BERLIN—Adidas is selling Reebok to Authentic Brands Group(ABG) for up to 2.1 billion euros ($2.5 billion) as the German sporting goods company concentrates on its core brand after a deal that did not deliver. Adidas bought Reebok for $3.8 billion in 2006 to help compete with arch-rival Nike, but its sluggish performance prompted repeated calls from investors to sell the U.S. and Canada focused brand. In the meantime, Adidas managed to eat into Nike’s dominance in the United States with its own brand, helped by partnership with celebrities like Kanye West, Beyonce, and Pharrell Williams. Reebok will keep its headquarters in Boston and continue operations in North and Latin America, Asia-Pacific, Europe, and Russia, the U.S. brand management firm said in a statement, adding it will work closely with Adidas during the transition. Over 11 years, ABG has amassed more than 30 labels sold in some 6,000 stores. Its brands …