With 1c as substrate, several chiral dienes were subsequently evaluated for the asymmetric hydrogenation at 40 °C (Scheme 3). Chiral diene 3b exhibited high reactivity, but resulted in a very low ee (Scheme 3). When a bulkier chiral diene 3c was utilized, the ee could be reached to 71%. Moreover, chiral dienes 3d and3e derived boranes could also promote this reaction efficiently, but only yielded moderate ee’s.
Scheme 3 The screen of chiral dienes