Benzyne dimerisation

The kinetics of benzyne dimerisation to biphenylene have been the object of two studies. In the first a limit was determined for the rate coefficient (k > 7x 10 I.mole". see" ) , while the authors of the second arrived at a more accurate value (/ = (4.6 1.2) X 10 l.mole". sec" ) and checked its in-variancy over the range 90-200°C (benzyne was obtained in situ by decomposition of aromatic derivatives). 

The pyrolysis of a number of compounds at temperatures around 600— 800° and at pressures of the order of 10 2 mm. has been shown to give rise to benzyne. These compounds include for example indanetrione 29>, and phthalic anhydride 30 38>. The dimerisation of benzyne to yield biphenylene has been used preparatively 31 33>, an(j the pyrolysis of tetrafluorophthalic anhydride 34>, and tetrachlorophthalic anhydride 3i-33) gave the corresponding octahalobiphenylenes. In the case of the pyrolysis of tetrachlorophthalic anhydride some hexachlorobenzene is also formed, while the pyrolysis of tetrabromophthalic anhydride results in the formation of hexabromobenzene but no octabromobiphenylene. The disproportionation of tetrabromobenzyne to form carbon and bromine is a function of the high temperature involved and, as we shall see later, both tetrabromo- and tetraiodo-benzyne behave normally in solution. 

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 m/e 76 peak declined and the m/e 152 peak increased rapidly with time, indicating the progressive dimerisation of benzyne to the more stable biphenylene (105, above). 

Immobilisation prevents the benzyne intermediates from dimerisation and allows the alternative transformation which in solution is too slow to compete with dimerisation. Applications of functionalised polymers for monoprotection (many examples were contributed by Lezno, A"6.146-150 further examples see Refs. - SZ) or monoactivation of symmetrically difunctionalised molecules as well as their use to promote the formation of large rings are based on this site isolation or pseudodilution effect . ... 

Investigation of the nature and activity of catalytic species in copper(i) chloride-promoted dimerisation of acetylene indicates that the mechanism either involves insertion of a C2H unit into a copper-chlorine bond, or the reaction of CgH with two copper atoms to give a 3-centre, 2-electron-bonded species. Cyclic oligomerisation of diphenylacetylene in the presence of AlPha proceeds through ir-complexes as intermediates benzyne intermediates are suggested, on the basis of observed products, in oligomerisation of but-2-yne and of oct-4-yne in the presence of n-butyl- or /7-tolyl-chromium compounds. 

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