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 513

324157, and for the corresponding esters R1O NDC: from the oxime ethers 325158. Pyrolysis of aminoalkylidene compounds proceeds similarly to give the aminovinylidenes but, depending on the groups present, these may isomerize to aminoalkynes (‘ynamines’), ketenimines or nitriles159. Thus, for example, FVP of 326 at 650 °C results in loss of CO2 and MeCN to give ketenimines, RNDCDCH2, as the final products, except for R D H, where the aminoalkyne, HC C NH2 is more stable. Loss of CO2 and MeCN from 327 occurs to give the ynamine, PhC C NH2, at 750 °C, but at 850 °C the isomeric phenylacetonitrile is the product isolated in 76% yield. For the more highly substituted example 328, FVP at 650 750 °C gives the stable ynamine 329. A different process is observed for cases such as 330 where FVP at 300 °C results in loss of HX to give the ketenimine compounds 331160. At higher temperatures these lose only CO2 to afford the vinylnitrenes 332, which can rearrange to either the azirines 333 or the bis-imine 334 of carbon suboxide.

With a ketene dithioacetal function in the 4-position as in 335, FVP results in loss of CO2 and PhCN to give 336, which rearranges to the dithioalkyne 337161. The

(324)X = NOH

(325)X = NOR1

Ph

Me N

corresponding process for both the cyclic analogues 338162 and 339163 proceeds similarly to give first 340 and then the cycloalkynes 341. However, these now eliminate ethylene to give the cumulenes 342, which in the nitrogen case isomerizes to thioformyl cyanide 39. Reactions of this type can sometimes take an unexpected course, and FVP of 343 at 800 °C gives only a trace of the expected cyclic trimer of cyclohexyne 345 formed via 344. The major product is the pyrrole carboxylic acid 346164.

For the isomeric 2H-isoxazolin-5-ones 347, elimination of a nitrile is impossible and only CO2 is lost upon pyrolysis to give the iminocarbenes 348, which cyclize as shown to provide a useful synthesis of fused imidazoles 349. A wide range of examples has been examined as illustrated by the products 350 353, which are generally formed in almost quantitative yield using FVP at 500 600 °C165,166.

In contrast to the isoxazolones, the isomeric 4H-oxazolin-5-ones which lack the weak N O bond usually lose only CO upon pyrolysis. Thus, FVP of a variety of compounds 354 at 600 °C gives the acylketenimines 355167. The 2H-oxazolin-5-ones do lose CO2 to give iminocarbenes, which may undergo rearrangement, as in the formation of 357 from

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 515

FVP of 356168. Alternatively, as in the pyrolysis of 358, the carbene 359 can be trapped by intraor intermolecular addition to give 360 and 361, respectively169.

MeO2 C C C CO2 Me

Me

G. Meldrum’s Acid Derivatives

Pyrolysis of Meldrum’s acid (2,2-dimethyl-1,3-dioxane-4,6-dione) 362 proceeds by loss of acetone and CO2 to give ketene. Because of the ready availability of the starting material and the ease with which it can be functionalized at the acidic 5-position, pyrolysis of Meldrum’s acid derivatives has been widely studied. Pyrolysis of the 5-formyl and 5-acyl derivatives 363 gives formyl or acylketenes 364, which can be trapped in a number of ways170,171. In many cases, loss of acetone and CO2 is accompanied by loss of CO to give a carbene, and this is illustrated by FVP of 365 at 560 °C which affords the ˛-diketone 368 by way of ketene 366 and carbene 367172.

The majority of work has involved derivatives 369 with a double-bonded group at C-5 which provide access to either the cumulenes 370 or, by further loss of CO, the carbenes 371. FVP of the acenaphthylene compound 372 at 860 °C gives the vinylidene, which rearranges to ethynylacenaphthylene 311151. The formation of cyclopentylideneketene 374 by FVP of 373 at 500 °C is complicated by simultaneous production of the isomer 377

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 517

O

Me

Ph

O

O

(369)

O

O

Me

Me

O

O

(372)

whose formation is explained by loss of acetone from the tautomeric form 375 to give 376, which then loses CO2. At lower temperatures 376 can be isolated173,174. The unsaturated analogue, cyclopentadienylideneketene (380) may be formed by FVP of either 378 at 650 °C175 or 379 at 550 °C176. Under these conditions it loses CO to give the carbene 381 which isomerizes to benzyne, and in the latter study carrying out the pyrolysis over bitumen as a hydrogen source gave benzene.

Cyclization of a variety of alkylideneketenes generated by the Meldrum’s acid method has provided a number of useful syntheses of nitrogen heterocycles. Thus, for example, FVP of 382 leads to the azepinone 383177, and the indole compounds 384 may give

CO2 H

O

O

Me

Me

O

Pyrolysis of the cyclic aminoalkylidene Meldrum’s acids 411 and their six-membered ring analogues at 600 °C proceeds with cyclization of the intermediate 412 on to the N- alkyl group to afford pyrrolizidinones 413185,186. In the case of the N-methyl compound 411 R D H , the unusual alkylidenemalonic anhydride 414 resulting from loss of only acetone could be detected from FVP at 450 °C185. FVP of the N-allylpiperidine compound 415 provides convenient access to the bicyclic azepinone system 416187. The pyrazolyland pyrrolylamino compounds 417 give the fused pyridazinones 418 on FVP at 700 °C188.

The major pyrolytic process for the methylthioamino compounds 419 is loss of acetone

CO2 and MeSH to give the cumulenes 420. For R D H this isomerizes to cyanoketene117, while in other cases 420 can be isolated189. For 419 R D Ph an additional product is the quinolone 421. Replacement of the amino group of 419 by Me, H or SMe leads to a change in behaviour to give dihydrothiophene-3-ones as the products, as illustrated by the conversion of 422 to 423190, and of 424 to 425190,191 on FVP at 600 625 °C.

The 5-imino Meldrum’s acid derivatives 426 428 have also been investigated. Heating the oxime 426 in boiling toluene provides a route to nitrosoketene, ODN CHDCDO,