C-H bond cleavage reaction

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. 

Ethylene and Acetylene. On nickel (111), both ethylene and acetylene are irreversibly chemisorbed neither can be thermally desorbed. We also find that trimethylphosphine cannot displace ethylene or acetylene from these surfaces. There have been suggestions that ethylene and acetylene are not present on the surface as molecules but as molecular fragments. Many ultra-high vacuum studies of ethylene and of acetylene chemisorption on nickel crystal planes have been reported. Most of these studies seem to implicate states in which C-H bond cleavage reactions have accompanied the basic chemisorption process (19). 

This section details selected catalytic C-H bond cleavage reactions, in addition to strategies for cleaving sp and sp C-H bonds. The activation of polar C-H bonds is also described. 

Various vinylsilanes, olefins or acetylenes insert into the ortho C-H bond of aromatic ketones in the presence of catalytic amount of ruthenium complexes in high yields [21,22], The C-H bond cleavage reaction of aromatic ketones also involves orthometallation which is promoted by prerequisite coordination of the carbonyl group to ruthenium (Scheme 14.9) [21], This type of reaction has a wide generality for aromatic and alkenyl ketones with a variety of alkenes. 

On the other hand, the sp C-H bond is much less reactive, and the catalytic cleavage reaction is stdl uncommon [23,24], As a rare example, the sp C-H bond in 2,6-dimethylbenzoisocyanide takes place by RuH(naphthyl)(DMPE)2 to give 7-methylindole, though the efficiency is poor (TON = 3.5) (Scheme 14.10) [23]. In this reaction, isocyanide coordinates to the coordinatively unsaturated ruthenium(O) center, and the C-H bond cleavage reaction takes place. 

Key Strategies for C-H Bond Cleavage Reactions 14.2.2.1 Coordinative Unsaturation... 

By virtue of the intramolecular anchoring bonding, sp C-H bond activation may become possible. Treatment of Ru(l,5-COD)(l,3,5-COT) with (ortho)-substituted phenols resulted in the successive O-H and sp C-H bond-cleavage reactions, giving... 

Jones and co-workers used this scheme to study C-C versus C-H bond cleavage reactions involving Ni(0) complex 2, which is reminiscent of the chemistry involved with hydrocyanation of 1,3-butadiene.126 At room temperature, compounds 3 and 4 form more rapidly than 5. Over time, especially at temperatures above 25 °C, compound 5 disappears from the reaction mixture and compounds 3 and 4 are the only products. When BPh3 was added to complex 2, compound 6 formed exclusively and rapidly, even at -30 °C. There was no formation of complexes 3 and 4, which would involve C-H activation, indicating that in the presence of a Lewis acid, C-C activation is faster than OA of a C-H bond. 

The activation parameters (TMP A// +7kcalmoH A5 —39 cal. K-. mor TXP A// +17kcalmor A5 -25cal.K .mor, the rate law (rate = fc[(por)Rh ] [CH4]) and large kinetic isotope effects (fcn/fe >8) derived from kinetic studies of the reactions in Fig. 58 (166, 167), are consistent with the proposed concerted four-centered pathway with a near linear transition state in Fig. 59(a). Similar kinetic smdies suggest that toluene and methane C—H bond cleavage reactions (and also H—H bond reactions of H2) proceed via related mechanisms (166, 168, 169). 

It is probably true that all reactions studied involve electron abstraction or hydrogen abstraction. However, formally, a wide variety of organic oxidation reactions are represented O—H, N—H, S—H, and C—H bond cleavage (reactions (6), (7), (8), and (10)), oxidative coupling (reactions (5), (9), and (10)), and electron abstraction (reactions (l)-(4)). 

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