Generation of benzynes

Aryl halides on treatment with a strong base like KNH2 and C6H5li generate benzyne. 

Phthaloyl peroxide (2.93) on photolytic decomposition generates benzyne via lactone intermediate. 

Thermal decomposition of benzothiadiazole-1,1-dioxide (2.94) generates benzyne. 

Benzyne can also be generated from 1,2-bromofluorobenzene via the formation of Grignard reagent or organolithium reagent. 

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A method for the generation of benzyne involves heating the diazonium salt from o aminobenzoic acid (benzenediazonium 2 carboxylate) Using curved arrows show how this sub stance forms benzyne What two inorganic compounds are formed in this reaction" ... 

There are several methods for generation of benzyne in addition to base-catalyzed elimination of hydrogen halide from a halobenzene, and some of these are more generally applicable for preparative work. Probably the most convenient method is diazotization of o-aminobenzoic acid. Concerted loss of nitrogen and carbon dioxide follows diazotization and generates benzyne. Benzyne can be formed in this manner in the presence of a variety of compounds with which it reacts rapidly. 

Benzyne itself has been isolated in solid argon at 8 K, and much evidence for the existence of arynes has come from trapping experiments and spectroscopy. Thus generation of benzyne (101) in the presence of furan (102) leads to the formation of the Diels-Alder (p. 197) adduct (103), which undergoes ready acid-catalysed... 

DiphenyI-3-ethylpentane, 48, 82 Diphenyliodonium-2-carboxylate, as reagent for generation of benzyne,... 

Generation of benzyne in the presence of 1,4,2-benzodithiazine gave a low yield of thianthrene among a complex product mixture (82CC612). 

The addition of benzyne to Cgg leads to the formation of [2+2] qfcloadducts [330-332]. The in situ generation of benzyne can be achieved by diazotization of anthraniKc acid at room temperature with isoamyl nitrite, by diazotization of the potassium salt of anfhranilic acid in the presence of 18-crown-6, or by the oxidation of 1-aminobenzotriazole with lead tetraacetate (Scheme 4.54). Adduct formation occurs in the 1,2-position, leading to the corresponding ring-closed structure. 

The generation of benzyne in the presence of nitrosobenzene produced 45% of 9-phenylcarbazole the process may proceed via 9-hydroxycarbazole, as shown, with final loss of oxygen in some manner. The production of 3-bromo-9-phenylcarbazole from 4-bromonitrosobenzene substantiates the sequence proposed, as does the observation that tetrabromo- (and tetra-chloro-) benzyne react with nitrobenzenes to produce low to moderate yields of tetrahalo-9-hydroxycarbazoles. ° ... 

The procedure demonstrates a safe and simple method for the generation of benzyne from a stable and easily prepared precursor and its use in the synthesis of a hitherto difficultly accessible hydrocarbon. 

The presence of benzyne has been more directly demonstrated by trapping experiments. For example, the generation of benzyne in the presence of anthracene gives the Diels-Alder adduct, triptycene (Equation 7.94).187... 

Benzyne is an important reactive intermediate especially useful for the construction of polycyclic compounds via cycloaddition reactions with various dienes. Several benzyne precursors, including diphenyliodonium-2-carboxylate [ 1 ], have been previously used for the generation of benzyne by thermal decomposition. More recently, several new precursors that generate benzyne quantitatively under very mild conditions have been developed [105 -108]. An efficient benzyne precursor, iodonium triflate 109, can be readily prepared by the reaction of l,2-bis(trimethylsilyl)benzene 108 with [(diacetoxy)iodo]benzene in the presence of trifluoromethanesulfonic acid (Scheme 47) [105]. 

Elimination of iodobenzene alone from 2-phenyliodonio benzoate leads to generation of another transient 1,4-dipole which was trapped by phosphaalkynes [55], A new precursor, 2-trimethylsilyl-diphenyliodonium triflate, obtained from 1,2-bis-trimethylsilylbenzene, (diacetoxyiodo)benzene and triflic acid, permitted the generation of benzyne with tetrabutylammonium fluoride, at room temperature [56] ... 

Phenyl[2-(trimethylsilyl)phenyl]iodonium triflate (226) has emerged as a superior benzyne precursor i.e., admixture of 226 with tetra-n-butylammonium fluoride (TBAF) enables the generation of benzyne in dichloromethane at room temperature (95JCS(CC)983). When organo azides or -nitrones are available in the reaction medium, high yields of the benzotriazoles 227 or benzisoxazolines 228 are obtained (Scheme 62) (98HC205). 

We reported that the palladium-catalyzed reaction of arynes with bis-jr-allyl palladium complex afforded 1,2-diallylated derivatives of benzene in good yields (Scheme 32) [77]. The reaction of 104 with allyltributylstan-nane 3a and allyl chloride 11 in acetonitrile in the presence of 2.5 mol % Pd2(dba)3.CHCl3/dppf catalyst at 40 °C for 12 h afforded 1,2-diallyl benzene 105 in 76% yield. The generation of benzyne 106 takes place presumably first from 104 under the conditions of the palladium catalysis, which reacts with the complex 2 in a manner similar to the diallylation of activated olefins (refer Scheme 29). 

The pyrolytic generation of benzyne in parallel to ionic fragmentation reactions has received considerable attention in the literature. One of the procedures that has been used for benzyne generation is the pyrolysis of biphenylene. °>54)Thebenz5me ion (mjelG) has a relative intensity of 25% as compared to the biphenylene molecule ion at 70 eV. Under pyrolytic conditions biphenylene pelds a small amount of benz5uie although most of... 

Generation of benzyne for Diek-AUer reaction, (a) Org. Syn. procedure. A... 

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