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Frontier molecular orbital analysis, [4+2] cycloadditions, endo selectivity, electrocyclic reactions, and sigmatropic rearrangements
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Master concerted, single-step pericyclic reactions through a frontier-molecular-orbital (FMO) framework: HOMOโLUMO interactions, the WoodwardโHoffmann rules, and orbital symmetry control of stereochemistry.
Diels-Alder [4+2] cycloaddition
Electrocyclic reactions โ 4ฯ conrotatory (thermal) vs disrotatory (photochemical), 6ฯ disrotatory (thermal) vs conrotatory (photochemical).
Sigmatropic rearrangements โ Claisen [3,3] of allyl vinyl ethers and Cope [3,3] of 1,5-hexadienes; the chair-like transition state.
(a) Predict the product of the Diels-Alder reaction between (E,E)-2,4-hexadiene and maleic anhydride. (b) Explain why the product is a single diastereomer (specify cis or trans methyl groups). (c) Why does (Z,E)-2,4-hexadiene react more slowly?
(a) Product โ A cyclohexene fused to the anhydride. Both methyl groups end up cis on the ring, and both anhydride C=O groups are cis to each other (and endo with respect to the new ring).
(b) Stereospecificity โ The Diels-Alder is concerted and suprafacial/suprafacial. Substituents that are cis on the diene stay cis in the product; substituents that are cis on the dienophile stay cis. (E,E) diene โ both methyl substituents project on the same face โ cis methyls.
(c) Kinetic effect of (Z,E) โ A diene must reach the s-cis conformation to cyclize. (Z,E)-2,4-hexadiene has a methyl group blocking the s-cis rotation (severe A^1,3 strain), so the reactive conformation is destabilized and the rate drops.
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For the [4+2] cycloaddition of cyclopentadiene with methyl acrylate (CHโ=CHโCOโCHโ), draw the major endo product and explain the endo selectivity using FMO theory.
Major product โ Bicyclo[2.2.1]hept-2-ene (norbornene) with the โCOโCHโ group on the endo face (pointing toward the residual ฯ bond, i.e., the same side as the bridging methylene).
FMO rationale โ The dienophile's LUMO (lowered by the conjugated ester) interacts with the diene's HOMO. In the endo transition state, secondary (non-bonding) orbital overlap between the carbonyl ฯ* and the diene's p-orbitals at C2/C3 stabilizes the TS by ~2โ3 kcal/mol relative to the exo TS, so endo dominates kinetically. (Exo is often more thermodynamically stable but is not formed under typical conditions because Diels-Alder reactions at moderate temperature are kinetically controlled.)
Classify each pericyclic process and predict whether it proceeds thermally or photochemically: (a) ring-closure of (2E,4Z,6E)-octa-2,4,6-triene to cis-5,6-dimethyl-1,3-cyclohexadiene; (b) the Cope rearrangement of 1,5-hexadiene; (c) the [2+2] dimerization of two ethylene molecules.
(a) 6ฯ electrocyclization โ 6ฯ electrocyclic; thermal โ disrotatory; photochemical โ conrotatory. Producing the cis 5,6-dimethyl product from the (E,Z,E) triene requires the disrotatory mode โ thermal.
(b) [3,3]-Sigmatropic โ The Cope rearrangement is allowed thermally through a chair-like 6-membered TS (6 electrons; suprafacial/suprafacial; aromatic Hรผckel TS). Thermal.
(c) [2+2] cycloaddition โ 4 electrons. Thermally forbidden (would require antarafacial overlap). It is allowed photochemically, where excitation moves an electron into the ฯ* and the s/s mode becomes Mรถbius-allowed.