NMR‐Spectroscopic Investigations of Potentially Planarized Nonafulvenes

Chai, Steven K.; Bönzli, Peter; Neuenschwander, Markus

doi:10.1002/hlca.19950780119

Cited by 6 publications

(2 citation statements)

References 21 publications

(24 reference statements)

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“…Figure ). Thus, partial aromaticity, suggested for the whole molecule 10 , ,,− is limited to the three-membered ring moiety only, and the π C  C orbital occupation proves to be of intermediate nature.…”

Section: Resultsmentioning

confidence: 93%

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Quantification of the (Anti)Aromaticity of Fulvalenes Subjected to π-Electron Cross-Delocalization

2007

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Fulvalenes 3-12 were theoretically studied at the ab initio level of theory. For the global minima structures, the occupation of the bonding (pi)C=C orbital of the interring C=C double bond obtained by NBO analysis quantitatively proves pi-electron cross-delocalization resulting in, at least partially, 2- or 6pi-electron aromaticity and 8pi-electron antiaromaticity for appropriate moieties. The cross-conjugation was quantified by the corresponding occupation numbers and lengths of the interring C=C double bonds, while the aromaticity or antiaromaticity due to cross-delocalization of the pi-electrons was visualized and quantified by through-space NMR shielding surfaces.

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Section: Resultsmentioning

confidence: 93%

“…Otherwise, 1 H chemical shifts residing in the aromatic region, 13 C chemical shifts resonating at ca. 110 ppm, or large 3 J H,H coupling constants of similar size have all been used for assessing the extent of delocalization. − …”

Section: Introductionmentioning

confidence: 99%

Quantification of the (Anti)Aromaticity of Fulvalenes Subjected to π-Electron Cross-Delocalization

2007

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The Fulvalenes

Halton

2005

Eur J Org Chem

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The fulvalenes are the class of hydrocarbons obtained by formally cross‐conjugating two rings through a common exocyclic double bond. Although the first report of such a compound dates to 1915, study of the series began in earnest only in the 1950s, and even today many of the unsubstituted parent hydrocarbons remain unknown. The following report, the first to specifically address this class of molecules, summarises the information available from research published to the end of 2004 and offers some suggestions for future study. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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Synthetic Attempts towards Polymers with Pentafulvene Structural Units

Zhang

Neuenschwander

1995

Helvetica Chimica Acta

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Synthetic attempts towards fully conjugated polymers 9 with pentafulvene-diyl structural units are described. Cationic polymerization of pentafulvenes la (R = X = Me) and lb (R = X = MeS) nearly quantitatively gives polymers 8a and Sb with typical M , and M , values of 38800 and 53750, respectively, for 8a, and 12000 and 35900, respectively, for 8b. Key step of the conversion 8a + 9a (Scheme 6 ) is a quantitative bromination Sa -+ 32a, the structure of 32a being confirmed by analytical data as well as by spectroscopic comparison with model compound 23. Best results in view of two-fold the HBr elimination 32a + 9a are obtained with Et,N, but so far elimination has not been complete. Synthetic sequences are optimized with model compound 21 (Scheme 4). Here again, bromination 21 + 23 is quantitative, while two-fold HBr elimination 23 -+ 22 with Et,N proceeds in 51 YO yield. Dibromide 23 easily undergoes HBr elimination followed by a Br shift to give bromide 29. Contrary to cationic polymerization, anionic polymerization of simple pentafulvenes 1 to 2 (which would be attractive in view of the conjugated polymers 3) is not successful: For pentafulvene lb (R = X = MeS), the main reaction is Diels-Alder-type dimerization l b -+ 1% (Scheme 2), even under anionic conditions. 1. Introduction. -Pentafulvenes ( = 5-methylidenecyclopenta-1,3-dienes) 1 are cyclic cross-conjugated molecules with a five-membered ring whose electronic properties are strongly varying with exocyclic substituents [&6] influencing the extent of 71-delocalization as well as the HOMO-LUMO gap. If these substituent effects would be operative in polymers containing pentafulvene-diyl structural units as well, then polymers with considerably varying electronic properties could in principle be available. In fact, semiempirical calculations [7] predict that some types of long-chain polyenes with fulvene-diyl or fulvalene-diyl structural units should be characterized by a relatively small energy difference between the HOMO and the LUMO. Therefore, they are very attractive in view of polymers with unique electrical and optical properties. Furthermore, polymers of this type or their dihydro precursors (see Scheme 1 ) should be of interest in view of the synthesis of 'nonclassical non-alternant polymers ' [8].Some synthetic pathways to polymers with pentafulvene-and pentafulvalene-(5-(cyclopenta-2,4-dien-l-ylidene)cyclopenta-1,3-diene)-derived structural units are summarized in Scheme 1. Considering the fact that pentafulvenes 1 are easily attacked by nucleophiles at C(6) to give substituted cyclopentadienides and, after protonation, substituted cyclopentadienes (for examples, see [9-1 l]), anionic polymerization of pentafulvenes 1 to 2 followed by HX elimination (+ 3; Route A ) should be an attractive procedure. On

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NMR‐Spectroscopic Investigations of Potentially Planarized Nonafulvenes

Abstract: 'H-and ' k -N M R spectra of nonafulvene l e and nonafulvenes 2 and 3 have been assigned, high-resolution 'H-NMR spectra of 2 (600 MHz,

Cited by 6 publications

References 21 publications

Quantification of the (Anti)Aromaticity of Fulvalenes Subjected to π-Electron Cross-Delocalization

Quantification of the (Anti)Aromaticity of Fulvalenes Subjected to π-Electron Cross-Delocalization

The Fulvalenes

Synthetic Attempts towards Polymers with Pentafulvene Structural Units

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