Stereochemistry and Topicity

This symposium addressed several important issues in bromine chemistry. A major part has been devoted to stereochemistry and mechanism of electrophilic bromination of olefins. Other topics included new selective methods of bromination and oxybromination, brominations in presence of solid supports and catalysts, organobromine compounds as synthons, recent developments in brominated fire retardants and toxicological and environmental aspects of brominated compounds. 

This whole area of spectroscopy touches on many different topics and can only be approached confidently with a reasonable working knowledge of basic NMR, stereochemistry and certain aspects of physical chemistry. 

The palladium-allylation of ambident aromatic heterocycles is covered by Professor Moreno-Mafias and Dr. Pleixats (Barcelona, Spain) in the second chapter of this volume. The preference for carbon versus oxygen, nitrogen, and sulfur allylation is discussed from the diverse viewpoints of regioselectivity, kinetic versus thermodynamic control, mechanisms, stereochemistry, and synthetic targets in the first general survey of this topic. 

Since Pasteur s time, stereochemistry has experienced an enormous intellectual growth and has also found widespread industrial application. In recent years, a spate of articles, reviews, books, and international conferences and symposia have dealt with the role of chirality in chemistry, and three new journals have been specifically devoted to this topic Chirality, by Wiley-Liss in 1989, Tetrahedron Asymmetry, by Pergamon Press in 1990, and Enantiomer, by Gordon and Breach in 1996. Much of the research reported in these media, though motivated to some degree by market forces—notably by the demand of pharmaceutical industry for enantiopure drugs4—serves as a reminder that molecular chirality remains the centerpiece of stereochemistry and allied branches of science. 

Vocabulary of Stereochemistry and Stereoselective Synthesis II Topicity, Asymmetric Synthesis... 

Chapter 5, Polymer Synthesis, is probably most useful in a second-year chemistry course that touches on the topics of stereochemistry and some organic chemistry. 

Stereochemistry and asymmetric synthesis are topics with which chemists traditionally have been concerned (1 ). In recent years there has been a virtual explosion of literature in the area of asymmetric organic synthesis that has fortuitously paralleled the increased awareness of insect pheromone stereochemistry. Many useful reviews of asymmetric synthesis exist (2, 2> 4, 5, 0 and this paper will only briefly direct the reader s attention to examples of reported syntheses by type that may be of potential general use for pheromone synthesis. It should be clear even to the casual reader that this field is in need of almost annual review and current literature would have to be consulted in the face of an original problem in synthesis. 

This review deals with the replacement of substituents in the vinylic position hy anionic or neutral nucleophiles. Its division according to mechanistic routes suffers from the fact that for many systems there is a strong connection and mutual intercalation between several routes, but we will try to show the similarities in the behaviour of different systems and to discuss the various criteria which have been used for differentiation between the mechanistic pathways. Some topics, e.g. the stereochemistry and the element effect, are discussed in greater detail than others, especially when the data could be collected in convenient tables. No attempt has been made to cover all the synthetically used vinylic substitution reactions of which reviews are available, e.g. on /3-chloro-vinyl ketones (Kochetkov, 1952, 1961 Kochetkov et al., 1961 Pohland and Benson, 1966), fluoro-olefins (Chambers and Mobbs, 1965) or tetracyanoethylene (Cairns et al., 1958 Cairns and McKusick, 1961). 

The prediction of chemical shifts in H-NMR spectroscopy is usually more problematic than in C-NMR. Experimental conditions can have an influence on the chemical shifts in H-NMR spectroscopy and structural effects are difficult to estimate. In particular, stereochemistry and 3D effects have been addressed in the context of empirical H-NMR chemical shift prediction only in a few specific situations [81,82]. Most of the available databases lack stereochemical labeling, assignments for diastereo-topic protons, and suitable representations for the 3D environment of hydrogen nuclei [83]. This is the point where local RDF descriptors seemed to be a promising tool. 

Ab initio building can be done in many ways, but a few points are common to all techniques the fact that endpoints have to match is a very strong constraint. Furthermore, Van der Waals clashes have to be prevented, and all rules about proper stereochemistry and energetics should be obeyed. Many articles have been published about these topics [45-51]. 

N. G. Gaylord, Reduction with Complex Metal Hydrides. Interscience, New York (1956) H. C. Brown, Boranes in Organic Chemistry, Cornell Univ. Press, Ithaca, NY (1972), pp 209-250 H. 0. House, Modem Synthetic Reactions, Benjamin, New York (1972), p 49 Chem Soc Rev 5 23 (1976) Tetr 35 449 (1979) (stereochemistry and mechanism) Topics Stereochem 11 53 (1979) (stereochemistry) JACS 103 4540 (1981) (stereochemistry of cyclohexanone reductions) J. Seyden Penne, Reductions by the Alumino- and Borohydrides in Organic Synthesis, VCH-Lavoisier (1991), Chpt 2 Comprehensive Organic Synthesis, Eds. B. M. Trost and I. Fleming, Pergamon, Oxford (1991), Vol 8, Parts 1.1 and 1.7 TL 34 5483 (1993) (stereochemistry) ... 

Pertinent to these fascinating subjects, quite a number of excellent articles have appeared in Topics in Stereochemistry and elsewhere (27-30) this may justify the exclusion of these topics from the present review. 

Five of the next seven chapters cover the reactions of hydrocarbons—compounds that contain only carbon and hydrogen. The other two chapters treat topics that are so important to the study of organic reactions that each deserves its own chapter. The first of these is stereochemistry and the second is electron delocalization and resonance. 

The above account of copper(II) stereochemistry and electronic properties (Sections 53,4 and 53.4.4) establishes the uniqueness of the regular octahedral (and trigon octahedral) stereochemistries and the novel temperature variability of this stereochemistry (Sections 53.4.2.li and x) and the corresponding ESR spectra. " In order to understand these properties it is necessary to examine the Jahn-Teller theorem in a little more detail, as the consequences of this theorem extend beyond the above two topics and will cover the following (i) temperature variable (fluxional) copper(II) pseudo stereochemistries (ii) non temperature variable, static copper(II) stereochemistries (iii) the plasticity effect and varying tetragonal distortions (iv) the second order Jahn-Teller effect " and (v) the cooperative" and noncooperative " Jahn-Teller effects. For this reason the application of the Jahn-Teller theorem to the coordination chemistry of the copper(II) ion will be described" " and extended to the above topics." " " ... 

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