An Absolute Asymmetric Synthesis of the [2 + 2] Cycloadduct via Single Crystal-to- Single Crystal Transformation by Charge-Transfer Excitation of Solid-State Molecular Complexes Composed of Arylolefins and Bis[1,2,5]thiadiazolotetracyanoquino-dimethane
The title electron acceptor, bis[ 1,2,5] thiadiazolotetracyanoquinodimethane (BTDA, 1), formed weak electrondonor-acceptor (EDA) complexes with arylolefins such as styrene (ST) and divinylbenzenes (DVs). Upon chargetransfer (CT) excitation of these complexes in MeCN, the [2 + 2]-type cycloadducts (2) were formed via a single
electron transfer. Similar cycloaddition reactions were efficiently induced when the solid-state molecular complexes
of 1 and arylolefins were irradiated. In contrast to the close similarities in solution photochemistry, the solid-state
reactivities of three isomeric divinylbenzenes (oDV, mDV, pDV) were quite dissimilar because of the different molecular
overlaps of 1 and DVs in the crystal. The apparent reactivity of the oDV- crystal was much higher than that in solution
as in the case of the ST-1 crystal, and adduct 2o was formed via the single crystal-to-single crystal transformation.
Because of the asymmetric crystal structure in oDV- , the optically pure product with 95% ee was obtained from the
achiral components without any external chiral source. On the other hand, the incomplete conversion of the mDV-1
crystal to 2m is due to the crystal-to-amorphous transformation, and the 1:2 adduct 6 was formed on irradiation of
the pDV-1 crystal which could not be obtained on excitation of the EDA complex in MeCN.
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