Stereochemistry of reaction paths at carbonyl centres

Results of recent experimental and theoretical studies of nucleophilic addition to carbonyl groups are described. The reaction paths found by different methods for different nucleophiles show some striking similarities that appear to be characteristic for the reaction type.     

An Eclipsed C(sp3)-CH3 Bond in a Crystalline Hydrated Tricyclic Orthoamide: Evidence for C?H…︁O hydrogen bonds

Accurate, low-temperature (81 K) X-ray analyses have been made for two crystalline modifications of the tricyclic orthoamide 1b : a cubic trihydrate in space group Pa3 (Z = 8), where the molecule has crystallographic threefold rotation symmetry, and an anhydrous monoclinic form in space group P2₁/c (Z = 8) where two symmetry-independent molecules have different configurations, one ail-trans. (as in the cubic trihydrate), the other cis, cis, trans. In the cubic trihydrate, each orthoamide molecule is attached to a triad of H₂o molecules by OH…?N H-bonds. A remarkable feature of this structure is the nearly eclipsed conformation about the central C-CH₃ bond. In the anhydrous crystal, both types of molecule have the normal staggered orientation of their Me groups. The reversal of the Me orientation in the trihydrate is attributed to C? H…?O H-bonding, which must be much stronger and more directionally specific than has been previously assumed.     

X-Ray Study of the Deformation Density in Tetrafluoroterephthalodinitrile: Weak Bonding Density in the C,F-Bond

A careful deformation density study of tetrafluoroterephthalodinitrile at 98K has been made from X-ray diffraction measurements. Prominent ‘bonding density’ peaks are found at or near the mid-points of the C, C- and C, N-bonds but not for the C, F-bonds, which show only weak density. Similarly weak bonding densities for C, F-bonds are also found for 1, 1, 4, 4-tetrafluorocyclohexane. The possible significance of these results in terms of bonding theory is briefly discussed.     

Die Kristallstruktur des l, 6-8, 13-Bis-oxido-[14]-annulens

Crystals of 1,6-8, 13-bis-oxido-[14]-annulene are monoclinic, a = 9,29, b = 9,74, c = 11,46 Å, β = 90,0°, space group P 2₁/c, with 4 molecules in the unit cell. The structure has been solved by direct methods and refined by full-matrix least-squares analysis of three-dimensional intensity data. The cis-configuration of the two oxygen bridges is confirmed, and the observed molecular parameters are in full accord with the aromatic properties of the molecule.     

Ammonium cyanate shows N-H...N hydrogen bonding, not N-H...O

The transformation of ammonium cyanate into urea, first studied over 170 years ago by W?hler and Liebig, has an important place in the history of chemistry. To understand the nature of this solid state reaction, knowledge of the crystal structure of ammonium cyanate is a prerequisite. Employing neutron powder diffraction, we demonstrate conclusively that, in the structure of ammonium cyanate, the NH(4)(+) cation forms N-H...N hydrogen bonds to four cyanate N atoms at alternate corners of a distorted cube, rather than our previously proposed alternative arrangement with N-H...O hydrogen bonds to cyanate O atoms at the other four corners.     

Langsame Inversion am pyramidal gebundenen Stickstoff: Kristallstrukturanalyse des trans-N-Methoxy-3, 3-di-methoxycarbonyl-5-cyan-1, 2-oxazolidins

Crystals of two interconvertible, diastereomeric N-methoxy-3, 3-di-methoxy-carbonyl-5-cyano-1, 2-oxazolidines have been examined by X-ray analysis. The lower melting isomer, m.p. 55°, is shown to have trans configuration of the methoxy and cyano substituents. Some features of the molecular topography are discussed.     

Directional Preferences of Approach of Nucleophiles to Sulfonium Ions

An analysis of the crystal environments of sulfonium ions shows that short nonbonded contacts (secondary bonds) to sulfur tend to occur along the extensions of the C-S primary bonds. Non-bonded contacts to the C (a)-atoms of sulfonium ions are not especially short and do not show any striking directional preference, except in sulfonium ions derived from methionine where a specific intramolecular interaction between a carboxyl O-atom and the C (γ)-atom may represent an incipient stage of the internal nucleophilic displacement leading to formation of homoserine and mercaptan.