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

Abstract

Alkylation of iridium 5,10,15,20-tetrakistolylporphyrinato carbonyl chloride, Ir(ttp)Cl(CO) (1), with 1°, 2° alkyl halides was achieved to give (ttp)Ir-alkyls in good yields under air and water compatible conditions by utilizing KOH as the cheap reducing agent. The reaction rate followed the order: RCl < RBr < RI (R = alkyl), and suggests an SN2 pathway by [Ir(I)(ttp)](-). Ir(ttp)-adamantyl was obtained under N2 when 1-bromoadamantane was utilized, which could only undergo bromine atom transfer pathway. Mechanistic investigations reveal a substrate dependent pathway of SN2 or halogen atom transfer.

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

“…Chemical shifts were referenced with the residual solvent protons in CDCl 3 (δ 7.26 ppm) and C 6 D 6 (δ 7.15 ppm). All Ir­(ttp)-alkyl compounds are known compounds, and their characterization data are in accordance with those previously reported. ,,, Unless otherwise specified, the coupling constant J refers to H–H coupling.…”

“…Ir­(ttp)­Cl­(CO) ( 1a ; 11.6 mg, 0.0125 mmol), KOH (7.0 mg, 0.125 mmol) and n -octanol ( 2k ; 162.8 mg, 1.25 mmol) were added into benzonitrile (1.5 mL) in a Teflon screw-capped tube, degassed for three freeze–pump–thaw cycles, refilled with N 2 , and heated at 150 °C for 24 h. After the excess alcohol and benzonitrile were removed by vacuum distillation, the solid mixture was then extracted with CH 2 Cl 2 and filtered through a short pipet column chromatograph on silica gel with CH 2 Cl 2 to give the red crude product. The crude product was further purified by a precoated silica gel TLC plate with CH 2 Cl 2 /hexane (1/1) to give Ir­(ttp)­( n -octyl) ( 3k ; 8.5 mg, 0.0087 mmol) in 70% yield. 1 H NMR (CDCl 3 , 400 MHz): δ −5.54 (t, 2 H, J = 8.4 Hz), −4.44 (quint, 2 H, J = 7.8 Hz), −1.51 (quint, 2 H, J = 7.5 Hz), −0.46 (quint, 2 H, J = 7.6 Hz), 0.10 (quint, 2 H, J = 7.6 Hz), 0.55 (quint, 2 H, J = 7.6 Hz), 0.63 (t, 3 H, J = 7.2 Hz), 0.87 (sext, 2 H, J = 7.8 Hz), 2.70 (s, 12 H), 7.53 (t, 8 H, J = 6.9 Hz), 7.98 (d, 4 H, J = 7.6 Hz), 8.05 (d, 4 H, J = 7.6 Hz), 8.51 (s, 8 H).…”

“…All Ir(ttp)alkyl compounds are known compounds, and their characterization data are in accordance with those previously reported. 5,6,11,13 Unless otherwise specified, the coupling constant J refers to H−H coupling.…”

“…5 Recently, we have expanded the scope of alkylating substrates to alkyl halides, including 1°, 2°, and 3°a lkyls, for the preparation of Ir(ttp)-alkyls from Ir(ttp)(CO)Cl in good yields under user-friendly aerobic conditions (Scheme 1c). 6 Generally, under neutral or basic conditions alkyl alcohols are not good alkylating agents due to the poor leaving ability of hydroxide and the strong carbon−OH bond strengths (such as 92.0 kcal/mol in Me−OH and 79.9 kcal/mol in PhCH 2 − OH). 7 Therefore, both acid-catalyzed nucleophilic substitutions and conversion of the hydroxyl group into better leaving groups followed by substitution are popular strategies.…”

“…In 2008, our group developed the base-promoted benzylic C–H bond activation of various PhCH 3 species by Ir­(ttp)­(CO)Cl (ttp = 5,10,15,20-tetraphenylporphyrinato dianion) providing different Ir­(ttp)­Bn species (Scheme c) . Recently, we have expanded the scope of alkylating substrates to alkyl halides, including 1°, 2°, and 3° alkyls, for the preparation of Ir­(ttp)-alkyls from Ir­(ttp)­(CO)Cl in good yields under user-friendly aerobic conditions (Scheme c) …”