C-H activation of benzene

A related base-promoted C-H activation of benzene by Pt(II) was recently reported. With the tridentate monoanionic amido pineer ligand N3 , the triflate complex N3 Pt(OTf), depicted in Scheme 8 was shown to activate benzene in the presence of base (35). It was noted that the chloro complex N3 PtCl was not reactive under these conditions. The activity of the triflate complex again appears to result from the higher lability of triflate which can allow for coordination of the hydrocarbon. 

Decarbonylation of a-acyliron complexes by photolysis (Section 3.5) is the primary method for the preparation of a-aryliron complexes. Alternatively, cr-aryliron complexes can be prepared by the method of Beletskaya from FpNa (5). For example, the /7-tolyl analog has been recently prepared in order to study the C H activation of benzene (equation 3). ... 

An alternative to the coupling of olefins with aryl halides is C-H activation of benzenes and addition to alkynes. A Pd" catalyst was shown to catalyze this reaction in an acidic medium such as CF3COOH, even at room temperature (eq. (17)) [83],... 

In 1983, Watson reported that Lu(III) and Y(III) methyl complexes mediate C—H activation of benzene and methane.59 For benzene activation, Cp 2Lu(Me), which is in equilibrium with the species Cp 2 Lu (,u - Me)Lu (Me)Cp 2, reacts to release methane and produce Cp 2Lu(Ph) (Equation 11.7). Kinetic studies revealed a rate equation — d[Lu-Me] = (ki + fc2[C6H6])[Lu-Me] that is consistent with two... 

The reaction chemistry of the hydridovinyl complexes has, however, often proven unexpectedly complicated, owing to their propensity to effect intra and zfzf rmolecular C-H activations. For instance, under an atmosphere of ethylene, benzene solutions of 202 undergo a thermal (60 °C) reaction to afford exclusively 210. This reaction is presumed to involve the intermediacy of Tp Ir(C6H5)2 (f/ -C2H4), an unobserved product of the C—H activation of benzene. This proposal is supported by the fact that Tp Ir(C6H5)2(N2) (211) can be isolated (Section II-C.2), and converts cleanly to 210 under comparable conditions. 

Scheme 22 Proposed mechanisms for the oxidatively induced C-H activation of benzene...

A subsequent study by Goddard and Periana [31] in the intermolecular C—H activation of benzene and methane by [Ir(acac )2(X)] complexes (X = CH3COO and OH) also facilitates an easy bond activation process and demonstrates a similar ambiphilic bonding character. An acetate-assisted deprotonation via a 6-membered transition state was computed as the lowest-energy process in both cases. This underlines again the high potential of such ambiphilic character for an easier C—H activation process. 

While we concentrated mainly on the catalytic applications of ruthenium PNP and PNN pincer complexes described above, we have also established the metal-ligand cooperation in C-H and H-H activation reactions by Ir-PNP pincer complexes. The Ir-PNP pincer complex 27, which was formed by C-H activation of benzene by a dearomatized Ir(I) complex, reacts with H2 to provide a trans-dihydride complex exclusively. Use of D2 revealed, surprisingly, formation of D-Ir-H in 28, while one D atom is attached to pyridinylmethylenic carbon (Scheme 9). Further studies... 

The first computational study of heteroatom-assisted intermolecular C-H activation was reported in 2000 by Sakaki and coworkers, who studied the C-H activation of benzene and methane at M(02CH)2 (M = Pd, Pt) at the MP4(SDQ) level [32]. For benzene, this involved a two-step process with k -k displacement... 

Figure 1.11 Computed reaction profile for C-H activation of benzene at PdfK -COjHjj MP4(SDQ) energies are in kcalmoh and selected distances in A [32].

This and subsequent papers by Gorelsky [37] employed the activation strain model [38] to analyze trends in the CMD activation barriers (see Figure 1.14 for C-H activation of benzene). In this approach, the activation energy, for... 

Figure 1.14 The activation strain model illustrated for C-H activation of benzene at Pd(Ph)(K -OAc)(PMej) with component energies indicated in kcal mol [37a].

The speciation of the Pd(OAc)2 catalyst in such reactions is a key point. For example, Hartwig and Tan have shown that the Pd(OAc)2-catalyzed direct aryla-tion of 2-bromotoluenes and benzene proceeds more efficiently in DMA solvent in the absence of added phosphine ligands [54]. A preformed dianionic dimer, [ArPdBr2]2 , was also active. B3LYP calculations showed that C-H activation of benzene at Pd(Ph)(K -OPiv)(DMA) has a computed free energy barrier of 31 kcal mol" compared to a barrier of 42 kcal mol at Pd(Ph)(K -OPiv)(P Bu3). 

Ess, Periana, Goddard, and coworkers subsequently compared four-membered and six-membered C-H activation transition states in a B3LYP study on the C-H activation of benzene at Ir(acac)2(X) species (X = OH, OAc) [66]. An activation strain model was used to compare the performance of hydroxide and acetate, where the latter could access both four-membered and six-membered transition states, depending on whether the proton transfers onto the Ir-bound or pendant oxygen, respectively. C-H activation via a six-membered process is favored and is associated with a lower term compared to the more constrained four-membered transition states. The acetate and hydroxide four-membered transition states are very similar. 

More recently, further computational studies of the C-H activation of benzene at Ir [71] and Rh [72] have appeared, while Jones and coworkers compared aryl C(sp )-H bond activation and alkyl C(sp )-H bond activation... 

Scheme 15 C—H activation of benzene, leading to the formation of (Cp AI)3Ni HAIPh...

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