Cycloheptatrienes electrocyclic reactions

Electrocyclic reactions have been performed with three of the bridged [ 11 Jan-nulenones. Both 11 and 13, which both contain a cycloheptatriene unit, undergo Diels-Alder additions with dienophiles via their norcaradiene valence tautomers 41 and 43 and yield adducts of the type 42 and 44. Annulenone 13 was found to react only with 4-phenyl-l,2,4-triazoline-3,5-dione whilst 11 underwent reaction with a variety of dienophiles. 3,8-Methano[ 11 Jannulenone 12 contains a tetraene system and undergoes addition reactions, apparently of the (8 + 2)-type, at the termini of the tetraene system. Thus with maleic anhydride the adduct 46, the valence tautomer of the initial adduct 45, was isolated. 

The literature until 1970 is reviewed comprehensively in Houben-Weyl, Vol. 4/3, pp 509-527 for the most recent reviews, see refs 96,97 and 157. The transformation of cycloheptatriene 1 into norcaradiene 2 (or vice versa) is a bn-electron symmetry-allowed disrotatory electrocyclic reaction ... 

Sol 1. Isomerization of substituted cycloheptatrienes takes place through sequences of [1,5] sigmatropic rearrangements and electrocyclic reactions. 

The reaction of carbenoids with aromatic systems was first reported by Buchner and coworkers in the 1890s.6 The reaction offers a direct entry to cycloheptatrienes and has been used to synthesize tropones, tropolones and azulenes.6 Neither the thermal nor copper-catalyzed reactions, however, proceed in good yield. The problems associated with these transformations were clearly demonstrated in a recent reexamination of die thermal decomposition of ethyl diazoacetate in excess anisole (137).129 A careful analysis of the reaction mixture revealed the presence of seven components (138-144) in 34% overall yield (Scheme 29). The cycloheptatrienes (138M142) were considered to be formed by cyclopropanation followed by electrocyclic ring opening of the resulting norcaradienes. A mixture of products arose because the cyclopropanation was not regioselective and, also, the initially formed cycloheptatrienes were labile under the reaction conditions. 

Norcaradiene formation from a cycloheptatriene corresponds to a 1,3-ring-closure. On the basis of this reaction, aminocyclopropane 317 was obtained as a solid in 94% yield from piperidine and the tropylium ion 315 (equation 74). A rapid equilibrium between 316 and 317 was postulated in solution. Electrocyclic 1,3-bond connections also were involved in the fluctual behaviour of 9-azabarbaralanes " , in the formation of homoazepines (from nitrenes and cycloheptatriene" " ) and in a 6-azabenz[10]annulene system . ... 

The reaction appears to be a 1,5-shift of a cycloheptatriene (the retro-electrocyclization product of the norcaradiene) ring carbon over the cyclopentadiene system which is followed by a 1,5-hydrogen shift. 

Two possible pathways were envisioned for the reaction (a) cyclopropane to propylene-like rearrangement followed by 1,5-hydrogen shifts, that can equilibrate C4 and C6 as well as C3 and Cl with a slower 1,5-deuterium shift (due to the primary isotope effect) and (b) the second pathway would involve a retro-electrocyclization to a cycloheptatriene destroying the aromatic it system in the process, and this undergoes a 1,5-deuterium shift to the 1,2-benzocycloheptatriene which subsequently undergoes a 1,5-hydrogen shift to equilibrate C4 and C6 but also must equilibrate C3 with Cl. Further, a 1,5-deuterium shift in the 7-deuterio material gives the isomer from path (a) (Scheme 12.8). 

The cation (26) was obtained by protonation of the corresponding azulene in dichloromethane. A tropylium ion-mediated a-cyanation of amines was described. The key step is a hydride transfer from the amine to the cation, resulting in cycloheptatriene and an iminium ion, the latter then reacting with cyanide to give the aminonitrile. The dehydrofropylium-Co2(CO)6 ion has been prepared as a BF4 salt. Various measures suggest that the ion is weakly aromatic, with about 25% of the aromaticity of the tropylium ion. Computational analysis of a number of annulenes predicts that the Mobius dication (CH)i4+ should be stable under persistent ion conditions. In particular, this dication is stable towards reactions such as cis-trans isomerization and electrocyclic rearrangement that limit the lifetime of other Mobius annulenes. 

The Buchner reaction describes cyclopropanation of an aromatic double bond with the a-ketocarbene derived from an a-diazocarbonyl compound 1 to produce an unstable norcaradiene intermediate 2, which is in thermal equilibrium with the more stable cycloheptatriene tautomer 3. This tautomer undergoes thermally or photochemically induced electrocyclic ring opening to give other cycloheptatriene isomers A-6)... 

Arenes suffer dearomatization via cyclopropanation upon reaction with a-diazocarbonyl compounds (Btlchner reaction) [76]. Initially formed norcaradiene products are usually present in equilibrium with cycloheptatrienes formed via electrocyclic cyclopropane ring opening. The reaction is dramatically promoted by transition metal catalysts (usually Cu(I) or Rh(II) complexes) that give metal-stabilized carbenoids upon reaction with diazo compounds. Inter- and intramolecular manifolds are known, and asymmetric variants employing substrate control and chiral transition metal catalysts have been developed [77]. Effective chiral catalysts for intramolecular Buchner reactions include Rh Cmandelate), rhodium carboxamidates, and Cu(I)-bis(oxazolines). While enantioselectivities as high as 95% have been reported, more modest levels of asymmetric induction are typically observed. 

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