Theoretical benzannulation

The three different benzoxepins are simply assigned by the position of the oxygen 1 -benzoxepin, 2-benzoxepin, 3-benzoxepin. Among the four possible dibenzoxepins only dibenz[6,d]oxepin and dibenz[6,/]oxepin are of importance whereas the two other isomers are only of theoretical interest because they contain unfavorable o-quinoid structures. Benzannulation across all of the C-C double bonds leads to tribenz[6,rf,/]oxepin. 

Nearly 25 years after its discovery the mechanism of the benzannulation reaction has been theoretically and experimentally elucidated in detail. The most predominant outcome of this reaction is the formation of the 4-methoxyphe-nol or 4-methoxy-l-naphthol skeleton coordinated to a Cr(CO)3 fragment. Therefore the mechanism leading to this type of product will be discussed first. 

Despite the undeniable synthetic value of the benzannulation reaction of aryl and alkenyl Fischer carbene complexes, the details of its mechanism at the molecular level remain to be ascertained. Indeed, although a relatively large number of theoretical studies have been directed to the study of the molecular and electronic structure of Fischer carbene complexes [22], few studies have been devoted to the analysis of the reaction mechanisms of processes involving this kind of complexes [23-30]. The aim of this work is to present a summary of our theoretical research on the reaction mechanism of the Dotz reaction between ethyne and vinyl-substituted hydroxycarbene species to yield p-hydroxyphenol. 

The mechanism of the benzannulation reaction involves the stepwise construction of the aromatic ring within the coordination sphere of the chromium template (Scheme 3). This idea is strongly supported by experimental evidence for model intermediates and by theoretical calculations [9] highlighting the course of the reaction (see below). 

The effect of benzannulation on the position of the tautomeric equilibrium has been evaluated using the PMO semiempirical calculations (90ZOR1387) and revealed the following trends (a) the transition from 2-hydroxypyridine to 2-hydro-xyquinoline and 1-hydroxyisoquinoline leads to increased stability of the oxo tautomer (b) transition from 2-hydroxypyridine to 3-hydroxyisoquinoline shifts the equilibrium toward the hydroxy tautomer and (c) the relative stability of the oxo tautomer of 1-hydroxyisoquinoline is higher compared to that of 2-hydroxyquino-line. These theoretical results are in good agreement with the experimental data. 

Sastry, G. N. (2001). Theoretical studies on the effect of sequential benzannulation to corannulene. Journal of Molecular Structure (Theochem), 543,1-10. 

Asao, Sato, and co-workers reported that AuCl-catalyzed benzannulation of o-alkynylphenyl ketones 11 (R H) with benzenediazonium 2-carboxylate 51 gave a variety of anthracene derivatives 52. On the other hand, the reaction of o-alkynylbenzaldehyde 11 (R = H) afforded triptycyl ketones 53 (Scheme 15.21) [33]. The reaction proceeded most probably through [4 - - 2] cycloaddition between ben-zopyrylium intermediate and benzene. Zhu et al. investigated the mechanism of this reaction by theoretical calculation and reported that AuCl catalyst is more effective than AuCls, due to the lower activation free energy of the rate-determining step [34],... 

The thermal [3-i-2-i-l]benzannulation reaction of a,/3-unsaturated alkenyl- or aryl-carbene complexes with alkynes represents the unique and most synthetically valuable reaction of chromium carbenes [20]. A variety of densely substituted benze-noid compounds is accessible by this Cr(CO)3-templated one-pot cyclization reaction. Experimental and theoretical studies support a stepwise C-C bond formation according to a mechanism (Scheme 11.6) that starts with a reversible decarbonyla-... 

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