Benzene annelation

The reaction of the chloro ester 1-Me with 1,2-dinucleophilic 1,2-disubstituted benzene derivatives 161 in most cases gave the benzene-annelated six-member-ed heterocycles 162 in moderate yield (Scheme 50) [22b, 26,73]. The course of the domino-Michael addition-cyclization is also very sensitive to the nature of the base and the reaction conditions applied. Thus, only the primary Michael adducts... 

Sakai, S. Theoretical study on the aromaticity of benzenes annelated to small rings, 7. Phys. Chem. A 2002,106, 11526-11532. 

Structural aspects of heterocyclic systems that contain a metal heteroatom have been little explored. Such a wealth of data is available for carbocycles that these will be used as initial structural models for the organometallic systems. The questions related to the effect of replacing a —CHg— group in a carbocycle by a metal atom are many-sided. In the next few paragraphs the structures of carbocycles and their benzene-annelated derivatives will be briefly reviewed, followed by a discussion of the problems introduced, or changes that may be expected, upon heteroatom replacement. Then the data observed for organometallic systems will be discussed in terms of the points raised in these sections. 

Benzene annelation significantly increases the barrier to ring inversion processes. Saturated seven-membered rings can invert by pseudorotation (pseudorotation barrier is 2 kcal/mole) which has not been measured by NMR techniques. The AG for the ring inversion process for the benzene-annellated derivatives of cycloheptane (which probably exists in a chair form) was determined to be 10.9 kcal/mole at Tj, = — 57°C (186, 298, 322). Equilibria in tricyclic organic derivatives have been studied more extensively. Several different inversion processes have been reported and depend on structural type. The following are examples of ring inversion processes. 

The same tricyclic skeleton with an annelated benzene ring was prepared as a precursor to scopadulcic acid B (Scheme 25), starting from an ortfto-iodobenzoyl-substituted methylenecycloheptene derivative, and the o-iodobenzyl derivative with a shorter tether, led to the benzene-annelated lower homologue of the tricycle (Scheme 25). 

Annelation of a benzene ring on to the [Z>] faee of the heteroeyelie ring does not have any pronouneed effeet upon the ehemieal shifts of the heteroeyelie protons (cf. Table 8). The rather unexpeeted heteroatom sequenee for shifts to progressively lower field for both H-2 and H-3 remains NHsolvent dependent and as in pyrrole it is also eoupled to the ring protons with Ji,2 = 2.4Hz and Ji,3 = 2.1 Hz. The assignment of the benzenoid protons H-5 and H-6 has eaused some eonfusion in the literature as they have almost... 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

In 80, where 77 is fused to two benzene rings in such a way that no canonical form can be written in which both benzene rings have six electrons, the aromaticity is reduced by annellation, as shown by the fact that the molecule rapidly converts to the more stable 81, in which both benzene rings can be fully aromatic (this is similar to the cycloheptatriene-norcaradiene conversions discussed on p. 1448). 

Poly(l,4-naphthylenevinylene) (106) is accessible via the Wessling polymerization procedure. Lenz, Karasz, Wegner et al. have published the synthesis of PNV 106, starting from l,4-bis(chloromethyl)naphthalene [127, 128]. The poly(l,4-naphthylenevinylene) (106) displays an optical absorption energy of 2.05 eV, slightly red-shifted by about 0,3 eV relative to the parent PPV 60-system, due to the electronic effect of the annelated benzene ring. 

In connection with the chemistry of the reactive transient species, nitrene, the chemistry of azepines is well documented u. Also, the chemistry of oxepins has been widely developed due to the recent interest in the valence isomerization between benzene oxide and oxepin and in the metabolism of aromatic hydrocarbons 2). In sharp contrast to these two heteropins, the chemistry of thiepins still remains an unexplored field because of the pronounced thermal instability of the thiepin ring due to ready sulfur extrusion. Although several thiepin derivatives annelated with aromatic ring(s) have been synthesized, the parent thiepin has never been characterized even as a transient species3). 

Thiepins stabilized by two annelated benzene rings could generally be prepared by the usual elimination processes, and some examples are shown in the following scheme 7,8 9,10). 

It is well known that unstable molecules can be stabilized by varying the aromatic ring annelation. This is also the case for thiepins. The rise in thermal stability of the thiepin ring with increasing number of annelated benzene rings is seen from the following comparison. While the unsubstituted parent thiepin 1 is considered to be an extremely reactive species and could not be detected so far, benzo[6]thiepin 4) has been... 

Table 3. Effect of annelation site of benzene and naphthalene on the thermal stability of thiepins...

The photolysis of benzo-annellated quinone diazides such as 2m should preferentially lead to products with intact benzene rings. Monochromatic irradiation... 

The significant behaviour of the compound 12 on electron impact ionization, i.e. the formation of a dominating ionic species, m/z 92, 14, has been rationalized as shown in scheme 1885. It should be noted that the intermediacy of the ionized iminocarbene was proposed along with the possibility that an oxygen transfer takes place directly from the nitro group to the annelated benzene ring. Similarly, ionized 2- and 3-azafulven-6-ones were demonstrated to be formed from the appropriate heterocycles85. 

Notably, stabilization of the cation-radical center prevents a proton from being expelled. The cation-radical of tris(bicyclopentyl)annelated benzene is not prone to proton loss, entirely, owing to the spin-charge location more or less in the aromatic (nodal) phase (Rathore et al. 1998b Scheme 1.21). 

Geometries of Annelated Benzenes Theory Versus Experiment.240... 

The Mills-Nixon hypothesis that small ring annelation on benzene would induce bond fixation (bond alternation) by trapping out one Kekul6 tautomer is a casualty of early twentieth century structural chemistry. Due to a lack of direct methods for analyzing molecular structure, structural postulates of that time were often supported by an analysis of product distributions. An experimental observable such as product selectivity or isomer count was correlated to an unobservable structural feature derived on the basis of a chemical model. Classical successes of this method are van t Hoff s proof of the tetrahedral carbon atom and Fischer s proof for the configuration of sugars. In the case of Mills and Nixon, however, the paradigm broke down. 

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