Silanones matrix isolation

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. 

Up to 1986 several silanones have been matrix-isolated and their IR spectra were recorded1. Most of these silanones are accessible by intermolecular reactions in the matrix which take place under specific conditions, that allow the diffusion of the reagents. 

No silanones 71, i.e. compounds with silicon-oxygen double bonds, have been isolated yet either neat or even in dilute solution, but matrix-isolation techniques have recently allowed their direct observation. Consequently, most of our experimental knowledge of silanone properties still originates in studies on transients19. Theory, being a primary source of reliable fundamental information, is therefore extremely valuable to the study of these species. 

No stable silanones are known at this time. The only direct spectroscopic evidence has been obtained by the matrix-isolation technique. Except for this, silanone chemistry is limited to the transformation of transient intermediates. 

DFT calculations predict a very small barrier (ca. 1 kcal/mol) for the rearrangement of silanone oxide 3f to dioxasilirane Although ab initio calculations at the MP2 level of theory predict a somewhat larger barrier (ca. 6 kcal/mol), it is safe to say that the barrier for the 3 4 rearrangement is much smaller than the barrier for the rearrangement of 5 to 6, while the former rearrangement is considerably more exothermic. This explains why, even under the conditions of matrix isolation, only the siladioxiranes 4, and not the proposed primary adducts of molecular oxygen and silylenes 1, the silanone oxides 3, are observed. 

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. 

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