Chem 125. Advanced Organic Chemistry. 13. Cycloadditions and Sigmatropic Rearrangments.

The script begins with a recap of chapter 5, focusing on pericyclic reactions, which include electrocyclic reactions, cyclo additions, and sigmatropic rearrangements. The lecturer emphasizes that these reactions are characterized by the absence of intermediates and involve a reorganization of electrons and bonds to form products. The Diels-Alder reaction is introduced as a classic example of a cyclo addition reaction, involving the interaction of an alkene with a diene to form a six-membered ring. The importance of substituents in facilitating this reaction is highlighted, with a distinction made between normal and inverse electron demand Diels-Alder reactions. The concept of homo-lumo interactions as a key aspect of pericyclic reactions is also introduced.

This paragraph delves deeper into cyclo addition reactions, specifically the Diels-Alder reaction, and the importance of stereochemistry and orbital overlap in determining the feasibility of these reactions. The lecturer explains how the interaction between the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of another is crucial for bond formation. The concept of suprafacial and antarafacial interactions is introduced, with examples provided to illustrate how these interactions can lead to the formation of new sigma bonds in the product. The stereospecificity of the Diels-Alder reaction is also discussed, highlighting how the reaction results in the retention of the relative stereochemistry of the reactants in the product.

The script discusses the influence of stereochemistry on the course of reactions, particularly in the context of the Diels-Alder reaction. It explains how the presence of chirality can lead to the preferential formation of one enantiomer over another, a concept that is central to the development of chiral auxiliaries and catalysts in organic synthesis. The importance of controlling the stereochemical outcome of reactions to establish new stereocenters is emphasized, with examples given to illustrate the principles of syn and anti addition in the context of the Diels-Alder reaction.

This section of the script examines 2+2 cycloaddition reactions, contrasting the allowed and forbidden types based on the principles of orbital overlap and symmetry. The lecturer explains why a simple [pi2s + pi2s] cycloaddition between two alkenes is not permitted due to the lack of concurrent orbital overlap. However, an example of a [pi2s + pi2a] cycloaddition involving cyclopentadiene and dichlorocarbene is provided to illustrate how such reactions can occur through an antarafacial mechanism. The unique reactivity and geometry of cumulated systems like dichlorocarbene are also highlighted.

The script introduces the concept of photochemical reactions, explaining how the excitation of electrons from the HOMO to the LUMO can alter the allowedness of certain reactions. Specifically, it discusses how a [pi2s + pi2s] cycloaddition, which is forbidden in the ground state, becomes allowed upon photoexcitation due to the changed orbital interactions. The stereochemical consequences of photochemical reactions are also explored, with examples provided to illustrate how the stereochemistry of reactants can be retained in the products formed under photochemical conditions.

This paragraph provides an overview of sigmatropic rearrangements, a type of pericyclic reaction where a sigma bond migrates across a conjugated system. The lecturer outlines the general concept and provides examples of common sigmatropic rearrangements, such as the Cope and Claisen rearrangements. The term 'sigmatropic' is explained in terms of the number of atoms the sigma bond moves over, with the rearrangement order indicating the distance of migration. The importance of these rearrangements in organic chemistry and their characteristic thermal or photochemical activation is also discussed.

The script delves into the orbital symmetry interactions involved in sigmatropic rearrangements, using the example of a [3,3] sigmatropic rearrangement to illustrate the concept. The lecturer explains how the process can be visualized as involving a pentadienyl anion and a proton, with the sigma bond migration occurring through a suprafacial process. The role of the HOMO and LUMO in facilitating the rearrangement is highlighted, with an emphasis on the importance of orbital overlap and symmetry for the reaction to proceed.

This section discusses the stereochemical implications of sigmatropic rearrangements, emphasizing the direct relationship between the stereochemistry of the reactants and the products. The lecturer explains how specific stereochemical features, such as the configuration of stereocenters or double bonds, are conserved during the rearrangement process. The concept of suprafacial migration and its stereochemical consequences are explored, with examples provided to illustrate the predictability of product stereochemistry based on the reactant configuration.

The script concludes with a discussion of forbidden sigmatropic rearrangements, specifically focusing on the [1,3] sigmatropic rearrangement. The lecturer explains why a suprafacial migration is not allowed in this case, using the concept of orbital overlap and symmetry to illustrate the geometrical impossibility of such a reaction. The unique case of a migrating carbon atom, which can overcome this restriction due to the p-orbital's sign reversal, is also mentioned. The experimental verification of these theoretical concepts and their significance in organic chemistry, leading to a Nobel Prize, is highlighted.