Novel aromatic systems. 4. Cyclobutadiene dications

Oláh, George A.; Staral, John S.

doi:10.1021/ja00436a037

Cited by 51 publications

(18 citation statements)

References 9 publications

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“…The planar D 4 h structure is destabilized compared with the puckered D 2 d structure because of the unfavorable 1,3‐antibonding interactions in the former 21. Moreover, the experimental 13 C NMR chemical shifts of [(MeC) 4 ] + are in better agreement with those calculated for the D 2 d rather than the D 4 h structure 20. 21a, 22, 23 Consequently, it was concluded that in small doubly charged four‐membered ring systems, such as the cyclobutadiene dication and cyclobutadiene dianion, the Coulomb repulsive interaction between the two charges becomes more important than the advantage gained by the π delocalization and therefore may overcome the effects of Hückel aromatic stabilization 21a.…”

Section: Species With Two π Electronssupporting

confidence: 75%

“…(Scheme ; E=C, R=Me) 18. The same research group subsequently reported the generation of tetraphenyl‐,19 1,2‐difluoro‐3,4‐diphenyl‐,20 and 1,2‐diphenylcyclobutadiene20 dications under conditions for stable ions. However, the synthesis of compounds of this type that were stable at room temperature was not achieved; therefore, real structural information is not available for the discussion of the bonding nature of such intriguing species.…”

Section: Species With Two π Electronsmentioning

confidence: 99%

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Aromaticity of Group 14 Organometallics: Experimental Aspects

2007

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The long story of aromatic compounds has extended over almost two centuries, since the discovery by Faraday of "bicarburet of hydrogen", or C(6)H(6), now called benzene. Since then, the chemistry of aromatic compounds has been developed extensively; this is reflected in the synthesis of novel classes of aromatic derivatives including charged species, nonclassical (Möbius, three-dimensional, homo-, metalla-) aromatics, and fullerenes. The theory of aromaticity has also undergone a spectacular evolution since the first definition of aromaticity by Hückel; the classification of aromaticity now requires the consideration of versatile criteria: energetic, structural, magnetic, among others. In this Review, we discuss the current state of affairs in the chemistry of aromatic compounds of the heavier Group 14 elements, the latest experimental achievements, as well as future prospects in the field.

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Section: Species With Two π Electronssupporting

confidence: 75%

Section: Species With Two π Electronsmentioning

confidence: 99%

Aromaticity of Group 14 Organometallics: Experimental Aspects

2007

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“…An intrinsically symmetrical structure for A would be the heptamethyltropylium ion 24. The 'H-NMR chemical shift observed for ion A (83.08) is close to the one reported by Schmid and * :W-oo2 4 24'…”

Section: Resultssupporting

confidence: 86%

Strained Mono‐ and Dications. On the Structures of [(CCH₂)₇H]^2⊕ and [(CCH₃)₇CH₃]^2⊕ Dications

Carnadi

Giordano

HELDEWEG

et al. 1981

Israel Journal of Chemistry

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The reactions of alcohol 1 with HF‐SbF5 and of alcohol 8 with FSO3H‐SbF5 at low temperatures afforded dications [(CCH3)7H]2⊕ and [(CCH3)7CH3]2⊕, respectively. Both NMR (1H and 13C) data and analysis of the products, obtained upon quenching of the cations, were used to discuss the possible structures of these cations. Both ions showed a remarkable NMR behaviour, viz. an equivalency of all methylgroups on the one hand and all skeleton carbon atoms on the other hand, indicating a high symmetry or a rapid (on the NMR time scale) process. Using model compounds the weighted average 13C‐NMR chemical shifts of reasonable structures for the ions were calculated and compared with the experimentally obtained values. The conclusion is drawn that the former reaction presumably leads to dications 34 and 36 and the latter reaction to dication 51.

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“…Table 2 ing for the ring is consistent with substantial cyclobutendiylium character for this moiety (14)(15)(16)(17). If one turns one's attention to the pendent guaiazulene groups in bisguaiazulenylsquaraine (Table l), then several carbons show sizeable downfield (10 ppm) shifts from the parent guaiazulene.…”

Section: Fig 2 the 250 Mhz 'H Nmr Spectrum Of Bis(4-dibutylamino-2-mentioning

confidence: 60%

Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines

Kazmaier¹,

Hamer²,

Burt³

1990

Can. J. Chem.

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Can. J. Chem. 68, 530 (1990). The proton and I3C NMR of azulene and dialkylaminohydroxyphenyl squaraines have been investigated. The four-membered ring carbon resonances occur in the 180-190 ppm range and are attributed to the cyclobutenediylium character of this structure. Some charge delocalization from the central ring to the pendent aromatic substituents was also observed, indicative of a substituent effect of the squaraine moiety on aminoaromatic systems equivalent to that of a formyl or acetyl group. Hydroxy squaraines 10 and 11 show the presence of two isomeric squaraine species in the proton NMR in deuterochloroform. Saturation transfer experiments confirm that these species are readily interconverted, and cisltrans isomerism (9a 9b) is consistent with these observations. The free energy difference between the two isomers at 318 K is calculated to be 3.0 and 3.2 kJ/mol for hydroxysquaraines 10 and 11 respectively.Key words: squaraines, saturation transfer, NMR. On a examink les spectres RMN du proton et du I3c de l'azulkne et des dialkylaminohydroxyphCnyl squara'ines. Les resonances des carbones du cycle 5 quatre chainons sont observkes dans la plage allant de 180 a 190 ppm et elles sont attribukes au caractere cyclobutenediylium de cette structure. On a aussi observe une certaine dClocalisation de la charge du noyau central vers les substituants attaches au noyau aromatique; ces observations suggerent qu'il existe un effet de substituant de la portion squaraine sur les systkmes aminoaromatiques equivalent a celui d'un groupement formyle ou acyle. Dans les spectres RMN du proton des hydroxy squarai'nes 10 et 11 en solution dans le deutkrochloroforme, on observe la prksence de deux especes isomkres. Des expkriences de transfert de saturation confirment que ces especes se transforment facilement l'une dans I'autre; une isomkrie cislrrans (90 S 9b) est en accord avec ces observations. On a calcule que, a 318 K, les diffkrences d'knergie libre entre les deux isomeres sont de 3,O et de 3,2kJ/mol respectivement pour les hydroxysquarai'nes 10 et 11.

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Novel aromatic systems. 4. Cyclobutadiene dications

Cited by 51 publications

References 9 publications

Aromaticity of Group 14 Organometallics: Experimental Aspects

Aromaticity of Group 14 Organometallics: Experimental Aspects

Strained Mono‐ and Dications. On the Structures of [(CCH₂)₇H]^2⊕ and [(CCH₃)₇CH₃]^2⊕ Dications

Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines

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Novel aromatic systems. 4. Cyclobutadiene dications

Cited by 51 publications

References 9 publications

Aromaticity of Group 14 Organometallics: Experimental Aspects

Aromaticity of Group 14 Organometallics: Experimental Aspects

Strained Mono‐ and Dications. On the Structures of [(CCH2)7H]2⊕ and [(CCH3)7CH3]2⊕ Dications

Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines

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Strained Mono‐ and Dications. On the Structures of [(CCH₂)₇H]^2⊕ and [(CCH₃)₇CH₃]^2⊕ Dications