Chiral-at-Iron Catalyst for Highly Enantioselective and Diastereoselective Hetero-Diels-Alder Reaction.

This study demonstrates that chiral-at-iron complexes, in which all coordinated ligands are achiral and the overall chirality the consequence of a stereogenic iron center, are capable of catalyzing asymmetric transformations with very high enantioselectivities. The catalyst is based on a previously reported design (J. Am. Chem. Soc. 2017, 139, 4322), in which iron(II) is surrounded by two configurationally inert achiral bidentate N-(2-pyridyl)-substituted N-heterocyclic carbenes in a C2 -symmetric fashion and complemented by two labile acetonitriles. By replacing mesityl with more bulky 2,6-diisopropylphenyl substituents at the NHC ligands, the steric hindrance at the catalytic site was increased, thereby providing a markedly improved asymmetric induction. The new chiral-at-iron catalyst was applied to the inverse electron demand hetero-Diels-Alder reaction between ß,γ-unsaturated α-ketoester and enol ethers provide 3,4-dihydro-2H-pyrans in high yields with excellent diastereoselectivities (up to 99 : 1 dr) and excellent enantioselectivities (up to 98 % ee). Other electron rich dienophiles are also suitable as demonstrated for a reaction with a vinyl azide.

Complementing Pyridine-2,6-bis(oxazoline) with Cyclometalated N-Heterocyclic Carbene for Asymmetric Ruthenium Catalysis.

A strategy for expanding the utility of chiral pyridine-2,6-bis(oxazoline) (pybox) ligands for asymmetric transition metal catalysis is introduced by adding a bidentate ligand to modulate the electronic properties and asymmetric induction. Specifically, a ruthenium(II) pybox fragment is combined with a cyclometalated N-heterocyclic carbene (NHC) ligand to generate catalysts for enantioselective transition metal nitrenoid chemistry, including ring contraction to chiral 2H-azirines (up to 97 % ee with 2000 TON) and enantioselective C(sp3 )-H aminations (up to 97 % ee with 50 TON).

Intramolecular C(sp3 )-H Bond Oxygenation by Transition-Metal Acylnitrenoids.

This study demonstrates for the first time that easily accessible transition-metal acylnitrenoids can be used for controlled direct C(sp3 )-H oxygenations. Specifically, a ruthenium catalyst activates N-benzoyloxycarbamates as nitrene precursors towards regioselective intramolecular C-H oxygenations to provide cyclic carbonates, hydroxylated carbamates, or 1,2-diols. The method can be applied to the chemoselective C-H oxygenation of benzylic, allylic, and propargylic C(sp3 )-H bonds. The reaction can be performed in an enantioselective fashion and switched in a catalyst-controlled fashion between C-H oxygenation and C-H amination. This work provides a new reaction mode for the regiocontrolled and stereocontrolled conversion of C(sp3 )-H into C(sp3 )-O bonds.

Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis.

A new class of chiral iron catalysts is introduced that contains exclusively achiral ligands with the overall chirality being the result of a stereogenic iron center. Specifically, iron(II) is cis-coordinated to two N-(2-pyridyl)-substituted N-heterocyclic carbene (PyNHC) ligands in a bidentate fashion in addition to two monodentate acetonitriles, and the dicationic complex is complemented by two hexafluorophosphate ions. Depending on the helical twist of the PyNHC ligands, the metal center adopts either a Λ or Δ absolute configuration. Importantly, the two PyNHC ligands are constitutionally and configurationally inert, while the two acetonitriles are labile and allow asymmetric transition metal catalysis. This is demonstrated with an enantioselective Cannizzaro reaction (96% yield, 88% ee) and an asymmetric Nazarov cyclization (89% yield, >20:1 dr, 83% ee).

Non-C2-Symmetric Chiral-at-Ruthenium Catalyst for Highly Efficient Enantioselective Intramolecular C(sp3)-H Amidation.

A new class of chiral ruthenium catalysts is introduced in which ruthenium is cyclometalated by two 7-methyl-1,7-phenanthrolinium heterocycles, resulting in chelating pyridylidene remote N-heterocyclic carbene ligands (rNHCs). The overall chirality results from a stereogenic metal center featuring either a Λ or Δ absolute configuration. This work features the importance of the relative metal-centered stereochemistry. Only the non-C2-symmetric chiral-at-ruthenium complexes display unprecedented catalytic activity for the intramolecular C(sp3)-H amidation of 1,4,2-dioxazol-5-ones to provide chiral γ-lactams with up to 99:1 er and catalyst loadings down to 0.005 mol % (up to 11 200 TON), while the C2-symmetric diastereomer favors an undesired Curtius-type rearrangement. DFT calculations elucidate the origins of the superior C-H amidation reactivity displayed by the non-C2-symmetric catalysts compared to related C2-symmetric counterparts.

Tris(cyclopentadienyl)lanthanide Complexes as Catalysts for Hydroboration Reaction toward Aldehydes and Ketones.

It was found that homoleptic cyclopentadienyl lanthanide complexes Cp3Ln (Ln = Y (1), Yb (2), Sm (3), Nd (4), La (5), Cp = cyclopentadienyl) can be employed as excellent catalysts for the hydroboration of various aldehydes and ketones toward pinacolborane. These robust lanthanide catalysts exhibited high reactivity with low catalyst loadings (0.01-1 mol %) under mild conditions and good functional group tolerability. These complexes also demonstrated uniquely carbonyl-selective hydroboration in the presence of alkenes and alkynes.

Catalytic addition of amines to carbodiimides by bis(ß-diketiminate)lanthanide(II) complexes and mechanistic studies.

Reduction reactions of bis(ß-diketiminate)lanthanide(III) chlorides formed in situ by reactions of anhydrous LnCl3 with 2 equiv. of sodium salt of the ß-diketiminate ligand in THF with a Na/K alloy afforded a series of bis(ß-diketiminate)lanthanide(II) complexes LnL2(THF)n (L = L(2,6-Me2) = [N(2,6-Me2C6H3)C(Me)]2CH(-), n = 1, Ln = Eu (1); L = L(2,4,6-Me3) = [N(2,4,6-Me3C6H2)C(Me)]2CH(-), n = 1, Ln = Eu (2); L = L(2,6-iPr2) = [N(2,6-(i)Pr2C6H3)C(Me)]2CH(-), n = 0, Ln = Eu (3), Sm (4); L = L(2,6-ipr2)(Ph) = [(2,6-(i)Pr2C6H3)NC(Me)CHC(Me)N(C6H5)](-), n = 0, Ln = Eu (5), Yb (6); L = L(2-Me) = [N(2-MeC6H4)C(Me)]2CH(-), n = 1, Ln = Yb (7)) in high yields. All the complexes, especially the complexes of Sm(II) (4) and Eu(II) (5), were found to be excellent pre-catalysts for catalytic addition of amines to carbodiimides to multi-substituted guanidines with a wide scope of substrates. The activity depends both on the central metals and the ligands with the active sequence of Yb(II) < Eu(II) and Eu(II) < Sm(II) and L(2,6-Me2) < L(2,4,6-Me3) ∼ L(2,6-iPr2) < L(2,6-ipr2)(Ph) for the ligands. The mechanistic study by the isolation of guanidinate species and their reactivity revealed that Eu(II) monoguanidinate complexes Eu(L(2,6-Me2))[(C6H5N)C(NHCy)(NCy)](DME) (8) and Eu(L(2,6-ipr2)Ph)[(C6H5N)C(NHCy)(NCy)](THF)2 (9) should be the key active intermediates for the systems with Eu(II) complexes and a Yb(III) bis(guanidinate) complex Yb(L(2-Me))[(C6H5N)C(NHCy)(NCy)]2 (11) for the system using a Yb(II) complex.