Silanone dimerization

The silenes 10 and 14 were chosen for studies of the behavior of the new compounds towards benzaldehyde. As products of a standard Peterson mechanism, mixtures of the stereoisomers of the silanone dimers 18 and 19, respectively, were obtained, besides 2,2-bis(trimethylsilyl)styrene (17) (Scheme 9). 

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... 

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... 

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. 

When a toluene solution of a mixture of cyclotrisilane 34 and cyclohexyl isocyanate (or f-butyl isocyante) was heated at 70 °C, cyclic di- and trisiloxanes 37 and 38, i.e. the cyclic dimer and trimer of the silanone 36, were obtained together with the corresponding isonitrile RN=C. The formation of 37 as well as 38 was completely suppressed in the presence of hexamethylcyclotrisiloxane (19 D3) instead, quantitative conversion of 35 into 39, the formal insertion product of the silanone 36 into the Si—O bond of D3, occurred (Scheme 14). Since neither cyclodisiloxane 37 nor cyclotrisiloxane 38 reacted with D3 under the reaction conditions, the possibility that 37 or 38 is the precursor of 39 was ruled out. Whereas the oxidation of 35 with cyclohexyl and t-butyl isocyanates proceeded with exclusive formation of 37 and 38 (as the silicon-containing compounds) the reaction of 35 with phenyl isocyanate resulted in the formation of 37 in low yield. Furthermore, in this case the presence of D3 did not totally suppress the formation of 37. According to the authors, these results indicate that the oxidation of 35 with cyclohexyl and f-butyl isocyanates appears to use other reaction channels than that with phenyl isocyanate. 

Cyclic siloxanes are important precursors in the silicon industry, being formally dimers or trimers of silanone (R2Si=0), a known intermediate. Cyclic siloxanes have been synthesized by four routes, the conventional methods being the condensation of silanediol or the hydrolysis of species such as halosilanes or aminosilanes (Scheme l)16-20. Alternatively, oxidation of disilene by triplet oxygen (equation l)21-27 or oxidation of oxadisiliranes by singlet oxygen (equation 2)28-31 may be utilized. 

Thermolysis of 2,1,3-silaphosphaoxetane 38 furnishes the transient silanone, which immediately dimerizes to the l,3-disila-2,4-dioxetane 65 (Scheme 4) <19960M1845>. 

FIGURE 14. Schematic illustrations of passive-active oxidations of an Si(100)-2 x 1 surface with silanone intermediates (a) reactant, (b) on-top, (c) silanone, (d) product and (e) dimer bridge... 

Very few reactions of silanones have been proposed. Left to its own devices, H2Si = O will dimerize with no calculated barrier and an enthalpic gain of 106 kcal/mol. The dimer is also subject to facile insertion by silanone, and thus the observation of D3 and D4 products is often cited as evidence for silanone intermediacy. 

In the case of the reaction of 1 with COS the intermediate silathione dimerizes to the dithiadisiletane derivative III. In the reaction of 1 with RNCS the silathione does not dimerize, but it is trapped by the respective isothiocyanate in form of the formal [2+2] cycloaddition product IV. The intermediates Cp 2Si=X (X = O, S) (2) could not be isolated but derivatized by trapping reactions. In the presence of t-butyl methyl ketone the silanone (Cp 2Si=0) reacts in an ene-type reaction [9] to give the addition product 5 the silathione (Cp 2Si=S) is transformed in a first step to an addition product analogous to 5, in a further step this species reacts with 1 in an oxidative addition process to form the final product 4 (see Scheme 2). 

Compound 23 decomposes on thermolysis at 160°C in a sila-Wittig-type reaction to furnish the transient silanone ls2Si=0, which immediately dimerizes to the l,3-disila-2,4-dioxetane 24, and the phosphaalkenes 25 and 25A (Scheme 6). l,3-Disila-2,4-dioxetanes are well known from the work of West et al. [28]. Because of the drastic reaction conditions, the expected phosphaalkene 25 partially isomerizes under phenyl and silyl group migration to 25A. However, such an isomerization process was previously unknown for phosphaalkenes,... 

The potential energy surface for the head-to-tail dimerization of silanone (equation 25) was also studied137. The fascinating structure of the product 33 was discussed in Section IV.C.2.b. The dimerization is calculated to be highly exothermic (109.4 kcal mol-1 at... 

The evidence is the formation of the expected methanol adducts of the silanone and of the silene when methanol is present in the irradiated mixture. In an inert solvent the expected 2 4-2 cycloaddition product of the silanone and the silene is formed quantitatively76. If the suspected silene and silanone transients were present in the solution in a truly free form, one might expect the formation of some silene dimer and some silanone... 

It has been proposed298 that bis(trimethylsilyl)silanone is formed, presumably via a transient 2-silaoxetane, when tetraphenyl-a-pyrone reacts with l,l-bis(trimethylsilyl)-2-(trimethylsiloxy)-2-phenylsilene, generated from its dimer in a refluxing solvent, since the corresponding cyclotrisiloxane was produced (equation 196). 

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