2-Nortricyclyl cation

The participation of the cyclopropyl group is at its maximum in its bisected conformation. Hence, 3-nortricyclyl cations 28 would be expected to be highly stabilized, as the... 

Comparison of the absorptions for the cationic center of the 3-nortricyclyl cation 28,... 

The 3-nortricyclyl cation 100 can be generated by ionization of either 3-nortricyclyl derivatives (101) (Section II.F), 2-norbornen-5-yl substrates (102) and by protonation of norbornadienyl derivatives (103). 

Due to the skeleton rigidity in the 3-nortricyclyl cations the a-delocalization between the C—C bonds of the cyclopropane ring and the vacant p-orbital is highly restricted. This example can be regarded as charge delocalization with minimum motion of nuclei. 

Nortricyclyl and Quadricyclyl Systems.—The studies of the kinetics of acid-catalysed hydration of exo- and endo-norbornen-2-ols, referred to above, have been extended to include 3-nortricyclanol, 1-methyl-cis- and -rra s-3-nortricyclanols, and 3-methyl-3-nortricyclanol. Solvent deuterium-isotope effects are interpreted in favour of an A-S 2 hydration of the three-membered carbon ring of the secondary alcohols, or an A-1 mechanism for the tertiary alcohol in which case the rate-determining step is the formation of a carbocation (probably the 3-methyl-3-nortricyclyl cation) by elimination of water. 

From a consideration of molecular geometries, the 7-norbornyl system (119) is regarded as a better model for 3-nortricyclyl (120) than is 2-norbomyl p-nitrobenzo-ate. A comparison firstly of the ratio of the rates of solvolysis of (120) and (119), where major relative rate enhancements occur with increasing electron demand by the aromatic ring, and secondly of the values so obtained with those for (96) and (97), has led to the conclusion that the stabilization of cyclopropylcarbinyl is not the result of a-bridging. Thus, for example, the rigidity of the 3-nortricyclyl cation forces... 

The products of the buffered acetolysis of 2-deuterio-eiido-norborn-5-en-2-yl p-bromobenzenesulphonate (91) have been analysed by mass spectrometric, C and H n.m.r. procedures, and the results discussed in a clearly written and interesting paper. The deuterium initially on C-2 had migrated partially on to C-1 (30%) in the deuteriated cxo-norborn-5-en-2-yl acetates so formed, and was equally and uniquely distributed between C-1 and C-6 of the deuteriated nortricyclyl acetate, the major solvolysis product. Within the limits of detection products derived from C( 1 -> 7) migration or from fragmentation were not formed. The results are consistent with the formation of the symmetrical nortricyclyl cation (92), or ion pair, from the norborn-5-en-2-yl cation, the isomerization being competitive with the reaction of these intermediates with solvent [n.b. C-1 and C-6 in (92) are equivalent positions that correspond to the C-1 and C-2 positions in the norbomenyl system]. 

The radical cations 15 + and 16 + add methanol exclusively from the exo face forming exo-methoxynorbornyl and nortricyclyl free radicals, which undergo rapid cyclopropylcarbinyl-butenyl interconversions (119 120 121 see... 

Ion pair return to positions more remote than the adjacent carbon is quite common with homoallyl-cyclopropylcarbinyl cations, cf. compounds (92)-(94)l05 lol Even the vinyl tosylate (96) is produced from either of the allenic isomers (95) and (97)108 In contrast, 5-norbornen-2-yl brosylate (98) does not yield any nortricyclyl brosylate (99) although scrambling of a deuterium label, (98a) (98b), is ca. 12 times faster than solvolysis109. ... 

Electrophilic addition of hydrohalic acids (hydrogen fluoride, hydrogen bromide, hydrogen chloride), carboxylic acids (e.g. acetic acid) and halogens (bromine, chlorine, iodine) to bicy-cloheptadiene gave a homoallyl cation which underwent rearrangement to nortricyclyl derivatives, i.e. 5-substituted tricyclo[2.2.1.0 ]heptanes. ... 

The major products of solvolysis of 2-exo- and 2-endo-norbomenyl or nortricyclyl halides or sulphonates are nortricyclyl derivatives 2-exo-norbomenyl derivatives are formed with a small yield. The predominant formation of nortricyclyl derivatives is associated with its higher stability compared to the norbomene one Even in the direct equilibrium between norbomene and nortricyclene the tricyclic isomer is predominant (3 1) at reflux temperature. This difference in the stability of products can be partially reflected on the activation energy for partitioning the cationic intermediates into bicyclic and tricyclic solvolysis products. 

The 2-exo-norbomenyl isomer can be assumed to yield first ion 171 while the nortricyclyl isomers are likely to yield directly the symmetrical cation 173. 

Exo brosylate 176 forms the ion 181 which is attacked by a nucleophile on four sides. The less active endo epimer 175 also forms the ion 181, but with a parallel Sfj2-substitution which reduces the amount of nortricyclyl compounds and the fraction of rearranged norbomenyl derivatives. The authors maintain that the concept of equilibrated homoallylic cations is not necessary at all. 

Other Systems.— The addition of formic, acetic, and chloroacetic acids to quadri-cyclane at 20 °C affords the corresponding esters of exo-2-norbornenol and nortri-cyclenol. Solvent effects and the results obtained using the 0-deuteriated carboxylic acids are interpreted in terms of the six-centre addition mechanism (97) the minor pathway to nortricyclyl products, in the absence of added mineral acid, is the result of leakage from (97) to the norbornenyl cation. ... 

The solvolysis of 1-quadricyclylcarbinyl 3,5-dinitrobenzoate (121) gives the isomeric alcohols (122)-< 124) and their dinitrobenzoates. The rate ratio for (122) cyclo-propylcarbinyl 3,5-dinitrobenzoate is found to be 1.2 x 10 the large stabilization of the 1-quadricyclylcarbinyl cation (125) is estimated to arise from strain relief (70—75%) and cyclopropyl-cyclopropyl interaction (25—30%). The lack of additivity as a result of cyclopropyl enhancement in solvolysis rates in the series 7-norbomenyl, 3-nortricyclyl, and 7-quadricyclyl (k i = 1 6 x 10 10 respectively) has been explained in terms of antibonding interactions across the base of the tetracycle in a frontier orbital analysis of the problem. ... 

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