t. Chirality and Chiral Homogeneity in Nature 2. Biotic Theories 3. Abiotic Theories 3.1. Chance Mechanisms 3.1.1. Models for Spontaneous Symmetry Breaking 3.1.2. Spontaneous Resolution on Crystallization 3.1.3.Asymmetric Syntheses in Chiral Crystals 3.1.4. Asymmetric Adsorption and Catalysis on Quartz 3.1.5.Asymmetric Adsorption and Polymerization on Clays 3.1.6. Conclusions 3.2. Determinate Mechanisms 3.2.1. Regional and/or Temporal Processes 3.2.1.1.Electric, Magnetic and Gravitational Fields 3.2.1.2.Circularly Polarized Eight 3.2.1.2.1.Asymmetric Photoequilibration 3.2.1.2.2.Photochemical Asymmetric Synthesis 3.2.1.2.3.Asymmetric Photolysis 3.2.1.2.4.Sources of Circularly Polarized Eight 3.2.2. Universal Processes 3.2.2.l.The Violation of Parity 3.2.2.2.The Vester-Ulbricht (V-U) ttypothesis 3.2.2.3.Radioracemization 3.2.2.4.Direct Effects of Chiral Particles 3.2.2.4.1 .Electrons 3.2.2.4.2.Protons 3.2.2.4.3.Positrons and Muons 3.2.2.5.Weak Interactions, Neutral Currents and Parity Violating Energy Differences 3.2.3. Conclusions 4. The Amplification of Enantiomeric Excesses

The origin and amplification of biomolecular chirality

In 1836 the first organoselenium compound, diethyl selenide, was prepared by Löwig,1 and it was isolated in the pure form in 1869.2 Early selenium chemistry involved the synthesis of simple aliphatic compounds such as selenols (RSeH), selenides (RSeR), and diselenides (RSeSeR); however, because of their malodorous nature, these compounds were difficult to handle. This, combined with the instability of certain derivatives and difficulties in purification, meant that selenium chemistry was slow to develop. By the 1950s, the number of known selenium compounds had increased significantly, but it was not until the 1970s, when several new reactions leading to novel compounds with unusual properties were discovered, that selenium chemistry began to attract more general interest.3-9 Aryl-substituted compounds were synthesized that were found to be less volatile and more pleasant to handle than the earlier aliphatic compounds. Compounds containing selenium in high oxidation states are relatively easy to manipulate using modern techniques.4c Organoselenium chemistry has now become a well-established field of research, and recent advances have been brought about by the potential technical applications of selenium compounds. Today selenium compounds find application in many areas including organic synthesis,4 biochemistry,5 xerography,6 the synthesis of conducting materials7 and semiconductors,8 and ligand chemistry.4c,9 Many of these aspects of selenium chemistry are wellcovered elsewhere in the literature; however, the subject of hypervalency has not attracted much attention and is the focus of this review.10

Organoselenium chemistry: role of intramolecular interactions.

Origins of Chiral Homogeneity in Nature

Big and little Meccano

The synthesis and characterization of homoleptic zinc(II), cadmium(II), and mercury(II) selenolates incorporating the intramolecularly chelating oxazoline ligand are described. The derivatives, M[Se(Ox)](2) [M = Zn (1), Cd (2), Hg (3); Ox = 2-(4,4-dimethyl-2-oxazolinyl)benzene], were prepared in good yield via the metathesis reactions of MCl(2) with a lithium areneselenolate, OxSe(-)Li(+). The mercury complex 3 was also synthesized by treating the corresponding diselenide with elemental mercury. The complexes are quite stable and highly soluble in common nonpolar organic solvents. X-ray diffraction results show that the complexes are monomeric in the solid state. The geometry around the metal ion in all complexes is found to be distorted tetrahedral. The crystal structure of Zn[Se(Ox)](2) (1) shows that the complex is "helically" chiral and enantiomerically pure. The spontaneous splitting of the racemates indicates the solubility differences between the racemates and pure enantiomers. (1)H, (13)C, and (77)Se NMR measurements indicate that complex 1 retains its "helical" structure in solution. Crystal data (Mo Kalpha; 293(2) K) are as follows: 1, monoclinic space group P2(1), a = 9.3900(12) A, b = 11.618(2) A, c = 10.8822(14) A, beta = 98.245(8) degrees, Z = 2; 2, orthorhombic space group Pbca, a = 12.777(6) A, b = 17.841(10) A, c = 21.010(8) A, Z = 8; 3, monoclinic space group P2(1)/c, a = 9.087(2) A, b = 11.889(2) A, c = 22.456(4) A, beta = 98.780(13) degrees, Z = 4.

Synthesis and Structural Characterization of Monomeric Selenolato Complexes of Zinc, Cadmium, and Mercury.

Synthesis and conformational behaviour of 1,9,17-triaza[2.2.2]metacyclophane-2, 10, 18-trione derivatives

Synthesis and conformational behaviour of tri-3-methyltrianthranilides. A new example of spontaneous resolution and inclusion compound formation on crystallisation

X-ray structural studies of the phenyl thio- and seleno-imines derived from 2,4-xylenol reveal, in both molecules, a syn configuration with X ⋯ O distances of 2·604 A(X = S) and 2·575 A(X = Se).

Kwamena A. Woode

Papers

Synthesis and conformational behaviour of 1,9,17-triaza[2.2.2]metacyclophane-2, 10, 18-trione derivatives

Synthesis and conformational behaviour of tri-3-methyltrianthranilides. A new example of spontaneous resolution and inclusion compound formation on crystallisation

X-Ray evidence for partial bonding between sulphur or selenium and oxygen in thio- and seleno-imines