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Published June 8, 2016 | public
Journal Article

In Situ Catalyst Modification in Atom Transfer Radical Reactions with Ruthenium Benzylidene Complexes


Ruthenium benzylidene complexes are well known as olefin metathesis catalysts. Several reports have demonstrated the ability of these catalysts to also facilitate atom transfer radical (ATR) reactions, such as atom transfer radical addition (ATRA) and atom transfer radical polymerization (ATRP). However, while the mechanism of olefin metathesis with ruthenium benzylidenes has been well-studied, the mechanism by which ruthenium benzylidenes promote ATR reactions remains unknown. To probe this question, we have analyzed seven different ruthenium benzylidene complexes for ATR reactivity. Kinetic studies by 1H NMR revealed that ruthenium benzylidene complexes are rapidly converted into new ATRA-active, metathesis-inactive species under typical ATRA conditions. When ruthenium benzylidene complexes were activated prior to substrate addition, the resulting activated species exhibited enhanced kinetic reactivity in ATRA with no significant difference in overall product yield compared to the original complexes. Even at low temperature, where the original intact complexes did not catalyze the reaction, pre-activated catalysts successfully reacted. Only the ruthenium benzylidene complexes that could be rapidly transformed into ATRA-active species could successfully catalyze ATRP, whereas other complexes preferred redox-initiated free radical polymerization. Kinetic measurements along with additional mechanistic and computational studies show that a metathesis-inactive ruthenium species, generated in situ from the ruthenium benzylidene complexes is the active catalyst in ATR reactions. Based on data from ^1H, ^(13)C, and ^(31)P NMR spectroscopy, and X-ray crystallography, we suspect that this ATRA-active species is a Ru_xCl_y(Pcy_3)_z complex.

Additional Information

© 2016 American Chemical Society. Publication Date (Web): May 17, 2016. The research described was financially supported by the ONR (Award N00014-12-1-0596) and the NIH NIGMS (Award F32GM108145; postdoctoral fellowship to K.M.E.). The authors thank Materia, Inc. for generous donation of catalysts 1, 2, 3, 6, and 7. The authors also thank Dr. Michael Haibach (Grubbs group, Caltech) and Dr. Peter Dornan (Grubbs group, Caltech) for the helpful discussions. Calculations were performed using the NSF funded (OCI-1053575) Extreme Science and Engineering Discovery Environment (XSEDE) and the UCLA Hoffman2 Cluster.

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