Chiral Aluminum Lewis Acids in Organic Synthesis

Abstract

The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Ponndorf-Verley reduction of ketones with chiral aluminum alkoxides which resulted in low induction in the alcohol products [1]. Subsequent developments in the area were sparse until over a decade later when chiral aluminum Lewis acids began to be explored in polymerization reactions, with the first report describing the polymerization of benzofuran with catalysts prepared from and ethylaluminum dichloride and a variety of chiral compounds including /3-phenylalanine [2]. Curiously, these reports did not precipitate further studies at the time because the next development in the field did not occur until nearly two decades later when Hashimoto, Komeshima and Koga reported that a catalyst derived from ethylaluminum dichloride and menthol catalyzed the asymmetric Diels-Alder reaction shown in Sch. 1 [3,4]. This is especially curious because the discovery that a Diels-Alder reaction could be accelerated by aluminum chloride was known at the time the polymerization work appeared [5]. Perhaps it was because of this long delay, that the report of this asymmetric catalytic Diels-Alder reaction was to become the inspiration for the dramatic increase in activity in this field that we have witnessed in the twenty years since its appearance. It is the intent of this review to present the development of the field of asymmetric catalytic synthesis with chiral aluminum Lewis acids that includes those reports that have appeared in the literature up to the end of 1998. This review will not cover polymerization reactions or supported reactions. The latter will appear in a separate chapter in this handbook. 15 rnol % catalyst 4 toluene, -78 OC, 3 h 0 2 1.1 equiv 3 69 % yield exo : endo = 98 : 2 72 % ee 1 EtAlC12 . OAIC12 A A 5 4 Scheme 1

Addition Reactions of Me3SiCN with Aldehydes Catalyzed by Aluminum Complexes Containing in their Coordination Sphere O, S, and N Ligands

“…The Lewis acid properties of organic aluminum compounds are well established in organic chemistry. They are responsible for a plethora of organic reactions, polymerizations, and catalytic cycles . Among those compounds, Lewis acidic derivatives prepared from AlCl 3 are primarily used, although poor selectivity and control of reactivity are often the major issues in using these compounds .…”

Synthesis and Characterization of Dialkylaluminum Amidates and Their Ring-Opening Polymerization of ε-Caprolactone

“…Alkylaluminum compounds, which tend to be highly air sensitive, are very important reagents widely employed in organic synthesis, as cocatalysts in olefin polymerization, and as initiators in the ring-opening polymerization (ROP) of cyclic esters or copolymerization of CO 2 and epoxides . Recently, the reactivity and catalytic behavior of alkylaluminum complexes have been extensively reported .…”

Isolation and Chemistry of Aluminum Compounds Containing (S)-3,3‘-Bis(triphenylsilyl)-1,1‘-bi-2,2‘-naphthoxide Ligands

“…Significant structural and chemical studies have also been reported from the Barron group . A continuing challenge centers on the development of chiral analogues for the synthesis of optically pure compounds . A series of aluminum reagents containing C 2 -symmetric, chiral aryloxides were also synthesized and have been applied to asymmetric Claisen rearrangements, 2 aldol reactions, and aldehyde alkylations.…”