A Study on the Photochemical Dimerization of Coumarins in the Solid State

Solid-state photochemical behavior of 28 substituted coumarins has been investigated. Of these twelve underwent photodimerization and this is remarkable in light of the inertness of coumarin itself in the solid state. X-ray crystallographic investigation of eight coumarins was undertaken with the view of understanding the role of packing in the crystal on their solid-state reactivity. Important findings include the identification of acetoxy and chloro substituents as useful “crystal engineering” groups and the results pertaining to subtler aspects of topochemical postulates. X-ray crystal structure analyses of 7-chlorocoumarin and 7-methoxycoumarin reveal packing modes which are not commonly met. The former is arranged in a \beta type packing, the center-center distance between the reactive double bonds being 4.454 \AA, which lies beyond the so far accepted limit of 3.5-4.2 \AA. The reactive double bonds of 7-methoxycoumarin, on the other hand, are rotated by $65^O$ with respect to each other with the center-center distance between the double bonds being 3.83 \AA. Inspite of these unfavorable arrangements photodimerization of the above two coumarins in the solid state occurs through a topochemical process with large dimer yields. A careful analysis of the X-ray crystallographic resulta obtained from our investigations reveals that the two double bonds in the reactive crystals may be displaced with respect to each other both along the molecular plane as well as along the double bond axis. Thus the normally accepted dictums that in the photoreactive crystals the doube bonds should be within a distance of 4.2 \AA, and that they be parallel are no longer operational

Topochemical photodimerization of 7-acetoxycoumarin: the acetoxy group as a steering agent

Among the various substituted coumarins investigated only 7-acetoxycoumarin is observed to photodimerize topochemically in the crystalline state and this observation may be of importance in the context of "crystal engineering"

A study in crystal engineering: solid-state photodimerization of chloro- and methyl-coumarins

Use of chloro and methyl substitution in crystal engineering and their interchangeability in terms of mode of packing have been examined in a series of substituted coumarins. Photoreactivity in the solid state lists been correlated with the crystallograhic structures of these coumarins. The packing of chloro-substituted aromatic compounds has been investigated by analysing the arrangement of 132 compounds. Results substantiate the use of the chloro group as a steering agent and show that the chloro and methyl groups are not always interchangeable

Topochemical solid state photodimerization of nonideally oriented monomers: 7-chlorocoumarin and 7-methoxycoumarin

Photodimerizations of 7-chlorocoumarin and 7-methoxycoumarin in the solid state were studied; syn head-to-head and syn head-to-tail dimers are the respective products of irradiation in the solid state. X-ray crystal structure analyses of 7-chlorocoumarin and 7-methoxycoumarin reveal unusual features. 7-Chlorocoumarin is arranged in a $\beta$-type packing, the centre-to-centre distance between the reactive double bonds being $4.454 \AA$, which lies outside the limit accepted at present of $3.5-4.2 \AA$. In contrast, the reactive double bonds of 7-methoxycoumarin are rotated by about 65° with respect to each other with the centre-to-centre distance between the double bonds being $3.83 \AA$. In spite of these unfavourable arrangements photodimerization of these two coumarins in the solid state occurs through a topochemical process with large dimer yields

A study in Crystal engineering: Solid-state photodimerization of chloro- and methyl-coumarins

Use of chloro and methyl substitution in crystal engineering and their interchangeability in terms of mode of packing have been examined in a series of substituted coumarins. Photoreactivity in the solid state lists been correlated with the crystallograhic structures of these coumarins. The packing of chloro-substituted aromatic compounds has been investigated by analysing the arrangement of 132 compounds. Results substantiate the use of the chloro group as a steering agent and show that the chloro and methyl groups are not always interchangeable

Topochemical solid state photodimerization of non-ideally oriented monomers: 7-chlorocoumarin and 7-methoxycoumarin

Photodimerizations of 7-chlorocoumarin and 7-methoxycoumarin in the solid state were studied; syn head-to-head and syn head-to-tail dimers are the respective products of irradiation in the solid state. X-ray crystal structure analyses of 7-chlorocoumarin and 7-methoxycoumarin reveal unusual features. 7-Chlorocoumarin is arranged in a β-type packing, the centre-to-centre distance between the reactive double bonds being 4.454 Å, which lies outside the limit accepted at present of 3.5 - 4.2 Å. In contrast, the reactive double bonds of 7-methoxycoumarin are rotated by about 65° with respect to each other with the centre-to-centre distance between the double bonds being 3.83 Å. In spite of these unfavourable arrangements photodimerization of these two coumarins in the solid state occurs through a topochemical process with large dimer yields