Why use a silicon tether? Intramolecularity offers reduced entropic factors Higher yields Certain reactions that could not be carried out intermolecularly are possible u High Regioselectivity Function of tether length Certain cases offer route to opposite regioisomer obtained from intermolecular variant u Increased Stereoselectivity Function of tether length and steric bulk of alkyl substituents on silicon u Functional Group Protection Serves as protecting group before and after reaction Protodesilylation, Tamao oxidation, allylsilane additions, and transmetallations are possible Diels-Alder To Form 5-Atom Tether Ring 60-170°c favors exo-cyclization ieburth S M. Fensterbank, L.J. Org. Chem. 1992, 57, 5279 33A-023/149611:14PMOH SiR2Cl O Si R R Si O R R Si O R R OH SiR2Cl O Si R R ❏ Increased Reactivity ❏ High Regioselectivity ❏ Increased Stereoselectivity ❏ Functional Group Protection ❏ Facile Refunctionalization H O Si Why use a silicon tether? - Intramolecularity offers reduced entropic factors - Higher yields - Certain reactions that could not be carried out intermolecularly are possible - Function of tether length - Certain cases offer route to opposite regioisomer obtained from intermolecular variant - Function of tether length and steric bulk of alkyl substituents on silicon - Serves as protecting group before and after reaction - Protodesilylation, Tamao oxidation, allylsilane additions, and transmetallations are possible Et3N H O Si 160 - 170 °C + R = Me 2 : 1 R = Ph R = t-Bu 1 : 4 Note: Bulkier substituent favors exo-cyclization Et3N 190 °C + R = Me 4 : 1 R = Ph R = t-Bu >20 : 1 R R R 10 : 1 Sieburth, S. M.; Fensterbank, L. J. Org. Chem. 1992, 57, 5279. 1 : 1 R Diels-Alder To Form 5-Atom Tether Ring: 33A-02 3/14/96 11:14 PM