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I. Cycloadditions

I. Cycloadditions. (b) 4 p + 2 p (known as a Diels-Alder reaction) more prevalent cycloaddition 4 p system is the diene 2 p system is the dienophile the product is known as a Diels-Alder adduct On board: Examples Regiochemistry - use resonance to rationalize selectivity

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I. Cycloadditions

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  1. I. Cycloadditions • (b) 4p + 2p (known as a Diels-Alder reaction) • more prevalent cycloaddition • 4p system is the diene • 2p system is the dienophile • the product is known as a Diels-Alder adduct • On board: • Examples • Regiochemistry - use resonance to rationalize selectivity • Stereochemistry with substituted dienes and/or substituted • dienophiles: • 1. Intracomponent control 2. Intercomponent control: exo vs. endo transition states

  2. Intercomponent Control: • Most important orbitals to focus on are the HOMO of one • component and the LUMO of the other component. • The most energetically important interaction is between a • filled orbital of one component and an empty orbital of the • other component. • It does not matter which component you choose to designate • as the HOMO or LUMO; either choice will lead to the same • analysis. • Commonly, however, the energy levels that best match will • have the strongest interaction. • Typically, use the HOMO of the diene and the LUMO of the • dienophile.

  3. II. Electrocyclizations • Only a single component (starting material) • Involves the rearrangement of p bonds to form one new s bond. • Important Questions: • Which conjugated p systems work or don’t work? • What is the basis for stereochemical control? • Why do heat and light give different (opposite) stereochemical results?

  4. 1. Minimal structural criteria for electrocyclizations: To work, a p system must be conjugated without interruption. Conjugated system must be able to achieve planarity. Conjugated system must have a chromophore (molecular antenna that absorbs light) for photochemistry.

  5. 2 & 3. Stereochemistry: Thermal vs. Photochemistry • Can be understood by reliance on a model which utilizes molecular orbitals. • Only 2 M.O.s are necessary to predict product: HOMO & LUMO The model that uses these orbitals has the following rules: • Thermal reactions utilize the HOMO of the conjugated system. Photochemical reactions utilize the LUMO of the conjugated system (the excited state of the system). 2. The lobes rotate (which leads to the stereochemistry of the the product) in such a way as to maintain maximal overlap (bonding). Lobes can rotate either disrotatory or conrotatory.

  6. 2 & 3. Stereochemistry: Thermal vs. Photochemistry Summary: # of double bonds Participating Thermal (HOMO)Photochemical (LUMO) odd disrotatory conrotatory even conrotatory disrotatory Note: Thermal and photochemical conditions have opposite direction of rotation of lobes. Odd and even # of p bonds have opposite direction of rotation of lobes.

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