Carbodications. 5.1 Ring opening of the cyclopropanecarbonyl cation in superacid

Abstract

The cyclopropanecarbonyl cation (11) was prepared from cyclopropanecarbonyl chloride in 1∶1 HF–SbF₅, 1∶1 FSO₃H–SbF₅, and 4∶1 FSO₃H–SbF₅. Ring opening occurred in the strongest superacids 1∶1 HF–SbF₅ and (much slower) 1∶1 FSO₃H–SbF₅, but not in 4∶1 FSO₃H–SbF₅. The crotyl (2) and methacryloyl (14) cations were formed in 1∶1 FSO₃H–SbF₅, but very little or no 14 accompanied 2 in 1∶1 HF–SbF₅. Thus, 2 is formed by acid catalysis only, whereas formation of 14 involves base catalysis supplementing the acid catalysis in superacids. Dehydrochlorination of the 4-chlorobutanoyl cation in HF–SbF₅ and H/D exchange at C3 of 2 (involving attack by the acid at C3 of 3-butenoyl cation) in 1∶1 DF–SbF₅, both reported before, cannot involve intramolecular assistance with the formation of ring-hydronated 11 as intermediate. Instead, a 1,4 acyl alkyl dication in a tight ion pair is indicated by the results. Reaction in 1∶1 FSO₃H–SbF₅ under CO pressure followed by methanol quenching gave the methyl esters of glutaric (major) and methylsuccinic acid (minor); at least the latter should be formed by an S_N2-like attack by CO. The reaction of 11 in deuterated superacids 1∶1 DF–SbF₅ and 1∶1 FSO₃D–SbF₅ was much slower than the reaction in the corresponding protio-acids. At the same time, H/D exchange in the ring of unreacted 11 was observed. The extent of exchange could be assessed for the reaction in 1∶1 FSO₃H–SbF₅, where conversion to 2 was small. The deuteration of the ring in this medium is similar in rate to the ring cleavage. Together with the observed rate reduction in the deuterated acids, this result suggests that H/D exchange in 11 and its ring opening do not occur on the same reaction pathway.