Different configurations are available to perform a reaction under reflux at the microscale.
With Kontes-Williamson equipment, the reaction tubes can be used directly. Since the solvent volume is very small, the vapors rise along the tube, cool on the wall and fall back to the bottom of the tube, thus establishing reflux.
Another possibility is to use a reaction tube, flask or conical vial with an air condenser. If the amount of solvent used is not too large and the boiling point of the solvent is not too low, this can be a good option.
Finally, for very volatile solvents, the safest option is to use a conical vial with a water-condenser that will prevent the vapors generated by heating the reaction from passing into the atmosphere.
When anhydrous conditions are required in the reaction to reflux setups, it will be necessary either to couple a drying tube to prevent humidity or to adapt some of the components of the assembly by filling them with the corresponding desiccant.