Aromatic and antiaromatic interactions in rigid polycyclic systems : an orbital symmetry model description Citation for published version (APA): Schipper, P. (1977). Aromatic and antiaromatic interactions in rigid polycyclic systems : an orbital symmetry model description. Technische Hogeschool Eindhoven. https://doi.org/10.6100/IR8747 DOI: 10.6100/IR8747 Document status and date: Published: 01/01/1977 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 28. Sep. 2021 AROMATIC AND ANTIAROMATIC INTERACTIONS IN RIGID POLYCYCLIC SYSTEMS AN ORBITAL SYMMETRY MODEL DESCRIPTION PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL EINDHOVEN, OP GEZAG VAN DE RECTOR MAGNIFICUS, PROF.DR. P. VAN DER LEEDEN, VOOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN DEKANEN IN HET OPENBAAR TE VERDEDIGEN OP DINSDAG 14JUNI 1977 TE 16.00 UUR DOOR PIETER SCHIPPER GEBOREN TE ROTTERDAM DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTORS PROF. DR. H.M. BUCK en PROF. DR. G.C.A. SCHUIT Aan mijn moeder Ter nagedachtenis aan mijn vader Aan Taos CONTENTS Chapter 1 General introduction 7 References Chapter 2 Generation of 7-norbornenyl cations and the 15 mechanism of their reaction with nucleophiles 2.1. Introduction 2.2. Preparation of norbornadienes substituted at the 7-position with elements of group VA 2.3. Generation of the 7-triphenylphosphonio- 7-norbornenyl dication: a nonclassical dication 2.4. Experimental References Chapter 3 A Diels-Alder intermediate in the ionization 30 reaction of 9-chloro-9-methoxy-endo- lo- [4.2.1.o2•5]nona-3,7-diene 3.1. Introduction 3.2. Synthesis 3.3. Generation and properties of cations derived from 9-chloro-9-methoxy-endo-tri cyclo[4.2.1.o2 •5]nonanes 3.4. Reaction products 3.5. Kinetics of the reaction of pyridine with various 9-chloro-9-methoxy[4.2.1.o2 •5]- nonanes 3. 6. Discussion 3.7. Experimental References Chapter 4 Antiaromatic interaction in the 9-methoxy-9- 58 bicyolo[4.2.1]nona-2,4,7-trienyl cation 4.1. Introduction 4,2, Reaction products 4,3, Kinetics of the reaction of pyridine with 9-chloro-9-methoxybicyclo [4,2.1] nonanes 4.4. Generation and properties of cations derived from 9-chloro-9-methoxybicyclo­ [4.2.1]nonanes and -bicyclo[4,4.1]un ... de canes 4,5. Discussion 4.6. Experimental References Chapter 5 Stereospecific reactions of the 9-phenyl­ 83 seleno-9-bicyclo [4. 2. 1] nona-2 ,4, 7-trienyl anion 5.1. Introduction 5.2. Preparation of the 9-phenylseleno-9-bi- cyclo[4.2.1]nona-2,4,7-trienyl anion 5.3. Stereospecific reactions 5,4, Discussion 5.5. Experimental References Summary 97 Samenvatting 100 Curriculum vitae 103 Dankwoord 104 CHAPTER 1 General introduction One of the most significant advances in the study of sol­ volytic mechanisms was the recognition by Winstein and Lucas 1 that substituents which are not directly connected to the re­ action center can strongly affect the rate and the stereo­ chemical course of a reaction. If this phenomenon arises from an intramolecular nucleophilic attack of the functional group at the reaction center, it is known as "neighbouring group participation"2 • This participation may result in an increase in reaction rate. In this case the neighbouring group provides "anchimeric assistance". Three major classes of neighbouring groups can be chosen on the basis of the kind of electrons available for participation: the nonbonding electrons as present on oxygen, sulphur, nitrogen, phosphorus and the halo­ gens; electrons provide? by a rr system and a electrons of a saturated bond. An early example of a chemical consequence resulting from R R TsO H 3 !AcOH. R R TsO A cO H 7 the participation of the rr electrons of a double bond is the enhanced rate of acetolysis of 3S-cholesteryl sulphonates (1), which proceeds 500 times faster than the acetolysis of satur ated 3B-cholestanyl compounds (2) 3 • The former process proceeds • with retention, whereas the latter one occurs with inversion of configuration4 • The intermediate 3 has been described as a homoallylic cations, the homologue of an allylic cation, which has an additional methylene group between the double bond and the cationic center. It was further demonstrated that a proper orientation of the leaving group for backside attack by the double bond is critical. The 3a-cholesteryl derivatives (1a) are less reactive than the 3S derivatives (1) and give, as the principal product on acetolysis, cholesta-3,5-diene6. R R H OTs When the double bond is situated in a symmetric position with respect to the reaction center, even larger rate enhance­ ments were observed 7 • The neighbouring group participation of the double bond of anti-7-norbornenyl tosylate (5) results in a rate acceleration of 10 11 with respect to the saturated analogue (4). The intermediate cation (6) has been described as a homoaromatic species, in which the overlap of p orbitals 4 5 gives rise to a set of molecular orbitals which are similar to the cyclopropenium cation. Three types of homocyclopropenium ions can be chosen on the basis of the number of interruptions of the a framelc. The mono-, bis- and trishomocyclopropenyl cations, respectively 8 exemplified by the cyclobutenyl cationlc,a (7), the norbornen­ yl cation (6)9 and the [3.1.0]hexenyl cation (8)w. A w. ~ 7 6 8 The concept of homoaromaticity has been extended to bi­ cycloaromaticity by Goldstein and Hoffmann in a series of papers in which they attempt to elucidate the nature of the interaction between ~ systems 1 1 • In their study the fundamen­ tal building block of an unsaturated compound is an intact conjugated polyene segment, which is designated in Figure 1 as an unbroken line, called a ribbon. Of the great variety of topologies which may be envisaged for the linkage of several ribbons, four have been selected. These topologies, the peri­ cyclic, the spirocyclic, the longicyclic and the laticyclic, are depicted in Figure 1 together with some examples. For the pericyclic topology Hlickel's 4n+2 ~-electron prerequisite for cyclic stabilization does apply. The other topologies are pre- dicted to be stabilized if there are more stabiliz than destabilizing interactions between the ribbons. An interaction between two ribbons is stabilized, when 4n+2 ~ electrons are involved. Thus, the 7-norbornenyl cation (6) as well as the 7-norbornadienyl cation (9), respectively examples of the pericyclic and longicyclic topology, are stabilized ions. The traditional tests for relative stabilities of inter­ mediates are most indirect. The rate of solvolysis of a neu­ tral precursor is usually compar~d with that of an appropria­ tely hydrogenated derivative. However, the solvolysis of an unsaturated compound can give rise to rearranged products. In 9 Pericyclic topology (......_____ ···~ .n C.,,__,; ~.. ~ 0 6 8 Spirocyclic topology Longicyclic topology c____.:.. D .. 9 Laticyclic topology n ... n ,. 'l...g ...... .. ~... J Figure 1 this case it is difficult to decide which kind of activated complex is involved in the rate-determining step. Alternative­ ly, the character of the activated complex is correlated to the structure of the corresponding ion which may be observed as a long-lived species in superacid media. In this thesis the neighbouring group participation of rr and a electrons in polycyclic systems has been studied. Charge­ stabilizing groups were introduced at the reaction center in order to prevent rearrangement reactions. In this way it was possible to identify the intermediates and to ascertain their relative order of stability. Thus, for the first time, the study of antiaromatic interactions appeared to be accessible along kinetic and spectroscopic lines. Chapter 2 deals with the introduction of charge-stabiliz- 10 ing groups, derived from elements of group VA and VIA, at the 7~position of the norbornenyl system. Reaction of triphenyl­ phosphine with the homoaromatic 7-norbornadienyl cation afford­ ed 2,3-substituted products as a consequence of charge deloca· lization, whereas the related reaction with the classical 7- methoxy-7-norbornenyl cation produced the ?-substituted tri­ phenylphosphonium salt.