Is it possible to tape a conversation between employee and boss without them knowing

What is the reason behind this?

If it is a legal reason even if you do record a conversation the tapes cannot be used.

In some states it is legal to tape a conversation as long as one person is aware of it. In others both people have to be aware. It is illegal to tape a conversation where neither party is aware without a court order.

Is it legal to to tape conversations between employee and boss, without them knowing first. Then use it against them

No
Its only legal if YOU are one of the parties involved...

They way you are speaking makes me believe that you are a 3rd party who wants to record a conversation between 2 people without their consent - this is illegal.

What state is this in? Like others pointed out, some states allow you to tape the conversation IF one of the parties gives you their consent.

is it legal to to tape conversations between employee and boss, without them knowing first. then use it against them

You could/will be arrested for blackmail if you do that type of behavior.

It depends, if you need it to save your innocence or life then use to protect yourself. If you in court, it can come to your favour to win the case.... But if you want to use it for fun then its totally wrong

Can you provide us with something that shows your statement is legally accurate? States have varying laws regarding what you can or cannot tape. "My innocence was on the line" is not a legal defense for breaking the law.