New solvent-assisted mechanistic routes were located for the (THF)W(CO)(5)-promoted endo-and exo-cycloisomerization of 4-pentyn-1-ol using the B3LYP/6-31G* (with the LANL2DZ relativistic pseudopotential for W) theory level. A mixed model was used by explicitly including a THF molecule as a component of the reactive system and taking into account the effect of bulk solvent by means of the PCM-UAHF method. This THF molecule plays a crucial role by making the endo route leading to the cyclic carbene complex the most favorable one because of an important catalytic effect. These new mechanisms allow a satisfactory rationalization of experimental facts.