1.3.5- cycloheptatrienes

Cyclohepta-l,3,5-trienes (CHTs) are important bn-electron-rich ligands of transition-metal complexes especially in metal carbonyl complexes to balance 

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C-jHg. A slightly coloured liquid, b.p. 90 C, obtained by a Diels-Alder reaction between cyclopcntadiene and ethyne at approximately 150°C. At temperatures in excess of 450°C it rearranges to cycloheptatriene (tropilidene). 

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

Progress has been made toward enantioselective and highly regioselective Michael type alkylations of 2-cyclohexen-l -one using alkylcuprates with chiral auxiliary ligands, e. g., anions of either enantiomer of N-[2-(dimethylamino)ethyl]ephedrine (E. J. Corey, 1986), of (S)-2-(methoxymethyl)pyrrolidine (from L-proline R. K. EHeter, 1987) or of chiramt (= (R,R)-N-(l-phenylethyl)-7-[(l-phenylethyl)iinino]-l,3,5-cycloheptatrien-l-amine, a chiral aminotro-ponimine G. M. Villacorta, 1988). Enantioselectivities of up to 95% have been reported. 

Cycloheptatriene lacks cyclic conjugation interrupted by CH2 group... 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is we are not comparing the stability of the two to each other Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic Cycloheptatrienyl cation although aromatic is still a carbocation and reasonably reac tive toward nucleophiles Its special stability does not imply a rock like passivity but rather a much greater ease of formation than expected on the basis of the Lewis struc ture drawn for it A number of observations indicate that cycloheptatrienyl cation is far more stable than most other carbocations To emphasize its aromatic nature chemists often write the structure of cycloheptatrienyl cation m the Robinson circle m a ring style... 

There is a striking difference in the acidity of cyclopentadiene compared with cycloheptatriene Cycloheptatriene has a pK of 36 which makes it 10 times weaker m acid strength than cyclopentadiene... 

By analogy to additions of divalent carbon to the Cio aromatic framework, the molecule Cgi was expected to have the norcaradi-ene (II) or the cycloheptatriene (III) structure. Although an X-ray structure was not available, the UV-visible spectrum, NMR spectrum, and cyclic voltammetry supported the cycloheptatriene (III) structure. The researchers then calculated the relative molecular mechanics energies of II and III and found the cycloheptatriene structure stabilized by 31 kcal/mol with respect to the norcaradi-ene structure. Although the calculations do not confirm the structures, they provide additional supporting evidence. 

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

Cycloheptatrienes are in many cases in rapid equilibrium with an isomeric bicy-clo[4.1.0]heptadiene. The thermodynamics of the valence isomerism has been studied in a number of instances, and some of the data are given below. Calculate the equilibrium constant for each case at 25°C. Calculate the temperature at which K= for each system. Are the signs of the enthalpy and entropy as you would expect them to be Can you discern any pattern of substituent effects from the data ... 

Bicyclo[2.2.1]heptadiene rearranges at elevated temperatures to cycloheptatriene and toluene. The reaction is facilitated by substituents at C-7 such as phenyl and alkoxy, in which case cycloheptatrienes are the dominant products. 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

An especially interesting case of the hexatriene-cyclohexadiene type interconversion is the rapid equilibrium between cycloheptatrienes and bicyclo[4.1.0]hepta-2,4-dienes ... 

The thermal rearrangements of methyl-substituted cycloheptatrienes have been proposed to proceed by sigmatropic migration of the norcaradiene valence tautomer. The first step is an electrocyclization analogous to those discussed in Section 11.1. 

These results indieate that deaminoeolehinol methyl ether is 9 12 13 14-tetramethoxy-3 4 5 6-dibenz-d = -cycloheptatriene (XI j... 

Arene (e.g. benzene, substituted benzenes) cycloheptatriene, cycloocta-l,3,5-triene Tropylium (cycloheptatrienyl)... 

Treatment of cycloheptatriene complexes of the type [M(tj -C7H8)(CO)3] (M = Cr, Mo, W) with Ph3C" "BF4 results in hydride abstraction to give orange-coloured tj -cycloheptatrienyl (or tropy-lium) complexes ... 

Although the chemistry of zirconium in its lower oxidation states is still relatively unexplored, it is developing. Examples which offer the possibility of further exploitation include the blue, paramagnetic zirconium(III) compound 32) [L2Zr(/r-Cl)2ZrL2] L = C5H3(SiMe3)2-l,3, and the sandwich and half-sandwich compounds derived from cycloheptatriene red... 

As in the previous group, a potentially productive route into C7-ring chemistry is provided by the reduction of a metal halide with Na/Hg in thf in the presence of cycloheptatriene. WithMoCl5, [Mo(p -C7H7)(p -C7H9)] is produced and a variety of derivatives have already been obtained. 

One compound, either 7-bromo-1,3,5-cycloheptatriene or 5-bromo-1,3-cyclopentadiene, dissociates into ions when dissolved in water. Which molecule do you think displays this behavior How do you think the other molecule behaves when mixed with water ... 

Cycloheptatriene might be expected to be even more acidic, since seven resonance contributors can be drawn for its conjugate base. However, the fact that several resonance contributors can be drawn for a molecule does not necessarily guarantee that it will actually be resonance stabihzed (see also Chapter 12, Problem 9). 

Energies for cyclopentadiene and cycloheptatriene are available.) Is the reaction energy consistent with the other data Explain. 

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