Preorganized receptor

The doubly functionalized calix[4]arene 9, bearing two chiral arms , was prepared recently [48]. This neutral system proved not only to be a good receptor for a-phenylglycine anions, it was also found to display good enantioselectivity towards the L-isomer fCa(L)//< l(D) = 4.76. It was thus proposed by the authors, that this presumably preorganized receptor could function as a chromogenic sensor for nonracemic a-phenylglycine anions. 

Enantioselective recognition between chiral hosts and guests is of great interest. The required difference in stability between diastereoisomeric complexes often relies on three-point interactions between a well-preorganized receptor and the substrate. Hydrogen bonding as a directional interaction is of particular importance in this context, often in combination with steric host-guest complementarity. 

H-binding sites like C(0)-NH fragments in a preorganized receptor molecule should increase the selectivity and efficiency of anion complexation. Using this approach a novel class of neutral anion receptors that contain a unique combination of an immobilized Lewis acidic binding site (U02 ) and the presence of additional amide C(0)-NH groups which can form a favorable H-bond with a coordinated anion guest, has been developed [ref 27 and 28]. 

Fig. 4. Recognition and preorganization of hosts (receptors) on complex formation (21).

From the kinetic point of view the facts are different and the order is reverse, ie, the rigid highly preorganized spherands are slow, as contrasted with the flexible barely preorganized podands that are fast both in formation and decomposition of the receptor—substrate (host—guest) complex (20,21). 

Klein Gebbink, R.J.M., Martens, C.F., Feiters, M.C., Karlin, K.D. and Nolte, R.J.M. (1997) Novel molecular receptors capable of forming Cu-2-0-2 complexes. Effect of preorganization on 0-2 binding. Chem. Commun., 389-390. 

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