Cationic bis(oxazoline) and pyridyl-bis(oxazoline)Cu(II) and Zn(II) Lewis acid catalysts. A comparative study in catalysis of Diels-Alder and aldol reactions

Water, an Essential Element for a ZnII -Catalyzed Asymmetric Quinone Diels-Alder Reaction: Multi-Selective Construction of Highly Functionalized cis-Decalins

A Zn^II complex of a C₂ -chiral bisamidine-type sp² N bidentate ligand (L_R ) possessing two dioxolane rings at both ends catalyzes a highly efficient quinone asymmetric Diels-Alder reaction (qADA) between o-alkoxy-p-benzoquinones and 1-alkoxy-1,3-butadienes to construct highly functionalized chiral cis-decalins, proceeding in up to a >99:1 enantiomer ratio with a high generality in the presence of H₂ O (H₂ O:Zn^II =4-6:1). In the absence of water, little reaction occurs. The loading amount of the chiral ligand can be minimized to 0.02 mol % with a higher Zn/L_R ratio. This first success is ascribed to a supramolecular 3D arrangement of substrates, in which two protons of an "H₂ O-Zn^II " reactive species make a linear hydrogen bond network with a dioxolane oxygen atom and one-point-binding diene; the Zn^II atom captures the electron-accepting two-points-binding quinone fixed on the other dioxolane oxygen atom via an n-π* attractive interaction. The mechanisms has been supported by ¹ H NMR study, kinetics, X-ray crystallographic analyses of the related ZnL_R complexes, and ligand and substrate structure-reactivity-selectivity relationship.

Unraveling and Manipulating the Stereospecific Retro-Aldol Reaction in the Organocatalytic Asymmetric Aldol Reaction of Isatin and Cyclohexanone

An l-pyroglutamic acid-derived bifunctional organocatalyst was designed and applied in an organocatalytic asymmetric direct aldol reaction between isatins and cyclohexanone, in which an erosion of enantiomeric excess of aldol adduct was unexpectedly observed. Through closely monitoring the reaction and performing extensive control experiments, it was determined that the erosion of ee was attributed to a rare stereospecific retro-aldol process. Moreover, effective manipulation of the retro-aldol process by tuning the use of starting materials was ultimately accomplished, leading to evidently upgraded enantioselectivity and functional group tolerance. This study demonstrates the impact of the hidden reaction pathway on the enantioselectivity in asymmetric transformation.

Using chiral ionic liquid additives to enhance asymmetric induction in a Diels–Alder reaction

A bis-oxazoline ligand has been complexed using Cu(ii) and Zn(ii) trifluoromethanesulfonate and a range of chiral ionic liquid (CIL) additives based on natural products were used as a co-catalyst for a Diels–Alder reaction.

Chelated [Zn(cyclam)]2+Lewis acid improves the reactivity of the electrochemical reduction of CO2by Mn catalysts with bulky bipyridine ligands

This communication reports the use of a soluble Lewis acid complex, [Zn(cyclam)]²⁺(cyclam = 1,4,8,11-tetraazacyclotetradecane) as a co-catalyst coupled with Mn(Mesbpy)(CO)₃Br (Mesbpy = 6,6′-dimesityl-2,2′-bipyridine) for the electrochemical reduction of CO₂to CO.

Cascade radical reaction of substrates with a carbon-carbon triple bond as a radical acceptor

The limitation of hydroxamate ester as a chiral Lewis acid coordination moiety was first shown in an intermolecular reaction involving a radical addition and sequential allylation processes. Next, the effect of hydroxamate ester was studied in the cascade addition–cyclization–trapping reaction of substrates with a carbon–carbon triple bond as a radical acceptor. When substrates with a methacryloyl moiety and a carbon–carbon triple bond as two polarity-different radical acceptors were employed, the cascade reaction proceeded effectively. A high level of enantioselectivity was also obtained by a proper combination of chiral Lewis acid and these substrates.

Asymmetric Diels-Alder and Ficini reactions with alkylidene β-ketoesters catalyzed by chiral ruthenium PNNP complexes: mechanistic insight

Hydride abstraction from the β-position of the enolato ligand of the previously reported complex [Ru(3a-H)(PNNP)]PF(6) (5a; 3a-H is the enolate of 2-tert-butoxycarbonylcyclopentanone) with (Ph(3)C)PF(6) gives the dicationic complex [Ru(6a)(PNNP)](2+) (7a) as a single diastereoisomer, which contains the unsaturated β-ketoester 2-tert-butoxycarbonyl-2-cyclopenten-1-one (6a) as a chelating ligand. The methyl analogue 2-methoxycarbonylcyclopentanone (3b) gives [Ru(3b-H)(PNNP)]PF(6) as a mixture of noninterconverting diastereoisomers (ester group of 3b trans to P, 5b; or to N, 5c), which were separated by column chromatography. Hydride abstraction from 5b (or 5c) yields diastereomerically pure [Ru(6b)(PNNP)](2+) (7b or 7c). Complexes 7b and 7c do not interconvert at room temperature in CD(2)Cl(2) and form opposite enantiomers of the Diels-Alder adduct upon reaction with Dane's diene (1 equiv). X-ray studies of 7a, 5b, and 5c give insight into the origin of enantioselection and the sense of asymmetric induction in the previously reported asymmetric Diels-Alder and Ficini cycloaddition reactions with 2,3-disubstituted butadienes and ynamides, respectively. Stoichiometric reactions (substrate coordination, cycloaddition, and product displacement) between [Ru(OEt(2))(2)(PNNP)](2+) (2), 6b (or 6a), and Dane's diene (15, to give estrone derivatives) or N-benzyl-N-(cyclohexylethynyl)-4-methylbenzenesulfonamide (17, to give cyclobutenamides) suggest that product displacement from the catalyst is turnover limiting.

Ruthenium-Catalyzed Asymmetric Epoxidation of Olefins Using H2O2, Part I: Synthesis of New ChiralN,N,N-Tridentate Pybox and Pyboxazine Ligands and Their Ruthenium Complexes

The synthesis of chiral tridentate N,N,N-pyridine-2,6-bisoxazolines 3 (pybox ligands) and N,N,N-pyridine-2,6-bisoxazines 4 (pyboxazine ligands) is described in detail. These novel ligands constitute a useful toolbox for the application in asymmetric catalysis. Compounds 3 and 4 are conveniently prepared by cyclization of enantiomerically pure alpha- or beta-amino alcohols with dimethyl pyridine-2,6-dicarboximidate. The corresponding ruthenium complexes are efficient asymmetric epoxidation catalysts and have been prepared in good yield and fully characterized by spectroscopic means. Four of these ruthenium complexes have been characterized by X-ray crystallography. For the first time the molecular structure of a pyboxazine complex [2,6-bis-[(4S)-4-phenyl-5,6-dihydro-4H-[1,3]oxazinyl]pyridine](pyridine-2,6-dicarboxylate)ruthenium (S)-2 aa, is presented.

Coupling of bulky, electron-deficient partners in aryl amination in the preparation of tridentate bis(oxazoline) ligands for asymmetric catalysis

A new class of tridentate bis(oxazoline) ligands 7, in which an N-phenylaniline unit links the two oxazoline rings, has been prepared. The key step in their synthesis is a Hartwig-Buchwald type Pd-catalyzed aryl amination between the two bulky o-substituted coupling partners, 2-(2'-bromophenyl)oxazolines 8 and 2-(o-aminophenyl)oxazolines 9. By varying the substituent on the coupling partners, a range of 10 ligands has been prepared in good yield. During the synthesis of 2-(o-aminophenyl)oxazolines 9a-d, a number of products of unexpected side reactions were isolated in two of the three steps. Alternatively, the required 2-(o-aminophenyl)oxazolines 9 were obtained by a DAST-promoted cyclodehydration of hydroxyamides 12a-d without formation of any byproducts.

Enantioselective oxidative coupling of the titanium enolate of 3-phenylacetyl-2-oxazolidinone

Enantioselective oxidative coupling of titanium and ytterbium enolates of 1 bound to chiral diol, e.g., TADDOL 6, and bisoxazoline ligands with ferrocenium cation as oxidant affords dimers 2 with moderate to good enantioselectivities. Reaction: see text.

Chiral CO(2)-Synthons via Catalytic Asymmetric Hetero-Diels-Alder Reactions of Ketomalonate and Dienes

A catalytic enantioselective approach for the formation of chiral CO(2)-synthons is presented. The described methodology is based on the reaction of dienes with diethyl ketomalonate using C(2)-symmetric bisoxazolines as the chiral ligands and copper(II) and zinc(II) as the Lewis acids. For cyclic dienes the reaction proceeds in good yield and with up to 93% ee for 1,3-cyclohexadiene, while for cyclopentadiene the reaction also proceeds well at low temperature, but increasing the temperature leads to a retro-Diels-Alder reaction. The reaction has been studied under different conditions and for various dienes, and it has been found that for activated dienes both the hetero-Diels-Alder and Mukaiyama aldol products are isolated. The compound formed by the enantioselective hetero-Diels-Alder reaction of 1,3-cyclohexadiene with ketomalonate has been converted to both the CO(2)-synthon formed in principle from the [2+4] cycloaddition reaction of CO(2) and 1,3-cyclohexadiene, and attractive optically active 1,4-disubstituted cyclohexene diols. The absolute configuration of the hetero-Diels-Alder adduct has been assigned on the basis of the structure of (1S)-camphanic ester of a 1,4-disubstituted cyclohexene diol which gave suitable crystals for X-ray analysis. The reactions have also been analyzed from a theoretical point of view. First, the [2+4] cycloaddition reaction of CO(2) with 1,3-cyclohexadiene has been investigated, and then the relative stability of the two major isomers of bidentate ketomalonate coordinated to a copper(II) or zinc(II) Lewis acid dication was investigated applying density functional theory calculations. Finally, the energetics of the [2+4] hetero-Diels-Alder addition mechanisms typical of the reaction with normal dienes catalyzed by C(2)-symmetric bisoxazoline-zinc(II) complexes using semiempirical calculations are presented.