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The unimolecular reactions of ions in a mass spectrometer are remarkably well correlated with both photochemical and thermochemical reactions of the corresponding neutrals. The reactivity correlations are seldom quantitative and exceptions to the correlation should be expected when heteroatoms or delocalization effects convey unusual stability to the ions in the mass spectrometer as compared to their neutral analogs. In spite of the special effects on ionic stability and the relatively large amount of energy that is available in 70 eV electron impact, it appears that mass spectra will be an increasingly useful guide to new photochemical and thermochemical reactions. [ABSTRACT FROM AUTHOR]

Benzenoid (chemical) isomers are, in a strict sense, the benzenoid systems compatible with a formula CnHs≡(n; s). The cardinality of CnHs, viz. CnHs≡n, s is the number of isomers pertaining to the particular formula. The generation of benzenoid isomers (aufbau) is treated and some fundamental principles are formulated in this connection. Several propositions are proved for special classes of benzenoids defined in relation to the place of their formulas in the Dias periodic table (for benzenoid hydrocarbons). Constant-isomer series for benzenoids are treated in particular. They are represented by certain CnHs formulas for which n; s=n1; s1=n2; s2=…, where (nk; sk) pertains to the k times circumscribed CnHs isomers. General formulations for the constant-isomer series are reported in two schemes referred to as the Harary-Harborth picture and the Balaban picture. It is demonstrated how the cardinality n; s for a constant-isomer series can be split into two parts, and explicit mathematical formulas are given for one of these parts. Computational results are reported for many benzenoid isomers, especially for the constant-isomer series, both collected from literature and original supplements. Most of the new results account for the classifications according to the symmetry groups of the benzenoids and their Δ values (color excess). [ABSTRACT FROM AUTHOR]

A synthon model of organic chemistry is described. A synthon is determined as a molecular graph in which a certain part of vertices is distinguished and those vertices are called the virtual vertices. The virtual vertices correspond to functional groups or molecular fragments that do not directly participate in chemical transformations (though these transformations may be substantially influenced by them). The synthon precursors or successors are formed from a given synthon by special sort of reaction graph in such a way that all valence states of atoms (vertices) in a precursor or successor are classified as stable valence states. The program PEGAS represents an implementation of the synthon model. Its basic features, scopes, and algorithms are briefly outlined. The effectiveness of the program PEGAS is illustrated by many examples of interest for organic chemists. [ABSTRACT FROM AUTHOR]

In this article we have described the different types of reactions that have been investigated by means of optically active compounds. It remains to determine the scope of the different mechanisms suggested, and to formulate stereochemical rules for use in further synthetic work. [ABSTRACT FROM AUTHOR]

It is evident from the results presented above that the cis-(−N=N−) bond has powerful coordinating properties towards low valent transition metals. Due to the unique electronic structure of the diazeno group, many different modes of interaction have been observed and it seems promising that this ability of the N=N function may be utilized in catalytic processes. Since diazene itself is a proposed intermediate in biological nitrogen-fixation, the observed modes of coordination are of some relevance to this problem. The results obtained from the cycloaddition reactions are of general importance for the investigation of the factors determining photochemical or thermal reactions of coordinated ligands. The ability of iron carbonyls to activate the diazeno group towards attack by unsaturated molecules like alkynes, offers a convenient synthesis of novel organic heterocycles. Together with the recently reported conversion of coordinated dinitrogen into diazeno and diazenato complexes4), these reactions may be key steps in the incorporation of N2 into organic compounds. [ABSTRACT FROM AUTHOR]

A computer program has been developed which automatically generates sequences of chemical reactions. As reactions are treated quite formally, all conceivable reactions are obtained irrespective of whether they axe already known or are without precedence. This program is especially useful for planning organic syntheses and fox generating the products of starting materials. The system is in very active development but the present version can already serve as a tool for the chemist in studying problems of immediate interest. Examples for applications are given to demonstrate the flexibility and performance of the program. [ABSTRACT FROM AUTHOR]

The above examples of the unique possibilities of stereo chemically fixed aromatic rings in multibridged, multilayered and multistepped hydrocarbon compounds show that these classes of compounds are most appropriate for the study of intramolecular, steric and electronic interactions, from which too little quantitative relations are known. Again, it follows that further synthetic approaches are not only possible, but also necessary for a more accurate differentiation and analysis of the frequently overlapping electronic and steric effects. More progress in this field is sure to be expected, above all when synthetic and spectroscopic chemists, and physicists adhere to interdisciplinary, collaborative work. [ABSTRACT FROM AUTHOR]

While in the early days of organic mass spectrometry the ortho effect was preferentially used as a diagnostic tool for structure elucidation, the recent research activities have concentrated on mechanisms and related aspects. Among these the following were the most important topics: hydrogen transfer and especially the question of concerted or multistep reaction; the influence of the structure on the selectivity of hydrogen migration via higher-membered transition states; the apparent cleavage of carbon/heteroatom bonds induced by hydrogen transfer; the redox processes which formally correspond to photochemical transformations, but which are completely different according to the reaction mechanisms; the different types of cyclization reactions; and the field of exchange processes preceding or accompanying unimolecular decompositions of ions in the gas phase. If a prognosis concerning the future activities in the field of electron impact induced ortho effects may be risked, the following aspects will probably become more important in addition to the topics discussed in this review:(i)Kinetic studies in the time range of nanoseconds to elucidate the time dependence of the different ortho effect reactions;(ii)ion/molecule reactions with the scope to obtain more information concerning the reactivities of open and closed shell systems;(iii)comparative studies in the fields of mass spectrometry, photochemistry and electrochemistry. In principle all these studies may extend our knowledge concerning the inherent properties of isolated ionic species. [ABSTRACT FROM AUTHOR]

A series of azophenol-dyed macrocycles, which accommodate selectively a given metal or ammonium ion in their cavity and undergo a concurrent color change, were designed and synthesized in order to study molecular recognition in biological systems. Selective coloration of some azophenol hosts and selective fluorescent emission of benzothiazolyl crowns for metal cations are observed to be useful for lithium analysis. Amine selective coloration with some azophenol hosts including enantiomeric discrimination are reported and X-ray crystal structures of sec.-amine inclusion complexes are illustrated. Selective coloration with two acerands is capable of discriminating the substitution pattern of alkylamines. [ABSTRACT FROM AUTHOR]

A review is given of the recent developments of the chemistry of cyclotriveratrylenes and cryptophanes, and of their use in the study of host-guest interactions. Experiments dealing with the complexation of tetrahedral species including neutral molecules (halogenomethanes and alkanes) and organic soft cations (tri- and tetramethyl ammonium cations, acetylcholine) are discussed. Prospective applications of cyclotriveratrylenes and cryptophanes to the design of ferroelectric liquid crystals and of three-dimensional charge tranfer materials are presented. [ABSTRACT FROM AUTHOR]

An increasing appreciation of molecular architecture and design will provide a greater impetus to tackle the exciting and elaborate structural targets that will confront chemistry in the twenty-first Century. It is, therefore, of the upmost importance that methods by which outwardly complex chemical systems can be assembled selectively, rapidly, and efficiently, are devised. Towards this goal, a trebly diastereoselective tandem Diels-Alder reaction sequence has been developed. Subsequently, we have demonstrated how this synthetic methodology can be utilized for the synthesis of novel macropolycyclic molecular structures. In each cycloaddition, the stereoelectronic characteristics, that are inherent in part of the rigid bicyclic frameworks of both bisdienophile and bisdiene building blocks, are used to dictate their respective modes of reaction. The outcome of this substrate-directed approach to organic synthesis allows the rapid and highly-controlled assembly of an increasing range of apparently complex molecular structures. The diastereoselectivities that are witnessed in each cycloaddition are rationalized as involving kinetically-controlled transition-state effects. The rationalization is based upon maximal vicinal staggering and the subsequent minimization of torsional strain within the rigid bicyclic framework in the transition state. In this survey, the application of the repetitive Diels-Alder approach for the synthesis of a wide range of macropolycyclic molecular structures is examined. The potential of these compounds as ideal precursors in the preparation of intriguing hydrocarbons is revealed. The pivotal role played by the sequential use of mild and forcing conditions — most notably the use of thermally- and high pressure-promoted Diels-Alder reactions — to produce oligomeric compounds in a predictable and stepwise manner is documented. Throughout these discussions, special attention has been given to the methods employed for the purification and characterization of this new class of unnatural products. [ABSTRACT FROM AUTHOR]

Structure-activity relationships for biologically active congeneric compounds were examined by using free-energy-related physicochemical parameters and regression technique. The steric effects involved in such biological activities were expressible and separable from other factors as steric parameters depending upon the situations. In this article, examples for enzyme reactions, enzyme inhibitions, insecticides, fungicides, herbicides, plant growth regulators and synthetic sweetners, mostly from our own laboratory are reviewed. The steric parameters used here are Taft Es, Hancock Esc, Verloop STERIMOL and van der Waals molar volume. [ABSTRACT FROM AUTHOR]

This section reviews the molecular shape descriptors developed by Amoore, Allinger, Simon et al. and Testa and Purcell. The illustrative examples discussed refer to the odour similarity and cardiotoxic aglycones. One has stressed the methods based on the reference structure because, correctly formulated, these methods seem to offer promising perspectives to model the steric effects in biological systems. Finally, a short discussion of possible connections between steric and other substituent constants (relevant in the context of multicollinearity in QSAR) is included. [ABSTRACT FROM AUTHOR]

The definition of steric parameters initially based on Van der Waals raddi (rv) is described. Tables of various sets of rv are given. These different types of rv are shown to be interrelated. The rv values of Bondi are chosen as the standard set. Substituents are classified according to the degree of conformational dependence of their steric effect. Values of the steric parameters v are obtained directly from rv whenever possible. For symmetric MZn groups they are obtained from calculated rv values. For planar π-bonded groups equations are derived which permit the calculation of both v and the corresponding delocalized electrical effect parameters. Values of v for other grups are based on kinetic data scaled by correlation with groups for which v is known. Over 300 values of v are reported. [ABSTRACT FROM AUTHOR]

This chapter deals with the description of the main topological indices and of related indicatros for molecular constitution used in structure-activity relationships (QSAR). The topological indices are numerical quantities based on various invariants or characteristics of molecular graphs. For the convenience of the discussion, these indices are classified according to their logical derivation from topological invariants, rather than according to their chronological development. [ABSTRACT FROM AUTHOR]