Benzyne with 2,1,3-benzoselenadiazoles

A series of adducts analogous to 132 has been prepared from benzyne and substituted 2,l,3-benzoselenadiazoles.84a The adduct 133, from 5,6-dimethyl-2,l,3-benzoselenadiazole (129), undergoes a further remarkable rearrangement either thermally or photochemically to give the selenocyanate 136. The corresponding compound 137 is obtained directly from the reaction of benzyne with 130, which implies that the adduct 134 is even more labile... 

The reaction of benzyne with certain selena- and thiadiazoles has been studied extensively and revealed some interesting mechanistic twists and turns. With benzoselenadiazole... 

In the variable temperature photoelectron spectroscopy of 1,2,3-selenadiazole (101) at 5 x 1(T2 mbar between 450 and 750 °C, acetylene is always formed along with selenoketene (105). At 800 °C, acetylene is the sole product. 1,2,3-Benzoselenadiazole (106) does not produce benzyne even at 800 °C. The reaction product was found to be the highly reactive 6-fulveneselone (107). The PE spectra of (101), (106) and their decomposition products are very similar to those of their corresponding sulfur analogs (Scheme 45) (80CB3187)... 

Benzyne, generated either by oxidation of 1-aminobenzotriazole with lead tetraacetate or by decomposition of benzenediazonium-2-carboxylate, was efficiently trapped by (2) to give 88% of the 1,2-benzisoselenazole (26) however, it was trapped in only 5-10% yield by (1). The series of adducts analogous to (26), prepared from benzyne and substituted benzoselenadiazoles (81JCS(P1)607), occurred via attack of the benzyne at the selenium atom followed by reorganization of the intermediate according to Scheme 6. Benzyne addition to dimethylthiadiazole produced methyl derivatives of quinoline and 1,2-benzisothiazole (82CC299). 

Cycloaddition of 2,1,3-benzoselenadiazole (40) with benzyne (Scheme 23) is followed by rearrangement to provide the benzisoselenazole (41) <83T835>. The cycloaddition of dimethyl- or diphenylselenadiazole with benzyne gives the 3-methyl- or 3-phenylbenzisoselenazole, respectively, refuting a previous report of rearranged structure as the 1,3-benzoselenazole <88JCS(P1)2141>. 

Benzoselenadiazole (128) behaves as a heterodiene toward dimethyl acetylenedicarboxylate, with which it gives the quinoxaline 124 and selenium. But 128 reacts differently with benzyne (generated from 4 or from 9) to give the 1,2-benzisoselenazole derivative 132 (88%) and a small amount of a cis,trans stereoisomer of 132.82 The analogous adduct 131 is obtained in lower yield from benzyne and 2,1,3-benzothiadiazole (127). The structure of these benzyne adducts is strikingly reminiscent of 135, which is obtained from a photochemical addition of dimethyl acetylenedicarboxylate to 126 via a nitrile oxide intermediate.84 However, for reasons given elsewhere,82 a nitrile selenide is unlikely to be an intermediate in the formation of 132, which is better explained by the mechanism outlined in Scheme 16. As in the case of thiophen (Section V,B), this is a 1,3-cycloaddition (in one or two steps) of benzyne to the heterocycle, enabled by the use of d orbitals on the sulfur or selenium atom. 

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