Silenes bimolecular reactions

In this section we will summarize bimolecular reactions of silenes with alkenes, alkynes and dienes which might be regarded nominally as cycloaddition reactions. 

The focus of the section on silene reaction kinetics is mainly on studies of bimolecular reactions of transient silene derivatives, because little absolute kinetic data exist for the reactions of stable derivatives and there have been few quantitative studies of the kinetics of unimolecular isomerizations such as ,Z-isomerization and pericyclic rearrangements, although a number of examples of such reactions are of course well known. In contrast, most of the studies of disilene reaction kinetics that have been reported have employed kinetically stable derivatives, and E,Z-isomerization has thus been fairly well characterized. The paucity of absolute rate data for unimolecular isomerizations of transient silenes and disilenes is most likely due to the fact that it is comparatively difficult to obtain reliable data of this type for transient species whose bimolecular reactions (including dimerization) are so characteristically rapid, unless the unimolecular process is itself relatively facile. Such instances are rare, at least for transient silenes and disilenes at ambient temperatures. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

Several kinds of bimolecular reactions appear to be characteristic of silenes. In the following we shall group them according to their presumed mechanisms, but few, if any, of these have been firmly established (Chart 9). 

An alternative method of isolating 1 or 2 without sacrificing their reactivity is the matrix-isolation technique. In a low-temperature inert gas matrix, reactive molecules are immobilized, and thus bimolecular reactions are inhibited. In addition, the low temperatures prevent reactions with activation barriers larger than a few kcal/mol. In most of our experiments, argon matrices at 10 K have been used. Under these conditions, the diffusion of even small molecules like CO or O2 is effectively suppressed. Warming the argon matrix from 10 K to temperatures above 30 K allows small molecules to slowly diffuse. Under these conditions, bimolecular reactions are observed, if the activation barrier is small enough. Thus, reactions of matrix-isolated reactive species such as silenes and silylenes can be effectively controlled by variation of the matrix temperature. 

Over the past ten years, absolute rate data have been reported on the kinetics of several bimolecular silene reactions in solution, including both head-to-tail and head-to-head dimerization the [l,2]-addition reactions of nucleophilic reagents such as water, aliphatic alcohols, alkoxysilanes, carboxylic acids and amines and the ene-addition, [2 + 2]-cycloaddition and/or [4 + 2]-cycloaddition of ketones, aldehydes, esters, alkenes, dienes and oxygen. The normal outcomes of these reactions are summarized in Scheme 1. 

Diphenylsilene (19a), produced by photolysis of 1,1-diphenyl- or 1,1,2-triphenylsilacyclobutane (17a and 18, respectively equation 11), has been particularly well studied, and absolute rate constants have been reported for a wide variety of silene trapping reactions in various solvents at room temperature (see Table 3)40-46. Not all of these have been accompanied by product studies, unfortunately. A number of other transient silenes have been characterized as well with solution-phase kinetic data for a range of bimolecular silene trapping reactions, though much less extensively than 19a. These include the cyclic l,3,5-(l-sila)hexatriene derivatives 21a-c (formed by photolysis... 

ConUn and coworkers have smdied the kinetics of the dimerization of the silene 369 by laser flash photolysis. 369 gives in a clean reaction exclusively the linear dimer 370 (equation 103). Second-order kinetics were observed with a bimolecular rate constant of... 

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