At the outset, the model reaction involving 4-chlorobenzaldehyde (
1a) and 3-bromocyclohexene (
2a) was performed in the presence of bismuth (3 equiv.) and various additives (2 equiv.) in aqueous DMF at room temperature for 24 h, attempting to find the optimum reaction conditions. As summarized in
Table 1, among the several additives screened (such as LiCl [
14], LiI [
15], NaI [
16], NH
4Cl [
17], NH
4Br [
18], and NH
4I [
19]; entries 2–7), the use of NH
4I as reaction additive afforded the desired product
3a with the highest product yields (95% NMR yield, 93% isolated yield; entry 7). In sharp contrast, the reaction proceeded sluggishly in the absence of an additive (entry 1). It should be mentioned that the use of LiI, NH
4Cl, and NH
4Br as reaction additive also delivered the product
3a in reasonable yields (entries 3, 5, and 6). By using NH
4I as optimum reaction additive, it was observed that conducting the reactions with reduced or prolonged reaction time led to decreased product yield (entries 8 and 9). Moreover, the addition reactionwas found to be sensitive to temperature, as eroded reaction performance was detected when the reactions were carried out at elevated temperature (entries 10 and 11). Furthermore, survey of other aqueous solvent systems showed that the reactions worked more efficiently in aqueous media containing more polar organic solvents (DMA and DMSO; entries 16 and 17) than in aqueous media using relatively less polar organic solvents (THF, CH
3CN,
etc.; entries 12–15). In view that the highest product yield was obtained by employing DMF/H
2O as reaction media, thus the following reactions were performed in aqueous DMF by utilizing NH
4I as reaction additive (entry 7). More importantly, the desired product
3a was obtained with excellent diastereoselectivity (>99:1
syn:
anti) under the optimized reaction conditions, with the formation of
syn-
3a as the exclusive diastereomer. Additionally, a 10 mmol scale of the model reaction by using 1.4 g aldehyde
1a could also work with a good efficiency to provide the corresponding product
3a in 81% yield (entry 7, footnote c).