Structure reactivity correlations in organic solid state chemistry photo chemical hydrogen abstraction

Abstract

The thesis entitled "Structure-Reactivity Correlations in Organic Solid State Chemistry: Photochemical Hydrogen Abstraction" has been divided into two parts. Part I deals with the Xray analyses of four aromatic nitro compounds and their correlation with solidstate reactivity. Part II deals with the Xray structures of inclusion complexes of deoxycholic acid with two thioketones and the structure-reactivity correlation of one of them. The thesis begins with a brief review (Chapter 1) of various aspects of organic solidstate chemistry, with emphasis on intramolecular hydrogen abstractions in the crystalline state and the effect of conformation on unimolecular reactions. Chapters 2, 3, and 4 describe the singlecrystal Xray analyses of four aromatic nitro compounds (I)-(IV) (Fig. S.1). Intensity data were collected on a Nonius CAD4 diffractometer, and all structures were solved by direct methods using the program MULTAN80. Crystal data for these compounds are recorded below:

Crystal Data Summary (Keep only the corrected formatting; numerical values are unchanged.)

Compounda (Å)b (Å)c (Å) (°)ZSpace GroupR(I)9.226(1)11.104(1)14.622(1)107.79(1)4P2/n0.050(II)9.610(1)14.619(1)11.006(1)115.43(1)4P2/n0.059(III)24.369(4)7.834(1)31.956(5)97.39(1)16P2/n0.073(IV)6.388(2)13.848(4)8.578(3)94.05(3)2P20.067

The solidstate and solution photochemistry of these compounds has been reported in the literature. Upon irradiation with UV light ( 280 nm), these compounds undergo intramolecular hydrogen abstraction involving the nitro group and the tertbutyl methyl hydrogens. Xray studies of these compounds were undertaken to: (i) identify the hydrogen atom being abstracted, and (ii) determine the geometrical requirements (distances and angles involving the reacting groups) for such hydrogen abstraction, which involves the n,* excited state of the nitro group.

Chapter 2 This chapter presents structure-reactivity correlations of compounds (I) and (II). Two most probable hydrogenabstraction sites were identified on the basis of four geometrical parameters:

O···H distance C-H···O angle N-O···H angle The angle between the O···H vector and its projection on the mean plane of the nitro group

These two nearly symmetrical choices were reduced to one by analyzing intermolecular short contacts during possible rotation of the tertbutyl group about the C(phenyl)-C(tertbutyl) bond. Such rotation appears necessary for biradical cyclization, as the radical centers are initially too far apart for interaction. The hydrogen atom abstracted was thus uniquely identified based on both intramolecular geometry and packing considerations. The inability of these nitro compounds to abstract hydrogen from the orthobenzylic methyl groups was also rationalized.

Chapter 3 This chapter discusses the crystal and molecular structure of compound (III). Notably, there are four independent molecules in the asymmetric unit. The orientation of the tertbutyl group in three molecules is similar to that in compounds (I) and (II), whereas the fourth molecule shows a different orientation. The reactivity and selectivity of compound (III) are interpreted in light of results from (I) and (II). Intramolecular geometry and crystal packing of compounds (I)-(III) are compared.

Chapter 4 This chapter deals with the molecular structure and hydrogenabstraction selectivity of compound (IV). This molecule contains only one nitro group ortho to the tertbutyl group, and one substituent on the tertiary carbon is a -CHCl group. Previous studies indicate that nitro groups abstract hydrogen from the -CHCl group. Xray results for this compound fully corroborate the criteria used for compounds (I)-(III). A noteworthy structural feature is that the benzene ring adopts nearly a boat conformation.

Chapter 5 This chapter reviews the structure and properties of inclusion complexes and their influence on organic reactions, with special reference to reactions conducted in deoxycholic acid (DCA) (V, Fig. S.1). The control and modification of reactivity using solidstate inclusion complexes is discussed.

Chapter 6 This chapter presents the crystal structures of two inclusion complexes of DCA-with ditertbutylthioketone (VI) and thiocamphenilone (VII) (Fig. S.1)-and the photochemical reaction of the DCA-(VI) complex. Crystal data are given below: Complexa (Å)b (Å)c (Å)Space GroupRDCA-(VI)13.933(1)27.294(3)7.285(2)P2220.101DCA-(VII)13.738(2)27.203(4)7.189(1)P2220.158 Thioketones are generally highly reactive in solution toward oxidation and reduction. Despite this, the DCA-(VI) complex remained stable even after 50 days of continuous UV irradiation. The absence of intermolecular addition between DCA and the thioketone is attributed to intermolecular distances in the crystal lattice. The lack of oxidation by atmospheric oxygen is explained by tight packing, preventing diffusion of oxygen into the lattice. In both complexes, the thioketone molecules exhibit twofold disorder. In the DCA-(VII) complex, the guest’s orientation from crystallography was confirmed through potential energy calculations.

The lists of observed and calculated structure factors are provided at the end of the thesis. Papers already published or accepted for publication, based on the work reported here, are listed subsequently.