Elon Musk quietly donated nearly $6 billion last year. Where did it go?

In an inconspicuous fashion — via a regulatory filing with the Securities and Exchange Commission — Elon Musk, Tesla’s chief executive, disclosed on Monday that he gave nearly $6 billion worth of the company’s shares to charity last year, instantly propelling him into the upper ranks of philanthropic donors.

But the document gave little information about where he directed his wealth, the DealBook newsletter reports.

Mr. Musk donated just over five million shares in November, according to the filing. Based on the average price of Tesla shares on the days he sold, that is worth roughly $5.7 billion. That would make him the second-biggest donor last year, behind only Bill Gates and Melinda French Gates and their $15 billion in giving, according to the Chronicle of Philanthropy.

In November and December, Mr. Musk sold more than $16 billion worth of shares, much of which was meant to cover tax obligations after he exercised stock options. At the time, he wrote on Twitter that he was likely to pay more than $11 billion in taxes, one of the biggest individual bills in U.S. history. The stock donation could be useful in defraying that bill.

The question is where those shares went. The donation came as Mr. Musk argued with populist lawmakers like Senators Bernie Sanders, the independent from Vermont, and Elizabeth Warren, Democrat of Massachusetts, about wealth inequality and taxes. Several observers noted that weeks before his donation, Mr. Musk tweeted that he would give $6 billion if the United Nations could prove that money could help solve world hunger. Days afterward, the U.N.’s World Food Program outlined how it would spend $6.6 billion to help avert famine.

Mr. Musk’s other disclosed philanthropic efforts include millions in gifts to Texas municipalities and a $100 million prize for developing technologies to remove carbon dioxide from the air.