Lecture Outline Diels-Alder Reaction

Lecture outline Diels-Alder Reaction C C C C C C C C C C C C dienophile (2 π-e– component) diene (4 π-e– component) the dienophile is shown above as an alkene, but it can also be a diene, alkyne, –N=N–, etc. 1. The reaction is stereospecifically syn with respect to each reactant That the dienophile stereochemistry is preserved is illustrated by reaction of 1,3butadiene with fumaronitrile (trans-1,2-dicyanoethylene) and maleonitrile (cis-1,2-dicyanoethylene). NC + CN trans CN + CN cis What do these results imply about the mechanism of this reaction? What stereochemical outcome would you expect if the reaction were concerted? What if it were stepwise? That the diene stereochemistry is preserved is illustrated by rxn of diethyl acetylenedicarboxylate ("DEAD" ... seriously) with E,E- and E,Z-2,4-hexadiene CH3 + CO2Et C C CO2Et CH3 CH3 + CH3 CO2Et C C CO2Et What product would you expect from the Z,Z-diene? 2. The stereochemistry is clearly most consistent with a concerted mechanism — for all the bond making and breaking processes to occur simultaneously, the orbitals must all be aligned properly... The diene and dienophile can't very well come together in the same plane, now can they? The new σ-bonds have to be formed by initial overlap of p-orbitals, thus (these drawings are meant to show p-orbital orientations only — they're not meant to represent actual π-MOs Dienophile must approach from above or below (not in) plane of diene cyclohexene is formed in boat conformation 3. Another consequence of concert is that the diene must be s-cis, otherwise the reaction cannot occur. Draw the product that would be produced if the diene reacted in its s-trans conformation (assuming the dienophile could even reach the ends of the diene, which it couldn't do, even if it wanted to, which it doesn't.) s-trans s-cis — dienes that cannot be s-cis do not react, e.g.... 3-methylenecyclopentene a cis,cis-1,4-disubstituted-1,3-butadiene — and dienes constrained to be s-cis react rapidly, e.g.... 1,3-cyclopentadiene 1,2-dimethylenecylcohexane Of course, a D-A reaction of 1,3-cyclopentadiene produces a bicyclic product + C(CN)2 CN ≡ CN CN C(CN)2 CN "endo" "exo" 4. In contrast to the Diels-Alder reaction, a "4+2" cycloaddition, neither 2+2 nor 4+4 cycloadditions work. To understand the difference, we need to consider the orbitals of the reacting components — First, the Diels-Alder in its most basic form — Butadiene Now the 2+2 and 4+4 — Ethylene