Benzyne trapping with dienes

The carborane analog of benzyne, 1,2-dehydrocarborane, can be generated similarly from phenyl[o-(trimethylsilyl)carboranyl]iodonium acetate by treatment with CsF in ether and trapped with dienes such as anthracene, naphthalene, norbornadiene and 2,5-dimethylfuran to give the respective 1,2-dehydrocarborane adducts in high yield [934]. 

Trapping of an intermediate. In some cases, the suspected intermediate is known to be one that reacts in a given way with a certain compound. The intermediate can then be trapped by running the reaction in the presence of that compound. For example, benzynes (p. 646) react with dienes in the Diels-Alder reaction (5-47). In any reaction where a benzyne is a suspected intermediate, the addition of a diene and the detection of the Diels-Alder adduct indicate that the benzyne was probably present. 

The treatment of reagent 109 with tetrabutylammonium fluoride in dichloro-methane at room temperature generates benzyne, which can be trapped with the appropriate diene to afford the benzyne adducts 110-114 in high yields (Scheme 48) [105, 106]. Similarly, 3- and 4-methylbenzynes can be efficiently... 

Photolysis or thermolysis of diarylzirconocenes or arylalkylzirconocenes lead to reactive aryne-zirconocene complexes which can be trapped with several types of electron-donating ligands including dienes, phosphines, and carbon monoxide [13-15]. After being postulated by VoPpin et al. [14] and Erker [13], benzyne-zirconocene 1 (Eq. 1) was fully characterized by Buchwald et al. [16] by X-ray crystallographic studies of its trimethylphosphine complex adduct ... 

Furan has generally been observed to be a good diene in Diels-Alder reactions with arynes, and has been widely used in trapping aryne intermediates. Upon treatment with one equivalent of lanthanum, l-halo-2-iodobenzenes effectively generate benzynes, which are trapped with furan to afford the Diels-Alder product 99 (Equation 12.35) [58]. 

This mechanism has been confirmed using isotopically labelled fluorobenzene. Benzynes have been trapped by dienes in Diels-Alder reactions. They can also be generated by strong bases such as sodium amide in liquid ammonia in this way bromobenzene can be used without the interference of metal-halogen exchange which is the preferred course with lithium alkyls. 

Kitamura and coworkers reported that treatment of (phenyl)[2-(trimethylsilyl) phenyl]iodonium triflate with TBAF results in the formation of benzyne, which can be trapped with furan and other dienes in cycloaddition reactimis [35, 267, 268]. This feature was recently utilized with an iodonium salt containing an ort/io-(pyridyldiisopropyl)silyl group [269]. Diaryliodonium salts are also useful in covalent grafting of carbon surfaces useful in materials chemistry [270-273]. 

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. 

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-... 

When 11 decomposes in the presence of an added nucleophile, the benzyne intermediate is trapped by the nucleophile as it is formed. Or, if a conjugated diene is present, benzyne will react with it by a [4 + 2] cycloaddition. In the absence of other compounds with which it can react, benzyne will undergo [2 + 2] cycloaddition to itself ... 

Whether or not branching occurs at an intermediate, its existence may be demonstrated by running the reaction in the presence of a reagent designed to intercept that intermediate to yield a new and characteristic product. Observations at a qualitative level can be extremely informative (e.g. formation of cyclo-adducts when aryl halides are treated with a strong base in the presence of conjugated dienes to trap a benzyne intermediate) but even more information maybe obtained from quantitative experiments, especially when product analyses are coupled with rate measurements. 

Oxazoles are anodier class of heteroaromatic dienes which readily undergo Diels-Alder reactions with benzynes. For example, slow, simultaneous injection of solutions of triazole (483) and lead tetraacetate to a solution of oxazole (515) in CH2CI2 at 0 C afforded cycloadduct (516) in essentially quantitative yield. The latter is a convenient source for the unstable isobenzofiiran (517), which can be trapped by [4 + 2] cycloaddition to a variety of dienophiles, e.g. by -methylmaleimide (Scheme 121). 

Generation of benzyne (see also 1, 560). Campbell and Rees1 point out that the generation of benzyne from this precursor by oxidation with lead tetraacetate differs from other methods in that, in the absence of a trapping agent, the dimer, diphenylene, is formed in yields as high as 83%. The trimer, triphenylene, is formed in less than 0.5% yield. Addition reactions of benzyne generated in this way with 1,3-dienes have been discussed.2... 

The intensely yellow compound melts at 127°C. Its solution exhibits a blue-green fluorescence. 1,3-Diphenylisobenzofiiran proves to be an extremely reactive diene [12] in [4+2] cycloadditions. It is used to trap unstable alkenes or acetylenes, e.g. 1,2-dehydrobenzene (benzyne) with formation of the adduct 4 ... 

Isobenzofurans are especially useful dienes toward arynes. 1,3-Diphenylisobenzofuran has long been used as a trap for benzynes, but only recently has the utility of this reaction been demonstrated through the generation of simpler isobenzofurans and their trimethylsilyl derivatives d47 Treatment of acetal 258 with one equivalent of MeLi and a catalytic amount of diisopropylamine in ether gave solutions of259, and subsequent addition of 0-... 

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