Benzyne from Acetylene

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

In our study of the formation and reactions of arynes at high tempera- 

The striking similarity in the nature and some of the relative concentrations of the products in the three reactions strongly suggests common mechanisms and intermediates for their formation. Phthalic anhydride gives benzyne upon pyrolysis the parallel behavior of acetylene suggests that it also forms benzyne at high temperatures. 

To test this hypothesis, phthalic anhydride, acetylene, and acetylene-d2 were separately reacted with hexafluorobenzene at 690° under the same conditions as those used with acetylene alone. Phthalic anhydride gave tetrafluoronaphthalene, by 1,4-addition, and hexafluorobiphenyl, by insertion of benzyne, in a 1 5 ratio as estimated from the low-voltage mass spectrum and directly-coupled gas chromatography-mass spectrometry  

Acetylene with hexafluorobenzene also gave tetrafluoronaphthalene and hexafluorobiphenyl, though a 4 1 ratio, and also tetrafluoro-anthracene. Acetylene-d2 under the same conditions gave tetrafluoro-naphthalene- 4 and hexafluorobiphenyl-d4 in a 9 1 ratio, as well as tetrafluoroanthraeene-d6. 

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The reaotion of benzyne from the pyrolysis of phthalic anhydride with chlorobenzene was expected to give chlorobiphenyls by insertion and naphthalene and chloronaphthalenes by 1,2- and 1,4-addition and rearomatization with respective loss of chloroacetylene and acetylene. 

The major products from the reaction of arynes with thiophene and benzothiophene by addition and insertion are shown in Table 11. Benzyne from phthalic anhydride reacted with thiophene at 690° to give naphthalene and benzothiophene by 1,4-addition and loss of sulfur, and by 1,2-addition and loss of acetylene, respectively, as well as phenyl-thiophene by insertion (Fields and Meyerson, 1966d, 1967e) (Scheme 19). The ratio of naphthalene to benzothiophene was about 9 1, nearly the same preference for 1,4-over 1,2-addition as was inferred from the reaction of benzyne with dichlorobenzenes and pyridine at the same temperature, and again reflects the strong tendency of benzyne to act as a dienophile. 

Benzyne probably forms from acetylene by cycloaddition reaction of acetylene and diacetylene, concerted or stepwise (Woodward and Katz, 1959 Woodward and Hoffmann, 1965a, b) ... 

This evidence that benzyne is at least one of the intermediates in acetylene pyrolysis has many implications. However, as the ratios of products from acetylene and hexafluorobenzene differ appreciably from those obtained from phthalic anhydride, it might be best at this point to call acetylene a benzynoid precursor. Additional data will be needed and are being accumulated to determine to what extent acetylene reactions proceed at high temperatures through a benzyne intermediate (Fields and Meyerson, 1967a). 

N-Benzylaniline (14%) was obtained from JV-benzylaziridine and benzyne (from fluorobenzene and butyllithium).35 The strongly basic conditions may cause the elimination of acetylene from an intermediate N-vinylaniline (33) as outlined in Scheme 5. 

Benzyne trapping agents s. 19, 910 Benzotropolone-l, 2 -quinones from o-quinones via l, 2 -dihydroxybenzotropolones s. 18, 914 Pyrroles by 1,3-dipolar addition from acetylene derivatives... 

Carbene reactions are also involved when electron-deficient acetylenes are heated with carbon disulphide.Full details have now been published for these addition reactions, in which intermediates of type (70 R = CF, or C02Me) are postulated. Generation of benzyne, from 1-aminobenzotriazole and lead tetra-acetate, in the presence of carbon disulphide, also leads to a 1,3-dithiole carbene, and hence to a variety of complex products. ... 

Benzyne, generated from diphenyliodonium 2-carboxyIate, reacts with various thiophenes by addition to the sulfur and /3-carbon to give, after loss of an acetylene moiety, benzo[Z)]thiophenes in low (<4%) yield (Scheme 52) (81CC124). 

It should be noted that CASSCF methods inherently tend to give an unbalanced description, since all the electron correlation recovered is in die active space, but none in the inactive space, or between the active and inactive electrons. This is not a problem if all the valence electrons are included in the active space, but this is only possible for small systems. If only part of die valence electrons are included in the active space, the CASSCF methods tend to overestimate the importance of biradical structures. Consider for example acetylene where the hydrogens have been bent 60° away from hnearity (this may be considered a model for ort/zo-benzyne). The in-plane jt-orbital now acquires significant biradical character. The true structure may be described as a hnear combination of the three configurations shown in Figure 4.11. 

In the case of 1,3-diphenylisoindole (29), Diels-Alder addition with maleic anhydride is readily reversible, and the position of equilibrium is found to be markedly dependent on the solvent. In ether, for example, the expected adduet (117) is formed in 72% yield, whereas in aeetonitrile solution the adduet is almost completely dissociated to its components. Similarly, the addition product (118) of maleic anhydride and l,3-diphenyl-2-methjdi.soindole is found to be completely dissociated on warming in methanol. The Diels-Alder products (119 and 120) formed by the addition of dimethyl acetylene-dicarboxylate and benzyne respectively to 1,3-diphcnylisoindole, show no tendency to revert to starting materials. An attempt to extrude carbethoxynitrene by thermal and photochemical methods from (121), prepared from the adduct (120) by treatment with butyl-lithium followed by ethyl chloroform ate, was unsuccessful. 

With one exception, naphthalen-l,4-imines with a double bond between C-2 and C-3 are not known to dissociate thermally by either possible retro-Diels-Alder pathway (the reverse of reactions described in Section III, A, 1 and 2), and the enthalpy requirements for the formation of a benzyne or an acylic acetylene are doubtless unfavorable. However, the mass spectra of compounds 93-99 reveal one important fragmentation of the molecular ions to be loss of dimethyl acetylene-dicarboxylate, and another fragmentation pathway involves the formation of nitrilium ions MeC=NR and PhC=NR from 93-95 and 96-99, respectively. ... 

As expected, the formal C—C triple bond in benzyne is significantly weaker than in unstrained alkynes, the C=C stretching vibrations of which usually fall in the region 2150 cm. Nevertheless, o-benzyne is better described as a strained alkyne rather than a biradical, which is evident from the large singlet-triplet splitting of 37.5 0.3 kcal/mol as well as the alkyne-like reactivity (e.g., in Diels-Alder reactions). The enthalpy of formation of 4 was determined to be 106.6 3.0 kcal/mol by Wenthold and Squires. For the C=C bond length a value of 124 2 pm was found experimentally, " which comes closer to a typical C C triple bond (120.3 pm in acetylene) rather than a C C double bond (133.9 pm in ethylene). 

In 1972 Berry and co-worker detected 3,4-pyridyne (101) by MS. Trapping experiments also provided evidence for the existence of this intermediate, although the chemistry of 101 differs considerably from that of o-benzyne. Thus, neither anthracene nor dimethylfulvene form Diels-Alder adducts with 101. Nam and Leroi were able to generate 101 in nitrogen matrices at 13 K and characterized it by IR spectroscopy. Irradiation of 3,4-pyridinedicarboxylic anhydride (103) with 1 > 340 nm results in formation of 101, which upon short wavelength photolysis (k > 210 nm) fragments to buta-l,3-diyne (104) and HCN, and to acetylene (105) and cyanoacetylene (106, Scheme 16.24). The assignment of an intense... 

The arsenin analogues of quinoline and acridine have been prepared. Compound (124) is prepared from the Diels-Alder adduct of arsenin with benzyne by abstraction of acetylene with 3,6-di(2 -pyridyl)-2,3,5,6-tetrazine (equation 26). It is an unstable oil which can be... 

Biphenyl, benzocyclooctatetraene (9), and benzobicyclo[2,2,2]octatriene (10) resulted from the reaction of benzyne (by decomposition of benzene-diazonium carboxylate) with benzene at 45° (Miller and Stiles, 1963). Both 9 and 10 have been found to go to naphthalene and acetylene 9 on photolysis (Fonken, 1963), and 10 in a sealed tube at 300° (Miller and Stiles, 1963). 

We propose rather that thiophyne is formed by intramolecular dehydrogenation, and that the minor pyrolysis products of thiophene arise from the reaction of thiophyne with thiophene (Scheme 22). Phenylthiophene is a major product from benzyne and thiophene, and therefore suggests the intervention of benzyne in thiophene pyrolysis. A likely source of benzyne in this system is the 1,4-addition of thiophyne to thiophene followed by loss of acetylene and sulfur (Scheme 23). Such a scheme parallels the behavior of benzene, which at 690° gives a small amount of naphthalene, arising presumably from intramolecular... 

Tetrachloronaphthalene (26) and tetrachlorobenzothiophene (27) from tetrachlorophthalic anhydride formed by addition of tetrachloro-benzyne to thiophene and subsequent loss of sulphur and acetylene, respectively (Fields and Meyerson, 1967e) ... 

Alternatively, tetrafluoroanthracene could form by the reaction of benzyne with tetrafluoronaphthalene. In that case, however, some difluoroanthracene should also form by 1,4-addition of benzyne to the fluorinated ring. No difluoroanthracene or difluoroanthracene-d8 was found in the products from the reactions of hexafluorobenzene with acetylene and acetylene-d2, respectively. 

In one case a substituted phenanthrene (160) is also formed. Formally, this must arise from coupling of one acetylene with two benzynes. Insertion of a benzyne moiety into a nickelaindene is a reasonable mechanism. 

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