Homocyclopropenyl cation

In the light of the appreciable puckering found in the seven-membered ring of [16], Childs (1984) recalculated the expected chemical shifts for the exo and endo H(8) protons of [12]. He calculated the difference in chemical shift (A6) to be 6.9 ppm which is in good agreement with the observed AS - 5.86 ppm. However, his calculations revealed that both the exo and endo protons are shielded. This surprising result is opposed to the accepted intuitive view that in an aromatic/homoaromatic system protons with the H(8)(exo) orientation should be deshielded and those with the H(8)(cndo) orientation shielded. This result closely parallels the analogous calculated data for the homocyclopropenyl cation [2] (Schindler, 1987). 

Basicity. Computational studies indicate that protonation of tetrahedrane would lead to the homocyclopropenyl cation, the stability of which, taken with the release of the great strain in tetrahedrane, results in a big enthalpy drop. This was calculated (B3LYP) to correspond to a proton affinity of 1053 kJ mol (298 K enthalpy of reaction) [80]. A CBS-4 M (cf Tetrahedrane Acidity above) calculation by the author gave a value of 1004 kJ mol. The gas-phase basicity... 

The calculated potential curves for the (CH)3CH2 system show that the cyclo-propenylcarbinyl, cyclobutenyl, and bicyclobutyl cations are surrounded by relatively low-energy barriers and that they would readily collapse to the most stable conformer, the puckered homocyclopropenyl cation (5). ... 

The first direct observation of the tris-homocyclopropenyl cation (420) has been accomplished by n.mr. examination of the product formed on dissolution of 3-chloro-... 

Miscellaneous Valence Isomerizations 4-31G calculations at optimum STO-3G molecular geometries show the forbidden pathway in the degenerate rearrangement of the homocyclopropenyl cation to be of lower energy than the allowed one (Scheme 45) because of interactions with the asymmetric Walsh cyclopropane orbitals. ... 

Cyclopropyl-allyl rearrangement has been employed as a synthetic tool and studies of homocyclopropenyl cations have led to the stable dication (319)... 

Winstein S, Sonnenberg J, deVries L (1959) The Tris-homocyclopropenyl cation. J Am Chem Soc 81 6523-6524... 

Recent MP2(full)/6-31 IG optimizations on the homocyclopropenyl cation gave a puckered Cs geometry with... 

SbFj has also been found useful in the preparation of homoaromatic cations. For example, the simplest 2ir monohomoaromatic cations, the homocyclopropenyl cations, can be prepared from corresponding 3-halocyclobutenes in SbFj (eq 31). ... 

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