Bond cyclic diacetylenes

The photoelectron spectra of cyclic diacetylenes, 4a and 6a and a reference compound, cyclooctyne were measured in order to study the proximity interaction of acetylenic bonds . The observed values of the lowest vertical ionization potentials were in the order 6a > 4a > cyclooctyne. The photoelectron bands were assigned by scmiempirical calculations, MINDO/2 and SPINDO, using X-ray crystallographical data, and their relative sequence and positions were explained in terms of through-bond and through-space interactions between 7t and a orbitals. 

The cyclic diacetylene 5,6,1 l,12-tetradehydrotetrabenzo[a,c,g,i]cyclododecahexa-ene 89 is particularly interesting for its two triple bonds are closely fixed and cross each other, and the four j-p-carbons occupy the apices of an expected tetrahedrane (90). In view of this, 89 was synthesized from bis(2-bromophenyl)acetylene (88) by... 

In the case of cyclic diynes more structural data are available, probably due to the fact that diynes crystallize better. The most relevant structural parameters of cyclic diacetylenes have been collected in Tables 8-2-8-4. The most strained diynes are those with an eight-membered ring, 1,5-cyclooctadiyne (56) [25] and 5,6,ll,12-tetradehydrodibenzo[fl,e]cyclooctene (98) [42]. The sp centers in both hydrocarbons are strongly bent like those listed in Table 8-1. The trans-annular distances between the triple bonds amount to 2.597 A (56) and 2.617 A (98), respectively (Table 8-2). A further consequence of the imposed strain is an elongation of the other bonds the bonds between the sp atoms in 56 are stretched to 1.57 A, those between the sp centers in 98 to 1.43 A. In the tetrasila compound 99 most of the strain is relieved in the long Si-Si bonds [43]. Consequently the bond angles at the sp centers deviate only 11-16° from linearity. 

Ikble 8-4 Comparison of selected bond lengths in cyclic diacetylenes with 11- and 12-membered rings. 

Only two general methods have been developed for the synthesis of the macrocyclic annulenes.9 The first of these, developed by Sondheimer and co-workers, involves the oxidative coupling of a suitable terminal diacetylene to a macrocyclic polyacetylene of required ring size, using typically cupric acetate in pyridine. The cyclic compound is then transformed to a dehydroannulene, usually by prototropic rearrangement effected by potassium i-butoxide. Finally, partial catalytic hydrogenation of the triple bonds to double bonds leads to the annulene. 

The jp-carbons of 4a are deshielded by 14-5 p.p.m. from the chemical shift (8T3 p.p.m.) of those of cyclotridecyne. This remarkable effect has been ascribed to partial olefinic character of the triple bonds caused by large molecular deformation . In fact the c/.y-oIefinic configuration has recently been confirmed by X-ray structure analysis of 25 As to the chemical shifts of acetylenic jp-carbons in cyclic tetraacetylenes, both of the inner and outer sp-carbons of 8b are deshielded by ca. 3 p.p.m. relative to the corresponding carbons of acyclic diacetylenes (Figure 5). 

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