Optimization of the reaction conditions for the synthesis of 3a^a.

Entry	Catalyst (2 mol%)	Conditions	Yieldb (%)
1	SiO2	8 h, 100 °C	56
2	SiO2@Ag	8 h, 100 °C	65
3	SiO2@Au	8 h, 100 °C	67
4	SiO 2 @Au–Ag	8 h, 100 °C	97
5	—	8 h, 100 °C	32

^aReaction conditions: amine 1a (1.0 mmol), carboxylic acid 2a (5 mmol).

^bIsolated yield.