Alkane C-H activation

J.-Y. Saillard, Theoretical Aspects of Alkane C-H Activation by Organometallics, in Selective Hydrocarbon Activation (J. A. Davies, P. L. Watson, J. F. Liebman, A. Greenberg, Edits.), p. 207 ff, VCH Publishers, New York 1990. 

Another strategy for developing active and stable oxidation catalysts that operate via the alkane C-H activation reaction is to use metals that can be readily dis-... 

Bergman and co-workers used IR laser-based TRIR spectroscopy to investigate alkane C—H activation with ( 7 -C5Me5)Rh(CO)2 in liquid Kr and liquid Xe in the presence of cyclohexane 42-44) and neopentane 45). This permitted the detection of the noble gas intermediates. 

Detailed theoretical work has shown that alkane C-H activation by the 3Pi excited state Hg proceeds by initial formation of a weak [Hg (alkane)] exciplex, followed by C-H cleavage via a mechanism described as a mixture of oxidative addition and H atom abstraction. While alkanes react exclusively via C-H bond activation, sensitization of monosubstituted arenes ArCH2 -X (Ar = aryl X = H, alkyl, OR, NR2) occurs with substantial C-X bond cleavage (equation 25). 

One of the earliest reports of alkane C-H activation was made by Shilov in 1969 in which H/D exchange was reported between methane and a D20/CH3C00D solvent in the presence of K2PtCl4 [57]. While the mechanistic details of this exchange were not entirely clear, the work stood as an isolated example of alkane activation for many years. Alkane activation by platinum was not reported again until 1986, when Whitesides found that (Cy2PCH2CH2PCy2)Pt(neopentyl)H lost neopentane and activated a variety of alkanes at 50°C (Eq. 12) [58,59]. These reactions are believed to proceed by way of an initial reductive elimination to... 

A. Company, J. Lloret, L. G6mez, M. Costas, in Alkane C-H Activation by Single-Site Metal Catalysis (ed. P. J. Perez), Springer, Dordrecht, 2012, Chapter 5. 

Control of reactivity by catalysis provides the capability to shift to lower cost feedstocks. In the twentieth century, advances in catalysis have allowed the substitution of acetylene with olefins and subsequently with synthesis gas as primary feedstocks. For example, production of acrylic acid, traditionally produced by the Reppe process from acetylene and CO, has now been replaced by catalytic oxidation of propylene. The emergence of paraffins, the hydrocarbon feedstock of the future, will depend on development of catalysts for selective alkane C-H activation (2). 

Hydrocarbon C-H bond activation with [Pt(Tpx)] complexes has been widely investigated by Templeton s group.186 It has been hypothesized that the first step in alkane C-H activation by platinum ) scorpionates involves coordination of the alkane to a coordinatively unsaturated metal center to form a a-bound alkane intermediate Pt(R-H) which is thought to interconvert rapidly with a PtIV(R)(H) oxidative addition product (Fig. 8.19). [PtR(H)2(Tpx)J species have been isolated and structurally characterized. When... 

Four-coordinate Rh(I) hydroxide and related complexes have been demonstrated to initiate aromatic C—H activation (Equation (11.13)).97,100 In contrast to the proposed mechanisms for the Ru(II) and Ir(III) reactions, mechanistic studies for the Rh(I) systems suggest the possibility of an initial exchange between RO (via Rh—OR heterolytic cleavage), coordination of the substrate undergoing C—H activation, and deprotonation by free RO (Scheme 11.47). Alkane C—H activation by late transition metal complexes via 1,2-CH addition across metal-heteroatom bonds has yet to be demonstrated. 

Although we have focused on the photochemistry of Rh(Cp )(CO)2 (Cp =Cp or Cp ) so far, the most complete understanding of C-H reactions came in parallel TRIR studies on Rh(Tp )(CO>2 (Tp = HB(3,5 -Me2pyrazol-l-yl)3, and extensive reviews cover this work." We choose this example since this investigation built upon matrix-isolation and liquid Xe studies. A combination of ultrafast TRIR and ns-step-scan FTIR spectroscopy was used to unravel the mechanism of alkane C-H activation by Rh(Tp An alkane complex was formed within the earliest... 

In 2000, Periana reported an Ir(iil)(acac)3 complex that could activate benzene and catalyze anti-Markovnikov additions of olefins to the aryl C-H bond. An iridium-phenyl complex was shown to be an intermediate. This complex was converted to an Ir(lll) derivative containing an iridium-methyl bond, a species which underwent alkane C-H activation (Equation (18)). ... 

Perez PJ (2013) Alkane C-H activation by single-site metal catalysis, vol 38. Springer, The Netherlands... 

The alkane C-H activation reactivities of the ferryl complexes with the two isomeric pentadentate bispidines and were compared with other extensively studied pentadentate amine/pyridine ligands [20a]. The interesting observation is that, in contrast to the sulfoxidation reaction (see Section 6.6.3), there is no correlation of the redox potentials (H -coupled electron... 

Alkane C-H activation by a cationic methyl-lr " complex oxidative addition (left side) rather than a-bond metathesis (right side). Related Si-H activation (by ortho-metallation) below goes to stable Ir. ... 

The active species resulting from O2 activation is an Fe =0 or Fe -0" complex, and the intimate mechanism that has been proposed for alkane C-H activation is shown below ... 

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