Water‐Catalyzed Dehalogenation Reactions of the Isomer of CBr₄ and Its Reaction Products and a Comparison to Analogous Reactions of the Isomers of Di‐ and Trihalomethanes

Abstract

A combined experimental and theoretical study of the UV photolysis of a typical tetrahalomethane, CBr4, in water and acetonitrile/water was performed. Ultraviolet photolysis of low concentrations of CBr4 in water mostly leads to the production of four HBr leaving groups and CO2. Picosecond time-resolved resonance Raman (Ps-TR3) experiments and ab initio calculations indicate that water-catalyzed O-H insertion/HBr elimination of the isomer of CBr4 and subsequent reactions of its products lead to the formation of these products. The UV photolyses of di-, tri-, and tetrahalomethanes at low concentrations in water-solvated environments are compared to one another. This comparison enables a general reaction scheme to be deduced that can account for the different products produced by UV photolysis of low concentrations of di-, tri-, and tetrahalomethanes in water. The fate of the (halo)formaldehyde intermediate in the chemical reaction mechanism is the key to determining how many strong acid leaving groups are produced and which carbon atom final product is likely formed by UV photolysis of a polyhalomethane at low concentrations in a water-solvated environment.

Minisci C−H Alkylation of Heteroarenes Enabled by Dual Photoredox/Bromide Catalysis in Micellar Solutions**

“…Detailed mechanistic studies on the role of CBr 4 co‐catalyst are ongoing, but preliminary results suggest that, through the photosensitized hydrolysis of CBr 4 , it may provide the starting concentration of bromide anions at the early stage of the process. Although the light‐induced reactivity of this compound is usually associated with mesolytic bond cleavage [52–55] or homolytic dissociation to CBr 3 and Br radicals, [56, 57] it has been shown that, in the aqueous conditions, the photoinduced hydrolytic pathway to HBr prevails [58] . Alternatively, the reduction of CBr 4 by excited Ir III *‐photocatalyst can be considered leading to Br − , the CBr 3 radical and Ir IV ‐complex.…”

“…Although the light-induced reactivity of this compound is usually associated with mesolytic bond cleavage [52][53][54][55] or homolytic dissociation to CBr 3 and Br radicals, [56,57] it has been shown that, in the aqueous conditions, the photoinduced hydrolytic pathway to HBr prevails. [58] Alternatively,t he reduction of CBr 4 by excited Ir III *-photocatalyst can be considered leading to Br À , the CBr 3 radicala nd Ir IV -complex. The last two speciesm ay undergo SET to recover Ir III and produce CBr 3 cation,w hich reacts with water to give the tribromomethanol and ap roton.C on-tinuouspHm easurements strongly support suchs teady generation of HBr.U pon the interaction of CBr 4 ,w ater,p hotocatalyst and light, the reaction mixture gradually become acidic (see Supporting Information).…”

Brominated Methanes as Photoresponsive Molecular Storage of Elemental Br2

“…On the other hand, similar to the case of anisole (1; Table 1, entry 1), phenol (18) gave 2,4-dibromophenol in 93 % yield (Table 3, entry 3). 2,4-Di-tert-butylphenol (19) underwent a monobromination reaction at the 6-position to give compound 19Br in 62 % yield, but 4,6-dibrominated 19Br 2 , which had an oxidized dienone structure, was also obtained in 28 % yield (Table 3, entry 4). The structure of compound 19Br 2 was determined by spectroscopic measurements and by single-crystal X-ray crystallography.…”

“…[15c] North and co-workers reported a computational study of the atmospheric oxidation of CH 2 Br 2 and CHBr 3 that was initiated by UV photolysis, [16] but no example has been reported for the experimental photodecomposition of the lessbrominated CH 2 Br 2 . Because the photodecomposition of brominated methanes is still an ambiguous qualitative issue, [15][16][17][18][19] the use of their photodecomposed products has hardly attracted any attention as practically available elemental sources in organic synthesis and material sciences. Herein, we report that brominated methanes, in particular CH 2 Br 2 , upon exposure to UV light efficiently generate elemental Br 2 , which is practically available for a variety of applications.…”

Spectroscopic and computational studies of matrix-isolated iso-CXBr3 (X=F, Cl, Br): Structure, properties, and photochemistry of substituted iso-tribromomethanes

“…Later, the same group examined the photolysis of CBr4 in water and acetonitrile/water solution, and showed that the water catalyzed O-H insertion/H-Br elimination reaction of iso-CBr4 led to the final products: HBr and CO2. [27] The TR3 experiments revealed that, following UV photolysis, iso-CBr4 is formed on the ps timescale, and persists for several ns in acetonitrile/water solution.…”