What temperature of water will kill yeast?
There are multiple temperature points per yeast family might be of Interest:


The growth limit temperature, see this paper.  It indicates as high as 45&deg;C for some and 20&deg;C for others.
'Injury' temperature (for bread and brew generally thought be around 120&deg;F)
Death temperature. 


Keep in mind for bread: Bread Yeast at higher temperatures produces off-flavours that may be undesirable to your goal.  If you are trying to speed up your dough, you may want to add the yeast to water with some food and let develop for a while before mixing in the flour.