Disilenes thermal isomerization

The first cyclic disilene, hexakisUerf-butyldimethylsilyljcyclotetrasilene (51), is synthesized in 13.6% yield by the reduction of a 1 2 mixture of tetrabromodisilane 86 and dibromosilane 87 with lithium naphthalenide in THF [Eq. (12)].23 Interestingly, the initial product of the reductive condensation is not cyclotetrasilene 51 but bicyclo[1.1.0]tetrasilane 88. Thus, reduction of a mixture of 86 and 87 with lithium naphthalenide at -78 °C followed by hydrolysis at 0 °C affords mainly a hydrolysis product of bicyclotetrasilane 88, 89 [Eq. (13)]. As discussed in Section IV.A.4 in more detail, compound 51 should be formed by the thermal isomerization of 88 during workup at higher temperatures. 

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. 

Thermal E-Z isomerization was also observed for kinetically stabilized disilenes 5-9 (equation l)19-22. The n bond strength was estimated to range from 24.7 to 30.6 kcalmol-1. These data are in good agreement with those of 1,2-dimethyl-1,2-diphenyldisilene and those predicted by ab initio calculations for H2Si=SiH2 (22-28 kcalmol-1)23,24. 

The trans isomers of two disilenes were observed to undergo photoisomerization to mixtures enriched in the cis isomers when irradiated at either 254 or 350 nm (equation 28), and the kinetics of the thermal cis-trans isomerization have been determined164. 

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). 

Absolute rate constants and Arrhenius parameters have been determined for the thermal E,Z-isomerization of the stable disilene derivatives 92-96 in deuteriated aromatic solvents or THF-ds solution by XH or 29Si NMR spectroscopy133-136. With 1,2-dialkyl- and 1,2-diamino-l,2-dimesityldisilenes such as 92a-94, the (E)-isomers are considerably more stable than the (Z)-isomers, and so rate constants for E,Z-isomerization were determined after first generating mixtures enriched in the (Z)-isomer by UV-irradiation of the (El-isomer, and then monitoring the recovery of the solution to its equilibrium composition. On the other hand, little difference in thermodynamic stability is observed between the (Eland (Z)-isomers of tetraaryldisilenes such as 95a,b, and E,Z-isomerization kinetics were hence determined starting from solutions prepared from the individual, pure (or almost... 

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. 

Several kinds of thermal unimolecular isomerization reactions of disilenes are now known. 

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