Chiral ureas with two electronegative substituents at 1-N: an unusual case of coexisting pyramidal and almost planar 1-N atoms in the same crystal.

XRD studies of structure of N-acetoxy-N-methoxyurea and N,N-bis(methoxycarbonyl)-N-methoxyimide have revealed that in N-methoxy-N-X-ureas (X = OAc, Cl, OMe, N(+)C(5)H(5)) the additional shortening of N-OMe bond took place, which arising from an n(O(Me))-sigma*(N-X) anomeric orbital interaction. XRD studies of N-chloro-N-ethoxyurea crystal have revealed the presence of two kinds of anomeric nitrogen configuration in the O-N-Cl group in the form of a pyramidal configuration and a planar configuration for same 1-N nitrogen atom. XRD studies of N-4-chlorobenzoyloxy-N-ethoxyurea have revealed that the degree of pyramidality of the 1-N nitrogen in N-aroyloxy-N-alkoxyureas is tuned by orientation of benzoyl group with respect to the N-O bond, which in turn depends of size of N-alkoxy group.

Racemic compound against racemic conglomerate formation: the crystal properties of allylbenzylmethylphenylphosphonium iodide as compared with the nitrogen analogue.

Although racemic allylbenzylmethylphenylammonium iodide 1b displays spontaneous resolution crystallizing in a homochiral mode as a conglomerate, its phosphonium analogue rac-2b crystallizes in a heterochiral lattice forming racemic compound in the solid state. The more pronounced conformation freedom for 2b molecules manifests itself by multiple molecules crystallization (Z' = 3) of (S)-2b with the three independent molecules having different conformation.

Bis-ortho-substitution by methyl groups dramatically increases the racemization barrier of Tröger bases.

We have shown through racemization kinetics studies that the enantiomerization barriers of the bis-ortho-methyl substituted Tröger bases 2 and 3 in acidic media are raised by 30 kJ mol(-1) relative to the parent compound 1, that is 130.4(4) and 131.6(4) kJ mol(-1), respectively (105 degrees C, pH 1, ethylene glycol). The enantiomerization barrier of para-methoxy-para-nitro substituted Tröger base 4 was determined by dynamic capillary electrophoresis to 96.3(2) kJ mol(-1) (25 degrees C, pH 2.2, H(2)O), which is lower by 5 kJ mol(-1) relative to 1. The influence of deutero-substitution on the racemization rates was also studied. The influence of steric and electronic factors on the enantiomerization barrier was investigated by quantum-mechanical (DFT) calculations. It is shown that enantiomerization takes place in two steps: ring-opening and further interconversion of the monocyclic intermediate. For the interconversion to occur a transition state has to be passed which is sensitive to steric effects. Ortho-substitution by methyl groups significantly increases the energy of this state. Thus, compounds 2 and 3 are the simplest Tröger bases which are configurationally stable in acidic media.

Solid-state ESR differentiation between racemate versus enantiomer.

Noticeable differences between solid-state ESR spectra of enantiopure and racemic single crystals as well as powders of 2,2,5,5-tetramethylpyrrolidine-3-carboxamide-1-oxyl have been observed. Possible reasons of this fact are proposed.