Trimethylamine oxide, anhydrous

Trimethylamine oxide is normally available as a hydrate, and for the present preparation it is necessary to convert it to its anhydrous form. A convenient way of doing this is as follows. A solution of 45.0 g. of trimethylamine oxide dihydrate (supplied by Beacon Chemicals) is dissolved in 300 ml. of warm dimethyl-formamide and placed in a three-necked flask set up for distillation. At atmospheric pressure the flask is heated and solvent distilled off until the boiling point reaches 152-153°. Then the pressure is reduced using a water aspirator, and the remainder of the solvent is distilled. At the end of the distillation the temperature of the bath is slowly raised to 120°. The residual anhydrous trimethylamine oxide (30 g.) can be dissolved in 100 ml. of chloroform and may remain in the same flask for use in the present preparation. 

The currently available methods for the synthesis of the title compounds are confined to the preparation of homo-1,1-dihalo-1-alkenes 180 while only a few reports are available for mixed 1,1-dihalo-1-alkenes of defined stereochemistry 18u. As the hy-droboration reaction proceeds in a stereospecific manner, the hydroboration-oxi-dation-bromination-debromoboration sequence of 1-chloro-l-alkynes produces selectively (Z)-l-bromo-l -chloro-l-alkenes (Eq. 116),82>. The oxidation with anhydrous trimethylamine oxide of the alkenylborane prior to the addition of bromine is necessary to avoid the competing transfer of one of 1,2-dimethylpropyl group from boron to the adjacent carbon atom. Similar reaction sequence provides 1,1-dibromo-l-alkenes (Eq. 117)182). 

Oxidation of orgauoalumiuum compounds. Organoaluminum compounds can be oxidized conveniently to the corresponding alcohols in nearly quantitative yield by anhydrous trimethylamine oxide (4 hours, 140°). ... 

For dehydration 10 g. of the amine oxide dihydrate is placed in a 150-ml. flask which has a long, wide neck. Some boiling chips are added, and the compound is heated in an oil bath under a water pump vacuum of 10-12 mm. A calcium chloride tube is placed between the water pump and the flask. At an oil bath temperature of 120°, the amine oxide dihydrate melts and begins to bubble. The temperature is raised very slowly. After most of the water is off, the temperature of the bath is maintained at 140-150° for 10 minutes. A new calcium chloride tube is placed in the apparatus, and it is again heated under vacuum. At 180° sublimation starts, and anhydrous amine oxide collects in the neck of the flask. The mixture is held at a bath temperature of 190-200° for 1.5 hours to complete the sublimation. The yield of anhydrous trimethylamine oxide is around 95%. The material is extremely hygroscopic. A sample in a capillary tube melts at 208° after subliming around 180°. 

Similar migration of the other two R groups and hydrolysis of the B-0 bonds leads to the alcohol and boric acid. Retention of configuration is observed in R. Boranes can also be oxidized to borates in good yields with oxygen," with sodium perborate NaBOs," " and with trimethylamine oxide, either anhydrous" or in the form of the dibydrate." The reaction with oxygen is free radical in nature." ... 

Hydroboration of 1-trimethylsilyl-l-alkynes with BH3 SMe2 in a 3 1 ratio, followed by oxidation of the resultant trivinylborane with anhydrous trimethylamine oxide, provides an operationally simple, one-pot synthesis of alkyl-substituted acylsilanes in good yield from readily available starting materials. ... 

Monohydroboration of bis(trimethylsilyl)acetylene and oxidation of the resultant trivinylborane with anhydrous trimethylamine oxide followed by hydrolytic workup affords [(trimethylsilyl)acetyl]trimethylsilane, which contains both a- and 3-ketosi-lane structural features. This reagent is a versatile synthon for stereoselective syntheses of functionalized trisubstituted olefins. 

Anhydrous trimethylamine oxide is obtained by removing the water from the hydrate at 120-150°/12 mm.176 Industrially, trimethylamine oxide hydrate is prepared at a lower temperature (—40°) than that mentioned above.177... 

Anhydrous trimethylamine oxide added with stirring in three portions to a soln. of diisopropyl (4R)-/ri7 5-2-[(Z)-but-2-en-2-yl]-l, 3,2-dioxaborolane-4,5-dicarboxylate in dry dichloromethane at 10, after 1 h at room temp, the mixture cooled to 0°, volatiles removed under reduced pressure, the residue re-dissolved in dichloromethane, cooled to —78°, treated with benzaldehyde, stirred for 10-14 h while the temp, was raised to - 50°, and quenched with phosphate buffer (pH 7) - (3S,4S)-(-)-j 7w-4-hydroxy-3-methyl-4-phenylbutan-2-one. Y 75% (e.e. 65%). Both (E)- and (Z)-dioxaborolanes gave the 5 y/2-ketol. F.e.s. T. Basile et al., J. Chem. Soc. Perkin Trans. 1 1989, 1025-9. 

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

All reactions are carried out under N2 atmosphere with use of standard Schlenk techniques. Solvents are dried appropriately before use. Ceo (99.5%, Southern Chemical Group, LLC) is used without further purification. Anhydrous trimethylamine A-oxide (mp 225-230°C) was obtained from Me3N0-2H20 (98%, Aldrich) by sublimation (3 times) at 90-100°C under vacuum. 

Oxidation of organoboranes (6, 624 8, 507). A new synthesis of acylsilanes (2) involves oxidation of the hydroboration products of alkynylsilanes (I).1 Oxidation with the commercially available dihydrate of trimethylamine N-oxidc is sluggish however, anhydrous reagent2 is satisfactory for this purpose (equation I). 

RCHiBr - ECHO.1 Anhydrous trimethylamine N-oxide2 in DMSO can oxidize primary bromides (but not the chlorides) to aldehydes at 25° in yields of about 80% (5 hours reaction). Allylic and benzylic chlorides and bromides are oxidized more rapidly at 0° in a mixed solvent of DMS0/CH2C12. 

Trimethylamine N-oxide, either anhydrous or as its readily available dihydrate, smoothly oxidizes a wide variety of alkyl, cycloalkyl, aryl and heterocyclic boron derivatives to the corresponding oiganyl-oxyboranes (equation 22) which, in Ae case of the dihydrate, are hydrolyzed in the reaction mixture. Anhydrous trimediylamine N-oxide is simply prepared and this reagent must be used for the oxidation of a enylboranes if prior hydrolysis is not to compete with oxidation. > Alkynylboranes are not oxidized by trimethylamine N-oxide. ... 

Anhydrous trimethylamine N-oxide has been suggested as an alternative, neutral oxidant, although with dialkoxysilanes relatively high reaction temperatures are required. Alkyltrifluorosilanes, on the other hand, undergo cleavage with this oxidant at room temperature. ... 

Caution. Anhydrous trimethylamine-N-oxide is obtained by vacuum sublimation of the commercial dihydrate Aldrich Chemical Company)... 

A mixture of polymeric dicarbonyldichlororuthenium(II) (0.15 g, 0.64 mmol), 2,2 -bipyridine (0.35 g, 2.2 mmol) and anhydrous trimethylamine-iV-oxide (—0.2 g, 2.5 mmol) is heated under reflux in degassed 2-meth-oxyethanol for 2 h under nitrogen. Work-up as described above gives [Ru(bpy)3] [PFJ2. Yield 0.44 g (70%). 

Alkylborinic esters, obtained from alkylboronic esters and an organometallic reagent, are converted into the corresponding ketones by the reaction with dichloromethyl methyl ether in the presence of a hindered base, followed by oxidation with hydrogen peroxide in pH 8 buffer or with anhydrous trimethylamine Ar-oxide18. 

Borane-dimethyl sulphide solution (73 mmol, 2.74 m in THF) was added dropwise, with stirring, to the neat alkynylsilane (200mmol), keeping the reaction temperature below 20°C by use of an ice bath. The mixture was stirred for 2 h at 0-5 °C. At this time, the solution of the trialkenyl borane was diluted with THF (200 ml), and then anhydrous trimethylamine A/-oxide (240 mmol, dried by azeotropic distillation with toluene) was added over 5 min at ambient temperature. The resultant slurry was heated (bath temperature 75 °C) for 4h, and then cooled to ambient temperature. The enol borinate thus formed was hydrolysed by the addition of water (100 ml), followed by stirring for 5 min. The layers were separated, and the aqueous phase was extracted with ether (2x 100ml). The combined organic extracts were washed with brine and dried. Concentration and distillation gave the acylsilane (75-91%). 

CH3)3N—B(C2Hj)2CsCCH3 is a white solid (mp = 33°), which can be crystallized from cold pentane (— 78°). The air- and moisture-sensitive compound is very soluble in diethyl ether and in pentane. The amine-borane can be stored indefinitely in an inert gas atmosphere (Ar, N2) in closed vessels at room temperature 1 mol of (CH3)3N—B(C2H5)2C=CCH3 reacts with an excess of anhydrous trimethylamine N-oxide in boiling toluene liberating 3 mol of (CH3)3N. ... 

Compound A, stable to oxygen and to alkaline hydroperoxide below 100°, reacts with anhydrous trimethylamine N-oxide under liberation of about 9 mol (CH3)3N per mol of A. Bromine reacts with neat compound A or CCI4 solutions of A to give HBr and various brominated C2B3 compounds. Compound A does not react with nitric acid up to 85°. Compound A is also stable toward pyridine at room temperature, and it shows no reaction with metallic sodium on heating in the absence of any solvent at < 150°. 

The solid white decaethyl-2,6,8,10-tetracarbadecaborane(10) (C) can be exposed to air and moisture for only a short time without changing its appearance and composition at room temperature. Compound C is stable to iodine at room temperature. The solution of C in THF is attacked by metallic potassium to form a pale-yellow-colored mixture. Compound C reacts analogously to A with anhydrous (CH3)3NO above 75°. In refluxing toluene 1 mol of C liberates more than 16 mol of trimethylamine from the N-oxide reagent. 

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