl radical isomerization. Relevant results of calculation via Eqns. (6.7, 6.8,... 

Aromatic C-H bonds are not broken in radical halogenation, because they are a little stronger than aliphatic C-H bonds. When benzene reacts photochemically with chlorine, a radical addition process takes place, and the mixture of stereoisomerir hexachloro-cydohexanes (S.78) includes one isomer which has powerful insecticidal properties but which, unlike some chlorinated insecticides, is readily biodegradable. 

IR spectroscopy also indicated qualitatively that organosiloxanes were incorporated into the silica framework. The appearance of peaks at 3050 cm1 and 1430 cm1 for Ti-MCM-41-Ph is due to the presence of phenyl groups (data not shown) while a peak at 2987 cm1 for Ti-MCM-41-Me (data not shown) corresponded the presence of aliphatic C-H bonds. 

The single known example of a deprotonation of an alkylamido ligand suggests that a potentially convenient synthetic pathway may not be generally useful owing to the low acidity of the aliphatic C—H bonds (equation 99). 

Studies examining H-abstraction by various oxygen-centred radicals have been reported. Activation energies114 and rate constants115 for the processes of abstraction from alkyl, vinyl, and aryl hydrocarbons have been calculated with abstraction from aliphatic C—H bonds proceeding with the highest activation energies. 

Probing C—H addition/elimination in Pt(ll)/Pt(IV) systems The importance of oxidative addition of aromatic and aliphatic C—H bonds to Pt(II) centers and its microscopic reverse, reductive elimination of C—H from Pt(IV) species, is ubiquitous in the context of both catalysis and synthesis. It is thus inevitable that the chemical, mechanistic, and kinetic facets of such reactions have become a prominent focus of group 10 poly(pyrazolyl)borate research, although this remains a relatively nascent area. 

Nitrogen [186], oxygen [187], and sulfur nucleophiles [170] also act as nice Michael donors in the tandem MCI reactions [Eq. (106)]. The a-oxyalkylidene carbene 116, generated by the reaction with phenoxide anion, shows a high selectivity for 1,5-insertion to the aromatic over the aliphatic C-H bonds to give the benzofuran. 

P-450-catalyzed hydroxylations of aliphatic C—H bonds most often involve a nonconcerted mechanism (Figure 8), which occurs in two steps (1) an abstraction of the hydrogen atom by the P-450 active oxygen complex, which exhibits a free radical-like reactivity, and (2) an oxidation of the substrate-derived free radical formed in this step by the Fe(IV)—OH intermediate [34,37,38],... 

The band intensities arising from the aliphatic C—H bonds are of particular interest, since they depend on the atomic weights of the atoms to which the other three valences of the carbon are linked. The peaks around 2930 and 2860 cm-1 are due mainly to the asymmetric and symmetric stretching of alkyl CH2 groups, expected at 2926 and 2853 + 10 cm-1. The CH3 groups are expected to give asymmetric and symmetric stretching bands at 2962 and 2872 cm-1. 

The metamorphic alteration of hydrocarbon structural units has been studied by Robin et al. (1977) 21), who documented the expected disappearance of aliphatic C—H bonds with increasing levels of maturity, and also demonstrated the disappearance of aromatic C—H bonds as structures approach extreme levels of carbonization. In extreme cases when the carbon content exceeds 91 % by weight and the H/C atomic ratio is only 0.4, aliphatic and G=0 bonds have vanished, and aromatic C=C are the main components remaining in IR spectra. The... 

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