A Ni-catalyzed [2 + 2 + 2] cycloaddition reaction between allene-ynes and various mono-, diand tri-substituted allenes is described. This protocol effectively differentiates allenyl π components and shows broad substrate generality to give highly functionalized carbocycles. A DFT study played a key role in revealing a detailed reaction mechanism that controls site-, regio-and stereoselectivity, which are thought to originate in the substituent effect on π-bonds in the early transition state. Figure 3. Eight possible transition states and their activation energies.Scheme 5. Effect of R 6 in regioselective [2 + 2 + 2] cycloaddition and substrate scope. Scheme 6. Reaction using 1 a,o with 2 h.
The front cover picture, provided by Shigeru Arai and co‐workers, illustrates new protocols for the [2+2+2] cycloaddition reaction between allene‐ynes and simple allenes to create highly functionalized nitrogen heterocycles. DFT calculations play a key role to design new reaction pathways and control regio‐, site‐ and stereoselectivities of the cycloadducts. Details of this work can be found in the Communication on pages 4882–4887 (S. Arai, A. Izaki, Y. Amako, M. Nakajima, M. Uchiyama, A. Nishida, Adv. Synth. Catal. 2019, 361, 4882–4887; DOI: 10.1002/adsc.201900719).
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