Microwave-Assisted Enantioselective Heck Reactions: Expediting High
Reaction Speed and Preparative Convenience

Abstract

Palladium-catalyzed asymmetric Heck reactions have been performed using high temperature conditions with the aid of controlled microwave dielectric heating. Significant enantioselectivities of up to 92% ee were obtained under non-inert conditions utilizing the previously reported thermostable palladium-phosphine­oxazoline catalytic system. With microwave irradiation, reaction times of hours instead of days were obtained. Enantiomeric purities and conversions were found to be strongly dependent on both the choice of the aryl triflate, the reaction temperature, the solvent and the base. Examples of asymmetric arylations of electron-rich as well as neutral cyclic alkenes are presented.

Key words

asymmetric catalysis - Heck reaction - arylations - regio­selectivity - palladium

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5

The generic name of this base is 2,8-bis(dimethylamino)naphthalene.

6

In THF, diisopropylamine gave an optimized ee of 94% (120 °C, 7 h), but the reaction suffered from incomplete conversion of 1a. With proton sponge as the base the best ee in THF was 93% (120 °C, 12 h).

9

Alternatively product 5a (Ar = Ph) may be formed by a trans Pd-β-hydride elimination from the depicted intermediate σ-complex (Figure [3] ).