Dimethyl boron tribromide

Good yields are generally observed, especially for methyl ethers. The combination of boron tribromide with dimethyl sulfide has been found to be particularly effective for cleaving aryl methyl ethers.91... 

Good yields are generally observed, especially for methyl ethers. The combination of boron tribromide with dimethyl sulfide has been found to be particularly effective for cleaving aryl methyl ethers.87 Trimethylsilyl iodide cleaves methyl ethers in a period of a few hours at room temperature.85 Benzyl and /-butyl systems are cleaved very rapidly, whereas secondary systems require longer times. The reaction presumably proceeds via an initially formed silyl oxonium ion ... 

General procedure for deprotection of mono- and polymethyl-aryl ethers with boron tribromide.41 To a 10-ml flask fitted with a septum and magnetic stirrer bar are added reactant (3.6 mmol) and 5 ml of dichloromethane. An inert atmosphere is established and maintained. This mixture is cooled in a dry ice/propan-2-ol bath and boron tribromide [0.13 ml, 1.32 mmol (for monomethyl ethers), or 0.38 ml, 4 mmol (for dimethyl ethers)] is added through the septum by use of a syringe. The cold bath is removed and the mixture stirred for 30 minutes, poured into ice water, stirred for 30 minutes, saturated with salt and extracted with dichloromethane. The extract is dried and concentrated. The purity of the product is established by h.p.l.c. analysis on a Waters Associates 6000A model using both refractive index and u.v. absorbance detectors with a Waters 3.9mm i.d. x 30cm p-Bondapack Ci8 reverse phase column. 

Dimethyl-4-bromo-27/-chromenes are accessible via a boron tribromide induced cyclization of phenol derivatives 31 (Scheme 9). The reaction is proposed to proceed via O-deprotection and subsequent elimination of hydroxide to form the intermediate zwitterionic species 32. Addition of bromide, pyran ring closure and protonation provides 4-bromo-2/7-chromenes in moderate yield (Scheme 9) < 1997H(45) 1131 >. 

In order to introduce a second hydroxyl group al C-l 1 of the ester (99) its sodium salt was treated with p-nitrobenzene diazonium chloride in methanol. The resulting product on methylation and then reduction with potassium dithionate afforded the amino ester (100). Diazotization and methanolysis produced ethyl ( )-camosate dimethyl ether (101), which on saponification produced camosic acid dimethyl ether (102). Wenkert [22] accomplished demethylation of dextrorotatory camosic dimethyl ether with boron tribromide. Thus the work of Meyer lacks only the... 

The reaction of ( + )-bulbocapnine methyl ether (42) with excess boron trichloride in methylene chloride containing 0.3% ethanol produces the catechol (43) together with the monophenol (44). However, with boron tribromide, it is the diphenol (45) which is produced. Both compounds (44) and (45) can be O-methylated with diazomethane to the dimethyl ether (46), and hydrolysis of (46) with dilute hydrochloric acid leads to (+)-corytuberine (47).43 Alternatively, methylation of (43), using diazomethane, affords (+)-corydine methyl ether (48). 

Dimethyl tellurium dibromide and boron tribromide formed a thermally unstable 2 1 adduct4. Calorimetric titrations indicated that 1 1 complexes are formed between diphenyl tellurium dihalides and aluminum tribromide or gallium trichloride5 6. 

Canadian chemists3 report that attempted demethylation of the dimethyl ether of pinosylvin (1) with boron tribromide gave the monodemethylated product in... 

Pyridinium tetrabromoborate in dimethyl sulphoxide has a B shift of +2 07, and addition of pyridinium bromide increases the shift to a limiting value of +13-0. The shift of pyridinium tetrabromoborate in liquid hydrogen bromide at — 75°C was +23 6 p.p.m. and was invariant of the addition of further amounts of bromide ion. The addition of tetraethylammonium bromide to boron tribromide in methylene dichloride caused a shift to a limiting value of + 23 8 p.p.m. 

Anisole added to 1,2-dichloroethane complex containing boron tribromide dimethyl sulphide complex (Nj) and the mixture refluxed (12 h)to give phenol. 

Reaction of the ArC,o unit with 1-pentynylmagnesium bromide occurred smoothly in tetrahyrofuran containing cuprous chloride without formation of alienic impurities. An analogous type of route was used for the synthesis of 1,3-dimethoxy-5-[(ZZ)-pentadec-8,11-dienyl]benzene, (15 2)-cardol dimethyl ether (ref. 105). For the synthesis of (15 2)-urushiol, 1,2-dihydroxy-3-[(ZZ)-penta-dec-8.11-dienyl]benzene and its dimethyl ether, 7-(2,3-dimethoxyphenyl)heptyl bromide and the corresponding 2,3-dihydroxy compound, obtained by demethylation with boron tribromide, have been used (ref. 149). 

Depending on the stoichiometric quantity of Br2 and the reaction temperature, electrophilic bro-mination of 1 with Brj in the presence of catalytic amounts of AICI3 gave 9-bromo-o-carborane (31, 95% yield), 9,12-dibromo-o-carborane (32), and 8,9,12-tribromo-o-carborane (33), as shown in Scheme 6.5 (Smith et al., 1965). Under similar reaction conditions, 1,2-dimethyl-o-carborane (28) was found to be tetrabrominated (34), presumably due to the electron donating effect of the methyl groups. 3-Bromo-o-carborane (35, 65% yield) was obtained by the reaction of 14 with a boron tribromide-methylsulflde complex [BBr3-S(CH3)3] (Li and Jones, 1990). 

Preparation by total demethylation of 4 -tert-butyl-2,5-dimethoxybenzophe-none (SM) with boron tribromide in methylene chloride, first at -30° for 30 min, then at 22° for 5 h (86%). SM was obtained by Friedel-Crafts acylation of hydroquinone dimethyl ether with p-tert-butylbenzoyl chloride in methylene chloride in the presence of alnmininm chloride at 0° for 8 h (84%) [1258], m.p. 115-116° [1258] H NMR [1258]. 

Methylating agents for phenolic groups are a) diazomethane (the reaction time varies from minutes to 24 hours), b) dimethyl sulphate (DMS) - potassium carbonate in acetone or dimethylformamide (DMF) and c) methyl iodide - potassium carbonate in acetone. For demethylation of a methyl ether group ortho to a methyl ester group boron tribromide is used, while demethylation of a methyl ether para to a methyl ester group is carried out with boron trichloride (see Scheme 8). 

Recently, we explored haloboration polymerization between boron tribromide and terminal diynes to give poly(organoboron halide)s as a polymeric Lewis acid, which has scarcely ever been known [13]. These poly(organoboron halide)s were also prepared by means of hydro-boration polymerization between dienes and monobromoborane as shown in Scheme 4 [14]. As a typical example, 1,7-octadiene was added to a 1.0 M dichloromethane solution of monobromoborane-dimethyl sulfide complex with stirring at 0°C under nitrogen. The molecular weight of the polymer obtained was increased when the feed ratio of monobromoborane to diene approached or slightly exceeded unity. 

---