Effect of Tethered, Axial Thioether Coordination on Rhodium(II)‐Catalyzed Silyl‐Hydrogen Insertion

Abstract

The effect of tethering a thioether to a rhodium(II) paddlewheel complex are evaluated in Si–H insertion reactions. This study provides further evidence for the benefits of incorporating thioethers into the rhodium paddlewheel motif. When using a tethered complex, the in situ carbenoid formation and subsequent Si–H insertion are not impeded, yields are improved by up to 12 % at elevated temperatures, and selectivity for diazo compounds is greater when compared to non‐tethered catalysts. Computational modelling of the complexes is also presented in order to rationalize the results.

Effects of Axial Solvent Coordination to Dirhodium Complexes on the Reactivity and Selectivity in C–H Insertion Reactions: A Computational Study

“…As a result of the communication between these three atoms, dirhodium carbene reactivity is potentially “tunable” by axial ligation opposite to the Rh–C bond, a possibility being actively investigated. For instance, many have designed dirhodium complexes with improved catalytic properties ,− by incorporating ligands with tethered axial donors (or ligands that shield the Rh core from axial coordination by external molecules). However, this direction is still in its infancy compared to that of modifying the electronic and steric properties of the bridging ligands …”

Tuning Rh(II)-Catalyzed Cyclopropanation with Tethered Thioether Ligands

“…As observed in our previous work, the mixed bis-substituted complexes 3a,b exhibited greatly diminished catalytic activity(Table 2, entry 7). 42,43 In this reaction, some carbene dimerization was observed along with a high percentage of unreacted EDA. The dual tether of the bis-complex likely blocks both axial sites, requiring displacement before catalysis is possible and thereby reducing reaction yield.…”

“…Higher overall yields were obtained when the tether length was one methylene unit compared to two methylene units, as evidenced by comparing yields of complexes from ligand 1a to those from ligand 1d. It is worthy to note that while ligands with pendant thioethers proceeded readily with one equivalent of the ligand under microwave conditions, synthesis of mixed ligand oxazolidinate/carboxylate complexes without axial coordination required excess ligand to drive the reaction forward, 43,46 suggesting that the tethered thioether facilitates ligand exchange. We have previously reported the X-ray structure for complex 2, 42 and we were able to obtain suitable crystals for X-ray analysis for complex 6 via slow evaporation of a 1:1 mixture of acetonitrile and toluene (Figure 3).…”

“…The ligand design was inspired by similar scaffolds reported by Bera 40 and Ball 41 , in which tethered axial coordination resulted in interesting results in α-diazocarbonyl mediated cyclopropanation and C-H insertion. Initial reports of our bespoke Rh II complexes proved that they were proficient in cyclopropanation 42 and Si-H insertion reactions 43 with donor/acceptor carbenes (Chart 1). Most importantly, we showed that dimerization could be minimized by incorporating tethered, axially coordinated thioether groups.…”

Melding of Experiment and Theory Illuminates Mechanisms of Metal-Catalyzed Rearrangements: Computational Approaches and Caveats

“…62,70 Preliminary evidence from our work indicates that including explicit solvent at the presumably vacant axial position of dirhodium complexes can modulate reactivity of dirhodium tetracarboxylates, which aligns with a number of experimental studies that show that axial coordination can influence electronic communication between the two Rh atoms in these complexes. [136][137][138][139][140] In this case, however, the question to be addressed with microsolvation was well-defined; it was specific to a particular position where solvent could bind. Any electron donor (including reactants) could, in principle, coordinate the vacant, axial coordination site of dirhodium complexes, which may (or may not) have a non-negligible effect on reaction barriers.…”