Reaction intermediates silanones

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. 

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. 

In recent years remarkable progress has been made in the chemistry of organosilicon compounds containing a double bond to group 14 and group 15 elements. The reaction of a silylene-isocyanide complex with nitrile oxide generated the intermediate silanone, which with phenyl isothiocyanate afforded 1,3,2-oxathiasiletane 63, which is sensitive to hydrolysis (Equation 10) <2000CL244>. 

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... 

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. 

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,... 

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... 

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. 

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. 

A recent study of the photolysis of simple diazoalkanes 314 or diazirines 315, compounds known to lead to the formation of silenes under inert conditions, led, in oxygen-doped argon matrices, via the silene 316 to the siladioxirane 317. While previously postulated as an intermediate in silene oxidations, this is important experimental evidence for this intermediate. Continued photolysis of the system led to a compound identified as the silanone-formaldehyde complex 318, which on further irradiation led to the silanol-aldehyde 319. The latter compound itself underwent further photochemical oxidation leading to the silanediol 320160. The reactions are summarized in Scheme 58. Detailed infrared studies, including the use of isotopes, and calculations, were used to establish the structures of the compounds. 

Silane thiones have been shown to be formed as intermediates in a range of reactions, and either oligomerize or react with other suitable acceptors. Thus, pyrolysing the propar-gylthiosilane 9 at 600 °C gives the silanethione which dimerises, while in the presence of ketene at 800 °C the 4-membered ring 10 results. The latter decomposes at 900 °C to give silanone and thioketene (equation 26)51. 

The pathway of the reaction of 1 with the above heterocumulenes is discussed as follows (see Scheme 2) The first intermediate is a highly reactive [2+1] cycloaddition product or its ring-opened isomer, which easily loses CO or in the case of isothiocyanates the corresponding isonitrile RNC to give the silanone or silathione 2. The formation of CO was proved in the reaction of 1 with CO2 by reaction with the iron complex (HsC5)Fe(SMe2)3 BF4 [7]. In the reaction of 1 with isothiocyanates the formation of RNC was proved using NMR spectroscopy. The intermediates of the type Cp 2Si=X (X = O, S) are not stable and stabilized via different routes to the final products. In the reaction of 1 with CO2 the double bond species Cp 2Si=0 is transformed by CO2 to the formal [2+2]... 

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). 

Silanones, the silicon analogs of ketones, are produced via rDA reactions. One attempt to prepare a suitable DA precursor for retrograde decomposition to a silanone met with unexpected results. The desired DA reaction between 2,2-dimethyl-l-oxa-2-silacyclohexa-3,S-diene (251) and perfluoro-2-but-yne was complete in one day at room temperature. The observed product was o-bis(trifluoromethyl)ben-zene, as the initial adduct apparently underwent retrodiene decomposition to yield the intermediate dimethylsilanone (252) (equation 111). The occurrence of this retro ene process at room temperature was not consistent with the analogous extrusions of silenes and disilenes that require elevated temperatures. However, the reaction sequence was substantiated by comparison with its carbon analog in which tetramethylpyran and dimethyl acetylenedicarboxylate react at room temperature to afford only acetone and the corresponding phthalate. Stable adducts that extrude silanones are also known. Reactions of 2-sili yrans and nudeic anhydride provide stable adducts, such as (253), that decompose upon thermolysis... 

The intermediate formation of a silanone has been repeatedly postulated in the thermolytic decomposition of hydrosilylperoxides370,37, and the claims were supported by several trapping experiments. A recent critical reexamination372 came to the conclusion that transient silanones might be involved, but there is no need to include them in the mechanism of decomposition. The trapping products might be the results of other reactions and could well not involve silanones at all. 

Silanones have been often suggested as intermediates formed when the pyrolytic preparation of silenes is performed in the presence of ketones1 or oxygen293. These reactions have already been discussed (Sections III.B.3.C. and III.B.3.g) and will only be mentioned briefly here. 

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