TY - JOUR
T1 - Mechanistic Study of Cp∗CoIII/RhIII-Catalyzed Directed C-H Functionalization with Diazo Compounds
AU - Qu, Shuanglin
AU - Cramer, Chris
N1 - Publisher Copyright:
© 2016 American Chemical Society.
PY - 2017/1/20
Y1 - 2017/1/20
N2 - Density functional theory calculations have been performed to provide mechanistic insight into a series of Cp∗CoIII- and Cp∗RhIII-catalyzed directed C-H bond functionalizations with diazo-compound substrates. Co-based catalysis proceeds through five steps: C-H bond activation; C-C coupling via a concerted 1,2-aryl transfer; proto-demetalation; nucleophilic addition; and solvent-assisted methanol elimination. C-H bond activation is predicted to be reversible, consistent with deuterium-scrambling experiments. The higher Lewis acidity of Co compared to Rh for two otherwise identical catalysts increases the susceptibility of a coordinated carbonyl group to nucleophilic addition in the former, facilitating the formation of cyclized products not observed for Rh. Methanol elimination is predicted to be the turnover-limiting step for one substrate, and this is facilitated by solvent 2,2,2-trifluoroethanol (TFE) acting as a proton shuttle. Theory suggests that further tuning of acidity may offer opportunities for improving catalysis. We also assess the role of a pyridine group that leads to a different series of final steps in one Rh-based catalytic cycle, thereby enabling access to the otherwise suppressed cyclization product. Our study of an alternative Rh-based system having acetate ligands replaced with MeCN indicates that C-H bond activation is sensitive to those ligands and variation can affect which step is turnover-limiting.
AB - Density functional theory calculations have been performed to provide mechanistic insight into a series of Cp∗CoIII- and Cp∗RhIII-catalyzed directed C-H bond functionalizations with diazo-compound substrates. Co-based catalysis proceeds through five steps: C-H bond activation; C-C coupling via a concerted 1,2-aryl transfer; proto-demetalation; nucleophilic addition; and solvent-assisted methanol elimination. C-H bond activation is predicted to be reversible, consistent with deuterium-scrambling experiments. The higher Lewis acidity of Co compared to Rh for two otherwise identical catalysts increases the susceptibility of a coordinated carbonyl group to nucleophilic addition in the former, facilitating the formation of cyclized products not observed for Rh. Methanol elimination is predicted to be the turnover-limiting step for one substrate, and this is facilitated by solvent 2,2,2-trifluoroethanol (TFE) acting as a proton shuttle. Theory suggests that further tuning of acidity may offer opportunities for improving catalysis. We also assess the role of a pyridine group that leads to a different series of final steps in one Rh-based catalytic cycle, thereby enabling access to the otherwise suppressed cyclization product. Our study of an alternative Rh-based system having acetate ligands replaced with MeCN indicates that C-H bond activation is sensitive to those ligands and variation can affect which step is turnover-limiting.
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U2 - 10.1021/acs.joc.6b02962
DO - 10.1021/acs.joc.6b02962
M3 - Article
C2 - 28005358
AN - SCOPUS:85017476215
SN - 0022-3263
VL - 82
SP - 1195
EP - 1204
JO - Journal of Organic Chemistry
JF - Journal of Organic Chemistry
IS - 2
ER -