Brook silenes reactions

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 reactions with quadricyclane, shown in Eq. (31), gave products identical to those formed by the same silene reacting in a [2 + 2] manner with norbornene. Mixtures of exo endo isomers were frequently observed. Again, only silenes of the Auner type have been studied with this reagent,51-53,185,188 so it is not known whether the Wiberg- or Brook-type silenes will undergo this mode of cycloaddition. 

Brook et al. 5X1 observed such reactions during the formation of siienes by photolysis. Using radiation with A > 360 nm, they photolyzed acylsi-lanes such as 127, which bears a mesityl group attached to the carbonyl carbon. On prolonged photolysis of the initially formed silene 128, the C—H bond of the ortho methyl group of the mesityl group added to the silicon-carbon double bond to form the benzocyclobutane 129. Alternatively a 1,5-H shift would lead to the species 130, which would also yield the benzocyclobutane on electrocyclic rearrangement. 

The double bond in silenes is strongly polarized. They react with phosphorus ylides, as shown by Brook and MacMillan,45 like alkenes with the strongly polar C=C bond. Therefore, it is reasonable to suggest that the reaction also occur through the betaine intermediate (12) (Scheme 6). 

The facile photochemical sigmatropic 1,3-trimethylsilyl shift in polysilylacylsilanes from silicon to oxygen (equation 33) was utilized historically to prepare the first relatively stable silenes3 86 87. Silenes prepared by isomerization of acylpolysilanes bear, due to the synthetic approach, a trimethylsiloxy group at the sp2-hybridized carbon and relatively stable silenes of this type have in addition also at least one trimethylsilyl group at the silicon. These substituents strongly influence the physical properties and the chemical behaviour of these silenes. This is noticeable in many reactions in which these Brook -type silenes behave differently from simple silenes or silenes of the Wiberg type. 

Relatively little is known experimentally about the mechanism of the reaction. A widely accepted mechanism originally suggested by Brook and coworkers starts with the formation of a Si—Si bond, giving a carbon centred 1,4-biradical86. This 1,4-biradical then combines in a second step to the 1,2-disilacyclobutane. This mechanism is favoured by the calculations and is also corroborated by experiments the relatively stable silene 149... 

Alkoxysilanes are frequently used as scavenger reagents for silenes. They add regiospecifically to the Si=C bond. In the case of silacyclobutadiene 296 the reaction was shown to be also stereospecific. Thus Z-404 is the sole product of the addition of trimethylsilyl methyl ether (TMSOMe) to 296 (equation 122)163. Characteristically Brook -type silenes do not react with alkoxysilanes like TMSOMe. 

Alkynes also react with sterically, highly hindered Brook-type silenes giving 1-silacyclobut-2-enes in high yields95. This reaction is strictly regiospecific thus from E/Z-136 only a EjZ mixture of 457 is formed, whereas the regioisomer 458 was not detected (equation 146)95. 

Reaction of simple aromatic aldehydes with Brook - type silenes (Me3Si)2Si=C(0 SiMe3)R1 gives exclusively a cis/trans mixture of the 2-siloxetanes 469, the nominal [2 + 2] cycloadducts, which can be identified by NMR spectroscopy (equation 154)235. 

The reaction of thermolytically generated Brook -type silenes with acetone is believed to give initially the siloxetane 483, which undergoes diverse isomerizations or fragmentations depending on the substituent R, yielding products 484-491 (equation 161)". 

Cycloaddition reactions of acyl silanes appear to be rare, but Brook has shown that a-silyloxy bis(trimethylsilyl)silenes (52), generated photochemically from acyl tris(trimethylsilyl)silanes (vide infra, Section IV.A.4), undergo [2 + 2] and [4 + 2] cycloaddition reactions with ketones, and [4 + 2] cycloaddition reactions with less bulky acyl silanes, as illustrated in Scheme 8717,24 26 72 73,201. They do not, however, react with their parent acyl tris(trimethylsilyl)silanes. 

Brook and coworkers have already reported a similar reaction mode in the reaction of silene with oxygen53. In that case the intermediary silanone which arises from the... 

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