Alleghany is expected to pay $59 per share to $60 per share in cash and stock for Transatlantic, the source said.
Former Berkshire Hathaway
Brandon had earlier been working with Morgan Stanley
The deal, which was previously reported by Bloomberg, is expected to be announced on Monday.
Davis Selected Advisors, Transatlantic's largest shareholder, is supportive of the deal, the source said
Transatlantic called off a deal with rival Allied World Assurance Co Holdings Ltd
Alleghany and Transatlantic Holdings could not immediately be reached for comment by Reuters outside regular U.S. business hours. (Reporting by Paritosh Bansal, Additional reporting by Sakthi Prasad; Editing by Muralikumar Anantharaman)