Synthesis of Ladderanes by Nickel‐Catalyzed Cyclodimerization of Allenynes: Scope and Mechanism

We have developed a Ni‐catalyzed cyclodimerization of 1,6‐allenyenes which affords pentacyclic derivatives containing a [3]‐ladderane. Six new carbon‐carbon bonds are formed in a single operation which is fully atom‐economical. The reaction proceeds with an inexpensive catalyst and shows a broad scope in the alkyne, the allene and the tethering group. A computational and experimental study on the reaction mechanism suggests that this process proceeds by oxidative cyclometallation of the allenyne coordinated to Ni(0) followed by reductive elimination leading to cyclobutenes. Then, two of these molecules are coupled following a related cyclometallation‐reductive elimination sequence, the latter being the rate‐limiting step.