Cyclohepta-l,3,5-trienes

Reaction of dibromocyclopropane (39) with hot quinoline gives 1-ethoxy-cyclohepta-l,3,5-triene (37) in 32% yield. Dihalocyclopropanes prepai ed from larger ring enol ethers do not react with hot pyridine but afford products with hot quinoline formed by transannular reactions. 

The relative stereochemistry of tricycle-fused isoxazolines resulting from 1,3-dipolar cycloaddition of cyclo-1,3-diene-tethered nitrile oxides is cis-cis, whereas from cyclohepta-l,3,5-triene-tethered nitrile oxides the cis-trans isomer predominates (412). 

You will not be surprised, then, to learn that the tropylium ion is the major peak found in the MS of 7-substituted cyclohepta-l,3,5-trienes, but you may be surprised to find that the benzyl carbocation rearranges to the tropylium ion under MS conditions (Scheme 5.7). 

Asymmetric deprotonation of /V-(/-butoxycarbonyl)indolincs at the 2-position with s-butyllithium-(—)-sparteine has been reported.161 Results of an ab initio MO study of deprotonated 2,3-dihydrooxepin suggest that the allylic anion is 15 kcal mol 1 more stable than the vinylic anion, which is, in turn, 8 kcal mol 1 more stable than the vinyl anion of cyclohepta-l,3,5-triene.162... 

An extensive review of 6 + 4-cycloaddition reactions has been published.287 The use of a precatalyst in the chromium(0)-promoted 6 + 4-cycloaddition reaction of cyclohepta-l,3,5-triene with acychc dienes ensures that only stoichiometric quantities of the metal are involved in the cycloaddition (Scheme 59).288 The first example of an aqueous 6 + 4-cycloaddition of tropone with the water-soluble diene -([l-D-glucopyranosyloxyjbuta-1,3-diene (152) yields 2-(glucopyranosyloxy)bicyclo-[4.4.1]undecanones (153) and (154) with exo selectivity (Scheme 60).289 A new benzannulation sequence is based on chromium(0)-promoted 671 + 47i-photo-cycload-dition of O/ -thicpin- l, 1 -dioxidc)tricarbonylchromium(O) in the presence of a diene followed by a Ramberg-Backlund rearrangement of the cycloadduct.290... 

Aclohcpuidiciiyl complexes, with vanadium, 5, 50 Cycloheptadien-l-yne, in Pd(0) it-complexes, 8, 355 tj6-Cyclohepta-l,3,5-triene complex, as catalyst precursor,... 

Such rearrangements have been exploited to homologate cyclic dienes, to prepare bridged annulenes and cyclohepta-l,3,5-trienes (Scheme 3.13).27,28... 

One ligand that possesses multiple bonding sites is cyclohepta-l,3,5-triene (cht) that contains three double bonds ... 

This reacts with cycloocta-1,5-diene or ewdo-dicyclopentadiene to give [Cu(diolefin)2]03S-CF3 complexes, and with cyclooctatetrene, cyclo-dodeca-l,5,9-triene, cycloocta-1,3-diene, or norbomylene to give Cu(olefin)03S-CF3 complexes cyclohepta-l,3,5-triene can replace benzene in the parent complex yielding Cu(CFgS03) J(triolefin). 

With respect to the carbene, arylcyclopropane formation occurs with rather low stereospecificity. In most cases the ewt/o-isomer predominates, which is in accordance with the general observation made when arylmethylenes from other sources are added to alkenes. Thus, photolysis of phenyldiazomethane dissolved in cyclohexene gave 7-phenylbicyclo[4.1.0]heptane (2) in 34% yield with a endojexo ratio of 1.1. Preference for the e (/o-isomer is also observed when monoarylcarbenes are reacted with cyclohepta-l,3,5-triene, but when 1-naphthylcar-bene, 2-naphthylcarbene and 9-anthrylcarbene are reacted with cycloocta-l,3,5,7-tetraene the exo-isomer predominates. ... 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

In a similar manner, transfer of oxocarbenes from a-diazo ketones to benzene mediated by tetrakis(trifluoroacetato)dirhodium(II) yields, in some cases, 7-(l-oxoalkyl)cyclohepta-l,3,5-trienes. These compounds undergo facile acid-catalyzed isomerization to benzyl ketones 36. In other cases, the latter products are formed directly under the conditions of the carbenoid reaction " for examples, see Houben-Weyl, Vol. E19b, pl313. 

When diazoacetonitrile was photolyzed in refluxing benzene, bicyclo[4.1.0]hepta-2,4-diene-7-carbonitrile (5) was formed, which rapidly tautomerized to cyclohepta-l,3,5-triene-7-carbo-... 

Similarly, thermolysis (150°C) of 3-diazodecafluoropentane in the presence of benzene yielded 7,7-bis(pentafluoroethyl)cyclohepta-l,3,5-triene in 22% yield, rather than the valence tautomeric norcaradiene derivative.27... 

Associative and dissociative paths have also been established from studies of the kinetics of reactions of [Fe(CO)3( y -dienone)] with tertiary phosphines and now for polyenes/ which is a very useful route to [Fe(CO)g(polyene)] complexes. Both paths as shown in Scheme 3 lead to an initial dissociation of the organic carbonyl, and the rate constants for the unimolecular path increase with increasing donor ability of group R (Ph < H < Me) where the incoming polyene is cyclohepta-l,3,5-triene (L). 

Interratingly, irradiation for 6 h of the 1 1 complex 61 compc d of 8 a and 58 which had been prepared by keeping a solution of 8a and an equimolar amount of 58 in benzene-hexane at room temperature for 12 h (colorless needles of mp 133 to 136 °C) gave the optically active photocyclization product (—)-bk yclo[4.2.0]oct-7-en-2-one (62) Qa]p —60.6 (c 0.18, CHCI3)] in quantitative yield The photocyclization reaction is in contrast to the photodimerization of 58 in 59. In the case of 61, the photodimerization of 58 is probably prevented by steric hindrance. It is almost certain that one optical conformer of 58 is included in 61, but a real proof of this fact requires an X-ray structural study in the future. However, the formation of the optically active 62 is valuable, because photoreaction of 58 in solution does not give any intramolecular photocyclization product It has been reported that irradiation of bicyclo[5.1.0]octa-3,5-dien-2-one (63) in methanol leads to a mixture of racemic tricyclo[4.2.0.0 ]oct-7-en-2-one (65), 37, and cyclohepta-l,3,5-triene Control of the reaction was also tried in expectation... 

Kinetic studies of the 1,3 cycloadditions of tetracyanoethylene (TCNE) to tricarbonyl (Tj -cyclohepta-l,3,5-triene) iron 45 (R = H) and related complexes to give adducts of type 46 have confirmed their concerted nature. The small rate increases on changing from dichloromethane to the more polar nitromethane solvent ruled out an ionic intermediate, and a free radical mechanism had been previously excluded. Comparison with free cyclohepta-l,3,5-triene showed that coordination toFe(CO)3 increased... 

All the potential products formed by removing a hydrogen from cyclo-penta-1,3-diene and from cyclohepta-l,3,5-triene can be drawn with numerous resonance structures, hut only the six-Tr-elecfron cyclopentadienyl anion and cycloheptatrienyl cation are predicted by the 4n -1- 2 rule to be aromatic (Figure 9.3). 

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