Hückel and Möbius aromaticity in bicyclic systems Citation for published version (APA): Gillissen, H. M. J. (1982). Hückel and Möbius aromaticity in bicyclic systems. Technische Hogeschool Eindhoven. https://doi.org/10.6100/IR69545 DOI: 10.6100/IR69545 Document status and date: Published: 01/01/1982 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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CERFONTAIN CONTENTS Chapter I General Introduetion 7 Heferences and Notea 11 Chapter 11 Intramolecular electron transfer in 9-(arylseleno)bicyclo[4.2.1]nona-2,4,7- trien-9-yl and -bicyclo [4.2.1]nona-2,4- dien-9-yl carbanions promoted via the aryl ligands 13 11.1 Introduetion 13 I I. 2 Quench reeulte of 9-arylaeleno eubetituted carbanione 15 I I. 3 Structural aaeignment of the quenah produate 23 II.4 Direct 1 H NMR obeervation of anionio eolutione 27 I I. 5 Discussion 28 II.6 E:r:perimental 35 Referenaee and No tee 49 Chapter 111 Reductive eliminatien and skeletal re­ arrangement of S-hydroxy selenide deri­ vatives of bicyclo ~.2.1]nona-2,4,7- triene in a super acid medium 51 III.l Introduetion 51 III. 2 Rearrangement of S-hydro:cy eele- nides 54 III.3 Diecuesion 56 III.4 E:cperimental 58 Referenaee and Notes 62 Chapter IV Cationic derivatives of bicyclo[4.2.1J­ nona-2,4,7-triene as modelsystems for ground-state Möbius aromaticity 64 IV.1 Synthetia aonsidePations 64 IV.2 StePeoahemistPy of pPotonation 68 IV.3 NMR speatPosaopia investigations of aaPboaations undeP ~ong ~ife aonditions 10 IV.4 Disaussion 18 IV. 5 ExpePimental 81 RefePenaes and Notes 94 Sun:unary 97 Samenvatting 99 Curriculum vitae 101 Dankwoord 102 CHAPTER I GENERAL INTRODUCTION A concept, which has been of special interest for or­ 1 ganic chemists, is the Hückel rule , which involves that for ground-state molecules with a cyclic array of atomie orbitals, 4n+2 electrens result in aromaticity and thermo­ dynamica! stability. Fundamental in Hückel's reasoning is the large energy difference between ground-state and excited state(s) in a ring with 4n+2 electrons, whereas 4n electrens result in a smal! energy separation. This aspect has been nicely demonstrated in a quantitative way for the cycliza­ tion of butsdiene using complete VB and MO calculations by van der Lugt and Oosterhoff2 , The same is also true fora cyclic array of orbitals with 4n and 4n+Z electrons, respec­ tively, when this interchange is accompanied with an odd number of sign inversions for the orbitals in the ring. In the latter case, we are generally speaking of Möbius aroma­ ticity: 4n electrens result in aromaticity. Using simple MO calculations, the Möbius aromaticity concept has been explored for a variety of reactions, which are commonly 3 4 known as Woodward-Hoffmann reactions ' • It attracks attention that energy lowering of transi­ tion states and stabilization of intermediates has been 5 hardly explained with the concept of Möbius aromaticity , although the thermal conrotatory conversions and the thermal sigmatropie shifts with inversion of the migrating carbon are distinct examples for 4n electron aromaticity (Figure I). 7 b Figure I: Transition state for a thermal aonrotatory con­ version (~) and a thermal sigmatropie shift with inversion of the migrating carbon (~). Of course, ground-state Möbius aromaticity has never been observed in consequence of the steric strain, which imposes a sign inversion on a small cyclic polyene. One possible way to generate ground-state 4n electron aromaticity is the preparation of bishomocyclic systems l· Sign inversion may then be realized by a simple orientation of the carbon p-orbital, which is homoconjugated with the two neighbouring carbons. 3 1 In fact, this resembles the transition state for the ther­ mal [1 ,3] C and [1 ,4] C sigmatropie shifts. In spite of the fact that the extended VB theory as was ~utlined by Oosterhoff et al. 6 is a direct tool in the search for the fundamentals determining aromaticity in the general sense, Möbius molecules seem to be exceptious or in other words their existence looks rare, whereas molecules possessing Hückel aromaticity are present in an overwhelming 8 quantity. A similar situation is encountered in antiaroma­ ticity7, i.e. 4n electrans in a Hückel arrangement or 4n+2 electrans in a Möbius cycle. From the work of Schipper and Buak 8 it appeared that if geometrical restrictions are di­ minished, the system escapes the 4n electron Hückel anti­ aramaticity via 4n-2 electron Hückel aromaticity and the simultaneous formation of a localized double bond (~). Befare this fascinating observation Buak et aZ. 9 investiga­ ted the thermal D,j]shifts in cyclic systems with a pertu­ bation approach and INDO calculations. It was clearly shown that the concept of Hückel aromaticity controls the supra­ facial shift. Unfortunately, the antarafacial shift was not discussed in a quantitative way. We may expect that the an­ tarafacial shift results in Möbius aromaticity for the transition state. Hückel vs. Möbius aromaticity is then re­ flected in the electron density on the migrating hydrogen. The value of this concept is clearly demonstrated when the shift occurs between non-bonding carbons e.g. in the supra­ fadal [1, s] H shift in cycloheptatr.iene the hydragen carries one electron, whereas in the suprafacial [1, 7] H shift, which is only of theoretica! interest, the electron density on hydragen increases. In this thesis the concept of aromaticity is fully explored with a number of experimental ex~mples. The systems derived are cations and anions which demonstrate bishomo­ conjugation resulting in Hückel and Möbius aromaticity. The various structures were established by 1 H and 13C NMR mea­ surements. Generally, the ions are derivatives of bicyclo- ~.2.1]nona-2,4,7-triene. Chapter II deals with the chemistry of the bicyclo­ ~.2.1] nona-2,4,7-trien-9-yl carbanion. According to GoLd­ 10 stein's theoretica! model , this carbanion should besta- 9 11 bilized by longicyclic interactions • The carbanions are generated by the cleavage of arylselenoketals. This strate­ gy leads to the introduetion of an arylseleno-group at the charge center. It is shown that the chemistry of the carb­ anions is dominated by the stereospecific transposition of the negative charge from the anti-Cg carbanion to the aryl ring of the introduced substituent via proton transfer. Electrophiles tend to react with the èyn-Cg carbanion. This result is compatible with the notion of a bishomoaromatic interaction12 between Cg and the diene moiety. In Chapter III the syn-c10 carbocation derivatives of bicyclo[4.2.1]nona-2,4,7-triene are introducedas possible model systems for the investigation of ground-state Möbius aromaticity. 6-Hydroxy selenide derivatives of bicyclo~.2.1]­ nona-2,4,7-triene were investigated in super acid media. It is demonstrated that neighbouring group participation13 by selenium preelucles the formation of free c,o-carbocations. Instead, exocyclic olefins and selenenyl cations are produced in a solvent cage. This initiates a rearrangement reaction for which a mechanism is given. Chapter IV describes the generation and investigation 13 1 of a-hetero-substituted c10 -cations. C and H NMR spec­ troscopy suggest that the empty orbital at c1 0 is o.rienta­ ted perpendicularly with respect to the mirror plane of the cations. The saturated analogues of these cations also adopt this configuration. The chemica! shift differences between the unsaturated cations and the saturated derivatives may suggest a certain degree of charge delocalization via Möbius aromaticity.