Thomas H. Lee Partners agreed to buy a stake in HighTower, betting on a wealth-management company that has been expanding with the purchase of investment-advisory firms. The private equity firm is acquiring a "significant" piece of the Chicago-based company, which oversees about $50 billion in client assets, HighTower’s chief executive, Elliot Weissbluth, said Wednesday. HighTower was advised by JPMorgan Chase & Co. on the deal and Weissbluth declined to comment on the terms. The deal comes as independent financial advisers, which HighTower serves, are attracting assets spurred by regulatory changes and as buyout firms look to deploy capital.