A mechanistic study of the chromium(II)-mediated transformations of trichloromethyl alkyls and carbinols: evidence for carbene, carbyne, and carbenoid intermediates

A trinuclear chromium(iii) chlorocarbyne

Reduction of CCl₄ by CrCl₂ in THF afforded a trinuclear chromium(iii) carbyne [CrCl(thf)₂]₃(μ₃-CCl)(μ-Cl)₃. The chlorocarbyne complex reacted with aldehydes to afford chloroallylic alcohols and terminal alkynes. The mechanistic study proposed two competitive pathways via an α-chlorovinyl intermediate.

Formation and characterization of a reactive chromium(v)-oxo complex: mechanistic insight into hydrogen-atom transfer reactions

A mononuclear Cr(v)-oxo complex, [CrV(O)(6-COO--tpa)](BF4)2 (1; 6-COO--tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of a Cr(iii) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was achieved using ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance Raman spectroscopies. The reduction potential (Ered) of 1 was determined to be 1.23 V vs. SCE in acetonitrile based on analysis of the electron-transfer (ET) equilibrium between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine). The reorganization energy (λ) of 1 was also determined to be 1.03 eV in ET reactions from phenol derivatives to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(iv)-oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alcohol derivatives (R-BA) with different oxidation potentials were employed as substrates, corresponding aldehydes were obtained as the 2e--oxidized products in moderate yields as determined from 1H NMR and GC-MS measurements. One-step UV-vis spectral changes were observed in the course of the oxidation reactions of BA derivatives by 1 and a kinetic isotope effect (KIE) was observed in the oxidation reactions for deuterated BA derivatives at the benzylic position as substrates. These results indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidation of trimethoxy-BA (Eox = 1.22 V) by 1, trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET-proton transfer (ET/PT), depending on the redox potentials of R-BA.

Microwave-assisted reduction of F-BODIPYs and dipyrrins to generate dipyrromethanes

The reduction of BODIPYs and dipyrrins to dipyrromethanes, via a reaction involving ethylene glycol and sodium methoxide, is reported. When benzyl alcohol is used in place of ethylene glycol, the addition of 2,4-dinitrophenylhydrazine to the reaction mixture after microwave irradiation results in the production of 1-benzylidene-2-(2,4-dinitrophenyl)hydrazone, indicating concomitant production of aldehyde alongside the dipyrromethane.

Generation of Nucleophilic Chromium Acetylides from gem-Trichloroalkanes and Chromium Chloride: Synthesis of Propargyl Alcohols

Nucleophilic mixed chromium(II) and chromium(III) acetylides are generated from the smooth reduction of primary 1,1,1-trichloroalkanes with chromium(II) chloride in the presence of an excess amount of triethylamine at room temperature. These species arise from chromium(III) vinylidene carbenoids. It has been demonstrated that uncommon low-valent Cr(II) acetylides are formed by C-H insertion of Cr(II)Cl(2) into terminal alkynes, formed in situ through the Fritsch- Buttenberg-Wiechell (FBW) rearrangement, whereas Cr(III) acetylides are concomitantly generated by HCl elimination from the chromium(III) vinylidene carbenoid. Both divergent pathways result, overall, in the formation of nucleophilic acetylides. In situ trapping with electrophilic aldehydes afforded propargyl alcohols. Furthermore, deuteration experiments and the use of deuterium labeled 1,1,1-trichloroalkane substrates demonstrated the prevalence of low-valent Cr(II) acetylides, potentially useful, yet highly elusive synthetic intermediates.