Annulenes Heteroannulenes

Heteroannulenes electrophilic reactions, 7, 726 tr-excessive synthesis, 7, 727 nucleophilic reactions, 7, 727 Hetero[l IJannulenes structure, 7, 715 Hetero[ 12]annulenes pyridine-like methano-bridged, 7, 715 Hetero[ 13]annulenes unrestricted structure, 7, 716... 

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). 

Thermal bond transposition has been utilized successfully in the synthesis of a number of relatively heat-insensitive families of heteroannulenes, such as the triheteronin frame (20) constructed thermally from the tetracyclic valence tautomer and the tetrabenzo heteronins (18) obtained by thermal bond transposition of their spirostructured valence tautomers (113). Similarly, thermally induced bridge extrusion (C02) has been utilized as a means of preparing dihydrobenzazonine (112) (the direct photoprogenitor of aza[13]annulene 36a) from cycloadducts (104) (73TL3805). 

Edgar Heilbronner in 1964 worked out that such a twisted system (Mobius system) would be aromatic if it contained 4n conjugated tr-electrons. Curiously, no actual examples were identified until relatively recently, when it was proposed that [12], [16] and [20] annulenes have several higher energy conformations which adopt this mode. In 2003, the first crystal structure of a true stable Mobius [16] annulene was completed, and various heteroannulenes were identified as existing in Mobius form. 

The bridged hetero[ll]annulenes (120) have been obtained as shown. Evidence from their n.m.r. spectra suggests that they exist in the jy -form (121). The bright red thienocyclobutadiene (122) was obtained as shown. Among other heteroannulenes synthesized using Wittig olefin syntheses are a thia[ll]annulene, an oxa[13]annulene, the oxa[15]annulene... 

This includes heteroannulenes, which comply with the HUCKEL rule, i.e. which possess An + 2) -electrons delocalized over the ring. The most important group of these compounds derives from [6]annulene (benzene). They are known as heteroarenes, e.g. furan, thiophene, pyrrole, pyridine, and the pyrylium and thiinium ions. As regards stability and reactivity, they can be compared to the corresponding benzenoid compounds [1]. 

The principle developed by Vogel of methano- or oxy-bridging of medium-sized annulenes [8] to strengthen the perimeter has proved advantageous in the synthesis of larger-membered heteroannulenes of various structures. 

The methano-bridged aza[10]annulene 20 [9], a 10 r-analogue of pyridine, is a stable, yellow compound with a quinoline-like odour and a flattened perimeter (UV Amax 364 nm). The NMR data confirm the aromatic character of 20. The perimeter protons combine the features of a-substituted 1,6-methano[10]annulenes and quinoline (see p 318) the upheld shift of the bridge H-atoms (< ch2 = -0.40 / +0.65) is characteristic. It is caused by the shielding effect of the diatropic heteroarene system. The heteroannulene 20 is less basis (pX = 3.20) than pyridine (pX = 5.23) or quinoline pKa = 4.94). 

Unbridged analogues of pyridine containing 14 r- and 18 r-electrons are known (Schroder 1979). The aza[14]annulene 23 [12] is a thermally stable, violet compound (UV Jlmax = 620 nm) and shows signals in the NMR spectrum which are due to conformationally mobile intra- and extraannular protons with a shift difference of 9.5 ppm. This indicates the presence of a flat heteroannulene perimeter with a delocalized, strongly diatropic r-system. 

Many annulenes have been obtained by photochemical electrocyclic processes. Thus [lOJannulene was obtained from c/5-9,10-dihydronaphthaIene, and various heteroannulenes have been prepared by very similar procedures. ... 

The aromaticity of annulenes and heteroannulenes can be described with the Hiickel rule. Due to their closed-shell structures, annulenes with 4N + 2 electrons are not distorted Dnh symmetry) and show strong diamagnetic ring currents while singlet 47V annulenes are often distorted and have paratropic character. However, the Hiickel rule cannot be applied in polycyclic systems, where, for example. 

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