Reactions stepwise

Preparation by reaction of tantalum pentachloride with pure lithium dimethylamide in pentane is unsafe. Initial non-reaction is followed by explosion during manipulation of the slurry. Presence of ether or dimethylamine gives smooth reaction. Stepwise displacement of chlorine, starting with the free amine, is also recommended. 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

In denitrification, part of the biological nitrogen cycle, nitrate in the soil is converted via four enzymatic reactions stepwise to nitrite, nitric oxide and nitrous oxide to finally yield gaseous nitrogen. 

First Reaction (Stepwise Addition Polymerization) Preparation of the Polyfamide Acid)... 

There is far less information in the scientific literature about template copolymerization than about template homopolymerization. As in the case of template homopolymerization, template copolymerization can be realized according to different types of reaction stepwise (template polycondensation), copolyaddition, radical or ionic polymerization, ring-opening copolymerization, etc. 

Dimeric versions of these triazines have also been reported to undergo the Diels-Alder reaction with preformed enamines to give 2,2 -bipyridines <2005MOL265>. It is possible to carry out the reaction stepwise or in one pot however, the yields are variable for both sequences (10-64%). 

When dealing with catalysis it is best, however, to classify polymerization reactions according to the mechanism of chain propagation (2). One may distinguish in this way between chain-reaction polymerization and step-reaction (stepwise) polymerization. The essential features of these classes are shown in Table I (15). The diflFerences between the two types of polymerization are also evident from equations of rate (Rp) and average degree of polymerization (DP). For a free-radical polymerization of vinyl compounds (an example of a chain reaction), Rp and DP are functions of monomer and catalyst concentration (Equations 9 and 10) ... 

Density functional theory calculations have been used to estimate the mechanistic course of hydration and hydrolysis reactions of a-oxocarboxylic acid derivatives (esters and amides), with solvent effects allowed for. For the uncatalysed reactions, stepwise and concerted routes are comparable in energy, whereas catalysis via water assistance significantly favours the stepwise process. 

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