Reactions of an Isolable Dialkylstannylene with Propynoates and Benzyne

The
reactions of stable monomeric dialkylstannylene 1 with
methyl and ethyl propynoates give the corresponding 1:2 adducts,
alkenyl­(alkynyl)­stannane 2 and 3 in high yields, while 1 does not react with parent
acetylene or common mono- and disubstituted acetylenes such as phenylacetylene,
trimethylsilylacetylene, diethyl 2-butynedioate, etc. Notably, 2 and 3 have the Z-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 1 adds to in situ generated benzyne, forming a 1:1 adduct
having a unique 3-stanna-1-silaindane ring system.