We thus started to explore the practicability of the proposed conjugated strong base-promoted method in
Scheme 1. Initially, a reaction of propylene glycol
1a, propargylic alcohol
2a, and CO
2 in CH3CNunder the given conditions was investigated (
Table 1). To our delight, when 1, 8-diazabicyclo[5.4.0]undec-7-ene (DBU) was employed as the catalyst, the corresponding products,
i.e., propylene carbonate (PC,
3a) and
α-hydroxy ketone
4a were obtained in 61% and 58% yields, respectively (entry 1). At the same time, several typical guanidine bases were also examined as catalyst. As seen from the results (entries 2–4), theyalso showed catalytic activity for the transformation. 1, 5-Diazabicyclo[4.3.0]non-5-ene (DBN) possessing the similar structure with DBU gave the equivalent level result (entry 2). The other strong base such as 1, 5, 7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 1, 1, 3, 3-tetramethylguanidine (TMG) gave lower yields (entries 3 and 4). As we all know, the basicity strength of the catalyst plays a key role in the coupling reaction of CO
2 and propargylic alcohol, which is the ratedetermining step in the three-component reaction [
32,
39]. Generally, the weak basicity of catalysts resulted in low activity, while the strong base exhibited a higher conversion and a lower yield because of some side reactions [
38]. The basicity of the guanidine bases (entries 1–4) followed the order: TBD > DBN > DBU > TMG [
39-
41]. However, their catalytic activity showed as DBU, DBN > TBD > TMG, an action from both basicity and nucleophilicity indicating a good agreement with the previous works [
38,
42]. Additionally, both K
2CO
3 and Cs
2CO
3 led to the lower yields (entries 5 and 6). The unconjugated base,
i.e ., Et
3N exhibited significantly a lower catalytic activity than that of guanidine bases (entries 7
vs. 1–4). Clearly, the guanidine bases displayed better performance than that of the other basic catalysts.