Molecular complexes, chiral interactions

Hawkins and Loren225 reported simple chiral arylalkyldichloroborane catalysts 352 which were effectively used in the cycloadditions of acrylates lib and 350 to cyclopen-tadiene, affording adducts 351a and 351b, respectively (equation 99). A crystal structure of the molecular complex between methyl crotonate and the catalyst allowed the authors to rationalize the outcome of the reaction. One face of methyl crotonate is blocked by tt-tt donor-acceptor interactions, as becomes clear from the structure of complex 353. The cycloadduct of methyl acrylate and cyclopentadiene (5 equivalents) was obtained with 97% ee, using the same catalyst. Three years later, the authors reported that the cycloadduct was obtained with 99.5% ee in the presence of 10 equivalents of cyclopentadiene226. 

In spite of these problems, induction or disappearance of optical activity in products affords a sensitive measure of the degree to which cholestric liquid-crystalline or other chiral solvents Interact with reacting solutes. When absolute rotations are large, as they are for JBN (30) even small interactions can be detected. On a molecular level, these interactions can be considered dlastereoraeric and arising via differential solvation (52) of the R or form of BN by enantioraerically pure neighbors. The macro order Imposed by a cholesteric phase may result in more complex Interactions. It is these which we seek to probe. 

A. Latini et al., Short range interactions within molecular complexes formed in supersonic beams Structural effects and chiral discrimination. Chem. Eur. J. 6, 1042-1049 (2000)... 

Hawkins et al. described a simple and efficient catalyst for the Diels-Alder reaction based on a chiral alkyldichloroborane (Eq. 15) [16]. A molecular complex between methyl crotonate and the chiral catalyst have been isolated for the first time. A study of the crystal structure of the complex enabled the authors to propose a model predicting the approach of the diene on one of the faces of the methyl crotonate, because the other face is protected by n-n donor-acceptor interactions. This secondary attractive substrate-catalyst interaction is the basis of the stereocontrol. 

Metals in Molecular Biology—Synthesis, Photophysical and Chiral Properties of Tris(1,10-Phenanthroline)Chromium(lll) Metal Complex DNA Interactions and Reactivity... 

Sellergren, B. Molecular imprinting by noncovalent interactions—enantioselectivity and binding-capacity of polymers prepared tmder conditions favoring the formation of template complexes. Makromolek. Chem.—Macromol. Chem. Phys. 1989, 190, 2703-2711. Kempe, M. Mosbach, K. Directed resolution of naproxen on a non-covalently molecularly imprinted chiral stationary phase. J. Chromatogr. 1994, 664, 276-279. 

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