Pyrolysis reactions, 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. 

Phenylisosilacyanide is produced either by irradiation of triazidophenyl-silane in matrix isolation or by pyrolysis followed by trapping in noble gas matrix (Eq. 3). The reaction with /-butanol leads to the expected product6 ... 

In early studies, flash vacuum pyrolysis, a method that has proven very valuable in preparative studies of closed-shell compounds,was regarded as the method of choice for the production of radicals for matrix isolation studies. " The disadvantage of this method, which is very well suited for preparative studies of closed-shell compounds, is that the reaction occurs on the walls of a hot tube whose surface may trap radicals (this problem may be alleviated by coating the inside of the tube with gold ). Also, unless a very low vacuum can be maintained in the pyrolysis mbe, collisions between radicals may lead to gas-phase dimerization. 

The parent silabenzene 24 was first matrix-isolated by our group in 198035 by pyrolysis of precursors 25 and 26, which yield the expected silabenzene by retro-ene fragmentation. Later, it could be shown that in analogy to carbon chemistry the hydrogen elimination from silacyclohexadiene 27 also gives the silaaromatic 2436. This reaction is allowed by the Woodward-Hoffmann rules. In accordance with the Woodward-Hoffmann rules, it could be demonstrated that silabenzene 24 is not accessible by pyrolysis of the conjugated silacyclohexadiene 28 (equation 8). 

Gaseous mixtures of disilane 1 and argon (1 1000-2000) were subjected to flash pyrolysis at various temperatures and pressures. After leaving the hot zone the reaction products were directly condensed onto a Csl or BaF2 window at 10 K. The matrix-isolated products were studied by IR and UVA IS spectroscopy. Under the conditions of high-vacuum flash pyrolysis only trimethylsilane (2) and small amounts of acetylene were detected. Any C2H2Si isomer that might have formed was too unstable to be detected under these pyrolysis conditions. 

The formal substitution of a saturated carbon atom in compounds 8, 13 and 16 by silicon results in precursors 21, 17 and 20, which could all be synthesized. In contrast to the results above, pulsed flash pyrolysis of these oligosilanes gave rise to the formation of only one C2H4Si2 isomer, namely 18. Actually, irradiation of matrix-isolated 2-silylsilacyclopropenylidene (18) led to silylene 19 in analogy to reaction 14—>15. 

As mentioned in Section 1.07.3(b), irradiation (254 nm, 4 min) of 1,2,3-selenadiazole (27) in argon matrix produced a reactive intermediate benzoselenirene (28) (see Equation (6)), the formation of which was confirmed by IR spectra. The matrix-isolated benzoselenirene (28) undergoes ready rearrangement to 6-fulveneselone (29) on further irradiation (Equation (8)). Benzoselenirene (28) is also formed in gas-phase pyrolysis (560°C, ca. I03 min) of 1,2,3-benzoselenadiazole (27) if the reaction temperature is kept a little lower than required for complete fragmentation of (27). Selenirene (28) was found to undergo thermal rearrangement to (29), the IR spectrum of which is identical with that obtained in the exhaustive photolysis of (27). With increasing temperature, the amount of the new product steadily decreased and at 700°C its IR bands were at the limit of detection. 

Pyrolysis of 32 was carried out at 850 °C. The trimer was sublimed at ca 105 °C with argon as a carrier gas. Under these conditions, pure quinone methide 31 was matrix isolated on a 7.6 K KBr target as evidenced by the IR spectrum. In a similar experiment, the pyrolysis of trimer 32 was carried out without argon. When the neat quinone methide 31 was warmed above —92°C, new IR bands appeared, and the absorptions due to the quinone methide decreased. These newly formed bands became much stronger when the target was warmed further to —65 °C, and they are attributed to the formation of dimer 34 and trimer 32 by comparison with the IR spectrum of the trimer obtained in the preparative FVP work. Moreover, a low temperature NMR experiment revealed the existence of dimer 34. After the target was warmed to room temperature, additional bands due to the tetramer appeared. It was readily concluded that dimerization would be the first step of reaction of quinone methide 31. 

Evidence for additional silene-to-silylene isomerizations on simple systems is now available. Thus the pyrolysis of a silabicyclo[2.2.2]octadiene precursor for 1-chlorosil-ene229 produces a mixture of matrix-isolated 1-chlorosilene and chloromethylsilylene. In this instance the thermal rearrangement of the silene to the silylene is probably facilitated by the increased exothermicity of the reaction, since halogenated silylenes are particularly stable (equation 95). 

In some cases, cyclic and linear PDMS have been combined to form conetworks, and unsaturated cyclic side-chain fragments have been placed into polysiloxanes to make them thermoreactive. Interactions between ring polymers have also been analyzed and related to loops in chromatin. Also, some cyclic oligosiloxanes having polar end groups show liquid-crystal-line behavior, specifically smectic A and E phases. Finally, several PDMS cyclics have been exposed to vacuum pyrolysis and the products analyzed by matrix-isolation spectroscopy. The pyrolysis products obtained under a variety of conditions identified the radical reactions that were involved. 

A matrix-isolation spectral study of the intermediates and products of the pyrolysis, and also the pyrolytic oxidation of diborane(6), revealed no observable intermediates and only B and H2 as products of the former reaction, and only boroxin, [-B(H)-0-]3, as the product of the latter reaction. The study sought to observe such species as HBO, and, although the results did not eliminate the possibility of intermediacy of the latter, they were not observed [7]. 

Tetrafluoro-p-xylylene 23 has been matrix isolated by flash vacuum pyrolysis of the corresponding octafluoro[2.2]paracyclophane followed by condensation at 30 K with a large excess of argon. This is a further example of the use of matrix isolation to stabilize a highly reactive species produced in a thermal reaction, which then becomes the starting material for photochemical studies. Irradiation of 23 at 248 nm resulted in partial conversion to a new species, which was identified as the tetrafluoroheptafulvene 24 by comparison of the experimental and computed IR transitions. Several possible reaction mechanisms for this rearrangement were proposed, but no intermediate species was detected in the matrix photolysis experiments, so a final conclusion as to the mechanism could not be drawn. 

In the past two years, we have been able to isolate four C2H2Si isomers in an argon matrix after pulsed flash pyrolysis of 2-ethynyl-l,l,l-trimethyldisilane [3]. As was proposed earlier, another access to the C2H2Si hypersurface consists of the reaction of silicon atoms with acetylene [5]. Based on this information about the C2H2Si isomers, it was obvious to take the Si/acetylene system to refine our silicon evaporation techniques. 

Both reactions yield the crystalline product 12. Pyrolysis experiments show that 12 eliminates two equivalents of anthracene at about 950°C giving an identical product as isolated in matrix from the pyrolysis of 10 the nature of this compound is not yet known and is currently under investigation [4, 5]. 

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