You can do it, but you have to do the logic on the smartthings side. As of this writing, Alexa routines can only be triggered from a sensor, lock, or a flic or echo button, not from a switch or a thermostat.
So create a smartthings Automation based on the temperature and have it turn on a virtual sensor.
Then have that virtual sensor opening be the trigger for your Alexa routine.
You do this by using a virtual device which is BOTH a sensor and a switch. When you turn the switch on, it makes the sensor look like it’s open. When you turn the switch off, it makes the sensor look like it’s closed.
Here is the community FAQ on how to do this. It includes a link to a virtual device of this type that you can use. (The topic title is a clickable link)
FAQ: Can I trigger an Echo Action without Speaking to It?
This is a very common method for using an Alexa routine as an intermediary between smartthings and a device that smartthings does not control. You put all the logic on the smartthings side in smartthings automations or webcore and then the last step of that is to turn on the virtual sensor to trigger the Alexa routine.
This method used to work great up until they introduced the new smartthings skill in April 2020. Since that time it works fine most of the time, but every once in a while it will glitch for a few days and just not work. 
And then it will start working again. So just be aware of that as a potential issue.