Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0473 536

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 503

in undergoing loss of allyl radical, intramolecular hydrogen atom abstraction and then cyclization to the isoindolones 250107.

D. Acyl Phosphorus Ylides

The acyl phosphorus ylides of general structure 253 are unreactive crystalline solids whose stability is explained by the large contribution from the phosphonium enolate resonance form 254. It has long been known that they undergo ready pyrolytic elimination of Ph3PO in an ‘intramolecular Wittig reaction’ to give alkynes 255. This reaction has recently been the subject of extensive investigations and a summary of the most important examples is given in Table 1. Since the starting ylides 253 are readily prepared from a

a Terminal alkyne R2C CH formed.

b 1:1 mixture of E and Z isomers formed.

simple phosphonium salt 251 and an acid chloride 252, this represents a valuable synthetic route to alkynes, and in many cases the widely differing volatility of Ph3PO and the product means that using FVP they are collected in different regions of the cold trap in pure form avoiding the need for subsequent separation.

In cases where R1 is a stabilizing group (entries 1 11) conventional pyrolysis is satisfactory, but the use of FVP has allowed the extension of the reaction to cases with R1 D H or alkyl (entries 13, 18 20) where conventional pyrolysis does not work. For R1 D CO2Et, simply increasing the furnace temperature leads to loss of the ester group to give terminal alkynes and 1,3-diynes (entries 15, 17) as already noted in Section IV.B. The E Z isomerization which occurs for styrylalkynes at higher temperatures (entry 20) was also noted previously in Section II.C.

It is notable that this reaction is not possible with elimination of an ester oxygen (253,

R2 D O alkyl). Where there is a choice between two aldehyde or ketone carbonyls as in 256, a mixture of the two possible isomeric products 257 and 258 is obtained136. In view of this, it is surprising that when an additional carbonyl group is introduced, there is good

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 505

selectivity for elimination across the central position to give diacylalkynes (entry 21). With the tetraoxo ylides (entry 22), selectivity is again poor and a mixture is produced. An unusual observation in this case is that conventional pyrolysis is successful while FVP is not.

The oxalyldiylides 259 undergo twofold elimination of Ph3PO to give diynes 260 but this requires the high FVP temperature of 900 °C and is only useful for aromatic examples133. In several cases ylides have been designed from which the alkynes may undergo further reactions in situ leading to useful syntheses. Thus, while the ylides 261 give the expected alkynes 262 upon FVP at 700 °C, increasing the temperature to 850 °C leads to benzofurans and benzothiophenes 263 by loss of Mež followed by cyclization137. It should be noted here that for aliphatic groups R1, cyclization is followed by interesting radical reactions of this group leading to products 263 with R2 6D R1. This approach can be extended to tandem cyclization as illustrated by FVP of 264 at 850 °C to give the benzothienobenzofuran 265 and the reaction of 266 under similar conditions to afford 267138. At 500 °C, FVP of ylides 268 with an o-methylcinnamoyl group gives the expected enynes, but at 900 °C the alkenylnaphthalenes 64 resulting from further cyclization are obtained40.

Certain classes of acyl ylides have given more unexpected results. For the amino acid derived phthalimidoacyl compounds 269, for example, Ph3PO is lost between the ylide and a CO of the phthalimide to give the pyrroloisoindolediones 270 upon FVP at 500 °C139. In the case of the benzotriazolyl tributylphosphonium ylide 271, FVP at 450 °C results in

MeS

PPh3

S

Ph3 P O

mechanism appears to involve initial formation of a carbene which can rearrange directly to 272 or the isomeric 1,2,3-benzotriazine which loses N2 and rearranges to give 273. The reason for the phosphine rather than the phosphine oxide being eliminated in this case is unclear. The ylides 274 stabilized by both ester and sulphinyl groups undergo loss of Ph3PO on FVP at 600 °C to give vinylsulphides 277 as the main products141. The initially formed carbenes 275 apparently undergo intramolecular CH insertion to give the ˇ-lactones 276 which readily lose CO2.

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 511

308 at 860 °C, but these are accompanied by a much larger quantity of the isomeric fluorene150.

The case of the phenanthrene anhydride 309 is interesting since the carbene insertion product 310 is highly strained and undergoes ring opening under the conditions used (FVP, 870 °C) to give a mixture of the isomeric ethynylacenaphthylenes 311 and 312151,152. Tetraphenylphthalic anhydride (313) fragments by a similar mechanism upon FVP at 840 °C with carbene insertion into a phenyl CH to give the triphenylbenzopentalene (314) in 74% yield153. By starting from quinoline-fused anhydrides, a range of nitrogen heterocycles can be obtained. Thus, FVP of 315 at 800 °C provides access to the novel indenoindole 316154, and under the same conditions the o-tolyl and benzyl compounds 317 and 319 give benzocarbazoles155. In the first case, 318 is formed in 77% yield, but in the second, the expected product 321 is only obtained in 30% yield and is accompanied by the isomer 320 (60%). An unexpected result is obtained for the pyridinefused anhydride 322156. FVP at 900 °C gives mainly benzonitrile and diphenylbutadiyne, PhC C C CPh, whose formation is explained by cycloreversion of the aryne formed by loss of CO2 and CO.

O

O

O

+

(312)

Ph

O

Ph

Ph

Ph O

F. Isoxazolones and Oxazolones

The pyrolysis of 4H-isoxazolin-5-ones of general structure 323 proceeds readily with loss of CO2 and RCN to give the corresponding carbenes XDC:. This has provided a convenient route for the generation of fulminic acid, HO NDC:, by pyrolysis of oximes