Iridium porphyrins were found to be good catalysts for the carbon−carbon σ-bond hydrogenation of [2.2]paracyclophane using water in neutral conditions. Mechanistic investigations reveal the promoting effects of iridium porphyrin hydride, Ir III (ttp)H, for efficient hydrogenation in the catalysis. The bimolecular reductive elimination from Ir III (ttp)H and carbon−carbon bond activation intermediates speeds up the hydrogenation process. KEYWORDS: C−C bond hydrogenation, water, iridium porphyrins, iridium hydride, bimolecular reductive elimination C atalytic aliphatic carbon−carbon σ-bond cleavage with transition-metal complexes in homogeneous solution is challenging due to the sterically hindered C−C bond by the surrounding C−H bonds. 1 Chelation assistance is a common strategy employed in homogeneous catalytic carbon−carbon bond activation (CCA). 2 Strain-relief is another method to achieve ring expansion, 3a arylation, 3b and polymerization, 3c accompanied by a CCA process. However, these examples involve functionalized organic substrates. In contrast, examples of CCA in unfunctionalized hydrocarbon substrates are limited. 4 Our group has reported the stoichiometric aliphatic CCA of nitrile (C(α)-C(β)) and TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) (C(Me)-C(ring)) with Rh II (tmp) (tmp = tetramesitylporphyrinato dianion). 5a,b The carbon−carbon bond of c-octane can be cleaved via a Rh II (ttp)-catalyzed 1,2-addition of Rh III (ttp)H (ttp = tetratolylporphyrinato dianion). 5c Recently, we have developed the catalytic carbon−carbon σ-bond hydrogenation of [2.2]paracyclophane (PCP) using water as the convenient hydrogen source in neutral conditions (Scheme 1). 5d Kinetic studies reveal a bimetalloradical CCA of PCP with Rh II (tmp) in the stoichiometric reaction. We envisioned that this CCA process would be more favorable by replacing the rhodium porphyrin complexes with iridium congeners, taking the advantage of forming the stronger Ir−C bond (62 kcal/mol) than the Rh−C bond (56.9 kcal/mol) in the CCA step. 6 However, the chemistry of iridium(II) porphyrins is little explored, with the spin density equilibrating with porphyrin π radical in sterically bulky monomeric iridium(II) porphyrins, reducing its metalloradical character. 7 There are only a few reports on stoichiometric bond activation with iridium(II) porphyrins, which are limited to H−H, C−H and aryl C-X (X = Cl, Br and I) bonds. 8 Stoichiometric C(CO)-C(α) bond activation of ketones by proposed Ir III (ttp)OH intermediate has been documented. 9 Here, we report an iridium porphyrin catalyzed carbon−carbon σ-bond hydrogenation of PCP using water in neutral conditions with nearly quantitative yields and faster rate than the reported rhodium system. Furthermore, Ir III (ttp)H plays an important promoting role in the catalysis. Mechanistic studies reveal the Ir III (ttp)H undergoes rapid bimolecular reductive elimination with the CCA product Ir III (ttp)R to give the hydrogenation product in accounting the faster hydrogenation.Initially, we explored the ca...

Cobalt porphyrins were found to catalyze the transfer hydrogenation of the carbon−carbon σ bond of [2.2]paracyclophane (PCP) with the solvent DMF serving as the hydrogenating agent. Successful trapping experiments with benzene solvent and the kinetic isotope effect (4.9) suggested the presence of benzyl radical intermediates in undergoing hydrogen atom transfer from DMF as the rate-limiting step. The rate law was established by initial rate measurements to be rate = k obs [Co II (ttp)]- [PCP].C atalytic carbon−carbon bond activation (CCCA) is the key chemical transformation in hydrocracking, turning crude oil into petroleum. 1 CCCA holds the potential to convert heavy polymeric residues and biomass into lighter, economically valuable chemicals. 2 Despite the usefulness of CCCA, examples with transition-metal complexes in homogeneous media remain limited. 3 The small number of literature reports on CCCA reflects the inertness of the C−C σ bond relative to the C−H bond. 4 CCCA with transition-metal complexes in homogeneous media mainly employs strategies such as chelation assistance, 5 ring strain relief, 6 and carbonyl functionality 7 to generate organometallic intermediates, followed by subsequent rearrangement of the carbon skeleton 8 or M−C σ-bond hydrogenation with H 2 9 to complete the catalytic cycle. Our group has been interested in carbon−carbon bond activation (CCA) of organic substrates and has reported several stoichiometric examples. 10 Recently, we have developed rhodium and iridium metalloporphyrin (M(por), M = Rh, Ir) catalyzed C−C σ-bond hydrogenation of [2.2]paracyclophane (1) with water as the hydrogenating agent. 11 In light of these successes, we wish to extend the catalysis to a much less reactive but more easily accessible and cheaper cobalt porphyrin catalyst. Co(II) porphyrin is expected to have a lower reactivity than the corresponding rhodium and iridium porphyrin analogues since (1) Co−C bonds are generally weaker 12 and (2) Co II (por) metalloporphyrin radical has a lower SOMO energy level. 13 As a result of low reactivity, CCA by cobalt complexes remains scarce in the literature. 14

Primary alcohols were successfully utilized as the alkylating reagents to conveniently access rhodium porphyrin alkyl complexes in up to 91% yields under basic conditions. Mechanistic investigations suggest two possible pathways for the C–O bond cleavage: (1) nucleophilic substitution with rhodium­(I) porphyrin anion and (2) a borrowing hydrogen pathway via rhodium­(III) porphyrin hydride.

Rhodium porphyrin catalyzed hydrogenation of the aliphatic carbon-carbon σ-bond of [2.2]paracyclophane with water has been examined with a variety of tetraarylporphyrins and axial ligands. Mechanistic investigations show that Rh(ttp)H, which can be derived from the reaction of [Rh(ttp)] with water without a sacrificial reductant, plays an important role in promoting bimetallic reductive elimination to give the hydrogenation product.