Reactivity and Structure Concepts in Organic Chemistry

Transcription

1 Reactivity and Structure Concepts in Organic Chemistry Volume 30 Editors: Klaus Hafner Charles W Rees Barry M. Trost Jean-Marie Lehn P. von Rague Schleyer Rudolf Zahradnik

2 Michinori Oki The Chemistry of Rotational Isomers With 3 Figures and 42 Tables Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona Budapest

3 Professor Michineri Oki Department of Chemistry Okayama University efscience Okayama 700, Japan Library of congress Cataloging in Publication Data Oki, Mid!inori The d!emistry of rotational isomers I Mid!inori Oki, p. cm. (Reactivity and structure; v. 30), Includes bibliographical references and index. I. Isomerism. II. Title. III. Series. QD S41.2'252-dc CIP This wod: is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specificall y the rights of translation, reprinting, reuse ofillustrations, recitation, broadcasting, reproduction on microfilms orin any otherway, and storage in data banks. Duplication ofthis publication or parts there{)f is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version,and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. ISBN ISBN (d look) 1> C Springer-Verlag Berlin Heidelberg 1993 Soflco\'cr rellrint of the hartlco\ er 1st etlition 1993 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specifi c statement, that sud! names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Macmillan India Ltd., Bangalore, India I 0

4 List of Editors Professor Or. Klaus Hafner Institut fur Organische Chemie derth Darmstadt PetersenstraBe 15, D-6100 Darmstadt Professor Dr. Jean-Marie Lehn Institut de Chimie, Universite de Strasbourg I, rue Blaise Pascal, B.P. 296/R8, F Strasbourg-Cedex Professor Dr. Charles W. Rees, F. R. S. Hofmann Professor of Organic Chemistry, Department of Chemistry Imperial College of Science and Technology South Kensington, London SW7 2A Y, England Professor Dr. Paul v. Rague Schleyer Lehrstuhl fur Organische Chemie der Universitat Erlangen-Niimberg HenkestraBe 42, D-8250 Erlangen Professor Barry M. Trost Department of Chemistry, Stanford University Stanford, CA , U.S.A. Professor Dr. Rudolf Zahradnik Tschechoslowakische Akademie der Wissenschaften J.-HeyrovskY-lnstitut flir Physikal. Chemie und Elektrochemie Machova 7, Praha 2, 1SEC

5 Preface The concept of internal rotation was developed as early as 1875 but was never thought meaningful in organic chemistry until recent times. Indeed, rotation about a single bond in organic compounds has been assumed for a long time to be free and it was only about 50 years ago when the concept of restricted rotation was established from physical measurements. Even after learning of the presence of rotational isomers by spectroscopic measurements, organic chemists tended to neglect the presence of such isomers. The presence of atropisomers in biphenyls has been a very special case and they give differences only under chiral conditions: usually we distinguish diastereomers but not enantiomers. However, knowledge of rotational isomers has been accumulating rapidly in recent years. Even organic chemists cannot ignore the difference in rotational isomers today. This is, on one hand, caused by advances in spectroscopy and, on the other, by advances in asymmetric syntheses. We were fortunate to be able to work on two series of compounds that give stable rotational isomers at room temperature and to be able to examine the differences in their reactivities. This book is the compilation of our recent work in this field together with some historic background and related areas. Chapter 1 describes the historical concept of "free rotation" about a single bond, development of the concept of restricted rotation and rotational isomers, and isolation of stable rotational isomers at room temperature. This chapter also deals with the concepts which still prevail among some chemists today but should be changed: internal compensation and chemical properties of diastereomeric rotational isomers. Chapter 2 describes factors that affect populations of rotational isomers. Since the two series that give stable rotational isomers at room temperature feature the close proximity of two groups in a molecule in the ground state, it is possible to observe very weak molecular interactions, that cannot be otherwise detected, as unusual population ratios of rotational isomers. These series of compounds serve as excellent probes for detecting weak molecular interactions which are theoretically possible but are difficult to observe. Chapter 3 describes factors that affect the height of the barrier to rotation, after briefly mentioning about the method for determining the barrier height. An interesting point here is that a bulky substituent does not necessarily increase the height of the barrier. Emphasis is put on the fact that the barrier height is the difference in energies between the ground state and the transition state for rotation.

6 viii Preface Chapter 4 describes the differences in the reactivities between rotational isomers. It is also pointed out that the diastereotopic methyls such as those in a tert-butyl group can show different reactivities. The origin of the difference in reactivities is, of course, intra(and/or inter) molecular interactions which accelerate or retard formation of reaction intermediates or transition states. Readers will notice that there is even a case in which rotational isomers react via different mechanisms from each other under the same conditions. It will be a great pleasure for the author if this book helps organic chemists realize that a mixture of diastereomeric rotational isomers should be considered a mixture of differently reacting isomers in the laboratory and, if they are considering reactions under chiral conditions, any of the rotational isomers can behave differently. It is also hoped that this book causes further development of chemistry in the field of rotational isomers by motivating young chemists. There is one thing which should be mentioned here concerning the presentation of this book. Showing stereochemical structures is very space-demanding. Because we are dealing with rotational isomers in this book, it is normally necessary to show three or more stereochemical drawings for one compound which can sometimes fill a page of printing. To avoid this situation, after a time, only one isomer of the two possible enantiomers is shown. Therefore, the readers are advised not to take these structures as if they are separated into optical isomers but take them as showing the possible pair of enantiomers. The author owes a great deal to former and present associates in The University of Tokyo and Okayama University of Science, without whose enthusiasm this kind of chemistry could not have been explored. The author also wishes to thank Professor Michael Hanack at Tiibingen, Professor Horst Kessler in Miinchen, and Professor Jan Sandstrom in Lund for their reading of the manuscript. Okayama, Japan, May, 1992 Michinori Oki

7 Table of Contents 1 Introduction The "Free Rotation" Concept Recognition of Rotational Isomers Atropisomers Isolation of Rotational Isomers Factors Affecting Barrier Heights Triptycene Derivatives Arylftuorenes Concepts That Need Modification Are There Dextrorotatory and Levorotatory Groups? Reactivity of Rotational Isomers References Rotamer Populations Estimation of Rotamer Populations Vibration Spectroscopy Nuclear Magnetic Resonance Spectroscopy Factors That Affect Rotamer Populations Information on Molecular Interactions Obtained from Rotamer Populations Triptycenes Long-Range Coupling in NMR Spectra Steric Repulsion n-n* Interactions n-q* Interactions Hydrogen Bonding Involving a Methyl Group Arylftuorenes XH-n Interactions n-n* Interactions References Barriers to Rotation Estimation of Barriers to Rotation Coalescence Method Line Shape Analysis Saturation Transfer The 2D-NMR Spectroscopy

8 x Table of Contents Classical Kinetics Reliability of the Data High Barriers to Rotation in the 9-Arylfluorene Series Isolation of Rotational Isomers Factors Affecting the Rotational Barriers Effects of the Steric Size of the Substituents Ground State Destabilization Ground State Stabilization Solvent Effects High Barriers to Rotation in 9-Substituted Triptycene Series Triptycenes Carrying a 9-tert-Alkyl Substituent Triptycenes Carrying a 9-sec-Alkyl Substituent Triptycenes Carrying a 9-prim-Alkyl Substituent Atropisomers About a Carbon-to-Heteroatom Bond High Barriers to Rotation in Miscellaneous Compounds Atropisomerism About an Sp3_Sp3 Bond Atropisomerism About an Sp3_Sp 2 Bond References Reactivity of Rotational Isomers Reactivity of 9-Arylfluorenes Additions and Eliminations Additions to Olefins Addition to Carbonyl Groups Elimination Reactions Substitution Reactions Acid-Base Properties Acids and Bases in Substituents of the Aryl Group Deprotonation at the 9-Position of Fluorene Other Types of Reaction Radical Reactions of 9-Arylfluorene Rotamers Nitrenes and Carbenes in Arylfluorene Rotamers Reactivity of Substituted Triptycenes Radical-Forming Reactions Substitution of a tert-butyl Group and Related Reactions Thermolysis of Peroxyesters Cation-Forming Reactions Lewis Acid-Catalyzed Ionization of Halogen Compounds Intramolecular Friedel-Crafts Acylation Diazotization of Amines Other Types of Reactions Reactions of Chlorides with tert-butyllithium Chlorodecarboxylation Bromine Addition Reactions References