Curtin–Hammett Paradigm for Stereocontrol in Organocatalysis by Diarylprolinol Ether Catalysts

Detailed mechanistic study of two reactions catalyzed by diarylprolinol ether catalysts, the conjugate addition of aldehydes to nitro-olefins and the α-chlorination of aldehydes, leads to the proposal that the stereochemical outcome in these cases is not determined by the transition state of the step in which the stereogenic center is formed from enamine attack on the electrophile but instead is correlated with the relative stability and reactivity of diastereomeric intermediates downstream in the catalytic cycle. This combination of kinetic and thermodynamic factors illustrates a remarkable Curtin–Hammett scenario that can result in either an enhancement or an erosion of the selectivity that would be predicted by the transition state for enamine attack on the electrophile. Evidence is offered to suggest that this concept may represent a general phenomenon for pyrrolidine-based catalysts lacking an acidic directing proton. Implications for catalyst and reaction design are discussed

Relative Tendency of Carbonyl Compounds To Form Enamines

Equilibria between carbonyl compounds and their enamines (from <i>O</i>-TBDPS-derived prolinol) have been examined by NMR spectroscopy in DMSO-<i>d</i>₆. By comparing the exchange reactions between pairs (enamine A + carbonyl B → carbonyl A + enamine B), a quite general scale of the tendency of carbonyl groups to form enamines has been established. Aldehydes quickly give enamines that are relatively more stable than those of ketones, but there are exceptions to this expected rule; for example, 1,3-dihydroxyacetone acetals or 3,5-dioxacyclohexanones (2-phenyl-1,3-dioxan-5-one and 2,2-dimethyl-1,3-dioxan-5-one) show a greater tendency to afford enamines than many α-substituted aldehydes

Intermediacy of Ni–Ni Species in sp² C–O Bond Cleavage of Aryl Esters: Relevance in Catalytic C–Si Bond Formation

Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C–O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C–O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy₃-catalyzed silylation of aryl pivalates with CuF₂/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C–O bond cleavage and their relevance in C–Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C–O functionalization reactions employing monodentate phosphines

Intermediacy of Ni–Ni Species in sp² C–O Bond Cleavage of Aryl Esters: Relevance in Catalytic C–Si Bond Formation

Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C–O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C–O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy₃-catalyzed silylation of aryl pivalates with CuF₂/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C–O bond cleavage and their relevance in C–Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C–O functionalization reactions employing monodentate phosphines

Intermediacy of Ni–Ni Species in sp² C–O Bond Cleavage of Aryl Esters: Relevance in Catalytic C–Si Bond Formation

Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C–O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C–O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy₃-catalyzed silylation of aryl pivalates with CuF₂/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C–O bond cleavage and their relevance in C–Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C–O functionalization reactions employing monodentate phosphines

Intermediacy of Ni–Ni Species in sp² C–O Bond Cleavage of Aryl Esters: Relevance in Catalytic C–Si Bond Formation

Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C–O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C–O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy₃-catalyzed silylation of aryl pivalates with CuF₂/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C–O bond cleavage and their relevance in C–Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C–O functionalization reactions employing monodentate phosphines