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SMALL RING HETEROCYCLES

Part 3

Oxiranes, Arene Oxides, Oxaziridines, Dioxetanes, Thietanes, Thietes,

Thiazetes, and Others

n S s-x r y 0-0 n

0 n 0

Edited by

Alfred Hassner DEPARTMENT OF CHEMISTRY

STATE UNIVERSITY OF NEW YORK AT BINGHAMTON

and

BAR-ILAN UNIVERSITY, RAMAT-GAN ISRAEL

A N INTERSCIENCL@ PUBLICATION

JOHN WILEY AND SONS NEW YORK * CHICHESTER - BRISBANE - TORONTO * SINGAPORE

SMALL RING HETEROCYCLES - PART 3

This is the Forty-Second Volume in the Series

THE CHEMISTRY OF HETEROCYCLIC COMPOUNDS

THE CHEMISTRY OF HETEROCYCLIC COMPOUNDS

A SERIES OF MONOGRAPHS

ARNOLD WEISSBERGER and EDWARD C. TAYLOR

Editors

SMALL RING HETEROCYCLES

Part 3

Oxiranes, Arene Oxides, Oxaziridines, Dioxetanes, Thietanes, Thietes,

Thiazetes, and Others

n S s-x r y 0-0 n

0 n 0

Edited by

Alfred Hassner DEPARTMENT OF CHEMISTRY

STATE UNIVERSITY OF NEW YORK AT BINGHAMTON

and

BAR-ILAN UNIVERSITY, RAMAT-GAN ISRAEL

A N INTERSCIENCL@ PUBLICATION

JOHN WILEY AND SONS NEW YORK * CHICHESTER - BRISBANE - TORONTO * SINGAPORE

An Intcrscience@ Publication Copyright 0 1985 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada.

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of thc copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc.

Libmry of Congress Cataloging in Publication Data:

Main entry under title: Small ring heterocycles.

(The Chemistry of heterocyclic compounds, ISSN 0069-

“An Interscience publication.” Includes indexes. 1 . Heterocyclic compounds. 2. Ring formation

3 1 5 4 ; ~ . 42, pt . 3- )

(Chemistry) I. Hassner, Alfred, 1930- 11. Series: Chemistry of heterocyclic compounds; v. 42, pt. 3, etc.

QD400 .S5 1 15 547‘.59 82-4790 ISBN 0-471-05624-3 AACRZ

The Chemistry of Heterocyclic Compounds

The chemistry of heterocyclic compounds is one of the most complex branches of organic chemistry. I t is equally interesting for its theoretical implications, for the diversity of its synthetic procedures, and for the physiological and industrial significance of heterocyclic compounds.

A field of such importance and intrinsic difficulty should be made as readily accessible as possible, and the lack of a modern, detailed and comprehensive presentation of heterocyclic chemistry is therefore keenly felt. It is the intention of the present series to fill this gap by expert presentations of the various branches of heterocyclic chemistry. The subdivisions have been designed to cover the field in its entirety by monographs which reflect the importance and the interrelations of the various compounds, and accommodate the specific interests of the authors.

In order t o continue t o make heterocyclic chemistry as readily accessible as possible, new editions are planned for those areas where the respective volumes in the first edition have become obsolete by overwhelming progress. If, however, the changes are not too great so that the first editions can be brought up-to-date by supplementary volumes, supplements to the respective volumes will be published in the first edition.

ARNOLD WEISSBLRGER Research Laboratories Eastman Kodak Company Rochester, New York

EDWARD c. TAYLOR Princeton University Princeton, New Jersey

Preface

The chemistry of small ring compounds (three- and four-membered rings) has played a considerable role in the development of modern organic chemistry. Fore- most among these reactive molecules are the small ring heterocycles. The presence of one or more heteroatoms in these strained rings imparts a measurable dipole moment to such molecules. It also adds a new dimension of intrinsic difficulty concerning the synthesis and stability of such heterocyclic analogs of cyclopropanes and cyclobutanes. If one considers the compressed bond angles (near 60" in three- membered rings and near 90" in four-membered rings), the mere synthetic challenge, especially for the unsaturated analogs of these heterocycles, seems enormous. Indeed, the small ring heterocycles possess much greater reactivity toward a variety of reagents than do their five- or six-membered ring analogs.

The overwhelming amount of recent research literature in this field has made it necessary to divide this treatise on small ring heterocycles into several parts, with three- and four-membered rings sometimes interspersed. The current volume con- stitutes Part 3 in the series.

Part 1 includes the three-membered rings containing one nitrogen or sulfur; thus it consists of chapters on Aziridines, Azirines, and Three-Membered Rings Contain- ing Sulfur, which includes Thiiranes, Thiirenes, as well as their respective Oxides, Dioxides, and Onium salts.

Part 2 is devoted largely to four-membered rings containing nitrogen, for instance Azetidines, Azetines, Azetidinones (0-Lactams), Diazetidines, and

Diazetines, as well as the three-membered ring Diaziridines. In this third part of the series the important group of three-membered rings

containing oxygen, as well as four-membered rings containing two oxygens or one sulfur, are reviewed.

Oxiranes, often referred to as Epoxides, are covered in the first chapter. They are among the most studied and stereochemically very valuable heterocycles. Recent advances in chiral epoxidation of olefins have made chiral oxiranes available as building blocks for important natural products. Unfortunately, coverage of the chemistry of oxiranes related to natural products is beyond the scope of this volume, and only the important basic features of oxirane chemistry can be dealt with here. However, a separate chapter on the biologically important arene oxides has been included. These compounds that undergo valence tautomerism with oxepins and that were unknown at the time of the 1964 edition of Three- and Four-Membered Rings Heterocycles now occupy an important role in understand- ing the metabolism of carcinogenic polycyclic aromatic compounds.

The third chapter on Oxaziridines is an expos6 of this labile ring system containing both an oxygen and a nitrogen heteroatom and its equilibration with the isomeric nitrone system.

viii Preface

Chapter 4 deals with the chemistry of the energyrich 1,2-dioxetane ring system; the isomeric 1,3-dioxetane systems still await exploration. The exploration of 1,2-dioxetane chemistry is also of recent vintage and the fact that they are cyclic peroxides explains their high reactivity. It is now established that such heterocycles are involved in bioluminescence.

The final chapter involves the important chemistry of Four-Membered Ring Sulfur Heterocycles. This is a subject of considerable enormity. The subject matter covered includes not only the better known Thietanes and Thietes, but also the corresponding Sulfoxides, Sulfones, and cyclic Sulfonium salts, as well as the functionalized Thietanones, Iminothietanes, and Methylene-Thietanes. Furthermore, four-membered sulfur heterocycles containing additional heteroatoms such as N, 0, S, P, Si, and other elements, as well as selenetanes and telluretanes, are also discussed in this review. Specialists will appreciate the inclusion of four-membered ring sulfur compounds devoid of carbon, such as the dithiophosphonates (S2P2 ring system), which are useful in the preparation of thiocarbonyls, the S2N, ring system, and others.

There has been a great deal of recent progress on regio- and stereoselectivity, as well as on photochemistry of these three- and four-membered rings. What is even more intriguing is their use as synthons for other functional groups as well as for larger ring heterocycles. Furthermore, there has been increasing interest in the biological properties and polymerization behavior of such molecules.

An effort was made to briefly present the general state of the art and to empha- size research results of the past 15-20 years. Such an undertaking makes it necessary to be more selective than all-inclusive. Often it became more realistic to build on existing reviews of the subject.

Editing this volume is especially meaningful to me because I had the privilege of being involved firsthand in the exciting explorations of some of these heterocycles during the past 20 years.

I am indebted to the authors of the chapters for their splendid cooperation and patience and to my secretary, Joyce Scotto, for her invaluable help.

Most of all, this book is devoted to my wife, Cyd, whose love has sustained me through this effort, and to the loving memory of our daughter, Erica, cruelly torn from us at a tender age.

ALFRED HASSNER

Binghamton, New York September 1984

Contents

1 . OXIRANES

M. Bartdk and K . L. Lbng

2. ARENE OXIDES-OXEPINS

Derek R. Boyd and Donald M. Jerina

3. OXAZIRIDINES

Makhluf J. Haddadin and Jeremiah P. Freeman

4. 1,2-DIOXETANES AND a-PEROXYLACTONES

Waldemar Adam and Faris Yany

5 . FOUR-MEMBERED SULFUR HETEROCYCLES

D. C. Dittmer and T. C. Sedergran

Author Index

Subject Index

1

197

283

35 1

431

769

855

ix

CHAPTER I

Oxiranes M. BARTOK AND K. L. LANG

Department of Organic Chemistry, Josef Attila University, Szeged, Hungary

I . Introduction . . . . . . . . . . . . . . . . 11. Physical Properties of Oxiranes . . . . . . . . . .

1. Theoretical Models . . . . . . . . . . . . 2. Molecular Geometry . . . . . . . . . . . 3. Energetics . . . . . . . . . . . . . .

A . Heats of1:ormationandCombustion . . . . . B. Ring Strain . . . . . . . . . . . . . C. Ionization Potential . . . . . . . . . . D. Conformational Energy of the Oxirane Ring . . .

4 . Spectroscopic Properties . . . . . . . . . . A. Microwave Spectroscopy . . . . . . . . . B. IR Spectroscopy . . . . . . . . . . . C. UV Spectroscopy . . . . . . . . . . . D. NMRStudies . . . . . . . . . . . .

a. Shielding, Chemical Shift . . . . . . . b. Coupling Constants . . . . . . . . . c. Shift Technique . . . . . . . . . . d. NMR in an Oriented phase . . . . . . . e. Resonance of Other Nuclei . . . . . . .

5 . Other Physical Measurements . . . . . . . . . A. Diffraction Measurements . . . . . . . . B. Raman Spectroscopy

D. Optical Rotatory Dispersion . . . . . . . . E. Mass Spectrometry . . . . . . . . . . I;. Basicity . . . . . . . . . . . . . . G. Photoelectron Spectroscopy . . . . . . . .

1. Oxidation of Alkenes . . . . . . . . . . . A . Oxidation with Organic Peracids . . . . . . B. Oxidation with Hydrogen Peroxide . . . . . .

a. Oxidation with Alkaline Hydrogen Peroxide . . b.

C. Dipole Moment Measurements . . . . . . .

111. Synthesis of Oxiranes . . . . . . . . . . . . .

Oxidation with Hydrogen Peroxide and a Catalyst

. . . . 3

. . . . 4

. . . . 4 5 6 6 I I 8 8 8 8 9

10 1 0 11 11 12

. . . . 12 13 13 13 13 14

. . . . 14 14 14 15

. . . . 15 15 25

. . . . 25 29

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. . . .

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. . . .

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. . . .

1

2 Oxiranes

C . Oxidation with Organic Hydroperoxides . . . . . . . . D . Oxidation with Molecular Oxygen . . . . . . . . . .

a . Oxidation with Oxygen with Metal Complex Catalysis . . . b . Oxidation withoxygen without aCatalyst . . . . . .

E Other Oxidation Methods 2 . Preparation of Oxiranes from 1, 2.Difunctional Compounds by 1, 3.

Elimination . . . . . . . . . . . . . . . . . . 3 . Preparation of Oxiranes from Carbonyl Compounds by Formation of

Carbon-Carbon Bonds . . . . . . . . . . . . . . . A . Darzen’s Reaction . . . . . . . . . . . . . . . B . Reaction with Diazoalkanes . . . . . . . . . . . . C . Reaction with Sulfonium Ylides . Corey Synthesis . . . . . . D . Other Oxirane Syntheses . . . . . . . . . . . . .

IV . Reactions of Oxiranes . . . . . . . . . . . . . . . . . 1 . Deoxygenation . . . . . . . . . . . . . . . . . 2 . Rearrangements and Isomerizations . . . . . . . . . . .

A . BaseCatalyzed Rearrangements . . . . . . . . . . . B . AcidCatalyzed Rearrangements . . . . . . . . . . . C . Rearrangements Induced by Heterogeneous Catalysts and Metal

D . Other Rearrangements . . . . . . . . . . . . . 3 . Oxidation . . . . . . . . . . . . . . . . . . . 4 . Reduction . . . . . . . . . . . . . . . . . .

A . Reduction with Metal Hydrides . . . . . . . . . . . B . Dissolving Metal Reduction . . . . . . . . . . . . C . Catalytic Hydrogenolysis . . . . . . . . . . . . . Ring Transformation of Oxiranes into other Heterocyclic Compounds . A . Ring Transformation into other Three-Membered Heterocycles . . B . Ring Expansion into Four-Membered Heterocycles . . . . . C . Ring Expansion into Five-Membered Heterocycles . . . . .

. . . . . . . . . . . . . .

Complexes . . . . . . . . . . . . . . . . .

5 .

30 34 34 36 38

4 0

47 4 7 5 1 5 2 5 4 5 7 5 8 6 1 6 2 65

7 1 75 76 77 7 8 83 8 3 87 87 8 8 90

. . . . . . . D Ring Expansion into Six-Membered Heterocycles 96 6 Reaction with Organometallic Compounds 9 8

A Reaction with Grignard Reagents 99 B . Reactions with Alkylmagnesium and Alkylaluminium Compounds . 101 C . Reaction with Lithiumdiorganocopper Reagents . . . . . . 106 D . Reaction with Alkyl- or Aryllithium . . . . . . . . . 110 E . Reaction with other Organometallic Compounds . . . . . . 113

7 . Other Reactions involving C-0 Bond Opening . . . . . . . . 115 A . Hydrolysis . . . . . . . . . . . . . . . . . 118 R . TrazsfQrrr?ationswith AlroholsandPhenols . . . . . . . 119 C . Transformations with SulfurContaining Nucleophiles . . . . 120 D . Reaction with Halogen Acids . . . . . . . . . . . . 1 2 1 E . 1 2 2 I; . Reaction with Ammonia, Amines, and their Derivatives . . . . 1 2 3 G . Miscellaneous . . . . . . . . . . . . . . . . 125

8 . Photochemistry . . . . . . . . . . . . . . . . . 126 A . Alkyloxiranes . . . . . . . . . . . . . . . . 127 B . Unsaturated Oxiranes . . . . . . . . . . . . . . 1 2 9 C . Epoxyketones . . . . . . . . . . . . . . . . 1 3 1

a . a$-Epoxyketones . . . . . . . . . . . . . 131

c . a$-Unsaturated y,6 -Epoxyketones . . . . . . . . 1 3 9 D . Salts and Esters of Arylglycidic Acids . . . . . . . . . 140 E . Aryloxiranes . . . . . . . . . . . . . . . . 1 4 1

. . . . . . . . . . . . . . . . . . . . .

Reaction with Carboxylic Acids and their Derivatives . . . . .

b . P , y-Epoxyketones . . . . . . . . . . . . 136

Introduction 3

9. Thermally Induced Reactions . . . . . . . . . . . . . 145 A. Alkyl- and Alkenyloxiranes . . . . . . . . . . . . 145 B. Oxiranes Containing a Croup Stabilizing the Ylide Intermediate. . 146

a. Aryl-Substituted Oxiranes . . . . . . . . . . . 146 b. Alkenyl- and Alkynyloxiranes . . . . . . . . . . 147

C. Miscellaneous . . . . . . . . . . . . . . . . 150 10. Polymerization . . . . . . . . . . . . . . . . . 151

V. Abbreviations . . . . . . . . . . . . . . . . . . . 152 VI. References . . . . . . . . . . . . . . . . . . . . 152

I. INTRODUCTION

The synthesis and chemical reactions of cyclic ethers with 3-6 ring atoms have been subjected to wide-range study. The class name for the three-membered ring monocyclic ethers is oxiranes. To preserve the name of a specific complex structure, the prefix epoxy is used in the substitutive nomenclature. Additive nomenclature is still used for oxiranes when they are described as the oxides of unsaturated compounds (ethylene oxide, styrene oxide). To describe oxirane formation, the expression epoxidation is still used instead of the more correct oxiranation. By virtue of the relative ease of their preparation and their readiness to undergo many kinds of reactions, the oxiranes stand out, and, are accordingly, thc oxacycloalkanes of most practical importance.

The main task of this review is to present an account of the synthesis and most characteristic properties of the oxiranes. We shall deal mainly with the formation and chemistry of the oxiranes without going into details into the synthesis and reactions of oxiranes containing other functional groups.

(oxirane) (ethylene oxide)

The older literature on the synthesis and chemistry of the oxiranes was reviewed very systematically and in great detail by Rosowsky,' in a 1964 monograph that appeared in the series, The Chemistry of Heterocyclic Compounds. This was rapidly followed by two additional general reviews of the subject.233 Interest in the chemistry of the oxiranes did not wane in the 1970s, as indicated by the publication of several more monographs!-7 The most recent comprehensive survey of oxiranes was published in the series, The Chemistry of Functional Groups.8

Of the monographs that have appeared in the past two years, only those that give a more general account of the oxiranes have been mentioned. In the following section, we describe the reviews of individual special areas relating to the preparation and reactions of the oxiranes.

Since some of the above monographs give a detailed treatment of the earlier results but merely touch on the more recent ones, we attempt to present the

4 Oxiranes

tremendous development now taking place in the chemistry of the oxiranes on the basis of experimental data mainly reported in the past few years (up to the end of 1982). Regarding results published after 1964, only those that are of a more general nature and those that were not dealt with by previous reviews for various reasons will be mentioned.

Every year, a huge number of publications on oxiranes appear in the literature. As a consequence of the rapid development in synthetic methods, the investigation of chemical reactions, and hence utilization of oxiranes, oxiranes occupy a central position among cyclic organic compounds. For the above reasons, it is difficult to achieve completeness. We apologize in advance if we have inadvertantly omitted reference to work that the reader feels should have been included.

11. PHYSICAL PROPERTIES OF OXIRANES

1. Theoretical Models

Because the oxirane contains a strained ring consisting only of two carbon atoms and one oxygen atom, a series of theoretical calculations and physical measurements have been performed in order to determine the exact molecular structure. The simplicity of the molecule permits the rather complex quantum-chemical calculations. It follows from the strained nature of the ring that it is a rigid one; good comparisons may therefore be made between the geometric data obtained via the various spectroscopic and other physical methods.

The strain in the three-membered ring is one of its most important properties. It is also the basis of the explanation of many of the features of the molecule, for example, the high reactivity in ring-opening reactions and the low electron-donor ability. These properties can be explained in terms of the hybridization of the bonding and nonbonding electron orbitals and the angular strain in the transition state. A short account of the MO model with reference to the oxiranes is found in the work of Rosowsky.'

The ground state and low excited state of the oxirane molecule have been described by a semiempirical SCF-MO method.' Ab initio FSGO calculations have confirmed the chemical experience that the C-C bond displays an increasing tendency to bend the shorter the bond length, while the C-C bond is more flexible than the C-0 bond." An ab initio MO method in combination with the Moller- Plesset perturbation theory (MP3) gives the result that, of the three C2H40 isomers, acetaldehyde and vinyl alcohol (45 kJ/mol) are much more stable than the high- energy oxirane (1 14 kJ/mol). The ionization and excitation energies of oxirane together with its electron affinity have been calculated with a semiempirical HAM/3 method; the results fit well with those obtained from the electron spectrum." The structures and energetics of the possible C2H40 isomers have also been calculated via STO-3G and 4-31G programmes;" these data conform well with those derived from the rotation ~pec t rum. '~

Physical Properties of Oxiranes 5

For substituted oxiranes, the effect of the substituent on the electronic structure of the ring has been computed with the CND0/2 method on the basis of orbital hybr id iza t i~n . '~ . '~ The calculations revealed that the C-C bond is stronger than the C - 0 bond and that the C-0 bond is weaker when it is adjacent to a substituent. The computed dipole moment, total energy, electron configuration, and bonding energy are in good agreement with the experimentally found data. Quantum- mechanical studies on the triplet-state isomeric methyloxiranes have been d e s ~ r i b e d . ' ~ ~ ~ lSb

In the past ten years, many investigations and theoretical calculations have been made with regard to the chemical nature of the bent bond. In contrast with molecular orbital theory, classical theory considers the bent bond to be connected only with the localized bond or the bond orbital. The Foster-Bogs method has been employed for the indirect generation of the localized orbitals and the resulting orbitals have subsequently been analyzed from the aspect of the bent

The FSGO (floating-spherical-Gaussian orbital) rnethodl8 in an ab initio computation procedure results in direct bond orbitals without base functions restricted to the atoms. When the degree of bond bending was calculated with this method, the following results were obtained:"

l7

1. 2. bond length. 3 .

4. difference. 5. orbitals adjacent to the ring.

Reference will be made in the relavant subsections to the calculations necessary for a theoretical interpretation of the results of the individual experimental methods (microwave, photoelectron spectroscopy, nmr data, etc.).

The C-C bond orbitals are more diffuse than in the similar unstrained systems.

The degree of bending of the C-C bond is in a simple correlation with the

The C-0 bond is less flexible than the C-C bond.

The center of the C - 0 bond can be well followed via the electronegativity

The computation does not lead any closer to the factors influencing the C-H

2. Molecular Geometry

A consequence of the rigid molecular skeleton is in close agreement with molecular geometry data obtained by different procedures. Information on the exact bond lengths and bond angles is provided primarily by microwave spectroscopy, electron diffraction, and, more recently, the nmr spectrum in the liquid-crystal phase. A fuller survey of the individual methods will be given in the appropriate sections; here, only the geometric data yielded by the various procedures are listed (Table 1). All of the measurements clearly demonstrate that the plane of the hetero-ring is perpendicular to the plane defined by the four hydrogen atoms. The two carbon atoms of the ring are symmetrically raised above the plane of the four hydrogen atoms.

6 Oxiranes

TABLE 1. GEOMETRIC DATA OF THE OXIRANE RING MEASURED BY DIFFERENT METHODS

Reference

19 20 21 1 1 3

c-c 1.54 1.472 1.4728 1.47 1.483 c-0 1.43 1.436 1.4363 1.44 1.433 C-H 1.05 1.082 1.0802 1.08 1.088 CY 6 7" 6 1" 24' 61"41' 6 1" 24' 62.3" P 51" 26' 59" 18' 59"9' 59" 18' 58.85" 7 - 159'25' 158'5' 158'6' 155.3" 6 117"28' 116"41' 116"51' 116' 15' 114.5"

The C-C bond distance lies between those for a single (1.548) and a double (1.33 8) bond; similarly, the H-C-H bond angle lies between those for tetrahedral (109'28') and trigonal (120') bonding.13 The same situation is observed for the C-C-C bond angle in the event of substitution:

CH, CH3 7' HpC-CH

r/ H2C =CH

CH3 f - 1 H,C-CH2

'd 11 2.4°22 1 24.8°23

121OZ4 118* 3""

This suggests the model in Ref. 1, in which the two carbon atoms are apparently elevated above the plane of the four hydrogen atoms in response to the presence of the oxygen atom (in olefins the two caiboii atoms !is in thc plane of the hydrogen atoms).

3. Energetics

A. Heats of Formation and Combustion

The stability of the three-membered ring is surprising, particularly if the bond angles are considered and the situation is further complicated by the nature of the nonbonding H-H interactions.26 The heats of formation and combustion of oxirane are given in Table 2 and are compared with those of higher homologues.

Physical Properties of Oxiranes

TABLE 2. HEAT OF FORMATION AND COMBUSTION O F OXIRANE AND ITS HIGHER HOMOLOGUES

Heat of Formation" (kJ/mol) (kJ/mol)

Heat of Combustion'*

I

Oxirane 117.2 114.1 Oxetane - 106.7 Oxolane 28.0 23.6 Oxane 9.2 4.9

B. Ringstrain

An examination of oxirane ring strain" showed that it depends primarily on the structure of the ring and its basicity. The ring-strain energy has been calculated as the difference between the experimental heat of formation (obtained from the heat of combustion) and the calculated total bond energy. The results for various ring systems are listed in Table 3." In a study of the ring strain relating to ring-opening with a secondary amine (dibutylamine) in an aprotic solvent,30 it was found that the reactivity of methylthiirane is greater than that of methyloxirane.

C. Ionization Potential

Photoionization indicated an ionization potential of 10.565 eV,31 which is in excellent agreement with the value of 10.5 eV calculated from the UV ~pect rum.~ ' The oxiranes have higher ionization potentials than that observed for dimethyl ether (10.0 eV);33 this demonstrates the less effective nature of the delocalization of the 2p n-electron pair in the case of oxirane than in simple ethers. Support for this is provided by the experimental fact that the oxiranes exhibit a charge shift from the oxygen towards the carbon atoms,34 which may cause an increase in the ionization potential of the 2p n-electron pair.

Mass spectrometric measurements have been used to calculate the energy profile of the cation C2H50+.33 Cleavage of the C-C bond here requires about 61 kcal/mol more energy than the corresponding C-0 ring-opening. This finding is supported both by theoretical calculations on oxirane and by the experimental facts.34i35 Furthermore, the C-0 bond rupture is favored not only in the ground state, but in the various excited states.

TABLE 3 . RING-STRAIN ENERGY OF OXIRANE AND ITS ANALOGUES

Ring-Strain Energy (kJ/rnol)

Cyclopropane 104.7 Aziridine 58.6 Oxirane 54.4 Thiirane 37.7

8 Oxiranes

The structures and formation enthalpies of the three ions CzH4O' have been determined;36-38 at the same time, the mass spectrometric measurements revealed that there is a fourth such ion, with a formation enthalpy of 202-205 kcal/mol, which is probably a product of rupture of the C-C bond.39

D. Conformational Energy of the Oxirane Ring

Theoretical studies have indicated that the oxiranes display similar properties to compounds containing a double bond. For instance, the conformational energy of the A ring of 2,3-epoxylanostane is analogous to that of c y c l ~ h e x e n e . ~ ~

Dipole moment measurements have shown that, of the possible conformers of l-tert-butylpiperidine-4-spiro-2'-oxirane (l), the axial-0-conformer is preferred (1.1 kJ/mol). The conformational energy of the corresponding thiirane compound is 1.8 kJ/rnol more favorable than when the heteroatom is equatorial.

From nmr studies, a value of 1.13 kJ/mol was determined in the case of the cyclohexane deri~ative.~'

4. Spectroscopic Properties

A . Microwave Spectroscopy

This technique can be used extremely well to determine the geometry of the oxiranes. As long ago as the 1940s it was utilized in combination with dipole measurement to establish the bond lengths and bond angles in various substituted oxiranes. Simiiar data are found in the iiterature on deuterium-substituted o ~ i r a n e . ~ ~ ~ ~ ~

By means of microwave measurements, the ro, rs , and rm structures of oxygen- containing heterocycles have been determined.I3 The structure of 1,2- difluorooxirane has been investigated via microwave spectroscopy from the aspect of fluorine ~ubsti tution?~

B. IR Spectroscopy

Oxirane exhibits three intense IR bands ~ at 1265, 1165, and 865 cm-' 44,45 - the last of which is the asymmetrical ring-stretching vibration. The overlapping


band at 1250 cm-' can be regarded as characteristic of the oxirane This band is probably attributable to the symmetrical stretching vibration. Steroid oxiranes do not yield the 1250cm-' band, but bands are observed at 900-800cm-' for some and at 1050-1035 cm-' for others.47 If the oxirane ring is terminal, sharp bands are found at 910 and 840cm-' for fatty acids. With nonterminal oxirane rings, the absorption band is at 890cm-' for the trans compounds and around 830cm-' for the cis isomers.48 Perdeuterated ethylene derivatives have also been investigated in detail4' and the individual fundamental vibrations have been calculated.

A review of the IR frequencies of the three-membered heterocycles is found in Katritzky and Ambler.49 The IR frequencies were more recently studied by Potts." Georges1 recorded the IR spectra of 16 straight-chain oxiranes for analytical purposes and reported their refractive indices too. IR (4000-200 cm-') and Raman spectra have been taken in the solid and the liquid phases for the conformational examination of alkyl-substituted o x i r a n e ~ . ~ ~ ~ ~ ~ Studies have been made of the steric structure of oxirane-car box aldehyde^'^ and the low-temperature oxidation of cyclo- hexene in the presence of Co"' c h e l a t e ~ . ~ ~ Analyses have been carried out on the IR spectra of oxiranes in the region of 850cm-' and the vibrational energy levels.56 Steroid oxiranes have likewise been subjected to IR investigation." Hirose13 published the rotational spectra of 10 oxiranes together with their evaluation and, in conjunction with the microwave spectra, determined the r,,, r,, and r , structures of the compounds.

Bellamys8 and Jones et al.s9 have made excellent surveys of the IR frequencies of substituted oxiranes. In these compounds, two fundamental effects may be differentiated: one involves changes in the vibrations of the oxirane ring by the action of the substituents and the other involves the effect of the oxirane ring on the absorption bands of the functional groups of the substituents. Rosowsky' gives a good account of the variations in the ring frequencies of oxiranes as the substituents are changed. In general it may be said that there is not a close correlation between the position of the oxirane band and the stereolectronic factors, which is primarily indicative of the determining role of the strained ring. In the second case, an important role is played by the absorption bands of an aromatic or olefinic functional group in conjugation with the oxirane ring. Since the oxirane ring has properties intermediate between those of a saturated function and an olefin, it is expected that the stretching frequency of a carbonyl group in the case of an (YJ- epoxyketone will lie between the frequencies for the corresponding saturated and a,P-unsaturated ketones.

The concentration of an oxirane can be determined in the gas phase by the IR bands.60 Terminal oxiranes can be estimated with an accuracy of 1-2% in CC14 solution, via the bands at 6061 and 4545 cm-'.61

C. UV Spectroscopy

Oxygen-containing heterocyclic homologues of ethers exhibit two absorption bands, with the exception of oxirane which has only one absorption maximum at

10 Oxiranes

1713cm-'. At the same time, its ionization potential (10.565eV) is higher than that of dimethyl ether (10.0eV) (see Section II.3.C.). The energy of the difference between the absorption bands for the higher cyclic ethers almost corresponds to this energy and thus it is probable that the lone p electrons are equally stabilized in oxirane, presumably by hyperconjugation with the methylene groups.62

The A,,, values of substituted oxiranes are surveyed by Rosowsky.' In the majority of UV spectra of these systems, the conjugative effects are examined, which gives a reflection not of the ground state, but of the excited state. Accordingly, a comparative measurement is advisable, with a joint study of the corresponding olefins and cyclopropanes. Investigations of the UV spectra63 and acidities of substituted oxiranes and cyclopropanes have revealed that conjugation occurs in both systems and specific orientation of the cyclopropane ring is not necessary. This appears to refute the earlier assumption that a specific orientation is required for conjugation of the oxirane ring.@ The conjugative ability may be explained by the negative charge of the oxygen in the excited state, which is neglected in the ground-state calculation^.^^

D. Nrnr Studies

a. SHIELDING, CHEMICAL SHIFT

In the nmr spectrum of oxirane,66 the protons /3 to the oxygen display a small chemical shift compared to that of the /3 protons of larger cyclic ethers; this can only be explained by the shielding effect of the abnormal electron density, which assumes a low electron density around the oxygen, as suggested earlier on theoretical grounds. The proton chemical shifts for various substituted oxiranes are given in a number of reviews and

The oxirane ring exhibits similar anisotropy to that of cyclopropane. The literature contains only a single example of intense shielding above the ring: the chemical shift of proton 14 in 15/3,16/3-oxidobeyeral is found at 0.6ppm.69 Anisotropic shielding effects on the 19-methyl protons of steroids with the oxirane ring in various positions are listed by Tori.70 The chemical shifts correspond quantitatively to the ring current model; however, their values are rather small, and it is necessary tc take inte ccnsideratim the canfanrr?,atiana! change occwring i ~ ? the six- membered ring and the fact that an appreciable solvent effect can be observed even in chl~roforrn.~'

Tabulated compilations of the chemical shifts of the protons in oxiranes are frequently found in the l i t e r a t ~ r e , ~ ' - ~ ~ but structural conclusions are rarely drawn from the chemical shifts as the coupling constant is a more exact means of establishing the configuration. Configurations and conformations of oxiranes derived from prostaglandin intermediates have been determined from 'H shifts with the aid of the ring current

In phenyloxiranes, the protons adjacent to the aromatic ring absorb at lower field than the cis and truns protons. This low-field shift can be interpreted in terms of the ring current of the phenyl In 1,2-diphenyloxiranes there is resonance at lower field for the cis isomers. The effect of this has been calculated on


the basis of the ring current,79 but a quantitative description of the phenomenon has not succeeded.80 The phenyl ring conformation has been investigated on the same basis for many substituted oxiranes." Consideration was paid to the steric factors and solvent effects. The results agree well with the experimental proton shifts.

In the oxides of styrene, stilbene, and stilbazole, the chemical shifts of the cis protons of the oxirane ring are larger than those for the trans isomers. This can be explained by the polarization effect of the electric dipole moment of one of the CH protons on the other CH.80 The anisotropc shielding of the ring in the case of oxiranes substituted with an aromatic group can be utilized well in the determina- tion of the configuration.82

b. COUPLING CONSTANTS

The most important datum that can be determined from the nmr spectrum of an oxirane is the coupling constant. Since the spectra are generally complicated and contain an ABM or ABX multiplet that cannot be interpreted directly, computer simulation is necessary. Abundant data on the coupling constants are to be found in the reviews referred to in Section I1 .4.D.a.67'68

MortimerE3 has determined the coupling constants in the various three- membered rings; these are listed in Table 4. It may be stated generally that the cis vicinal coupling constant is larger than that in the corresponding trans case.84 This is in contrast with what is experienced for the larger rings, where Jmns > Jcis.67

The coupling constants strongly suggest the half-chair conformation in the 2,3- and 3,4-anhydropyranosides, in which the conformational effect of the oxirane ringa5 is similar to that of a double bond (2,3-unsaturated pyranoside derivatives).86 The 13C-'H coupling constant can be determined from the I3C satellites of the proton spectruma7 and from the 13C spectrum.83

c. SHIFT TECHNIQUE

The occurrence of an appreciable solvent effect was observed for spectra of o x i r a n e ~ . ~ ~ Substituents can be distinguished by the benzene-induced shift

Oxiranes can be studied well with the aid of lanthanide shift reagents since they are strong complex-formers. Primarily, the change in the chemical shift is inform- ative since the coupling constant can generally not be determined as a consequence of signal b r ~ a d e n i n g . ~ ~ - ~ '

Chiral shift reagents are conveniently applicable to establish optical purity;'-" whiie the europium complexes are useful in determining oxirane configura- t i o n ~ . ~ ' - ~ ~

TABLE 4. PROTON COUPLING CONSTANTS IN THREE- MEMBERED HETERO-RINGS

Jcis Jtrans Jgem

Oxirane 4.45 3.1 5.5 Thiirane 7.15 5.65 0.4 Aziridine 6.3 3.8 2.0

12 Oxiranes

d. NMR IN AN ORIENTED PHASE

For molecules dissolved in a nematic thermotropic liquid-crystal, the direct coupling constants can be determined and from these the molecular geometry can be calculated. If the 13C satellites are determined, not only the proton structure but the carbon skeleton of the molecule can be established. Oxirane has been measured in two l a b o r a t o r i e ~ . ~ ~ ~ ' ~ It proved possible to determine the orientation, the sign of the indirect coupling constant, and the geometry. Enantiomers can readily be determined by recording measurements in optically active liquid-crystals as solvents."'

e. RESONANCE OF OTHER NUCLEI

The 19F nmr spectra of the fluorine-substituted oxiranes have been reasonably well studied, for in the region of high chemical shifts, the fluorine nuclei readily yield spectra that can be interpreted directly. A further principle is that the long- range coupling constants can be measured well because of the large nature of the c ~ n s t a n t s . ' ~ ~ - ' ~ ~ In the case of a fluorinated substituent, the linear Taft correlation may be used to determine the inductive and resonance substituent constants.lo6

In 31P nmr measurements, good use may be made of the fact that the stereospecificity of the P-C-C-H coupling is well do~umented ."~ For example, 2 and 3 can readily be differentiated on the basis of the 31P-1H coupling:'08 for 2 3Jp-C-C-H = 4.5 Hz, and 3 3Jp-c-c-H = 5 .3 Hz.

0 0 MeO,; MeO, 11 M e 0 / ~ ~ M e O y ' H

Ph

H o Ph H O H

2 3

The resonance of the I3C nuclei may be employed extremely well in studies of oxirane structures.'09-"' Fr om the 13C spectra for 61 oxiranes an additivity rule was formulated for the chemical shifts.l12 Some examples are given in Table 5 .

The conformation of cycloheptene oxide has been examined via the 13C nmr spectra of deuterated compounds on the basis of the temperature-dependence of the chemical shifts of the individual signal^."^ In phenyl-substituted oxiranes, the 13C shifts have revealed the inductive and hyperconjugative effects of the oxirane ring, and thus the ring behaves as an electron-accept~r."~

'H-13C coupling constants have been measured for three-membered heterocycles and it has been observed that the coupling constant increases in the following

TABLE 5. CHEMICAL SHIFTS OF OXIRANES IN PPM _________ ~~ ~

Compound C-1 C-2

Ethylene oxide 40.8 -

1,2-Epoxypropane 47.8 48.0 1,2-Epoxypentane 46.8 52.0 1-Phenyloxirane 52.2 50.9 1,l-Diphenyloxirane 61.7 56.1


sequence: 0, N, S, CX2. This sequence correlates with the increase in the C-C bond length .l ''

From the 13C nmr data on 42 aliphatic oxiranes, an additivity relationship has been derived for calculation of the chemical shifts.116 An nmr investigation has been described on the addition products of indene derivatives and singlet oxygen.'0g Good reviews of the 13C nmr data may be found in the following references and in the papers cited thereir~.I '~- '~ '

Ex0 and endo oxiranes of bicyclo[2.2.l]heptane can be well differentiated on the basis of their 13C spectra.122 The effect of the molecular asymmetry on the chemical shift of the carbon in 0- and N-glycidyl compounds has been investigated.'23 In the study of stereoisomeric epoxyspirocyclohexane derivatives, the effects of the equatorial and axial oxiranes have been observed on the carbon atoms of the cyclohexane ring.'24

Relatively few publications have appeared on 1 7 0 measurements. The I7O chemical shifts have been measured for 21 oxiranes and compared with the corresponding 13C shifts.12' The results could be interpreted on the basis of the para- magnetic p and the diamagnetic y effects.

5. Other Physical Measurements

A. Diffraction Measurements

l-p-Bromophenyl-'26 and l -p -n i t r~pheny l - '~~ substituted oxiranes have been examined by x-ray diffraction. The dihedral angle between the phenyl ring and the oxirane ring was found to be 83" and 80.2", respectively. This can be explained through the pseudoconjugational interaction of the two rings.

B. Raman Spectroscopy

The differences between the IR and Raman spectra were examined as long ago as the 1 9 3 0 ~ . ' ~ ~ 1 ' ~ ~ M o u s ~ e r o n ' ~ ~ later performed measurements on many oxiranes and identified the individual vibration bands. More recently, the results of solid-, liquid-, and gas-phase studies on the Raman spectra of a l k ~ l - ~ ~ and vinyl-53 substituted oxiranes have been reviewed.

C. Dipole Moment Measurements

Oxirane has a higher boiling point (10.5") than cyclopropane (- 32.9"), which indicates its more polar character. Its dipole moment has been determined by numerous authors via the dielectric constant (e.g., R e f ~ . ' ~ l - ' ~ ~ ) and the Stark effect.40 The results agree well: 1.9D. The structures of aryl-substituted oxiranes have also been determined by means of dipole moment measurements.'34

14 Oxiranes

D. Optical Rotatory Dispersion

The oxirane ring has been found to exhibit an ORD curve at 290 nm; this has the opposite sign to that of the alkyl

E. Mass Spectrometry

The ring-opening modes have already been discussed in connection with the ionization potential of the oxirane ring (see Section 11.3). Mass spectrometric books present a detailed treatment of the properties of ~ x i r a n e s . ' ~ ~ - ' ~ ~ The individual fragmentation patterns have also been subjected to detailed discussion on the basis of the high-resolution mass spectra.I3'

In the mass spectra of the oxiranes of terminal and nonterminal alkenes, the following characteristic fragments have been found: M-29, M-43, M-57.I4O A similar result emerged from more recent measurements, in the course of which the mass spectra of 16 straight-chain aliphatic oxiranes containing 7-1 2 carbon atoms were recorded." The mass spectra of 12 substituted methyloxiranes and 15 substituted 1,2-diphenyloxiranes have been compared with those of the corresponding aldehydes from the aspect of fragmentati~n.'~' I t could be established that the oxirane fragmentation either gives rise to a symmetrical ion through the direct loss of one mole of aldehyde or ketone, or the molecular ion undergoes rearrangement to an isomeric carbonyl radical ion. Fragmentation of oxiranes has also been studied by chemical i o n i z a t i ~ n . ' ~ ~ In the mass spectra of simple oxiranes, the fragmentation does not indicate electron-impact-induced isomerization towards a 1 d e h ~ d e . I ~ ~ Even radicals with lifetimes shorter than IO-'sec have been detected in the impact-activation mass spectra of oxiranes.'@

F . Basicity

The basicities of cyclic ethers have been investigated widely, as they provide infermation not only on the natiure of the chemical reactions hut on theoretical questions as well. Earlier monographs may be consulted for analyses and conclusions relating to the relevant

G. Photoelectron Spectroscopy

From the photoelectron spectra, it is possible to calculate the ionization potential (see Section II.3.C). Ab initio SCF calculations145 have been used to ascribe the individual transitions in the photoelectron spectra of the 0 ~ i r a n e s . I ~ ~ The photoelectron spectra of halomethyloxiranes have also been p ~ b 1 i s h e d . l ~ ~

The photoelectron spectra have been utilized as the basis of a study of the effects of mono- and dialkyl, halomethyl, phenyl, and vinyl substituents on the

Synthesis of Oxiranes 15

ionization potentials of 0 ~ i r a n e s . l ~ ~ A halogen substituent in the methyl group of methyl oxirane stabilizes all of the levels and increases the ionization potential. In methyl- and halomethyl-oxiranes, the ionization potentials of all four oxirane-type orbitals correlate with the electronegativity of the substituent. It may be concluded from the resulting linear correlation that inductive and hyperconjugative effects are manifested jointly in the oxiranes.

111. SYNTHESIS OF OXIRANES

1. Oxidation of Alkenes

In both the synthetic organic laboratory and industry, the first and foremost procedure for the preparation of oxiranes is the direct oxidation of alkenes. Signifi- cant new results have been achieved in the development of methods of oxidizing alkenes in the liquid phase. The major aim is the attainment of an oxidation reaction under the mildest possible experimental conditions, which allows an increase in the selectivity of oxirane formation and permits the selective oxidation of more sensitive compounds. Since the various methods of preparing oxiranes were reviewed quite recently,' the individual oxidation procedures will be mainly illustrated here with some more recent examples. Surveys concentrating on stereo- controlled epoxidations and assymmetric synthetic methods have been published.8ai'bisc

A. Oxidation with Organic Peracids

The most frequently employed method for the conversion of alkenes to oxiranes is oxidation with organic per acid^.'>^^ 149-151a Th' is procedure was discovered by Prileshaev in 1909.152 The usual oxidants are perbenzoic acid and its substituted derivatives, but aliphatic acids are also used, predominantly in industrial syntheses. It will be seen that the range of organic peracid derivatives is being increasingly extended with compounds in which there is an -0OH moiety in conjugation with a C=O or C=N, for example, 4, 5 , and 6 . The oxidant is prepared from the corresponding acid by the addition of hydrogen peroxide; in preparations from weaker acids, the presence of catalytic amounts of mineral acids is sufficient. Because of the ease of decomposition of the perdcid, the latter is often prepared from the appropriate reagent in situ by the addition of hydrogen peroxide.

NH 0 0 II It I I

-C -0-0-H -NH-C-0-0-H -0-C-0-0-H

4 5 6

16 Oxiranes

Of the percarboxylic acids, commercially available m-chloroperbenzoic acid (MCPBA) is generally the most favored; it is sometimes used at high temperatures in the presence of a radical inhibitor'53 and the yield may be increased with peracid stabilizer^.'^^ Inert solvents such as CH2C12, CHC13, and benzene are most commonly employed in the reaction Eq. 1. In basic solvents, the reaction rate decreases in proportion to the rise in basicity. With acid-sensitive olefins and in the preparation of acid-sensitive oxiranes, buffers are utilized; recent work involves the advantageous use of an alkaline two-phase solvent.'55

CHCI, - R\/ (1) P--o-o-H 0 0 R-CH=CH2 +

c1 Oxidation with the peracid is an electrophilic addition in which the driving force

is provided by the electron-donor nature of the double bond and the electron- acceptor nature of the -C020H group. The alkene is the nucleophile, the peracid is the electrophilic partner, and in the final step of the reaction, the peroxidic oxygen behaves as a nucleophile too.

The reaction mechanism depends on the electrophilic or nucleophilic strengths of the two reactants. Quantitative studies have been performed on the basis of the linear free-energy correlation of the reaction rate and the structure of the alkene.lS6 Numerous authors have dealt with the role of the solvent in the r e a ~ t i o n . ' ~ ~ - ' ~ ' Even today, however, the fine details of the reaction mechanism have not been clarified in every respect. A stepwise mechanism is disproved by the stereochemical results. Oxygen transfer from the intermolecularly hydrogen-bonded peracid monomer'61 and 1,3-dipolar addition with a 1,2-dioxolane i r ~ t e r r n e d i a t e ' ~ ~ : ' ~ ~ are not confirmed by the experimental results. Investigations have been carried out in solvents of various polarities and structures in order to shed light on the structure of the transition complex.'64 A kinetic isotope effect has led to the proposal of an open-chain structure 7 with strong charge separation, from which the rate of ring- closure is greater than that of rotation about the C-C axis.'65

7

Dryuk'66 attempted to solve the existing contradictions by performing wide- ranging reaction-kinetic examinations. The results of these can be summarized as follows: the course of the reaction, which is in competition with the formation of H-bonded complexes, is governed by the nature of the electron-donor-acceptor complex (EDAC) formed between the alkene and the peracid. The entire process is influenced by solvation effects. Oxirane formation is accompanied not only by the direct formation of a rearranged product, but by the induced decomposition of the peracids (Eq. 1 a).

Synthesis of Oxiranes 17

/ 4 'd

\ 0 ,+c, ,

\ / ---H-0-0 C-Ar - ,C-C,

C Ar-C-0-0-H + C II E!Ac{ :' ' !I

(1 a) / \ I t 0

other transformation

In contrast with other electrophilic additions, the peracid epoxidation is syn- stereospecific. With sterically strongly hindered alkenes the reaction takes place on the less sterically hindered side. In other cases, the stereochemistry of the reaction is affected by polar effects or the geometry of the transition state. Important conclusions regarding the mechanism of the reaction can be drawn from the steric path- ways in the synthesis of the oxiranes. This has been dealt with comprehensively by Berti? who reviewed the topic up to 1971, with special emphasis on the peracid oxidation. A noteworthy account of the topic of peracid epoxidation is given in a review by Rebek.&

Ab initio molecular orbital studies have been carried out on the mechanism of epoxidation of alkenes with p e r a ~ i d s . ' ~ ~ ~ ' ~ ~ Numerous examples have been reported of the formation of products where the stereochemistry differs from the known general regularities; some characteristic instances of these will be presented below.

Two products, 13%

The stereoselective oxidation of cis- and trans-3-tert-butyl-4-cyanocyclohexenes can be explained on the basis of the energetic data and the geometry of the conformers Eq. lb.'69 An unexpected product is formed as a consequence of neighbor- ing-group parti~ipation.'~' The syn directing effect of the methoxycarbonyl group is manifested in the epoxidation of adjacent double bonds in dihydrophthalates, the main product being 8.17'

W ' - C O z M e b'

8

18 Oxiranes

In the case of a sterically hindered allylic methoxycarbonyl group, the epoxidation of substituted cyclohexenes-1,4-dienes occurs on the opposite side.'72 Further studies on the epoxidation of 0,y-unsaturated cyclohexenecarboxylic acids and esters indicate that the steric and polar effects of the COzMe group result mainly in anti-epoxidation while the carboxyl group in inert solvent exerts a syn directing effect . ' 73

OH I

9

The direction of MCPBA epoxidation changes from syn to anti between the small ring (n = 2, 3) and the medium ring (n = 4) allylic alcohols 9.174 The change from syn to anti direction is explained in terms of Witham's model for the transition-state geometry of peracid ep~xidation. '~ '

10

Epoxidation of acyclic allyl alcohols with peracid and Mo/TBHP displays an opposite stereospecificity to that for the V/TBHP s y ~ t e m . ' ~ ~ ' ' ~ ~ Trimethylsilyl- substituted allylic alcohols give threo-epoxyalcohols with MCPBA and eiythro- alcohols with VO(acac)2-TBHP, with high s te reo~elec t iv i ty . '~~~ In the stereospecific epoxidatjon of ris- and trans-ally1 alcoholq, formation of a transition state is assumed with the development of two H bonds: between the hydrogen atom of the hydroxy group of the allyl alcohol and the oxygen of the peracid, and between the hydrogen of the peracid OH and the oxygen of the ether An analysis of the diastereometric transition-state interactions for stereoselective epoxidation of acyclic allylic alcohols has been p ~ b l i s h e d . ' ~ ~ A conformational effect may be res- ponsible for the unexpected cis major productg5 in Eq. 2.

small ring heterocycles - download.e-bookshelf.de · advances in chiral epoxidation of olefins have...

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