Protonated cyclopropane Inversion

Proton shifts within protonated cyclopropanes also explain the rather low stereo-specificity at the origin of 1,2-ethyl shifts. (15, 2S)-(661) gives 3-ethyl-2-pentanol (674) with 54% inversion and with substantial redistribution of a label, as required by the conversion of the protonated cyclopropane (672c) to its enantiomer (tf72d)4S7-47a ... 

When trifluoroacetic acid was added to butylcyclopropane derivatives 12, Markovnikov products were obtained predominantly. Treatment of cyclopropane with concentrated sulfuric acid yielded propanol. With the deuterated acid, D2SO4, deuterium was found at all three carbon atoms presumably due to the formation of a protonated cyclopropane intermediate. When exo-tricyclo[3.2.1.0 ]octane (13) was treated with deuterated sulfuric acid an end-on attack of the deuterium ion at the most strained bond with inversion of configuration to give 14 was observed. 

An interesting variant of a geometric isomerization was observed for the 7,7-dimethylbicyclo[4.1.0]hept-2-ene system. The electron transfer reaction of the highly strained rrans-fused isomer (30) with 1-cyanonaphthalene rapidly converted it to the cw-fused system (31) [224], The observed rearrangement requires inversion at one of the tertiary cyclopropane carbons. This can be accomplished either by removal of a hydrogen (proton) or by cleavage of a cyclopropane or an allylic bond. 

Gassman and Creary have reported that the centra bond of succinic esters can be cleaved with sodium in liquid ammonia. This method gives dimethyl glutarate from dimethyl cis- and trans-cyclopropane-l,2-dicarboxylates. The C-1—C-2 cleavage of a 1,2-diphenylcycIopropane which is part of a more complex, tricyclic compound has been effected by lithium in ammonia. Although it is known that lithium-ammonia reduction of a cyclopropyl ketone cleaves the bond which will allow tlw best overlap with the carbonyl Tt-system, the steric course of protonation at the f(-carbon has not, until recently, been known. Reduction of (301) at — 78 °C gives (302X the product with inversion, and (303),... 

Czochralska [113] investigated the reduction of the optically active 2-phenyl-2-chloropropionic acid at a mercury cathode in acidic ethanolic solution. It was found that inversion took place during electrolytic reduction, and the yield of optically active product amounted to 77.2-92.2%. Annino and co-workers [112] investigated the electrolytic reduction on mercury of the optically active l-bromo-2,2-diphenylcyclopropanecarboxylic acid and its methyl ester, and also the reduction of l-bromo-l-methyl-2,2-diphenyl-cyclopropane in neutral, acidic, and alkaline ethanolic solutions. It was found that reduction of the acid took place with 26-35% inversion of the configuration in acidic solutions and with 31-38% retention in alkaline solutions. Reduction of methyl l-bromo-2,2-di-phenylcyclopropanecarboxylate took place with 30-56% inversion of the configuration irrespective of the composition of the solution, whereas reduction of l-bromo-l-methyl-2,2-diphenylcyclopropane took place with 21% retention. These results can be explained on the assumption that the molecule is subject to attack by electrons from the side of the halogen atom and that the whole stereochemistry of the process is determined by stereoselective reaction with proton donors of the free carbanion or of the carbanion screened by the electrode. 

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