Unactivated C—H bonds

In contrast to these results, low-temperature methane and argon matrix photolysis of [Cp M(CO)2] (Cp = Cp, Cp, 7/ -indenyl, M = Ir, Rh) gave indications of initial CO dissociation. No difference was seen in the behavior of 17 -indenyl and Cp. No evidence was found for C—H bond activation by [Cp Co(CO)2]. However, carbonyl exchange was the most rapid with the cobalt complex. Such compounds have also been studied in Nujol mull matrix at temperatures as low as 12 Similar studies have been undertaken to elucidate C—H activation 

11 Metal-Alkyl and Metal-Hydride Bond Formation and Fission 

A few systems have begun to reveal not only activation, but also functionalization of alkanes. A short-lived oxidative addition, /3-elimination, reductive elimina- 

While the activation of sp, sp, and heteroatom-substituted C—H bonds have become widely documented, the oxidative addition of alkane bonds remains restricted to only a few systems. Therefore the goal of understanding the chemistry of these complexes has attracted much study. The most studied systems involve the d fragment [Cp M(PMe3)], ( ) M = Rh, Ir. LCAO calculations based on nonlocal density functions reflected the more favorable reaction pathway for C—H bond activation by (9) than by [M(CO)4], (10), M = Ru, Complexes (9) 

Early transitionand lanthanide metal ions ° tend to show the greater aptitude toward gas-phase alkane activation. However, middle and late transition metal may be compelled to undergo these transformations. Bond 

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Thermolysis of m-PtH(CH2CMe3)(cy2PC2H2Pcy2) at 45-80°C yields a bent platinum(0) complex (Figure 3.13) that is intensely reactive to a whole range of unactivated C—H bonds in saturated and unsaturated hydrocarbons. 

Miura M, Nomura M (2002) Direct Arylation via Cleavage of Activated and Unactivated C-H Bonds. 219 211-241... 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates 286), rhodium(II) pivalate or some other carboxylates 287,288 and rhodium-(III) porphyrins 287 > proved to be well suited (Tables 19 and 20). In the era of copper catalysts, this reaction type ranked as a quite uncommon process 14), mainly because the yields were low, even in the absence of other functional groups in the substrate which would be more susceptible to carbenoid attack. For example, CuS04(CuCl)-catalyzed decomposition of ethyl diazoacetate in a large excess of cyclohexane was reported to give 24% (15%) of C/H insertion, but 40% (61 %) of the two carbene dimers 289). 

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

Another approach for the chemoselective and asymmetric iodination of unactivated C H bonds was reported with a palladium catalyst using a chiral auxiliary (Scheme 5.19). Excellent diastereoselectivities were induced by chelating the auxiliary to the palladium catalyst center followed by an electrophilic C—H activation and iodination. Studies showed that I2 acts as both the reactant and the activator to form the reactive catalyst precursor, Pd3(OAc)3. After the reaction was completed, the formed Pdl2 was precipitated from the solution and could be reused several times without losing reactivity and selectivity. 

As stoich. [Ru(0)((N 0)p7CH3CN it oxidised primary alcohols to aldehydes, secondary alcohols to ketones, alkenes to aldehydes, tetrahydrofuran to y-butyrolactone. Styrene, cis- and tran -stilbenes gave benzaldehyde and adamantane gave 1-adamantol exclusively, while cyclohexanol gave cyclohexanone, suggesting that the complex is an effective oxidant for unactivated C-H bonds [636]. Immobilisation of the catalyst within Nation films on a basal plane pyrohtic graphite electrode was achieved, but the... 

This section reviews those methods for constructing a C—C bond by direct insertion into an unactivated C-H bond that show synthetically useful selectivity. Both inter- and intramolecular reactions are considered. 

Dichlorocarbene, generated under phase transfer conditions, is reactive enough to insert into an unactivated C-H bond, yet discriminating enough to select for tertiary C-H bonds, as exemplified by the formation of rw-4a-(dichloromethyl)decahydronaphthalene9. 

Usually, carbon-carbon bonds are formed by coupling two carbons each of which are already functionalized in some way, as with the displacement of a C-Br with NaCN to form C-CN. It would be more efficient, and potentially less expensive and less polluting, if one of the partners could be an ordinary C-H bond. Intramolecular processes for carbene insertion into unactivated C-H bonds have been known for years. Practical intermolecular processes for C-C bond formation to a C-H bond are just starting to appear. 

Oxidation Catalyzed by Metalloporphyrins. Much attention has been devoted to the metal-catalyzed oxidation of unactivated C—H bonds in the homogeneous phase. The aim of these studies is to elucidate the molecular mechanism of enzyme-catalyzed oxygen atom transfer reactions. Additionally, such studies may eventually allow the development of simple catalytic systems useful in functionalization of organic compounds, especially in the oxidation of hydrocarbons. These methods should display high efficiency and specificity under mild conditions characteristic of enzymatic oxidations. 

Frey, P. A., The importance of organic radicals in enzymatic cleavage of unactivated C—H bonds. Chem. Rev. 90 1343, 1990. A brief review of coenzymes required to cleave un-reactive C—H bonds. 

Unactivated C—H bonds can be selectively replaced with C—C bonds in some cases. 

Carbenoid intermediates are highly reactive and capable of functionalization of unactivated C-H bonds [1]. In order for this reaction to be a practical transformation, however, the chemoselectivity of the reaction needed to be enhanced. Not only would it be necessary to distinguish between different C-H bonds, but... 

One of the most exciting features of these intermolecular C-H insertions is that the functionalization of unactivated C-H bonds can be efficiently achieved, leading to new strategies for the synthesis of chiral molecules. An example of this is the asymmetric synthesis of (+)-indatraline (13) shown in Eq. (6) [19]. Rh2(S-DOSP)4 catalyzed reaction of 11 with 1,4-cydohexadiene generated 12 in 93% ee, which was then readily converted to (+)-indatraline (13). 

The a-keto acid-dependent enzymes [206-208] catalyze a diverse array of reactions (Figure 26) involving functionalization of an unactivated C—H bond concomitant with the oxidative decarboxylation of a keto acid. For the hydroxylation... 

The chemical reactivity most associated with dioxiranes is the electrophilic transfer of oxygen to electron-rich substrates (e.g., epoxidation, N-oxidation) as well as oxygen insertion reactions into unactivated C-H bonds. The reactivity-selectivity relationships among these types of reactions has been examined in depth by Curci. The reaction kinetics are dependent upon a variety of factors, including electron-donor power of the substrate, electrophilicity of the dioxirane, and steric influences (95PAC811]. 

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