Dehydration boron trifluoride

Toluene reacts with carbon monoxide and butene-1 under pressure in the presence of hydrogen fluoride and boron trifluoride to give 4-methyl-j iYbutyrophenone which is reduced to the carbinol and dehydrated to the olefin. The latter is cycHzed and dehydrogenated over a special alumina-supported catalyst to give pure 2,6- dim ethyl n aph th a1 en e, free from isomers. It is also possible to isomerize various dim ethyl n aph th a1 en es to the... 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Strong acids are able to donate protons to a reactant and to take them back. Into this class fall the common acids, aluminum hahdes, and boron trifluoride. Also acid in nature are silica, alumina, alumi-nosihcates, metal sulfates and phosphates, and sulfonated ion exchange resins. They can transfer protons to hydrocarbons acting as weak bases. Zeolites are dehydrated aluminosilicates with small pores of narrow size distribution, to which is due their highly selective action since only molecules small enough to enter the pores can reacl . 

Treatment with triethylsilane and boron trifluoride etherate allows a variety of methyl (i-hydroxy-/3-ary lpropionates to be reduced to methyl ft -ary lpropionates in yields of 85-100% as part of a synthetic sequence leading to the preparation of indanones (Eq. 31).170 Small amounts of dehydration products formed simultaneously are reduced to the methyl -arylpropionates by mild catalytic hydrogenation.170... 

The electron-rich thiophene ring system can be elaborated into complex, fused thiophenes by acid-mediated intramolecular annelation reactions. For example, treatment of alcohol 96 with trimethylsilyl triflate promoted a Friedel-Crafts acylation and subsequent dehydration giving benzo[b]thiophene 97, a potential analgesic <00JMC765>. Treatment of ketone 98 with p-toluenesulfonic acid resulted in the formation of fused benzo[b]thiophene 99 <00T8153>. Another variant involved the cyclization of epoxide 100 to fused benzo[f>]thiophene 101 mediated by boron trifluoride-etherate . 

For example, the reaction of nitronates (123) with a zinc copper pair in ethanol followed by treatment of the intermediate with aqueous ammonium chloride a to give an equilibrium mixture of ketoximes (124) and their cyclic esters 125. Heating of this mixture b affords pyocoles (126). Successive treatment of nitronates (123) with boron trifluoride etherate and water c affords 1,4-diketones (127). Catalytic hydrogenation of acyl nitronates (123) over platinum dioxide d or 5% rhodium on aluminum oxide e gives a-hydroxypyrrolidines (128) or pyrrolidines 129, respectively. Finally, smooth dehydration of a-hydroxypyrrolidines (128) into pyrrolines (130f) can be performed. 

Levulinic acid, acting in its lactone form (XXXII) undergoes dehydration and two products have been obtained, - and /3-angelica lactones, (XXXVIII) and (XXXIX), respectively. a-Angelica lactone has been found to polymerize to a tacky resin by the catalytic agency of boron trifluoride. 

The dehydration of thiobenzoylthioglycolic acid (135, R = Ph, R = H) with acetic anhydride-boron trifluoride was initially described as yielding the meso-ionic l,3-dithiol-4-one (134, R = Ph, R = H), but subsequent studies showed that the product was in fact the 5-substituted derivative 134, R = Ph, R = COCHjSCSPh. Authentic meso-ionic l,3-dithiol-4-ones (134) have recently been prepared (85-90% yield) by the cyclodehydration of the acids (135) with acetic anhydride-triethylamine at 0°-10°. Examples include anhydro-4-hydroxy-2-phenyl-l,3-dithiolium hydroxide (134, R = Ph, R = H) described as scarlet needles, m.p. 113°—115° this compound is sensitive to moisture. Anhydro-4-hydroxy-2,5-diphenyl-1,3-dithiolium hydroxide (134, R = R = Ph) was obtained as gold lustered, deep violet needles,... 

A convenient route to 2-alkylthio-4-alkyl-4-hydroxy-5,6-dihydro-4/7-l,3-thiazine derivatives 176 is the reaction of A-alkyldithiocarbamates with a,/3-unsaturated ketones in the presence of boron trifluoride etherate at 0°C (Scheme 15) <2002HAC377>. The predominant diastereomer displayed a m-relationship between the hydroxyl group and the C-4 substituent. Subsequent dehydration led to two isomeric products 177 and 178 with an equilibrium mixture resulting in a ratio of 94 6 in the case of 2-benzylthio-4-hydroxy-4-methyl-5,6-dihydro-4/7-l,3-thiazine. 2-Phenyl-4-alkyl-4-hydroxy-5,6-dihydro-4/7-l,3-thiazine derivatives are similarly prepared by reacting thio-benzamide with o ,/3-unsaturated ketones at room temperature <2002EJP307>. 

Cycloadduct 212 is converted into the dihydrobenzofuran 213 when treated with boron trifluoride diethyl etherate in good yield <2000JA8155>. The formation of the dihydrobenzofuran proceeds by an initial ring opening followed by a subsequent dehydration and acid-catalyzed cyclopropyl ketone rearrangement (Equation 142). 

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

Dehydration of primary nitroalkanes with phenyl isocyanate or acetic anhydride in the presence of catalytic triethylamine affords nitrile oxides, which may be trapped as their 1,3-dipolar cycloadducts or allowed to dimerize to the corresponding furoxans. Other dehydrating agents that have been used include diketene, sulfuric acid and, when the a-methylene group is activated by electron-withdrawing groups, boron trifluoride in acetic anhydride, trifluoroacetic anhydride with triethylamine, and nitric acid in acetic acid. 

Aromatic compounds are usually nitrated in liquid phase by treatment with a mixture of concentrated nitric acid and concentrated sulfuric acid. Sulfuric acid serves as a dehydrating agent which prevents dilution of the nitric acid by the liberated water. Acetic anhydride in acetic acid and boron trifluoride also serve this purpose. 

CyclopropanecarboxaUehytte. Dehydration of a mixture of cis- and froni-cyclo-butane-l,2-diol (I) with boron trifluoride n-butyl etherate at 230° yields cyclopropane-carboxaldehyde (2) in 65-80% yield. Dehydration of (I) with p-toluenesulfonic acid gives (2) in somewhat lower yields (66%)- ... 

The monoaryl boric acids, RB(OH)2, arc usually isolated, as stated al >o x, l)y the action of water on the type RBXg, aitliough in certain cases this leads to the formation of the oxide RBO. The phenyl eom >ound has l)ecn obtained b " boiling with water the product of reaction from magnesium phenyl bromide and boron trifluoride. The most remarkable feature of the type RE(OH)2 is that the action of iiiercuric chloride upon them leads to the production of mercury aryl halides (RHgX). The anisyl and phenetyl compounds do not yield oxides when heated, or form salts, and tlie jS-naphthyl acid exists in two modifications. Dehydration of the acids in maw gives the oxides, RBO. 

By the Darzens reaction. -ionone afforded a C14 aldehyde which as the diethyl acetal underwent addition to ethyl vinyl ether in the presence of boron trifluoride etherate to yield a Cl6 acetal. After hydrolysis, loss of ethanol and formation of the diethyl acetal as before, reaction under acidic condiions with ethyl propenyl ether gave the unsaturated Cl9 aldehyde after hydrolysis and removal of ethanol. Reaction of two moles with ethyne dimagnesiuro bromide produced the C40 chain and dehydration of the diol, selective catalytic hydrogenation followed by isomerisation completed a remarkable technical synthesis of i-carotene. Further variations have involved the use of two moles of the C14 aldehyde and a Cl2 divinyl ether. An independent approach (ref.29) has utilised vitamin A (32) converted to a phosphonium salt, thence to the corresponding phosphoran, autoxidation of which afforded s-carotene ( scheme 16). 

Compound (80), prepared from 2-methylcyclopentane-l,3-dione and methyl 2-chloroacrylate followed by the sequence (76)-> (77)-> (78)- (79) (resolved)— (80), combined with (71) (prepared from m-methoxyphenylstyrene and diborane) to form the seco-steroid (82)/ Acetic anhydride-toluene-p-sulphonic acid then cyclized this stereoselectively to furnish the triacetate (83) which on saponification gave the triol (84). Interestingly, this compound reacted with toluene-p-sulphonic acid in alcohol to produce, by dehydration and change of configuration at C-14, the compound (85), which served as a source of various 8a-oestrone compounds. Compound (84) on treatment with boron trifluoride etherate underwent pinacol transformation in preference to dehydration to yield the ketone (86) this ketone was correlated with the known compound (87). 

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