Revenge of the Fallen film
Sentinel Prime led an expedition where it took him fourteen galactic convergences to get an echo of something.
Whether or not he is one of the original Primes or in the later Dynasty of Primes is uncertain. He does however seem to be one of the direct ancestors of Optimus Prime.
Sentinel Prime was mentioned in a rambling lecture of Sam Witwicky's while under an All Spark fragment-induced trance.