Carbene and Silicon Routes as Methods for the Generation and Dipolar Cycloaddition Reactions of Methyl Nitrile Ylide

摘要

Methyl nitrile ylide was generated by both a photochemical and desilylation route and has been characterized by UV spectroscopy and by its kinetic and cycloaddition behavior. The dipole is formed by the addition of standard dipolarophiles. The relative rates of formation of 1,3-di polar adducts and the relative rates of quenching of the transient absorption are in excellent agreement. Generation of the dipole from a silylthioimidate followed by dipole cycloaddition also gave rise to related cycloadducts. The ratio of cycloadducts obtained from an unsymmetrically substituted dipolarophile was found to be strikingly dependent on the purity of the silylthioimidate. The different product ratios result from the operation of an alternate mechanism which involves the thiophenol that is released in the decomposition of silylthioimidate to the nitrile ylide. The initially generated dipole is believed to react with excess thiophenol to give a half-capped dipole which undergoes a subsequent conjugate addition to the unsymmetrical dipolarophile.