Not even close. This is just a talking-point bouncing around the conservative media. And it’s not true.
They came up with this claim by not counting undisclosed donations, which are the main way that really large amounts of money are donated. The Koch brothers, to give a prominent example, paid $4,900,000 in disclosed donations in the 2012 election and $407,000,000 in undisclosed donations. Ignoring the undisclosed donations greatly distorts the amount of money they gave.
If you add in all the money that was donated, the Koch brothers donated $412,670,666 (I’m assuming the 666 was just their little joke). The top ten union donations combined added up to $153,473,251.