Selective Addition to Iridium of Aryl C−H Bonds Ortho to Coordinating Groups. Not Chelation-Assisted

Abstract

Precursors of the pincer-ligated iridium species, (PCP)Ir, react with nitrobenzene or acetophenone at ambient temperature to give O,C-chelated complexes resulting from addition of an aryl C-H bond and coordination of a nitro or acetyl oxygen. The C-H additions appear to be completely regioselective for the position ortho to the functional group; however, structural characterization and low-temperature NMR studies demonstrate that the reaction does not proceed via coordination of the functional group followed by C-H addition. In the case of nitrobenzene, kinetic preference for the para and meta positions is demonstrated at low temperature. Addition occurs more slowly at the ortho position, without assistance by the functional group; the ortho-C-H addition product is then trapped by chelation.

Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment

“…The two complexes are Ͻ20 kcal⅐mol Ϫ1 higher, with CH1 being preferred by 1.6 kcal⅐mol Ϫ1 . The TSs for the oxidative addition to the methyl C-H bond and the [1,2] shift are very similar in energy, with the former being slightly more favorable. The TS for oxidative addition to the methylene C-H bond is Ͼ4 kcal⅐mol Ϫ1 higher than that for the methyl C-H bond.…”

“…When coordination at the methyl carbon occurs, the most likely process is the oxidative addition to give the propyl hydride. However the [1,2] shift has a similar energy barrier, but, the passage from methyl coordination to methylene coordination being endothermic, this equilibrium is strongly shifted toward coordination at the methyl group (14:1). The critical point is that coordination at the secondary carbon leads preferentially to the [2,1] shift and not to the oxidative addition at the methylene C-H bond to yield the isopropyl-hydride complex.…”

“…These geometrical features of these propane complexes are in agreement with those calculated for TpRh(CO)(CH 4 ) (26). The TS for the methylene-methyl [1,2] shift (or [2,1] shift), TS12, also has a d 8 square planar rhodium with one weakly coordinated pyrazole ring (3.04 Å). The two H involved in the exchange are at similar distances from Rh, with the methyl hydrogen being slightly closer (2.285 Å) than the methylene hydrogen (2.320 Å).…”

“…The energy profile associated with the [1,2] shift and the oxidation addition at the primary and secondary carbon is given in Table 3 in terms of different types of energies (electronic energy, E, electronic energy corrected with zero-point energy, E ϩ ZPE, and Gibbs free energy value, G). Whichever type of energy is considered, the trends are similar and in very good agreement with the experimental results, except for the dissociation process (vide infra), and we will only discuss E values here.…”

“…In the past decade, useful processes have been developed that employ alkane activation via oxidative addition; significant examples include alkane dehydrogenation (1)(2)(3)(4), alkane and arene borylation (5)(6)(7)(8)(9), and alkane metathesis (10). In all of these developments, selectivity in activation has been a critical issue, and rhodium and iridium transition metal complexes have been found to show a strong preference for the activation of the methyl C-H bonds in alkanes, despite the presence of weaker methylene C-H bonds.…”