Carbodications. 5. Ring opening of the cyclopropanecarbonyl cation in superacid

The cyclopropanecarbonyl cation (II) 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 accompani ed 2 in 1 :1 HF-SbF5. Thus, 2 is formed by acid catalysis only, whereas for mation of 14 involves base catalysis supplementing the acid catalysis in su peracids. 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 intramolecul ar assistance with the formation of ring-hydronated 11 as intermediate. Ins tead, a 1,4 acyl alkyl dication in a tight ion pair is indicated by the res ults. Reaction in 1 : 1 FSO3H-SbF5 under CO pressure followed by methanol q uenching gave the methyl esters of glutaric (major) and methylsuccinic acid (minor); at least the latter should be formed by an S(N)2-like attack by C O. The reaction of 11 in deuterated superacids 1:1 DF-SbF5 and 1:1 FSO3D-Sb F5 was much slower than the reaction in the corresponding protio-acids. At the same time, H/D exchange in the ring of unreacted ii was observed. The e xtent of exchange could be assessed for the reaction in 1 : 1 FSO3H-SbF5, w here 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 re duction 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.