Annulenes General

Annulenes have been utilized in the investigation of the effect of adding (or removing) electrons to Ti-conjugated systems [lb]. Annulenes can be used in demonstrating the validity of the Hiickel theory and the experimental criteria of aromaticity [57], because the molecular frame can hold different numbers of electrons. 

Molecular orbital (MO) diagrams show that when a [4n]annulene is reduced to a [4n- -2]dianion, the LUMO level becomes fully populated, resulting in a stable ground state and an aromatic system. Conversely, the reduction of a [4n- -2]annulene affords a [4n] dianion, that possesses a low-lying LUMO, which can undergo a pseudo-Jahn-Teller distortion. These [4n]annulenes are predicted to be anti-aromatic [lb, 56-61]. 

In this section we describe the reduction of some classical annulenes , [8]-annulene 3, [12]annulene 4, [16]annulene 5 and [18]annulene 6. In all of these systems the reduction is accompanied by changes in the molecular frame. We shall concentrate on the NMR spectra of the diamagnetic anions and the EPR spectra of the [4n- -l]-radical anions, both of which serve as probes for the delocalization and planarity of these r-systems. The Hiickel theory [58], which only considers the 71-energy, neglecting the c-frame, can only describe planar, strain-free r-perimeters therefore, it is important to analyze the structure of the annulenes and their anions. 

Most annulenes are not completely planar, and except for the unique case of benzene, annulenes do not conform to the full symmetry [56, 57]. The adoption of nonplanar conformations is not only typical of the [4n]annulenes, but also 

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The annulenes generally are not stable compounds, but the [4n + 2] annulenes clearly show typical aromatic reactions. For instance [18] annulene has been converted to the nitro, ethanoyl, bromo, and carbaldehyde derivatives by electrophilic substitution reactions. 

The general term annulene has been coined to apply to completely conjugated mono cyclic hydrocarbons with more than six carbons Cyclobutadiene and benzene retain then-names but higher members of the group are named [jcjannulene where x is the number of carbons m the ring Thus cyclooctatetraene becomes [8]annulene cyclodecapentaene becomes [10] annulene and so on... 

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. The synthesis of annulenes has been extended well beyond the first two members of the series [4]annulene (cyclobutadiene) and [6]annulene (benzene). The generality of the Hiickel rule can be tested by considering the properties of members of the annulene series. 

The preparation of l,6-oxido[10]annulene, described simultaneously by Sondheimer and Shani4 0 and by Vogel, Biskup, Pretzer, and Boll, is illustrative of the rather general synthesis of aromatic 1,6-bridged [10]annulenes starting from 1,4,5,8-tetrahydronaphthalene. Apart from the present compound, the following bridged [10]annulenes have thus far been obtained by this approach l,6-methano[10J-annulene,2-8-9 the ll,ll-dihalo-l,6-methano[10]annulenes,9 10 and 1,6-imino [ 1 OJannulene.11... 

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 presently described procedure for the synthesis of [18]arinulene, although the overall yield is low by the standard normally set for Organic Syntheses, illustrates the above general route leading to the theoretically important macrocyclic annulenes, and in this way [14]-, [16]-, [18]-, [20]-, [22]-, and [24]annulenes have been prepared in pure crystalline form. 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

The interesting, heavily substituted, diatropically delocalized triaza[10]annulenone formulated as shown in (24) is a further known member of the general aza[10]annulene family <71JCS(C)2142>. 

Because of the enhanced rigidity of the higher-membered heteroannulenes, e.g. (36), imparted chiefly by the presence of properly positioned trans double bonds, these frames are generally less thermally labile than their heteronin counterparts although here too aromatic derivatives are substantially more heat resistant than their polyenic relatives. It was noted, for example (75PAC(44)69l), that while parent aza[13]annulene (37a) is thermally stable at 56 °C, its polyenic acetamide (36a) readily rearranges ( i/2< 1 h) under these conditions to what is believed to be a tricyclic isomer (96). 

Finally, thermal cycloaddition between benzocyclopropene and triazene (114) serves as a conveniently general means of constructing the bridged aza[10]annulene frame depicted in (115) (a direct relative of 23d), the preparation of the last two derivatives (115c, d) requiring the use of ultra-high pressure (1.5 kbar) (80TL7). 

Steric interactions generally predominate relative to ring current effects in carbon-13 NMR. Proton shifts of bridged [10]- and [14]annulenes, for example, clearly identify these compounds to be aromatic due to the typical deshieldings of about 2 ppm relative to comparable non-aromatic cyclopolyenes [401a]. In contrast, carbon-13 shifts of bridged annulenes (Table 4.58) are more closely related to those of comparable non-aromatic cyclopolyenes with similar steric interactions than to those of other annulenes [401 b]. 

The heavily annulated benzo- and furanothia[ll]annulenes depicted in 6182 and 6283 were synthesized by condensation of dialdehyde (47) with properly structured Wittig components. Further, treatment of 62 with maleic anhydride produced a mixture of endo and exo cycloadducts of general formula 63 whose hexatriene segment that was not directly bound to the heteroatom was found to be thermally labile, readily... 

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