NEW YORK (Reuters) - Troubled commodities brokerage Refco Inc. said on Monday it had filed for bankruptcy protection as it struck a deal to sell its core futures brokerage business to a group of private equity investors for $768 million (439 million pounds). New York-based Refco said it and certain subsidiaries filed for bankruptcy protection on Monday as part of the memorandum of understanding with investors. None of Refco's regulated subsidiaries -- its futures brokerage business, which is conducted through Refco LLC, Refco Overseas Ltd. and Refco Singapore Ltd, and its broker-dealer, Refco Securities LLC -- filed for bankruptcy...