2-Bicyclo alkyl cations

Attempts have been made to generate a number of 2-bicyclo[ .1.0]alkyl cations under stable ion conditions Ionization of bicyclo[3.1.0]hexan-2-ol 57 in SbF5/S02ClF even at -140 °C gave the rearranged cyclohexenyl cation 59, presumably through the bicyclop. 1.0]hex-2-yl cation 60 (equation 40). However, ionization of bicyclo[4.1.0]heptan-2-... 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

The validity of equation (12) has been checked for several families of alkyl halides for which D and E /x- are known (Ref. 32, see particularly figure 6 therein). It was thus found that for v = 0.1 V s the constant is equal to 0.3 eV at 20°C (expressing D in eV and the potentials in V). Equation (12) was then applied to the approximate determination of unknown BDEs in several series of compounds undergoing dissociative electron transfer, namely, TV-halosultams,32 sulfonium cations,33 vicinal dihalides,34 1,3-dihaloadamantes, 1,4-dihalo-bicyclo[2.2.2]octanes, and l,3-dihalobicyclo[l.l.l]pentanes.35 In the latter case, the mutual influence of the two halogens could be rationalized thanks to the conversion of the peak potential data to bond dissociation energies. 

The reaction of 1,3-disubstituted bicyclo[2.1.0]pentanes with tris(4-bromophenyl)ammoniumyl hexachloroantimonate (the latter in catalytic amounts) leads to the corresponding cyclopentene after 1,2-hydrogen or 1,2-alkyl migration in the intermediary 1,3-cation-radicals (Adam and Sahin 1994 Scheme 7.47). 

In the acid-catalyzed rearrangement of bicyclo[3.1.1]heptanones, two different cations, 1 and 2, lead to two different bicycloheptanones via 1234-1243 rearrangement. The ionic process involves sequential 1,2-alkyl shifts. Substituents at C2 and C4 determine which cationic intermediate is more stable. 

Bridgehead iodides, namely bicyclo[n.l.l]alkyl iodides, react with xenon difluoride in dichlo-romethane at ambient temperature to form the corresponding bridgehead fluorides in high yields.1617 The process apparently involves a cationic intermediate forming as a result of heterolytic cleavage of the appropriate difluoroiodoalkane and has been named "fluorode-iodination . 

Tiecco et al. found that 4-acylp5Tidines were prone to ipso-substitution reactions with 3°— alkyl radical intermediates such as 1-adamantyl and bicyclo[2,2,2]octyl radical <76CC329>. They reasoned that the reaction began with an electron transfer from S04 to the heteroaromatic base leading to radical cation 87. Addition of Ad to C4 then gave acetate 88,... 

With larger bicyclo[n. 1. OJalkyl cyclopropyl derivatives such as 8 and 9, their solvolytic behavior follows from that of the simple alkyl-substituted cyclopropyl derivatives. With smaller bicyclo[n.l.0]alkyl cyclopropyl derivatives such as 10 and 11, however, where a trans-a y cation cannot be accommodated in the ring, the order of reactivity is reversed. In both the [6.1.0] and [3.1.0] examples mentioned above, the rates are given relative to cyclopropyl tosylate. The much higher reactivity of the endo-[3.1.Qi] system (11) over the endo-[6.1.0] system (9) reflects the stability of the almost strain-free cyclohexenyl allylic cation versus the cyclononenyl allylic cation which possesses both torsional and transannular strain. 

A synthetically useful example of this process is the conversion of 117 to 120, which involves a 1,2-alkyl shift, and was part of Hwu s synthesis of (-)-solavetivone. 38 jhe alkyl fragment is actually part of the bicyclic ling system, one arm of the bicyclo[4.4.0]decane ring system. Reaction of the OH unit with the Lewis acid resulted in formation of the tertiary cation 118, which was followed by a 1,2-alkyls shift to give 119, where the new cation is stabilized by the adjacent silicon of the trimethylsilyl group. 39 Loss of the trimethylsilyl group from 119 gives spiran (120). 

Of the carbocyclic sesquiterpenes found in Eremophila, the most elaborate are the tetracyclic enr-zizaenes (89,90,92,93). One possible sequence for the assembly of such a nucleus is given in Scheme 24. Cyclization of 2E,6Z-farnesyl pyrophosphate between the 1 and 6 position would generate the bisabolonium cation equivalent which, after a hydride shift, could further cyclize to a spiro carbocation. The tricyclic ring system can then be assembled by invoking alkylation of the cyclohexene double bond. The tertiary carbocation generated incorporates a bicyclo[3.2.1]octane system which can rearrange in two ways leading to the tricyclic sesquiterpenes metabolites found in Eremophila. 

Interesting data have been obtained from the solvolysis of alkyl-substituted bicyclo[3,l,0]hexanes The high solvolytic activity of cis-epimers in comparison with trans-epimers, the stereochemistry of this reaction and the equilibration of deuterium in the products testify to the intermediate formation of trishomocyclo-propenyl cations upon acetolysis of cis-raonomethyl- and dimethylbicyclo[3,l,0]-hexyl-3-tosylates. 

Alkyl groups may also delocalize the unpaired electron and charge density in radical cations formed from strained alkanes. For example, radical cations of bicyclo[1.1.0]butane and l,3-dimethylbicyclo[1.1.0]butane have been detected in a Freon matrix following y-irradiation of the parent hydrocarbon (Figure 2.21). 

---