Cyclopropyl group charge delocalization into

Several substituted bicyclo[3.1.0]hexenyl cations 42 and 43 were prepared by Olah and coworkers73, and they all showed extensive charge delocalization into the cyclopropyl group as shown by the significantly deshielded l3C absorptions for the methylene carbons (89-124 ppm). 

VII. EXTENT OF POSITIVE CHARGE DELOCALIZATION INTO THE CYCLOPROPYL GROUP... 

Previously discussed spiro[2.5]oct-4-yl cation 34148 is also a cyclopropylmethyl cation with substantial positive-charge delocalization into the cyclopropyl group. Other representative cyclopropylmethyl cations that have been prepared in the... 

PouLTER has demonstrated that chrysanthemyl derivative 16-OPy I asymmetrically deuterated at the carbinyl position ionizes with 91 2% stereoselectivity from a conformation with the leaving group aligned antiparallel to the 1,3-cyclopropyl bond (68). Since rotation about the C1-C2 bond in the cyclopropylcarbinyl carbonium ion intermediate should be slow relative to nucleophilic capture, this selectivity is reflected in the position of the deuterium atom in the vinyl group of the two major products. The stereoselectivity is attributed to stabilization of the transition state for ionization by charge delocalization into the vinyl group which should be more favorable in the a/i/f-parallel alignment. Kinetic data also indicate that the double bond facilitates the ionization process. 

Several 1,4-disubstituted derivatives of the dication, 63, were successfully prepared. The 1,4-diphenyl, 1,4-dimethyl, 1,4-dicyclopropyl-substituted derivatives of carbodication 63 (65, 66, and 67) are exceptionally stable. The 1,4-dimethyl-1,4-cyclohexyl dication, devoid of the adjacent cyclopropyl groups, could not be prepared. The carbocationic center in all these dications are somewhat shielded as compared to their monocations. The C NMR chemical shifts of the carbocationic centers for the dications 65, 66, and 67 are 235.4, 293.4, and 260.8 ppm, respectively. The diprotonated ann -tricyclo(5.1.0.0 )octa-2,6-dione, 68, may be treated as a dicarboxonium ion, instead of dihydroxy dicarbenium ion, since the cabronyl carbon is shielded by only 25.2 ppm, much smaller than that observed for the protonated cyclohexanedione (34 ppm). The para carbons of the phenyl substituents in carbodication 65 are relatively shielded by about 5 ppm from that of the parent l,4-diphenylcyclohexane-l,4-diyl dication showing relatively less delocalization of the charge into the aromatic rings. 

The 9-Cyclopropyl-9-xanthyl cation 33 is so stable that the positive charge is hardly delocalized into the cyclopropyl group. The alpha and beta hydrogens of the cyclopropyl group show absorptions at <5 H 2.96,1.38 and 1.96, comparable to that of cyclopropylam-monium ion (<5 H 2.93 and 0.93), rather than a typical cyclopropylcarbinyl cation, such as dimethylcyclopropylcarbinylcation (<5 H 3.4, 4.0)64. 

The solvolysis product distribution and the kinetic data (solvent effect33) m = 0.583 — 0.505, substituent effect34) q = —2.98) were clearly consistent with a SNj ionization process involving the anchimeric assistance of the triple bond (kA) 35) leading to the resonance stabilized cation 63 with its positive charge delocalized through the adjacent triple bond into the aryl (or cyclopropyl) group 13). 

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