10.12 The Diels-Alder Reaction. Synthetic method for preparing compounds containing a cyclohexene ring
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1 10.12 The Diels-Alder Reaction Synthetic method for preparing compounds containing a cyclohexene ring
2 In general... + conjugated diene alkene (dienophile) cyclohexene
3 via transition state
4 Mechanistic features concerted mechanism cycloaddition pericyclic reaction a concerted reaction that proceeds through a cyclic transition state
5 Recall the general reaction... + conjugated diene alkene (dienophile) cyclohexene The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile.
6 What makes a reactive dienophile? The most reactive dienophiles have an electron-withdrawing group (EWG( EWG) ) directly attached to the double bond. EWG Typical EWGs C C C C N
7 Example H 2 C CHCH CH 2 + H 2 C CH CH benzene 100 C CH (100%)
8 Example H 2 C CHCH CH 2 + H 2 C CH CH via: benzene 100 C CH CH (100%)
9 Example H 2 C CHC CH 3 CH 2 + benzene 100 C H 3 C (100%)
10 Example H 2 C CHC CH 3 CH 2 + benzene 100 C H 3 C via: H 3 C (100%)
11 Acetylenic Dienophile H 2 C CHCH CH 2 + CH 3 CH 2 CC CCCH 2 CH 3 benzene 100 C CCH 2 CH 3 (98%) CCH 2 CH 3
12 Diels-Alder Reaction is Stereospecific* syn addition to alkene cis-trans relationship of substituents on alkene retained in cyclohexene product *A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.
13 Example C 6 H 5 CH H 2 C CHCH CH 2 + C C H H H only product H C 6 H 5 CH
14 Example H CH H 2 C CHCH CH 2 + C C C 6 H 5 H C 6 H 5 H only product H CH
15 Cyclic dienes yield bridged bicyclic Diels-Alder adducts.
16 H CCH 3 + C C CH 3 C H H CCH 3 H CCH 3
17 H CCH 3 H CCH 3 is the same as H CCH 3 CCH 3 H
18 10.13 The π Molecular rbitals of Ethylene and 1,3-Butadiene
19 rbitals and Chemical Reactions A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals of the reactants. Electrons flow from the highest occupied molecular orbital (HM) of one reactant to the lowest unoccupied molecular orbital (LUM) of the other.
20 rbitals and Chemical Reactions We can illustrate HM-LUM interactions by way of the Diels-Alder reaction between ethylene and 1,3-butadiene. We need only consider only the π electrons of ethylene and 1,3-butadiene. We can ignore the framework of σ bonds in each molecule.
21 The π Ms of Ethylene red and blue colors distinguish sign of wave function bonding π M is antisymmetric with respect to plane of molecule Bonding π orbital of ethylene; two electrons in this orbital
22 The π Ms of Ethylene Antibonding π orbital of ethylene; no electrons in this orbital LUM HM Bonding π orbital of ethylene; two electrons in this orbital
23 The π Ms of 1,3-Butadiene Four p orbitals contribute to the π system of 1,3- butadiene; therefore, there are four π molecular orbitals. Two of these orbitals are bonding; two are antibonding.
24 The Two Bonding π Ms of 1,3-Butadiene HM 4 π electrons; 2 in each orbital Lowest energy orbital
25 The Two Antibonding π Ms of 1,3-Butadiene Highest energy orbital LUM Both antibonding orbitals are vacant
26 10.14 A π Molecular rbital Analysis of the Diels-Alder Reaction
27 M Analysis of Diels-Alder Reaction Inasmuch as electron-withdrawing groups increase the reactivity of a dienophile, we assume electrons flow from the HM of the diene to the LUM of the dienophile.
28 M Analysis of Diels-Alder Reaction HM of 1,3-butadiene HM of 1,3-butadiene and LUM of ethylene are in phase with one another allows σ bond formation between the alkene and the diene LUM of ethylene (dienophile)
29 M Analysis of Diels-Alder Reaction HM of 1,3-butadiene LUM of ethylene (dienophile)
30 A "forbidden" reaction H 2 C H 2 C + CH 2 CH 2 The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not?
31 A "forbidden" reaction H 2 C H 2 C + CH 2 CH 2 HM-LUM mismatch of two ethylene molecules precludes single-step step formation of two new σ bonds HM of one ethylene molecule LUM of other ethylene molecule
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