Silylenes kinetics

In the course of this development, knowledge about low valent (in the sense of formal low oxidation states) reactive intermediates has significantly increased [26-30]. On the basis of numerous direct observations of silylenes (silanediyles), e.g., by matrix isolation techniques, the physical data and reactivities of these intermediates are now precisely known [31], The number of kinetic studies and theoretical articles on reactive intermediates of silicon is still continuously growing... 

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

Tokitoh et al. have synthesized a sterically hindered disilene TbtMesSiSiMesTbt 210 that is kinetically stable but thermally labile, thus providing the corresponding silylene TbtMesSi 211, whose reaction with isocyanides bearing bulky substituents has resulted in the formation of the first stable silylene isocyanide complexes TbtMesSiCNR (212, R = Tbt 213, R = Mes ) (Scheme 30).327... 

Corriu et al. have reported that the coupling reaction of 2-(iV,iV-dimethylaminomethyl)phenyllithium with (McvSi)vSiCI 53 affords 2-(iV,iV-dimethylaminomethyl)-l-[tris(trimethylsilyl)silyl]benzene 894. No evidence has been found that the intramolecular iV-ligand coordinates to the silicon atom of 894. Upon UV irradiation, the trisilane forms a transient silyene 895, which has been trapped with 2,3-dimethyl-2,3-butadiene and triethylsilane to give the oligosilanes 896 and 897 as well as 898-900, (Scheme 126).859 Apparently, the bulk on the two ligands is insufficient to provide kinetic stabilization of the silylene intermediate 895. 

For design purposes, Armaou and Christofides use the following kinetic model. Initially, SiH4 dissociates due to electron impact to form silylene (SiH2), silyl radicals (SiH3), and atomic hydrogen ... 

The fert-butyl group is known as one of the most effective kinetic protectors and many reactive molecules have been stabilized and even isolated by using this group. For example, the divalent center of silylenes and germylenes, the heavy atom analogues of carbenes, have been shown to be blocked by tcrt-butyl groups. 

Kinetic isotope effects (KIE) of silylene insertion reactions were determined in solution as well as in the gas phase. Steele and Weber39 found KIEs between 1.8 and 2.3 for the insertion of silylene 13 into various alcohols no significant variation of these values was observed with a change of solvent from cyclohexane to THF. These results are consistent with two mechanisms (Scheme 8), both of them proceeding via a nonlinear or triangular transition state. Mechanism B, which is favored over mechanism A by the authors, is in agreement with theoretical results, which, in general,... 

The sterically crowded, kinetically stable disilene 122 acts on heating as a source of silylene 123 that exhibits the usual reactivity of silylenes, as... 

R. Becerra and R. Walsh, Kinetics and mechanisms of silylene reactions a prototype for gas-phase acid/base chemistry, in Research in Chemical Kinetics (Eds. R. G. Compton and G. Hancock), Vol. 3, Elsevier, Amsterdam, 1995, p. 263. 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

In recent years the application of laser techniques has become an important tool in the study of the kinetics of relatively simple reactive organosilicon species (silylenes, silenes etc.). Gaspar and coworkers have reviewed the use of laser techniques to study the generation and reactions of silylenes14. 

The chapter table of contents contains subjects that were either unknown or merely distant hopes a decade ago, such as persistent silylenes, the dissociation of disilenes to silylenes and terminal silylene-transition metal complexes. The kinetics and spectroscopy of silylenes and theoretical treatments of silylene structure and reactivity have made such gigantic strides in the intervening years that they represent new vistas in our understanding. 

Steady-state kinetic analysis of a competition experiment led to the conclusion that the siloxolane is formed by reaction of a vinylsilirane intermediate with acetone, and that the vinylsilirane arises from addition of the free silylene to butadiene. Since silylenes are known to react more rapidly with acetone than with butadiene, the kinetic analysis further suggested that the carbonyl sila-ylide dissociates more rapidly than it rearranges to the silyl enol ether shown in equation 64140. 

There is a dramatic kinetic stabilization by bulky substituents of the three-membered ring products from silylene addition to jr-bonds. Addition of t-Bu2Si to ethylene led to the first silirane with no substituents on its ring carbon atoms as a distillable liquid 164. Interestingly, l,l-di(terf-butyl)silirane does not undergo photochemical or thermal silylene extrusion, but instead polymerizes. A distillable silirane was also reported from addition of t-Bu2Si to 2-methylstyrene165. 

Studies of the kinetics of cis-trans interconversion in disilene 33 indicate that it takes place by dissociation to the silylene and recombination, rather than by Si=Si bond rotation as is the case for other disilenes187. This isomerization occurs even at 50 °C. 

Despite the short lifetimes of most silylenes, improvements in flash photolysis techniques for their generation and time-resolved spectroscopic detection methods in the past decade have made possible direct kinetic measurements on the reactions of silylenes. The purpose of these kinetic studies has been to elucidate the mechanisms of silylene reactions. While considerable work remains to be done, transition state structures and activation barriers are emerging from these experiments, and aspects of silylene insertion and addition mechanisms have been revealed that were not uncovered by product studies and were, indeed, unexpected. 

Becerra and Walsh have presented a clear exposition of the mechanistic problems addressed by kinetic studies on silylenes and the conclusions that have been drawn to date323. Absolute rate studies have begun to throw light on the questions How do the structures and energies of silylenes affect their reactivity What are the transition state structures What is the balance between the electrophilic and nucleophilic character of the attacking silylene in the transition state What roles do steric and electronic effects play in silylene reactions ... 

All the kinetic experiments discussed thus far have been in the gas phase, whose advantages include the possibility of studying the effects of energy release through pressure-dependence studies, and the absence of complications due to solvents, such as the rapid complexation of silylenes. 

The reactions of transient silylenes are so rapid that most of the limited mechanistic information that has been obtained over the past quarter-century has been through indirect means. Direct measurements of silylene reaction rates by kinetic spectroscopy in the past decade have yielded important new insights. One can predict with some confidence an explosion of mechanistic studies of silylenes employing fast spectroscopies capable of providing more structural information than traditional electronic absorption and emission techniques. The nearly universal reversibility of silylene reactions remains to be fully exploited through kinetic studies of retro-reactions. The mechanisms of most silylene reactions remain to be fully elucidated, and this task will increase in urgency as silylenes see more use in synthesis. 

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