The Norcaradiene Walk Rearrangement

The strongest presumptive evidence for facile sigmatropic shifts that obey the Woodward-Hoffmann Rules is cited for molecules with a cyclic a frame in which the migrating bond is to carbon rather than to hydrogen [30, pp. 98-103]. A particularly intriguing example, discovered by Berson and Wilcott some 25 years ago [40], is illustrated in Fig. 8.10. 

Accordingly, the norcaradiene walk rearrangement j = 5) was assumed to proceed with retention at C7, along the WH-allowed pathway [41] until 1974, when Klarner and his coworkers showed that it occurs with inversion [42]. Since this pathway had been characterized as forbidden on the ground-state surface Klarner [43] initially discussed it in terms of biradical transition states, but the one-step nature of the reaction became evident when it was found that that there is no one-center epimerization at C [44]. Rearrangement with inversion is indeed generally observed. For example, it occurs in bicyclopentene (j = 3) [35, p. 9] and c 5-bicyclo[6.1.0]nona-2,4,6-triene (j = 7) [35, p. 22] as well. In these cases it was simply characterized as allowed whereas the forbiddenness of the inversion pathway for the norcaradiene walk was assumed [35, p. 15] to be mitigated by subjacent orbital interaction [45, 46]. 

Both reaction paths are analysed in Fig. 8.11 The reaction involves 12 mobile electrons assuming that neither TS is a biradical, all we need consider are six doubly occupied orbitals four constructed from the a bonds to the migrating C atom and two tt orbitals. Both transition structures have a mirror plane passing through C7, Ci and C4, and the symmetry of their occupied MOs is characterized accordingly in the diagram on either side of the anasymmetrized reactant-product pair. 

The WH-allowed and forbidden transition states differ in only one feature. In the former, the antisymmetric CH-bonding orbital is a and the p orbital 

Anasymmetrization of the occupied orbitals of norcaradiene is straightforward. The two bonds forming the cyclopropane ring involve the Pz and orbitals of C7 respectively in order for them not to vanish, the first MO must be taken as a and the second as a . The bonds to the substituent H atoms (a and b) both lie in the yz plane, so their positive and negative combinations both have to be a. C3 and C5 are reflected into each other if their Pz AOs are in-phase in the lower tt orbital they have to be taken out-of-phase in the upper. The occupied orbitals are thus anasymmetrized to [4xa - -2xa ]. As can be seen, both transition states correlate equally well with the anasymmetrized reactant-product pair, so both have to be characterized as allowed. The choice between them is evidently made by energetic factors that are not related to orbital symmetry, perhaps the better overlap of C7 s px than its Pz with the Pz AOs of C2 and Cq. 

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The observed stereochemistry of the norcaradiene walk rearrangement excludes orbital symmetry control (4) as well as a conformationally equilibrated diradical intermediate of type 46 (61). The stereoselective course of the walk re-... 

Walk rearrangements have been observed in many carbo- and heterocyclic norcaradiene cycloheptatriene systems upon thermal as well as photochemical excitation. Table 4 contains selected examples. In this connection, the very different thermal stability of ll,ll-dimethyl-l,6-methano[10]annulene and its radical anion is worth mentioning (59). The neutral hydrocarbon rearranges to 7,7-dimethyl-l,2-benzocycloheptatriene at temperatures between 150 and 190°C [Table 4, entry 3, X = C(CH3)2 log A = UA Ea = 35.9 kcal/mol]. The corresponding rearrangement of the radical anion produced by reduction of the hydrocarbon with potassium occurs at -110°C already. The activation energy is lowered here by about 25 kcal/mol over that of the hydrocarbon. The norcaradiene walk has also gained some synthetic importance, for example, in the preparation of the unsaturated bicycle 44, a precursor of heptalene (60). 

The stereochemistry of the norcaradiene walk, studied for the thermal as well as photochemical process, has attracted special attention. The thermal rearrangement is discussed first. 

Formation of 648 from 647 was experimentally discarded. Since the most basic site of 646 is the carbonyl group, both Bronsted and Lewis acids should first coordinate to this position. In the case of Bronsted acid, protonation could occur at the electron-rich 9-position, and this was followed by deprotonation at the 1 la-position, which is promoted by the carbonyl protonation. In contrast, the Lewis acid cannot add to the 9-position and thus the skeletal rearrangement to give 648 took place. This kind of rearrangement through norcaradiene tautomers, shown in Scheme 128, is called walk rearrangement in thermal reaction of cycloheptatrienes <2002CL260>. 

Upon heating at 300°C, 3,7,7-trimethylcycloheptatriene 36c rearranges to 2,7,7- and 1,7,7-trimethylcycloheptatriene 37c and 38c (36). The key reaction of this interconversion is a stepwise walk of C-7 in the norcaradiene form (36a 37a 38a), by way of consecutive [1,5] sigmatropic carbon shifts. The possibility of two subsequent [1,3] carbon shifts involving intermediate trimethyl-... 

Figure 5. Walk rearrangement of chiral tropilidenes proceeding with inversion in each step (si process) (66, 70). The term tropilidene is used as a general designation for a cyclohepta-triene-norcaradiene valency tautomeric pair. Since only relative configurations of tropili-denes have been assigned, each configuration depicted in this and the following figures may actually correspond to the opposite sign of optical rotation.

The comparison of the walk rearrangements in the vinylogous systems bi-cyclo[2.1.0] pentene, norcaradiene, and bicyclo[6.1.0] nonatriene is particularly instructive in the context of the borderline between concerted and nonconcerted processes. Each rearrangement turns out to occur with inversion at the migrating carbon atom. In the case of bicyclopentene and bicyclononatriene, the stereochemical course is in accord with the predictions of orbital symmetry (4), but this is not so for the norcaradiene. Obviously stereochemistry is no reliable criterion for the distinction between a concerted and nonconcerted process in these cases. 

The cyclic ether (219) was converted into (220), which underwent rearrangement to (221) in a manner analogous to the celebrated norcaradiene ring-walk stereochemical analysis of the rearrangement indicated predominant inversion of configuration for both the photolytic and thermal processes and so symmetry considerations of the Woodward-Hoffman type may not be relevant to these (or at least the thermal) processes.559 Esters of pinocarveol were converted into myrtenyl compounds by CuOAc.560 cis- and rrans-Verbenyl and verbanyl acetates,561 562 related compounds563 and 2-methylverbanone564 have been synthesized for evaluation as attractants for cockroaches. 

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