Benzynes rearrangement

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

Diallylsulfonium salts undergo intramolecular allylic rearrangement with strong bases to yield 1,5-dienes after reductive desulfurization. The straight-chain 1,5-dienes may be obtained by double sulfur extrusion with concomitant allylic rearrangements from diallyl disulfides. The first step is achieved with phosphines or phosphites, the second with benzyne. This procedure is especially suitable for the synthesis of acid sensitive olefins and has been used in oligoisoprene synthesis (G.M. Blackburn, 1969). 

Unfortunately, appearance energy measnrements become more complicated with larger substrates, where the cations are more prone to rearrangement during ionization. Eor example, numerous attempts have been made to measnre the energy for formation of o-benzyne cation, CgH4+ by using benzonitrile as a precursor (Eq. 5.6). 

The stereochemistry of dienes has been found to have a pronounced effect in the concerted cyclo-additions with benzyne 64>65h A concerted disrotatory cyclo-addition of tetrafluorobenzyne, leading for example with trans- (3-methylstyrene to (63, R = Me), is likely and in accord with the conservation of orbital symmetry 68>. However while the electro-cyclic rearrangement of (63, R = H) to (65, R = H) is not allowed, base catalysed prototropic rearrangement is possible. A carbanion (64, R = H) cannot have more than a transient existence in the reaction of tetrafluorobenzyne with styrene because no deuterium incorporation in (65) was detected when either the reaction mixture was quenched with deuterium oxide or when the reaction was conducted in the presence of a ten molar excess of deuteriopentafluorobenzene. 

Dehydrophenol 20i is a tautomeric form of carbene la, and a [1,3]-H migration should in principal interconvert these species. However, under the conditions of matrix isolation the benzynes 201—1 are thermally and photochemically stable towards rearrangement to the corresponding carbenes. UV irradiation of 20i results in a ring-opening and formation of so far unidentified acetylenic products. 

Benzyne (412) reacted with homoazulene (413) to provide benzo[h]homoheptalene (415), presumably via the rearrangement of the [2- + 2]adduct (414)141 >. 

It should be noted that the 1 1 adducts of benzyne with pyrrylmag-nesium iodide, and A -methyl- and iV-benzylpyrrole were isolated and characterized only as the hydrobromide (113), the methiodide (115), and the picrate of 102, respectively. The low yields of all these derivatives are due in part to further reactions of thenaphthalen-l,4-imines with benzyne (see Section III, F and G). Yields are better where the starting pjnrole has an electron-withdrawing N-substituent. Some analogous naphthalen-l,4-imines expected from 1,2,5-trisubstituted pyrroles apparently rearrange spontaneously to j3-naphthylamine derivatives under the conditions for their formation (see Section III, F). The related adducts 107 and 108 are formed from tetrahalobenzynes and N-methy Ipyrrole. 

Diazo-2-oxindole (21c) reacted with benzyne and dimethyl acetylene-dicarboxylate in dichloromethane at 41°C to give polycyclic ring systems of type 265 (73TL1417) (Scheme 78). The intermediate spiro adducts 264 could not be detected, but it is reasonable to suppose that the final products were obtained by [1,5]sigmatropic rearrangement of the carboxamido moiety. 

Detailed thermochemical data for the Bergman rearrangement were determined by Roth et al. from gas-phase NO trapping experiments. The activation barrier for ring opening of 28 to enediyne (Z)-16 was reported as 19.8 kcal/mol, the enthalpy of formation of (Z)-16 293 = 129.5 kcal/mol) is lower than that of p-benzyne... 

In their early contribution, Griitzmacher and Lohmann identified diethynyl-benzene (65) instead of 1,3-didehydronaphthalene (51) as the pyrolysis product of 1,3-dinitro- and 1,3-dibromonaphthalene, based on the high ionization potential measured (8.96 0.02 eV). Obviously, a rearrangement analogous to the ring opening of m-benzyne takes place for the annellated derivative as well (Schemes 16.10 and 16.18). 

Photoexciting 1,4-dibromobenzene in a molecular beam with 307-nm pump pulse photons leads to sequential loss of both bromine atoms within 100 fs the excited dibromide has a lifetime of 50 fs and the monobromide of 80 fs. The p-benzyne generated decays relatively slowly, showing a lifetime of 400 ps. It presumably leads to (Z)-3-hexene-l,5-diyne through a valence isomerization of the sort associated with the Bergman rearrangement. 

The rearrangements in these reactions result from the attack of the nucleophile at one or the other of the carbons of the extra bond in the intermediate. With benzyne the symmetry is such that no rearrangement would be detected. With substituted benzynes isomeric products may result. Thus 4-melhylbenzyne, 10, from the reaction of hydroxide ion with 4-chloro-l-meth-ylbenzene gives both 3- and 4-methylbenzenols ... 

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