Rhodium(I)/Chiral Diene Complexes Catalyzed Asymmetric Desymmetrization of Alkynyl-Tethered 2,5-Cyclohexadienones Through an Arylative Cyclization Cascade.

Cis-hydrobenzofurans, cis-hydroindoles, and cis-hydrindanes, privileged structural motifs found in numerous biologically active natural and synthetic compounds, are efficiently prepared by a Rh(I)-catalyzed cascade syn-arylation/1,4-addition protocol. This approach starts with the regioselective syn-arylation of the alkyne tethered to 2,5-hexadienone moieties, using a chiral Rh(I) catalyst generated in situ from a chiral bicyclo[2.2.1]hepatadiene ligand L4f. By forging two new carbon-carbon bonds and introducing two chiral centers, the resulting alkenylrhodium species undergoes desymmetrization via an intramolecular 1,4-addition reaction, delivering annulated products with high yields and enantioselectivities.

A Bis-Cyclopentadienyl Ligand-Supported Di-Iron Trihydride Motif as a Synthon for Access to Heterobimetallic Trinuclear Complexes.

Herein, we report the synthesis of a flexible bis-cyclopentadienyl ligand L (the doubly deprotonated form of H2L (1,3-bis(2,4-di-tert-butylcyclopentadienyldimethylsilyl)benzene)), demonstrating its ability to stabilize a series of di-iron hydrido complexes. Notably, this ligand facilitates the isolation of an unprecedented anionic cyclopentadienyl ligand-supported di-iron trihydride complex, LFe2(µ-H)3Li(THF) (2), functioning as a synthon for the [Fe2(µ-H)3]- core and providing access to heterobimetallic complexes 4-6 with coinage metals.

Isolable Tin(II) Hydrides Featuring Sterically Undemanding Pincer-Type Ligands and Their Dehydrogenative Sn-Sn Bond Forming Reactions to Distannynes Promoted by Lewis Acidic Tri-sec-butylborane.

Unlike isolable tin(II) hydrides supported by bulky ligands reported in the literature, this research describes the synthesis and characterization of thermally stable tin(II) hydrides LPhSnH (1-H) and MeLSnH (2-H) stabilized by sterically undemanding N,N,N-coordinating pincer-type ligands (LPh = 2,5-dipyridyl-3,4-diphenylpyrrolato; MeL = 2,5-bis(6-methylpyridyl)pyrrolato). The results from previous reports reveal that attempts to access tin(II) hydrides containing less-bulky ligands have had limited success, and decomposition to tin(I) distannynes often occurs. The key to the successful isolation of 1-H and 2-H is the identification of the role of Lewis acidic BsBu3, generated upon delivering hydride from commonly used hydride reagents M[BsBu3H] ("selectrides", M = Li or K). This study details compelling experimental evidence and theoretical results of the role played by BsBu3, which catalyzes the dehydrocoupling reactions of 1-H and 2-H to yield tin(I) distannynes LPhSn-SnLPh (12) and MeLSn-SnMeL (22) with the liberation of H2. To avoid the interference of BsBu3, 1-H and 2-H can be isolated in pure forms using pinacolborane as the hydride donor with LPhSnOMe (1-OMe) and MeLSnOMe (2-OMe) as reactants, respectively. DFT calculations and experimental observations indicate that the coordination of the Sn-H bond of 1-H to BsBu3 leaves an electrophilic tin center, rendering the nucleophilic attack by the second equivalent of 1-H forming a Sn-Sn bond.

Mononuclear Manganese(III) Superoxo Complexes: Synthesis, Characterization, and Reactivity.

Metal-superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe-, Co-, and Ni-superoxo complexes have been reported to date, well-defined Mn-superoxo complexes remain rather rare. Herein, we report two mononuclear MnIII-superoxo species, Mn(BDPP)(O2•-) (2, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDPBrP)(O2•-) (2', H2BDPBrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O2 into solutions of their MnII precursors, Mn(BDPP) (1) and Mn(BDPBrP) (1'), at -80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin MnIII center (SMn = 2) antiferromagnetically coupled to a superoxo radical ligand (SOO• = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2' were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form MnIII-hydroperoxo species, Mn(BDPP)(OOH) (5) and Mn(BDPBrP)(OOH) (5'). Complexes 5 and 5' can be independently prepared by the reactions of the isolated MnIII-aqua complexes, [Mn(BDPP)(H2O)]OTf (6) and [Mn(BDPBrP)(H2O)]OTf (6'), with H2O2 in the presence of NEt3. The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5'.

Reversible Laser-Induced Bending of Pseudorotaxane Crystals.

This study investigated the dynamic photoresponse of pseudorotaxane crystals with azobenzene and ferrocenyl groups in the axle component. X-ray crystallography showed pseudorotaxanes with a methylazobenzene group and a dibromophenylene ring in the cyclic component to exhibit twisting of the trans-azobenzene groups at torsion angles of 17° and 38°, respectively. Repeated alternating laser irradiation of the crystals at 360 and 445 nm produced bending of 20-30° in opposite directions, with no evidence of decay. Under 445 nm irradiation, bending took place within 0.3 s. A crystal of nonsubstituted pseudorotaxane showed bending of only 2° under 360 nm irradiation due to multiple π-π interactions between the planar trans-azobenzene groups. The pseudorotaxane crystals have two chromophores, bent rapidly and reversibly on irradiation at rates depending on the molecular structure.

Access to ß2-Amino Acids via Enantioselective 1,4-Arylation of ß-Nitroacrylates Catalyzed by Chiral Rhodium Catalysts.

The highly enantioselective conjugate addition of a variety of arylboronic acids to ß-nitroacrylates is reported to provide optically active α-aryl ß-nitropropionates in up to 70% yields and >99.5% ee's, which are useful building blocks for preparing chiral ß2-amino acids. The applicability of this transformation is demonstrated by converting 3aa into the ß2-amino acid 5 and transforming 3ap to ß-amino ester 7 via reduction and reductive N-alkylation. The latter compound is a precursor for preparing ent-ipatasertib.

Directed Self-Assembly of C4-Symmetric, Oxidovanadate-Centered, Vanadyl(V) Quadruplexes for Ba2+- and Hg2+-Specific Recognition, Transport, and Recovery.

Directed assembly of loosely, Na+-bound, oxidovanadate-centered quartets of C4-symmetry from tailor-made chiral N-salicylidene-vanadyl(V) complexes, for the first time, allows for highly efficient Ba2+- or Hg2+-specific detection (by 51V NMR and VCD), transport (forming a unique helical capsule or a capped square planar complex, respectively), and green recovery from an aqueous phase containing 4 different alkaline earth ions or from at least 10 different metal ions of similar size and charge capacity into the CHCl3 layer without interference from oxa- or oxophilic ions like Mg2+, Ca2+, Cu2+, Cd2+, and Pb2+.

Ligand-Unsupported Cuprophilicity in the Preparation of Dodecacopper(I) Complexes and Raman Studies.

Synthesis and characterization of two dodecacopper(I) extended metal atom chains (EMAC) assembled by two hexadentate bis(pyridylamido)amidinate-supported hexacopper(I) string complexes (monomers) via the ligand-unsupported cuprophilicity are described. In addition to short unsupported Cu-Cu contacts, two hexacopper fragments in these two dodecacopper EMACs show a bent conformation based on X-ray crystallography. Compared with their THF-bound hexacopper(I) monomers and protonated ligands, these ligand-unsupported cuprophilic interactions are shown to be weak by Raman spectroscopy. DFT calculations suggest the ligand-unsupported cuprophilicity originate from weak attractive orbital interactions, and the strength is estimated to be 2.4 kcal mol-1 .

Molecular ReI Cages: Structural and Luminescent Properties.

Versatile building block [{Re(CO)4 }3 (C3 N3 S3 )] (1 a; C3 N3 S3 =cyanurate trianion) reacts with linear dipyridyl ligands [i.e., pyrazine (pz), 4,4'-bipyridine (bpy), 1,2-bis(4-pyridyl)ethylene (bpe), bis(4-pyridyl)acetylene (bpa), and 1,4-bis(pyridyl-4-ylethynyl)benzene (bpb)] and a tripyridyl ligand [1,3,5-tris(4-pyridylethynyl)benzene (tpb)] to afford a series of molecular cages [{Re(CO)3 }6 (L)3 (C3 N3 S3 )2 ] [L=pz (2), bpy (3), bpe (4), bpa (5), bpb (6)] and [{Re(CO)3 }9 (tpb)3 (C3 N3 S3 )3 ] (7) under solvothermal conditions. Various structural dimensions and motifs can be systematically tuned and obtained by using different dipyridyl and tripyridyl ligands in the reactions. The molecular cages of hexanuclear complexes 2-6 containing dipyridyl ligands feature interesting trigonal-prismatic structures with different dimensions. Furthermore, nonanuclear complex 7 has a novel triangular-star structure, and three benzene rings of tpb ligands form a triple-decker arrangement with significant π⋅⋅⋅π interactions having distances of 3.490(1) and 3.528(1) Å. In addition, molecular cages 1-3 and 5-7 exhibit luminescence in the solid state, and their luminescent properties were also studied.

Synthesis of Quaternary-Carbon-Containing ß2,2 -Amino Acids by the RhI -Catalyzed Enantioselective Arylation of α-Substituted ß-Nitroacrylates.

An enantioselective RhI -catalyzed conjugate addition reaction of α-substituted ß-nitroacrylates with various arylboronic acids by using chiral RhI diene catalysts is described for the first time. The addition reaction proceeds under mild conditions in a range of common organic solvents and additives, and it affords the corresponding quaternary-carbon-containing α,α-disubstituted ß-nitropropionate products in up to 63 % yield and 99 % ee. Reaction of either (E)- or (Z)-ß-nitroacrylates provided the same enantiomer of the product, and a range of esters and aryl groups were tolerated. To demonstrate the utility of the method, ethyl (R)-1,1-methyl-1-phenyl-3-nitropropionate, prepared herein, was converted to the non-proteinogenic ß2,2 -amino acid, (R)-2-(aminomethyl)-2-phenylpropanoic acid, and to the ß2,2 -lactam, (R)-3-methyl-3-phenylazetidin-2-one. In addition, a tripeptide, which comprised l-phenylalanine, l-alanine, and ß2,2 -amino acid 7, was also synthesized.

Design and Synthesis of Chiral Diene Ligands for RhI -Catalyzed Enantioselective Arylation of N-DPP-protected Aldimines: Synthesis of the Antifungal Agent Bifonazole.

Herein we describe the design and synthesis of a novel family of bifunctional, chiral bicyclo[2.2.1]heptadiene ligands bearing aryl and secondary amido groups, and demonstrate their usefulness in the RhI -catalyzed enantioselective addition reaction of arylboronic acids to N-diphenylphosphinyl (N-DPP)-protected aldimines. Unlike the analogous RhI -catalysts comprising diene ligands substituted with aryl and carboxylic ester groups, or only with aryl groups, the addition reaction proceeded with high stereoselectivity. The protocol tolerated a range of N-DPP-aldimines and arylboronic acids, producing the desired optically active N-DPP-protected amines with yields between 31-99 % and with ee values up to 91-99 %. The synthetic utility of the method was demonstrated by the conversion of N-DPP-protected amine 3 ae into the antifungal agent, bifonazole (13).

Synthesis and Characterization of an Eclipsed Digermylene as a Building Block to Construct a Cyclic Octagermylene.

The preparation of an unprecedented GeI -GeI bonded digermylene [K2 {Ge2 (µ-κ2 :η2 :η4 -2,6-(2,6-i Pr2 C6 H3 -N)2 -4-CH3 C5 H2 N)2 }] in an eclipsed conformation stabilized by two bridging diamidopyridyl ligands is presented. Although it exhibits an eclipsed conformation, the Ge-Ge bond length is 2.5168(6) Å, which is shorter than those in the trans-bent and gauche digermylenes. In combination with two pendant amido groups, the GeI2 motif is employed as a building block to assemble the first example of octagermylene [Ge4 (µ-κ2 :κ1 -2,6-(2,6-i Pr2 C6 H3 -N)2 -4-CH3 C5 H2 N)2 ]2 showing a cyclic configuration and containing three distinct types of GeI -GeI bonds.

A Structurally Characterized Nonheme Cobalt-Hydroperoxo Complex Derived from Its Superoxo Intermediate via Hydrogen Atom Abstraction.

Bubbling O2 into a THF solution of CoII(BDPP) (1) at -90 °C generates an O2 adduct, Co(BDPP)(O2) (3). The resonance Raman and EPR investigations reveal that 3 contains a low spin cobalt(III) ion bound to a superoxo ligand. Significantly, at -90 °C, 3 can react with 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOH) to form a structurally characterized cobalt(III)-hydroperoxo complex, CoIII(BDPP)(OOH) (4) and TEMPO•. Our findings show that cobalt(III)-superoxo species are capable of performing hydrogen atom abstraction processes. Such a stepwise O2-activating process helps to rationalize cobalt-catalyzed aerobic oxidations and sheds light on the possible mechanism of action for Co-bleomycin.

Encapsulation of Formaldehyde and Hydrogen Cyanide in an Open-Cage Fullerene.

Reaction of C63 NO2 (Ph)2 (Py) (1) with o-phenylenediamine and pyridine produces a mixture of C63 H4 NO2 (Ph)2 (Py)(N2 C6 H4 ) (2) and H2 O@2. Compound 2 is a new open-cage fullerene containing a 20-membered heterocyclic orifice, which has been fully characterized by NMR spectroscopy, high-resolution mass spectrometry, and X-ray crystallography. The elliptical orifice of 2 spans 7.45 Šalong the major axis and 5.62 Šalong the minor axis, which is large enough to trap water and small organic molecules. Thus, heating a mixture of 2 and H2 O@2 with hydrogen cyanide and formaldehyde in chlorobenzene affords HCN@2 and H2 CO@2, respectively. The (1) H NMR spectroscopy reveals substantial upfield shifts for the endohedral species (δ=-1.30 to -11.30 ppm), owing to the strong shielding effect of the fullerene cage.

Reactivity of a Fe(III)-Bound Methoxide Supported with a Tris(thiolato)phosphine Ligand: Activation of C-Cl Bond in CH2Cl2 by Nucleophilic Attack of a Fe(III)-OCH3 Moiety.

Two mononuclear nonheme Fe(III) complexes, [PPh4][Fe(III)(PS3″)(OCH3)] (1) and [PPh4][Fe(III)(PS3″)(Cl)] (2), supported by a tris(benzenethiolato)phosphine derivative PS3″ (PS3″ = P(C6H3-3-Me3Si-2-S)3(3-)) have been synthesized and characterized. The structures resolved from X-ray crystallography show that Fe(III) centers in both complexes adopt distorted trigonal-bipyramidal geometry with a methoxide or a chloride binding in the axial position. The magnetic data for both are consistent with intermediate-spin Fe(III) centers with a C3 symmetry (S = 3/2 ground state). The bound methoxide in 1 is labile and can be replaced by a CH3CN molecule. The forming Fe(III)-CH3CN species can be further reduced by cobaltcene quantitatively to a stable Fe(II)-CH3CN complex, [Fe(PS3″)(CH3CN)](-). One-electron oxidation of 2 by ferrocenium gave a Fe(IV) analogue, [Fe(IV)(PS3″)(Cl)]. Importantly, the Fe(III)-OCH3 moiety in complex 1 acts as a strong nucleophile that activates the C-Cl bond in CH2Cl2, leading to the formation of complex 2 quantitatively. Complex 1 also reacts with other electrophiles, benzyl chloride and benzyl bromide, to generate Fe(III)-X species (X = Cl or Br). The reactions were investigated and monitored by UV-vis-NIR, NMR, and ESI-MS spectroscopies.

A Family of Multiply Bonded Dimolybdenum Boraamidinates with the Formal Mo-Mo Bond Orders of 3, 4, 4.5, and 5.

A boraamidinato ligand [PhB(N-2,6-(i) Pr2 C6 H3 )2 ](2-) was employed to stabilize a new family of multiply bonded dimolybdenum complexes [MoCl(µ-κ(2) -PhB(N-2,6-(i) Pr2 C6 H3 )2 )]2 (4) and [Mo(µ-κ(2) -PhB(N-2,6-(i) Pr2 C6 H3 )2 )]2 (n-) (n=0 (5), 1 (6), 2 (7)), with the respective formal Mo-Mo bond orders of 3, 4, 4.5, and 5. Each metal center in 5-7 is two-coordinate with respect to the ligands. Of particular interest is the quadruply bonded dimolybdenum complex 5, featuring an unprecedented angular conformation. The bent Mo2 N4 core of 5 distorts toward planarity upon reduction. As a result, compound 7 features a planar Mo2 N4 core, while that of 6 is still bent but less significantly than that of 5. Additionally, the Mo-Mo bond lengths of 4-7 systematically decrease as the valency of the central Mo2 units decreases. Complex 7 features the shortest Mo-Mo bond length (2.0106(5) Å) yet reported.

Direct synthesis of chiral 3-arylsuccinimides by rhodium-catalyzed enantioselective conjugate addition of arylboronic acids to maleimides.

Chiral rhodium catalysts comprising 2,5-diaryl- substituted bicyclo[2.2.1]diene ligands L1-L10 were utilized in the enantioselective 1,4-addition reaction of arylboronic acids to N-substituted maleimides. In the presence of 2.5 mol % of Rh(I) /L2, enantioenriched conjugate addition adducts were isolated in 72-99 % yields with 86-98 % ee. This protocol offers a convenient method to access a variety of 3-arylsuccinimides in a highly enantioselective manner. Maleimides with readily cleavable N-protecting groups were tolerated enabling the synthesis of useful synthetic intermediates. Pyrrolidine 4, a biologically active compound, and pyrrolidine 5, an ent-precursor to an HSD-1 inhibitor, were synthesized to demonstrate the utility of this method.

Ambient Stable Trigonal Bipyramidal Copper(III) Complexes Equipped with an Exchangeable Axial Ligand.

A stable trigonal bipyramidal copper(III) complex, [PPN][Cu((TMS)PS3)Cl] (1, wherein PPN represents bis(triphenylphosphine)iminium), was synthesized from CuCl2/PPNCl via intramolecular copper(II) disproportionation. Under ambient conditions, the axial chloride of 1 is exchangeable in solution thus making 1 serve as an intermediate to prepare trigonal bipyramidal copper(III) derivatives, e.g., [PPN][Cu((TMS)PS3)(N3)] (2) and [Cu((TMS)PS3)(DABCO)] (3). Diamagnetic complexes 1-3 were fully characterized by X-ray crystallography, NMR, UV-vis, and Cu K-edge absorption spectroscopy. A series of UV-vis titrations were performed to investigate the relative ligand affinity toward the [Cu((TMS)PS3)] moiety, verifying the 1:1 binding equilibrium between various ligands. Compared to known copper(III) compounds, Cu K-edge absorptions of 1-3 possess lower pre-edge energy and higher shakedown transition energy, which, respectively, attribute to the electron donation from (TMS)PS3(3-) ligand and their trigonal ligand field.

The Mo-Mo Quintuple Bond as a Ligand to Stabilize Transition-Metal Complexes.

Herein, we report the employment of the Mo-Mo quintuple bonded amidinate complex to stabilize Group 10 metal fragments {(Et3P)2M} (M=Pd, Pt) and give rise to the isolation of the unprecedented δ complexes. X-ray analysis unambiguously revealed short contacts between Pd or Pt and two Mo atoms and a slight elongation of the Mo-Mo quintuple bond in these two compounds. Computational studies show donation of the Mo-Mo quintuple-bond δ electrons to an empty σ orbital on Pd or Pt, and back-donation from a filled Pd or Pt dπ orbital into the Mo-Mo δ* level (LUMO), consistent with the Dewar-Chatt-Duncanson model.

Synthesis and characterization of Ni(III)N3S2 complexes as active site models for the oxidized form of nickel superoxide dismutase.

Nickel complexes, [Ni(H2BA(R)TPP)](ClO4)2 (R = Ph for 1 or iPr for 2), supported by a pentadentate ligand H2BA(R)TPP were synthesized and oxidized to form Ni(III) species having a N3S2 coordination environment to mimic the active site of the oxidized form of nickel superoxide dismutase (NiSODox). The Ni(III) species 2(+) exhibited a rhombic signal with g values at 2.15, 2.12 and 2.02 similar to that of NiSODox. DFT calculations revealed that 2(+) has an unpaired electron primarily located in the dz2 orbital of the Ni(III) center, which strongly overlaps with the pz orbital of the axial pyridine nitrogen of H2BA(Pr)TPP.