Acetyl tetrafluoroborate

The transient nature of carbocations arises from their extreme reactivity with nucleophiles. The use of low-nucleophilicity counterions, particularly tetrafluorobo-rates (B I, ), enabled Meerwein in the 1940s to prepare a series of oxonium and carboxonium ion salts, such as R30+BF4 and HC(OR)2+BF4, respectively.13 These Meerwein salts are effective alkylating agents, and they transfer alkyl cations in SN2-type reactions. However, simple alkyl cation salts (R 1 BF4 ) were not obtained in Meerwein s studies. The first acetyl tetrafluoroborate—that is, acetylium tetrafluor-oborate—was obtained by Seel14 in 1943 by reacting acetyl fluoride with boron trifluoride at low temperature [Eq. (3.1)]. 

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)]. 

Diketones. Silyl enol ethers are C-acylated by acetyl tetrafluoroborate to give 1,3-diketones. Yields are generally higher if the reagent is generated in situ from CH3COF and BF3.2 Examples ... 

The term Superelectrophile was suggested by Olah in the early seventies when it was noticed that acylation of chlorobenzene by acetyl tetrafluoroborate was substantially accelerated in the presence of the corresponding Brpnsted acid. ... 

Acetoxytropylium tetrafluoroborate, prepared by the addition of tropone to acetyl tetrafluoroborate, reacts with methyl-lithium and sodium cyclopentadienide to give adducts which on pyrolysis in the gas phase give heptafulvene and sesquifulvalene, respectively, in good yield. ... 

Treatment of tricarbonylh4-1-(ethoxycarbonyl)-l//-azepine]iron(0) (30) with acetic anhydride and tetrafluoroboric acid at 0 C (Method A) yields the 3-acetyl derivative 31.226 The acetyl derivative is also formed, but in lower yield, by the action of acetic anhydride and tin(IV) chloride (Method B) on complex 30. The 3-propanoyl derivative (20% mp 95-96 C) can be prepared similarly, whereas formylation to give aldehyde 32 is successful under Vilsmeier conditions. 

Acetyl-4-(4 -sulfophenyl-3-tetrazene, 2982 l-Ethyl-l,l,3,3-tetramethyltetrazenium tetrafluoroborate, 2586 3 -Phenyl-1 -(5 -tetrazoly 1)-1 -tetrazene, 2804 Potassium 3,5-dinitro-2(l-tetrazenyl)phenoxide, 2256 Tetraallyl-2-tetrazene, 3545 Tetramethyl-2-tetrazene, 1759 See HIGH NITROGEN COMPOUNDS... 

The same approach allows preparation of various pyrylium carboranes from the corresponding 4/f-pyran carboranes 174a,b and 175b by the action of acetyl perchlorate,245 perchloric acid,244 and triarylamine radical cation salts,244,245 as well as electrochemically.243 The oxidation of condensed 4H-pyran 345 with trityl perchlorate, 2,3,5,6-tetra-substituted 4f/-pyrans 431 and 153 with tropylium tetrafluoroborate or 153 with heterocyclic salt 393 led to useful preparations of pyrylium salts 394,330 395a,359 and 395b,360 respectively. 

The results of nitration reactions depend on reagents and the acidity of the medium. Thus fuming nitric acid in concentrated sulfuric acid converts (410a) to the 3-nitro derivative while with acetyl nitrate reaction occurs exclusively at C-4 (equation 50). The nitration in the 4-position can also conveniently be done with nitronium tetrafluoroborate. The reduction of these nitro compounds by hydrazine in the presence of palladium on charcoal provides a versatile route to 4-amino-7,6-borazarothieno[3,2-c]pyridines (75ACS(B)46l>. In the nitration of 4,5-borazarothieno[2,3-c]pyridines (e.g. 409a), peri effects from the substituents on the boron atom lead to formation of a considerable amount of the 2-nitro derivatives. 

There was particular interest in the analogous reactions of 1-Oacetyl-2,3,5-tri-0-benzoyl-4-bromo-/ -D-ribose (37), because of the potential to prepare from it compounds related to nucleocidin, which is a 4 -fluoroaden-osine derivative having antitrypanosomal activity. With silver fluoride in acetonitrile, this bromide also reacted mainly with inversion of configuration, and gave the l-lyxo fluoride 140 (53% isolated yield), whereas, with silver tetrafluoroborate, much more of the more interesting 1-0-acetyl-2,3,4-tri-0-benzoyl-4-fluoro-/ -D-ribose (141) was obtained.38... 

A combination of trityl tetrafluoroborate or perchlorate with 2,4,6-tri(/err-butyl)-pyridine instead of pyridine enables an almost quantitative tritylation of the secondary hydroxyl group in l,2 5,6-di-0-isopropylidene-a-D-glucofuranose in 10 min at room temperature [321]. The more readily available 2,6-di(/m-butyl)-4-methylpyridine was used [322] in combination with trityl perchlorate for the selective tritylation of methyl 2,6-di-0-acetyl-a-D-galactopyranoside at the equatorial OH-3. Combination... 

The N,P,0-ligand depicted in 338 in the N,P-chelate unit follows from L(x 4-cod)Pd(Cl)MeJ (04JCS(D)3251). With silver tetrafluoroborate in acetonitrile/methylene chloride, cationic 339 follows, which under carbon monoxide is transformed to acetylated 340. The product of interaction of ligand 341 with [(T)3-C3H5)Pd(Cl)]2 is an efficient catalyst of asymmetric allylic substitutions (04JOC5060). 

Nitration of furan with nitronium tetrafluoroborate produces nitrofuran 2.33 directly. Contrast this result to the two stage reaction necessary with acetyl nitrate, page 16. Explain these observations. 

All these cationic deactivation processes were performed with oxolane as the monomer and with various initiators such as triethyloxonium tetrafluoroborate and benzoyl, acetyl or propionyl hexafluoroantimonate. Efficient difunctional cationic initiators such as adipoyl- or terephthaloyl hexafluoroantimonate) can also be used 42 to synthesize bifunctional macromonomers containing at both chain ends a polymerizable double bond. 

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