Topochemical photodimerization of (E)-3-benzylidene-4-chromanone derivatives from β-type structures directed by halogen groups

Halogen substituent plays an important role in the crystalline packing of aromatic compounds. The [2+2] photocycloaddition of (E)-3-benzylidene-4-chromanones in the crystalline state was investigated, and halogen substitution has been adopted to organize molecules with proper arrangement for photodimerization. Not halogen bonds, but the electron-withdrawing property of halogen atoms can enhance the face-to-face π–π interactions. Therefore, F, Cl or Br substitution at the para position of phenyl gave rise to almost the same β-structures with face-to-face π-stacking. Only resulted β-structures can undergo photodimerization, which gave the syn-HH (syn-head-to-head) products with high regio-/stereoselectivity.