Lewis versus Brønsted Acid Activation of a Mn(IV) Catalyst for Alkene Oxidation

Abstract

Lewis acid (LA) activation by coordination to metal oxido species has emerged as a new strategy in catalytic oxidations. Despite the many reports of enhancement of performance in oxidation catalysis, direct evidence for LA-catalyst interactions under catalytically relevant conditions is lacking. Here, we show, using the oxidation of alkenes with H2O2 and the catalyst [Mn2(μ-O)3(tmtacn)2](PF6)2 (1), that Lewis acids commonly used to enhance catalytic activity, e.g., Sc(OTf)3, in fact undergo hydrolysis with adventitious water to release a strong Brønsted acid. The formation of Brønsted acids in situ is demonstrated using a combination of resonance Raman, UV/vis absorption spectroscopy, cyclic voltammetry, isotope labeling, and DFT calculations. The involvement of Brønsted acids in LA enhanced systems shown here holds implications for the conclusions reached in regard to the relevance of direct LA-metal oxido interactions under catalytic conditions.

Sc3+-Promoted O–O Bond Cleavage of a (μ-1,2-Peroxo)diiron(III) Species Formed from an Iron(II) Precursor and O2 to Generate a Complex with an FeIV2(μ-O)2 Core

“…These results contrast the recent report that adding 1 equiv of either Sc­(OTf) 3 or HClO 4 to [Fe III (β-BPMCN)­(OOH)] 2+ (where β-BPMCN = cis -β-bis­(pyridyl-2-methyl)- cis -1,2-diaminocyclohexane) generates an oxidant capable of hydroxylating cyclohexane within seconds at −40 °C . Browne and co-workers have also provided strong evidence that the Sc­(OTf) 3 -enhanced olefin epoxidation by the combination of [Mn IV 2 (μ-O) 3 (tmtacn) 2 ] 2+ (where tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) and H 2 O 2 results from Brønsted acid formation upon hydrolysis of Sc­(OTf) 3 by water present in the reaction mixture . In contrast to these two examples, our system is unique in that the added acid (Brønsted or Lewis) has a choice between attacking the oxo bridge versus the peroxide bridge in a diiron model framework, resulting in significantly different outcomes from Brønsted and Lewis acids.…”

“…70 Browne and co-workers have also provided strong evidence that the Sc(OTf) 3 -enhanced olefin epoxidation by the combination of [Mn IV 2 (μ-O) 3 (tmtacn) 2 ] 2+ (where tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) and H 2 O 2 results from Brønsted acid formation upon hydrolysis of Sc(OTf) 3 by water present in the reaction mixture. 71 In contrast to these two examples, our system is unique in that the added acid (Brønsted or Lewis) has a choice between attacking the oxo bridge versus the peroxide bridge in a diiron model framework, resulting in significantly different outcomes from Brønsted and Lewis acids. Most importantly, our investigations into the conversion of 2 to 3 establish that both O atoms of one O 2 end up becoming incorporated into the high-valent diiron For L = 6-Me 3 TPA and BnBQA (black), an Fe III −O−Fe IV species is formed upon protonation of the peroxo intermediate.…”

Lewis Acid Regulated Divergent Catalytic Reaction between Quinone Imine Ketals (QIKs) and 1,3‐Dicarbonyl Compounds: Switchable Access to Multiple Products Including 2‐Aryl‐1,3‐Dicarbonyl Compounds, Indoles, and Benzofurans

“…The reaction was initiated by the protonation of 1 a by TfOH generated in situ via the hydrolysis of M(OTf) n . [13] Next, nucleophilic displacement at the C4 position by trace amounts of water from the solvent produced the hydrolyzed intermediate D. Subsequent C3-site addition and aromatization course delivered compound 7 a.…”

Generation and Electron‐Transfer Reactivity of the Long‐Lived Photoexcited State of a Manganese(IV)‐Oxo‐Scandium Nitrate Complex

“…It was recently reported that Sc(OTf) 3 underwent rapid hydrolysis by residual water in MeCN (ca. 0.5–3 mM H 2 O) to generate HOTf under the conditions used for catalytic oxidations with H 2 O 2 and a Mn(II) complex [Mn 2 ( μ ‐O) 3 (tmtacn) 2 ](PF 6 ) 2 (tmtacn= N , N ′, N ′′‐trimethyl‐1,4,7‐triazacyclononane) [23] . It should be noted that [(Bn‐TPEN)Mn IV (O)] 2+ −(Sc(OTf) 3 ) 2 ( λ max =690 nm) is clearly different from a HOTf‐bound complex, [(Bn‐TPEN)Mn IV (O)] 2+ −(HOTf) 2 ( λ max =560 nm) in TFE/MeCN ( v/v =1 : 1) [24] .…”

“…. [23] It should be noted that [24] In addition, a CSI-MS spectrum of [(Bn-TPEN)Mn IV (O)] 2 + À Sc(NO 3 ) 3 ( Figure 3) clearly indicates the binding of Sc(NO 3 ) 3 rather than HNO 3 .…”