Brook-type silenes

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. 

In view of the evident reactivity of the Brook-type silenes toward carbonyl compounds and the fact that these silenes were prepared by the photolysis of acylsilanes, it is natural to ask why the silenes apparently did not react with their acylsilane precursors. This question has been answered recently. On the one hand, as shown in Scheme 19, the silene Ph2Si=C(OSiMe3)Ad apparently did add in a [2 + 2] manner to its acylsi-... 

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. 

Brook-type silenes with less than two trimethylsilyl groups at the silicon tend to be photolabile90,96. Thus silenes Me3SiR1Si=C(OSiMe3)Ad-l 137 undergo a... 

The addition of methanol to Brook-type silenes at room temperature using an excess of methanol was found to be a non-stereospecihc process, as is evident from its stereochemical outcome. Methanolysis of a 1 4 mixture of E/Z-392 gives a mixture of two diastereomeric methanol adducts SS- and RS-393 in a relative ratio of 2.5 1 (equation 114)95. 

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)". 

Brook -type silenes have been found to undergo [4+2] and [2+2] cycloadditions with imines R2C=NR to give silatetrahydroisoquinolines or silaazetidines, respectively248. If formed, the [4 + 2] adducts generally rearrange slowly in the dark or more rapidly when photolysed to the thermodynamically more stable [2 + 2] isomers. 

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