Mechanistic investigation of sustainable heme-inspired biocatalytic synthesis of cyclopropanes for challenging substrates

Abstract

Engineered heme proteins exhibit excellent sustainable catalytic carbene transfer reactivities toward olefins for value-added cyclopropanes. However, unactivated and electron-deficient olefins remain challenging in such reactions. To help design efficient heme-inspired biocatalysts for these difficult situations, a systematic quantum chemical mechanistic study was performed to investigate effects of olefin substituents, non-native amino acid axial ligands, and natural and non-natural macrocycles with the widely used ethyl diazoacetate. Results show that electron-deficient substrate ethyl acrylate has a much higher barrier than the electron-rich styrene. For styrene, the predicted barrier trend is consistent with experimentally used heme analogue cofactors, which can significantly reduce barriers. For ethyl acrylate, while the best non-native axial ligand only marginally improves the reactivity versus the native histidine model, a couple of computationally studied macrocycles can dramatically reduce barriers to the level comparable to styrene. These results will facilitate the development of better biocatalysts in this area.

Fig. 1. Fe^II-macrocycle catalyzed cyclopropanations of olefins in reactions 1–13.

The reaction features a concerted mechanism, in which X represents the substrate substituent, L represents the axial ligand, and the blue oval represents the macrocycle.

Fig. 2. Correlation between current and higher-level free energy barriers of reactions 1–13.

The trendline (in red) shows a strong correlation between the barriers obtained from the two different levels of calculations.

Fig. 3. Free energy diagram of cyclopropanation pathways of different olefins in reactions 1–3.

Key geometric parameters at transition state (in black), changes from reactants to transition state (in blue), atomic charge changes to transition state (in black), and charge transfers (in blue) as indicated by arrows and numbers in parentheses for reaction 2 (X = (CH₂)₅CH₃). Reaction barriers are shown around the TS energy levels. Atom color scheme: Fe, black; O, red; C, cyan; H, gray; N, blue.

Fig. 4. Key geometric parameters at transition state (in black) and changes from reactants to transition state (in blue), and atomic charge changes from reactants to transition state (in black) and charge transfers (in blue).

a Reaction 3; b Reaction 4; c Reaction 5; d Reaction 6. Atom color scheme: Fe, black; O, red; C, cyan; H, gray; N, blue; F, purple; S, yellow.

Fig. 5. Free energy diagram of the cyclopropanation pathways of all eight macrocycles investigated in this study.

Dashed lines throughout represent reactions of olefin 1 (X=Ph), and solid lines throughout represent reactions of olefin 3 (X = CO₂Et) with respective structures and energy barriers.

Fig. 6. Key geometric parameters at transition state (in black) and changes from reactants to transition state (in blue).

a Reaction 7; b Reaction 8; c Reaction 9; d Reaction 10; e Reaction 11; f Reaction 12; g Reaction 13. Atom color scheme: Fe, black; O, red; C, cyan; H, gray; N, blue.

Fig. 7. Plot of ΔG^‡ vs Δ|Q_C1| of data from this study and previous work (shown as black blocks).

Datapoints are colored based on their respective features and reaction numbers in the text.