Calcium amido-bisoxazoline complexes in asymmetric hydroamination/cyclisation catalysis

Jumping in the Chiral Pool: Asymmetric Hydroaminations with Early Metals

The application of early-metal-based catalysts featuring natural chiral pool motifs, such as amino acids, terpenes and alkaloids, in hydroamination reactions is discussed and compared to those beyond the chiral pool. In particular, alkaline (Li), alkaline earth (Mg, Ca), rare earth (Y, La, Nd, Sm, Lu), group IV (Ti, Zr, Hf) metal-, and tantalum-based catalytic systems are described, which in recent years improved considerably and have become more practical in their usability. Additional emphasis is directed towards their catalytic performance including yields and regio- as well as stereoselectivity in comparison with the group IV and V transition metals and more widely used rare earth metal-based catalysts.

Copper-catalyzed asymmetric 1,6-conjugate addition of in situ generated para-quinone methides with β-ketoesters

A Cu-catalyzed asymmetric 1,6-conjugate addition of in situ generated para-quinone methides (p-QMs) with β-ketoester has been developed to construct the ketoester skeleton bearing adjacent tertiary-quaternary carbon stereocenter in good yields...

Calcium catalyzed enantioselective intramolecular alkene hydroamination with chiral C2-symmetric bis-amide ligands

The chiral building block (R)-(+)-2,2'-diamino-1,1'-binaphthyl, (R)-BINAM, which is often used as backbone in privileged enantioselective catalysts, was converted to a series of N-substituted proligands R1-H2 (R = CH2tBu, C(H)Ph2, PPh2, dibenzosuberane, 8-quinoline). After double deprotonation with strong Mg or Ca bases, a series of alkaline earth (Ae) metal catalysts R1-Ae·(THF)n was obtained. Crystal structures of these C2-symmetric catalysts have been analyzed by quadrant models which show that the ligands with C(H)Ph2, dibenzosuberane and 8-quinoline substituents should give the best steric discrimination for the enantioselective intramolecular alkene hydroamination (IAH) of the aminoalkenes H2C[double bond, length as m-dash]CHCH2CR'2CH2NH2 (CR'2 = CPh2, CCy or CMe2). The dianionic R12- ligand in R1-Ae·(THF)n functions as reagent that deprotonates the aminoalkene substrate, while the monoanionic (R1-H)- ligand formed in this reaction functions as a chiral spectator ligand that controls the enantioselectivity of the ring closure reaction. Depending on the substituent R in the BINAM ligand, full cyclization of aminoalkenes to chiral pyrrolidine products as fast as 5 minutes was observed. Product analysis furnished enantioselectivities up to 57% ee, which marks the highest enantioselectivity reported for Ca catalyzed IAH. Higher selectivities are impeded by double protonation of the R12- ligand leading to complete loss of chiral information in the catalytically active species.

Synthesis and crystal structures of tetra-meric [2-(4,4-dimethyl-2-oxazolin-2-yl)anilido]sodium and tris-[2-(4,4-dimethyl-2-oxazolin-2-yl)anilido]ytterbium(III)

Reaction of 2-(4,4-dimethyl-2-oxazolin-2-yl)aniline (H2-L1) with one equivalent of Na[N(SiMe3)2] in toluene afforded pale-yellow crystals of tetra-meric poly[bis-[μ3-2-(4,4-dimethyl-2-oxazolin-2-yl)anilinido][μ2-2-(4,4-dimethyl-2-oxa-zolin-2-yl)aniline]tetra-sodium(I)], [Na4(C11H13N2O)4] n or [Na4(H-L1)4] n (2), in excellent yield. Subsequent reaction of [Na4(H-L1)4] n (2) with 1.33 equivalents of anhydrous YbCl3 in a 50:50 mixture of toluene-THF afforded yellow crystals of tris-[2-(4,4-dimethyl-2-oxazolin-2-yl)anilinido]ytterbium(III), [Yb(C11H13N2O)3] or Yb(H-L1)3 (3) in moderate yield. Direct reaction of three equivalents of 2-(4',4'-dimethyl-2'-oxazolin-yl)aniline (H2-L1) with Yb[N(SiMe3)2]3 in toluene resulted in elimination of hexa-methyl-disilazane, HN(SiMe3)2, and produced Yb(H-L1)3 (3) in excellent yield. The structure of 2 consists of tetra-meric Na4(H-L1)4 subunits in which each Na+ cation is bound to two H-L1 bridging bidentate ligands and these subunits are connected into a polymeric chain by two of the four oxazoline O atoms bridging to Na+ cations in the adjacent tetra-mer. This results in two 4-coordinate and two 5-coordinate Na+ cations within each tetra-meric unit. The structure of 3 consists of a distorted octa-hedron where the bite angle of ligand L1 ranges between 74.72 (11) and 77.79 (11) degrees. The oxazoline (and anilide) N atoms occupy meridional sites such that for one ligand an anilide nitro-gen is trans to an oxazoline nitro-gen while for the other two oxazoline N atoms are trans to each other. This results in a significantly longer Yb-N(oxazoline) distance [2.468 (3) Å] for the bond trans to the anilide compared to those for the oxazoline N atoms trans to one another [2.376 (3), 2.390 (3) Å].

Stoichiometric reactions and catalytic dehydrogenations of amine–boranes with calcium aryloxide

A calcium aryloxide complex reacts with amine–boranes to give unprecedented amine–borane coordinated complexes through Ca⋯H interactions, which serve as active species for catalytic dehydrogenation reactions.

Barium complexes with crown-ether-functionalised amidinate and iminoanilide ligands for the hydrophosphination of vinylarenes

Two barium-amide complexes bearing crown-ether-functionalised amidinate and iminoanilide ligands that act as competent precatalysts for the intermolecular hydrophosphination of vinylarenes are presented.

Calcium-catalyzed reactions of element-H bonds

Investigation on organocalcium catalysis is just unfolding during the past decade. Beside conventional Ca salts with strong electron-withdrawing counter anions that may serve as Lewis acid catalysts, many Ca complexes have also been designed recently and found to be good catalysts in activation of element-H (EH) bonds like transition metal catalysts. These findings are interesting and may attract the interest of the chemists. Due to the great abundance, non-toxicity, and biocompatible features of Ca element, Ca-catalyzed reactions can be of great significance from the viewpoint of industry. This short review summarizes the recent advances on Ca-catalyzed reactions of EH bonds. We hope that it may provide a useful guide for interested readers from both the academy and industry.

Bora-amidinate as a cooperative ligand in group 2 metal catalysis

Syntheses and crystal structures of the monomeric bora-amidinate (bam) complexes ^DIPPNBN-Mg·(THF)₃ and ^DIPPNBN-Ca·(THF)₄ are presented; ^DIPPNBN = HB[N(2,6-iPr₂-C₆H₃)]₂. The simplicity of their ¹H NMR spectra in THF-d₈ suggest that their monomeric solid state structures are retained in solution. ^DIPPNBN-Mg·(THF)₃ in C₆D₆, however, is in equilibrium with a dimeric species. Calculations (B3PW91/6-311++G**) reveal a very high localized negative charge (NPA: -1.103) on the N atoms in ^DIPPNBN-Mg. The strongly basic properties of the bam ligand are in agreement with catalytic activity of these complexes in the intramolecular alkene hydroamination. A mechanism is proposed in which the bam ligand is non-innocent and cooperative, playing an active role in substrate deprotonation and product protonation.

MAu2GeS4-Chalcogel (M = Co, Ni): Heterogeneous Intra- and Intermolecular Hydroamination Catalysts

High surface area macroporous chalcogenide aerogels (chalcogels) MAu₂GeS₄ (M = Co, Ni) were prepared from K₂Au₂GeS₄ precursor and Co(OAc)₂ or NiCl₂ by one-pot sol-gel metathesis reactions in aqueous media. The MAu₂GeS₄-chalcogels were screened for catalytic intramolecular hydroamination of 4-pentyn-1-amine substrate at different temperatures. 87% and 58% conversion was achieved at 100 °C, using CoAu₂GeS₄- and NiAu₂GeS₄-chalcogels respectively, and the reaction kinetics follows the first order. It was established that the catalytic performance of the aerogels is associated with the M²⁺ centers present in the structure. Intermolecular hydroamination of aniline with 1-R-4-ethynylbenzene (R = -H, -OCH₃, -Br, -F) was carried out at 100 °C using CoAu₂GeS₄-chalcogel catalyst, due to its promising catalytic performance. The CoAu₂GeS₄-chalcogel regioselectively converted the pair of substrates to respective Markovnikov products, (E)-1-(4-R-phenyl)-N-phenylethan-1-imine, with 38% to 60% conversion.

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

The large alkaline earths (Ae), calcium, strontium and barium, have in the past 15 years yielded a brand new generation of heteroleptic molecular catalysts for the production of fine chemicals. However, the integrity of these complexes is often plagued by ligand redistribution equilibria in solution. This personal account retraces the paths followed in our research group towards the design of stable heteroleptic alkalino-earth complexes, including the use of intramolecular noncovalent Ae···H-Si and Ae···F-C interactions. Their implementation as homogenous precatalysts for reactions such as the intramolecular and intermolecular hydroamination and hydrophosphination of activated alkenes, the hydrophosphonylation of ketones, and the dehydrogenative coupling of amines and hydrosilanes that enable the efficient and controlled formations of CP, CN, or SiN σ-bonds, is presented in a synthetic perspective that highlights their overall outstanding catalytic performance.

Recent developments in alkene hydro-functionalisation promoted by homogeneous catalysts based on earth abundant elements: formation of C-N, C-O and C-P bond

This Perspective article provides an overview of the recent advancements in the field of intra- and inter-molecular C-N, C-O and C-P bond formation by hydroamination, hydroalkoxylation, hydrophosphination, hydrophosphonylation or hydrophosphinylation of unactivated alkenes, including allenes, 1,3-dienes and strained alkenes, promoted by (chiral) homogeneous catalysts based on earth abundant elements of the s and p blocks, the first row transition metals and the rare-earth metals. The relevant literature from 2009 until late 2014 has been covered.

Mono- and Dinuclear Macrocyclic Calcium Complexes as Platforms for Mixed-Metal Complexes and Clusters

Mono- and dinuclear calcium complexes of the Schiff-base macrocycles H4L have been prepared and characterized spectroscopically and crystallographically. In the formation of Ca(THF)2(H2L(1)), Ca2(THF)2(μ-THF)(L(1)), and Ca2(THF)4(L(2)), the ligand framework adopts a bowl-shaped conformation instead of the conventional wedge, Pacman-shaped structure as seen with the anthracenyl-hinged complex Ca2(py)5(L(3)). The mononuclear calcium complex Ca(THF)2(H2L(1)) reacts with various equivalents of LiN(SiMe3)2 to form calcium/alkali metal clusters and dinuclear transition metal complexes when reacted subsequently with transition metal salts. The dinuclear calcium complex Ca2(THF)2(μ-THF)(L(1)), when reacted with various equivalents of NaOH, is shown to act as a platform for the formation of calcium/alkali metal hydroxide clusters, displaying alternate wedged and bowl-shaped conformations.

Cyclopentadienyl-based Mg complexes in the intramolecular hydroamination of aminoalkenes: mechanistic evidence for cationic versus neutral magnesium derivatives

Mechanistic evidence in the catalytic hydroamination of aminoalkenes for a cationic magnesium derivative.

Calcium complexes with imino-phosphinanilido chalcogenide ligands for heterofunctionalisation catalysis

Syntheses, characterisation and utilisation of the calcium complexes supported by imino-phosphinanilido chalcogenide ligands {Ph₂P(E)–N–C₆H₄–CHN(Dipp)} (E = S, Se) as molecular precatalysts for the heterofunctionalisation of styrene are reported.

Hydroamination of diphenylbutadiyne with secondary N-methyl-anilines using the dipotassium tetrakis(2,6-diisopropylanilino)calciate precatalyst

The approved precatalyst [K2Ca{N(H)Dipp}4] was employed to study the hydroamination of diphenylbutadiyne with N-methyl-anilines in tetrahydrofuran at room temperature. The hydroamination occurs regioselectively within a few hours yielding (N-methyl)-(N-aryl)-1,4-diphenylbut-1-ene-3-yne-1-ylamine with phenyl (1a), 4-tolyl (1b) and 4-fluorophenyl groups (1c). In all cases a mixture of E- and Z-isomers is obtained. The second hydroamination step requires drastically extended reaction times and is successful only for the reaction of diphenylbutadiyne with N-methyl-aniline and N-methyl-4-fluoroaniline giving 1,4-diphenyl-1,4-bis(N-methyl-anilino)buta-1,3-diene [R = H (2a) and F (2c)]; a mixture of E,E-, E,Z- and Z,Z-isomers is obtained. The X-ray structures of E-1a, E-1b and E-1c show a slightly shortened N-C bond to the alkene moieties. Due to enhanced steric strain the anilino units of Z,Z-2c and Z,Z-3 turn away from the butadiene unit and consequently, the lone pair at the planar nitrogen atoms slightly interacts with the adjacent aryl groups.

Zn-catalyzed enantio- and diastereoselective formal [4 + 2] cycloaddition involving two electron-deficient partners: asymmetric synthesis of piperidines from 1-azadienes and nitro-alkenes

We report a catalytic asymmetric synthesis of piperidines through [4 + 2] cycloaddition of 1-azadienes and nitro-alkenes. The reaction uses earth abundant Zn as catalyst and is highly diastereo- and regioselective. A novel BOPA ligand (F-BOPA) confers high reactivity and enantioselectivity in the process. The presence of ortho substitution on the arenes adjacent to the bis(oxazolines) was found to be particularly impactful, due to limiting the undesired coordination of 1-azadiene to the Lewis acid and thus allowing the reaction to be carried out at lower temperature. A series of secondary kinetic isotope effect studies using a range of ligands implicates a stepwise mechanism for the transformation, involving an initial Michael-type addition of the imine to the nitro-alkene followed by a cyclization event. The stepwise mechanism obviates the electronic requirement inherent to a concerted mechanism, explaining the successful cycloaddition between two electron-deficient partners.

Early main group metal catalysis: how important is the metal?

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal-alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca(2+) . The role of the metal was evaluated by a study using the metal-free catalysts: [Ph2 N(-) ][Me4 N(+) ] and [Ph3 C(-) ][Me4 N(+) ]. These "naked" amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of ArNCO and RNCNR (R=alkyl) by Ph2 NH. For the intramolecular hydroamination of unactivated CC bonds in H2 CCHCH2 CPh2 CH2 NH2 the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.

Anionic chiral tridentate N-donor pincer ligands in asymmetric catalysis

Tridentate monoanionic ligands known as "pincers" have gained a prominent place as ligands for transition metals and, more recently, for main-group metals and lanthanides. They have been widely employed as ancillary ligands for metal complexes studied inter alia in bond activation steps relevant to catalytic processes. The central formally anionic aryl or heteroaryl unit acts as an "anchor" in the coordination to the metal, which kinetically stabilizes the resulting complexes. Their stability, activity, and reactivity can be tuned by subtle modifications of substitution patterns on the pincer ligand or by modifying the donor atoms. The challenges in pincer ligand design for enantioselective catalysis have been met by their assembly from rigid heterocycles and chiral ligating units in the "wingtip" positions, which generally contain the stereochemical information. The resulting well-defined geometry and shape of the reactive sector of the molecular catalyst favor orientational control of the substrates. On the other hand, the kinetic stability allows reduced catalyst loadings. Recently, a new generation of tridentate anionic N(∧)N(∧)N pincer ligands has been developed which give rise to highly enantioselective transformations. Their applications in asymmetric catalysis have focused primarily on the asymmetric Nozaki-Hiyama-Kishi coupling of aldehydes with halogenated hydrocarbons as well as Lewis acid catalysis involving enantioselective electrophilic attack onto metal-activated β-keto esters, oxindoles, and related substrates. These include highly selective protocols for Friedel-Crafts alkylations with Michael acceptors, electrophilic fluorinations, trifluoromethylations, azidations, and alkylations and subsequent transformations. Increasingly, these stereodirecting ligands are being employed in other types of transformations, including hydrosilylations, cyclopropanations, and epoxidations. The stability and well-defined nature of the molecular catalysts have made them attractive targets for mechanistic studies into a wide range of these transformations, thus providing the type of insight required for a more rational approach to catalyst development. This Account reviews work performed by us and other groups in the field and places it into perspective in relation to general research efforts in enantioselective catalysis.

Reductive Amination of Acetals by Anilines in the Presence of Triethylsilane and Iodine

A mild and efficient method for N-alkylation of aromatic amines with various acetals such as aryl, alkyl, cyclic and acyclic acetals was developed. A number of aromatic amines bearing electron-donating or electron-withdrawing substituents were directly alkylated by acetals with excellent yields. The method uses a catalytic amount of I2 and triethylsilane as the hydride source without a metal present. Monoalkylation with excellent chemoselectivity was observed.

The first calcium-catalysed Nazarov cyclisation

The first calcium-catalysed Nazarov cyclisation is described.

Chiral lanthanide complexes: coordination chemistry, spectroscopy, and catalysis

Luminescent lanthanide complexes bearing amido-bisoxazoline ligands are reported. They were studied using time-resolved luminescence spectroscopy, and were probed for their activity in hydroamination/cyclisation and ring-opening polymerisation catalysis.

Heteroleptic alkyl and amide iminoanilide alkaline earth and divalent rare earth complexes for the catalysis of hydrophosphination and (cyclo)hydroamination reactions

[{N^N}M(X)(thf)n] alkyl (X=CH(SiMe3)2) and amide (X=N(SiMe3)2) complexes of alkaline earths (M=Ca, Sr, Ba) and divalent rare earths (Yb(II) and Eu(II) ) bearing an iminoanilide ligand ({N^N}(-)) are presented. Remarkably, these complexes proved to be kinetically stable in solution. X-ray diffraction studies allowed us to establish size-structure trends. Except for one case of oxidation with [{N^N}Yb(II){N(SiMe3)2}(thf)], all these complexes are stable under the catalytic conditions and constitute effective precatalysts for the cyclohydroamination of terminal aminoalkenes and the intermolecular hydroamination and intermolecular hydrophosphination of activated alkenes. Metals with equal sizes across alkaline earth and rare earth families display almost identical apparent catalytic activity and selectivity. Hydrocarbyl complexes are much better catalyst precursors than their amido analogues. In the case of cyclohydroamination, the apparent activity decreases with metal size: Ca>Sr>Ba, and the kinetic rate law agrees with R(CHA) =k[precatalyst](1)[aminoalkene](1). The intermolecular hydroamination and hydrophosphination of styrene are anti-Markovnikov regiospecific. In both cases, the apparent activity increases with the ionic radius (Ca<Sr<Ba) but the rate laws are different, and obey R(HA) =k[styrene](1)[amine](1)[precatalyst](1) and R(HP) =k[styrene](1)[HPPh2 ](0)[precatalyst](1), respectively. Mechanisms compatible with the rate laws and kinetic isotopic effects are proposed. [{N^N}Ba{N(SiMe3)2}(thf)2] (3) and [{N^N}Ba{CH(SiMe3)2}(thf)2] (10) are the first efficient Ba-based precatalysts for intermolecular hydroamination and hydrophosphination, and display activity values that are above those reported so far. The potential of the precatalysts for C-N and C-P bond formation is detailed and a rare cyclohydroamination-intermolecular hydroamination "domino" sequence is presented.