Silylene dissociation

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 reactivity of a silylene 103 with isocyanides was probed by Okazaki et a . in 1997. When disilene 102 is heated to 60 C in THF or CfiDf, it dissociates... 

Dissociation of the gases SiH4 and H2 by electron impact will create reactive species (radicals) and/or neutrals (Si2H6 and even higher-order silanes [195-198]). Atomic hydrogen is an important particle because it is formed in nearly all electron impact collisions, and the H-abstraction reaction [199, 200] of (di)silane is an important process, as is seen from sensitivity study. Dissociation of SiHa can create different SiH (with x = 0, 1,2, 3) radicals. Only silylene (SiH2) and... 

The photoelectron spectra of 3 and 25 (but not 1) showed evidence of thermolytic dissociation of the disilenes into silylenes in the spectrometer. This type of dissociation will be treated in Section VIII.B. 

Many of the fundamental reaction patterns of disilenes, except for thermal dissociation into silylenes, have been described in the previous reviews,2-5 but ongoing studies on the reactivity of disilenes continue to reveal a rich and diverse chemistry. 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

As shown in Scheme 3, disilene 27 undergoes thermal dissociation into silylene 36 under very mild conditions (about 50°C).22,59 This represents the first example of such dissociation. The formation of 36 was confirmed... 

The dissociation into silylenes observed in photoelectron spectroscopy for 3 and 25 has been described in Section VII. In addition, thermal dissociation of 25 has recently been noted.63 This requires a somewhat higher temperature than that for dissociation of 27. At 100°C, 25 shows no reactivity toward Et3SiH or (remarkably) toward EtOH, but at I20°C it reacts with both substrates to give silylene trapping products (Scheme... 

Earlier, it had been shown that 25 reacts with CH3OH under photolysis to give similar silylene trapping products this reaction may involve photochemical dissociation of 25 into silylenes.23a... 

These recent observations suggest that thermal dissociation of disilenes into silylenes may be a more general phenomenon than has previously been thought. Additional examples are likely to be reported in the future. 

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 authors proposed the following picture of the silylene anion-radical formation. Treatment of the starting material by the naphthalene anion-radical salt with lithium or sodium (the metals are denoted here as M) results in two-electron reduction of >Si=Si< bond with the formation of >SiM—MSi< intermediate. The existence of this intermediate was experimentally proven. The crown ether removes the alkali cation, leaving behind the >Si - Si< counterpart. This sharply increases electrostatic repulsion within the silicon-silicon bond and generates the driving force for its dissociation. In a control experiment, with the alkali cation inserted into the crown ether, >Si — Si< species does dissociate into two [>Si ] particles. 

Meanwhile, Tokitoh et al. " reported the first example of thermal dissociation of extremely hindered disilenes [Tbt(Mes)Si=Si(Mes)Tbt Tbt = 2,4,6-tris[bis(tri-methylsilyl)methyl]phenyl, Mes = 2,4,6-trimethylphenyl (mesityl) fZ)-137 cis isomer, (E)-137 trans isomer] into the corresponding silylene [Tbt(Mes)Si , 138] under very mild conditions (—70 °C) as shown in Scheme 14.59. 

Data obtained from collision-induced dissociation experiments did not allow for a distinction of the isomeric metal-silene and-silylene species however, structure-specific ion-molecule reactions of the complexes with labeled ethene were used to clearly differentiate between the metal silene and the silylene. In this intriguing study, Jacobson and coworkers also bracketed the bond dissociation energies of the isomeric ions. 

They found that the dissociation energy for the silylene Fe—Si(Me)H+ lies between 56 and 78 kcalmol-1, and that of the silene isomer Fe—H2Si=CH2+ between 55 and 70 kcalmol-1. The similarity of the two energy ranges once again demonstrates the relative lability of the metal-silylene bond and suggests a potential stabilization of molecules which contain silicon, jr-bonded to transition metals. 

There is now significant evidence that highly sterically hindered disilenes readily dissociate to the related silylenes, which then undergo their characteristic reactions166. 

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. 

Calculations indicate that silacyclopropenes might also be employed as silylene precursors in thermolysis reactions. The enthalpy of dissociation of unsubstituted 1-silacycloprop-2-ene is predicted to be 50.4 kcal mol-1, only ca 7 kcal mol-1 greater than for silirane81. 

The silaylides formed from coordination of silylenes to chlorocarbons are believed to undergo competitive rearrangement (to the formal Cl—C insertion product) and fragmentation (to the product of formal HC1 abstraction by the silylene)5. In addition, these silaylides are believed to undergo dissociation into the radical pairs that would result from direct chlorine atom abstraction by the silylene111. 

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. 

From the reactions of carbon monoxide in frozen matrices with Me2Si155 and several arylsilylenes Mes(R)Si (R = Mes, 2,6-diisopropylphenoxy, f-Bu)156, adducts were formed (see Section V.A.2). Theoretical calculations led to consideration of both a linear silaketene and a pyramidal n-donor base complex structure for the Me2Si(CO) adduct249. The observation that warming the carbon monoxide adducts of the arylsilylenes led to the formation of disilenes was interpreted as indicating the formation of a nonplanar complex that could dissociate as do other silylene-n-donor base complexes156. 

---