A study on the photochemical dimerization of coumarins in the solid state

Abstract

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 0-type packing, the center-center distance between the reactive double bonds being 4.454 A, which lies beyond the so far accepted limit of 3.5-4.2 A. The reactive double bonds of 7-methoxycoumarin, on the other hand, are rotated by 656 with respect to each other with the center-center distance between the double bonds being 3.83 A. 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 results 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 A and that they be parallel are no longer operational.

7-Amino-4-methylcoumarin

“…1. Bond lengths and angles in the coumarin moiety are normal (Gnanaguru, Ramasubbu, Venkatesan & Ramamurthy, 1985). The packing of the molecules in the unit cell viewed down the a axis is shown in Fig.…”

Structure of α-benzylidene-(±)-piperitone, an exception to topochemical behaviour

“…Also a few examples (Ariel, Askari, Scheffer, Trotter & Walsh, 1984) where, in spite of crystal-lattice alignment of the reactive double bonds, absence of [2+2] photocycloaddition in the solid state is known. In continuation of our studies on the subtler aspects (Gnanaguru, Ramasubbu, Venkatesan & Ramamurthy, 1985) of the topochemical principles we have been investigating the photobehaviour of dimerizable olefins in the solid state. The photobehaviour of benzylidene-(+)-piperitone reported here is exceptional viewed in the context of its structure in the solid state.…”

Fotoquímica de chalconas fluoradas no estado sólido

“…A fotólise de sistemas a,b-insaturados que no estado cristalino apresentam distâncias intermoleculares de cerca de 4 Å é capaz de levar à formação de ciclobutanos, como observado para ácido cinâmico e derivados 15 , cumarinas 16 , chalconas e outras cetonas a,binsaturadas 17,18 . A substituição por átomos de cloro, especialmente por dois áto-mos de cloro, em moléculas aromáticas tende a direcioná-las, quando no estado cristalino, a distâncias entre as ligações duplas de aproximadamente 4 Å 4 , permitindo assim a formação de fotodímeros 10 .…”

“…Assim, os movimentos atômicos que se seguem durante uma determinada reação exercem pressão sobre as paredes da cavidade, a qual torna-se distorcida, limitando tanto os movimentos de deslocamento molecular quanto as mudanças conformacionais que são permitidas durante a reação 12 . A nível molecular, tem sido mostrado que as reações no estado sólido somente podem ocorrer quando existe uma forte similaridade entre a estrutura dos reagentes, o estado de transição e o produto final [12][13][14] .A fotólise de sistemas a,b-insaturados que no estado cristalino apresentam distâncias intermoleculares de cerca de 4 Å é capaz de levar à formação de ciclobutanos, como observado para ácido cinâmico e derivados 15 , cumarinas 16 , chalconas e outras cetonas a,binsaturadas 17,18 . A substituição por átomos de cloro, especialmente por dois áto-mos de cloro, em moléculas aromáticas tende a direcioná-las, quando no estado cristalino, a distâncias entre as ligações duplas de aproximadamente 4 Å 4 , permitindo assim a formação de fotodímeros 10 .…”