Tribromide, boron complex

Boron tribromide reacts with ethers by the sequence shown (22). The boron complex... 

AMIDES Boron tribromide. Boron trifluoride etherate. 6-Chloro-l-p-chlorobenzene-sulfonyloxybenzotriazole. Diethylphosphoryl cyanide. Dihalobis(triphenylphosphine)-palladium(H). Dihalobis(triphenylphosphine)paUadium(ll) complexes. Palla-dium(II) chloride. Sodium amide. Trimethylsilyl isocyanate. Triphenylphosphine ditriflate. 

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). 

The reaction is irreversible and can be used to synthesize aUphatic and aromatic esters. In addition, there are no complications involving water removal or azeotrope formation. Boron tribromide can be used ia place of boron trichloride, but the bromide has a stronger tendency to halogenate the alkyl group of the alcohol (26). Boron tritiuoride does not give the ester, but gives either a complex or dehydrated product. 

An interesting cyclization is brought about in the two-step synthesis of compound 140 from the tribromide 141 (Scheme 56). The resolution of these complexes represents one of the first examples of optical resolution of triarylboranes. The crystal stmcture of 140b (R = Me, R = H) has been determined B-C distances are in the range 155.0-157.4 pm and the environment about the boron atom is close to trigonal-planar <1991TL6363>. 

Reaction of the iron complex salt 602 with the arylamine 921 in the presence of air led directly to the tricarbonyl(ri -4b,8a-dihydro-9H-carbazole)iron complex (923) by a one-pot C-C and C-N bond formation. Demetalation of complex 923 and subsequent aromatization by catalytic dehydrogenation afforded 3,4-dimethoxy-l-heptyl-2-methylcarbazole (924), a protected carbazoquinocin C. Finally, ether cleavage of 924 with boron tribromide followed by oxidation in air provided carbazoquinocin C (274) (640) (Scheme 5.120). 

The halides, GaQs, AlBrs, A1C13, and FeCl3, very probably also exist in dimeric form. With these halides, however, the complexation reaction with the azide is the fast step so that no square-root term is found in the kinetic equation. Arsenic trichloride, arsenic tribromide, phosphorus trichloride, and phosphorus pentachloride were without effect on the rate of decomposition of benzoyl azide. Finally, the authors have observed that the catalytic influence of boron tribromide was much higher than all the other halides. Its rate constant was estimated to be about 18 times larger than that for gallium chloride. 

Aluminum trichloride is the most commonly used catalyst, although aluminum tribromide is more efficient.1 For the rearrangement of l-broino-2-chloro-1,L2-lrifluoroethane (3) to 2-bromo-2-chloro-l,l,l-trifhioroethane (4). none of the following Lewis acids are effective iron(III) chloride. iron(III) bromide, antimony(III) chloride, antimony(V) chloride. tin(IV) chloride, titanium(IV) chloride, zinc(II) chloride, and boron trifluoride-diethyl ether complex.1" ... 

The catalysts used successfully in the liquid phase in the order of their efficiency are anti-niony(V) chloride, aluminum tribromide, aluminum trichloride, and iron(Ill) chloride. Zinc(ll) chloride, tin(ll) chloride, boron trifluoride diethyl ether complex, and aluminum trifluoridc do not catalyze the dismutation. 

Jensen et al.16 also studied the decomposition of 5-ethoxythiatriazole in dibutyl phthalate solution in the presence of trichloroacetic acid, tripentylamine, 4-benzylpyridine, anhydrous aluminum chloride, or trinitrobenzene, but practically no effect on the reaction rate was observed. However, later experiments have shown that the decomposition can indeed by enhanced catalytically by Lewis acids under conditions where the catalyst is not sequestered by complex formation with the solvent.19 Thus the addition of aluminum chloride to 5-phenylthiatriazole in benzene causes a brisk evolution of nitrogen at room temperature, and if instead boron tribromide is added, a rather violent reaction sets in. However, when esters or ethers are used as... 

Cleavage of the iron complex and the ben/yl ether functions is accomplished with boron tribromide solution. 

A comparison of catalytic activity of the triphenyl phosphate complexes of boron trifluoride,boron trichloride and boron tribromide showed that comparable rates of polymerization were obtained using the chloride or bromide complexes. The fluoride complex gave about half the rate of the bromide or chloride comparable molecular weights were obtained in all three cases. 

It was also of interest to us how changes in the organic residue of the carbyne ligand influence the stability and the behavior of carbyne complexes. Hence, we treated with boron tribromide a series of pentacarbonyl-[methoxy (aryl) carbene tungsten (0) complexes which were substituted at... 

I would emphasize that the reaction of the amino acid-carbene complex with boron tribromide represents a good possibility of again cleaving the carbenyl protective group under extremely mild conditions at —25°C. 

That experimental results cannot always be generalized is shown by the treatment of cis-(bromo) tetracarbonylQiydroxy(methyl) carbene]manga-nese with boron tribromide. This procedure does not lead to the analogous carbyne complex but rather to a product in which the hydrogen atom of the hydroxy group is substituted by a BBr2 residue (94) ... 

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. 

A new synthesis of dibenzoboroles by reductive cyclization of arylboron dibromides opens up access to several derivatives of this ring system (Scheme 6) <1996JA7981>. MonoaryIboron dibromides 61 and 62 were prepared by reaction of the appropriate aryl lithium compound 60 with boron tribromide in hexane. Reductive cyclization of arylboron dibromide 61 with an excess of lithium metal in diethyl ether gave bislithium dibenzoborole complex 63. At the time of writing, compound 63 was the first dibenzoborole dianion to be structurally characterized by X-ray... 

The synthesis of cyclopropyl carbyne complexes follows the general Fischer synthesis of carbyne complexes from alkoxycarbene complexes typical of transition metals of group 6 (Cr, Mo, W). Thus, addition of cyclopropyllithium to chromium and tungsten hexacar-bonyl followed by alkylation of the acylmetallate intermediate with triethyloxonium fluo-roborate gave cyclopropyl ethoxycarbene complexes which, upon subsequent reaction with boron tribromide at -25 °C, afforded the corresponding /ra 5 -bromotetracarbonyl cyclopropylcarbyne complexes (equation 91). However, whereas the monotungsten... 

The substrate arachidonic aeid, whieh often leads to formation of inflammatory prostaglandins, is stored in tissues as one of a number of phospholipids these compounds, as the name indicates, comprise complex phosphate-containing esters. The antiinflammatory corticosteroids inhibit the action of the enzyme, phospholipase A2, that frees arachidonic acid. The many undesired effects of those steroids has led to the search for non-steroidal inhibitors of that enzyme. A highly substituted indole derivative has shown good activity as a phospholipase A2 inhibitor. Alkylation of the anion from treatment of indole (32) with benzyl chloride affords the corresponding A-benzylated derivative (33). The methyl ether at the 4 position is then cleaved by means of boron tribromide to yield 34. Alkylation of the enolate from reaction of the phenol with sodium hydride with tert-butylbromoacetate affords the corresponding... 

Aluminum and boron halides are sometimes used to dealkylate alkyl aryl ethers to phenols. Boron tribromide cleaves aliphatic ethers to alcohols and alkyl halides, but the reaction has no preparative value in the aliphatic series. Aluminum halide and the ether first form a complex from which a molecule of alkyl halide is eliminated upon heating. 

Asymmetric Aldol Reactions. Reaction of (1) with Boron Tribromide in CH2CI2 affords, after removal of solvent and HBr, a complex (5) useful for the preparation of chiral enolates (eq 5). Complex (5) is moisture sensitive and is generally prepared immediately before use. For propionate derivatives, either syn or, less selectively, anti aldol adducts may be obtained by selection of the appropriate ester derivative and conditions. Thus reaction of f-butyl propionate with (5) and triethylamine produces the corresponding E 0) enolate, leading to formation of anti aldol adducts upon addition to an aldehyde (eq 6). Selectivities may be enhanced by substitution of the t-butyl ester with the (+)-menthyl ester. Conversely, reaction of 5-phenyl thiopropionate with (5) and Diisopropylethylamine affords the corresponding Z(0) enolates and syn aldol products (eq 7). ... 

An immediate color change to a very pale gold is observed. After S min the solution is evaporated to dryness with a rotary evaporator, and methanol (10 mL) is added to the residue to destroy any remaining boron tribromide. Purification of the product is accomplished by preparative scale TLC (see the footnote in the procedure in Section A) on fluorescent silica gel, eluting with petroleum ether (bp 30-60 Q. The complex [Os3H3(//3-CBr)(CO)9] (100mg, 90%) is obtained after extraction of the single colorless band with dichloromethane. 

Dibutyl ether is the only solvent suitable for this reaction. Dibutyl ether has relatively low volatility and complexes with trimethylaluminum without solvent decomposition. Dibutyl ether complexes with BBr, sulTiciently strongly to slow the rate of reaction ofBBrj with AIMe, to a safe rate. Boron tribromide reacts explosively with trimethylaluminum in diethyl ether or anisole solutions because of the weak complexation between these solvents and BBr,. Tetra-hydrofuran and p-dioxane undergo decom X)sition reactions with trimethylaluminum, and thus are unsuitable as solvents in this synthesis. 

---