Siloxy compounds

Apparently, the by-products of the metallic aluminum are different from the trimethylsilyl case again siloxy compounds can be collected from the walls of the reaction vessel and can be characterized by spectroscopic and X-ray structural means as [(tBu)Me2SiO]2 A1H 2 and HAl2[OSiMe2(fBu)]5 [34]. It is remarkable that in this case no Al[OSiMe2(tBu)]3 seems to form, and that, in contrast to the corresponding trimethylsilyl derivative, [(tBu)Me2SiO]2 A1H 2 is stable. 

A very useful option for synthesis of a-functional siloxy compounds is the addition of variously substituted (functional) organosilanes. Both aldehydes and ketones react under these reactions although either basic or acidic catalysts are required in some cases (see Scheme 32). 

A procedure for alkylation of C=0 double bonds in the presence of (metal-free) organocatalysts and non-metallic nucleophiles has been reported by the Iseki group for trifluoromethylation of aldehydes and ketones [185]. On the basis of a previous study of the Olah group [186, 187] which showed the suitability of non-chiral phase-transfer catalysts for trifluoromethylation of carbonyl compounds, Iseki et al. investigated the use of N-benzylcinchonium fluoride, 182, as a chiral catalyst. The reaction has been investigated with several aldehydes and aromatic ketones. Trifluoromethyltrimethylsilane, 181, was used as nucleophile. The reaction was, typically, performed at —78 °C with a catalytic amount (10-20 mol%) of 182, followed by subsequent hydrolysis of the siloxy compound and formation of the desired alcohols of type 183 (Scheme 6.82). 

Not only hydrido derivatives can enter into this reaction entries 68 and 69 show that siloxy compounds and C-silyl-substituted phosphine methylenes also undergo exchange (303, 306, 310). In the latter case, the silyl group with the most electronegative substituents moves to carbon. Respective examples are... 

Direct reductive cleavage by catalytic hydrogenolysis of a cyclopropane C-C-bond is only possible in exceptional cases, where the three membered ring is further activated by a phenyl or a vinyl group. With unpoisoned catalyst a subsequent reductive desil-oxylation occurs to afford esters like 124 (Eq. 48), whereas addition of small amounts of triethylamine allows isolation of the desired siloxy compounds (e.g. 125, Eq. 49). Interestingly, both reactions demonstrate that the cleavage of the cyclopropane bond proceeds non-stereoselectively with inversion and retention at C-l and C-2, respectively 79). 

A mixture of cyclohexanone (0.96 g. 10 mmol) and CF,TMS (1.7 g, 12 mmol) in THF (10 mL) cooled 10 0 C was treated with a catalytic amount of TBAF 3H2O (ca. 20 mg). Instantaneously, a yellow color developed with the initial evolution of TMSF, and the mixture was brought to rt and stirred. The resulting siloxy compound was then hydrolyzed with aq IICl. The mixture was extracted with EtjO (75 niL), dried (MgSO, ) and concentrated, The residue was either distilled or crystallized from a suitable solvent to give the product this alcohol, a colorless oil, solidified in the receiver during distillation yield 1.28 g (77 %) bp 72 73 C/40 Torr mp 59-61 C. 

The siloxy compound and benzyl chloride treated with tetrabutylammonium fluoride 3 HjO in THF and left at ambient temp, (for 16h) gave benzy l 4-bromophenyl ether... 

Phosphonyl and phosphinyl fluorides can be obtained from fluorophosphoranes by using the appropriate siloxy compound (6.191,6.192). The hydrolytic stability of fluorophosphoranes increases with the number of P-C bonds present, and generally the sequence is... 

The siloxy compound of cyclohexadiene is able to polymerize as shown in eq. (19.51). However, it is difficult to dehydrogenate the siloxy polymers by heating, but their acetoxy compounds are able to be dehydrogenated at 310-340 ""C to afford a high molecular weight poly-p-phenylene [95,95a]. 

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