Aluminum Catalysts from Chiral Alcohols

The first examples of an asymmetric Diels-Alder reaction of a non-chiral diene and a dienophile catalyzed by a chiral Lewis acid were reported by Koga and coworkers in 1979 (Sch. 1 and 16) [3]. The catalysts 4,142 and 143 were prepared from (-)-menthol, (+)-neomenthol and (+)-borneol. The reaction of methacrolein and cyclopentadiene mediated by catalyst 4 gave a 98 2 mixture of exo to endo products and upon separation of these diastereomers by chromatography the exo product 3 was obtained in 69 % yield and 72 % ee. The exo .endo ratios for the other reactions in Sch. 16 were not reported. Low asymmetric induction was observed for acrolein and methyl acrylate with all three catalysts. Moderate induction was observed in the reaction of methacrolein with catalyst 4, and with catalyst 142, but in the latter the enantiomer of 3 was the predominant product. The reaction of methyl acrylate with cyclopentadiene mediated by 10 mol % catalyst 4 was also reported by Kobayashi, Matsumura and Furukawa to give the cycloadduct 141 in 2.9 % ee at 30 °C [37]. These workers also reported that catalyst 4 will give optically active product from the reaction of cyclopentadiene and acrylonitrile, although the optical yield was not determined. 

A few other reactions have been reported with catalysts 4 and 143 these are shown in Sch. 17. Northcott and Valenta investigated the reaction of the exo-cyclic methyle-nedienophiles 159 and 161 and the benzoquinone 163 with cyclopentadiene [39]. The absolute configurations of the products were not determined for these three reactions but it was determined that eatalyst 4 and catalyst ent-143 gave the same major enantiomer of 164 but different major enantiomers of 160. 

As has been seen in the study of Koga and coworkers [3,38] (Sch. 16), the catalyst derived from menthol was superior to that derived from borneol. Kobayashi and coworkers reported early on that 1.0 equiv. catalyst 4 at 60 °C would effect the cycload- 

An alternative model proposed by Northcott and Valenta involves an s-cis conformation of the dienophile as indicated in structure 171 [39]. Shielding of the re face by the iso-propyl group requires a change in the coordination of aluminum to the dienophile oxygen such that the aluminum is coordinated by the oxygen on the same side as the carbon substituent of the aldehyde. This model is consistent with the observation of high induction with dienophiles 159 and 161 in which the conformation is locked s-cis compared with that for dienophile 163 which is locked s-trans (Sch. 17). A combined quantum mechanics-molecular mechanics study has been performed on the 

2 Aluminum Catalysts from Chiral Diols, Sulfonamides, and j8-Hydroxy-sulfonamides 

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The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Poimdorf-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 /5-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. 

The first report of an aldol reaction mediated by a chiral aluminum Lewis acid described catalysts generated from the pinene diol 12 and the bornane amino alcohol 14 [7], The catalysts were generated by the reaction of each substrate with diethylalu-minum chloride and were evaluated in the Muikaiyama aldol reaction of the ketene acetal 7 with wo-valeraldehyde. The most successful of the two was catalyst 13 which gave the aldol adduct 8 in 66 % enantiomeric excess (ee) but only in 15 % yield with 20 mol % catalyst. Very recently, this reaction has been re-investigated with catalysts generated from type 16 bornane diols [7], The aldol reaction of ketene acetal 10 with dihydrocinnamaldehyde gave the adduct 11 in 58 % ee and 50 % yield with 100 mol % catalyst (Sch. 2). 

Kobayashi et al. developed chiral Lewis acids derived from A -benzyldiphenylproli-nol and boron tribromide and used these successfully as catalysts in enantioselective Diels-Alder reactions [89]. The corresponding polymeric catalyst 71 was prepared and used for the Diels-Alder reaction of cyclopentadiene with methacrolein [90]. Different polymeric catalysts 72, 73, 74 were prepared from supported chiral amino alcohols and diols fimctionalized with boron, aluminum and titanium [88,90]. In these polymers copolymerization of styrene with a chiral auxiliary containing two polymerizable groups is a new approach to the preparation of crosslinked chiral polymeric ligands. This chiral monomer unit acts as chiral ligand and as a crosslink. 

Asymmetric, borane-modified MPV reduction of a variety of aromatic ketones to their corresponding alcohols has recently (43) been reported using a chiral aluminum alkoxide catalyst shown in Figure 5. This compound was formed in situ from aluminum isopropoxide and (R)-l,r-binapthyl-diol in... 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

As an early example of asymmetric aldol reaction mediated by chiral aluminum Lewis acids, Fujisawa and Shimizu reported the use of several catalysts generated from a series of chiral bornane type diols or the related amino alcohols (Scheme 6.22)... 

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