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Resonance structures of normal-demand dienes and dienophiles In general, the regioselectivity found for both normal and inverse electron-demand Diels–Alder reaction follows the ortho-para rule , so named, because the cyclohexene product bears substituents in positions that are analogous to the ortho and para positions of disubstituted arenes.
Release of nitrogen from six-membered, cyclic diazenes is common and often spontaneous at room temperature. Such a reaction can be utilized in click reactions where alkanes react with a 1,2,4,5-tetrazine in a diels alder then retro diels alder reaction with the loss of nitrogen. In this another example, the epoxide shown undergoes rDA at 0 °C.
The imine is often generated in situ from an amine and formaldehyde.An example is the reaction of cyclopentadiene with benzylamine to an aza norbornene. [9]The catalytic cycle starts with the reactions of the aromatic amine with formaldehyde to the imine and the reaction of the ketone with proline to the diene.
Other common classes of dienes are oxo- and aza- butadienes. [9] [11] The key quality of a good DA INV diene is a significantly lowered HOMO and LUMO, as compared to standard DA dienes. Below is a table showing a few commonly used DA INV dienes, their HOMO and LUMO energies, and some standard DA dienes, along with their respective MO energies.
The central ring can serve as a diene in Diels–Alder reactions with various dienophiles. For example, reaction with benzyne leads to 1,2,3,4-tetraphenylnaphthalene and reaction with diphenylacetylene leads to hexaphenylbenzene. [5] In this way, it is a precursor to graphene-like molecules, [6] such as coronene.
In the presence of very reactive dienes (for example 1,3-diphenylisobenzofuran) butadienesulfone behaves as a dienophile and forms the corresponding Diels-Alder adduct. [ 21 ] As early as 1938, Kurt Alder and co-workers reported Diels-Alder adducts from the isomeric 2-sulfolene with 1,3-butadiene and 2-sulfolene with cyclopentadiene .
Conjugated dienes are more stable than other dienes because of resonance. Unconjugated dienes have the double bonds separated by two or more single bonds. They are usually less stable than isomeric conjugated dienes. This can also be known as an isolated diene. Some dienes: A: 1,2-Propadiene, also known as allene, is the simplest cumulated diene.
Again because of their electrophilic character, the alkene portion of α,β-unsaturated carbonyls is good dienophiles in Diels–Alder reactions. They can be further activated by Lewis acids, which bind to the carbonyl oxygen. α,β-Unsaturated carbonyls are good ligands for low-valent metal complexes, examples being (bda)Fe(CO) 3 and tris ...