Norcaradienes formation

Typical carbene reactions, such as norcaradiene formation with solvent (20) must be accepted, as must the occasional cyclopropanation of the trapping reagents. 

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 . ... 

Photolysis in acetonitrile gives ortho- (10) andpara-phenylbenzaldehyde (11), as well as cis- (12) and fmws-diphenylstilbene (13). The mechanism for product formation is not clear, but the norcaradiene valence isomer is thought to be the most likely precursor, arising either via a secondary... 

Copper-catalyzed cyclopropanation of benzene and its derivatives by a diazoacetic ester yields a norcaradiene 230 which undergoes spontaneous ring opening to cyclo-heptariene 231. At the temperatures needed for successful cyclopropanation, sigma-tropic H-shifts leading to conjugated isomers of cycloheptatriene carboxylates cannot be avoided. The situation is complicated by the formation of regioisomers upon cyclopropanation of substituted benzenes, and separation of the cycloheptatriene isomers may became tedious if not impossible. 

The presence of a locked norcaradiene is however not a prerequisite for introduction of the central double bond of a cycloproparene. Exposure of the 1,6-diha-logenated cycloheptatriene 50 to n-BuLi also results in formation of benzocyclopropene (1) via 69, despite of the unfavorable position of the cyclohep-tatriene-norcaradiene equilibrium. The method is, however, of limited interest, since the most convenient access to 1,6-dihalogenocycloheptatrienes starts with benzocyclopropene (1). ... 

However, orbital factors may override thermodynamic control. For example, the regiochemistry of nucleophilic attack on the bridged norcaradiene radical cation 122 shows a significant deviation from thermodynamic control. Although attack on the cyclopropane ring should be favored by both release of ring strain and formation of delocalized free radicals (cf. Scheme 6.8), methanol attacks 122 " selectively at C2 (and C5), generating 123 and 124. There is little stereoselectivity Products derived from 123 and 124 were formed in comparable yields. ... 

The existence of the carbene intermediate has also been demonstrated by formation of dimers12,54, reaction with solvent (benzene) to form a norcaradiene [observed only with electron-deficient 3ZZ-pyrazoIes72,154,162 (Scheme 57)], by trapping with active alkenes55,56,162 and dienes55,72,162 (often... 

Where the carbon-carbon double bond is a part of an aromatic system, in general, cyclopropanation of diazoketones results in the formation of unstable cyclopropane adducts. For example, Saba140 has shown that in the intramolecular cyclopropanation of diazoketone 57 the norcaradiene ketone 58 can be detected by low-temperature NMR and can be trapped in a Diels Alder reaction with 4-phenyl-l,2,4-triazoline-3,5-dione (equation 69). In addition, Wenkert and Liu have isolated the stable norcaradiene 60 from the rhodium catalysed decomposition of diazoketone 59 (equation 70)105. Cyclopropyl ketones derived from intramolecular cyclopropanation of hetereoaromatic diazoketones are also known and two representative examples are shown in equations 71 and 72106. Rhodium(II) compounds are the most suitable catalysts for the cyclopropanation of aromatic diazoketones. 

In the case of 2-halotropones, for instance, the reaction is believed to begin with the attack of hydroxide ion onto C-l or C-3, and it results in the formation of benzoic acid or salicylaldehyde, respectively. The balance between the two processes is affected by the alkali concentration More dilute alkali favors the second route. According to Pietra (79ACR132), the key step of the first route is ring closure in the initial hydroxide adduct to give a norcaradiene intermediate. 

In one famous case, the release of ring strain is almost exactly counterbalanced by the formation of a a bond at the expense of a it bond. Cycloheptatriene exists in equilibrium with a blcyclic isomer known as norcaradiene. Usually cycloheptatriene is the major component of the equilibrium, but the norcaradiene structure is favoured if R is an electron-withdrawing group. 

Formation of 648 from 647 was experimentally discarded. Since the most basic site of 646 is the carbonyl group, both Bronsted and Lewis acids should first coordinate to this position. In the case of Bronsted acid, protonation could occur at the electron-rich 9-position, and this was followed by deprotonation at the 1 la-position, which is promoted by the carbonyl protonation. In contrast, the Lewis acid cannot add to the 9-position and thus the skeletal rearrangement to give 648 took place. This kind of rearrangement through norcaradiene tautomers, shown in Scheme 128, is called walk rearrangement in thermal reaction of cycloheptatrienes <2002CL260>. 

In the case of 10, the strain of the six-membered ring is small, as is the strain energy due to ring annelation. Utilizing heats of formations for cw-1,3-butadiene, gauche-Vmy cyc o-propane" and norcaradiene ", a homoaromatic stabilization energy of 4 kcal mol is calculated in line with a description of 10 as a cyclopropyl homoaromatic 6n electron system. 

The formation of norcaradiene derivatives with naphthalene [reaction (22)] lends some support to this scheme. This mechanism resembles a bimolecular two-step process suggested for the reaction of chloromethyl-aluminum compounds with olefins (199-201). On the other hand, a bimolecular one-step methylene transfer mechanism is generally accepted for the formation of cyclopropane derivatives by the reaction of halo-methylzinc compounds with olefins. This difference between the mechanism proposed for the cyclopropane formation from olefin and that for the ring expansion of aromatic compound may be ascribable to the difference in the stability of intermediates the benzenium ion (XXII) may be more stable than an alkylcarbonium ion (369). 

Allene 68, formed in situ from l-(2,4,6-cydoheptatrien-l-yl)-2-phenyl-acetylene (67) in the presence of DBU in diethyl ether, underwent dimerization to give compounds 69,70, and 71 in 27, 43, and 4% yields, respectively (79CL171). Heating of compound 69 with DBU in benzene afforded a 2 1 isomeric mixture of azulenes 73 and 74 in 90% yield. The formation of the azulene derivatives (73 and 74) probably proceeds via the norcaradiene tautomer (72) of 69 (Scheme 2). 

Manitto, P., Monti, D., Speranza, G. Rhodium(ii)-Cataiyzed Decomposition of 1-Diazo-4-(2-naphthyi)butan-2-one. Direct Chemicai Evidence for the Formation of the Norcaradiene System in the intramoiecuiar Buchner Reaction. J. Org. Chem. 1995, 60, 484-485. 

Cycloaddition of cyclopropene, 1-methylcyclopropene or 3,3-dimethylcyclopropene to 1,2,4-triazines leads initially to an azacycloheptatriene which is in equilibrium with the norcaradiene form. A second cycloaddition of a cyclopropene to the latter may then occur, e.g. formation... 

It has been shown experimentally (heats of hydrogenation, heats of formation) that there is significant homoaromatic stabilization in cycloheptatrienes such as 1, 7, 8, 9, 12 (in the latter three there is additional classical Huckel aromaticity), as well as in norcaradienes such as 2, 3, 10, 11. In cases where other energetic factors of the pair of valence tautomers 1 2 are very similar, the relative homoaromaticity determines the equilibrium position. ... 

The quantum yield for toluene formation is very low in solution but approaches unity in the gas phase at low pressures- . The toluene was suggested to be formed from vibrationally excited ground state molecules, following rapid internal conversion from the excited singlet state manifold, perhaps involving the intermediacy of norcaradiene... 

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