Intramolecular C-H activation

The bidentate ligand (dmpe) is believed to act in a hemilabile fashion, enabling a crucial intramolecular C-H activation to take place (Equation (37)). 

Rhodium(n) carboxamidates are clearly superior to all other types of catalysts in effecting highly chemo-, regio-, diastereo-, and enantioselective intramolecular C-H activation reactions of carbenoids derived from diazoacetates. Specifically, Rh2(4Y-MPPIM)4 is the catalyst of choice for C-H activation reactions of simple primary and secondary alkyl diazoacetates. Likewise, Rh2(4Y-MACIM)4 thus far has been the most successful catalyst with tertiary alkyl diazoacetates, whereas for primary acceptor-substituted diazoacetates with a pendant olefin side chain, Rh2(4A-MEOX)4 has proved to be highly selective. 

Perhaps, many of the intramolecular C-H activations that are known for metal ligand complexes are actually best described as electrophilic substitutions rather than oxidative additions. A very large number of palladium(II) complexes are known to react intramolecularly to the metallated species. 

Scheme 6.10 Reversible intramolecular C—H activation in an lr(l)-amine complex.

The formation of a carbene from (18) in the coordination sphere of (PCy3)2RhCl has been studied experimentally and computationally.24 Evidence for an intramolecular C-H activation is provided by isotope labelling and reaction rate studies. These results have been further confirmed by DFT calculations. 

Basuli, F., Bailey, B.C., Huffman, J.C. and Mindiola, D.J., (2005) Intramolecular C-H activation reactions derived from a terminal titanium neopentylidene functionality. Redox-controlled 1,2-addition and a-hydrogen abstraction reactions. Organometallics 24, 3321. 

Pfeffer, M. Selected applications to organic synthesis of intramolecular C—H activation... 

A plausible mechanism for the one-pot synthesis ofcarbazoles is shown in Scheme 5. It consists of two interlinked catalytic cycles. In the first cycle a classical Buchwald-Hartwig amination reaction occurs to generate an intermediate 5 which then enters the second cycle by oxidative addition to Pd(0). The resulting Pd(II) complex then undergoes intramolecular C-H activation to give a six-membered palladacycle which subsequently yields the carbazole by reductive elimination. 

Scheme 3. Intramolecular C-H activation toward mono- and di-o-alkylation of palladacycles and norbornene deinsertion...

Attempts to prepare titanaindanes and titanaindenes from thermolysis of 28a or 28b in the presence of alkenes and alkynes failed only the cyclometalated complexes 30a and 30b from intramolecular C-H activation in 29 were observed [Eq. (7)].42 A similar reaction has been observed for substituted Cp2Zr-benzyne complexes.2... 

The intramolecular C—H activation of ethylene in complexes of the type CpIr(L)(QH4) is induced by uv irradiation and is favored over reaction with the solvent the process evidently involves a cage complex intermediate.118... 

An interesting observation was made by Morris et al. [158,159] who reported that the electron-rich system [Ru(IBu )(PPh3)3H] showed no inclination for intramolecular C-H activation. It seems that the same electronic preferences are working for reductive elimination and C-X activation. 

Also developed by Hill is a photochemical system (equations 41 to 48) based on a polyoxoacid, H3PW12O40 (P)- The excited state of the acid probably oxidizes the alkane in the first step. The radical can then either attack the solvent to give an iminium radical, which leads to ketone on hydrolysis, or it can be oxidized to the carbonium ion, in which case attack on the solvent leads instead to the iV-alkyl-acetamide. If the substrate has two adjacent tertiary C—bonds, then alkenes tend to be formed, llie Barton reaction, normally known as an intramolecular C—H activation, can give some intermolecular reaction in some examples. Thus, when n-octyl nitrite is photolyzed in heptane, some nitrosoheptane is observed. ... 

---