Allenes, such as 1,2-propadiene and 1,2-butadiene, are becoming highly sought after building blocks for their ability to react with many different classes of substrates. In addition, the allene moiety itself is present in many bioactive natural products and pharmaceutical agents. Some advances include ionic addition; hydrometallation-, carbometallation-, or nucleometallation-initiated reactions; cycloisomerization and eliminative cyclization; cycloaddition; and cyclometallation. Due to their substituent-loading capability, the reactions of allenes may nicely show diversity in organic synthesis, and the axial chirality in allenes may open new doors on asymmetric synthesis.
