Iridium-Catalyzed Carbon–Carbon σ-Bond Hydrogenation with Water: Rate Enhancement with Iridium Hydride

Abstract

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...

User-friendly aerobic reductive alkylation of iridium(iii) porphyrin chloride with potassium hydroxide: scope and mechanism

“…[8][9][10] We have previously reported the selective carbonyl carbon and α-carbon bond activation (CCA) of acetophenones with Ir( por)Me 11 and carbon-carbon σ-bond hydrogenation of [2.2]paracyclophane with water catalyzed by Ir(ttp)-alkyls. 12 Therefore, a facile synthesis of iridium porphyrin alkyls for further development of iridium porphyrin-based bond activation is desirable.…”

Base-Promoted C–O Bond Cleavage of Primary Alcohols by Iridium(III) Porphyrin Chloride

“…Mechanistic investigations reveal that the active catalyst is the iridium porphyrin hydride. 2 Hence, developing efficient strategies to access various iridium porphyrin alkyls remain an attractive research field.…”

“…More interestingly, iridium porphyrin alkyls can catalyze the hydrogenolysis of the carbon–carbon σ bond of [2.2]­paracyclophane with H 2 O as the hydrogen source under neutral conditions. Mechanistic investigations reveal that the active catalyst is the iridium porphyrin hydride . Hence, developing efficient strategies to access various iridium porphyrin alkyls remain an attractive research field.…”

Carbon–Carbon σ-Bond Transfer Hydrogenation with DMF Catalyzed by Cobalt Porphyrins

“…Our group has been interested in carbon–carbon bond activation (CCA) of organic substrates and has reported several stoichiometric examples . 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 . 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.…”

“…We suspected that the hydrolysis of Co II (ttp) benzyl intermediates was slow in nonpolar benzene solvent; the more polar solvent DMF to facilitate hydrolysis of intermediates was then examined with 10 mol % catalyst, as in the case of rhodium and iridium analogues . To our delight, 2 was obtained selectively in DMF in a good yield of 81% (Table , entry 2).…”