Diels-Alder and Ene Reactions

The moderate Lewis acidity of ruthenium complexes was used to promote catalytic Diels-Alder reaction of dienes and acrolein derivatives [21-23]. The enantioselective Diels-Alder reaction of methacrolein with dienes was catalyzed with cationic ruthenium complexes containing an arene or cyclo-pentadienyl (Cp) ligand and a chiral ligand such as phosphinooxazoline, pyridyl-oxazoline, monoxidized 2,2 -bis(diphenylphosphino)-1, T-binaphthyl (BINPO)or l,2-bis[bis(pentafluorophenyl)phosphanyloxy]-l,2-diphenylethane (BIPHOP-F). The reaction gave the cycloadduct in high yields with excellent 

With the complex [(indenyl)Ru(acetone)(Me4BIPHOP-F)]SbF6 as catalyst [22], the reaction afforded the exo cycloadduct as the major product for the reaction of acrolein with cyclopentadiene, whereas this noncatalyzed reaction is known to give the endo derivative as the major product. Analogously the catalyst 41 performed the asymmetric 1,3-dipolar addition of nitrones with enals [24]. 

The ene reaction involving an aldehyde has also been performed with a (salen)ruthenium(II) catalyst [25]. The C-C coupling of an unsaturated carbonyl with aldehydes was also achieved with RuH2(PPh3)4 to give an a-meth-ylene-/5-hydroxyketone but in that case the reaction proceeds via the preliminary hydrometallation of the double bond [26]. 

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The allenyl moiety (2,3-aikadienyl system) in the carbonylation products is a reactive system and further reactions such as intramolecular Diels-Alder and ene reactions are possible by introducing another double bond at suitable positions of the starting 2-alkynyl carbonates. For example, the propargylic carbonate 33 which has l,8(or 1.9)-diene-3-yne system undergoes tandem carbonylation and intramolecular Diels-Alder reaction to afford the polycyclic compound 34 under mild conditions (60 C, 1 atm). The use of dppp as ligand is important. One of the double bonds of the allenyl ester behaves as part of the dieneflSj. 

More recently, bis(oxazoline)-metal complexes supported in micro- and mesoporous solids have been used as catalysts of hetero-Diels-Alder and ene reactions. 

The germene Me2Ge = C(SiMe3)2 22 was obtained by thermolysis of 2331 (E = Ge) [Eq. (5)]. A very complete mechanistic study of the Diels-Alder and ene-reactions has been performed using 22 (see Section II,C,8). The corresponding silene (E = Si) has also been obtained by the same route.31... 

TABLE 9.17 ENANTIOSELECTIVE HETERO DIELS-ALDER AND ENE REACTION PRODUCTS,... 

D. F. Taber, Intramolecular Diels-Alder and Ene Reactions, Springer-Verlag, New York, 1984 see also W. R. Roush, in Comprehensive Organic Synthesis , ed. L. A. Paquette, Pergamon Press, New York, 1991, vol. 

Scheme 9.10. Formation of lactams by intramolecular Diels-Alder and ene reaction [25, 28]...

Overman, L.E. Angew Chem Int Ed Engl, (1984), 23, 579 "Asymmetric Diels-Alder and Ene Reactions in Organic Synthesis" Oppolzer, W. Angew Chem Int Ed Engl, (1984), 23, 876 "Acetylene Equivalents in Cycloaddition Reactions"... 

Desimoni and Righetti have been thoroughly investigating the effect of solvents, acid catalysis and salts on hetero Diels-Alder and ene reactions of 1-oxa-1,3-butadienes for a long time [ 154-156]. Recently, for the cycloaddition of 2-154 and ethyl vinyl ether 2-83 in the presence of lithium perchlorate in different solvents as diethyl ether, acetonitrile, acetone, methanol, iso-propanol to give... 

A major problem in the reaction of a,/3-unsaturated carbonyl compounds and alkenes proves to be the competition between hetero Diels-Alder and ene reactions. Intramolecular cycloadditions of 1,6- and 1,7-dienes with ester and cyano groups at the double bond yield the ene product nearly exclusively, but with alkylidene- and benzylidene-ketoesters and 1,3-diketones the Diels-Alder reaction is preferred under thermal conditions, however under Lewis acid catalysis also ene reactions occur [12]. 

Schmittel, M. Keller, M. Kiau, S. Strittmatter, M. A surprising switch from the Myers-Saito cyclization to a novel biradical cyclization in enyne-aUenes formal Diels-Alder and ene reactions with high synthetic potential, Chem. Eur. J. 1997, 3, 807-816. 

Sila-, germa-, and stannaethenes Me2E=C(SiR3)2 (E = Si, Ge, Sn / SiR3 = SiMes, SiMePh2) undergo Diels-Alder and ene reactions as well as [2+2] cycloadditions with methyl derivatives of organic... 

Oppolzer, W, Asymmetric DIels-Alder- and ene reactions In organic synthesis. Angew. Chem. 1984, 96, 840-854. 

Leach, A. G., Houk, K. N. Diels-Alder and ene reactions of singlet oxygen, nitroso compounds and triazolinediones transition states and mechanisms from contemporary theory. Chem. Common. 2002,1243-1255. 

Diels-Alder and ene reactions of singlet oxygen, nitroso compounds, and triazo-linediones 02CC1243. 

High pressure in heterocyclic synthesis 85S1, 85S999 81H(16) 1367. Intramolecular Diels-Alder and ene reactions 84MI19. 

Asymmetric synthesis using Diels-Alder and ene reactions 84AG840. Asymmetric synthesis of heteroeycies 86T5157 79T2797. 

Evans, D.A., Johnson. J.S.. Burgey. C.S., and Campos, K.R., Reversal in enantioselectivity of tert-butyl versus phenyl-substituted / z4 (oxazoline) copper(II) catalyzed hetero Diels-Alder and ene reactions. Crystallographic and mechanistic studies. Tetrahedron Lett., 40, 2879, 1999. 

D. F. Taber, Intramolecular Diels-Alder and Ene Reactions, Springer, New York 1984. 

In studies which represent the first carefully documented example of the 4-n participation of an a,/3-unsaturated ester in a [4 + 2] cycloaddition,142 Snider and co-workers systematically investigated the competing thermal, intramolecular Diels-Alder and ene reactions of l-allylic-2,2-dimethyl ethylenetricarboxylates [Eq. (39)].142... 

We shall now leave six-electron cyclodadditions such as the Diels-Alder and ene reactions and move on to some four-electron cycloadditions. Clearly, four is not a (4 -i- 2) number, but when we described the Woodward-Hoffman rules on p. 892 we used the term thermally. All... 

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