Annulenes, configurations

The interconversion of the two forms involves a configurational change from to Z at at least one double bond. The activation energy for this process is only about lOkcal/mol. The crystal stmcture for [14]annulene shows the to be present in the... 

Recently, further examples for dehydro Diels-Alder reactions were published, which are believed to proceed via intermediates of the type 205. To explain the formation of products besides those of the type 206, the authors [132] proposed a remarkable reaction cascade and supported it by quantum-chemical calculations. Accordingly, an isonaphthalene of the type 205 undergoes an electrocyclic ring expansion to give a l,2-dehydro[10]annulene derivative, in which a configurational isomerization occurs followed by an electrocyclic ring closure, yielding a further isonaphthalene of the type 205, and aromatization. 

The diatropism of diazapyraceheptylenes (106), which follows from a comparison of the PMR spectra with that of 105a (Table VI), may be due to the effect of the cyano groups on the electronic configuration of the 7r-system. The chemical shift difference between the inside and outside protons clearly reveals the expected diamagnetic ring current in the annulene derivatives (110). The coupling constants of the seven-membered rings of 110 show that this part of the molecule is planar and delocalized. 

A theoretical prediction that has been borne out by experiment is that an annulene with 4n v electrons should have a paramagnetic circulation of electrons—that is, opposite in direction to that shown in Figure 22-4 for benzene. For example, [16]annulene, which has 4n electrons, is not very stable and exists as a very rapidly interconverting mixture of two configurational isomers ... 

An X-ray crystal structure of annulene-dione (64) indicates an anti,anti configuration between the methylene and sulfur bridges.102 Diprotonation gives highly localized positive charges in the dication (65), mainly due to unfavourable p-orbital overlap. 

Vinylic SN2 reactions are discussed on p. 198. Here, we restrict ourselves to nucleophilic susbstitutions at saturated centers. Aromaticity criteria then strongly disfavor retention of configuration. Indeed, the transition state leading to retention, 43, resembles a four-electron (two from the nucleophile and two from the CY bond) antiaromatic annulene. 

Annulene (Section 16.10) A name sometimes given to rings that contain alternating single and double bonds in a single Lewis structure. Anomers (Section 25.3) Diastereomers that are formed by cyclization of a carbohydrate with differing configurations at the new stereocenter. 

Paracyclophane-l,9,23,31-tetraene60 (41) is a rigid annulene with a well-defined ring configuration. The neutral cyclophane, which formally has 32 -electrons An)... 

The prototropic rearrangement of the cyclic tetramer 152, the pentamer 153 and the hexamer 154 on treatment with potassium r-butoxide gave the tetrakis-dehydro[24]- (157) , the pentakisdehydro[30]- (159) and the hexakisdehydro[36]-(161) annulenes . Catalytic half-reduction of the triple bonds in 157 and 159 gave [24]annulene (158) and [30]annulene (160), respectively , although their configurations were not firmly established. 

In the late 1960s, as part of an effort devoted to the preparation of high-order annulene polyoxides, J. A. Elix attempted a Wittig-type self-condensation of 5-formyl-2-furfurylphosphonium chloride 1.19 While the products that resulted were predominantly polymeric, small quantities of macrocyclic products were isolated. These latter consisted of trimers, tetramers, and pentamers (discussed in Chapters 2, 4, and 6, respectively), as well as a hexamer. Unfortunately, this hex-americ [36]annulene hexoxide product represented by structure 7.80, was only obtained in 0.05% yield (Scheme 7.7.1). Thus, Elix was unable to elucidate the exact configurational arrangement of the double bonds in this product. Still, as discussed below, this issue could be addressed at least in part, by spectroscopic means. 

For the first time in our work we were able to isolate in the case of ll-cyano-1,6-methano[10]annulene a roughly 1 1 mixture of the anti-endo-Sidduci 48 and anti-exo-adduct 49 whose structures and configurations were determined by X-ray crystallography. ... 

Balaban, A.T. (1971). Chemical Graphs. 12. Configurations of Annulenes. Tetrahedron, 27,6115-6131. 

The presence of (An + 2) electrons is a requirement for the existence of a diamagnetic circulation of electrons. Pople showed that an external magnetic field can induce a paramagnetic circulation in a 4mr electron system. Under such circumstances, the conclusions drawn from the configuration in Figure 3-3a are reversed (i.e., outer protons are shielded and inner protons deshielded). The spectrum of [16]annulene is consistent with this interpretation (inner protons at 8 10.3, outer at 5.2), but the most dramatic example is... 

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