Influence of Fluorine Substitution on the Unusual Solid-State [2 + 2] Photo-Cycloaddition Reaction between an Olefin and an Aromatic Ring

Solid-state
[2 + 2] photo-cycloaddition reactions observed so far
were exclusively between a pair of olefin bonds. Usually when the
phenyl–olefin bonds have been closely aligned, they were found
to be either photoinert or sliding of molecules takes place for [2
+ 2] cycloaddition reaction between olefins in the solid state, although
intramolecular phenyl–olefin reactions are well-known in solution.
In the crystal structure of [Zn2(ptol)4­(4spy)2] (ptol = para-toluate), the neighboring
4-styrylpyridine (4spy) ligands are organized in a head-to-tail manner.
On one side of the complex in the crystal structure, the olefin bonds
in the 4spy pairs are perfectly aligned to undergo cycloaddition reaction,
but on the other side, the olefin bond pairs are slightly offset and
found to be photoinert at 223 K forming only a dimer in single crystals.
The sliding of 4spy groups has been restricted by the steric hindrance
of the adjacent methyl group of the ptol ligands. A similar packing
of 2-fluoro-4′-styrylpyridine (2F-4spy) pairs was found in
[Zn2(ptol)4­(2F-4spy)2]. Again,
normal cycloaddition reaction occurs on one side of the 2F-4spy ligand
pairs, whereas the second offset 2F-4spy ligand pairs undergo a rare
[2 + 2] cycloaddition reaction between the fluorophenyl group and
olefin bond resulting in the formation of a one-dimensional coordination
polymers containing a bicyclic product in a quantitative yield. The
bicyclic ring in the photoproduct can be thermally cleaved back to
olefin and phenyl groups. These observations have been confirmed by
single-crystal X-ray crystallography, 1H NMR, and 19F NMR studies. Density functional theory calculations were
performed to elucidate the nature of the interactions between the
fluorophenyl and olefin groups. The greater reduction of aromaticity
of 2F-4spy in the excited singlet state compared to the 4spy system
may explain the observed reactivity difference between the two systems.
The improved reactivity in 2F-4spy may also be attributed to the fact
that the olefin–phenyl distance is shorter in 2F-4spy than
in 4spy (3.63 versus 3.69 Å). This solid state phenyl–olefin
photodimerization helps to pave the way for making new bicyclic derivatives.