With the intent to demonstrate that the charge of Z‐type ligands can be used to modulate the electrophilic character and catalytic properties of coordinated transition metals, we are now targeting complexes bearing polycationic antimony‐based Z‐type ligands. Toward this end, the dangling phosphine arm of ((
- NSF-PAR ID:
- 10101109
- Date Published:
- Journal Name:
- Chemical Communications
- Volume:
- 55
- Issue:
- 39
- ISSN:
- 1359-7345
- Page Range / eLocation ID:
- 5587 to 5590
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract o ‐(Ph2P)C6H4)3)SbCl2AuCl (1 ) was oxidized with hydrogen peroxide to afford [((o ‐(Ph2P)C6H4)2(o ‐Ph2PO)C6H4)SbAuCl2]+([2 a ]+) which was readily converted into the dicationic complex [((o ‐(Ph2P)C6H4)2(o ‐Ph2PO)C6H4)SbAuCl]2+([3 ]2+) by treatment with 2 equiv AgNTf2. Both experimental and computational results show that [3 ]2+possess a strong Au→Sb interaction reinforced by the dicationic character of the antimony center. The gold‐bound chloride anion of [3 ]2+is rather inert and necessitates the addition of excess AgNTf2to undergo activation. The activated complex, referred to as [4 ]2+[((o ‐(Ph2P)C6H4)2(o ‐Ph2PO)C6H4)SbAuNTf2]2+readily catalyzes both the polymerization and the hydroamination of styrene. This atypical reactivity underscores the strong σ‐accepting properties of the dicationic antimony ligand and its activating impact on the gold center. -
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Abstract With the intent to demonstrate that the charge of Z‐type ligands can be used to modulate the electrophilic character and catalytic properties of coordinated transition metals, we are now targeting complexes bearing polycationic antimony‐based Z‐type ligands. Toward this end, the dangling phosphine arm of ((
o ‐(Ph2P)C6H4)3)SbCl2AuCl (1 ) was oxidized with hydrogen peroxide to afford [((o ‐(Ph2P)C6H4)2(o ‐Ph2PO)C6H4)SbAuCl2]+([2 a ]+) which was readily converted into the dicationic complex [((o ‐(Ph2P)C6H4)2(o ‐Ph2PO)C6H4)SbAuCl]2+([3 ]2+) by treatment with 2 equiv AgNTf2. Both experimental and computational results show that [3 ]2+possess a strong Au→Sb interaction reinforced by the dicationic character of the antimony center. The gold‐bound chloride anion of [3 ]2+is rather inert and necessitates the addition of excess AgNTf2to undergo activation. The activated complex, referred to as [4 ]2+[((o ‐(Ph2P)C6H4)2(o ‐Ph2PO)C6H4)SbAuNTf2]2+readily catalyzes both the polymerization and the hydroamination of styrene. This atypical reactivity underscores the strong σ‐accepting properties of the dicationic antimony ligand and its activating impact on the gold center. -
Dihydropyrimidine dehydrogenase (DPD) catalyzes the initial step in the catabolism of the pyrimidines uracil and thymine. Crystal structures have revealed an elaborate subunit architecture consisting of two flavin cofactors, apparently linked by four Fe4S4 centers. Analysis of the DPD reaction(s) equilibrium position under anaerobic conditions revealed a reaction that favors dihydropyrimidine formation. Single-turnover analysis shows biphasic kinetics. The serine variant of the candidate general acid, cysteine 671, provided enhanced kinetic resolution for these phases. In the first event, one subunit of the DPD dimer takes up two electrons from NADPH in a reductive activation step. Spectrophotometric deconvolution suggests that thes electrons reside on one of the two flavins. That oxidation of the enzyme by dioxygen can be suppressed by the addition of pyrimidine, is consistent with these electrons residing on the FMN. The second phase involves further oxidation of NADPH and concomitant reduction of the pyrimidine substrate. During this phase no net reduction of DPD cofactors is observed indicating that the entire cofactor set acts as a wire, transmitting electrons from NADPH to the pyrimidine rapidly. This indicates that the availability of the proton from C671 general acid controls the transmittance of electrons from NADPH to the pyrimidine. Acid quench and HPLC product analysis of single-turnover reactions with limiting NADPH confirmed 2:1, NADPH:pyrimidine stoichiometry for the enzyme accounting for successive activation and pyrimidine reduction. These data support an alternating subunit model in which one protomer is activated and turns over before the other subunit can be activated and enter catalysis.more » « less
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Abstract A series of various solvents and additives were tested in enantioselective hydroamination/cyclization reactions of aminoalkenes catalyzed by a binaphtholate yttrium catalyst. The functional group tolerance of the catalyst and the influence on the reaction rate and enantioselectivity was studied. Some weakly coordinating polar solvents, such as Et2O, MTBE, and chlorobenzene led to slightly increased reaction rates compared to the less polar solvent benzene, presumably due to a better stabilization of the polar transition state. Stronger binding solvents and additives, such as THF, DMAP, pyrrolidine,
n ‐propylamine, and 1‐phenylethylamine, decrease the reaction rate and diminish the enantioselectivity of the hydroamination product. Some additives, such as THF, Et2O, MTBE, chloro‐ and bromobenzene, as well as (+)‐sparteine resulted in slightly higher enantioselectivities in the cyclization of the model substrateC ‐(1‐allylcyclohexyl)methylamine, although this observation was not generally true for other aminoalkene substrates. The reaction rates and enantioselectivities were depressed in the presence of (−)‐sparteine using the (R )‐binaphtholate‐ligated catalyst. In case ofC ‐(1‐allylcyclohexyl)methylamine, the enantioselectivity was switched from 76% ee favoring the (S )‐enantiomer of the hydroamination product when using (+)‐sparteine to 22% ee in favor of the (R )‐enantiomer when (−)‐sparteine was used. The rates of cyclization of aminoalkenes and the resulting enantioselectivities significantly depend on substrate concentration with the highest rate (13.6 h−1) and enantioselectivity (68% ee) observed in dilute conditions (0.05 M) compared to a concentrated solution (0.5 M, 5.0 h−1, 35% ee) for 2,2‐dimethylpent‐4‐enylamine. These observations indicate that the reaction mechanism is shifted in favor of a slower, less enantioselective catalytic cycle involving a higher coordinate species when higher substrate concentrations or stronger binding additives are present.magnified image -
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9CC01076A
