Fig. 6. (a) trans-Selective hydrostannation of propargylic alkynes using ruthenium chloride complex B as opposed to cationic variant A. (b) High regioselectivity is also observed in other unsymmetrical alkynes bearing protic functionality, with selectivity reversal in the case of an ester. (c) Selected experiments conducted to probe the role of hydrogen bonding in the observed regioselectivity. (d) Hypothesis for origin of regioselectivity (left), plus application of this hypothesis to explain observed regioselectivity in two previously reported ruthenium catalysed reactions. (c) and (d) Reprinted with permission from ref. 45. Copyright 2015 American Chemical Society.