Oxonium salts, acyl-, preparation

The preparation of alkoxy(l-alkynyl)carbene complexes (e.g., of compounds L M = C(OR1)—C=CR) mostly follows the two-step Fischer procedure,14 involving addition of a 1-lithio alkyne LiC=CR to a metal carbonyl L M(CO) to give a metal acylate L M=C(CTLi+) — C= CR14, which is subsequently alkylated with an oxonium salt [R130]BF4,15 an alkyl triflate R10S02C — CF3,16,22 or an alkyl fluorosulfonate R103SF16a,n ... 

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. 

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