The Putative Role of Homoconjugation in Radical Cations: Electron Transfer Photochemistry of 7-Methylenenorbornadiene and 7-Methylenequadricyclane

Abstract

Photoinduced electron transfer from 7-methylenenorbomadiene, MN, and 7-methylenequadricyclane, MQ, to an excited sensitizer/acceptor in the presence of methanol generates products of several structure types. All products require nucleophilic capture of the radical cations, MN,+ and MQ'+, by methanol, followed by rapid rearrangements of the resulting free radicals to C' as the key intermediate. The short lifetime of the primary products of capture is ascribed to the allylic nature of their C4-C5 bonds. The stereochemistry of the methoxy groups in the products indicates that the nucleophile attacks MQ*+ exclusively from the "exo" face ("backside" attack), whereas MN*+ shows, in addition, a limited degree of attack from the "endo" face.

Electron transfer photochemistry of 7-(spirocyclopropane)quadricyclane: capture of a radical cation and sequential cyclopropylcarbinyl rearrangements

“…The reaction is initiated by electron transfer from 1 to the excited singlet state of CNN, generating the corresponding radical ion pair. In analogy to the parent radical cation, Q ᭹ + , 5 and its 7-methylene and 7-keto derivatives, 4,6 the key radical cation, 1 ᭹ + , may undergo three reactions: back electron transfer, (unimolecular) valence isomerization to the corresponding norbornadiene radical cation, 5 ᭹ + , or (bimolecular) nucleophilic capture by methanol (Scheme 1). Back electron transfer lowers the efficiency of the overall reaction but does not affect the product distribution.…”

“…The conversion to the mono-adduct, 2, could occur directly by hydrogen abstraction or by a two-step pathway, electron transfer from the sensitizer radical anion (CNN ᭹ Ϫ ) to 2 ᭹ , followed by protonation of the resulting anion, 2 Ϫ . Both processes are well documented [3][4][5] and, occasionally, compete with each other. 5 The noticeable rate difference between the rearrangement of 6 ᭹ to 2 ᭹ and that of 2 ᭹ to 3 ᭹ (or between the conversion of 6 Ϫ to 2 Ϫ and that of 2 Ϫ to 3 Ϫ ) is ascribed to the more limited stabilization of intermediates bearing spin or charge on a primary carbon.…”

“…1,2 Methanol attacks several of these species in stereospecific fashion from the exo face; [3][4][5][6] at 5 M methanol, nucleophilic capture is considerably faster than the valence isomerization of Q ᭹ + into N ᭹ + . 1, 7 The primary free radicals formed by nucleophilic capture undergo rapid molecular rearrangements; for example, exo-3-methoxybicyclo[2.2.1]hept-5-en-2-yl (A ᭹ , X=CH 2 ) rearranges to anti-5-methoxytricyclo[2.2.1.0 2,6 ]heptan-3-yl (B ᭹ , X=CH 2 ) which, in turn, rearranges to syn-7-methoxybicyclo-[2.2.1]hept-5-en-2-yl, (C ᭹ , X=CH 2 ).…”

“…Free radicals B ᭹ and C ᭹ acquire a hydrogen atom (by reduction-protonation or hydrogen abstraction) or react with the sensitizer radical anion by aromatic substitution, generating a wide range of products. [3][4][5][6] Radical cations of 7-substituted derivatives undergo related reactions; thus, the electron transfer photoreaction of quadricyclanone in acetonitrile-methanol generates 7-syn-methoxybicyclo[2.2.1]hept-5-en-2-one via B ᭹ (X=C--O) and C ᭹ (X=C--O). 6 The quadricyclane system (1) bearing a 7-spirocyclopropane group is a particularly attractive substrate because the strained ring in the 7-position might permit two consecutive cyclopropylcarbinyl-to-allylcarbinyl rearrangements.…”

Electron Transfer Photochemistry of Homochrysanthemol: Intramolecular Nucleophilic Attack on the Cyclopropane Ring

“…Radical cations of molecules containing strained ring moieties as well as olefinic fragments have been the focus of much interest in recent years, including the conjugative and homoconjugative interactions between the two types of functions. Various substrates have been probed to delineate changes in the molecular geometry upon one-electron oxidation and to assess the spin and charge density distributions in the resulting radical cations , in some systems, this reaction is assisted by a nucleophile. − …”