Boron trifluoride oxonium salts preparation

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)]. 

Meerwein was the first to succeed in obtaining dioxolanylium ions of type 2, sufficiently stabilized as salts with non-polarizable anions that they could be isolated crystalline. The compounds can be prepared by splitting out of an anion from cyclic ortho esters or acetals wherein the required ring-system is already present. The ortho ester 1 reacts with antimony pentachloride or boron trifluoride, with splitting out of OR, to give 2. Acetals (3) from aldehydes can be converted, by hydride abstraction with triphenylmethyl or triethyl-oxonium fluorohorate, into salts (2) this reaction proceeds well only with acetals of the 1,3-dioxolane type (3) that have little steric hindrance. With acetals of the 1,3-dioxane type, formed from aldehydes, the reaction of hydride abstraction is not, as a rule, possible. In all such reactions, the anion involved is either SbClg or BF4 . 

First and foremost is the existence of isolable coordination compounds. By analogy with ammonium ions and the boron trifluoride adduct of ammonia (discovered by Gay-Lussac in 1809) it has been possible to isolate and characterize oxonium salts of alcohols, ethers, and ketones (49,176,314) and to prepare Lewis acid adducts of the ethers, sulfides, and phosphines (329), as well as many other compounds which fall under our definition of weak bases. 

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