| Abstract
| - Computed enthalpies of formation for various Lewis acid complexes with representative unsaturatedcompounds (aldehydes, imines, alkynes, and alkenes) provide a means to evaluate the applicability of aparticular catalyst in a catalytic reaction. As expected, main group Lewis acids such as BX3 show muchstronger complexes with heteroatoms than with carbon−carbon multiple bonds (σ-electrophilic Lewisacids). Gold(I) and copper(I) salts with non-nucleophilic anions increase the relative strength of coordinationto the carbon−carbon multiple bonds (π-electrophilic Lewis acids). As representative examples for theuse of σ-electrophilic Lewis acids in organic synthesis, the Lewis acid mediated allylation reactions ofaldehydes and imines with allylic organometallic reagents which give the corresponding homoallyl alcoholsand amines, respectively, are mentioned. The allylation method is applied for the synthesis of polycyclicether marine natural products, such as hemibrevetoxin B, gambierol, and brevetoxin B. As representativeexamples for the use of π-electrophilic Lewis acids in organic synthesis, the Zr-, Hf-, or Al-catalyzedtrans-stereoselective hydro- and carbosilylation/stannylation of alkynes is mentioned. This method isextended to σ−π chelation controlled reduction and allylation of certain alkynylaldehydes. Gold- andcopper-catalyzed benzannulation of ortho-alkynylaldehydes (and ketones) with alkynes (and alkenes) isdiscovered, which proceeds through the reverse electron demand Diels−Alder type [4 + 2] cycloadditioncatalyzed by the π-electrophilic Lewis acids. This reaction is applied for the short synthesis of (+)-ochromycinone. Palladium and platinum catalysts act as a σ- and/or π-electrophilic catalyst dependingon substrates and reaction conditions.
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