Heteroatom-bridged Annulenes

Badger, et al., many of these systems have, in fact, been prepared, section, a review of the various synthetic strategies used to prepare these macrocycles is provided. 

Like much of the work reviewed in this monograph, one of the major stimuli driving the preparation of the bridged [18]annulenes derived from a desire to study 

Calculations carried out on all of the molecules of this set (2.335-2.344) allowed predictions to be made as to which should be aromatic. The prediction for each macrocycle is listed in Table 2.3.1, along with the experimentally determined result (where available). This model, which was based on an index of aromatic stabilization per 7t-electron, did not take into account unfavorable effects owing to the larger size of NH and S relative to O. In spite of this obvious oversimplification, in those cases where experimental data are available, these calculations have proved to be remarkably accurate. 

Prior to this theoretical study, simple CPK modeling studies carried out by Badger, et al. on several of these macrocycles indicated that systems derived from two or more thiophene units would likely be non-planar, and thus most likely non-aromatic. If, however, one or fewer thiophene units were present, it was predicted that a planar, aromatic macrocycle would result. It was Badger s intent, therefore, to see whether one could in fact dial-in planarity and hence aromaticity as a result of effecting known changes in the within-ring steric factors. 

In terms of actual chemistry, the first example of this class of macrocycles to be prepared was, as mentioned above, the [ISjannulene-trisulfide derivative 2.335. This macrocycle was obtained by reacting thiophene-2,5-diacetic acid 2.346 with 

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A handful of other heteroatom-bridged annulene-type contracted porphyrins have appeared in the literature. The first of these were reported in 1973 by Cresp and Sargent and consisted of the [17]annulenone derivatives 2.400a-c. These were prepared using a procedure similar to that used by Badger, et to obtain... 

In 1964, Badger and coworkers described what was the first representative of a new class of heteroatom-bridged [18]annulenes. This they did with their report on the synthesis of the so-called [18]annulene trisulfide 2.335 (Figure 2.3.4). ° Macrocycle 2.335 represents just one of ten possible macrocycles (i.e., 2.335-2.344) of this type that could in theory be derived by formally substituting one, two or all three of the thiophene subunits with pyrrole and/or furan. Indeed, since the initial report of... 

Like normal porphyrin, the a-P -linked porphyrin isomers 3.160 and 3.161 may be considered as being heteroatom-bridged [18]annulenes. They display, for instance, properties consistent with a 4n + 2 aromatic formulation. In particular, H NMR spectra of both 3.160 and 3.161 indicate the presence of a sustained diamagnetic ring current. " This is evident from the upheld shift of the internal NH and CH protons as well as from the corresponding downfield shift of the... 

Full accounts have been given of the syntheses of heteroatom-bridged [17]annulenes and of thia-[17]- and -[21]-annulenes, and of the reactions between the dialdehyde (83) and the bis-ylide (84), in which the thiopyran (85) was formed as well as the expected thionins. ... 

Diatropic systems result when nitrogen heteroatoms are introduced in bridged [10]- (224, 225), [13]-(226-230), [14]- (230, 231), and [17]annulenes 232 and 233, which have been extensively reviewed.269... 

The porphyrin ring contains 22 electrons in its n orbitals. As explained above, the Hiickel rule cannot be applied to this electron count, because the molecule is not monocyclic. However, the porphyrin ring can be formally derived from neutral [18]annulene, by introduction of appropriate heteroatoms and bridges (Fig. 3). The macrocycle is thus shown to be aromatic in the Hiickel sense, and is denoted [18]porphyrin. As we will show in subsequent sections, this approach is readily generalized to other porphyrinoids, whose aromatic character can be predicted by defining a neutral annulenoid pathway in the macrocycle. These pathways will be... 

Alternatively, the porphyrin ring can be constructed starting from [16]annulene. In the first step, two electrons are added to form the corresponding [16]annulene dianion, which is transformed into porphyrin Cl") by adding bridges and heteroatoms. Unlike the structure derived from [18]annulene, the dianion-based model has a fourfold symmetry, and was considered suitable for the description of metal complexes [23] (see Sect. 2.3.2). In yet another approach [24], based on the so-called perimeter model, the porphyrin macrocycle is derived from the [20]annulene dication (I "). Both the [16]- and [20]annulene models were employed to describe electronic absorption spectra and magnetic circular dichroism of porphyrinoids [24, 25],... 

The simplest polycycles stem from two triply-bridged points. These systems are exemplified by Trost et al. s 4,8-dihydrodibenzo[oi,g7z]pentalene (69) [87, 88], a precursor for a purturbed [12]annulene dianion, and Mislow et al. s double-bridged biphenyl derivatives (generally shown as 70) [89], where X is methylene, carbonyl, or various heteroatoms. Other longer bridged biphenyls include the triple-bridged cyclophanes 71 made by Hubert and Dale [90], their unsym-metrical relatives by Cram and Reeves [91], and the recent polyalkynyl cyclo-phane (72) made by Rubin et al. [92], a proposed fullerene precursor. 

A final sub-group within the bridged 10 7r-electron systems includes those compounds bridged by heteroatoms. The synthesis and n.m.r. investigation of l,6-oxido-[10]annulene 137-139,142) and many of its derivatives 17,125,138,140) have accordingly been executed. The n.m.r. spectrum 17 of l,6-oxido-[10]annulene itself (30, see Table 6) consists of an AA BB system centered at t 2.65 (in CCI4 also quoted at t 2.52... 

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