9-Barbaralyl cation

It should be possible to further reduce the energy barriers to internal rearrangements of the barbaralyl cation or related polycyclic compounds by relatively small changes in the structure or by appropriate substitution. In this way, a situation should be reached in which... 

In 1970 Ahlberg, Harris and Winstein reported the preparation of 71 and 72, the first examples of bishomotropenylium ions167. These 1,4-bishomoaromatic cations were prepared by ionization of the corresponding barbaralyl systems as shown in Scheme 30. The formation of 71 and 72 proceeds by way of an initial barbaralyl cation, the structure and nature of which has been the subject of a considerable amount of work168-170. The initially formed unsubstituted barbaralyl cation rearranges to 71 at -125 °C. 

The 9-ferrocenyl-9-barbaralyl cation 32c was found to be stable even in trifluroacetic acid63b. The l3C NMR chemical shifts of the cation are similar to that of the protonated barbaralone, reflecting the high stabilizing ability of the ferrocenyl group. 

Extensive theoretical studies have been carried out in order to rationalize the rearrangement pathways in the cationic system92. The characterization of the potential surface of C9H9+ cations93 at MP2-, MP3- and MP4(SDQ)/6-31G levels showed that the open 9-barbaralyl cation 83a is more stable than the completely charge delocalized (nonclassical) structure of Z)1h symmetry, and the bicyclo[3.2.2]nona-3,6,8-trien-2-yl cation 83b by 6.9... 

The 9-barbaralyl cation 83a is stable only at or below -135 °C, and on warming to -125 °C it is irreversibly converted into 1,4-dihomotropylium cation 85 (equation 54). The... 

Similar rearrangements are also observed in 9-substituted and 4,9-dimethyl-substituted 9-barbaralyl cations. The mechanisms of such rearrangements have been thoroughly investigated.993-995... 

It is characterized by double-bifurcation reactions with three directly connected first-order transition states. These are two transition states 570 and transition state 562. The chemical behavior of cation 560 is determined by the stereocomposition of a cyclo-propylcarbynyl cation moiety in conjugation with two vinyl group. The 9-barbaralyl cation 560 is characterized by four weak C—C single bonds (bond orders = 0.54 and 0.84) and one strong nonbonded interaction that can be easily broken and closed. 

Cremer and coworkers investigated a number of potentially homoconjugated cyclopropyl compounds such as the monohomotropylium cation the 1,4- and 1,3-bishomotropylium cation the trishomotropylium cation , the barbaralyl cation and the cyclobutenyl cation. All these cations have the choice between a closed cyclopropyl structure (la), an open cyclopolyenyl structure (Ic) and an intermediate structure (Ib) as demonstrated in the case of the monohomotropylium cation. 

Schleyer and coworkers investigated the nature of the 9-barbaralyl cation by solvolysis studies of the corresponding tosylate. The acetolysis of 9-deuterio-9-barbaralyl tosylate gave 9-barbaralyl acetate with complete deuterium scrambling, whereas solvolysis in more nucleophilic aqueous acetone gave the product alcohols with the label exchanged to only positions C3, C7 and C9 (equation 50). Solvolysis of 4-deuteriobicyclo[3.2.2]nona-2,6,8-trien-4-yl 3,5-dinitrobenzoate also resulted in the formation of the 9-barbaralols with deuterium scrambled to positions 1, 2, 8, 4, 5 and 6, with only trace amounts at C3 and (equation 51). 

A counterpart of [1] among the carbocations is the totally degenerate 9-barbaralyl cation C,HJ[2] which is observed by its sharp singlet in the H-nmr spectrum at about — I35°C. This extremely reactive ion makes use of mechanisms other than that of [1] for its degeneracy. By C-nmr spectroscopy employing the powerful isotopic perturbation technique, ion [2] has recently been shown to have structure [2] rather than being nonclassical (Ahlberg etal., 1981). 

A number of other symmetrizing processes exist in addition to those mentioned above. One such process is the Cope rearrangement, which does not involve the cationic centre, but can act to scramble a label. However, the Cope rearrangement (20) does not seem to be in operation in the 9-barbaralyl cation [2] which rather involves divinylcydopropylcarbinyl cationic rearrangements to achieve six-fold partial degeneracy. 

The totally degenerate barbaralyl cation, a C,HJ ion, has been proposed to be a rapidly equilibrating 9-barbaralyl cation [2] or the rapidly equilibrating nonclassical [37] which has Djh symmetry. Ahiberg et al. (1981) have perturbed a i C-Iabelled barbaralyl cation by eight deuteriums P CH(CD)g] chemical shifts created by this perturbation show that the ion has structure [2]. 

The first barbaralyl cation to be directly observed in superacid by H-nmr spectroscopy was the 9-methyl-9-barbaralyl cation [428] (Ahlberg et al., 1970a) (see p. 358). 

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