Police say it happened shortly before 6:30 p.m. Tuesday in Coney Island.
Two suspects got into the vehicle while it was stopped at an intersection.
They mugged him, taking the gold chain he wore off his neck.
The men fled and the driver took off, but only got a few blocks when he realized his passenger lost consciousness.
The victim has been identified as Michael Shelonchik of Bergen Beach, Brooklyn.
He was pronounced dead at Maimonides Medical Center. His cause of death will be determined by the Medical Examiner's office.
It is unknown what sparked the assault and if Shelonchik knew the suspects.
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