C-H activation transition state

An investigation of the C—H activation transition states for electron-deficient aromatic substrates revealed that the transition state (T.S. ) for meta-C—H bond activation has a lower energy than the corresponding transition states for activation of the ortho- (T.S. ) andpara-C—H bonds (T.S. ). The higher energy of T.S., (vs. T.S., and T.S. ) is manifested in the catalytic... 

FIGURE 25.11 Computed reaction profile (kcal/mol) for the cyclometalation of dmba-H by Pd/OAc) via a 6-membered C—H activation transition state [20]. 

FIGURE 25.18 Computed C—H activation transition states along Pathways I-III described in Figure 25.17. Non participating H atoms are omitted for clarity and distances in A [29] Adapted with permission from Ref. [29]. Copyright (2006) American Chemical Society. 

Table 5 Transition State Geometries RHF-Optimized Bond Lengths and Bond Angles for C—H Activation Transition States"... 

Figure 1.2 C-H activation transition states derived from 4, involving internal (TS4A) and external (TS4B) deprotonation. Computed free energies in kcal mol" and key distances in...

Figure 1.9 Computed C-H activation transition state for C(sp )-H bond activation of oxazolines (R = = Bu, 25a R = Et, R = Pr,...

Free energies (at 413 K) for the enantio-determining C-H activation transition states leading to products ds-/trans-28 and 29 are indicated in kcalmoh [31a]. 

Figure 1.22 C-H activation transition state for cyclometalation of 2-phenylpyridine at c s-Ru(CI)2(IMe) in the presence of bicarbonate base [60].

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. 

Figure 1.39 Gp 9-catalyzed amination of A/-tert-butylbenzamide with organic azides, aiong with the structure of the AMLA-4 C-H activation transition state, TS60, with key distances in A.

Polar (dimethylacetamide (DMAC), DMF, NMP, THF) and nonpolar (toluene) solvents are suitable for these reactions and should be selected according to the polymer solubility. DMAc is not a suitable solvent for polymerization and always gives brown soluble material. Addition of carboxylic acid may be beneficial in nonpolar solvents owing to the high polarity of the C-H bond transition states. However, C-H bond activations have been accomplished in toluene and xylenes without carboxylic acids. ... 

Rh(TMP)- under these conditions, and in fact the selective activation of methane in benzene solution is a distinctive and unusual feature of this system, given that aryl C—H activation ought to be thermodynamically favored over alkyl C—H activation. The proposed linear transition state proposed in Fig. 8 is the key to this different reactivity. The corresponding trimolecular transition state for an arene would be expected to be bent, and this would be precluded by the bulky TMP... 

The extent to which the radicals react according to Eqs. 6 or 7 depends on the nature of Ri, Ra, and R3. If Ri = Rj = H and R3 = H through NO2, the ratio (6) (7) > 20. The addition reactions observed with these systems are characterized by strongly negative activation entropies, which can be rationalized in terms of immobilization of water molecules by the positive charge at C in the transition state [15]. That the transition state for addition has pronounced electron-transfer character concluded from the fact [15] that the rate constants for addition depend on the reduction potential of the nitrobenzene in a way describable by the Marcus relation for outer-sphere electron transfer. 

In systems where steric interference is not a factor, C-H insertion at a methylene site is strongly preferred over that at methyl sites. A striking example of this effect is the reaction with 4-ethyltoluene (Eq. 24) [137]. The only C-H activation product formed is 202, derived from C-H insertion at the methylene site. A Hammett study on the benzylic C-H activation indicated that the transition-state build-up of positive charge at the benzylic position is stabilized by resonance. 

The cyclization of tetrahydropyran 12 to 5-methyl-8-oxabicyclo[3.2.1]octan-6-one (13) is a particularly striking example of C-H activation by an adjacent heteroatom59. Although the ring size would be the same, and conformational differences in the transition states leading to both products should be minimal, none of the alternative bicyclic ketone 14 was observed. 

Enthalpies of activation, transition-state geometries, and primary semi-classical (without tunneling) kinetic isotope effects (KIEs) have been calculated for 11 bimolecu-lar identity proton-transfer reactions, four intramolecular proton transfers, four nonidentity proton-transfer reactions, 11 identity hydride transfers, and two 1,2-intramole-cular hydride shifts at the HF/6-311+G, MP2/6-311+G, and B3LYP/6-311+-1-G levels.134 It has been found that the KIEs are systematically smaller for hydride transfers than for proton transfers. The differences between proton and hydride transfers have been rationalized by modeling the central C H- C- unit of a proton-transfer transition state as a four-electron, three-centre (4-e 3-c) system and the same unit of a hydride-transfer transition state as a 2-e 3-c system. 

Ye S, Neese F. Nonheme oxo-iron(IY) intermediates form an oxyl radical upon approaching the C-H bond activation transition state. Proc Natl Acad Sci USA. 2011 108 1228-33. 

Other reactions. iV-BOC-protected azocane 281 was reacted with aryl diazoacetate 282 in the presence of 1 mol% of dirhodium tetraprolinate catalyst to produce a very efficient C-H insertion with a high diastereoselectivity and enantioselectivity (Scheme 118 <2003JA6462>). The erythro C-H insertion product 283a was formed in 90% ee and 90% de, which infers that the flexibility of the eight-member ring sterically favors the accommodation of the transition state for the C-H activation. 

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