Benzyne, trapping

Adding legs or walls to the two-dimensional cyclic cavity leads to the formation of three-dimensional cavities. Further addition of a cap to the cavity creates an enclosed cavity space. Such spaces are called molecular capsules, and the trapped guest is shielded from the outer environment. If unstable species are trapped in the molecular capsule, their lifetimes can be extended and their properties are easily measured. An example of an unstable species that is stabilized inside a molecular capsule is shown in Fig. 2.22. Photoirradiation of the benzocyclobutendiol in the molecular capsule at -196 °C converts it to benzyne via benzocyclopropenone. Although benzyne is usually quite unstable, benzyne trapped in the molecular capsule can be characterized with H-NMR and i C-NMR at -75°C. 

Benzyne trapping agents 2,5-Di-p-anisyl-3,4-diphenylcyclopentadienone. 1,3-Diphenyl-isobenzofurane. Tetraphylcyclopentadienone. 

However, when a solution of pentaphenylbismuth (4) in carbon tetrachloride was heated under reflux in the presence of 2,3,4,5-tetraphenylcyclopentadienone (67), an efficient benzyne trap,50 the benzyne adduct (68), 1,2,3,4-tetraphenylnaphthalene, was indeed obtained in 50% yield together with triphenylbismuth (76%). 1 Thus the tarry products may more likely originate directly from the decomposition of pentaphenylbismuth. 

Benzyne is a necessary intermediate in the diazotization of 2-[(2-acetoxyethyl)sulfinyl(and sulfonyl)]anilines 107 and 116, but a detailed study of the way it is formed gave some surprises . Products from the aprotic diazotization of 107 included biphenylene 67 and dibenzo-l,4-thioxin 112. EPR evidence for radical intermediates was obtained. Formation of 112 requires cleavage of the S—O bond, most probably via benzoxathiete 110 as shown. Although yields of 67 and 112 were low, when the diazotization was carried out in the presence of benzyne traps such as diphenylisobenzofuran or anthracene, respectable yields (30-40%) of the corresponding benzyne cycloadducts were obtained. 

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... 

Naphthalenes from cyclones Benzyne trapping agents... 

Further mechanistic evidence comes from trapping experiments. When bromobenzene is treated with KNH2 in the presence of a diene such as furan, a Diels-Alder reaction (Section 14.5) occurs, implying that the symmetrical intermediate is a benzyne, formed by elimination of HBr from bromobenzene. Ben-zyne is too reactive to be isolated as a pure compound but, in the presence of water, addition occurs to give the phenol, in the presence of a diene, Diels-Alder cycloaddition takes place. 

Benzyne, intermediate in preparation of phenyl t butyl ether, 46, 90 methods for generation of, 46, 112 trapping by tetraphenylcyclopenta-dienone, 46,112... 

In the context of this section it is important that Ruchardt and Tan (1970 a) found that (solid) benzenediazonium fluoroborate gave benzyne adducts with potassium acetate in the presence of aryne trapping agents such as tetracyclone or anthracene. This is, however, not the case if water is present (Cadogan, 1971). As a consequence of these observations, Cadogan et al. (1971) simplified the formation of arynes from diazonium ions by converting aniline or its substitution products into arynes in a... 

The unsubstituted benzyne (8.26) from 2-diazoniobenzenecarboxylate can be trapped in the absence of a Bronsted acid as biphenylene, i.e., dimeric benzyne (8.30 Ford, 1971), or by cycloaddition to a 4-71 donor such as furan to give 8.31 (Stiles et al., 1963). 

Species such as 5 and 6 are called benzynes (sometimes dehydrobenzenes), or more generally, arynes, and the mechanism is known as the benzyne mechanism. Benzynes are very reactive. Neither benzyne nor any other aryne has yet been isolated under ordinary conditions, but benzyne has been isolated in an argon matrix at 8 where its IR spectrum could be observed. In addition, benzynes can be trapped for example, they undergo the Diels-Alder reaction (see 15-58). It should be noted that the extra pair of electrons does not affect the aromaticity. The... 

The l-aminobenzotriazoles are desirable as benzyne precursors. The dianion of N-boc-1-aminobenzotriazole 34 reacts to introduce electrophiles at the 7 position to give 35 in excellent yields <96TL5615>. Pyrrolo[l,2-c][l,2,3]triazoles 37 are formed by trapping... 

Dehydrobenzene or benzyne 158 can be trapped by all manner of species. 1,2-Dehydro-o-carborane 159 has been shown to undergo many of the same reactions as its two-dimensional relative, 1,2-dehydrobenzene. Although dehydroaromatic molecules can be formed in a variety of ways, synthetic pathways to 1,2-dehydro-o-carborane are quite limited. An effective procedure reported so far78 first forms the dianion by deprotonation of o-carborane with 2 equiv. of butyllithium. Precipitated dilithium carborane is then treated with 1 equiv. of bromine at 0°C to form the soluble bromo anion 160. Thermolysis of 160 with anthracene, furan, and thiophene as substrates leads to the adducts 161-164.79 80 1,2-Dehydro-o-carborane reacts with norbomadiene to give both homo 2+4 and 2+2 addition, leading to three products 165-167, in a 7 1 ratio79. An acyclic diene, 2,3-dimethyl-... 

Benzyne itself has been isolated in solid argon at 8 K, and much evidence for the existence of arynes has come from trapping experiments and spectroscopy. Thus generation of benzyne (101) in the presence of furan (102) leads to the formation of the Diels-Alder (p. 197) adduct (103), which undergoes ready acid-catalysed... 

If benzyne is produced under conditions where there is no suitable species for it to react with, then it dimerises ( self-trapping ) very rapidly to the stable biphenylene (105) ... 

The formation of anthracene in reactions of 185 and 186 with benzyne, which was unexplained by Wittig et aZ., possibly is due to an alternative reaction of the intermediate zwitterion (202) with another molecule of benzjme or with a benzyne precursor. Benzyne reacted with the isoindole (206) to give the tetramethyltriptycene (208) and, in a separate run using excess of the benzyne precursor, W-benzylcarbazole. The latter product would appear to be made up of the iV-benzyl group from an intermediate anthracen-9,10-imine (207) and two molecules of benzyne. Mass spectral evidence also implicated the adduct 207, and the formation of 208 was attributed to benzyne-induced deamination of 207 to 1,4,9,10-tetramethylanthracene, which was trapped by further addition of benzyne across the 9- and 10-positions. 

Mixtures of xanthone (246) and 3,4-benzocoumarin (247) result when 2-aryliodoniobenzoates 245 are heated at higher temperature. This reaction involves fragmentation of 245 to benzyne, carbon dioxide, and iodoarene. Thus, in the absence of trapping agents, the benzyne intermediate reacts with 245 to form cyclized products 246 and 247 (64JOC1637) (Scheme 63). 

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