In the TV show Samurai Jack, he is transported to the future where most episodes involve him chasing the one who sent him there, and trying to get back to his time. What was the reason AKU sends him to the future?
Aku sent Jack to the future as a last ditch effort to stop Jack from killing him.
Before the final blow was struck, I tore open a portal in time, and flung him into the future where my evil is law.
This is especially clear in the first episode/movie, in which we see him beg with Jack before this. It's never directly said why Aku sent him to that particular era, but it's safe to say that sending him in the future would prevent him from tampering with the past -- and that Aku is strongest in the future, where he has had time to recover and take everything over.
Yes, this is stated at the start of every episode in the first four seasons.
Moreover, lending credence to Aku's decision, we see that 50 years further down the road, Aku has regained an immense amount of power, being able to destroy a whole army in seconds.
Clearly, Aku knew that he could recoup and hoped to become powerful enough to overcome the "foolish samurai wielding a magic sword".
"Long ago in a distant land, I, Aku, the shape-shifting Master of Darkness, unleashed an unspeakable evil! But a foolish Samurai warrior wielding a magic sword stepped forth to oppose me. Before the final blow was struck, I tore open a portal in time and flung him into the future, where my evil is law! Now the fool seeks to return to the past, and undo the future that is Aku!"
-- Aku, opening sequence
Aku saw him as a threat, and sent him to a time where Aku was (apparently) too powerful to be threatened by him.
From Wikipedia (emphasis mine):
After taking his father's magic katana, the prince challenges Aku to a duel and defeats him. However, before the prince can deal the killing blow, Aku creates a time portal and sends his opponent into the distant future, anticipating that he would be able to amass sufficient power to deal with the samurai later.
And from Wikia (emphasis mine):
Jack travelled to the castle of Aku and found his father enslaved and nearly skin and bones. He warned Jack that it was too early for him to fight Aku, but he didn't listen and continued on. Jack then confronted the sorcerer and the battle began. Jack managed to severely wound Aku, but before he could deal the final blow, Aku opened a time portal and threw Jack into it, sending him centuries into the future.
When Jack finally escaped the portal, he found himself in a dystopian future. After Jack escaped being crushed, he met three alien teens who called him "Jack." He asked them where he was and what was going on around him, and learned that Aku had conquered the world and spread his influence across the universe, enslaving many planets while using Earth as his base of operations.
What would you do if you had encountered a dungeon boss you cannot beat?
CAUTION: Tongue in cheek answer imminent, reader discretion is advised.
That's right, you would use your escape rope, lick your wounds, go out and grind in those levels some more until you think you can beat it.
That is what Aku actually did in this case.
During the conclusion of the initial battle between Jack and Aku, Aku realized that his power level is not enough to beat Jack, and carelessly he had no more health potions left as well, so Aku decided to escape to live and fight another day.
Since Jack is not some dungeon boss that will idly sit in some dungeon waiting for Aku to grind in those levels he delayed the encounter the only way possible, by sending Jack to the future, Aku used his metaphorical escape rope and sent Boss Jack back to his "dungeon".
Evidently either Aku got lazy and forgot about this encounter, or he is just not that good in power-leveling, by the time it's once again destined to battle the boss, he is not high level enough to steamroll over Jack by this point. Whether or not he did this to keep the boss battles interesting remains uncertain.
Tearing open a portal in time was obviously a last-ditch effort, so it stands to reason Aku didn't have a solid plan for it. It was the only way he could think of, at the time, to distance himself from Jack.
From the facts that Aku isn't shown to use such portals at any other occasion (AFAIR), and that he waited until the very end to do it, we can conjecture that it was a sort of experimental ability - Aku wasn't sure he could do it at all, let alone what would be the exact consequences.
Aku probably did consider that in the future he would have an advantage against Jack, but didn't engineer the exact destination - he just did what he could with his remaining strength, and that ended up sending Jack when it did.
Aku attacks Jack's village when Jack is a child and he is helped by his Mother and friends of his Father to escape so that he could return to defeat Aku at a later time.
After training and growing up, Jack gets his Father's sword and returns to his village and confronts Aku. Jack wins the battle and as he is about to defeat/kill Aku, Aku sends Jack into the future to escape the battle and so that Aku will be stronger and have a better chance of defeating Jack when they meet again in the future.
This is why Jack is trying to get
back to the past so that he can fight the weaker "past Aku" and finish the job, as "future Aku" has a lot of resources which makes defeating him more difficult.