Christopher A. Viehbacher, the former Sanofi SA chief executive who engineered its $20.1 billion buyout of Cambridge's Genzyme Corp. in 2011, has been named a senior partner and board member at PureTech, a Boston research and development firm that incubates and bankrolls biotech startups. The move suggests that Viehbacher, who was fired by French drug maker Sanofi last year after he moved his primary residence from Paris to Massachusetts, plans to keep a high profile in the Boston area, which has emerged as a global hub for the life sciences.