Cycloheptatrienes Diels-Alder reaction

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). 

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. 

Later on, it was demonstrated that these heterocycles can undergo Diels-Alder reactions in the presence of an electrophile (Ss or Mel) and dienophiles <2002T1573, 2003HAC560>. These phosphoms-containing heterocycles were found to produce, upon reaction with tricarbonyl(cycloheptatriene)molybdenum(0) or tricarbonyl(mesitylene) tungsten(O), cr-complexes of the type L2M(CO)4 or I,(M(CO)( instead of 7t-complexes <1998EJI1079>. Some derivatives of this heterocycle were also found to display remarkable antibacterial activity <2005BML937>. 

This will apply not only to Diels-Alder reactions, but to other Woodward-Hoff-mann symmetry-allowed reactions, as well. For example, the good donor-acceptor pair, cycloheptatriene and tropone, react to give only the allowed ([6 + 4], then [4 + 2]) adduct <517, while cycloheptatriene dimerizes to give not only the analogous adduct, 7, but the forbidden, [6 + 2] adduct, 8, as well as several other unidentified dimers18. ... 

Fig. 9 (a) Molecular shuttles based on reversible Diels-Alder reaction (al-a6) and sulfur-containing molecular shuttles (a7-al2) (b) methoxy-cycloheptatriene- and methoxy-acridane-based rotaxanes (c) imine-based rotaxanes (see text for details)... 

In the parent system 1, and in most substituted cycloheptatrienes, the corresponding nor-caradienes cannot be detected under standard conditions even with very sensitive analytical methods, but some of their reactions (notably the Diels-Alder reaction with dienophiles) are based on the norcaradiene form. 

More readily identifiable geometrical factors probably outweigh the contribution of the frontier orbitals in the remarkable reaction 6.47 between tetracyanoethylene and heptafulvalene giving the adduct 6.49 (see p. 261). The HOMO coefficients for heptafulvalene 6.420 (see p. 347) are highest at the central double bond, but a Diels-Alder reaction, with one bond forming at this site is impossible. The best reasonable possibility for a pericyclic cycloaddition, from the frontier orbital point of view, would be a Diels-Alder reaction across the 1,4-positions (HOMO coefficients of -0.199 and 0.253), but this evidently does not occur, probably because the carbon atoms are held too far apart. This is well-known to influence the rates of Diels-Alder reactions cyclopentadiene reacts much faster than cyclohexadiene, which reacts much faster than cycloheptatriene (see p. 302). The only remaining reaction is at the site which actually has the lowest frontier-orbital electron population, the antarafacial reaction across the 1, f-positions, which have HOMO coefficients of —0.199. 

Scheme 2.12. Pressure-induced Diels-Alder reaction of cycloheptatriene norcaradiene with Ceo as dienophile.

Butenyl)cycloheptatriene Intramolecular Diels-Alder Reactions... 

The activation entropy values are found to be zero or positive for the extrusion of nitrogen from the azo-compound (212), to give cycloheptatriene, in various solvents. The rate of nitrogen loss decreases with increasing solvent polarity and internal pressure of the solvent an inverse solvent dependence, of the same order of magnitude, is evident from previous studies on the bimolecular reverse (Diels-Alder) reaction. These and other considerations lend support to the continuity of mechanisms in these cases, and no distinction between concerted or radical pathways can be made from the sign of Another remarkable example has appeared that further... 

Reaction of cycloheptatriene with p-benzoquinone in toluene at 110°C gave the [4 + 2] adducts (282), the uic-ditropyl product (283), a small quantity of 7,7 -bitropyl, and hydroquinone. " More complex behaviour was observed for the reaction conducted in the absence of solvent in addition to the above compounds, the wic-ditropyl derivative (284) and the cage compound (285), formed by intramolecular Diels-Alder reaction of (283), were also isolated, all in very low yield. The formation of (283) appears to involve a biradical process. ... 

An outstanding example of a branching pathway exploited complementary cyclisation reactions to yield alternative molecular scaffolds (Scheme 1.5). A four-component Petasis condensation reaction was used to assemble flexible cyclisation precursors e.g. 14). Alternative cyclisation reactions were then used to yield products with distinct molecular scaffolds Pd-catalysed cyclisation (- 15a) enyne metathesis (- 15b) Ru-catalysed cycloheptatriene formation (- 15c) Au-catalysed cyclisation of the alcohol onto the alkyne (- 15d) base-induced cyclisation (- 15e) Pauson-Khand reaction 15f) and Miesenheimer [ 2,3]-sigmatropic rearrangement (not shown). Four of these cyclisation reactions could be used again to convert the enyne 15e into molecules with four further scaffolds (17a-d). In addition, Diels Alder reactions with 4-methyl-l,2,4-triazoline-3,5-dione converted the dienes 15b, 17a and 17d... 

PROBLEM 20.37 Now, here s an interesting variation on the reaction of Problem 20.36. Diels-Alder additions typically take place as shown in Problem 20.36, with the ring-closed partner (A in the answer to Problem 20.36) actually doing the addition reaction. However, the molecule shown below is an exception, as the Diels-Alder reaction takes place in the normal way to give the product shown. Draw an arrow formalism for the reaction and then explain why 1 reacts differently from other cycloheptatrienes. 

Cycloheptatriene gives (181) on reaction with bis(trifluoromethyl)thioketen, via an ene reaction, a cycloheptatriene-norcaradiene valence tautomerism, and an intramolecular Diels-Alder reaction. Cyclo-octa-1,5-diene gives (182) and (183). 

Pyrolysis of the spiro-compound (6) and its congeners did not result in the expected retro-Diels-Alder reaction but furnished thiophens, fluorene, and 2-(9-fluorenyl)thiophens. The anhydride (7) gave the retro-Diels-Alder product on heating, together with a cycloheptatriene formed by a rearrangement for which there is an earlier precedent. As these reactions are not given by the corresponding diester, subtle structural differences in the systems (7) clearly have a profound influence on the course of the thermolysis. 

The applicability of thioketones in the synthesis of heterocyclic compounds has been further demonstrated. The Diels-Alder reaction of thiobenzophenone with l,3>5-cycloheptatriene gave the (2 + 4) cycloadduct (101) in 40% yield. Cycloheptatrienethione (92) yielded the (8 + 2) cycloadduct (102) quantitatively by reaction with maleic anhydride, and the bicyclic compound (103) in 65% yield by reaction with dimethyl acetylenedicarboxylate. Compound (103) was considered to be a rearrangement product of the probably primarily formed (8 + 2) cycloadduct intermediate (104). " The in situ addition of nitrile oxides to aromatic thioketones resulted in formation of 1,4,2-oxathiazoles, " whereas 1,4,2-oxathiazolidines were the products in the reactions of some alicyclic thiones with nitrones. The formation of thio-ozonides (1,2,4-trithiolans) by treatment of thioketones with amines or chloramine T was investigated recently. ... 

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. 

---