Stereochemical memory versus Curtin-Hammett behavior in the rearrangement of 1,3-cyclopentanediyl radical cations derived from housanes through structural effects on conformational control.

The electron-transfer-catalyzed rearrangement of the housanes 1 affords regioselectively the two cyclopentenes 2 and 3 by 1,2-migration of a group at the methano bridge. Appropriate ring annelation in the intermediary cyclopentane-1,3-diyl radical cation 1(*+) changes the stereochemical course of the rearrangement from complete stereoselectivity (stereochemical memory) for the structurally simple housane 1b to partial loss of stereoselectivity through competing conformational interconversion for the tricyclic housane 1c. Additional cyclohexane annelation, as in the tetracyclic housane 1a, results in complete loss of stereocontrol through Curtin-Hammett behavior, as substantiated by the viscosity dependence on the product ratio of the rearrangement. Whereas in the radical cations 1b(*+) and 1c(*+) the 1,2-shifts (k(2) and k(3)) are faster than the conformational anti <==> syn change (k(1), k(-1)), the reverse applies for the radical cation 1a(*+). Such structural manipulation of conformational effects in radical cation rearrangements has hitherto not been documented.

Stereochemical memory in the regioselective and diastereoselective rearrangement of tricyclo[3.3.0.0 2,4]octanes (housanes) by electron transfer (1,3-cyclopentanediyl radical cations) and acid (cyclopentyl carbenium ions) and silver-ion catalysis.

The electron-transfer-catalyzed rearrangement of the housanes 5 affords regioselectively only the two cyclopentenes 6 (CH(3) migration) and 7 (R migration) by 1,2-migration of the two groups at the methano bridge to the methyl terminus. The 1,2-shift of the CH(3) group prevails, and the rearrangement ratio is essentially insensitive to the migratory aptitude of the R substituent. This stereochemical memory effect derives from the conformational impositions on the stereoelectronic requirements during the 1,2-migration in the 1,3-radical-cation intermediates. Similar regioselectivities and diastereoselectivities are observed for the TFA-catalyzed and silver(I)-ion-promoted rearrangements, whereas the rearrangement catalyzed by HClO(4) affords a complete reversal in the product selectivity and both the regioselectivity and the diastereoselectivity are much reduced. Migration to the phenyl terminus is favored to afford the 6' and 7' cyclopentenes, of which the former (CH(3) migration) dominates. For the minor regioisomer, only the cyclopentene 6 is formed by an exclusive 1,2-shift of the CH(3) group. This dichotomy in product selectivities is rationalized in terms of two distinct mechanisms for the various activation modes: a common one for the electron-transfer-induced, TFA-catalyzed, and silver(I)-ion-promoted rearrangements and a different one for HClO(4).

Rearrangement of annelated housanes to triquinane-like hydrocarbons by electron transfer (1,3-cyclopentanediyl radical cations) and acid catalysis (cyclopentyl carbenium ions).

The electron-transfer-catalyzed rearrangement of the annelated housane 4a on treatment with tris(p-bromophenyl)aminium hexachloroantimonate (TBA(*)(+)SbCl(6)(-)) affords regioselectively the two isomeric olefins endo-5a and 6a by 1,2 migration of the two groups at the methano bridge. Acid-catalyzed rearrangement gives in addition to endo 5a and 6a also the regioisomer endo-7a as major product. The formation of both rearrangement products endo-5a and 6a suggests a planar conformation for the radical-cation and carbocation intermediates. The regioselectivity is rationalized in terms of electronic stabilization of the radical versus cationic sites by the substituent at the rearrangement termini in the radical-cation and carbocation intermediates. Of interest for preparative purposes, the annelated housane 4a leads under electron-transfer conditions to unusual triquinane-related olefins by means of an unprecedented synthetic pathway.