Phenyltrimethylammonium Tribromide: A Versatile Reagent in Organic Synthesis

Biographical Sketches

Hong-Yan Lü was born in Handan, Hebei Province, P. R. of China. She completed her Bachelor degree in 2006 in chemistry at Hebei Normal University, P. R. of China. Presently she is working as a postgraduate at the Hebei Normal University, under the supervision of Professor Zhan-Hui Zhang. Her research interest focuses on the development of new synthetic methodologies for green chemistry.

Introduction

Phenyltrimethylammonium tribromide (PTAB) is known to be a convenient oxidizing and brominating agent. It is an orange crystal and easy to handle, with a melting point at 113-115 ˚C. [¹] It has been used for the oxidative transformation of trans-stilbene oxide to 2-phenyl-1,3-dioxane in the presence of various of 1,3-diols and a catalytic amount of SbBr₃, [²] for brominating the α-position of carbonyl compounds, [³-8] α′-bromination of α,β-unsaturated ketones, [9] and for the addition of bromine to alkenes. [¹0] It was also found to be useful for the chemoselective conversion of 3-alkoxyfurans to 2-alkoxy-3(2H)-furanones, oxidative ring-opening of 3-alkoxy-2,5-diphenylfurans to cis-2-alkoxy-2-butene-1,4-diones, [¹¹] and synthesis of imidazolines, [¹²] 3-bromo-2-styrylchromones, [¹³] nitro dibromo-phenols, [¹4] pyridazines, [¹5] phytoalexin cyclobrassinin, [¹6] p-bromo­dienone calixarene derivatives, [¹7] and 2-arylthia­zino[5,6-b]indoles. [¹8]

Phenyltrimethylammonium tribromide is commercially available now. It can be readily prepared from N,N-di­methylaniline and dimethyl sulfate, followed by treatment with 48% hydrobromic acid and bromine. [¹]

Scheme 1

Abstracts

(A) Oxidation of Secondary Alcohols to the Corresponding Carbonyl Compounds: Sayama et al. showed that PTAB is an available and chemoselective reagent for the oxidation of secondary alcohols and substituted 1,2-diols to the corresponding ketones, 1,2-diketones and α-ketols in the presence of catalytic amounts of SbBr3 or CuBr2 at room temperature. [¹9] The oxidative cleavage of the glycol C-C bond for 1,2-diols was not found. Furthermore, aliphatic primary alcohols were not affected under the same oxidative conditions.
(B) Selective Oxidation of Sulfides to Sulfoxides: An efficient procedure for the selective oxidation of various sulfides to the corresponding sulfoxides in aqueous pyridine solution was achieved using PTAB as oxidant. [²0] This method allowed ¹8O-labelled sulfoxides to be prepared without loss of isotope enrichment of the used ¹8O-water.
(C) Oxidative Conversion of 3-Alkoxyfurans to 2-Hydroxy-3(2H)- furanones and 2-Hydroxy-2-butene-1,4-diones: PATB can be applied for the oxidative ring opening of 3-alkyoxy-2,5-diphenylfurans to 2-hydroxy-2-butene-1,4-dions in t-BuOH at room temperature. The transformation of 3-alkyoxy-2,4,5-triphenylfurans to 2-hydroxy-2,4,5-triphenyl-3-(2H)-furanone was also accomplished with PTAB in t-BuOH under the same reaction conditions. [²¹]
(D) One-Pot α-Bromoacetalization of Carbonyl Compounds: A convenient and practical method for the one-pot α-bromoacetalization of carbonyl compound has been developed. [²²] The reaction was performed in tetrahydrofuran-ethylene glycol (1:1) with 1-2 equivalents of PTAB at room temperature to afford the corresponding α-bromoacetals in good to excellent yields.
(E) Aziridination of Alkenes: Dauban and co-workers showed that PATB can catalyze the intramolecular aziridination of N-chlor-amine salts of ω-unsaturated sulfonamides. [²³] The aziridination of the alkene can also be carried out employing an understoichiometric amount of chloramines-T trihydrate in the presence of 5 mol% ­PTAB. [²4]
(F) Coupling Reaction of Carbon Dioxide and Epoxides: A combination of SalenRu(PPh3)2 [²5] or metal porphyrin [²6] with PTAB has been used as novel and high efficient catalysts for the coupling reaction of carbon dioxide and epoxides to yield the corresponding cyclic carbonates.

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