Methods silylene mechanism

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

Vacuum pyrolysis of 12 has been recently studied by high-resolution mass spectrometry200. The ionization potential (IP = 9.17 0.10 eV) for silole 2 was found to be in excellent agreement with the IP (9.26 eV) calculated by the semiempirical PM3 method. Thermodynamic calculations on possible decomposition mechanisms of 12 have shown that 2 is probably formed from the primary intermediate silylene 9 via two consecutive hydrogen shifts (equation 3). 

From (trialkylstannyl)dimethylsilane and terminal alkynes this method gives 3,4-disubstituted siloles (28, R = H, R = alkyl, aryl or alkoxy groups) in moderate to good yields44. Catalysis by Pd is better than by Ni or Rh. The proposed mechanism involves a palladium-silylene species 30 as an intermediate. This reacts with an alkyne giving successively a palladasilacyclobutene (31) and a palladacyclohexadiene (32). In the final... 

The photolysis of trisilanes, RR/Si(SiMe3)2, or of cyclosilanes (RR Si) , is a well-established method for silylene generation2,3. Here we will describe only selected examples, concentrating especially on findings which bear on the mechanism of silylene elimination. 

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. 

The mechanism of the reaction of l//-phosphirene 7 with silylene 8 resulting in 1,3-phosphasilacyclobutane 9 was studied by ab initio and DFT theoretical methods. The reaction was found to proceed via 2-phospha-4-silabicy-clo[1.1.0]butane as an intermediate (Scheme 5) <2004AGE3474>. This reaction was also studied for higher substituted analogues by spectroscopic methods (see Section 2.2E10). 

Cyclohexene silacyclopropane 47 undergoes silver-catalyzed silylene transfer, acting as an efficient method for silacyclopropane 48 synthesis (Equation 5) <2004JOC4007>. Kinetic studies of the transfer reaction suggested a possible mechanism for silver-mediated silylene transfer with a kinetic order of one for 47 <2004JA9993>. 

By far the most important spectroscopic method for this purpose is IR spectroscopy. In combination with DFT or ab initio calculations matrix IR spectroscopy has become a very powerful tool for the reliable identification of reactive and unusual molecules. In addition, isotopic labeling with is frequently used to assign the IR spectra of oxidized species. However, a prerequisite for this technique is the availability of suitable photochemical or thermal precursor molecules of the reactive silicon species. During the last years, we have published details of the oxidation mechanism of alkyl-substituted silenes 2. °In this chapter, our mechanistic studies on the oxidation of silylenes 1 using the matrix-isolation technique are summarized. 

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