We selected biphenylene
1a and chalcone
2aa as model substrates to explore the reaction parameters (
Table 1). The issues of both chemo- and diastereoselectivity arised when coupling with unsaturated alkenes comparing to the alkynes. It was found that the reaction delivered to the double bond insertion selectivity
trans-product
3aa in 90% yield and excellent diastereoselective ratio (
trans/cis > 20:1) with Ni(cod)
2/PPh
3 catalysis using the DMA as solvent (entry 1). When decreasing the temperature to 60 ℃ in the DMA solvent, the
trans/cis value decreased to 9:1 (entry 2). Further study showed that the solvent effect was dramatic (entries 3–9). Low diastereoselective ratio was obtained when the DMF was used as solvent (entry 3). The starting materials were recovered with DCE as solvent (entry 4). Very interestingly, the DME, MeO
tBu, THF or dioxane gave another isomer of the [4 + 2] annulation product
cis-product
3aa in excellent yield, even with low diastereoselective ratio (entries 5–8). To our delight, excellent diastereoselectivity could be obtained for
cis-
3aa when the toluene was used as solvent (entry 9, 72% yield,
trans/cis = 1:9), higher yield and
dr value were obtained when the reaction temperature decreased to 60 ℃ (entry 10, 88% yield,
trans: cis = 1: > 10), further decrease the temperature to 30 ℃ in toluene led to low efficiency (entry 11). Up to now, the solvent-controlled diastereodivergent [4 + 2] cycloaddition system was established, in which the
trans-
3aa was obtained in polar solvent DMA (entry 1, 90% yield,
trans: cis > 20:1) and
cis-
3aa was obtained in toluene (entry 10, 88% yield,
trans: cis = 1: > 10). Noteworthy, these reaction conditions only afforded the C=C insertion products. Other ligands such as Bpy or
rac-BINAP did not promote the annulation (entries 12 and 13), and no corresponding products observed without the addition of PPh
3 (entries 14 and 15). Our control experiments also showed that the Ni(cod)
2 was essential for the transformation (entry 16).