Novel preparative uses of 1-halogeno- and 1,2-dihalogenoacetylenes

Rather than fearing and consequently avoiding the high reactivity and often explosive character of 1-halogeno- and 1,2-dihalogenoacetylenes, there are more and more examples in the modern literature which deliberately set out to use this reactivity imaginatively for synthetic purposes. 

2-Dichloroacetylene (100) is a good example to illustrate this point. Originally reported in 1930 [88], its dangerous nature has frequently been noted it ignites spontaneously and may explode violently, especially on contact with air. However, it was also noted relatively early 

Once in hand, 100 may be used in multitudinous ways. Dreatment with a thiol (both aliphatic and aromatic ones having been employed) in the presence of potassium hydride in tetrahydrofuran (THE) solution affords the corresponding bis(thioethers) 142 in high yield [94] (Eq. (26)). Dichloroacetylene behaves as a very electrophilic compound which is readily attacked even by bulky nucleophiles (with tert-butylthiol the product 142 is obtained in 98% yield) [95]  

When 142 (R = tert-Bu) is oxidized with m-chloroperbenzoic acid (MCPBA), the disulfone 143 is obtained in quantitative yield [96] (Eq. (27)). As expected, the latter shows high dienophilic activity in Diels-Alder additions, and since the sulfonyl moiety can be easily removed from the Diels-Alder adducts (also obtained in good to excellent yields) by a variety of methods, 143 serves as another equivalent for acetylene in [2 -I- 4] cycloadditions. 

The high propensity for addition to 1-halogeno- and 1,2-dihalogenoacetylenes has often been noted [4, 60], and 100 is no exception in this respect [93]. With secondary amines, the 

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