U.S. Corporations are not required to disclose their charitable contributions.
BusinessWeek has done an annual survey since 2003 (Late November early December) listing the largest 30 or so corporate contributors (cash and in-kind gifts). The information is based on about 200 returned questionnaires that BW sends to S&P 500 companies. This issue of BW also ranks the largest 50 individual charitable contributors.

Go to the BusinessWeek website and search for "top givers" in the search box in the upper right corner of the screen.

Take a look at The Committee Encouraging Corporate Philanthropy - CECP.

Come to the Undergraduate Library (Career Cluster) and use National Directory of Corporate Public Affairs; a book at the Reference Desk. 659.285 N213. It incudes sections on Corporate Foundations and Giving Programs.