Benzyne cycloaddition reactions

Benzyne shares a feature with A in the [2+2] cycloaddition reactions. The HOMO-LUMO interaction prefers the three-centered interaction (Scheme 4) [115]. This is in agreement with the calculated reaction path [116]. 

The 2+2 cycloadditions of benzyne to cis- and trani-propenyl ether gave cis- and fran -benzocyclobntanes as the main products, respectively [ 117,118], Stereospecific [2+2] cycloaddition reactions were observed between the benzyne species generated by the halogen-Uthium exchange reaction of ort/io-haloaryl triflates and the ketene silyl acetals (Scheme 23) [119],... 

Scheme 23 Stereospecific [2+2] cycloaddition reaction of a benzyne 3.2.2 Reactions of Cycloalkynes...

Derivatives of the naphthalen-l,4 -imine ring system (2) have become available only since the discovery of cycloaddition reactions of benzyne, on the one hand, and the recent rapid development of isoindole chemistry on the other. 

Photooxygenations of 1,4-dioxins and their benzo- and naphtho derivatives as well as the ozonolyses of 1,4-benzo-dioxins and the cycloaddition of dithiins and 1,4-benzodithiins have been reported <1996CHEC-II(6)447>. In more recent reports, 2-chloro-l,4-benzodithiin-l,l,4,4-tetraoxide 67 was employed as a very reactive dienophile <19970393, 2005JOC5221> and was suggested as a mild alternative to the use of benzyne in [4-1-2] cycloaddition reactions (Scheme 7) <1996T14247>. 

The 7r-nature of the central bond in bicyclo[1.1.0]bulane is aptly demonstrated by its cycloaddition reaction with benzyne in 1,2-dichloroethane, from which two compounds 29 and 30 were isolated in moderate yield in a ratio of 6 1. The structures of the major and the minor compounds were identified as 3-phenylcyclobutene (29) and benzobicyclo[2.2.1]hex-2-ene (30), respectively.44 Deuterium labeling experiments showed that both products resulted from attack on bicy-clo[1.1.0]butane from the endo side.44... 

There is some evidence that the cycloaddition reaction of benzynes with 1-substituted pyrroles may proceed in two steps via a zwitterionic intermediate and that the reaction is thermally reversible, for although 1-methylpyrrole yields the [w4 + w2] adduct (258) with... 

After [2n+2n] cycloaddition reaction of imine 271 and benzyne 272, the formed benzazetidine 273 underwent ring opening into its valence tautomer 274. Moisture attacked the tautomer 274 to give oxazepine 275. This method yielded 60-68% of the bicyclic 1,4-oxazepine 275. One extra mole of benzyne led to the formation of N-phenylated 276 (Scheme 48) <2003T6067>. 

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]. 

Benzyne generated at room temperature from phenyl[2-(trimethylsilyl)phenyl]iodonium trifluoromethanesulfo-nate has been trapped in a [4+2] cycloaddition reaction with diarylthiones. The initial product, a 6-aryl-477 dibenzo[, r/]thiopyran, is accompanied by 6-aryl-67/-dibenzo[, r/]thiopyran arising by 1,3-prototropic aromatization of the cycloadduct (Equation 114). The use of sterically congested thiones, such as thiopivalophenones, results in competition from a [2+2] cycloaddition and 2/7-benzo[A]thietes are the sole products <2000BCJ155>. 

Benzyne probably forms from acetylene by cycloaddition reaction of acetylene and diacetylene, concerted or stepwise (Woodward and Katz, 1959 Woodward and Hoffmann, 1965a, b) ... 

The reaction of thiobenzophenones with electron poor acetylenes [228] takes place at ambient temperature and provides excellent yields of 2H-thia-pyrans (Table 4, entry 5). Benzyne, whose reaction with C=S compounds was reviewed recently [229], was treated with various thiobenzophenones [230] a [4+2] cycloaddition was followed by a hydrogen shift (entry 6). 

Cycloaddition reactions with alkynic compounds are illustrated by the reaction of methyl propiolate with (78) to form (79), and by the reaction of benzyne with (78) to form (80)... 

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