Development of a new molecular recognition chemistry using Lewis acidic receptors

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The fascination of molecular recognition chemistry is extending continuously, especially as host-guest systems which mimic biologically occurring compounds exhibiting catalytic activities like enzymes. The discovery of crown ethers and cryptands and their inclusion systems by Prof. Lehn and Cram led them to be the winners of the Nobel Prize, and most of these systems are constituted by hydrogen-bonding networks between host and guest molecules. Comparing with these weak interactions, relatively strong coordination bonding between Lewis acidic receptors and Lewis bases has received less attention for molecular recognition, and has never been used for selective organic synthesis.In this context, we have developed several bulky organoaluminum reagents which have sterically hindered phenoxide ligands. These bulky aluminum reagents are monomeric in solution, thereby possess strong Lewis acidity to bind a variety of heteroatom-containing substrates such as carbonyl, carboxyl and amide compounds. The complexation is sometimes stereo-, regio-, and chemoselective, leading to selective organic reactions which could not be realized with other ordinary Lewis acicls such as AlCl_3, SnCl_4, BF_3, and TiCl_4. In particular, aluminum tris(2,6-diphenylphenoxide(ATPH) was explored recently for obtaining unprecedented reactivity and selectivity n several reactions. 1. Conjugate addition of carbanions to alpha, beta-unsaturated aldehydes 2. Exo-selective Diels-Alder reactions 3. Stereo-and asymmetric Claisen rearrangement 4.1,6-Addition to aromatic carbonyl substrates.