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Download fileTailoring Atropisomeric Maleimides for Stereospecific [2 + 2] PhotocycloadditionPhotochemical and Photophysical Investigations Leading to Visible-Light Photocatalysis
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posted on 2014-06-18, 00:00 authored by Elango Kumarasamy, Ramya Raghunathan, Steffen Jockusch, Angel Ugrinov, J. SivaguruAtropisomeric maleimides
were synthesized and employed for stereospecific
[2 + 2] photocycloaddition. Efficient reaction was observed under
direct irradiation, triplet-sensitized UV irradiation, and non-metal
catalyzed visible-light irradiation, leading to two regioisomeric
(exo/endo) photoproducts with complete
chemoselectivity (exclusive [2 + 2] photoproduct). High enantioselectivity
(ee > 98%) and diastereoselectivity (dr > 99:1%) were observed
under
the employed reaction conditions and were largely dependent on the
substituent on the maleimide double bond but minimally affected by
the substituents on the alkenyl tether. On the basis of detailed photophysical
studies, the triplet energies of the maleimides were estimated. The
triplet lifetimes appeared to be relatively short at room temperature
as a result of fast [2 + 2] photocycloaddition. For the visible-light
mediated reaction, triplet energy transfer occurred with a rate constant
close to the diffusion-limited value. The mechanism was established
by generation of singlet oxygen from the excited maleimides. The high
selectivity in the photoproduct upon reaction from the triplet excited
state was rationalized on the basis of conformational factors as well
as the type of diradical intermediate that was preferred during the
photoreaction.
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irradiationreaction conditionsphotoproductphotophysical studiesmaleimidetriplet lifetimesbasisEfficient reactiontriplet energy transferphotocycloadditionsinglet oxygenalkenyl tetherTailoring Atropisomeric MaleimidesHigh enantioselectivitysubstituentPhotophysical Investigationstriplet energiesUVroom temperature