Cyclopropane face-protonated

Ab initio theoretical calculations by Pople and co-workers101 and at the semiem-pirical level by Dewar and co-workers102 103 did not give consistent results. Subsequent ab initio calculations (SCF level including electron correlation) by Lischka and Kohler104 are inconsistent with earlier ab initio work. Their calculations have confirmed the stability of the isopropyl cation 2 and the instability of face-protonated cyclopropane 15. Edge-protonated cyclopropane 16 is found to be a saddle point on the... 

The existence of protonated cyclopropanes in solution was postulated by a number of workers, often in connection with the problem of 1,3-elimination (Skell and Starer, 1960 cf. also Nickon and Werstiuk, 1967). Such intermediates were preferably represented as rapidly equilibrating edge-protonated cyclopropanes, rather than as a face-protonated moiety (Baird and Aboderin, 1964 Benjamin et al., 1966 Deno and Lincoln,... 

Fig. 3.11. Examples of the possible non-transversal intersections in a molecular system (a), between the stable and unstable manifolds of two bond critical points in CHJ (b) between the stable and unstable manifolds of two ring critical points in a distorted structure of [l.l.l]propellane (c) between ihe unstable manifold of a bond critical point with the stable manifold of a ring critical point (the ring axis) in face-protonated cyclopropane.

Cleavage Gem-dihalides and monohalides have been dehalogenated to chiral monohalides in the presence of alkaloids [397, 398]. l,l-Diphenyl-2-bromo-2-carboxyl (bromo or methyl carboxylate) cyclopropanes are cathodically debromi-nated in the presence of alkaloid cations with enantioselectivities up to 45% ee. A mechanism is proposed whereby the alkaloid is adsorbed at the Hg cathode, which protonates face selectively the carbanion generated by 2e reduction from the bromide [399]. 

This reactivity proved to be a general process, providing the unique products in moderate yields following cyclopropanation and immediate treatment with silver tetrafluoroborate. These structures revealed that a cascade sequence was proceeding stereoselectively in every case to furnish a single product as the result of conrotatory 4jt electrocyclization, electrophilic aromatic substitution at the least hindered position on the arene moiety (para to the MeO) in favor of six-membered ring formation, and desilylation with protonation from the exo face of the bicyclic product. Dehydrochlorination to form a second cationic intermediate did not occur in this case, due to structural restrictions imposed by the bridged architecture of 81. 

To attempt to define in some detail the reaction of a proton with cyclopropane, calculations of the relative energy of several different configurations of protonated cyclopropane have been carried out (Table 4). These calculations have resulted in conflicting conclusions however, all levels of the various theories result in the face-... 

The addition of phenyllithium to cyclopropene occurred with 99% stereoselectivity to give cw-l-lithio-2-phenylcyclopropane, albeit in 3 /o yield. With 3-methylcyclopropene, the reaction was more efficient and, after protonation of the lithiocyclopropane, 2-methyl-1-phenyl-cyclopropane was obtained in 44.5% yield. The product is about 94% in the trans form thus attack of the organolithium occurs on the opposite face to that bearing the larger substituent at C3. When the cyclopropene is 3,3-disubstituted, e.g. 17, both faces are sufficiently hindered to slow the addition process and metalation of a vinylic hydrogen predominates. ... 

Pople et al. have made ab initio calculations for nine isomeric cations of the common CjH. formula. They have shown the least stable structures to be those of the face-1 and the edge-protonated II cyclopropanes which are by 10 kcal/mole less stable than the corner-protonated cyclopropane III. 

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