"It's just being comfortable," said Kubel, who missed two weeks this season with a strained left quadriceps. "I'm not searching or cheating on pitches. I finally feel like I have two legs under me and I'm using them now."
Kubel's big hit came shortly after Jay Bruce's second home run of the game gave the Reds a 3-2 lead in the top of the ninth.
Paul Goldschmidt lined a leadoff single in the bottom half that deflected off the glove of first baseman Joey Votto and into right field. Chapman (3-3) then issued consecutive walks to Miguel Montero and Cody Ross. With the infield drawn in, Kubel singled over second base for his third career game-ending hit and first since May 22, 2010.
"You hate to lose it like that," Reds manager Dusty Baker said. "Of late, we've lost quite a few of them like that. It seemed like Chapman was overthrowing some today."
Gerardo Parra hit a two-run homer for Arizona.
Bell, who had converted nine straight saves, came on in the ninth to protect a 2-1 lead. But he walked Brandon Phillips leading off the inning and Bruce followed with his team-leading 18th homer.
Bell, who has served as the Diamondbacks' closer since J.J. Putz went on the disabled list May 8 with a strained right elbow, has allowed a home run in each of his last five appearances.
Putz could return as early as this week.
"We might have to put some thought into that," Arizona manager Kirk Gibson said. "He's done a pretty good job for us also. Today wasn't his day and I got him out of there pretty quick. I recognized it."
Leake retired the first 15 batters he faced and scattered four hits over eight innings with a walk and a strikeout for the Reds, who have lost three straight.
"I wouldn't have been in that position unless Jay would have hit those two home runs," Leake said. "It's a tough one to swallow but we will be right back there tomorrow."
Corbin allowed two runs on four hits over eight innings and was handed his fourth straight no-decision in his quest to become the first Diamondbacks pitcher to win his first 10 decisions.
"Obviously, I'd love to get that 10th one and get it out of the way," Corbin said. "Fortunately, we came back and won the game."
Bruce gave the Reds a 1-0 lead when he led off the second inning with a line drive to the balcony overhanging center field, about 20 feet from where his ninth-inning shot landed.
The 472-foot drive was the longest home run at Chase Field this season and the fourth-longest ever by an opponent. Bruce has homered in seven of his last nine games.
"The big night offensively didn't really mean a whole lot," Bruce said.
Corbin broke up Leake's nascent bid for a perfect game when he looped an opposite-field double into the left-field corner with two outs in the sixth.
"It was a terrible swing," Corbin said.
Two pitches later, Parra drove a cut fastball just over the fence into the pool area in right-center for his second home run in two games to give Arizona a 2-1 lead.
With Robinson representing the potential tying run, the 23-year-old lefty struck out Shin-Soo Choo and Votto to escape the jam and turn the game over to the bullpen.
"I was coming inside early and he worked the count full like he always does," Corbin said. "He's a tough batter to face. If he's going to beat me, I was going to throw it as hard as I could by him and if he got me, he got me."
Chapman's blown save was his first since May 19 at Philadelphia. ... Arizona has six walk-off wins. ... Gibson tied Buck Showalter for third place on the franchise's victory list with his 250th. ... Bruce had his 13th career multihomer game and first this season. ... Two of Corbin's four hits this season have been doubles. ... Corbin's three walks were his most since issuing five free passes against Atlanta on May 14. ... Leake threw only 52 pitches over the first five innings and 92 total. ... Parra's home run was the second allowed by Leake in his past five starts. ... RHP Randall Delgado will make his second start for the Diamondbacks against RHP Mat Latos in Sunday's series finale. Latos is coming off his first loss of the season Tuesday against Pittsburgh.