With C—H Bond Cleavage

The cleavage of sp C—H bonds by oxidative addition only occurs with fairly acidic molecules, and others have argued that this is an unpromising route to the activation of normal saturated alkanes. The reductive elimination of methane from [PtHMeP2] (P=tertiary phosphine) has the stoicheiometries shown in equations (15) and (16) in the absence or presence of added L e.g. PPhg or 

It is clear from the nature of the products that in these metallation reactions Ir and Rh undergo oxidative addition, but with [MnMe(CO)s] the loss of CH4 means that oxidative addition, if it occurs, is followed by reductive elimination. The mechanistic details of these reactions remain obscure. 

Oxidative addition with cleavage of H—H, C—H, and Si—H bonds requires a co-ordinatively unsaturated metal species such as four-co-ordinate / -complexes. Thus, although nucleophilic anions such as [Mn(CO)5] react readily with alkyl halides, the reaction with HSiPhg is photoinduced. Irradiation gives [Mn(CO)4] , which can then undergo oxidative addition leading to cis [MnH(SiPhs)(CO)4]-. 

Factors affecting simple reductive elimination versus other forms of elimination (e.g. a), especially with respect to dihydrides, dialkyls, and mixed alkyl-hydrides (which eliminate fastest) of mono- and di-nuclear osmium complexes, have been discussed. In compounds such as [OsHMe(CO)4] or [OsMe2(CO)4] simple reductive elimination does not occur for example, a mixture of [OsH(CD3)(CO 4] and [OsD(CH3)(CO)4] gave CH4, CH3D, CD3H, and CD4, and a dinuclear mechanism is proposed.  

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A natural extension of oxidative addition reactions of Os3 clusters with C—H bond cleavage is that with Si—H, Ge—H, or Sn—H bond cleavage. Os3(CO)12 reacts at high temperatures (e.g., 140°C) or under uv photolysis with Me3SiH to give mono- and dinuclear compounds, exem-... 

With C-H Bond Cleavage.—The complexes [MH(2-naphthyl)(Me2PCH2CH2-PMc2)2] (M = Fe, Ru, or Os), abbreviated to [MH(Np)(dmpe)2], readily eliminate naphthalene to give a co-ordinatively unsaturated / -intermediate, which rapidly co-ordinates to free ligand, if this is available, or undergoes oxidative addition with C—H cleavage (Scheme Most of the oxidative addition reactions... 

The ethyl radical decomposes with C-H bond cleavage to ethylene and hydrogen atom... 

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

In aromatic combustion flames, cyclopentadienyl radicals (c-CgHj ) can be precursors for PAH formation. " At high temperatures, benzene is oxidized by reaction with an oxygen molecule to yield phenylperoxy (C6H5O2 ) radical, via the initial formation of the phenyl radical (by C-H bond cleavage) and then the rapid addition of O2 (reaction 6.16). After expulsion of CO from phenylperoxy radical, a resonance-stabilized cyclopentadienyl radical (c-CgHg ) is formed (reaction 6.16). 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

The alkylation of the sp3 C-H bonds adjacent to a heteroatom becomes more practical when the chelation assistance exists in the reaction system. The ruthenium-catalyzed alkylation of the sp3, C-H bond occurs in the reaction of benzyl(3-methylpyridin-2-yl)amine with 1-hexene (Equation (30)).35 The coordination of the pyridine nitrogen to the ruthenium complex assists the C-H bond cleavage. The ruthenium-catalyzed alkylation is much improved by use of 2-propanol as a solvent 36 The reaction of 2-(2-pyrrolidyl)pyridine with ethene affords the double alkylation product (Equation (31)). 

Palladium-catalyzed reaction of 2-hydroxy-2-methylpropiophenone with aryl bromides shows a unique multiple arylation via successive C-C and C-H bond cleavages, giving tetraarylethanes.96 For example, the reaction of 2-hydroxy-2-methylpropiophenone with bromobenzene in the presence of Pd(OAc)2, P(/-Bu)3, and CS2CO3 gives 1,1,2,2-tetraphenylethane quantitatively, together with l,4,4-triphenyl-7-methylisochroman-3-one (13% yield) (Equation (74)). 

For the oxidative addition pathway, however, it is not obvious why the C-H bond cleavage reaction should be more facile if the hydrocarbon first binds in the coordination sphere of the metal (Scheme 5, c). One argument could be that the equilibrium between the Pt(II) alkane complex and the five-coordinate Pt(IV) alkyl hydride has an intrinsically low activation barrier. Insight into this question together with detailed information about the mechanisms of these Pt(II) a-complex/Pt(IV) alkyl hydride interconversions has been gained via detailed studies of reductive elimination reactions from Pt(IV), as discussed below. 

The experimental data available to date consistently indicate that ligand dissociation precedes reductive elimination from six-coordinate platinum(IV). In the reverse direction (oxidative addition), it seems necessary that the hydrocarbon molecule coordinates in the square plane of platinum(II). C-H bond cleavage then forms a five-coordinate Pt(IV) species consistent with the principle of microscopic reversibility. 

Calculations performed for cyclopropanation with Fischer-type carbene complexes [28] indicate that the electrophilic attack of the carbene complex at the alkene and the final ring closure are concerted. Extrapolation from this result to the C-H insertion reaction (in which a a-bond instead of a 7i-bond is cleaved) suggests that C-H bond cleavage and the formation of the new C-C and C-H bonds might also be concerted (Figure 3.38). 

Exploiting the Lewis basic phosphoryl oxygen of Im, Terada reported the direct alkylation of a-diazoesters with N-acyl imines to afford P-amino-a-diazoesters in high yields and ee s (Scheme 5.12) [23]. Earlier, Johnston had observed that catalytic TfOH promoted aziridine formation (Aza-Darzens reaction) between diazoacetates and N-benzyl imines [24]. The authors propose that aziridine formation is circumvented through C—H bond cleavage by the phosphoryl oxygen of 1 (Intermediate A). However, as noted by the authors, the low nucleophilicity of N-acyl imines might also be considered as the cause of this selective transformation. 

Kinetics of naphthalene and substituted naphthalenes oxidation by stoich. RuOy CCl to phthalic acids suggest an initial second-order reaction giving a complex with a naphthalene-0-Ru(Vl) bond, followed by a slower decomposition of this intermediate involving C-H bond cleavage [371]. 

With saturated alkanes the only reaction induced by mercury triplets is C—H bond cleavage. Gunning and Strausz have recently presented a theory that C—H and C—Cl bond cleavage proceeds through a cyclic mechanism.502... 

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