The Norcaradiene Problem

Compounds having the norcaradiene structure are stable if one of the double bonds of the norcaradiene also belongs to a benzene ring or if C atoms 1 and 6 are fixed in position by a bridge. A similar effect that favors the norcaradiene form is produced by the incorporation of nitrogen atoms in positions 3 and 4. Cycloheptatriene and most of the simply substituted tropilidenes exist exclusively in the monocyclic triene form. It has been possible in some cases to detect a valence-tautomeric equilibrium between the norcaradiene and the cycloheptatriene ring system. Non-classical stabilization of compounds having norcaradiene or cycloheptatriene structure in the sence of a pseudoaromatic “homobenzene” is only very slight, if it occurs at all.     

Effects of t-butyl substituents on valence tautomerism of 7-t-butyl-7-cyano-1,3,5-cycloheptatrienes to the norcaradiene form

Introduction of t-butyl groups to the 2-, 3-, 2,4-, or 2,5-positions of 7-t-butyl-7-cyano-1,3,5-cycloheptatriene dramatically shifts the cycloheptatriene - norcaradiene equilibrium to the norcaradiene form. The nonbonded interaction is an important factor.     

Antarafacial role of an aromatic nucleus; a novel regiospecific [π4a + π2a] intramolecular cycloaddition of the ene-ketene with an aromatic ring

The thermally generated ene-ketene undergoes a facile and regiospecific intramolecular cycloaddition with N,N-dimethylaniline to result in the novel formation of the azulenone derivative via a norcaradiene intermediate.     

Rearrangement and hydrogen scrambling pathways of the toluene radical cation: a computational study

A computational study is undertaken to provide a unified picture for various rearrangement reactions and hydrogen scrambling pathways of the toluene radical cation (1). The geometries are optimized with the BHandHLYP density functional, and the energies are computed with the ab initio CCSD(T) method, in conjunction with the 6-311+G(d,p) basis set. In particular, four channels have been located, which may account for hydrogen scrambling, as they are found to have overall barriers lower than the observed threshold for hydrogen dissociation. These are a stepwise norcaradiene walk involved in the Hoffman mechanism, a rearrangement of 1 to the methylenecyclohexadiene radical cation (5) by successive [1,2]-H shifts via isotoluene radical cations, a series of [1,2]-H shifts in the cycloheptatriene radical cation (4), and a concerted norcaradiene walk. In addition, we have also investigated other pathways such as the suggested Dewar-Landman mechanism, which proceeds through 5, via two consecutive [1,2]-H shifts. This pathway is, however, found to be inactive as it involves too high reaction barriers. Moreover, a novel rearrangement pathway that connects 5 to the norcaradiene radical cation (3) has also been located in this work.     

8-Oxoheptafulvene III. One Step Synthesis of Heptafulvalene and Benzoheptafulvalenes

In the previous paper,¹ we have reported that 8-oxoheptafulvene (1) reacts with monocyclic tropones to from adducts (2) containing a novel norcaradiene and cycloheptatriene moieties according to apparent 8 + 2 cycloaddition process.     

Synthesis and properties of 5-(2,3-diphenylcyclopropenylidene)-1,6-methano-2(5h)-[10]- annulenone (or 10,11-diphenyl-4,9-methano[3.10]quinaren-3-one). Contribution of a homobenzene structure annelated on [3.6]quinaren-3-one

The title quinarenone 2 has been prepared and proved that the three-membered ring possesses a larger diatropicity than diphenylcyclopropenone and the seven-membered ring exists in a cycloheptatriene (not norcaradiene) tautomer having a contribution of a homobenzene structure. The rotational barrier about the intercyclic bond of 2 is 13.3 Kcal/mol.     

Singlet oxygenation of 7-aryl and 7-alkyl-1,3,5-cycloheptatrienes: substituent effects on the product distribution of tropilidene- and nocaradiene-derived endoperoxides

The N/T ratio, i.e. the proportion of norcaradiene ($\text{N}$) versys tropilidene ($\text{T}$) endoperoxides in the cycloaddition of singlet oxygen with 7-substituted 1,3,5-cycloheptrines, decreases in the order p-C1C₆H₄ > C₆H₅ > p-MeOC₆H₄ and t-Bu > iPr > Et > Me, prresumably reflecting the ability of these substituents in promoting the tropilidene to norcaradiene valence isomerization.     

Position and dienophile dependent Diels-Alder reactions of vinylcycloheptatrienes

While 1-vinyl- and 2-vinylcycloheptatrienes undergo Diels-Alder reactions exclusively from cycloheptatriene forms at the diene part including the vinyl group, 3-vinylcycloheptatriene reacts site-selectively from either cycloheptatriene or norcaradiene form depending on electron affinity of dienophiles.     

Generation and notable position dependent electrocyclization of cycloheptatrienylphenylcarbonium ions -electrocyclization of a norcaradiene

While (1- and 2-cycloheptatrienyl)phenylcarbonium ions undergo electrocyclizations through the cycloheptatriene forms giving dihydrobenz[a]azulenes, (3-cycloheptatrieny])-diphenylcarbonium ion does through the norcaradiene form. (7-Cycloheptatrienyl)diphenyl- carbnium ion undergoes a rearrangement. A possible rationalization is discussed.     

An unusual norcaradiene/tropylium rearrangement from a persistent amino-phosphonio-carbene

An amino-phosphonio-carbene featuring a bromobiphenyl backbone was prepared and spectroscopically characterized at low temperature. This carbene was found to readily rearrange upon warm up, affording an original tricyclic phospholium derivative, presumably via a norcaradiene/tropylium isomerization.     

Preparation of some 2,4,6-cycloheptatriene-1-yl ketones. Strong preference for the norcaradiene isomer in the case of the 2,4,6-cycloheptatriene-1-yl t-butyl ketone

t-Butyl, i-propyl and phenyl 2,4,6-cycloheptatriene-1-yl ketone ({=1a},{=c},{=d}, respectively) have been synthesized. At ?139°C, rather unexpectedly, the t-butyl ketone {=1a} exists in a 1:1 equilibrium with its norcaradiene isomer {=2a}.     

Homobenzene: homoaromaticity and homoantiaromaticity in cycloheptatrienes

Cycloheptatriene (C(s)) is firmly established to be a neutral homoaromatic molecule based on detailed analyses of geometric, energetic, and magnetic criteria. Substituents at the 7 (methylene) position, ranging from the electropositive BH2 to the electronegative F, favor the equatorial conformation but influence the aromaticity only to a small extent. By the same criteria, the planar transition state (C(2v)) for cycloheptatriene ring inversion is clearly antiaromatic. This is attributed to the involvement of the pseudo-2pi-electrons of the CH2 group with the 6pi-electrons of the ring to give an 8pi-electron system. Similarly, the participation of the CH2 groups into C(2v) cyclopentadiene and cyclononatetraene lead to significant 4n + 2 pi electron aromaticity. The cyclization of cycloheptatriene to norcaradiene proceeds via a highly aromatic transition structure, but norcaradiene itself is less aromatic than cycloheptatriene. An annelated cyclopropane ring does not function as effectively as a double bond in promoting cyclic electron delocalization.     

Total synthesis of the tropoloisoquinoline alkaloid pareitropone via alkynyliodonium salt chemistry and related studies

The first chemical synthesis of pareitropone, by a route featuring application of alkynyliodonium salt chemistry, is described. The key transform initiates with addition of an alkylidenecarbene, derived by intramolecular nucleophile addition to the alkynyliodonium moiety, to a proximal aromatic ring. This addition delivers a highly strained norcaradiene substructure that rapidly reorganizes to furnish the pareitropone skeleton.     

High pressure [4+2] and [2+2+2] reactions between cycloheptatriene and methyl propiolate

Under high pressure conditions, cycloheptatriene reacts with methyl propiolate to afford mono-, bis- and trisadducts all retaining, the norcaradiene structure. The four new compounds are formed via [4+21] and [2+2+2] cycloadditions. The latter are examples for the high pressure extension of the scope of homo-Diels-Alder reactions.     

Acid-catalyzed rearrangement of ethynylcycloheptatriene to phenylallene

7-Ethynylcycloheptatriene (1) cleanly isomerizes to phenylallene in the presence of acid. A mechanism involving the protonation of ethynylnorcaradiene, which is in equilibrium with 1, followed by the cleavage of a three-membered ring to give an arenium ion, is proposed. The rearrangement is accelerated by a factor of 370 by introducing tert-butyl groups on C-2 and C-5, indicating the importance of the equilibrium concentration of the norcaradiene form as a rate-controlling factor.     

Manganese(III) Acetate Catalyzed Oxidative Radical Additions of α‐Dicarbonyl Compounds to 1‐ and 2‐Phenylcyclohepta‐1,3,5‐triene

Manganese(III) acetate catalyzed oxidative radical‐addition reactions of α‐dicarbonyl compounds such as methyl acetoacetate ( 6 ), acetylacetone ( 7 ), and dimedone ( 8 ) to the mixture of 1‐ and 2‐phenylcyclohepta‐1,3,5‐triene ( 4 and 5 ) were investigated (Scheme 1). The 1‐phenylcyclohepta‐1,3,5‐triene ( 4 ) formed mainly [2+3] and [4+3] dihydrofuran addition products derived from cycloheptatriene and [2+3] dihydrofuran addition products derived from the norcaradiene structure. The 2‐phenylcyclohepta‐1,3,5‐triene ( 5 ) formed mainly [6+3] dihydrofuran addition products derived from cycloheptatriene and [4+3] dihydrofuran addition products derived from the norcaradiene structure. The structures of isolated products were established by their spectroscopic data (IR, ¹H‐ and ¹³C‐NMR, MS, and elemental analysis) and comparison with literature data. The formation mechanism of the products is discussed.     

Primary dependence of 7-aryl-2,5-di-t-butyl-1,3,5-cycloheptatriene valence tautomerism on the π-acceptor strength of the 7-aryl substituent

¹³C NMR studies showed that the population of the norcaradiene form of the title systems containing p-CH₃O, H, and p-CF₃ on the 7-aryl group increases in this order. The result is consistent with the prediction from the π-acceptor strength of the aryl group estimated by INDO calculations.     

Azulene–Naphthalene‐Type Rearrangements in Benz[a]azulene and Cyclohepta[b]indole

Flash vacuum pyrolysis (FVP) of benz[a]azulene yields phenanthrene and 2‐ethynylbiphenyl. FVP of cyclohepta[b]indole similarly yields phenanthridine and 2‐cyanobiphenyl. The reversibility of the reactions is demonstrated by FVP of 2‐ethynylbiphenyl and 2‐isocyanobiphenyl. All the observed reactions are in accord with the norcaradiene–vinylidene mechanism of the azulene–naphthalene rearrangement, whereas other proposed mechanisms are ruled out.     

Kinetics and mechanism of photolysis of 2,6-di-tert-butyl-1,4-benzoquinone diazide in benzene and deuterobenzene

Conclusions It has been established by kinetic and spectral methods that the cyclohexadiene carbene generated in the photolysis of 2,6-di-tert-butyl-1,4-benzoquinone diazide in benzene predominantly in the singlet state reacts with benzene to form a norcaradiene derivative, which isomerizes thermally to 4-phenylcyclohexadiene-2,5-one. The latter is converted thermally to 2,6-di-tert-butyl-4-phenylphenol.Translated from Izvestiya Akadeinii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2680–2685, December, 1986.     

Synthesis, structure and some reactions of a multi-bridged unsaturated cyclooctadecane derivative formally having two cycloheptatrienes

Double annulation of 1,6-bis(bromomethyl)-1,3,5-cycloheptatriene with diethyl acetonedicarboxylate under basic conditions provided the title cyclooctadecane derivative having formally two 1,3,5-cycloheptatriene moieties. An NMR study of the compound suggested that one moiety stays as a cycloheptatriene form and the other as a norcaradiene form. X-ray crystallographic analysis revealed that two methano bridges have syn- and anti-configurations to the central carbonyl bridge and also showed that one of the moieties having the syn-methano bridge stays a CHT form and the other having the anti-methano bridge stays an NCD form.