The deal, the largest ever in the healthcare sector, will be announced on Monday and is sure to draw political ire in a U.S. presidential election year because Pfizer would redomicile to Ireland, where Allergan is registered, in a so-called “inversion” that would slash its corporate tax rate. It will also reignite debate in the pharmaceutical industry over the role of research and development, with Allergan Chief Executive Brent Saunders, a prolific dealmaker and a skeptic of in-house drug discovery, joining the combined company in a position to influence its strategy.