Conformational interconversion hydrogen bonding

The principle of the method is that once a substance crystallizes, the interconversion of the various conformers stops and so the crystals correspond to one discrete conformation or another. This has established, that 1,2-dichloroethane crystallises exclusively in the anti form, while ethylene chlorohydrin crystallizes in a gauche form because of intra molecular hydrogen bonding. But when the molecule crystallizes in two or more conformations, we say that it shows polymorphism. 

These steric interactions become more pronounced when we consider the introduction of an additional chelate ring in those complexes containing three didentate en ligands. The A XkX form of an [M(en)3]n+ cation is estimated to be 7-8 kJ mol-1 more stable than the A 888 diastereomer. This becomes particularly important when we consider kinetically inert complex cations, such as [Co(en)3]3+, where there is a significant barrier to the interconversion of the diastereomers. In practice, the conformation of the chelate rings in [Co(en)3]3+ salts depends upon the nature of both the anions and any additional solvent molecules in the lattice which can form hydrogen bonds with the en ligands. We will return to this topic in Chapter 7, where we discuss some reactions of [Co(en)3]3+ salts in which an extraordinary steric control is exerted. 

A C2-symmetrical conformation stabilized by intramolecular S=0—H-O hydrogen bonds was also found in C2v-symmetrical tetratosylates 105b as the energy minimum (AG =11.6 kcal mol 1 at 247 K in CDC13 for the interconversion C2 —> C2).49... 

The parent calix[4]arenes are (exclusively) found in the so-called cone conformation, where all the OH groups point in one direction which is therefore stabilized by an intramolecular array of hydrogen bonds. From variable temperature NMR studies (showing at low temperature a pair of doublets and at high temperature a singlet for the protons of the methylene bridges) the energy barrier. AG for the interconversion between two... 

Methylene-bridged calrx[4]resorcinols (for synthetic reasons derived from 2-aUsylresorcinols) assume a cone conformation at lower temperatures, and AG = 12.0 kcalmoU was found at 298 °C for the cone cone interconversion . This lower value in comparison to those for calixphenols like 2a reflects the fact that no substituents have to pass the annulus during the ring inversion. In addition, the intramolecular hydrogen bonds between exo-OH groups are distinctly weaker than those of the cyclic array of endo-OH groups in 2a as shown by NMR (5 = 6.30 vs. 10.2) and IR (v = 3420 vs. 3140 cm- ). 

The (j,Y-bidentate chromium(m).ATP complex (74 M = Cr +) has been separated into four diastereoisomers by reverse-phase h.p.l.c. The six-membered rings of the A and A forms are puckered, with the adenylate able to adopt an axial conformation (which is stabilized by transannular hydrogen bonding) or an equatorial conformation, in each. CD spectroscopy was used to identify the screw sense of the resolved isomers, and the interconversion rates measured, the axial-equatorial interconversion rates for the A and A isomers being more rapid than the A-A isomerization. Both (74 M = Cr +) and the corresponding... 

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