Structure - reactivity correlations of photochemical & thermal reactions in the solid state

Abstract

The thesis entitled "Structure-Reactivity Correlations of Photochemical and Thermal Reactions in the Solid State" has been divided into two parts. Part I deals mainly with studies on the role of steering groups such as sulphur, chloro, and bromo groups to preorganize the reactive partners in the crystal and the influence of molecular topology on crystal packing. In Part II, two thermally induced molecular processes in the solid state are dealt with, namely nitrogen inversion and a nucleophilic substitution reaction. Attempts are made to correlate the observed reactivity in these cases with the anisotropic thermal motion. The thesis begins with a brief review (Chapter 1) of the various aspects of structure-reactivity correlations of photochemically induced reactions in the crystalline state. The main emphasis is on the steering abilities of various groups and different strategies adopted to preorganize the molecules that are conducive for photoreaction. Some interesting thermally induced reactions are also briefly dealt with in the later part of this chapter. The suggestion that sulphur could be a steering group prompted the author to investigate the photoreactivity and crystal structure analysis of two bis(butoxycarbonyl)-substituted tetrathiafulvalenes (Chapter 2). In the context of S···S interactions interpreted in terms of nucleophile-electrophile pairing, this photoreaction shown in Scheme 1 is interesting, and attempts were made to explain the photoreactivity in terms of crystal packing. The formation of a “cage product” (II) was understood as a result of double photocyclodimerisation where both pairs of the double bonds react at some reaction sites. The X-ray structural investigation of CISTTF shows that the molecules are packed in such a way that the two reactive partners are one over the other with a separation distance of 3.942 Å. The formation of (I) was the consequence of a mere [2+2] cycloaddition reaction, whereas II results from double cyclodimerisation reaction. The juxtaposition of the double bonds in the crystal lattice as evidenced by the crystallographic investigations was suitable for the formation of both the photoproducts. The lack of such an arrangement in TRATTF made it photoinert where the reactive partners are too far away to favour any dimerisation. The role of sulphur as a steering group in the photoreactive CISTTF could not be substantiated as there were no S···S short contacts observed in the crystal. The utility of chlorine as a steering group is examined in Chapter 3. The directionally anisotropic and attractive interactions of the chloro group observed in many crystal structures could be exploited to perform solid-state photochemical reactions otherwise inaccessible. Scheme 2 is an example of an attempt to utilize such interactions based on the observation that the unsubstituted reactant is photoinert in the crystalline state. The parachloro derivative (III) undergoes a topochemical dimerisation giving an anti head-to-tail dimer (IV) instead of the syn head-to-head dimer expected on the basis of the ability of the chloro group to induce ?-type packing. Extensive photochemical and X-ray crystallographic investigations. The results pertaining to the photodimerisations of two bromocoumarins and p-bromobenzylidene-dl-piperitone are presented in Chapter 4. Photochemical and crystallographic investigations carried out to examine the steering ability of bromine include two distinct molecular systems: one planar (VI & VII) and the other non-planar (VIII) (Scheme 3). Careful analyses of the molecular packing modes in the above systems reveal that the steering ability of the bromo group is manifested in planar coumarins, giving rise to syn head-to-head photoproduct. In (VIII), the molecules do not acquire ?-packing. However, the propensity of the bromo group to come close is clear from the short Br···Br contact of 3.549 Å present in the crystal structure of (VIII). The non-planarity of the benzylidene piperitone molecules seems to be an important factor leading to the observed centrosymmetric arrangements of the molecules. During the course of the studies on photochemical behaviour of piperitones (I), the author observed that the molecules isomerise in the solution phase. Since the conformational freedom is increased enormously in the liquid phase, it was interesting to determine the fate of electronically excited I in dilute solutions and in melt. The results are represented in Scheme 4. These compounds undergo trans-cis photoisomerisation followed by electrocyclic ring closure, giving rise to phenanthrene derivatives in the solution state. The trans-cis isomerisation is found to be a very efficient process and the intermediate dihydrophenanthrenes are found to be very unstable. The plausible Norrish type-II photoproducts are not observed. Irradiation of the melt or concentrated solution does not yield any photodimer. Some of the mechanistic aspects of these reactions are presented in Chapter 5, and the structure of a typical photoproduct has been determined by X-ray structural analysis. The conditions leading to the formation of 2 rather than 1 from the irradiation of I are also discussed. Chapter 6 deals with a thermally induced solid-state methyl migration reaction (Scheme 5). The original aim of the study was to understand the course of the reaction in the light of the anisotropic displacement parameters obtained from the structural investigations. It was observed by the candidate that the rearrangement of 4 to 5 takes place in the crystalline state. But the crystal structure analysis of 4 reveals that the packing is not conducive for an intermolecular methyl transfer, nor does the molecular geometry favour the intramolecular methyl transfer. In order to settle this problem, detailed isotopic labelling experiments were performed in collaboration with Prof. J. D. Dunitz. The reaction has been carried out with mixed crystals composed of 4 and deuterio-methylated 4-d?, and analyses by fast atom bombardment mass spectroscopy showed that the product consists of a 1:2:1 mixture of the non-, tri-, and hexadeuteriated species-the mixture expected if the solid-state reaction proceeds by intermolecular methyl transfers. From this result, together with slower rates of conversion in the single.