Bishomocyclopropenium cations

On the basis of several studies, including the application of the tool of increasing electron demand,ion 167 can be best described as a nonclassical symmetrical bridged 27i-bishomocyclopropenium cation as opposed to a rapidly equilibrating pair of cyclopropylmethyl cations, for example 171. The observed unusually large coupling constants at the C(7) position of 167... 

Structures which have been proposed for the 2-norbomen-7-yl cation include the symmetrical bishomocyclopropenium ion (406) (Winstein)321 and the rapidly equilibrating pair of tricyclic cations (407) (407 ) (H.C. Brown)327. The products de-... 

Despite considerable effort to prepare the parent bishomocyclopropenium ion (23), it remains unknown experimentally. 23 is calculated to be puckered (pucker angle 90°) and 6 kcal mol (MP4SDQ/ 6-31G //MP2/6-31G ) more stable than the nonhomoaromatic, but less strained, planar conformation 24. Since Roberts et al. first proposed the enhanced stability of the bishomocyclopropenyl cation 5, numerous 3,5-bridged derivatives (25) were prepared and fully characterized as homoaromatic both by experiment and theory. ... 

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