Alkane C—H bonds

The IR spectrum of an alkane is fairly uninformative because no functional groups are present and all absorptions are due to C-H and C-C bonds. Alkane C-H bonds show a strong absorption from 2850 to 2960 cm-1, and saturated C—C bonds show a number of bands in the 800 to 1300 cm-1 range. 

Thus, the initial result of the stoichiometric boiylation of arenes was discovered serendipitously. However, the development of this initial observation into the catalytic borylation of alkane C-H bonds was largely based on the design of complexes for the stoichiometric functionalization of alkanes and then the catalytic functionalization of alkanes. 

In contrast to the borylation of alkane C-H bonds, the coupling of aryl halides with amines was based on a literature precedent from another group published about a decade before our initial studies. Kosugi, Kameyama and Migita published the coupling of aryl halides with tin amides." Mechanistic studies we conducted on this process led us to the perhaps obvious realization that the reaction" could be conducted with amines and a silylamide base instead of tin amides (equation 4)." Surveys of bases with similar p a values led Janis Louie to conduct reactions with alkoxide bases. Similar studies were conducted at nearly the same time by Steve Buchwald and coworkers."... 

For transition-metal catalyzed hydroxylation of alkane C-H bonds, the reactions of alkanes with platinum(II) complexes were the most successful. In an aqueous solution of hexachloroplatinic acid and Na2PtCl4, alkanes were converted into a mixture of isomeric alkyl chlorides, alcohols, and ketones, and the platinum(IV) is reduced to platinum(II).7 The kinetics of the reaction with methane as the alkane have been described in detail.8... 

In addition to metal catalyzed oxygenation of nonactivated alkane C-H bonds, oxofunctionalization of C-H bonds can also occur in water by using dioxiranes.20 Alkylketones and alkylketoesters could be regioselectively oxidized at the 5-position of the aliphatic chain by dioxiranes generated in situ by oxone in a mixture of H20/MeCN... 

Although the activation and functionalization of C-H bonds of alkanes are the important, promising routes for synthesis of functionalized materials, it is difficult to achieve the functionalization of alkanes because they are unreactive due to the low reactivity of alkane C-H bonds. Carboxylation of alkanes to carboxylic acids is one of the interesting and important functionalization processes. 

The oxidative addition of alkane C-H bonds to Pt(II) has also been observed in these TpRa -based platinum systems. As shown in Scheme 19, methide abstraction from the anionic Pt(II) complex (K2-TpMe2)PtMe2 by the Lewis acid B(C6F5)3 resulted in C-H oxidative addition of the hydrocarbon solvent (88). When this was done in pentane solution, the pentyl(hydrido)platinum(IV) complex E (R = pentyl) was observed as a... 

Many related complexes of iridium and rhodium undergo the oxidative addition reaction of alkanes and arenes [1]. Alkane C-H bond oxidative addition and the reverse reaction is supposed to proceed via the intermediacy of c-alkane metal complexes [4], which might involve several bonding modes, as shown in Figure 19.5 (for an arene the favoured bonding mode is r 2 via the K-electrons). 

For many years the activation of unfunctionalized alkanes has been the Holy Grail of organic synthesis and, indeed, it has only been during the past few years that catalysts have evolved which allow an alkane C—H bond to be selectively... 

The enhanced propensity of dioxiranes to insert oxygen into unactivated alkane C—H bonds was ascribed initially to the high ring strain energy (SE) of dioxiranes that has sometimes exceeded 30 kcalmol" . Since the SE of these three-membered peroxides has recently been substantially reduced (SE = ca 11-18 kcalmoU ), a different explanation is required . 

The benzylic C-H bond is weaker and more reactive than primary alkane C-H bonds because of the stabilization of benzylic radicals (see Table 4-6 and Exercises 6-11 and 14-6). 

The first activation of an alkane C-H bond was described in 1969 [29]. Three decades were to pass until the development of the current catalytic procedures for dehydrogenation and C-O, C-C, and C-B bond-forming reactions. Progress has been slow. Nevertheless, significant advances in catalyst research were achieved in the 1990s, aided by the development of improved metal ligands and the increased understanding of the mechanism of transition metal-catalyzed C-H activation reactions. Further improvements of catalytic cycles are nec-... 

Equation (1) depicts an early example of an intermolecular addition of an alkane C-H bond to a low valent transition metal complex [12], Mechanistic investigations provided strong evidence that these reactions occur via concerted oxidative addition wherein the metal activates the C-H bond directly by formation of the dative bond, followed by formation of an alkylmetal hydride as the product (Boxl). Considering the overall low reactivity of alkanes, transition metals were able to make the C-H bonds more reactive or activate them via a new process. Many in the modern organometallic community equated C-H bond activation with the concerted oxidative addition mechanism [10b,c]. 

When employing higher alkanes as a feedstock, process economics are often determined more by selectivity than by conversion per single pass. Most of these reactions are therefore conducted in the liquid phase, which enables easier control of reaction rates and also of side and consecutive reactions by adjustment of the temperature. However, activating alkane C-H bonds always requires reactions... 

The fact that the final product 3-Tp Rh(CO)(H)(R) does not appear on the ultrafast time scale (<1 ns,) (Fig. 4) indicates a free energy barrier greater than 5.2 kcal/mol for the alkane C-H bond activation. Nanosecond step-scan FTIR experiments on the 3-Tp Rh(CO)2/cyclohexane system show that the remnant of the 2-Tp Rh(CO)(S) peak persists for 280 ns after photoexcitation, while the product CO stretch at 2032 cm-1 rises with a... 

Figure 8 A proposed reaction mechanism for the alkane C-H bond activation by j3-Tp Rh(CO)2 covering the ultrafast dynamics to nanosecond kinetics. 

The above discussion demonstrates that the activation barrier for the silane Si-H bond is relatively small compared to that for an alkane C-H bond. This is surprising in that one might have expected comparable energy barriers for activation of both the Si-H and C-H bonds based on the similar enthalpy of activation AH from macroscopic kinetic measurements (41-44). Clearly other mechanisms are at work to make up the energy barrier in the case of Si-H bond activation. To investigate if more intermediates are involved, which may provide an explanation for the reported apparent AH values, the reaction is followed extending into the nano- and microsecond regime. The experiments show that only the previously discussed ethyl-solvate appears on these time-resolved IR spectra. Its decay correlates very well with the product rise as displayed in Fig. 15. 

Lian T, Bromberg SE, Yang H, Proulx G, Bergman RG, Harris CB. Femtosecond IR studies of alkane C-H bond activation by organometallic compounds — direct observation of reactive intermediates in room temperature solutions. J Am Chem Soc 1996 118(15) 3769—3770. 

Asbury JB, Ghosh HN, Yeston JS, Bergman RG, Lian TQ. Sub-picosecond IR study of the reactive intermediate in an alkane C-H bond activation reaction by CpRh(CO)2. Organometallics 1998 17(16) 3417-3419. 

You may be confused the first time you see the IR spectrum of a terminal alkyne, R-OC-H, because you will see a strongish sharp peak at around 3300 cm"1 that looks just like an N-H stretch. The displacement of this peak from the usual C-H stretch at about 3000 cm"1 cannot be due to a change in the reduced mass and must be due to a marked increase in bond strength. The alkyne C-H bond is shorter and stronger than alkane C-H bonds. 

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