Brook silenes dimerization

The Brook silene 28, produced photochemically as shown in equation 18, dimerizes to yield a mixture of the 1,2-disilacyclobutane 29 and the acyclic ene-dimer 3064, the common mode of dimerization for the large majority of l,l-bis(trialkylsilyl)silenes that have been studied to date12. Conlin and coworkers determined the absolute rate constant for dimerization of 28 in cyclohexane solution, k, tm = 1.3 x 107 M 1 s 1 at 23 °C65. Arrhenius activation parameters for the reaction were determined over the 0-60 °C temperature range. The values obtained, a = 0.9 0.4 kJmol 1 and log(A/M 1 s 1) = 7 1, are consistent with the stepwise mechanism for head-to-head dimerization originally proposed by Baines and Brook (equation 19)64, provided that the rate of reversion of the... 

The dimerization of 49 (Ar = Mes) yields compound 54, whose X-ray crystal structure could be determined , via diradicals 52 and 53. This reaction may be considered as a special case of silene dimerization. Although the typical course of silene dimerization leads to head-to-tail adducts (cf Section II.A), highly substituted analogues often give the head-to-head isomer. Brook and Baines have proposed that many of these dimerizations also proceed via initial formation of a Si—Si bond to give 1,4-diradicals which, in some cases, isomerize by intramolecular hydrogen abstractions . ... 

Considerable insight into the broader question of how the kinetic and thermodynamic stability of the Si=C bond is affected by substituents at silicon and carbon was obtained in the late 1970s, when Brook and coworkers reported the isolation and characterization of the first stable silene derivatives (80), prepared by photolysis of the corresponding acylpolysilanes (79) as shown in equation 6017. The unique stability of these compounds is clearly due to steric stabilization effects to some extent, but it was suggested that the siloxy substituent at carbon also reduces the reactivity of these silenes toward dimerization as a result of resonance effects on the degree of positive charge character at silicon. 

The dimerization of the transient silenes 2a-2c is discussed in terms of a radical process, which was proved by Brook et al. for some similarly structured 2-alkyl- and 2-phenyl-2-trimethylsiloxy-l,l-... 

In contrast, l,l-silylsilenes ° ° ° and 2-siloxysilenes 3,86,90,180 j silenes of the Apeloig-Ishikawa-Oehme and Brook type as well as some 1-silaallenes , dimerize in a head-to-head mode yielding 1,2-disilacyclobutanes (equation 98). 

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