The billionaire chief executive of Las Vegas Sands Corp. has invested $475 million in the hotel and casino operator to avoid violating lender covenants. In a sign of creative cash-raising in a tight credit environment, Sheldon G. Adelson, who also is chairman and principal shareholder, and his family purchased convertible senior notes due October 2013 in a private transaction. The notes pay interest of 6.5 percent and convert into common shares at a price of $49.65.