Reactions of 1,3,5-Trisilacyclohexanes

Several studies reported on silabenzenes in which more than one carbon was snbsti-tuted by silicon. Maier and coworkers observed 1,4-disilabenzene (11) in a matrix and identified it by its IR absorbtion at 1273 cm and its electronic absorptions at 408, 340 and 275 nm . Evidence for the formation in the gas phase of 1,3,5-trisilabenzene (12) as a ligand in the dehydrogenation reaction of 1,3,5-trisilacyclohexane with [Cp,Fe]+, [Cp,Co]+ and [Cp,Ni]+ was recently reported by Bjamason and Amason. The strncture of 12 was assigned on the basis of the coUision-indnced dissociation spectrum and from deuterium-labelling experiments. ... 

Reaction of 1,3,5-trisilacyclohexane 290 containing two CCI2 groups with MeMgCl generated... 

Fig. 17. Si-NMR shift values of 1,3,5-trisilacyclohexanes and a schematic representation of their respective products from reaction with MeMgCl...

Si-fluorinated derivatives of 1,3,5-trisilacyclohexane behave in a totally similar fashion [124]. The presence of a CCI2 group causes reaction according to... 

The influence of Si-substituents on the formation and structure of the ylides of 1,3,5-trisilacyclohexanes were investigated in the products from reactions of Me3Si—PMe2 with C-chlorinated or C-brominated 1,3,5-trisilacyclohexanes containing only one CX group [133] ... 

Again, the application of isoprene as the quenching substrate is less effective and only low yields of the expected silacyclohexenes were obtained. Slight variations of reaction conditions lead to the formation of additional products. For example, a small amount of the 1,3,5-trisilacyclohexane 3 can be detected within 12 h in addition to tetramethyl-l,3-disilacyclobutane as the main product if the reaction of Me2SiCl2 is carried out in THF at -78 C using lithium powder with a higher content of sodium (Eq.4). 

Of special significance is the isolation of the cyclic compounds 1,3-disilacyclobutane and 1,3,5-trisilacyclohexane. Their formation is connected to the previously mentioned reactions by assuming that a further reaction occurs,... 

The reaction of different partly phenylated and fluorinated 1,3,5-trisilacyclohexanes with LiAlH yields the corresponding SiHPh compounds which can then be separated by means of HPL [65]. 

Further bromination of (HBrSi—CH2)3 is achieved in methylcyclohexane. However, the decreased reactivity means that the time required for complete reaction at room temperature is considerably longer. Partly brominated 1,3,5-trisilacyclohexanes exist at room temperature as colorless liquids which maintain an oily appearance as the extent of bromination is increased. 

One observes from the NMR spectra of the reaction mixtures, that the multiply methylated 1,3,5-trisilacyclohexanes are cis-trans mixtures. This is also indicated during gas chromatographic separation by the largely differing melting points of the isomers observed in the product cooling traps [31]. As a result, by addition of excess... 

If instead the reaction is conducted in the presence of MeLi (Table 41), methyla-tion of the SiH groups is accompagnied by attack on the molecular skeleton. Apart from methylated silanes including a large amount of SiMc4, carbosilanes containing more than two Si atoms were also produced, which comprised not only those compounds formed due to simple chain extension but also cyclic fully methylated 1,3,5-trisilacyclohexane. Even on addition of 12 mole equivalents of MeLi, the reaction ceases as soon as all products are Cl-free. Under no circumstances does methylation of bridging C atoms occur as is observed in reactions of perchlorinated carbosilanes with MeLi [122]. 

Reactions of Partly Chlorinated 1,3,5-Trisilacyclohexanes with MeMgCl... 

Reactions of Si-Methylated, Partly C-Brominated 1,3,5-Trisilacyclohexanes with BuLi and EtMgBr... 

In reactions with organometallic agents, the Si-methylated partly C-brominated 1,3,5-trisilacyclohexanes exhibit some similarities — and a number of differences — to their Si-chlorinated C-chlorinated analogues. For example, the 2,2,4,4-tetrabromo-hexamethyl-l,3,5-trisilacyclohexane yields with n-BuLi the dialkylated derivative. 

Reactions of (HBrSi—CH2)3 with KFe(CO)2cp or KMn(CO)5 generate dimetal substituted derivatives, and only in the case of KCo(CO) is the formation of the trimetallo species 3 achieved, along with a tetrasubstituted 1,3,5-trisilacyclohexane species. The compounds are presented in Table 65. The thermal stability of these Co(CO)4-substituted 1,3,5-trisilacyclohexanes decreases as the number of Co(CO)4 substituents coordinated to the ring increases. 

The investigations described so far indicate that reactions of metal carbonylates in general lead only to the coordination of two metal carbonyl groups to the 1,3,5-trisilacyclohexanes. From the chemistry of silylcobalt compounds it is known that silanes of the type R SiH (n = 1-3) cleave dinuclear Co2(CO)g, producing silyl-cobalt-carbonyl complexes [166, 167, 171]. 

Utilizing the method just described, higher metal-carbonyl-substituted 1,3,5-trisilacyclohexanes can be achieved, as the reactions of compounds 3, 379 and 376 with Co2(CO)8 show [163]. 

If the reaction mixture is run over an AI2O3 chromatographic column, H2PtCl6 is sufficiently removed [74], and formation of highly viscous polymeric reaction products is prevented. Yet, even under these conditions and with a mole ratio of mono-silylethyne to l,3,5-trimethyl-l,3,5-trisilacyclohexane of 1 1, it was not possible to prevent completely the formation of bis- and trissubstituted 1,3,5-trisilacyclohexanes [76]. 

While only 1,2-disilaalkanes emerge from the catalytic addition of Si—H groups to Si-substituted C=C double bonds [72], 1,1- as well as 1,2-disubstituted products arise from the analogous addition to alkynes [73]. The corresponding reaction of 1,1,3,3,5-pentamethyl-l,3,5-trisilacyclohexane with HC — C—SiMe2—CH2CI proceeds without difficulty to... 

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