Walgreen ( WAG) said Wednesday that Chairman David Bernauer plans to step down from his executive duties at the company, though he will remain head of the board. The Deerfield, Ill.-based drugstore giant didn't provide a reason for the move, which is effective immediately. Jeffrey Rein will continue as Walgreen president and chief executive and is assuming Bernauer's executive responsibilities. Bernauer, who started his career with Walgreen in 1966, has been a member of the company's board since 1999 and was named chairman in 2003. He also served as CEO until July of last year. Earlier Wednesday, Walgreen announced a $1 billion stock buyback plan to be executed over the next five years.