Regio- and diastereoselective rearrangement of cyclopentane-1,3-diyl radical cations generated by electron transfer

Abstract

Cyclopentane-1,3-diyl radical cations are readily available from azoalkanes or bicyclo[2.2.0]pentanes (housanes) by electron transfer. These short-lived intermediates may stabilize by numerous chemical processes, predominantly by 1,2-migration to form the corresponding cyclopentene derivatives after back electron transfer. This review focuses on the regio-and diastereoselectivities in the rearrangement of the strained cyclopentane-1,3-diyl radical cations and explores the mechanism of this novel 1,2-migration. This highly selective transformation offers attractive opportunities for the convenient and efficient synthesis of complex polycyclic structures.

Cyclopropylcarbinyl-Type Ring Openings. Reconciling the Chemistry of Neutral Radicals and Radical Anions

“…[3][4][5][6][7][8][9][10][11] There are fewer examples of bond cleavage in radical ions which can be classified as rearrangements, i.e., bond breaking leads to a single species, a distonic radical ion (in which the charge and spin reside on the same species). [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Compared to neutral free radicals, our understanding of the mechanism and kinetics associated with rearrangements of radical ions, and consequently, their use as mechanistic probes or clocks, is in the dark ages. Sometimes the "rearrangement" of a radical ion is more complex than a simple, first-order process (e.g., ring opening radical cations generated from cyclopropyl arenes).…”

Role of Hyperconjugation in the 1,2-Shift Reactivity of Bicyclo[2.1.0]pentane and Cyclopropane Radical Cations: A Computational Study

“…The above findings suggest a possible relationship between the hyperfine coupling and the subsequent migrating ability of a hydrogen atom or substituent at the single methylene bridge of the 1a – 1d reactant radical cations, this migration providing the basis for synthetic applications. ,, Moreover, since the magnitude of the β-hydrogen coupling in radicals is a direct measure of hyperconjugation, this transfer of spin and/or charge into the σ orbital of the endo substituent could well provide part of the driving force for the reaction. Since hyperfine coupling constants are usually the only experimental data that are available to characterize the structures of radical cations, comparison is made between the computational and experimental results for the 1 H hyperfine coupling constants both in the housane radical cations 1a – 1d and in the series of structurally related methyl-substituted cyclopropane radical cations 2a – 2d (Chart ).…”

“…Although radical cations have been studied extensively over the past three decades, understanding their reactivity in terms of electronic structure has not advanced as rapidly as mechanistic details, and achieving this nexus remains a daunting challenge . One class of radical cation reactions that readily lends itself to probing this relationship is the stereoselective ring-opening of bicyclo[2.1.0]pentanes to cyclopentenes. − These rearrangements have found application as efficient steps in the organic synthesis of polycyclic compounds ,, and are characterized by a 1,2 shift of a hydrogen atom or substituent from the single methylene bridge to the radical cation site at the bridgehead carbons.…”

“…Also, a possible 1,3-shift involving the migration of a β-hydrogen from the dimethylene bridge cannot a priori be excluded . However, parallel studies on the 5-methylbicyclo[2.1.0]pentane epimers reveal a remarkable stereochemical memory effect , in which the pattern of reactivity can be linked directly to the structure of the reactant. − Thus, the exo-5-methylbicyclo[2.1.0]pentane radical cation 1b is transformed to the 1-methylcyclopentene radical cation by migration of the endo hydrogen in a 1,2 shift, whereas the endo-5-methylbicyclo[2.1.0]pentane radical cation 1c rearranges to the 3-methylcyclopentene radical cation by migration of the endo methyl substituent. , It is significant that, in each case, the migrating entity from the methylene bridge is the endo rather than the exo substituent, irrespective of whether the substituent is a hydrogen atom, a methyl group, − or the pendant CH 2 group of a spiro-substituted ring structure. , …”

Rearrangement of Annelated Housanes to Triquinane-Like Hydrocarbons by Electron Transfer (1,3-Cyclopentanediyl Radical Cations) and Acid Catalysis (Cyclopentyl Carbenium Ions)

“…The remarkable chemical behavior of the 1,3-cyclopentanediyl radical cations derived from bicyclo[2.1.0]pentanes (housanes) by electron-transfer oxidation is their regioselective and diastereoselective rearrangement to cyclopentenes, if properly substituted . For example, the housane 1 (Scheme ) afforded on treatment with triarylaminium hexachloroantimonate (Ar 3 N • + SbCl 6 - ) exclusively the cyclopentene 2 ; the regioisomeric 2 ‘ was not observed .…”