Phosphorus ylides, pyrolysis

Flash vacuum pyrolysis (600 °C) of arylmethylsulfonyl-stabilized phosphorus ylides... 

A new stabilized phosphorus ylide 357 designed to undergo thermal tandem cyclization has been prepared. Upon flash vacuum pyrolysis at 850 °C, loss of PhsPO and Me radical results in tandem cyclization to give [l]ben-zothieno[3,2- ][l]benzothiophene 356 (Equation 7) <1998J(P1)3937>. 

TABLE 1. Pyrolysis of acyl phosphorus ylides 253 to afford alkynes 255... 

Heterocyclic phosphorus ylides (e.g., 114, R = Me) containing an azepinedione nucleus have been prepared, although in low yield (13-26%), by flash vacuum pyrolysis (FVP) of the corresponding open-chain ylide precursor 113 (Equation 15) <2001TL141>. The authors noted, however, that the azepinediones were isolated from the inlet residue rather than from the cold trap after the pyrolysis tube. 

A series of cyano(arylcarbamoyl)phosphorus ylides (6) and cyano(arylthiocarbamo-yl)phosphorus ylides (7) have been prepared and fully characterized.33 Pyrolytic reaction products obtained by FVP have shown that thermal extrusion of PI13PO or PI13PS occurs (Scheme 5). Kinetic study of the gas-phase pyrolysis of each ylide by a static method showed that these reactions are unimolecular and first order with no significant substituent effect, but the thiocarbamoyl ylides (7) react 40-65 times more rapidly than their carbamoyl analogues (6). 

Heterocyclic phosphorus ylides (e.g. 5, R = Me) have also been prepared, although in low yield, by flash vacuum pyrolysis of the corresponding open-chain ylide precursors 4 [01TL141]. 

There are also cases where addition of thiyi radicals to unsaturated carbon-carbon bonds is the crucial step for thiophene ring formation. Flash vacuum pyrolysis (FVP) of the phosphorus ylide 90 affords initially the alkyne 91, which produces thiyi radical with loss of methyl radical. Cyclization of the resulting radical affords thienothiophene 92 as the final product with loss of one more methyl radical (Scheme 20) <1995SL53>. FVP of ylide 93 results in a multistep cascade reaction leading to 7-(2-benzothienyl)benzofuran 94 (Scheme 21) <2001SL228>. 

Flash vacuum pyrolysis of a sulflnyl-stabilized phosphorus ylide (85) gave a carbene (86) by loss of phosphine oxide. The carbene then extruded carbon dioxide to give the vinyl sulfide (87). The corresponding a-diazo sulfones were also examined and similarly proceed to give vinyl sulfones. ... 

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