The board of directors of OpenAI, the high-flying artificial intelligence start-up, said in a note to employees on Sunday night that its former chief, Sam Altman, would not be returning to his job, while naming his second interim replacement in two days.

Hours later, in another head-spinning move, Microsoft said it was hiring Mr. Altman and Greg Brockman, OpenAI’s president and a company co-founder who quit in solidarity with Mr. Altman. The two men will lead an advanced research lab at Microsoft.

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