Chapman came to the plate with two outs in and sent a drive over the bullpen beyond the fence in left-center to give San Francisco a 4-0 lead.
Mason Black made the lead stand up by allowing only one run in 4 1/3 innings in his home debut, giving up a solo homer to Elly De La Cruz in the fourth.
The bullpen did the rest, sending the Reds to their ninth loss in 10 games.
Taylor Rogers (1-1) replaced Black with two runners on in the fifth. He got out of the jam by striking out pinch-hitter Stuart Fairchild with the bases loaded to earn the win.
Black was helped by outfielder Heliot Ramos, who threw out two runners from right field trying to stretch hits into doubles and then made a diving catch in left field in the fifth inning.
Ramos threw out Will Benson in the second and Jake Fraley in the fourth and then robbed Benson of a hit after moving spots in the fifth inning. Mike Yastrzemski replaced Ramos in right field and threw Jeimer Candelario out trying to stretch a hit into a double to end sixth.
TRAINER’S ROOM
Reds: LHP Alex Young (back) retired all three batters on a rehab assignment at Triple-A Louisville on Friday and is expected to pitch again in the minors Sunday
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