Designer Lewis Acids for Selective Organic Synthesis Hisashi Yamamoto, Keiji Maruoka, Kazuaki Ishihara School of Engineering, Nagoya University, Abstract: In view of the ever expanding repertoire of Lewis acids available as proton substitutes in current synthetic organic methodology, our goal was to engineer an artificial proton substitute possessing unique topology; effective as an artificial enzymefor chemical reactions, by harnessing the high reactivity of the metal atom towards oxygen and nitrogen. Such a concept was realized by examining the recognition ability of specially designed metal receptors for various oxygen- and nitrogen-containing substrates. 1. Introduction An enzyme is typically a large molecule, large enough to support the substrate, whereas chemical reagents are composed of much smaller molecular makeup. Still, the much smaller molecular apparatus of commercial chemical reagents is expected to induce reactions with selectivities comparable to that of a large enzyme. These set of observations therefore beg the question: are enzymatic reactions really an appropriate catalysis system for laboratory chemical reactions? A case in point is the important role of hydrogen bonding during enzymatic reactions. In the course of such processes, the giant template of the enzyme will specify quite accurately the position and direction of a proton for hydrogen bonding, before and after the reaction. However, a proton by itself cannot behave in this fashion. As a perfect sphere, it has no directional selectivity for hydrogen bonding outside the domain of the enzyme, thus it is unable act as a "delicate finger" in an ordinary organic reaction, as it does in the enzymatic transformation. Therefore obliviously one can suspect whether an appropriate substitute for the proton might induce chemical reactions capable of selectivities comparable to those afforded by enzymes. An excellent candidate as a proton substitute is a Lewis acid. The observation that main group organometallic compounds have high reactivity, inspired us to devise a new series of reagents based on these metals, viz.: novel "designer Lewis acids" for organic synthesis. Since an organometallic compound would have multiple ligands around the metal, the structural design of such a catalyst could be quite flexible. Our goal, then, was to engineer an artificial proton of a specific shape, which could be utilized as an effective tool for chemical reactions, by harnessing the high reactivity of the metal atom towards oxygen and nitrogen (ref. 1). Such a concept was initially studied by examining the influence of a specially designed Lewis acids on a typical organic reaction: the nucleophilic addition to a carbonyl substrate. In 1985, we reported enantioselective cyclization of prochiral unsaturated aldehydes using chiral zinc reagent 1 derived from dimethylzinc and enantiomerically pure (R)-binaphthol (ref. 2). Although the reagent 1 is rather unstable, the system is flexible and has important potential for further design in asymmetric metal catalysis. 912 ( 40 ) J. Synth. Org. Chem., Jpn. From this pioneering work, there is no doubt that the carefully designed chiral Lewis acid may have vast potential for the asymmetric synthesis of carbon skeletons. Choice of an appropriate metal and design of suitable chiral ligands may be the most important factors for effective catalysis. Thus herein is a report on the design of chiral Lewis acids conducted in our laboratory since 1985. 2. Chiral Aluminum Reagent In view of the well-established capacity of aluminum reagents to enhance the reaction rate and selectivity of various organic reactions (ref. 3), the utilization of a chiral aluminum catalyst should, in principle, lead to asymmetric induction. However, until recently little work has appeared in the area of asymmetric synthesis using chiral aluminum reagents (ref. 4). The first reliable chiral aluminum reagents of types (R)-2 and (S)-2 were devised for enantioselective activation of carbonyl groups based on the concept of diastereoselective activation of carbonyl moieties with the exceptionally bulky organoaluminum reagents, namely MAD and MAT (ref. 5). The sterically hindered, enantiomerically pure (R)-(+)-3,3'-bis(triarylsilyl)binaphthol (R)-3 required for the preparation of (R)-2 can be synthesized in two steps from (R)-(+)-3,3'- dibromobinaphthol by bis-triarylsilylation and subsequent intramolecular 1,3-rearrangement of the triarylsilyl groups as shown in Scheme 1 (ref. 6). Reaction of (R)-3 in toluene with trimethylaluminum Scheme 1 (R)-2 (S)-2 (R)-3 produced the chiral organoaluminum reagent (R)-2 quantitatively. Its molecular weight, found cryoscopically in benzene, corresponds closely with the value calculated for the monomeric species 2. The modified chiral organoaluminum reagents, (R)-2 and (S)-2 were shown to be highly effective as chiral Lewis acid catalysts in the asymmetric hetero Diels-Alder reaction (ref. 7). Reaction of various aldehydes with activated dienes under the influence of a catalytic amount of 2 (5-10 mol%) at -20 •Ž, after exposure of the resulting hetero Diels-Alder adducts to trifluoroacetic acid, gave predominantly cis- dihydropyrone 5 in high yield with excellent enantioselectivity. The enantioface differentiation of 4 5 Vol.52, No.11 (November 1994) ( 41 ) 913 prochiral aldehydes is controllable by judicious choice of the size of trialkylsilyl moiety in 2, thereby allowing the rational design of the catalyst for asymmetric induction. In fact, switching the triarylsilyl substituent (Ar = Ph or 3,5-Xylyl) to the tert-butyldimethylsilyl or trimethylsilyl group led to a substantial loss of enantio as well as cis selectivity in the hetero Diels-Alder reaction of benzaldehyde and activated diene 4. In marked contrast, the chiral organoaluminum reagent derived from trimethylaluminum and (R)-(+)-3,3'-dialkylbinaphthol (alkyl = H, Me, or Ph) could be utilized, but only as a stoichiometric reagent and results were disappointing both in terms of reactivity and enantioselectivity for this hetero Diels-Alder reaction. An interesting method for the preparation of chiral aluminum reagents has appeared recently. The chiral organoaluminum reagent, (R)-2 or (S)-2 can be generated in situ from the corresponding racemate (±)-2 by diastereoselective complexation with certain chiral ketones (Scheme 2) (ref. 8). Among several terpene-derived chiral ketones, 3-bromocamphor was found to be the most satisfactory. The hetero Diels-Alder reaction of benzaldehyde and 2,4-dimethy1-1-methoxy-3-trimethylsiloxy-1,3- butadiene (4) with 0.1 equiv of (±)-2 (Ar = Ph) and d-bromocamphor at -78 •Ž gave rise to cis-adduct 5 as the major product with 82% ee. Although the level of asymmetric induction attained does not yet match that acquired with the enantiomerically pure 2 (Ar = Ph, 95% ee), one recrystallization of the cis- adduct 5 of 82% ee from hexane gave essentially enantiomerically pure 5, thereby enhancing the practicality of this method. This study demonstrates the potential for broad application of the in situ generated chiral catalyst via diastereoselective complexation in asymmetric synthesis. Scheme 2 (S)-2 (R)-2 Since the enantioselective activation of carbonyl with the chiral aluminum, (R)-2 or (S)-2, had been demonstrated, the asymmetric ene reaction of electron-deficient aldehydes with various alkenes, by employing the latter reagent, could also be considered a feasible transformation (ref. 9). Indeed in the presence of powdered 4A molecular sieves, the chiral aluminum reagent, (R)-2 or (S)-2 can be utilized as a catalyst without any loss of enantioselectivity. The concept of the enantioselective activation of carbonyl groups with the bulky, chiral aluminums, (R)-2 or (S)-2 has been further extended to the enantioselective activation of an ether 914 ( 42 ) J. Synth. Org. Chem., Jpn. oxygen, which gave rise to the first successful example of the asymmetric Claisen rearrangement of allylic vinyl ethers 6 catalyzed by (R)-2 or (S)-2 (Scheme 3) (ref. 10). This method provides a facile asymmetric synthesis of various acylsilanes 7 or 8 (X = SiR3) and acylgermane 7 (X = GeMe3) with high enantiomeric purity (Table 1). Among the various trialkylsilyl substituents of 2, use of a bulkier t- butyldiphenylsilyl group gives rise to the highest enantioselectivity. Conformational analysis of two possible chairlike transition-state structures of an ally! vinyl ether substrate 6 reveals that the chiral organoaluminum reagent 2 can discriminate between these two conformers, A and B, which differ each other only in the orientation of a-methylene groups of ethers. Scheme 3 A 7 6 B 8 Table 1. Asymmetric Claisen Rearrangement of Allylic Vinyl Ethers 6 Notably, the asymmetric Claisen rearrangement of cis-allylic a-(trimethylsilyl)vinyl ethers with (R)-2 produced optically active acylsilanes with the same absolute configuration as those produced from trans-allylic a-(trimethylsilyl)vinyl ethers (ref. 11). Vol.52, No.11 (November 1994) ( 43 ) 915 3. Chiral Titanium Reagent A new type of chiral helical titanium catalyst of type 9 has been rationally designed with the expectation that a high level of asymmetric induction can be achieved by way of an efficient enantioface recognition of prochiral substrates using a fixed helical conformation of a chiral ligand. Such an idea originates from a characteristic helix conformation found in various naturally occurring substances, such as the secondary structures of DNA, polypeptides (proteins and collagens), and starch. (P)-9 (M)-9 The