Editing Diels–Alder reaction (section)

===The diene===

The [[diene]] component of the Diels–Alder reaction can be either open-chain or cyclic, and it can host many different types of substituents.<ref name=cb474/> It must, however, be able to exist in the s-''cis'' conformation, since this is the only conformer that can participate in the reaction.<!--[[File:S-cis-s-trans conformation.png|thumbnail|right|s-''cis'' and s-''trans'' conformations of butadiene]]--> Though butadienes are typically more stable in the s-''trans'' conformation, for most cases energy difference is small (~2–5 kcal/mol).<ref>[[#Carey|Carey]], Part A, p. 149</ref>

A bulky substituent at the C2 or C3 position can increase reaction rate by destabilizing the s-''trans'' conformation and forcing the diene into the reactive s-''cis'' conformation. 2-''tert''-butyl-buta-1,3-diene, for example, is 27 times more reactive than simple butadiene.<ref name=cb474/><ref name="Ditertiobutylbutadiène">{{cite journal |last=Backer |first=H. J. |year=1939 |title=Le 2,3-Ditertiobutylbutadiène |journal=Recueil des Travaux Chimiques des Pays-Bas |volume=58 |issue=7 |pages=643–661 |doi=10.1002/recl.19390580712}}</ref> Conversely, a diene having bulky substituents at both C2 and C3 is less reactive because the steric interactions between the substituents destabilize the s-''cis'' conformation.<ref name="Ditertiobutylbutadiène"/>

Dienes with bulky terminal substituents (C1 and C4) decrease the rate of reaction, presumably by impeding the approach of the diene and dienophile.<ref>{{cite journal |last1=Craig |first1=D. |last2=Shipman |first2=J. J. |last3=Fowler |first3=R. B. |year=1961 |title=The Rate of Reaction of Maleic Anhydride with 1,3-Dienes as Related to Diene Conformation |journal=Journal of the American Chemical Society |volume=83 |issue=13 |pages=2885–2891 |doi=10.1021/ja01474a023}}</ref>

An especially reactive diene is 1-methoxy-3-trimethylsiloxy-buta-1,3-diene, otherwise known as [[Danishefsky's diene]].<ref>{{cite journal |last1=Danishefsky |first1=S. |last2=Kitahara |first2=T. |year=1974 |title=Useful diene for the Diels–Alder reaction |journal=Journal of the American Chemical Society |volume=96 |issue=25 |pages=7807–7808 |doi=10.1021/ja00832a031}}</ref> It has particular synthetic utility as means of furnishing α,β–unsaturated [[cyclohexenone]] systems by elimination of the 1-methoxy substituent after deprotection of the enol silyl ether. Other synthetically useful derivatives of Danishefsky's diene include 1,3-alkoxy-1-trimethylsiloxy-1,3-butadienes (Brassard dienes)<ref>{{cite journal |last1=Savard |first1=J. |last2=Brassard |first2=P. |year=1979 |title=Regiospecific syntheses of quinones using vinylketene acetals derived from unsaturated esters |journal=Tetrahedron Letters |volume=20 |issue=51 |pages=4911–4914 |doi=10.1016/S0040-4039(01)86747-2}}</ref> and 1-dialkylamino-3-trimethylsiloxy-1,3-butadienes (Rawal dienes).<ref>{{cite journal |last1=Kozmin |first1=S. A. |last2=Rawal |first2=V. H. |year=1997 |title=Preparation and Diels−Alder Reactivity of 1-Amino-3-siloxy-1,3-butadienes |journal=Journal of Organic Chemistry |volume=62 |issue=16 |pages=5252–5253 |doi=10.1021/jo970438q}}</ref> The increased reactivity of these and similar dienes is a result of synergistic contributions from donor groups at C1 and C3, raising the HOMO significantly above that of a comparable monosubstituted diene.<ref name="AngChem2002">{{cite journal |last1=Nicolaou |first1=K. C. |last2=Snyder |first2=S. A. |last3=Montagnon |first3=T. |last4=Vassilikogiannakis |first4=G. |year=2002 |title=The Diels-Alder Reaction in Total Synthesis |journal=Angewandte Chemie International Edition |volume=41 |issue=10 |pages=1668–1698 |doi=10.1002/1521-3773(20020517)41:10<1668::AID-ANIE1668>3.0.CO;2-Z|pmid=19750686 }}</ref>
[[File:Named dienes.png|400px|center|General form of Danishefsky, Brassard, and Rawal dienes]]

Unstable (and thus highly reactive) dienes can be synthetically useful, e.g. ''o''-[[quinodimethane]]s can be generated in situ.  In contrast, stable dienes, such as [[naphthalene]], require forcing conditions and/or highly reactive dienophiles, such as [[N-Phenylmaleimide|''N''-phenylmaleimide]]. [[Anthracene]], being less aromatic (and therefore more reactive for Diels–Alder syntheses) in its central ring can form a 9,10 adduct with [[maleic anhydride]] at 80&nbsp;°C and even with [[acetylene]], a weak dienophile, at 250&nbsp;°C.<ref>Margareta Avram (1983). ''Chimie organica'' p. 318-323. Editura Academiei Republicii Socialiste România</ref>