Reaction mechanism generator applications

C. Chevalier, W.J. Pitz, J. Wamatz, C.K. Westbrook and H. Melenk, Hydrocarbon Ignition Automatic Generation of Reaction Mechanisms and Application to Modeling of Engine Knock, 24th Symp. (Int.) Comb. (1992) pp. 93-101. 

Methods for the generation and use of enantiomerically pure vinylketene complexes have been well developed in recent years. This has allowed stereocontrol to be exerted over subsequent reactions to yield optically active organic or organometallic products. It has also allowed investigations to be carried out into the reaction mechanisms of several fundamental processes. This area is currently ripe for further exploration and would be of particular interest in its application to other metal systems. We trust that the work presented here will serve as a platform for further new and exciting discoveries. 

The reaction has broad applications and a large number of secondary and especially tertiary amines was prepared in isolated yields ranging from 60% to 84% [1136]. Although the mechanism of this reaction is not clear it is likely that the key step is reduction of the acid by borane, generated in situ from sodium borohydride and the acid, to an aldehyde which reacts with the amine as described in the section on reductive amination (p. 134-136). 

The use of ac electrolysis in all its variations is certainly an interesting and valuable technique for study of the mechanism of electron transfer reactions. The generation of a short-lived redox pair as chemical intermediates is an important feature of the ac electrolysis. In the future it may even be developed to synthetic applications irrespective of the mechanistic details. In some cases it could be a convenient alternative to photochemical reactions. In other cases it represents a new reaction type which has no precedent. 

Yet the similarity of the chemistries of these two elements with those of Mo, W, Ru, and Os makes them attractive models for probing reaction mechanisms for compounds of metals in the middle of the transition series. Also, as this chemistry has developed it has become clear that there may be unique applications of these two metals, particularly for rhenium. This review will outline the new mechanistic understanding of organic oxidation chemistry that has been generated over the past dozen years by using high-valent metal oxo compounds of technetium and rhenium. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

In the process of catalyst development, the key to efficient reaction optimization is frequently rapid access to catalysts with diverse chiral environments. Therefore, it is important to design ligands that can be generated from common intermediates in a few steps. While it is often useful to synthesize elaborate ligands to probe asymmetric induction and reaction mechanisms, few researchers are willing to perform multistep ligand syntheses in the application of catalysts. 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

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