Hydroalumination Cross Coupling Chemistry Using Enynols & Uncatalyzed Diels-Alder Reactions of Siloxacyclopentenes Prepared from Hydrosilylation of Enynols
The purpose behind this metal diene is three fold. First, a large metal helps keep the diene in the reactive s-cis confirmation, a requirement for a Diels-Alder reaction. Second, the presence of a bulky metal can also help control the diene and dienophile from generating the endo Diels-Alder product. The endo product is traditionally the most favored product due to the fact it results from pi bond overlap between the diene and dienophile 0.2. The endo product also results in a syn addition cycloadduct 0.3. By blocking the endo approach using a bulky metal a synthetically viable pathway to the exo product is achieved 0.4, resulting in the anti addition product 0.5 (see Figure 0.2). The traditional diene 0.2 allows the dienophile access to its pi orbitals, resulting in an endo product 0.3. However the diene 0.4 that has a metal attached (if the metal is bulky enough) can block the dienophile resulting in the exo product 0.5. Third, by placing a metal on the diene, one of the terminal carbons (1 or 4) becomes more electronegative than the other thereby helping to direct regiochemistry.*
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