Benzyne with 2,1,3-benzothiadiazoles

Another heterocyclic molecule that can serve as a benzyne precursor is benzothiadiazole-1,1-dioxide, which decomposes with elimination of sulfur dioxide and nitrogen. ... 

It is known that benzenediazonium-2-carboxylate decomposes to give benzyne via the zwitter-ion (128) 161>. We therefore checked that benzyne is involved in our reactions by carrying out reactions with cinnamaldehyde using benzyne generated from benzothiadiazole-1,1-dioxide 162), diphenyhodonium-2-carboxylate i 3,164)( weil as fr0m anthranilic acid165). Flavene was isolated from each reaction and hence our reactions do involve aiynes and are not arynoid 13 8). 

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. 

The sulfur analogs of these selenium compounds react less efficiently with benzyne. Thus the yield of396 (S in place of Se) was only 5% from the benzothiadiazole. However, with the parent thiadiazoles 406 the yields are improved. Thus 1,2-benzoisothiazoles 409 are formed in 25-75% yield, depending on probably by the mechanism shown. Selenium... 

The title other methods does not mean that these methods are unimportant, but rather includes a few specific approaches to the synthesis of biaryls. First of these involves the benzynes as reactive species, capable to react with a number of arenes, including themselves, to give the respective biaryls in various yields [94-97]. As the benzyne precursors, benzenediazonium carboxylate (59a) [94], diphenyIiodonium-2-carboxylate (521) [96], benzothiadiazole dioxide (522), and 1-aminobenzotriazole (523) have been used. More conveniently, the benzynes have been in situ generated upon the treatment of anthranilic acid (58) with alkyl nitrites (via 59a), Scheme 50. 

In experiments aimed at the preparation of stable benzyne complexes of platinum of type (51), 1,2,3-benzothiadiazole 1,1-dioxides (52 R=H or Me), which are known to fragment under mild conditions, were used as a potential source of benzyne. However, their interaction with tetrakis-(triphenylphosphine)platinum in benzene did not giye the required benzyne complexes, but resulted in excellent yields of yellow crystalline azocomplexes of structure (53) or possibly (54 R = H or Me). ... 

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