The
reactions of stable monomeric dialkylstannylene <b>1</b> with
methyl and ethyl propynoates give the corresponding 1:2 adducts,
alkenyl(alkynyl)stannane <b>2</b> and <b>3</b> in high yields, while <b>1</b> does not react with parent
acetylene or common mono- and disubstituted acetylenes such as phenylacetylene,
trimethylsilylacetylene, diethyl 2-butynedioate, etc. Notably, <b>2</b> and <b>3</b> have the <i>Z</i>-configuration
of the alkenyl moieties, in contrast to similar adducts obtained by
the known reactions of silylenes with terminal acetylenes. It is suggested
that the formation of a carbonyl oxygen-coordinate cyclic zwitterion
as a key intermediate is essential for the reactions. Stannylene <b>1</b> adds to in situ generated benzyne, forming a 1:1 adduct
having a unique 3-stanna-1-silaindane ring system
