7-Norbornadienyl cation

For some homoaromatic carbocations the NICS values and chemical shifts have been calculated.105,106 IGLO-HF and GIAO-MP2 calculated 13C NMR chemical shifts for bishomoaromatic 7-norbornenyl 90 and 7-norbornadienyl cation 91 have also been reported.107... 

There is no general theoretical study for trialkyl-substituted cations R3E, which investigates the relationship of the classical planar trigonal structure to isomeric complexes RE /R2 and its relative energy compared to the dissociation products, the singly coordinated four-valence-electron species R E and the hydrocarbon R2. The only exceptions are 7-norbornadienyl cations 37 for which the germyl and silyl cation has been intensively studied theoretically by Radom and Nicolaides. ... 

These observations, when compared with spectra of the 7-norbornenyl and 7-norbornadienyl cations, confirmed the protonated cyclopropane structure [4] for the norbornyl cation. Average chemical shifts and coupling constants estimated for C(l), C(2) using equilibrating classical models were not in accord with the experimental i C-nmr data (Olah, 1976). 

In order to explain the above mentioned observations within [3.2.0] cations, we propose the formation of 33 and 34 to proceed via an intermediate [2.2.1]cation, in which a bridge-flip takes place. Such a bridge-flip has been proposed by Winstein for several 7-substituted norbornadienyl cations, interchanging the bound and unbound vinyl group (Scheme 19). In the parent 7-norbornadienyl cation he... 

For the 7-norbornadienyl cation 236, various formulae were suggested from chemical data. However, the first PMR study of 7-norbomadienyl fluoroborate obtained from 7-chloronorbornadiene showed this salt to be unsymmetrical because the PMR signals showed four types of protons (2 2 2 1). Such a spectrum corresponds either to formula 428 or to 226. 

A further prediction of the r-complex theory is that the n complex should have the geometry indicated in (60), the hydrogen at the 7-position being tilted out of the CCC plane because the adjacent carbon atom is quadri-covalent and should therefore have a tetrahedral geometry. This carbon should also be tilted over toward the double bond to which it is covalently bound. Both these predictions have been confirmed in the case of the analogous 7-norbornadienyl cation (62), which can be obtained as a stable species in super acids. 

A series of 7-substituted norbornadienes have been synthesized by quenching the 7-norbornadienyl cation with various nucleophiles of sulphur (SMOj), arsenic (AsPhj), and antimony (SbPhg). 7-t-Butylnorbornadiene, prepared by reaction of t-butyl-lithium with 7-t-butoxynorbornadiene, has been transformed to, inter alia, (128) and (129). By a similar route, (20) and related compounds, required for study of their photoelectron spectra, were prepared (Scheme 5). Propenylidene-norbornadienes were accessible by the routes shown in Scheme 6. They provide a novel photochemical synthesis of dihydro-azulenes, azulenes, and of vinylogous heptafulvenes. 

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