Benzyne dimer

You can t isolate the benzyne because it reacts with itself to give a benzyne dimer having a four-membered ring between two benzene rings, there is a peak at 152 for the dimer but also a strong peak at 76, which is benzyne itself. 

Benzyne dimerizes to biphenylene when generated in the absence of either a nucleophile or a reactive unsaturated compound. The lifetime of benzyne is estimated to be on the order of a few seconds in solution near room temperature. ... 

Biphenylene was observed among the products of the reaction of o-chloronitrobenzene with disodium ditelluride, and is thought to arise from benzyne dimerization. The major product, however was phenazine (102). 

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). 

When benzyne is generated in the absence of another reactive molecule it dimerizes to biphenylene.132 In the presence of dienes, benzyne is a very reactive dienophile and [4+2] cycloaddition products are formed. The adducts with furans can be converted to polycyclic aromatic compounds by elimination of water. Similarly, cyclopentadienones can give a new aromatic ring by loss of carbon monoxide. Pyrones give adducts that can aromatize by loss of C02, as illustrated by Entry 7 in Scheme 11.9. 

Benzyne generated from 2-carboxybenzenediazonium chloride reacted with sulfur monochloiide to give dibenzothiophene 13 (8-10%) and thiantherene 14 (26-35%) (1989SUL83). A mechanism involving the addition of sulfur mono-chloride to benzyne with the formation of betaine 15 followed by the elimination of SCI2 to afford benzothiirene 16 and a further reaction with another benzyne molecule or dimerization to thianthrene 14 is given in Scheme 8. 

When benzyne is generated in the absence of another reactive molecule, it dimerizes to biphenylene.123 124 125 126 In the presence of dienes, benzyne is a very reactive dienophile, and [4 + 2] cycloaddition products are formed. 

The thermal reaction between two molecules of olefin to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the olefins are the same or different, but the reaction is not a general one for olefins.921 Dimerization of like olefins occurs with the following compounds F2C=CX2 (X = F or Cl) and certain other fluorinated alkenes (though not F2C=CH2), allenes (to give derivatives of 97),922 benzynes (to give biphenylene deriv-... 

The widely differing product distributions frorn the two anhydrides reflect the difference in structure and reactivity of the two naphthalynes. 2-3-Naphthalyne (14) behaves like benzyne, giving anthracene by 1,2-and 1,4-addition, and phenylnaphthalene by insertion, as well as some binaphthylene (15) by dimerization ... 

In one of the first contributions to the field, Friedman reported on the ability of silver to influence the reactivity of benzyne toward benzene (Scheme 2.1). In the absence of silver, benzyne 2, generated in situ from 1, reacted with benzene in a [4+2] manner to afford adduct 3 in addition to small amounts of dimerization product 4.6 However, in the presence of substochiometric amounts of silver ions, the course of the reaction was altered, leading to formation of biphenyl 5 and benzocycloctatriene 6. A contingent of other metal ions including T1+, Cu +, Cu2 +, Hg22 +, and Hg2 + were unable to influence product formation in a similar manner. 

Aromatic compounds are dechlorinated by the general mechanism shown in Sch. 1. Electron transfer to a ir-antibonding orbital forms an aromatic radical anion, which then ejects Cl" to give an aromatic radical. This radical picks up a second electron to give a very basic cx-anion, which abstracts a proton either from NH3 or from a more acidic source like water, when water is present. If water is not present, then an NH2 anion can be formed. The presence of ME can lead to the formation of aminated products via the benzyne mechanism. Aminated products were formed in dry NH3 but not when water was present [24], A further reduction via radical anion formation and proton abstraction can give dihydroaromatics or tetrahydroaromatics, or dimerization may occur. In soils, both water and... 

Pyridinium-3-olates and pyrylium-3-olates react with a variety of monoenes, dienes, and trienes. Each of these systems react readily with dienophiles to yield cycloadducts of type 433. For Z = NMe, an electron-withdrawing X group is required in the dienophile, but with Z = 0 or A-(2-pyridyl) even unactivated alkenes react. 1-Phenylpyridi-nium-3-olate and benzyne give 434 dienes give adducts of type 435. Fulvenes behave as trienes to give adducts across the 2,6-positions 436. 2-Benzothiopyrylium-4-olate 437 gives a thermal dimer across the 1,3-positions. 

An organocopper intermediate was detected by Lewin and Cohen in the reaction of / -iodotoluene with copper in a good complexing solvent (184). Analysis of protonated aliquots from a reaction performed in quinoline indicated an accumulation of />-tolylcopper to a maximum of 43% after 95 hours, at which point the iodide was consumed, and then a slow decrease to by dimerization. Other experiments also indicate the formation of an arylcopper compound in Ullmann reactions (127,141, 210). The isolation of deuterated products, presumably from the decomposition of an intermediate organocopper species in deuterated benzene and cyclohexane, suggested decomposition to free radicals (127). Decompositions of certain o-haloarylcopper intermediates by a benzyne mechanism cannot be totally excluded. The formation of a dichlorobenzene and by-products such as dibenzofuran and triphenylene from only the ortho isomer of the chloroiodobenzenes in Ullmann coupling reactions (210)... 

Cyclopentadienone is an elusive compound that has been sought for many years but with little success. Molecular orbital calculations predict that it should be highly reactive, and so it is it exists only as the dimer. The tetraphenyl derivative of this compound is to be synthesized in this experiment. This derivative is stable, and reacts readily with dienophiles. It is used not only for the synthesis of highly aromatic, highly arylated compounds, but also for examination of the mechanism of the Diels-Alder reaction itself. Tetraphenylcyclopentadienone has been carefully studied by means of molecular orbital methods in attempts to understand its unusual reactivity, color, and dipole moment. In Chapter 48 this highly reactive molecule is used to trap the fleeting benzyne to form tetraphenylnaphthalene. Indeed, this reaction constitutes evidence that benzyne does exist. 

A special reaction of this type is the formation of benzyne. o-Fluorobromobenzene reacts with lithium amalgam in ether or furan with an intermediate formation of benzyne. In the former medium, diphenylene and small amounts of triphenylene and 9,10-dimer-cura-9,10-dihydroanthracene were formed, whereas the major product (76%) in furan was 1,4-dihydronaphthalene-1,4-endoxide [64]. 

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