From σ- to π-Electrophilic Lewis Acids. Application to Selective Organic Transformations

Abstract

Computed enthalpies of formation for various Lewis acid complexes with representative unsaturated compounds (aldehydes, imines, alkynes, and alkenes) provide a means to evaluate the applicability of a particular catalyst in a catalytic reaction. As expected, main group Lewis acids such as BX3 show much stronger complexes with heteroatoms than with carbon-carbon multiple bonds (sigma-electrophilic Lewis acids). Gold(I) and copper(I) salts with non-nucleophilic anions increase the relative strength of coordination to the carbon-carbon multiple bonds (pi-electrophilic Lewis acids). As representative examples for the use of sigma-electrophilic Lewis acids in organic synthesis, the Lewis acid mediated allylation reactions of aldehydes and imines with allylic organometallic reagents which give the corresponding homoallyl alcohols and amines, respectively, are mentioned. The allylation method is applied for the synthesis of polycyclic ether marine natural products, such as hemibrevetoxin B, gambierol, and brevetoxin B. As representative examples for the use of pi-electrophilic Lewis acids in organic synthesis, the Zr-, Hf-, or Al-catalyzed trans-stereoselective hydro- and carbosilylation/stannylation of alkynes is mentioned. This method is extended to sigma-pi chelation controlled reduction and allylation of certain alkynylaldehydes. Gold- and copper-catalyzed benzannulation of ortho-alkynylaldehydes (and ketones) with alkynes (and alkenes) is discovered, which proceeds through the reverse electron demand Diels-Alder type [4+2] cycloaddition catalyzed by the pi-electrophilic Lewis acids. This reaction is applied for the short synthesis of (+)-ochromycinone. Palladium and platinum catalysts act as a sigma- and/or pi-electrophilic catalyst depending on substrates and reaction conditions.