HONG KONG — A mysterious Chinese buyer surprised the financial world last year when it swooped in to buy a $9 billion chunk of Russia’s state oil company. Few outside China had ever heard of the firm, called CEFC China Energy, or could explain how it would complete — or even pay for — such an expensive and politically sensitive deal.

The company’s young chairman, Ye Jianming, cast himself as a new kind of private entrepreneur in China. He had somehow become a big player in a strategic sector long dominated by big state companies. He appeared to have the government’s blessing and a relationship with a group tied to China’s military, according to experts.

His company invested so much in the Czech Republic that the Czech president named him an economic adviser.

But Mr. Ye now appears to have come under pressure from the government of President Xi Jinping, joining other major corporate figures in China.