Snagging the power of cyclic allene intermediates

Strained cyclic compounds have long been curiosities in organic chemistry, but chemists have not explored many ways to harness their reactivity. Although cyclic allenes racemize spontaneously, chemists have been able to take stereoselective control over the mixtures, but only by using large amounts of chiral compounds. A new study shows that chemists can catalytically take advantage of this process to build stereospecific complexity into target organic compounds, which are useful in agrochemistry and drug development. Neil Garg, Kendall Houk, and coworkers at the University of California, Los Angeles, have managed to grab strained cyclic allene intermediates and use them in catalytic asymmetric reactions to make tricyclic lactams of mainly one enantiomer (Nature 2020, DOI: 10.1038/s41586-020-2701-2).The team reacted a benzyl triazinone and a cyclic allene precursor in the presence of a nickel catalyst with an iron cyclopentadienyl-based chiral ligand to give two intermediates: a strained cyclic allene and an organometallic intermediate based on the Ni catalyst. These species quickly merge to form a chiral adduct through olefin insertion, to give the product (shown) an as high as 85% yield and 94% enantiomeric purity. Garg and coworkers crystallized the product and found that the reaction makes the opposite enantiomer—not the one they expected. Through calculations, they found that the transition state leading to the favored enantiomer forms through stabilization from the solvent, making it slightly lower in energy. This research shows that strained intermediates can be used to make highly enantiomerically pure compounds, Garg says.