Disilenes. thermal dissociation

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... 

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. 

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. 

Kinetic studies on the thermal dissociation of 137 revealed that the unusual behavior of 137 in contrast to the stable disilenes obviously resulted from the pre-... 

As discussed in the previous section, thermal dissociation of disilenes into the corresponding silylenes may occur if the BDE of the disilenes is small. As shown in review OW, a facile thermal dissociation of disilene 27 into silylene 127 occurs at 50 °C [Eq. (49)],61,91 The formation of silylene 127 is evidenced by its trapping by methanol, triethylsilane, and 2,3-dimethyl-1,3-butadiene. The activation enthalpy and entropy for the dissociation of (Z)-27 to 127 are 25.5kcalmol-1 and 7.8 cal mol-1 K-1 respectively.91 The activation free energy for the dissociation at 323 K (22.9 kcal mol-1) is much smaller than that for the Z-to-E isomerization of 26 (27.8 kcal mol-1), indicating that the E,Z-isomerization of 27 should occur via the pathway (2) in Eq. (47) rather than pathway (1) in Eq. (48). 

The evidence that ,Z-isomerization of 92-95 proceeds by Si=Si bond rotation and not a mechanism involving silylene intermediates, produced by cleavage of the Si=Si bond followed by recombination, rests upon the fact that no trapping products consistent with the intermediacy of the corresponding diarylsilylenes could be detected upon heating the disilenes in the presence of known silylene traps such as methanol, triethylsilane or 2,3-dimethyl-l,3-butadiene. In fact, one tetraaryldisilene has been shown to isomerize by this mechanism, the 1,2-dimesityl-l,2-bis(2,4,6-tris[bis(trimethylsilyl)methyl]phenyl derivatives (E)- and (Z)-97a (equation 70)142,143. Arrhenius parameters for the thermal dissociation of (E)- and (Z)-97a to diarylsilylene 98 are listed in equation 70. 

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