X-ray crystallographic investigation of strained small rings and some photoreactive coumarins

Abstract

The thesis entitled, “X?ray Crystallographic Investigations of Strained Small Rings and Some Photoreactive Coumarins” has been divided into two parts. Part I deals with the structural investigations of four cyclopropane derivatives and part II, the photochemistry and structure of four coumarin derivatives. The strained ring system discussed in this thesis is the cyclopropane ring. The properties of cyclopropane, so different from other higher cycloalkanes, has for a long time been attributed to the Baeyer’s strain. However, as experimental evidence accumulated, it became clear that these anomalies were due to the peculiar bonding in cyclopropane. Many theoretical models were proposed which are used in providing qualitative information on the geometrical perturbation caused by various substituents on the ring. Chapter I of this thesis is a brief review dealing with the nature of bonding and interaction of substituents with cyclopropane. Various models in which the substituent can interact, the possible conformational preferences which a substituent can undertake and the extent to which bond?length asymmetry is introduced by these substituents are brought into focus. Chapter II discusses the structure solution of three cyclopropane derivatives. Each of these has four substituents in common (Figure S.1). Since all these are strongly electron withdrawing, a drastic change in the geometry of the ring can be expected. The crystal data of compounds (I), (II) and (III) and the final R?factors are recorded below. I???II???III a?14.662(6) Å?7.886(5)?10.694(8) Å b?10.492(5)?22.011(8)?11.743(8) c?7.375(3)?8.200(3)?12.658(8) ??–?103.12(5)°?113.20(7)° Z?4?4?4 space group?Pna2??P2?/a?P2?/c R?0.040?0.042?0.054 The extent to which these groups have affected the geometry has been explored on the basis of the molecular orbital model for cyclopropane and the conformations taken up by these groups. As expected from an analysis of MO model for cyclopropane, the bond flanked between the strongly electron?withdrawing cyano and carboethoxy groups is significantly longer than the average bond length found in cyclopropane. Moreover, the lengthening is shown to depend on the conformation taken up by these substituents. Chapter III: The molecular structure of Feist’s acid is shown in (Figure S.2). The molecule provides a simple system for the investigation of the effect of both electron?withdrawing and donating groups on the cyclopropane ring. For this reason the structure of the molecule has been reinvestigated. The compound crystallizes in P1 with a = 4.482(1), b = 7.697(1), c = 9.168(3) Å, ? = 96.42(2), ? = 99.89(2) and ? = 77.41(1)°, the number of molecules in the unit cell being 2. The structure has been solved by direct methods and refined to an R value of 0.038. The carboxyl groups are disordered in the crystal lattice. The geometry of the ring is found to be in close agreement with that of methylenecyclopropane indicating that the influence of the carboxyl groups on the cyclopropane geometry is not significant. Part II of the thesis is concerned with the solid?state photochemical and crystallographic investigations of several coumarin derivatives synthesized by the author (Figure S.3). It is known that the crystals of coumarin do not photolyse owing to the unfavourable molecular packing in the crystal lattice. Thus the moiety provides a simple molecular framework for finding out the effect of substituents on molecular packing favourable for inducing photoreactivity. Chapter IV is a brief review dealing with the various aspects of solid?state organic photochemistry. The differences in behaviour of compounds in solid state and fluid state, both in unimolecular as well as bimolecular reactions are discussed. One of the many coumarin derivatives which underwent dimerization in solid state is 7?methoxycoumarin (Chapter V). A single dimer corresponding to syn?head?to?tail configuration (~90% yield) was obtained. The crystal data of the monomer and dimer are: 7?methoxycoumarin: a = 6.834(3), b = 10.672(4), c = 12.600(7) Å ? = 108.19(3), ? = 95.23(4), ? = 95.22(3)°. The compound crystallizes in P1 with Z = 4. Dimer of 7?methoxycoumarin: a = 14.767(3), b = 12.068(3), c = 18.798(4) Å. The molecule crystallizes in Pbcn with Z = 8. The structures of monomer and dimer have been refined to an R value of 0.056 and 0.050 respectively. The two molecules in the asymmetric unit of the monomer crystal are the nearest neighbours with their reactive double bonds not properly oriented for dimerization. Chapter VI discusses the structure–reactivity relationship of two coumarin derivatives, 8?methoxycoumarin (8M) and 4?chlorocoumarin (4Cl). The crystal data are presented below: 8M: a = 7.529(1), b = 13.803(2), c = 16.176(2) Å, ? = 102.07(2)°, P2?/a, Z = 8. 4Cl: a = 7.271(1), b = 12.816(3), c = 9.078(2) Å, ? = 111.93(3)°, P2?/n, Z = 4. The packing of the reactive monomeric units in 4Cl again, as in the 7?methoxy derivative, is not suitable for a topochemical reaction. Surprisingly, however, both 8M and 4Cl crystals underwent photodimerization to yield about 50% and 40% dimers respectively. The dimers, being sparingly soluble in most organic solvents, could not be crystallized for undertaking crystal structure analysis. However, the structure of the dimers could be determined unambiguously from the NMR spectra. Substitution of an acetoxy group at the 7?position produced, on the other hand, a favourable molecular packing resulting in topochemical reactivity (Chapter VII). 7?acetoxycoumarin crystallizes in P2?/c with a = 3.833(1), b = 22.675(4), c = 9.791(2) Å and ? = 96.23(2)° with Z = 4. A single dimer at yields greater than 80% is obtained whose structure has again been deduced from its NMR spectrum unambiguously. The dimer, syn?head?to?head in configuration, has a 1:1 correspondence to the geometrical relationships existing between two nearest monomers in the monomer crystal lattice. The influence of an acetoxy group in steering the molecules into the ??type packed structure has been explored. The final Chapter (Chapter VIII) of this thesis is devoted to a discussion of the results of the crystal analyses of the 7?methoxycoumarin, 8?methoxycoumarin and 4?chlorocoumarin as well as 7?acetoxycoumarin with particular reference to the packing modes of these molecules in the respective crystal lattices and the observed photoreactivity of these crystals. A probable mechanism by which photodimers could be obtained from 7?methoxycoumarin, 8?methoxycoumarin and 4?chlorocoumarin is briefly mentioned. The strategic importance of certain groups in solid?state photochemical reactions is discussed.