Radialenes trienes

To demonstrate the quality of this simple approach we show in Figures 6 and 7 the PE spectra of 1,3-butadiene 2, (3 )-hexa-l,3,5-triene 161, (3E, 5 )-ocla-1,3,5,7-tetraene 234, [3]radialene 173, 3,4-dimethylidenecyclobutene 174 and fulvene 175. The observed positions I j1 of the tt-bands are collected in the third column of Table 2, and the eigenvalues xj obtained from standard HMO models in the fourth. A least-squares calculation yields the linear regression... 

It appears that neither the lithium carbenoid pathway nor the cyclopropanation of buta-trienes are general routes to [3]radialenes. More successful is the cyclotrimerization of 1,1-dihaloalkenes via copper or nickel carbenoids, provided the substituents at the other end of the C=C double bond are not too small. Thus, tris(fluoren-9-ylidene)cyclopropane 27 was formed besides butatriene 28 from the (l-bromo-l-alkenyl)cuprate 26 generated in situ from (9-dibromomethylene)fluorene (Scheme 3)10. The cuprate complexes formed... 

Cyclononatriene, the smallest cyclic [3]cumulene isolated so far, polymerizes when its solutions are concentrated57. On the other hand, several radialenes have been isolated which represent cyclodimers of seven- and eight-membered 1,2,3-trienes (Scheme 9). 

Cyclodimerization of cumulenes can take place either by thermal activation (see Section 1.3.1.1.2.) or by metal catalysis. Nickel catalysts promote the cyclodimerization of buta-1,2,3-trienes (e.g., 8) to 4-radialenes (e.g., 9).21,22... 

Cyclodimerization of unsymmetrically substituted butatricncs such as 12 give both head-to-head and hcad-to-tail cycloaddition products. The structure of the head-to-head dimer was confirmed by its independent synthesis from the mixed cycloaddition of cumulenes 8 and 10,21 22 These dimerizations proceed by discrete nickel cyclopentanes which was established by the isolation of the 2-bispyridinenickel complex of the l.l,4,4-tetramelhylbuta-l,2,3-triene dimer.23 4-Radialenes with extended conjugation, potential organic conductors and semiconductors, have been prepared by similar methods as illustrated by the examples below.24,25... 

The dimerization of 19 also occurs in the presence of rhodium salts with stable crystalline rhodium complexes of 19 that can be isolated.26 it is interesting to note that the thermal dimerization of 19 gives only a 25% yield of the cyclodimer 20.27 The metal-catalyzed dimerization of cyclohepta-l,2,3-triene in the presence of tetrakis(triphenylphosphane)nickel(0) gives tricyclic 4-radialene 21.28 The structure of this cydobutane was confirmed by X-ray diffraction. 

A polycyclic system with a l,3-dioxa-2-silacyclohepta-4,6-diene fragment was obtained in low yield from bis(diazocarbonyl)silane 671. In the first stage, a biscarbene or a biscarbenoid, generated in the presence of copper triflate or palladium acetate, forms 1,2-diacylcyclopropene, then rearranging into cyclic cumulene triene 672 by migration of a silicon atom to an oxygen atom. The latter dimerizes to [4]-radialen 673 or reacts with furan to afford 674 (Scheme 32) (89AG175()). 

The electronic structure of Cgo is best described as a fusion of [5]radialene and cyclohexa-l,3,5-triene substructures (Taylor, 1992 Hirsch, 1994). It is known that the double-bond character in the five-memberend rings in is low. Methano bridging can take place, however, as we have already seen (8.28-8.30), at the [6,5] or the [6,6] ring junctions and, in addition, valence isomerization is possible in both cases. This results in four isomeric methanofullerenes and eight, if the two substituents at the methano C-atom are different. Structure and naming of the four isomers are shown in Figure 8-4. 

A first preparation of cumulene (buta-2,3,4-triene)-carboxylic acids (50) features condensations between the anion (49) and ketones or a-keto-esters, the latter electrophiles leading to cumulene diacids (Scheme 8)Thermolysis of these acids can be used to prepare [4]radialenes. 

The formation of dodecamethyl[6]radialene (72) by a Ni(0)-mediated [2+2+2] cycloaddition of 2,5-dimethylbuta-2,3,4-triene (70) has already been mentioned (see Section 4.2.2). Much better yields could be obtained, when 3,4-dibromo(or diiodo)-2,5-dimethylhexa-2,4-diene was exposed to a Ni(0) complex. Here, the [3]cumulene 70 is a likely reaction intermediate, and the product distribution ([4]radialene 71, [6]radialene 72, and its isomer 73 see Scheme 4.14) depends strongly on the solvent, with donor solvents favoring the formation of 72. With DMF as the solvent and in s/fw-generated Ni(PPh3)4 as a mediator, a yield of 63 and 50% could be achieved for 72 from the dibromo- and the diiododiene, respectively [65, 66]. 

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