Zusammenfassung
The twofold cyclizing Wittig reaction of the bis-aldehyde 6 with the ylide of the bis-phosphonium salt 7 yields tetraepoxy[26]annulene(4.2.2.2) 4, which exists in the two isomeric forms 4a (EE,Z,E,Z) and 4b (EE,Z,E,E). Annulene 4a is a highly dynamic system down to -80. Temperature-dependent H-1-NMR spectra of 4a establish that the (E,E)-buta-1,3-dien-1,4-diyl as well as the (E)-ethen-1,2-diyl ...
Zusammenfassung
The twofold cyclizing Wittig reaction of the bis-aldehyde 6 with the ylide of the bis-phosphonium salt 7 yields tetraepoxy[26]annulene(4.2.2.2) 4, which exists in the two isomeric forms 4a (EE,Z,E,Z) and 4b (EE,Z,E,E). Annulene 4a is a highly dynamic system down to -80. Temperature-dependent H-1-NMR spectra of 4a establish that the (E,E)-buta-1,3-dien-1,4-diyl as well as the (E)-ethen-1,2-diyl bridges rotate around the adjacent sigma -bonds in a synchronous manner. Isomer 4b, for steric reasons, is rigid. By Wittig reaction of the bis-aldehyde 8 with the ylide of the bis-phosphonium salt 9, the tetraepoxy[30]annulene(4.4.4.2) 5 is obtained. which exists also in two isomeric forms, 5a and 5b. Only 5a (EE,ZE,EE,Z) can be isolated in pure form. Like 4a, 5a is highly dynamic, the (E,E)-buta-1,3-dien-1,4-diyl as well as the opposite (E)-ethen-1.2-diyl bridge being able to rotate down to -80. The H-1-NMR spectrum at -80 indicates that 5a exists in the stable conformation 5a '. The 26- and 30-membered annulenes belong to the most expanded neutral annulencs known hitherto their H-1-NMR spectra confirm that they still have diatropic, aromatic character.
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