Disilenes spectroscopy

Using CO-saturated hydrocarbon matrices, Pearsall and West" photolyzed sily-lene precursors at 77 K and monitored CO coordination to the silylenes by UV-vis spectroscopy (Scheme 13). Bis(trimethylsilyl)silanes 44a-c or SifiMcji were irradiated at 254 nm to create silylenes 45a-d, which reacted with CO, causing new peaks to ca. 290 and 350 nm, which were attributed to complex 46a-d, a resonance structure of silaketene 47a-d. Silylene adducts form fairly weak bonds, as seen by warming of the matrices. In the case of silylene adducts where one R = Mes, the CO dissociates and the corresponding disilene 48a-c peaks in the UV-vis spectra observed upon warming (R2 = Me most likely produced silane rings Si, Me6. etc.). 

The highly hindered disilene 2 did not react with white phosphorus, even under forcing conditions. With disilene 3, which is more hindered than 1 but less so than 2, the reaction with P4 was more complicated. It proceeded slowly, producing small amounts of both stereoisomers of the bicyclobutane compounds 70 and 70. The major product, however, was a more complex compound containing four phosphorus and four silicon atoms, also obtained as a mixture of two stereoisomers. Two-dimensional 31P NMR spectroscopy established the probable structures to be 71.98... 

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. 

In 1981, West et al. synthesized the first stable disilene 1 via the dimerization of the corresponding silylene generated by the photolysis of a trisilane and characterized the structure by conventional spectroscopies [Eq. (2)].5 Availability of 1 and other stable disilenes has stimulated theoretical and experimental studies of various aspects of disilenes such as their bonding and structure, spectroscopic properties, reactivities, applications to the synthesis of novel types of organosilicon compounds, etc. 

As a typical unimolecular reaction of disilenes, the /i,Z-isomcrization is discussed first. In contrast to the isomerization of an alkene that occurs via the rotation around the C = C double bond with an activation energy of ca. 60kcalmol-1 the E,Z-isomerization of disilenes is known to occur more easily. As shown in review OW, the E,Z-isomerization in aryl-substituted disilenes 3,4,20,26, and 27 proceeds under mild conditions to allow the kinetic studies at 40-80 °C by NMR spectroscopy. Recently, the T,Z-isomerization between tetrakis(trialkylsilyl)disilenes ( )- and (Z)-33 was found to occur more rapidly with the rates of the NMR time scale at 30 °C 63... 

Of the more specialized methods for the formation of acyclic disilenes, two new methods deserve mention. Kira and coworkers reported that photolysis of the silirene 1 afforded the diaminosilylene 223. In contrast to theoretical predictions24-27, the silylene 2 apparently does not undergo dimerization to afford the bridged dimer 3 as is usually preferred in the presence of electronegative substituents but rather, according to the results of variable-temperature electronic spectroscopy, furnishes the disilene 4 (equation 4)23. 

Electron and 29 Si NMR spectroscopy, in particular, have provided valuable information for the description of the bonding in disilenes. In this section, only a few general principles and new developments are mentioned. The interested reader is referred to the comprehensive review published by Okazaki and West in 1996 for detailed information about the earlier work6. 

Si NMR spectroscopy provides diagnostic data for the formation of the RSi=SiR unit and supports the assignment of a disilyne formed by the dehalogenation of a disilene with lithium naphthahde (equation 14.7). ... 

In the preparation of the disilene 4 from the 1,2-dichlorodisilane 6 and lithium we observed that, in spite of the use of stoichiometric amounts of the two components, the yield of 4 mostly did not exceed 40 % and that half of the dichlorodisilane 6 was always recovered. Close monitoring of the reaction by H-NMR spectroscopy showed that at first the dehalogenation of 6 to the disilene 4 occurs on the lithium surface until a maximum of about 40 % of 4 is achieved. Thereafter, the amount of 4 decreases continuously while increasing amounts of 1,3,5-triisopropylbenzene are concomitantly detected in the reaction mixture. Since at the end of the reaction the lithium has been consumed completely, it is reasonable to suggest that, with increasing concentrations of 4 in the... 

The reaction between K2Fe(CO)4 and a tetrachlorotetrasilane resulted in the formation of an iron complex with a disilene ligand (Equation (33)). Both the (E)- and (Z)-isomers were formed. The reaction, when monitored by H NMR spectroscopy, indicated the initial formation of the (Z)-isomer with isomerization to the more stable ( )-isomer over time. Kinetic studies were also performed, and suggested that the ( )-isomer is >3kcalmol more stable than the (Z)-isomer. Theoretical calculations for similar molecules with varying R groups indicated that the stability of the molecule is dependent on the bulkiness of the R group. 

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