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Synthesis

Triple adamantane expands the boundary of phosphine ligands

Trisubstituted phosphine sets a new standard for electron-releasing character in an alkane-based ligand

by Stephen K. Ritter
May 30, 2016 | APPEARED IN VOLUME 94, ISSUE 22

Phosphine ligands have played a supporting role in the success of many a transition-metal catalyst, helping to tune their activity, selectivity, and stability. Among the examples, tri(tert-butyl)phosphine has long been characterized as the most electron-releasing alkylphosphine ligand for metals. Liye Chen, Peng Ren, and Brad P. Carrow of Princeton University have succeeded in raising the bar with tri(1-adamantyl)phosphine, PAd3, a ligand that now provides a bridge between trialkylphosphines and the stronger electron-releasing N-heterocyclic carbene ligands (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b03215). Adamantane is an unusual rigid C10 tricyclic alkane. Researchers have made phosphine ligands containing two adamantyl groups before, and adamantane is occasionally used as a bulky substituent for N-heterocyclic carbenes. But installing a third bulky adamantyl group on phosphorus has been a challenge, one that researchers took on with complex approaches. Carrow’s group pulled it off with a simple substitution reaction between commercially available HPAd2 and AdCO2CH3. The researchers demonstrated the utility of the surprisingly stable PAd3 by forming the palladium catalyst shown above and using it for Suzuki-Miyaura couplings, including using chlorinated heteroarenes to make drug intermediates and derivatives, with good results.

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