Flash vacuum pyrolysis, limitations

Methylbenzofuran-3-carbaldehydes undergo ready condensation with Meldrum s acid (isopropylidene malonate) to afford arylmethylene derivatives 83. These on flash vacuum pyrolysis at 500-600 C give 3-dibenzofuranols 84 (Scheme 21). The arylmethylene derivative, e.g., 85, presumably undergoes conversion to a methylene ketene (86, Scheme 22) on pyrolysis, which undergoes a [1,5-H] shift and subsequent cyclization and tautomerization, yielding the dibenzofuranol 87. The derived methyl ether 88 has been converted by mild acetylation with acetyl chloride and aluminum chloride and subsequent boron trichloride-induced demethylation to the natural product ruscodibenzofuran (8). A limitation is imposed on this method because 3-acetyl-2-methyldibenzofurans fail to condense with Meldrum s acid so that l-methyl-3-dibenzofuranols are not available by this method. ... 

A recent example of a cyclopentene annulation utilizing flash-vacuum pyrolysis led to hydrocarbons of medium molecular weight, e.g. 39 and 40. The practical limit of this particular method is defined by the volatility of the starting vinylcyclopropanes under high vacuum (approximately 400-500 MW at lO Torr). 

Maquestiau and co-workers investigated thermal rearrangement of iV(l)-acyl-1,2,4-triazoles to oxazoles using flash vacuum pyrolysis (FVP). In this case, FVP of an A (l)-acyl-1,2,4-triazole 83 leads to a 5-aryloxazole 86. It was assumed that a [1,5] sigmatropic shift of the acyl group in 83 produced 84, followed by loss of nitrogen to yield the diradical 85, which cycUzed to 86 (Scheme 1.25). A limited number of examples were investigated, and the results are shown in Table 1.4. The yields of 86 are excellent for iV(l)-aroyl-l,2,4-triazoles but modest in the case of A(l)-acetyl-1,2,4-triazole. Nonetheless, the author noted that this represented a one-step synthesis of a 5-susbstituted oxazole in contrast to the conventional four-step syntheses usually employed. 

As an alternative to thermal decomposition, CVD, and laser-assisted H elimination, the flash pyrolysis of soluble alkylpolysilyne in a vacuum is reported recently. Unless they are under vacuum conditions, polysilanes and polysilyne may convert to silicon carbide (SiC). The flash process, which is a rapid removal of volatile organic substances and H2 gas, enables the control of the dimension of Si structure from 2D to 3D. Flash pyrolysis of alkylpolysilyne in a vacuum above 500 °C leads to the formation of poly- Si with a minimal amount of SiH termini, although the size of the crystals is limited to the range between several pm and several nm. 

---