Highly Selective Nucleophilic 4‐Aryl‐2,3‐allenylation of Malonates

Summary of main observation and conclusion Allenes are a class of very important compounds and the development of straightforward, efficient, and highly enantioselective synthetic strategies for allenes have attracted extensive interests. Along this line, it is well known that aryl‐substituted allenes may be readily racemized, thus, difficult to prepare in high ee. Herein, an efficient palladium‐catalyzed nucleophilic allenylation of malonates with racemic 4‐aryl‐2,3‐butadienyl carbonates has been developed. The selectivity issue of mono‐ vs. bis‐allenylation with 2‐non‐substituted malonates has been addressed. By utilizing (R)‐(–)‐DTBM‐SEGPHOS (5,5'‐bis[di(3,5‐di‐t‐butyl‐4‐methoxyphenyl)phosphino]‐4,4'‐bi‐1,3‐benzodioxole) as a chiral ligand, various aryl‐substituted allenes and bisallenes have been prepared with good to excellent yields with high chemoselectivity and enantioselectivity under mild reaction conditions. Au‐catalyzed cycloisomerization and APK (allenic Pauson–Khand) reactions affording optically active mono‐ and bicyclic products have been demonstrated. A Pd(0)‐catalyzed synthesis of the easily racemizable aryl‐substituted allenes via highly selective nucleophilic allenylation of 2‐non‐substituted and 2‐substituted malonates with allenylic carbonates has been developed.