So I've just been doing the brotherhood quest line I know danse went rogue through it but I didn't realise it was as he is a synth(nice twist Beth). I was reading up on him after getting this info and there is somethings that just don't make sense for him been a synth though.

He grew up in the wasteland and joined the brotherhood there. (Synths don't grow).

The fact he doesn't know he is a synth, but he still has all memories of bos. If he had a memory wipe he would not remember any of this, and if be was a synth placed he would know he is a synth. Am I missing something in this quest line that should have giving me an answer or confused myself

Most likely the Railroad was involved. His memories of being a synth wiped, and false ones implanted. That's why he has memories of growing up, and doesn't realise he's a synth.

Pretty cruel twist for Danse to wind up joining a group that hates synths and wants to destroy the very people that probably helped him escape the Institute.

Or he was memory wiped before joining the BOS.

Danse says he use to be an trader in rivet city so he could have been memory wiped and placed there by the railroad only to then join BOS, wouldn't be the first a wiped synth went "bad"

Yeah, I believe he would have been wiped before joining the Brotherhood. The memories of Rivet City could be real, but anything before that is most likely implanted.

He could also be a replacement for the real Danse, we don't actually know if his earlier memories are an implanted RR reprogrammed memories and he joined BOS not knowing what he was or if the original Danse was killed or experimented on by the Institute while he was replaced by a Synth.

They borrowed liberally from Battlestar Galactica- including plot holes, apparently.