Silanones reactions

It appeared that if heated at 160°C in a sila-Wittig-type reaction, 26 actually decomposes into the transient silanone Is2Si=0, which immediately dimerizes to 27 and the phosphaalkenes 28 and 28 (Scheme 6).14... 

The adducts 41 from 1 and ketones or thiobenzophenone undergo interesting photochemical cycloreversion to afford a silanone or silanethione intermediate 42 in addition to silene 43 both of these intermediates are trapped by ethanol, as shown in Eq. (14).68 71 In the reaction with the thiobenzophenone adduct 41 (R = Ph, X = S), the intermediate silene 43 (R = Ph) was detected by Si NMR.71... 

More recently, Belzner et al. reported a new type of oxygen transfer reaction from isocyanates to bis[2-(dimethylaminomethyl)phenyl]silylene (8)18 which was thermally generated from the corresponding cyclotrisilane 7, and they obtained some convincing results of the involvement of silanone 9 (Scheme 3). However, they found that silanone 9 is not stable enough to be isolated. Only cyclic di- and trisiloxanes 10 and 11 (i.e., the cyclic dimer and trimer of the silanone 9) were obtained together with the corresponding isonitrile as other main products when... 

Since the silanone intermediates have been postulated as key intermediates in reports on the synthesis and reactions of organosilicon compounds,27 the isolation of a stable silanone and its characterization should be one of the most fascinating subjects in the future chemistry of low-coordinate organosilicon compounds. 

As in the case of silanone 9, the reaction of the silylene bis[2-(dimethylamino-methyl) phenyl]silanediyl (8) with phenyl isothiocyanate was examined.29 In this reaction the expected silanethione 36 was obtained as a single product even in the presence of (Me2SiO)3, no insertion product of 36 into a Si-O bond of (Me2SiO)3 was observed (Scheme 10). 

Some remarkable chemistry is observed when silenes react with heteroatom systems, in particular carbonyl compounds (]>C=0) and imines Q>C=N—R). The reaction with ketones was first described by Sommer (203), who postulated formation of an intermediate siloxetane which could not be observed and hence was considered to be unstable even at room temperature, decomposing spontaneously to a silanone (normally isolated as its trimer and other oligomers) and the observed alkene [Eq. (14)]. Many efforts have been made to demonstrate the existence of the siloxetane, but it is only very recently that claims have been advanced for the isolation of this species. In one case (86) an alternative formulation for the product obtained has been advanced (204). In a second case (121) involving reaction of a highly hindered silene with cyclopentadienones,... 

Scheme 26. Reaction of 1 with C02 and trapping of the silanone (Me5C5)2Si = O.

The discussion continues regarding the role of silanone and cyclodisiloxanes as reactive intermediates in the formation of Si-O-Si bond.25 In studies of the reaction of dimethyldichlorosilane, phenylmethyldichlorosilane, or diphenyldichlorosilane with dimethyl sulfoxide in the presence of 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane, Weber and co-workers obtained products of the insertion of diorganosiloxy unit into the cyclic siloxane, accompanied... 

This last reaction is interesting because a four membered ring, a silaoxetane, has never actually been observed. It thermally decomposes to a molecule of olefine and a molecule of silanone RjSiK). 

Silanoncs (R2Si=0). As already written above, an attempt to prepare a four membered silaoxetane ring has not been very successful because of its instability but these reactions have been used conveniently to generate silanone S. 

One reaction of silanone is the insertion reaction as exemplified above. 

In the reaction of lithiated alkylbenzothiazoles with BTSP, the formation of methylated products always competes with the formation of silyl ethers. The demethylation of BTSP can tentatively be conceived as being derived from the nucleophilic attack of alkylben-zothiazole anion at the methyl group of BTSP with displacement of silanone and silanolate anion, while silyl ethers would form by nucleophilic attack of the alkylbenzothiazole anion at the oxygen of BTSP (Scheme 8/. ... 

Photolysis of 2,3-disiloxetanes 72 liberates the corresponding silene 73 and silanone 74 in a retro-[2 + 2] reaction52. The transient species are trapped by ethanol to give 75 and 76. In the absence of traps the silene undergoes a 1,3-hydrogen shift to give 77. Ene reaction of the initially formed intermediates, or cleavage of the 1,3-disiloxetane 78 yields the silyl ether 79 (equation 17). 

A siloxetane 526 as an intermediate from a [2 + 2] cycloaddition of silene 241 with acetone has been formulated by lshikawa134. It extrudes a silanone equivalent to give the vinyl ether 527. The second regioisomeric silene 242 generated together with 241 by photolysis of 240 undergoes an ene reaction instead (equation 179)134. 

All known silenes react violently with molecular oxygen with the exception of the stable 1-silaallenes which are air-stable. Along with carbonyl compounds the products of the oxidation of silenes are cyclic siloxanes from the corresponding silanone, depending on the reaction conditions3,8,28. For example, when the stable adamantylsilene 150 is... 

Evidence for the formation of silanones depends on two types of experiments. In the first, silanones are generated as unstable intermediates in reactions, and their formation is inferred from the isolation of trapping products with suitable substrates. The second approach is based on their generation in a low temperature matrix and their characterization by infrared spectroscopy which reveals v(si=o) at ca 1200 cm-1. These two groups of experiments are described below. 

When a mixture of methylsilane (or dimethylsilane) deposited with ozone in an argon matrix at 17 K was photolyzed, the corresponding silanone, MeHSi=0 (or Me2Si=0), was generated and characterized by their infrared spectra. In the case of the parent silane, H3SiH, a similar photolytic reaction with ozone in an Ar matrix led to the identification of SiO, H2Si=0 (2), (HO)HSi=0 (silanoic acid) and (HO)2Si=0 (silicic acid)18. 

It is well known that thermal decomposition of allyl-substituted silanes proceeds by retro-ene reaction with formation of transient species having a Si=C bond, such as silaben-zene, silatoluene and dimethylsilaethylene4b e. The kinetic data on the gas-phase pyrolysis of a similar allyloxysilane derivative, (l,l-dimethylallyloxy)dimethylsilane (16), and the results on thermolysis of allyloxydimethylsilane (17) in a flow system both indicate the participation of an intermediate silanone, (CH3)2Si=0 (10), as shown in Scheme 523. 

In 1989, Nefedov and coworkers have reinvestigated the thermolysis of the above-mentioned allyloxysilane derivatives 16-18 and of 2,2,6-trimethyl-2-silapyrane (21) using vacuum pyrolysis and matrix isolation techniques23. IR spectroscopic studies on the products isolated in the matrices enabled them to probe directly the intermediacy of 10 in these reactions and to discuss its thermal stability. Only in the case of allyloxydimethylsilane (17) did they find direct spectroscopic evidence for the formation of 10 by observation of its most intense band at 798 cm-1 in the matrix IR spectrum of the pyrolysis products. In all other cases silanone 10 was not detected and it was assumed that it is thermally unstable, undergoing fragmentation into SiO and CH3 radicals as shown in Schemes 7, 8 and 9 (the species actually observed in the matrix are indicated). In this paper, Nefedov and coworkers have reaffirmed the thermal and kinetic stability of dimethylsilanone 10 in the gas phase, which they had previously described19. 

Most recently, Milstein and coworkers reported a new type of homogeneous catalytic reaction that generates silanones from secondary silanols under extremely mild conditions. The platinum complex (dmpe)Pt(Me)OTf) (27) (dmpe = Me2PCH2CH2PMe2, OTf = OSO2CF3 was treated with an equimolar amount of the silanol, (i-PrhSilKOII) (28), in acetone to yield a new dimeric hydrido-bridged complex 29 and the trimer 30 of diisopropylsilanone, 31 (Scheme 12)29. 

The generation of methane in the reaction was evidenced by the 111 NMR spectrum of the reaction mixture. It was also shown that the newly obtained complex 29 reacts catalytically with silanol 28 to give the trimer 30 (presumably from trimerization of 31) with the evolution of hydrogen gas. In the presence of MesSiOMe the same reaction resulted in the formation of an insertion product of the intermediate silanone 31 as shown in the lower part of Scheme 12. The proposed catalytic cycle for the dehydrogenation of 28 with 29 is shown in Scheme 13. It should be noted, however, that spectroscopic evidence for the proposed silanones was not presented. 

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