Unexpected formation of alkynyl-substituted 3-ylidenedihydrobenzo[ d]isothiazole 1,1-dioxides

The isothiazole chemistry has been some progress from the year 1956 and since then, their chemical and physical properties have been extensively studied. Due to their particular reactivity, isothiazoles have recently emerged as useful synthetic compounds and their applications in the search for alternative synthetic strategies and the development of novel molecular structures have been steadily growing [1]. The synthetic versatility of isothiazole has stemmed also from the interest in the biological and pharmacological properties of its derivatives [2]. A Pd-mediated one-step synthesis of benzoisothiazoles, involving the coupling of bromosulfoximine with terminal alkynes under Sonogashira coupling conditions, has parcially described [3].
Different ylidene dihydrobenzoisothiazole motif has been synthesised in one-pot conditions. Herein, we report palladium-copper metal catalyzed cyclization of 2-iodo-N-phenyl-N-((trimethylsilyl)ethynyl) benzensulfonamide in presence of terminal alkyne under Sonogashira coupling condition to afford substituted alkynyl ylidene dihydrobenzo[d]isothiazole dioxide. The resulting hydrobenzoisothiazole selectively gives the Z stereoisomer. X-ray molecular structures of compounds (Z)-3-(1,3-bis(trimethylsilyl) prop-2-yne-1-ylidene)-2-phenyl -2,3-dihydro benzo[d]isothiazole-1,1-dioxide, (Z)-3-(3-(3,5-dimethoxy phenyl)-1-(trmethylsilyl)prop-2-yne-1-phenyl-2,3-dihydrobenzo[d]isothiazole 1,1-dioxide and (Z)-3-(3-(2-methoxy phenyl)-1-(trmethylsilyl)prop-2-yne-1-phenyl-2,3-dihydrobenzo[d]isothiazole 1,1-dioxide are obtained.
The mechanism f these reactions had been proposed.