Organic Photochemistry

V. Ramamurthy

• Publisher: CRC Press
• Year: 1997
• eBook ISBN: 9781000612127
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Features surveys of all areas of organic, inorganic, physical and biological photochemistry. The text serves as a source of scientific findings pertinent to chemistry and biochemistry. It addresses the state of developments in the field, employing reviews of active research, including recent innovations, techniques and applications.

1

The Photochemistry of Sulfoxides and Related Compounds

William S. Jenks, Daniel D. Gregory, Yushen Guo, Woojae Lee, and Troy Tetzlaff

Iowa State University, Ames, Iowa

I. INTRODUCTION

The history of sulfoxide photochemistry dates back at least to the early 1960s, but this important functional group has received substantially less attention than some of the other chromophores whose chemistry was explored in those years [1,2]. On the other hand, the sulfoxide’s chiral nature has brought its thermal chemistry into greater exposure [3,4,5]. Much of that is due to the relative ease of preparation of optically pure samples and their utility as chiral auxiliaries, directing the stereochemistry of subsequent synthetic steps. Also, the sulfoxide is an intermediate oxidation state of sulfur, which can be made achiral or to have different reactivity by easily achievable oxidations and reductions.

We have returned to examination of the sulfoxide with the purpose of developing a model based on the primary photochemical processes of the excited states. This review is generally organized in a way to reflect that approach. The reactions are broken down by the type of first step, but it will be seen that a variety of different products can arise from the subsequent chemistry of intermediates. Within each section, work is organized roughly in chronological fashion, though observations that are obviously tied to one another are discussed together. In addition to cataloging transformations and reporting proposed mechanisms, we have, in places, suggested alternatives. In most such cases our job has been much easier by the benefit of time and a broader group of examples.

Unlike the carbonyl, the unconjugated sulfoxide is not associated with a long wavelength absorption of its own. Therefore most of what is reported herein has to do with conjugated sulfoxides. Aliphatic sulfoxides are treated separately, though there are many commonalities with aromatic sulfoxide chemistry. The aliphatic compounds have been examined by gas phase physical chemistry methods, which also justify a separate treatment.

We begin with a brief discussion of the sulfoxide chromophore itself. It will be seen that, just as the sulfoxide bond does not lend itself to simple description, the chromophore is fairly well understood phenomenologically but not yet well described in the shorthand photochemists have developed for other functional groups. Section III, Section IV, Section V, Section VI and Section VII constitute the bulk of the paper and develop the chemistry of conjugated sulfoxides by reaction type. The photochemistry of aliphatic sulfoxides is discussed in Section VIII, with emphasis on gas phase reactions.

In some of the more complex molecules, there are instances in which the low energy chromophore is probably not localized on the sulfoxide, yet the sulfinyl (SO) group is involved in the observed transformations. Some attempt will be made to differentiate photochemical reactions of molecules that merely happen to contain a sulfinyl group and those for which the sulfoxide is the critical functional group and/or chromophore. Section IX is reserved for the chemistry of ketosulfoxides in which the chemistry is clearly carbonyl but the reaction involves of sulfinyl site. The photochemistry of sulfenic esters (R-S-O-R′) appears throughout the text, particularly in Section III—the discussion of sulfoxide α-cleavage reactions. A short additional section on these sulfoxide isomers is also included at the end.

While we have attempted to be fairly comprehensive, inevitably there may be important contributions that are missed from time to time, and we apologize to any authors whose work we may have slighted. A point of notation: we have chosen to draw the sulfoxides as R₂S=O but are aware that there are others who prefer different representations. In some cases, where stereochemistry is being specifically implied, we have used a single-bond hash or wedge for clarity. This is only to show spacial relationships and does not denote any change in oxidation state. Finally, because of the multitude of sulfur oxidation states, and the relative unfamiliarity of many photochemists with their nomenclature, Figure 1 is intended as a quick reference guide.

II. THE SULFOXIDE CHROMOPHORE

The sulfoxide chromophore, in the absence of conjugated aromatic groups, has a relatively high excitation energy; the absorption spectra of aliphatic sulfox-ides have been examined by several authors [6,7,8,9,10,11]. Perhaps because of the more complex description of the sulfoxide bond, a meaningful simple orbital description (e.g., nπ*, ππ*) has not been settled upon. Alkyl sulfoxides do not fluoresce to a detectable amount in solution, so the best estimate of the singlet energy derives from the extrapolated onset of absorption of a gas phase sample of dimethyl sulfoxide (DMSO) [12]. The value so obtained is 105 kcal/mol. A similar method was used to estimate a triplet energy of 83 kcal/mol. Xe was added to the sample as a heavy atom to induce S₀→T₁ absorption, which appeared as a shoulder on the original absorption spectrum. (Gas phase fluorescence of DMSO has been reported, but without the details of a spectrum [13]).

Figure 1 Illustrations of sulphur-containing functional groups.

An attempt to observe the S₀ → T₁ absorption for di-tert-butyl sulfoxide in solution was made using a similar, but flawed, method [14]. Very concentrated solutions of the sulfoxide in iodomethane showed an absorption at 455 nm (63 kcal/mol). This absorption was interpreted—in combination with chemistry that was sensitized by anthraquinone (E_T ca. 63 kcal/mol)—to indicate a triplet energy of that magnitude, even though it was known that the sulfoxide is thermally reactive with iodomethane. Given the rather higher triplet energy from the gas phase results of Gollnick, and the higher energy phosphorescence observed for various aromatic sulfoxides (vide infra), we are skeptical of this value and view it as likely an artifact.

Reliable calculations on the excited states of DMSO and related derivatives are not yet available. Nonetheless, both CNDO/2 [12] and RHF/6-31G(d,p) calculations agree that the occupied frontier molecular orbitals have qualitatively higher coefficients at oxygen than do the first few unoccupied orbitals. The canonical MO picture is consistent, at least, with the blue shift of the absorption spectrum observed on increasing the polarity of the solvent. The canonical HOMO at RHF/6-31G(d,p) is π-antibonding between S and O but is σ-bonding between C and S. The LUMO is localized more on the sulfur but is π-antibonding along SO and σ-antibonding along CS. The HOMO in the valence bond picture is the sulfur lone pair.

T...

Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Series Introduction
• Preface
• Table of Contents
• Contributors
• 1. The Photochemistry of Sulfoxides and Related Compounds
• 2. The Photochemistry of Pyrazoles and Isothiazoles
• 3. Photochemistry of (S-Hetero)cyclic Unsaturated Carbonyl Compounds
• 4. Photochemistry of Conjugated Polyalkynes
• 5. Photochemistry and Photophysics of Carbocations
• 6. Regioselective and Stereoselective [2 + 2] Photocycloadditions
• 7. Photoinduced Redox Reactions in Organic Synthesis
• 8. Photochemical Reactions on Semiconductor Particles for Organic Synthesis
• 9. Photophysics and Photochemistry of Fullerene Materials
• 10. Use of Photophysical Probes to Study Dynamic Processes in Supramolecular Structures
• 11. Photophysical and Photochemical Properties of Squaraines in Homogeneous and Heterogeneous Media
• 12. Absorption, Fluorescence Emission, and Photophysics of Squaraines
• Index