Is there any halachic/hashkafic parameter that prevents a married man from starting up a new business with a single women. The pair would have to work closely for long hours and would share experiences of accomplishment, success and failure throughout their working relationship. This seems innapropriate to me but i was wondering if i am over reacting.

Without a doubt, it is clearly wrong, both hashkafa and halacha-wise, for any man, especially a married one, to have a close relationship with a women, other than his wife. The Shulchan Aruch states explicitly: “one must distance himself from women very, very much!” (Even HaEzer 21, 1), and working alone, side by side in a pleasant and friendly atmosphere (as a work-environment should be), is clearly not “distancing”, and surely not “very, very much”. This is not just a suggestion for a happy marriage, but an actual prohibition. Even working alone together with a female secretary for long hours, is a problem (even if they are careful in observing the laws of yichud, never to be alone in a room, just a man and a woman, if there’s no probability of somebody else entering by surprise), so how much more so, one shouldn’t have a partnership as equals, working closely for many hours. Inevitably, part of that special man-woman relationship and chemistry, the joking, laughing and unique friendship which should be totally reserved for one’s wife, will be “spent” on another, “competing” woman, and will undoubtedly come at the expense of his spouse. In fact, I heard from someone close to the police department, that they have a problem dealing with many “affairs” which arose out of having a man and woman on duty together, sometimes, when at least one of them was married. A happy marriage and healthy family life, is a lot more important than making money, especially when there are many kosher ways of making a living, like finding a male business partner. May the zchut of shalom bayit grant you parnassa tova in the best possible way! With Love of Israel, Rav Ari Shvat