Unexpected switching between addition and substitution in the rhodium-catalysed reaction between tin hydrides and propargyl ethers

The course of rhodium-catalysed reactions between triorganotin hydrides and alkynes depends both on the nature of the alkyne and of the tin hydride. T ributyltin hydride undergoes non-regioselective addition to propargylic alc ohols but regiospecific addition to propargylic ethers. The regioselectivit y is lost with triethyl- or triphenyltin hydrides, while use of trimethylti n hydride leads to substitution of the acidic hydrogen by the trimethyltin moiety.