Synthesis of ß-Amino Acid Derivatives via Enantioselective Lewis Base Catalyzed N-Allylation of Halogenated Amides with Morita-Baylis-Hillman Carbonates.

Trifluoro- and trichloroacetamides serving as pronucleophiles undergo enantioselective Lewis base catalyzed N-allylation with Morita-Baylis-Hillman carbonates to produce enantioenriched ß-amino acid derivatives. The reactions proceed as a kinetic resolution to give the allylation products and the remaining carbonates in good yields and high enantioselectivity. The obtained products are amenable to diastereoselective derivatization to produce a library of spiro-isoxazoline lactams.

Synthesis of Sterically Encumbered Alkaline-Earth Metal Amides Applying the In Situ Grignard Reagent Formation.

Magnesium and calcium are too inert to deprotonate amines directly. For the synthesis of bulky amides alternative strategies are required and in the past, N-bound trialkylsilyl groups have been used to ease metalation reactions. The in situ Grignard reagent formation in stirred suspensions of magnesium or calcium with hydryl halide and imine in THF allows the synthesis of a plethora of amides with bulky silyl-free substituents. Ball milling protocols partially favor competitive side reactions such as aza-pinacol coupling reactions. Calcium is the advantageous choice for the in situ Grignard reagent formation and subsequent addition onto the imines yielding bulky calcium bis(amides) whereas the stronger reducing heavier alkaline-earth metals strontium and barium are less selective and hence, the aza-pinacol coupling reaction becomes competitive. Exemplary, the solid-state molecular structures of [(Et2 O)Mg(N(Ph)(CHPh2 )2 ] and [(Et2 O)2 Ca(N(Ph)(CHPh2 )2 ] have been determined.

Neutral, Heteroleptic [Cu(I)(PPh3 )2 (4H-imidazolato)] Complexes: Ligand Exchange Reactivity, Redox Properties, Excited-State Dynamics.

Cu(I) 4H-imidazolate complexes are rare examples of Cu(I) complexes with chelating anionic ligands and are potent photosensitizers with unique absorption and photoredox properties. In this contribution, five novel heteroleptic Cu(I) complexes with monodentate triphenylphosphine co-ligands are investigated. As a consequence of the anionic 4H-imidazolate ligand and in contrast to comparable complexes with neutral ligands, these complexes are more stable than their homoleptic bis(4H-imidazolato)Cu(I) congeners. Here, the ligand exchange reactivity was studied by 31 P-,19 F-, and variable temperature NMR and the ground state structural and electronic properties by X-ray diffraction, absorption spectroscopy, and cyclic voltammetry. The excited-state dynamics were investigated by femto- and nanosecond transient absorption spectroscopy. The observed differences, with respect to chelating bisphosphine bearing congeners, are often due to the increased geometric flexibility of the triphenylphosphines. These observations render the investigated complexes interesting candidates for photo(redox)reactions not accessible with chelating bisphosphine ligands.

Organocatalyzed Ring-Opening Polymerization of (S)-3-Benzylmorpholine-2,5-Dione.

A 3-benzylmorpholine-2,5-dione monomer is synthesized from the natural amino acid l-phenylalanine and characterized by means of nuclear magnetic resonance and infrared spectroscopy, electrospray ionization mass spectrometry, and elemental analysis. Subsequent to preliminary polymerization studies, a well-defined poly(ester amide) homopolymer is synthesized via ring-opening polymerization using a binary catalyst system comprising 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and a 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea (TU) cocatalyst with a feed ratio of M/I/DBU/TU = 100/1/1/10. Kinetic studies reveal high controllability of the dispersities and molar masses up to conversions of almost 80%. Analysis by mass spectrometry hints toward excellent end-group fidelity at these conditions. In consequence, utilization of hydroxyl-functionalized poly(ethylene glycol) and poly(2-ethyl-2-oxazoline) as macroinitiators results in amphiphilic block copolymers. Bulk miscibility of the building blocks is indicated by differential scanning calorimetry investigations. As more and more promising new drugs are based on hydrophobic molecules featuring aromatic moieties, the novel polyesteramides seem highly promising materials to be used as potential drug delivery vehicles.

Synthesis and Structural Characterization of p-Carboranylamidine Derivatives.

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R-N=C=N-R (R = iPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C2H10B10[C(NiPr)2Li(DME)]2 (2) and p-C2H10B10[C(NCy)2Li(THF)2]2 (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me3SiCl) provided the silylated neutral bis(amidine) derivatives p-C2H10B10[C{iPrN(SiMe3)}(=NiPr)]2 (4) and p-C2H10B10[C{CyN(SiMe3)}(=NCy)]2 (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.

Templated Total Synthesis of Cu(I)-Methanobactin OB3b.

Methanobactin OB3b (Mbn-OB3b) is a unique natural product with stunning affinity for copper ions (Ka ≈Cu(I) 1034 ). Here, we report the first total synthesis of Cu(I)-bound methanobactin OB3b featuring as key transformations a cyclodehydration-thioacylation sequence, to generate the conjugated heterocyclic systems, and a copper-templated cyclization, to complete the caged structure of the very sensitive target compound.

In Situ Grignard Metalation Method for the Synthesis of Hauser Bases.

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot procedure to quickly produce multigram amounts of Hauser bases R2 N-MgBr which are valuable and vastly used metalation reagents and novel electrolytes for magnesium batteries. During addition of bromoethane to a suspension of Mg metal and secondary amine at room temperature in an ethereal solvent, a smooth reaction yields R2 N-MgBr under evolution of ethane within a few hours. A Schlenk equilibrium is operative, interconverting the Hauser bases into their solvated homoleptic congeners Mg(NR2 )2 and MgBr2 depending on the solvent. Scope and preconditions are studied, and side reactions limiting the yield have been investigated. DOSY NMR experiments and X-ray crystal structures of characteristic examples clarify aggregation in solution and the solid state.

Noble Metal Complexes of a Bis-Caffeine Containing NHC Ligand.

N-Heterocyclic carbenes (NHCs) have seen more and more use over the years. The go-to systems that are usually considered are derivatives of benzimidazole or imidazole. Caffeine possesses an imidazole unit and was already utilized as a carbene-type ligand; however, its use within a tridentate bis-NHC system has-to the best of our knowledge-not been reported so far. The synthesis of the ligand is straightforward and metal complexes are readily available via silver-salt metathesis. A platinum(II) and a palladium(II) complex were isolated and a crystal structure of the former was examined. For the Pt(II) complex, luminescence is observed in solid state as well as in solution.

A Planar Five-Membered Aromatic Ring Stabilized by Only Two π-Electrons.

Many chemicals known today are partially or fully aromatic, since a ring framework experiences additional stabilization through the delocalization of π-electrons. While aromatic rings with equal numbers of π-electrons and ring atoms such as benzene are particularly stable, those with the minimally required two π-electrons are very rare and yet remain limited to three- and four-membered rings if not stabilized in the coordination sphere of heavy metals. Here we report the facile synthesis of a dipotassium cyclopentagallene, a unique example of a five-membered aromatic ring stabilized by only two π-electrons. Single-crystal X-ray diffraction revealed a planar Ga5 ring with almost equal gallium-gallium bond lengths, which together with computational and spectroscopic data confirm its aromatic character. Our results prove that aromatic stabilization goes far beyond what has previously been assumed as minimum π-electron count in a five-atom ring fragment.

Forty Years after the Discovery of Its Nucleolytic Activity: [Cu(phen)2 ]2+ Shows Unattended DNA Cleavage Activity upon Fluorination.

[Cu(phen)2 ]2+ (phen=1,10-phenanthroline) is the first and still one of the most efficient artificial nucleases. In general, when the phen ligand is modified, the nucleolytic activity of its CuII complex is significantly reduced. This is most likely due to higher steric bulk of such ligands and thus lower affinity to DNA. CuII complexes with phen ligands having fluorinated substituents (F, CF3 , SF5 , SCF3 ) surprisingly showed excellent DNA cleavage activity-in contrast to the unsubstituted [Cu(phen)2 ]2+ -in the absence of the otherwise required classical, bioabundant external reducing agents like thiols or ascorbate. This nucleolytic activity correlates well with the half-wave potentials E1/2 of the complexes. Cancer cell studies show cytotoxic effects of all complexes with fluorinated ligands in the low µm range, whereas they were less toxic towards healthy cells (fibroblasts).

The Supramolecular Structural Chemistry of Pentafluorosulfanyl and Tetrafluorosulfanylene Compounds.

The analysis of crystal structures of SF5 - or SF4 -containing molecules revealed that these groups are often surrounded by hydrogen or other fluorine atoms. Even though fluorine prefers F⋅⋅⋅H over F⋅⋅⋅F contacts, the latter appeared to be important in many compounds. In a significant number of datasets, the closest F⋅⋅⋅F contacts are below 95 % of the van der Waals distance of two F atoms. Moreover, a number of repeating structural motifs formed by contacts between SF5 groups was identified, including different supramolecular dimers and infinite chains. Among SF4 -containing molecules, the study focused on SF4 Cl compounds, including the first solid-state structure analyses of these reactive species. Additionally, electrostatic potential surfaces of a series of Ph-SF5 derivatives were calculated, pointing out the substituent influence on the ability of F⋅⋅⋅X contact formation (X=F or other electronegative atom). Interaction energies were calculated for different dimeric arrangements of Ph-SF5 , which were extracted from experimental crystal structure determinations.

Synthesis and Structure of Alkaline Earth Bis{hydrido-tris(3,5-diisopropyl-pyrazol-1-yl)borate} Complexes: Ae(TpiPr2)2 (Ae = Mg, Ca, Sr, Ba).

The synthesis and structural characterization of Ae(TpiPr2)2 (Ae = Mg, Ca, Sr, Ba; TpiPr2 = hydrido-tris(3,5-diisopropyl-pyrazol-1-yl)borate) are reported. In the crystalline state, the alkaline earth metal centers are six-coordinate, even the small Mg2+ ion, with two κ3-N,N',N''-TpiPr2 ligands, disposed in a bent arrangement (B···Ae···B < 180°). However, contrary to the analogous Ln(TpiPr2)2 (Ln = Sm, Eu, Tm, Yb) compounds, which all exhibit a bent-metallocene structure close to Cs symmetry, the Ae(TpiPr2)2 compounds exhibit a greater structural variation. The smallest Mg(TpiPr2)2 has crystallographically imposed C2 symmetry, requiring both bending and twisting of the two TpiPr2 ligands, while with the similarly sized Ca2+ and Sr2+, the structures are back toward the bent-metallocene Cs symmetry. Despite the structural variations, the B···M···B bending angle follows a linear size-dependence for all divalent metal ions going from Mg2+ to Sm2+, decreasing with increasing metal ion size. The complex of the largest metal ion, Ba2+, forms an almost linear structure, B···Ba···B 167.5°. However, the "linearity" is not due to the compound approaching the linear metallocene-like geometry, but is the result of the pyrazolyl groups significantly tipping toward the metal center, approaching "side-on" coordination. An attempt to rationalize the observed structural variations is made.

Dithiocarbamatocarboxylate (DTCC) Ligands-Building Blocks for Hard/Soft-Heterobimetallic Coordination Polymers.

Four zinc/platinum(II) heterobimetallic coordination polymers with dithiocarboxylate-functionalized carboxylate (DTCC) ligands were prepared by different synthetic approaches and characterized by elemental analyses, IR and NMR (1H, 13C, and 195Pt) spectroscopy, thermal analyses coupled with mass spectrometry (TG/DTA/MS), and single-crystal and powder X-ray diffraction studies. Sequential syntheses via the carboxylic acid substituted platinum dithiocarbamates and zinc acetate revealed crystalline products in good to excellent yields. One-pot preparations from potassium DTCC salts, K2PtCl4, and ZnCl2 led to comparable products, thus demonstrating the high selectivity of the two donor groups toward platinum(II) and zinc. [SSC-N(Me)CH2COO]2- and [SSC-N(CH2COO)(CH2COOH)]2-, DTCC ligands derived from sarcosine and iminodiacetic acid, respectively, were found to form planar zigzag chains with zinc and platinum(II) ions, while the use of the l-proline-derived ligand [SSC-NC4H7COO]2- results in a helical structure. In the case of [SSC-N(CH2COO)2]3-, two-dimensional arrays are formed. All compounds are thermally stable up to 240-400 °C under nitrogen.

In the Mists of a Fungal Metabolite: An Unexpected Reaction of 2,4,5-Trimethoxyphenylglyoxylic Acid.

The reactions of phenylglyoxylic acids during the synthesis and biological evaluation of fungal metabolites led to the discovery of hitherto unknown compounds with a p-quinone methide (p-QM) structure. The formation of these p-QMs using 13C-labelled starting materials revealed a key-step of this reaction being a retro-Friedel-Crafts alkylation.

Making the SF5 Group More Accessible: A Gas-Reagent-Free Approach to Aryl Tetrafluoro-λ6 -sulfanyl Chlorides.

Modern pentafluorosulfanyl (SF5 ) chemistry has an Achilles heel: synthetic accessibility. Herein, we present the first approach to aryl-SF4 Cl compounds (key intermediates in state-of-the-art aryl-SF5 synthesis) that overcomes the reliance on hazardous fluorinating reagents and/or gas reagents (e.g. Cl2 ) by employing easy-to-handle trichloroisocyanuric acid, potassium fluoride, and catalytic amounts of acid. These simple, mild conditions allow direct access to aryl-SF4 Cl intermediates that either have not been or cannot be demonstrated using previous methods. Furthermore, the same approach provides access to aryl-SF3 and aryl-SeF3 compounds, which extend the applications of this chemistry beyond arene SF5 -functionalization, and demonstrate its ability to address a more general oxidative fluorination problem.

A Tunable Trifluoromethyliodonium Reagent.

Four complexes of MCl4 (M=Ti, Zr, Hf) with the hypervalent trifluoromethyl iodine reagent trifluoromethyl-1,3-dihydro-3,3-dimethyl-1,2-benziodoxole (1,=L) are described. With TiCl4 , an I-O bond cleavage occurs, leading to the formation of the trifluoromethyliodonium alcoholate complexes [Ti2 Cl6 (L)4 ]Cl2 (2 a) and Ti2 Cl8 (L) (2 b). Reactions with ZrCl4 and HfCl4 form the complexes ZrCl4 (L)2 (3) and HfCl4 (L)2 (4), respectively, wherein the original I-O bond is retained and elongated compared to that in free 1. Therefore, the reactivity of 1 can be easily and practically fine-tuned by addition of different metal chlorides, following the order ZrCl4 /HfCl4

Synthesis and Crystal Structures of the First Antimony(III) Aziridinides.

The first antimony(III) aziridinyl derivatives are reported. Treatment of anhydrous SbCl3 with N-lithioaziridine Li(Azn) (Azn = NC2H4) afforded the structurally unique heterobimetallic lithium/antimony(III) amide complex [Li3Sb(µ3-Cl)2(µ-Azn)4(THF)2]∞ (1). Homoleptic Sb2(Azn)6 (2) has become available for the first time through an amide group exchange reaction between Sb(NMe2)3 and 3 equiv of aziridine. The low-melting Sb2(Azn)6 exhibits a "weak dimer" structure in the crystal.

A Structurally Characterized Organometallic Plutonium(IV) Complex.

The blood-red plutonocene complex Pu(1,3-COT'')(1,4-COT'') (4; COT''=η8 -bis(trimethylsilyl)cyclooctatetraenyl) has been synthesized by oxidation of the anionic sandwich complex Li[Pu(1,4-COT'')2 ] (3) with anhydrous cobalt(II) chloride. The first crystal structure determination of an organoplutonium(IV) complex revealed an asymmetric sandwich structure for 4 where one COT'' ring is 1,3-substituted while the other retains the original 1,4-substitution pattern. The electronic structure of 4 has been elucidated by a computational study, revealing a probable cause for the unexpected silyl group migration.

Scandium-Mediated Formation of a Bis(tetrahydropentalene).

The reactivity of Li[Sc(COT'')2 ] (1; COT''=1,4-bis(trimethylsilyl)cyclooctatetraenyl) towards CoCl2 is considerably different from that of related lanthanide triple-decker sandwich complexes. In addition to the expected triple-decker complex Sc2 (COT'')3 (2), the complex Sc2 {µ-BTHP}(COT'')2 (3) is formed, which comprises the novel BTHP2- ligand (BTHP2- =bis(3,5-bis(trimethylsilyl)-1,3a,6,6a-tetrahydropentalene-1-yl)diide or bis(2,7-bis(trimethylsilyl)bicyclo[3.3.0]octa-2,7-dien-4-yl)diide, C16 H10 (SiMe3 )42- ). The formation of 3 is likely facilitated by the fact that scandium prefers η8 ,η3  coordination rather than highly symmetric η8 ,η8  coordination, and the η3 -coordinated COT'' ligand in 1 is activated owing to a loss of aromaticity. Acid hydrolysis of 3 leads to air-stable H2 BTHP (4).

Binucleating Jäger-type {(N2O2)2}4- ligands: magnetic and electronic interactions of Fe(II), Ni(II) and Cu(II) across an in-plane TTF-bridge.

The simultaneous presence of different electrophores provides an interesting playground for responsive materials. Herein, we present the incorporation of a twice-reversibly oxidizable tetrathiafulvalene (TTF) unit into a binucleating ligand, bridging two metal centers in a fully conjugated plane. A two-step synthesis scheme gave the D2h symmetric Schiff base-like ligand H4L in moderate yields from which the corresponding copper(II) [Cu2L], nickel(II) [Ni2L], [Ni2L(py)4] and iron(II) complexes [Fe2L(py)4], [Fe2L(dmap)4] and [Fe2L(bpee)2]·1 Tol could be obtained. Characterization was performed through 1H-NMR, IR, UV-vis and 57Fe-Mössbauer spectroscopy, SQUID magnetometry and cyclic voltammetry, supported by density functional theory (DFT) calculations. Single crystal X-ray analysis of [Ni2L(py)4] revealed six-coordinate paramagnetic centers, whereas [Ni2L] underwent gradual coordination induced spin state switching (CISSS) in solution. The magnetic independence of both metal centers is echoed by close-to-ideal Curie-plots of the [Cu2L] system and the gradual spin crossover of all iron(II) compounds. By contrast, cyclic voltammetry measurements in solution indicated oxidation-dependent TTF-metal interactions, as well as metal-metal interactions. The reversible TTF-borne events in H4L and [Ni2L] are overlaid with metal-borne events in the case of [Fe2L(py)4], as is corroborated by an analysis of the frontier orbital landscapes and through diagnostic spectral features upon chemical oxidation.