Direct Observation of Deuterium Migration in Crystalline-State Reaction by Single Crystal Neutron Diffraction IV. “Hula-Twist” Rotation of a Long Alkyl Radical Produced by Photoirradiation

When the crystal of [(R)-1,2-bis(ethoxycarbonyl)ethyl](pyridine)bis(dimethylglyoximato)cobalt(III) was exposed to a xenon lamp, the chiral 1,2-bis(ethoxycarbonyl)ethyl group was partly inverted to the
opposite configuration and finally the racemic group was produced with retention of the single crystal form.
To make clear the mechanism, the hydrogen atom bonded to the chiral carbon of the chiral group was
exchanged with the deuterium atom and the crystal was exposed to the xenon lamp for 3 days. The crystal
after irradiation was analyzed by neutron diffraction. About 33% of the (R)-isomers were inverted to the (S)
isomers in a crystal. The deuterium atom in the (S)-isomer was bonded to the same chiral carbon atom.
This result clearly indicates that the inversion proceeds in the three steps; (i) the Co−C bond was
homolytically cleaved by photoirradiation and the 1,2-bis(ethoxycarbonyl)ethyl radical and Co(II) were
produced, (ii) the radical rotated by 180° directing the C−D bond to the cobalt atom and the opposite
plane of the radical faced to the cobalt atom, and (iii) the radical made a bond with Co(II). Because the
peripheral atoms of the long radical occupy approximately the same positions in the process of the radical
rotation, the crystal was not decomposed. The above rotation is a good example of hula-twist rotation in
the process of photoisomerization of polyenes such as rhodopsin.