Hexamethyl radialene

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

Hexamethyl[3]radialene (25) does not undergo Diels-Alder-reactions with the typical electron-poor dienophiles, probably because of the full substitution at the diene termini. With TCNE, however, a violet-blue charge-transfer complex is formed which disappears within 30 min at room temperature to form a 1 1 adduct (82% yield) to which structure 55 was assigned9. Similar observations were made with tris(2-adamantylidene)cyclopropane (34), but in this case cycloaddition product 56 (81% yield) was identified its allenic moiety is clearly indicated by IR and 13C NMR data12. 

The parent [6]radialene has no stabilizing aromatic feature and is predicted to have a relatively low-lying HOMO as compared to the lower members of the series, 1-399. This may explain why it is so highly reactive that it is difficult to handle. As far as the chemical behavior of the [6]radialenes is concerned, it is the hexamethyl derivative 150... 

It should be noted that 175 is the chromium complex of 154, the intermediate postulated in the thermal isomerization of hexamethyl[6]radialene (150). 

Trimethylenecyclopropane, [3]radialene (169), which is one of the benzene isomers and the smallest member of the radialene family, is characterized by a cyclic arrangement of cross-conjugated Ti-electron systems. The synthesis and properties of radialenes have been reviewed recently by Hopf and Maas The C—C and C=C bond lengths in 169,1.453 (3) and 1.343 (3) A, determined by are similar to those in its hexamethyl derivative... 

As shown in Eq. 9, if we can remove two electrons from the tris (dimethyl-amino) cyclopropenium ion 11), then we will get the hetero (3) radialene system 14) which is iso-ji-electronic with the hexamethyl (3) radialene (75) s3mthesized by Kobrich and Heinemann in 1965. In accordance with this expectation, it has been found that one-electron oxidation of the diamagnetic tris(dimethylamino)cyclopropenium ion with concentrated sulphuric acid is easily effected to give the radical cation (75). 

From the ESR spectrum, the spin density q for the radical cation (76) is obtained as 0.25, which is very close to the q value (0.28) of the hexamethyl (3) radialene radical anion. Although the effects of bonding and charge must be considered, the n spin population of the two radical ions is not expected to differ greatly in view of the similarity of the HMO energy diagrams of both radical ions.i >... 

Relatively few structural studies of radialenes have been carried out, and most of these are X-ray crystallographic studies. The first structure study of a radialene was, however, a gas electron diffraction study of 3 that appeared in 1968. The molecule was found to be planar with Dsh symmetry, in agreement with information from IR and Raman spectroscopic measurements. To the best of our knowledge only two structures of substituted [3]radialene have been reported since then. In both molecules all six hydrogens are equally substituted in one case with methyl groups (7) and in the other with trimethylsilylethynyl groups(8) see Figure 10. In hexamethyl[3]radialene (7) the... 

Since the [6]radialenes are triple-diene systems, it comes as no surprise that they have been used in multiple Diels-Alder reactions. In fact, after a first 1 1 addition with 150, leading to 161, has taken place, the reaction could proceed in two fashions—a linear course of addition leading to a ara-xylylene 162, and an angular route which produces an crf/ic-xylylene intermediate 163 (equation 19)102-103 Whereas for the hexamethyl compound 150 only products formed by the linear route have been detected with a sizeable number of dienophiles (X=X inter alia TCNE, maleic anhydride, benzoquinone, 1,4-naphthoquinone, acrolein, methyl acrylate ), the parent system 4 undergoes threefold Diels-Alder addition in a star-shaped manner leading to 164 with dimethyl acetylenedicarboxylate and to 165 with fumaroyl chloride followed by methanolysis (equation 20). ... 

In this chapter we also should mention the PE spectra of such elusive species as [3]radialene (66) and hexamethyl[3]radialene (67) In the latter compound... 

The first radialene to be reported was all-( )-7,8,9,10,ll,12-hexamethyl[6] radialene [1]. The first syntheses of the parent systems [3]radialene 2 [2], [4]radi-alene 3 [3], and [6]radialene 5 [4-6] were reported in 1965,1962 and 1977/1978, respectively, yet unsubstituted [5]radialene 4 (see below, however note added in proof) as well as monocyclic [ ]radialenes with n>6 are still unknown. On the other hand, significant research activities have been devoted recently to the so-called expanded radialenes, which have general structural formulae such as 6 and 7 ( > 1), with ethyne, butadiyne, or benzene spacers being inserted between the corners of the parent [ ]radialenes they are covered in Chapter 9 of this book. 

Various alkyl- and/or aryl-substituted [3]radialenes couldbe prepared from 1,1-dihaloalkenes via organometallic intermediates. These transformations, hov rever, often give the radialene in low yield due to competing reaction pathways. The first radialene, hexamethyl[3]radialene (35), was obtained from dibromoalkene 32 and -butyl lithium in very low yield, the lithium carbenoid 33 and the [3]cumulene 34 being the likely reaction intermediates (Scheme 4.8) [32]. Preferential cyclo-propanation of the outer double bonds of the [3]cumulene by the carbenoid (or carbene) is a major factor for the complexity of the product mixture. 

In contrast to the parent [6]radialene, alkyl-substituted derivatives 113 and 115 are quite stable compounds that can even be handled in the presence of oxygen. All-( )-7,8,9,10,ll,12-hexamethyl[6]radialene (113) was the first radialene ever reported. It was obtained by dehalogenation of hexakis(l-chloroethyl)- [la] or hexakis(l-bromoethyl)benzene [lb] (112) with magnesium in methanol (Scheme 4.23). A recent GC/MS study of the product mixture has revealed the concomitant formation of several diastereoisomers of 113, which, however, could not be... 

The only other DA studies of [6] radialenes were conducted with hexamethyl derivative 200 (Scheme 12.44), and its hexaethyl analog [74]. In contrast to the examples described for the parent [6]radialene, only linear cycloadducts were observed. For example, when Saur, Sustmann, and coworkers reacted hexamethyl [6]radialene (200) with an excess of TCNE, /j-xylylene 202 was isolated in 92%... 

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