Z-Selective ruthenium metathesis catalysts: Comparison of nitrate and nitrite X-type ligands
Graphical abstract
Two new Ru-based metathesis catalysts, 3 and 4, have been synthesized for the purpose of comparing their catalytic properties to those of their cis-selective nitrate analogues, 1 and 2. Although catalysts 3 and 4 exhibited slower initiation rates than 1 and 2, they maintained high cis-selectivity in homodimerization and ring-opening metathesis polymerization reactions. Furthermore, the nitrite catalysts displayed higher cis-selectivity than 2 for ring-opening metathesis polymerizations, and 4 delivered higher yields of polymer.
Introduction
With increasing control of stereo and chemoselectivity, transition metal-catalyzed olefin metathesis is rapidly becoming ubiquitous and a preferred method for constructing carbon–carbon double bonds [1]. This process has gained widespread applicability in a variety of fields including organic synthesis, biochemistry, and materials science [2]. Transition metal catalysts that could selectively produce the kinetically favored cis-products remained elusive until the discovery of group VI-based systems by Schrock and Hoveyda [3]. cis-Selective Ru-based metathesis catalysts were developed soon thereafter, all containing a N-heterocyclic carbene (NHC) ligand [4]. In 2011, we reported that cis-selectivity could be achieved with a ruthenium catalyst where the N-adamantyl substituent of an NHC has undergone C–H activation at Ru to impose unique geometrical constraints [5]. During olefin metathesis, side-bound ruthenacycles are formed where the N-aryl NHC substituent dictates a cis-conformation of metallacycle substituents, resulting in production of the corresponding Z-olefin [6], [7]. Later N-adamantyl analogues with a bidentate nitrate ligand (catalysts 1 and 2) displayed greater activity and stability [8], [9]. In 2013, the Jensen and Hoveyda groups independently reported cis-selective Ru-based metathesis catalysts with [H2IMes2] (H2I = imidazolidinylidene, Mes = mesityl) NHC ligands, but different X-type ligands [10], [11].
In order to further probe the effect and role of the nitrate ligand in catalyst activity, stability, and selectivity of the Grubbs’ systems, we herein report the synthesis of the nitrite analogues of these catalysts, 3 and 4, and their reactivities for homodimerization and ring-opening metathesis polymerization reactions [12] (see Fig. 1).
Section snippets
Materials and methods
Unless otherwise stated, solvents and reagents were of reagent quality, obtained from commercial sources and used without further purification. Reactions involving catalysts 1–4 were carried out in a nitrogen-filled glovebox. Substrates for homodimerization were degassed by sparging with Ar(g) and liquids were filtered through a short plug of basic alumina prior to use. THF was purified by passage through solvent purification columns and degassed prior to use.
Preparation of catalyst 3
In a N2-filled glovebox, reaction
Synthesis
Previous studies demonstrated the nitrato X-type ligand on catalysts 1 and 2 exchanges to form the corresponding iodo complexes 5 and 6 (Scheme 1) upon exposure to excess NaI in THF [5a]. It was found that these iodo complexes could be readily converted to the corresponding nitrito complexes, 3 and 4, using excess AgNO2 in benzene. Trituration with pentane/ether afforded the pure catalysts. The 1H NMR spectra of 3 and 4 were recorded in C6D6. Both complexes showed a characteristic singlet at
Conclusion
In summary, salt metathesis of I-ruthenium complexes 5 and 6 with AgNO2 results in stable, chelated Z-selective ruthenium olefin metathesis catalysts 3 and 4. The nitrite-containing catalysts are slower initiating than the nitrito analogues and, therefore, have lower conversions at early time points in homodimerization and ring-opening metathesis polymerization. Both types of X-ligands result in exceptional Z-selectivity. This high Z-selectivity is retained at longer reaction times where, in
Acknowledgments
This work was financially supported by the NIH (R01GM031332) and the NSF (CHE-1212767). NMR spectra were obtained on instruments funded by the NIH (RR027690). Materia is gratefully acknowledged for donation of metathesis catalysts 1 and 2. We thank Naseem Torian for high-resolution mass spectrometry studies. The authors would like to thank Brendan Quigley for helpful discussions and assistance with NMR experiments and Dr. B. Keith Keitz for initial experiments.
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