Arynes pyrolysis

The pyrolyses of 2-halogenobenzoates proceed at temperatures around 250° and yield xanthones U. The pyrolysis of sodium penta-fluorobenzoate thus yields octafluoroxanthone (18) 3G.37). Although these reactions are known to involve arynes precise mechanistic details are not known but may well involve the following scheme. 

Methods such as the pyrolysis of (106), that do not require strongly basic conditions, have been used to generate arynes in bulk for preparative purposes, and another, even better, method is oxidation of l-aminobenzotriazole (107) with lead tetraacetate ... 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

In a study of the formation and reactions of arynes at high temperatures. Fields and Meyerson plyrolyzed a thiophene solution of phthalic anhydride at 690° by mass spectrometry and gas chromatography they found benzene, naphthalene, benzo[b]thiophene, phenylthiophenes, bithienyls, thienothiophene 1, and naphthothiophene in the pyrolysis products. Pyrolysis of thiophene itself produced benzo-[Z lthiophene, thienothiophene 1, phenylthiophene, and bithienyl. The... 

Didehydropyridine (97) is less well investigated, and the existence of this intermediate is not yet established definitively. Aryne 97 has been postulated as an intermediate in the gas-phase pyrolysis of 2,3-pyridinedicarboxylic anhydride (107) at 600 °C, which hnally yields (3-ethinyl acrylonitrile (108). Indirect evidence stems from trapping experiments with benzene and thiophene. Photolysis of matrix isolated 107 does not produce 97 (Scheme 16.25). Nam and Leroi ... 

A number of research groups have explored vapour phase pyrolytic reactions using apparatus designs (available from, for example, Aldrich), which allow for controlled vapour flow (and hence heat contact time), under moderate to high vacuum conditions. These techniques enable preparative procedures to be developed which involve the trapping of reaction intermediates (e.g. carbenes, nitrenes, arynes, etc.), and also the study of very short-lived species by the interfacing of the pyrolysis unit with, for example, a mass spectrometer. This specialist area has been comprehensively reviewed.26... 

With the discovery of benzyne formation by pyrolysis of phthalic anhydride, a new field was opened for the investigation of aryne reactions at high temperatures. A first concern was to determine the generality of aryne formation from aromatic acid anhydrides. Such syntheses could be of considerable significance because of the enormous quantities of aromatic mono- and polyanhydrides available from petroleum aromatics by oxidation. 

Tetrachlorophthalic anhydride gave a relatively low yield of products derived from tetrachlorobenzyne. The pyrolysis tube was badly carbonized evidently extensive decomposition of the anhydride, the aryne, or the chlorinated products had occurred. Tetrabromophthalic anhydride gave a still lower yield of products. These are not listed in the table because none of them retained all four bromine atoms and could be definitely ascribed to reactions of tetrabromobenzyne, although tri-bromo- and dibromonaphthalene were present in appreciable amounts. Tetraphenylphthalic anhydride also gave low yields of products of the reaction of tetraphenylbenzyne with pyridine. This was not because of the stability of the anhydride and its reluctance to form the aryne, but rather because the aryne preferred to stabilize itself intramolecularly. The behavior of tetraphenylphthalic anhydride is discussed in another Section. 

Pyrolysis of 1,2-dicarboxylic anhydrides in the p5urole series does not lead to the formation of five membered arynes.75) Anhydrides like 45 do... 

Titanacycle derivatives (Scheme 543) are obtained predominantly as one isomer of the probably expected products, as a result of the reaction between a titanium aryne intermediate, generated in the pyrolysis of bis-Cp diaryl complexes with diphenylacetylene. The reaction with Cr(CO)3(CH3CN)3 affords a heterobinuclear Ti-arene-Cr... 

Aryne formation by elimination of lithium fluoride limits the stability of 113 a polymer is formed, but attempts to trap the aryne (114) were unsuccessful.352 Decarboxylation of metal salts of tetrafluoroisonicotinic add occurs,357 giving the corresponding metal derivative, but pyrolysis of the disilver salt (115) appears to produce the aryne (114), since small amounts of diazabiphenylenes may be isolated from the reaction product.358 359... 

Benzoxazoles. - Both o- and m-halogenobenzanilides (413) are converted into the amidines (415) by potassium amide in liquid ammonia these are formed by aryne cyclization to 2-phenylbenzoxazole (414) and subsequent aminoly-sis." The conversion of acetophenone oxime into 2-methylbenzoxazole by the action of phosphorus oxychloride involves a Beckmann rearrangement. Pyrolysis of aryl azidoformates, ArOaCNa, gives benzoxazol-2-ones. The azide (416) is converted into the benzoxazole (417) on heating. The... 

Best and Wege59have reported the first total synthesis of Mansonone F and this is described in Scheme 10. Phenol (111)60 was made to react with 2-chloroacetyl-5-methylfuran (112) in dimethylsulfoxide and sodium methoxide to yield (113). Ketalization of (113) followed by catalytic reduction and basic hydrolysis afforded anthranilic acid (114). Diazotization followed by pyrolysis with propene oxide in 1,2-dichloroethane probably yielded aryne (115), which undergoes intramolecular Diels-Alder reaction producing the adduct (116). Deoxygenation and then acid hydrolysis afforded the product (117). This was subjected to Grignard reaction. The resulting tertiary alcohol on nitration yielded the nitro compound (118) which was subjected to reduction and oxidation respectively to obtain (119). It yielded Mansonone F (120) on dehydration. 

Although arynes are reluctant to rearrange, examples of ring contraction of an aryne have been recorded. An example of an aryne rearrangement to a carbene has been recorded it is probable that the aryne and carbene are in equilibrium under the vigorous conditions (flash vacuum pyrolysis at 960 C) of the reaction . A similar ring contraction in a heterocyclic system probably proceeds via a similar mechanism. ... 

Phenanthryne (and methyl and benzo analogs) are produced in good yield by a Type C method, very low pressure pyrolysis at 700-900 of the 9,10-dicarboxylic anhydrides. The arynes were identified by mass spectrometry and by products derived from reaction with benzene or 1,3-butadiene. 

The conversions of 144 - 145 and 146 -> 147 involve carbene insertion into a methyl and phenyl group, respectively. In the o-tolyl anhydride 162 both options are possible, but in fact its pyrolysis gave only products derived from interaction with adjacent methyl group. The products were fluorene 168 (49%) and the two benz[ ]indenes 169 and 170 (11% each). The initial intermediate is aryne 163, possibly the source of fluorene as shown, though... 

The interconversion of the 1,2-, 1,3-, and 1,4-didehydrobenzenes has been studied. A 1,2- to 1,3-didehydrobenzene conversion has been postulated to occur during pyrolysis (900 °C) of the phenyl substituted aryne precursors as shown in Scheme 7.4. Extremely high temperatures are required for benzyne interconversion. 

One of the best routes to arynes is the oxidation of aminotriazoles (49j) with lead tetraacetate. Variations involve generating the intermediate nitrene 57 or nitrenoid with other oxidizing agents or from other precursors (49k-49n). By analogy the nitrene 58 or nitrenoid obtained by oxidation of 1-aminoindazole (59) might be expected to fragment to arynes by loss of Nj and a nitrile. In fact this reaction constitutes an excellent synthesis of the previously discussed aryne precursors, the 1,2,3-benztriazines (49e). A vibrationally excited form of 49c is presumably an intermediate in the pyrolysis of sulfoximides of 2-aminoindazoles (60) to arynes. 

Loss of two molecules of CO leads to arynes upon photolysis or severe thermolysis of benzocyclobutenedione (78) and plasmolysis or pyrolysis-mass spectroscopy of acenaphthenequinone (79). With both precursors the process appears to be stepwise as shown except, once again, under pyrolysis-mass spectroscopy conditions, which is consistent with a concerted loss of CO. ... 

The first two precursors in this category to be discovered (see Table 5), 112 and 113, contain the relatively weak C-I and C-Hg bonds which undergo either photolysis or thermolysis to yield benzyne (1). The latter process is noteworthy as one of the first generations of this intermediate in the gas phase. Similar o-iodoorganometallic intermediates may be involved in the formation of arynes from o-diiodobenzene derivatives in the presence of zinc or copper. Both direct thermolysis and photolysis of 114 also lead to arynes, presumably via a stepwise mechanism involving o-iodophenyl radicals (47). The extent to which the latter go on to arynes depends not only on the nature of the aryl residue but, as mentioned in Section ILl.C, on the source of the radical center. Pyrolysis of dibromo (115), iodonitro (116), nitrobromo (117), and dinitro (118) aromatic compounds in a mass spectrometer also produces the corresponding arynes and didehydro compounds. 

Pyrolysis of silver phthalates (122) also leads to products which have suggested the intermediacy of either arynes or the related diradicals (le). Mercuric phthalates (123) lose COj in a stepwise manner to give the trimeric phenylmercury carboxylates (124) n = 3) and the o-phenylenemercury trimers (125) n = 3) before going on to aryne-type products. Although it has been claimed that the monomeric form of 124 decomposes to benzyne (1), both the trimer and hexamer of 125 apparently do not. 

Finally, benzyne has been postulated to be formed in small amounts during the pyrolysis of chlorobenzene, " nitrobenzene, " benzotrifluoride, " and even benzene itself " " by the m ramolecular elimination of HCl, HNOj, HCF3, and respectively. Plasmolysis of halobenzenes, benzene, and phenyl-acetylene also provides arynes by a similar process. " /n ermolecular dehydrogenation either with other arynes [equation (11)] " or nitrenes [equa-... 

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