Window air conditioner between living room and enclosed porch
If you have a forced air furnace for heating, you could install the window unit in another window and run the furnace fan to circulate the conditioned air throughout the house. My son did this in his first house. As for using the window you mentioned, you could do it your way if you can insure good air flow in and out of the porch area to keep that area the same temperature as the outside. Window fan/fans, may be needed. If the porch area heats and has  high humidity that will affect the operation of the A/C unit and could ruin the porch area. All window A/C units produce lots of heat and humidity when operating. So you will have to make sure you address these issues. A split system or other type of cooling unit would be preferred. Depending on where you live a 12,000 BTU unit may be a little undersized.