Table 3. Catalytic CO₂ Hydrogenation with 1, Screening of the Effect of Lewis Acid (LA) Co-catalyst under Various Conditions^a.

entry	1/DBU	1/LA	LA/DBU	TONb	yield (%)c
1	1/2000	1/100	0.05	1104	54.8
2d	1/2000	1/100	0.05	1988	98.7
3e	1/2000	1/100	0.05	85	4.2
4	1/2000	1/200	0.1	135	6.4
5	1/2000	1/50	0.025	678	33.7
6	1/5000	1/250	0.05	238	4.7

^aReaction conditions: catalyst 1, 2–5 μmol; DBU, 10 mmol; LA = LiOTf, 0.25–1.0 mmol; THF, 5.5 mL; H₂/CO₂ (2:1), 75 bar total pressure; 80 °C, 24 h.

^bTON = (mmol formate)/(mmol catalyst).

^cYield = [(mmol formate)/(mmol DBU)] × 100. The amount of formate was calculated from the integration of the corresponding ¹H NMR signal in D₂O against an internal standard (DMF).

^dAs above, 48 h.

^eAs above, 100 °C, 24 h. All experiments were repeated at least twice to check for reproducibility; average error, ca. 6%.