Aryne dimers

The preferred site of deprotonation of di- or polysubstituted arenes is not easy to predict. In 1,3-disubstituted benzenes in which both substituents facilitate ortho-metalation, deprotonation will usually occur between these two groups [181, 365, 408, 416-419] (Scheme 5.45). Dialkylamino groups, however, can sometimes deactivate ortho positions (fourth reaction, Scheme 5.45), but this does not always happen [181, 420], 3-Chloroanisole [411] and 3-fluoroanisole [421] are deprotonated by organolithium compounds between the two functional groups, but the lithiated arenes dimerize readily at -78 °C, presumably via intermediate aryne formation (last example, Scheme 5.45). 

Brown et al. [36] attempted to approach pentalene (2) by a thermolytic ring-contraction-cyclization of 3-ethenylbenzyne 41, which, in turn, was generated by flash vacuum thermolysis of 3-ethenylphthalic anhydride 40. Indeed, the pentalene dimer 43 ( 50%) was formed along with phenylacetylene 44 ( 50%) in 80% overall yield (Scheme 6). The failure to detect monomeric pentalene (2) is in accord with the observation of de Mayo et al. that 1-methylpentalene (37) dimerizes above —140 °C [29]. The formation of phenylacetylene 44 was unexpected, and it is as yet unclear as to whether it arises by migration of two hydrogens in the aryne 41 or the intermediate carbene 42, or whether it is a secondary product formed from pentalene (2). 

Arynes are very reactive intermediates which enter into a large variety of bimolecular reactions including dimerization. With the exception of apparently slow reactions such as equation (13), no unimolecular decompositions of arynes are known. The direct detection of arynes, therefore, is possible in the gas phase or in dispersed, inert media. Utilizing such conditions the ultraviolet and infrared absorption spectra, ionization potential, enthalpy of formation, rate of dimerization, and approximate lifetimes of arynes have been measured. Most claims for the existence of... 

Since carbonation led to both the 3- and 4-carboxylic acids, 574 and 575, respectively, a transhalogenation mechanism via the o-bromolithium compound 576 was postulated. Decomposition of this intermediate to the aryne 572 would be expected " and may be indicated by the isolation of an apparently dimeric acid of undetermined structure, perhaps 577. 

In 2005, Yoshida et al. disclosed that distannylation of arynes was accompanied by dimerization of arynes by use of phosphite, ETPO (4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octane), as a ligand for a palladium catalyst to produce diverse 2,2 -bis(trimethylstannyl)biaryls 51 in one step (Scheme 28.19) [25]. The dimerizative insertion into an Sn—Sn a-bond should be initiated by the formation of pallada-cyclopentadiene 52 from the palladium complex and two molar amounts of arynes. 

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