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 493

IV. COMPOUNDS CONTAINING C=O DOUBLE BONDS

The pyrolysis of carboxylic acids and their derivatives in the period up to 1980 was reviewed in a previous volume of this series84.

A. Aldehydes and Ketones

The characteristic thermal reaction of these compounds is decarbonylation and new examples include the formation of the ruthenocene compounds 158 by FVP of 157 at 640 °C85, and the decarbonylation of the cyclopentenone 159 to give the synthetically

494 R. Alan Aitken and Andrew W. Thomas

useful 1,3-diene 160 in 72% yield upon FVP at 630 °C86. Remarkably, if the last reaction is conducted at 740°C, there is complete degradation to give a high yield of p-cresol. The transannular aldol reaction which occurs upon FVP of dione 161 at 500 °C to afford 162 is explained by the electrocyclic reaction of the functions as in 163 to give the enol 164, which is then activated towards attack on the other ketone function87. The alkoxycyclobutenones 165 are in equilibrium with their ring-opened vinylketene isomers 166 under FVP conditions above 400°C. At higher temperatures the alkoxy group migrates

elimination

B. Esters and Thioesters

The pyrolysis of benzyl benzoate has been examined in detail under FVP conditions over the range 750 900 °C89. While the main products are the expected diphenylmethane, toluene, biphenyl and bibenzyl, these are accompanied by a number of interesting minor products.

The thermal elimination of acetic acid from alkyl acetates bearing a ˇ-hydrogen atom, which proceeds by a six-membered ring transition state, has been widely used as a method

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 495

of alkene preparation as exemplified by conversion of 169 into 170 by pyrolysis at 450°C in a nitrogen stream90, and the formation of the anti-Bredt alkene 172 from 171 at 400°C91. The method has also been used to form CDN double bonds in a few cases such as that of the highly reactive 2-azacyclopentadienone 174, formed by FVP of 173 at 350°C and trapped in a matrix92. By the use of suitable polyacetates, dienes and trienes can be obtained and Trahanovsky has described the generation of the interesting trienes 176 and 178 by FVP of the corresponding acetates 175 and 177 at 900 °C and 860 °C, respectively93.

Access to 1,3-dienes can also be gained by 1,4-elimination of acetic or benzoic acid from suitable ester substrates and this has been examined in detail for furan compounds by Trahanovsky and coworkers. Thus, the ‘furanoradialene’ 180 can be prepared in 13% yield by FVP of the diacetate 179 at 570 °C or in 30% yield from the corresponding dibenzoate at 610 °C94. These reactions are not as straightforward as they might appear since it is likely that migration of the acetoxy group to the 2-position by a [3,3] sigmatropic process followed by 1,2-elimination is involved rather than direct 1,4-elimination. While the benzoate 181 gives only 182, isolated in 51% yield as its dimer 183 upon FVP at 640 °C, the isomeric compound 184 gives both 183 and the cyclobutenone 187 under similar conditions95. This second product results from two sequential migrations of the benzoate group to give 185, which then suffers ˛-elimination of benzoic acid. The

resulting allenylketene 186 finally cyclizes to give 187 in 21% yield. The benzofuran system 188 behaves in a similar way to 181, giving 189 upon FVP at 630 °C96, and the method has recently been extended to more hindered compounds such as 191 formed from the corresponding benzoate 19097. In the absence of an adjacent methyl group as in 192, the 1,3-migration still occurs to give 193 but this undergoes ˛-elimination to give the oxacarbene 194, which rearranges to provide the first synthesis of the methylenebenzocyclobutenone 19598.

206, while, under similar conditions, the 1-ester 207 first rearranges to the 2H isomer 208 which then gives 209, a product also accessible from 210.

Pyrolysis of dialkyl oxalates is a well known method for the generation of alkyl radicals and FVP of 211 at 700 900 °C has been used to generate the o-allylbenzyl radical and examine its reactivity which is dominated by cyclization to give tetrahydronaphthalene 212104. The enol ester 213 gives the bicyclic ketone 215 on FVP at 420 °C by way of initial isomerization to the 1,3-diketone 214 whose enol form is ideally set up for an intramolecular Diels Alder reaction105. An intramolecular Diels Alder reaction is the first step in the remarkable pyrolytic conversion of the perfluoronaphthyl propiolate 216 into a mixture of the isomeric naphthocyclobutenones 217 and 218 and the naphthocyclopropene 219 upon FVP at 550 °C106. This is then followed by loss of CO and a series of 1,2- fluorine shifts to give the observed products. A number of useful synthetic procedures based on cyclization of ester-containing phenoxy radicals have been developed. Thus,

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 499

O

O

2

O

O O

O

Me

Me

O O

Me

10. Pyrolysis involving compounds with CDC, CDN and CDO double bonds 501

conditions, the diethyl ester fragments to give ethynylpyrrole 231. This is explained by twofold loss of ethylene to give the diacid, which then dehydrates to the anhydride, and this loses CO2 and CO to give the product. Complete loss of an ethyl ester group is also observed when acetylenic esters RC C CO2Et are pyrolysed at 750 °C111. The formation of both PhC CH and PhC CD from PhC C CO2CH2CD3 points to a combination of different mechanisms operating. FVP of the cyano silyl ester 232 provides an unusual route to trimethylsilylaniline 234 by migration of the silyl group from O to N with loss of CO2 to give 233, which then cyclizes112.

C. Acid Chlorides and Amides

The characteristic pyrolytic process for acid chlorides is loss of HCl and this occurs for the bicyclic example 235 on FVP at 800 °C to give cyclohexa-1,2-diene 238 by way of the ketene 236 and carbene 237 as shown113. The product can be directly observed by low-temperature IR and forms a [2 C 2] dimer on warming up. FVP of o-toluoyl chloride 239 at 630 °C also results in loss of HCl to provide a dependable large-scale synthesis of benzocyclobutenone 240 in excellent yield114. Pyrolysis of trichloroacetyl chloride, Cl3C COCl, over a bed of zinc at 420 °C results in dechlorination