Radical cations and triplet states of 1,2-disubstituted cyclopropanes: comparison of potential surfaces

Abstract

Radical ion pairs generated by photo-induced electron transfer from 1,2-disubstituted cyclopropanes to various acceptors undergo return electron transfer in pairs of singlet and triplet multiplicity. The pair energies relative to the reactant ground states and to accessible triplet states, respectively, determine the competition between the recombination pathways. The potential surfaces of the radical cations and triplet states of 1,2-diphenyl-, 1, and 1,2-dimethylcyclopropane, 2, have been examined by density functional theory calculations. The radical cation surfaces have minima at geometries that retain significant bonding between C-1 and C-2, preventing geometric isomerization of the radical cations. The triplet potential surfaces are dissociative with minimal rotational differentiation at long distances between C-1 and C-2.

Radical cations of disubstituted cyclopropanes: stereoelectronic effects on hyperfine coupling

“…The relative energies of cis ‐ and trans ‐ 5 •+ are marginally different, cis ‐ 5 •+ being 0.53 kcal/mol higher in energy. Concerning other conformers, the entire hypersurface of 5 •+ has been calculated for the full range of rotational conformers between C 1 and C 2 distances of 191.4 pm to 2.614 Å; the geometric isomers cis ‐ and trans ‐ 5 •+ were the only minima encountered . If stereoelectronic effects were to be present, they should manifest themselves in the 1 H hyperfine coupling constants of the geminal protons, which are non‐equivalent in cis ‐ 5 •+ .…”

Photochemically Induced Ring Opening of Spirocyclopropyl Oxindoles: Evidence for a Triplet 1,3‐Diradical Intermediate and Deracemization by a Chiral Sensitizer

“…Spirocyclopropyl oxindoles with the general structure I (Scheme 2) exhibit al actam motif which enables hydrogen bonding to sensitizers 1 and 2.Itwas hypothesized that the compounds might undergo ring opening upon triplet sensitization and form 1,3-diradical II as an achiral intermediate.I fs o, ad eracemization as generally described in Scheme 1w as conceivable for this substrate class.A lthough the ring opening of cyclopropanes upon triplet sensitization has some precedents, [19] there is also evidence that the compounds can undergo ring opening by as ingle electron transfer process. [20] Both reductive and oxidative quenching of aphotoexcited chromophore are possible and aracemization/ deracemization could also occur for example via putative radical cation III.…”

Enantioselective Visible‐Light‐Mediated Formation of 3‐Cyclopropylquinolones by Triplet‐Sensitized Deracemization

“…This hypothesis is supported by the association behavior (in [D 6 ]benzene at 25 8 8C) of 6a and ent-6a towards sensitizer 4,with ent-6a exhibiting an almost tenfold higher association constant K a than 6a (Scheme 3). On the contrary,the 1,3-diradical 8,which is al ikely intermediate in this process, [21,22] is expected to decay preferentially to product ent-6a owing to the geometric constraints in complex 4·8. Interestingly, preliminary DFT calculations [20] revealed that-in strong contrast to the allene deracemization [12b] -the distances between the two chromophores (thioxanthone carbonyl group and quinolone double bond) within 4·ent-6a and 4·6a are not significantly different.…”

“…Akinetic preference during the sensitization event is therefore not to be expected. On the contrary,the 1,3-diradical 8,which is al ikely intermediate in this process, [21,22] is expected to decay preferentially to product ent-6a owing to the geometric constraints in complex 4·8. [8][9][10] In other words,t he enantioselective cyclopropane formation within 4·8 does not match the intrinsic preference of the sensitization event.…”