Controlled orientation of metalloporphyrins and regioselective epoxidations in thermotropic liquid crystals

The orientation of a porphyrin molecule in a thermotropic nematic liquid crystal can be controlled by molecular design. Thus, the porphyrin plane of tetraphenylporphyrin (TPP) is aligned parallel to the director of a liquid crystal whereas a porphyrin with orthogonal substitution of mesogenic 4-n-butoxybiphenyl appendages (MesogenP) is oriented in a perpendicular conformation. The alignment was determined by time resolved EPR spectroscopy of the triplet state of the free base porphyrin. Manganese TPP and MesogenP porphyrins then were used as catalysts for the epoxidation of alkenes using iodosobenze as oxidant. Reaction yields and regioselectivity for enlongated substrates such as cis-stilbene and 4-vinylbiphenyl were dependent on the alignment of the alkene and its carbon-carbon double bond relative to the director and metalloporphyrin catalyst.