Acyl tetrafluoroborates

The reaction of [Fe(cycloheptatriene)(CO)3] with acyl tetrafluoroborates yields only cationic complex (29), which can then be converted to the acylated product (30) in high yield via the methoxide adduct (31 Scheme 20).40... 

The acylation of methylenecyclobutanc and 1-melhylcycIobutene with acyl tetrafluoroborates leads to the corresponding / -fluoro ketones 3 and 4.4 9-492... 

Acylation of Methylenecyclobutanc by Acyl Tetrafluoroborates General Procedure ... 

An equimolar amount of dry BFj was passed into a solution of the respective acyl fluoride (0.13 mol) in abs hexane (50 mL) at —40 to — 45 C with stirring. To a suspension of the obtained acyl tetrafluoroborate at the same temperature was added methylenecyclobutane (6.8 g, 0.1 mol) over 1.5 min. The mixture was stirred for 2 min until a yellow-orange color appeared a solution of EtjN (15niL) in abs McOH (20 mL, cooled to — 60 to — 50 C) was added, and the mixture was then poured into a mixture of sat. aq NaHCOj and extracted with EtjO. The organic layer was washed with aq NaHCOj and HjO, dried (Na2S04) and distilled. 

The transient nature of carbocations arises from their extreme reactivity with nucleophiles. The use of low nucleophilicity gegenions, particularly tetrafluoroborates (BF4 ) enabled Meerwein in the forties to prepare a series of oxonium and carbo-xonium ion salts, such as R30+BF4- and HC(OR)2+ respectively. These Meerwein salts are effective alkylating agents, and transfer alkyl cations in SN2 type reactions. However, simple alkyl cation salts (R+BF4 ) were not obtained in Meerwein s studies. The first acyl tetrafluoroborate, i. e. acetylium tetrafluoroborate was obtained by Seel16 in 1943 by reacting acetyl fluoride with boron trifluoride at low temperature. 

Acylation of 1,3,3-trimethylcyclopropene with an acyl tetrafluoroborate, such as pivaloyl te-trafluoroborate, gave a methylenedihydrofuran 3a. The reaction is highly dependent on the... 

Cyclopentenones. Russian chemists report a new synthesis of 2-cyclo-pentenones by the reaction of alkynes with an acyl tetrafluoroborate, formed in situ from an acyl chloride and silver tetrafluoroborate ... 

Acyl-3.4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes 1, on heating at 250-280 C for a short time without solvent, rearrange to the 1-acyl-1-benzazepines 2 (Method A).23-38 In some cases, rearrangement is accompanied by minor amounts of Ar-aeyl-l-naphthylamine and, at higher temperatures, the acylnaphthylatnine can become the major product (see Section 3.2.2.6.). In the presence of silver(I) tetrafluoroborate (Method B) rearrangement takes place at lower temperatures but the yields of benzazepine are inferior as the silver(I) ion also catalyzes the reverse reaction (see Section 3.2.2.1.). 

The thermally induced electrocyclic ring opening of 2-alkyl- and 2-acyl-3.4-benzo-2-azabicyclo-[3.2.0]hepta-3,6-dienes 2 to l//-l-benzazepines 1 (see Section 3.2.1.4.1.1.) are photorever-sible.23,37 38 Also, l-acyl-1//-benzazepines 1 (R = acyl), in refluxing xylene in the presence of silver(I) tetrafluoroborate, are in thermal equilibrium with their valence isomers the 2-acyl-3,4-benzo-2-azabicyclo[3.2.Oj nepia-3,6-uienes 2 (R = acyl).23-38... 

Arylation of activated double bonds with diazonium salts in the presence of copper catalysts is known as the Meerwin reaction. The reaction is postulated to either proceed through an organocopper intermediate or through a chlorine atom transfer from chiral CuCl complex to the a-acyl radical intermediate. Brunner and Doyle carried out the addition of mesityldiazonium tetrafluoroborate with methyl acrylate using catalytic amounts of a Cu(I)-bisoxazoline ligand complex and were able to obtain 19.5% ee for the product (data not shown) [79]. Since the mechanism of the Meerwin reaction is unclear, it is difficult to rationalize the low ee s obtained and to plan for further modifications. 

Volume 75 concludes with six procedures for the preparation of valuable building blocks. The first, 6,7-DIHYDROCYCLOPENTA-l,3-DIOXIN-5(4H)-ONE, serves as an effective /3-keto vinyl cation equivalent when subjected to reductive and alkylative 1,3-carbonyl transpositions. 3-CYCLOPENTENE-l-CARBOXYLIC ACID, the second procedure in this series, is prepared via the reaction of dimethyl malonate and cis-l,4-dichloro-2-butene, followed by hydrolysis and decarboxylation. The use of tetrahaloarenes as diaryne equivalents for the potential construction of molecular belts, collars, and strips is demonstrated with the preparation of anti- and syn-l,4,5,8-TETRAHYDROANTHRACENE 1,4 5,8-DIEPOXIDES. Also of potential interest to the organic materials community is 8,8-DICYANOHEPTAFULVENE, prepared by the condensation of cycloheptatrienylium tetrafluoroborate with bromomalononitrile. The preparation of 2-PHENYL-l-PYRROLINE, an important heterocycle for the synthesis of a variety of alkaloids and pyrroloisoquinoline antidepressants, illustrates the utility of the inexpensive N-vinylpyrrolidin-2-one as an effective 3-aminopropyl carbanion equivalent. The final preparation in Volume 75, cis-4a(S), 8a(R)-PERHYDRO-6(2H)-ISOQUINOLINONES, il lustrates the conversion of quinine via oxidative degradation to meroquinene esters that are subsequently cyclized to N-acylated cis-perhydroisoquinolones and as such represent attractive building blocks now readily available in the pool of chiral substrates. 

Secondary nitramines are conveniently prepared from the nitrolysis of A, A-dialkylamides with nitronium salts in acetonitrile or ethyl acetate at 20 °C where the acyl group is converted into an acylium tetrafluoroborate (Equation 5.14). Problems can occur if commercial nitronium salts like the tetrafluoroborate are used without purification. The presence of nitrosonium salts can then lead to nitrosamines via nitrosolysis. Yields of secondary nitramine up to 90 % have been reported with solutions of nitronium tetrafluoroborate in acetonitrile di-n-butylnitramine is obtained in 82 % yield from the nitrolysis of corresponding acetamide. ... 

The AgBp4 method was extended to cyclizations onto the alkoxyl side chain. Novel heterocycles A-acyl-3,4-dihydro-lP -2,l-benzoxazines (25) and A-acyl-4,5-dihydro-l//,3//-2,1-benzoxazepines (26) were synthesized by the treatment of open-chain A-chloro-A-(2-phenylethyloxy)- and A-chloro-A-(3-phenylpropyloxy)amides (24, n = 2, 3) with silver tetrafluoroborate in benzene (Scheme 2). Optimal yields of 25 were ultimately obtained with AgBp4 in ether or TPIP. ... 

Triazoles with alkyl, aryl, or acyl substituents on N(l) or N(4) can be readily quaternized with powerful quaternizing agents such as trialkyloxonium tetrafluoroborates. Quaternization takes place such that maximum separation of the substituents occurs. 

The electrophilic substitution of the 3-aryl compounds (265, R = Ar, R = H) exemplified by the formation of 5-bromo- (265, R = Ar, R = Br) and 5-nitro derivatives (265, R = Ar, R = NOj) has been put forward as evidence against the meso-ionic formulation 265. lliis approach is unacceptable since ground state charge distribution cannot be deduced from reaction products. The aluminum-amalgam reduction of meso-ionic l,2,3-thiadiazol-4-ones (265) yields either N-mercaptoacetyl-A-arylhydrazines or Ar-acyl-A-arylbydrazines. Triethyl-oxonium tetrafluoroborate and meso-ionic l,2,3-thiadiazol-4-ones (265) yield 1,2,3-thiadiazolium tetrafluoroborates (267). The effect of solvent on the ultraviolet spectra of meso-ionic l,2,3-thiadiazol-4-ones (265) has been reported. ... 

An interesting example is the acylation of various terminal acetylenes with preformed acylium tetrafluoroborates in the presence of aromatics 126... 

Acetylium tetrafluoroborate was prepared in this way as early as 194324. and the structures of at least two acyl cations have been investigated by X-ray diffraction methods25-27. As expected, CH ,-C=0+ is linear about the central carbon, with a CO bond length of 1.12 A, just shorter than in carbon monoxide, and a very short C-C bond length of 1.38 A25 26. Structurally, therefore, the acyl cations closely resemble the isoelectronic nitriles. 

Benzoxathiolium salts have proven to be effective masked acylating agents (79S223). 2-Substituted 1,3-benzoxathiolium tetrafluoroborates have now been utilized in the preparation of esters. Reaction of the salt (334) with two equivalents of an alcohol gave the 2-alkoxy-2-alkyl-benzoxathiole (335). Hydrolysis of (335) with red mercury(II) oxide and boron trifluoride etherate in aqueous THF delivered ethyl benzoate in excellent yield (Scheme 72). 

Alkylation and acylation of 1,2,3-triazines have not yet been studied extensively. Protonation of monocyclic 1,2,3-triazines usually takes place at N-2. The tetrafluoroborate salt of 5-chloro-4,6-bis(dimethylamino)-l,2,3-triazine could be isolated. Similarly, reaction of... 

In 1943, Seel observed14 the first stable acyl cation. Acetyl fluoride with boron trifluoride gave a complex (decomposition point 20°C) that was characterized as the acetyl tetrafluoroborate salt 331 [Eq. (3.94)]. 

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