Anthony Linden

A polymeric two-dimensional mixed-metal network. Crystal structure and magnetic properties of {[P(Ph)4][MnCr(ox)3]}

Abstract The mixed-metal ferromagnet {[P(Ph) 4 ][MnCr(ox) 3 ]} n , where Ph is phenyl and ox is oxalate, has been prepared and a two-dimensional network structure, extended by Mn(II)-ox-Cr(III) bridges, has been determined from single crystal X-ray data. Crystal data: space group R 3 c , a = b =18.783(3), c =57.283(24) A, α=β=90, γ=120°, Z =24 (C 30 H 20 O 12 PCrMn). The magnetic susceptibility data obey the Curie-Weiss law in the temperature range 260–20 K with a positive Weiss constant of 10.5 K. The temperature dependence of the molar magnetization exhibits a magnetic phase transition at T c =5.9 K. The structure is discussed in relation to the strategy for preparing molecular based ferromagnets and, in addition, it is a solution to the question of the dimensionality of the [MM′(ox) 3 ] n network, which in principle can extend two- or three-dimensionally to the crystal lattice. The optical absorption spectra of the single crystals are assigned to the ‘CrO 6 ’ chromophores. Their polarization patterns reflect the electric dipole selection rules for D 3 symmetry. A strong site selective luminescence from the chromium(III) 2 E states is observed at low temperature and the system may be suitable for studying energy transfer mechanisms.

Asymmetric aza-Henry reaction under phase transfer catalysis: an experimental and theoretical study.

An efficient catalytic asymmetric aza-Henry reaction under phase transfer conditions is presented. The method is based on the reaction of the respective nitroalkane with alpha-amido sulfones effected by CsOH x H2O base in toluene as solvent and in the presence of cinchone-derived ammonium catalysts. This direct aza-Henry reaction presents as interesting features its validity for both nonenolizable and enolizable aldehyde-derived azomethines and the tolerance of nitroalkanes, other than nitromethane, for the production of beta-nitroamines. The synthetic value of the methodology described is demonstrated by providing (a) a direct route for the asymmetric synthesis of differently substituted 1,2-diamines and (b) a new asymmetric synthesis of gamma-amino alpha,beta-unsaturated esters through a catalytic, highly enantioselective formal addition of functionalized alkenyl groups to azomethines. Finally, a preferred TS that nicely fits the observed enantioselectivity has been identified. Most remarkable, an unusual hydrogen bond pattern for the catalyst-nitrocompound-imine complex is predicted, where the catalyst OH group interacts with the NO2 group of the nitrocompound.

Multiethynyl Corannulenes: Synthesis, Structure, and Properties

Syntheses, crystal structures, ab initio density functional theory computations, and photophysical properties of 1,6-di-, 1,2,5,6-tetra-, and 1,3,5,7,9-pentaethynyl-substituted corannulenes (classes 3, 4, and 5, respectively) are reported. Classes 3 and 4 were prepared from the corresponding corannulenyl bromides and terminal alkynes in excellent yields (nine examples, with yields of 57-92%) using the Sonogarshira reaction. Class 5 was prepared from 1,3,5,7,9-pentacholorocorannulene and trimethylalkynylstannanes using a modification of Nolans procedure (8 examples, with yields of 45-93%). The molecular packing in crystals of 1,6-diphenylethynyl-2,5-dimethylcorannulene (3-Ph2) displays a polar columnar structure with all of the molecule bowls oriented in the same direction. Similarly, 1,2,5,6-tetrakis(3,5-dimethylphenylethynyl)corannulene [4-Ar(c)5] and 1,3,5,7,9-pentakis(3,5-dimethylphenylethynyl)corannulene [5-Ar(c)5] form columnar structures, but the bowls are oriented in opposing directions. Additionally, the number of attached alkynyl arms is correlated with an increase in bowl depth of the corrannulene nucleus. Most of the aryl derivatives displayed high-quantum-efficiency solution luminescence and variable emission wavelengths that were dependent on the nature of the substitution.

Unprecedented Selectivity via Electronic Substrate Recognition in the 1,4‐Addition to Cyclic Olefins Using a Chiral Disulfoxide Rhodium Catalyst

From zero to hero? Sulfoxides are generally not considered useful ligand entities in asymmetric metal catalysis. However, a chiral disulfoxide as a chelating ligand in the rhodium-catalyzed 1,4-addition of aryl boronic acids to cyclic, alpha,beta-unsaturated ketones and esters gives impressive catalytic results, thus opening the door to future applications of this new chiral ligand class.

Identification and Characterization of a New Family of Catalytically Highly Active Imidazolin-2-ylidenes

A new class of easily accessible and stable imidazolin-2-ylidenes has been synthesized where the side chains are comprised of substituted naphthyl units. Introduction of the naphthyl groups generates C 2 -symmetric ( rac) and C s- symmetric ( meso) atropisomers, and interconversion between the isomers is studied in detail both experimentally and computationally. Complete characterization of the carbenes includes rare examples of crystallographically characterized saturated NHC structures. Steric properties of the ligands and an investigation of their stability are also presented. In catalysis, the new ligands show versatility comparable to the most widely used NHCs IMes/SIMes or IPr/SIPr. Excellent catalytic results are obtained when either the NHC salts (ring-opening alkylation of epoxides), NHC-modified palladium compounds (C-C and C-N cross-couplings), or NHC-ruthenium complexes (ring-closing metathesis, RCM) are employed. In several cases, this new ligand family provides catalytic systems of higher reactivity than that observed with previously reported NHC compounds.

Matching the Chirality of Monodentate N-heterocyclic Carbene Ligands: A Case Study on Well-Defined Palladium Complexes for the Asymmetric α-Arylation of Amides

N-Heterocyclic carbene ligands derived from C(2)-symmetric diamines with naphthyl side chains are introduced as chiral monodentate ligands, and their palladium complexes (NHC)Pd(cin)Cl are prepared. These compounds exist as a mixture of diastereomers, and the palladium complexes can be successfully separated and their absolute stereochemistry assigned. When used in the asymmetric intramolecular alpha-arylation of amides, oxindoles with quaternary carbon centers can be obtained in high yield and selectivity when correctly matching the chirality of the NHC complexes.

Molecular Spur Gears Comprising Triptycene Rotators and Bibenzimidazole-Based Stators

Dynamic gearing of molecular spur gears, the most common type of mechanical gear, is elucidated. Molecular design and conformational analysis show that derivatives of 4,4-bis(triptycen-9-ylethynyl)bibenzimidazole represent suitable constructs to investigate gearing behavior of collateral triptycene (Tp) groups. To test this design, DFT calculations (B97-D/Def2-TZVP) were employed and the results suggest that these molecules undergo geared rotation preferentially to gear slippage. Synthesis of derivatives was carried out, providing a series of molecular spur gears, including the first desymmetrized spur gear molecules, which were subsequently subjected to stereochemical analysis.

Room-temperature synthesis of tetra-ortho-substituted biaryls by NHC-catalyzed Suzuki-Miyaura couplings

Transition-metal-catalyzed cross couplings have become some of the most powerful and widely used methods to construct C C bonds. Among them, the Suzuki–Miyaura coupling, has emerged as a particularly attractive and practical tool for synthetic organic chemistry. Indeed, over the last decade, several limitations of this methodology have been successfully addressed by using bulky, electron-rich monodentate phosphines or sterically demanding NHC ligands (NHC= N-heterocyclic carbene). One of the few challenges remaining in the Suzuki–Miyaura coupling reaction involves transformations with sterically demanding substrates that lead to tetra-ortho-substituted products. Especially in cases where aryl chlorides are used, the relatively poor nucleophilicity of the arylboron reagents results in diminished catalytic activities. In 2004, Glorius and co-workers showed for the first time that aryl chlorides can indeed be coupled to aryl boronic acids to generate such tetra-orthosubstituted biaryls at elevated temperature (110 8C) by employing a very bulky, yet flexible derivative of their bioxazoline-derived NHC ligands in combination with a Pd metal salt. More recently in 2009 and following the same concept of flexible steric bulk of the NHC ligand, Organ and coworkers used the complex Pd-PEPPSI-IPent as the catalyst for the Suzuki–Miyaura couplings to form bulky tetra-orthosubstituted biaryls at milder conditions (65 8C). Since then, various other ligand systems have been shown to effect similar couplings involving aryl chlorides when appropriate heating is employed. To date, systems that work at room temperature have not been reported for the construction of these important tetra-ortho-substituted biaryl structures by way of the Suzuki–Miyaura coupling. Recently, we have presented a new class of saturated NHC ligands with naphthyl-derived side chains that showed excellent reactivities in a variety of catalytic applications. In related studies, we noticed that a ligand with a cyclooctyl group in position 2 of the naphthalene moieties led to significantly increased reactivity. On the basis of these observations, we now report the application of such NHC ligand systems in the palladium-catalyzed Suzuki–Miyaura couplings to give tetra-ortho substituted biaryls and present conclusive evidence concerning the reasons leading to their superior behavior in these reactions. Reaction of NHC ligands with saturated and unsaturated N-heterocycles incorporating 2or 2,7-cyclooctyl groups on the naphthalene side chains with a Pd ACHTUNGTRENNUNG(cin)Cl dimer (cin =cinnamyl) and appropriate workup gave the four complexes depicted in Table 1 in good yield as single isomers (anti-configured). To explore the effect of these new NHC ligands on biaryl formation in difficult Suzuki–Miyaura couplings, we chose the reaction between 2,4,6-trimethylphenyl chloride and 2,6dimethylphenyl boronic acid (Table 1). Under optimized reaction conditions, the four new catalyst systems were benchmarked against the commercially available, SIPr/IPrmodified congeners (Nolan s catalysts) as well as Organ s Pd-PEPPSI-IPent system, currently the most powerful precatalyst for such transformations. At room temperature, these reference systems resulted in low product yields (Table 1, entries 1–5, GC yields). In entries 4 and 5 in Table 1, we used Organ s previously reported reaction conditions, which deteriorated the reaction outcome. Gratifyingly, all catalysts incorporating the new NHC structures showed higher conversions and yields than the benchmark systems. Among the four substructures tested, anti-C clearly stands out as being particularly effective as it shows both high conversions and yields at room temperature. We then proceeded in evaluating the coupling of a variety of hindered aryl bromides (Table 2) and aryl chlorides (Table 3) employing precatalyst anti-C. As can be seen from the data reported in Table 2, high isolated product yields were normally obtained at room temperature within short reaction times when employing aryl bromides. In entry 2 in Table 2, where the coupling proceeded very slowly at room temperature, slight heating (65 8C) was applied, leading to a [a] L. Wu, E. Drinkel, F. Gaggia, S. Capolicchio, Priv.-Doz. Dr. A. Linden, Prof. Dr. R. Dorta Organisch-chemisches Institut, Universit t Z rich Winterthurerstrasse 190, 8057 Z rich (Switzerland) E-mail : dorta@oci.uzh.ch [b] L. Falivene, Prof. Dr. L. Cavallo Dipartimento di Chimica Universit di Salerno Via Ponte don Melillo, 84084 Fisciano (Italy) [] New Address: School of Biomedical, Biochemical and Chemical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009 (Australia) [**] NHC=N-heterocyclic carbene. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102442.

Impact of NHC ligand conformation and solvent concentration on the ruthenium-catalyzed ring-closing metathesis reaction

Two saturated N-heterocyclic carbene ligands with substituted naphthyl side chains were used for the preparation of Blechert-type ruthenium metathesis precatalysts. The resulting conformers of the complexes were separated and unambiguously assigned by X-ray diffraction studies. All new complexes were compared in terms of activity to the original, SIMes-derived Blechert catalyst and were shown to be superior. A study on the impact of solvent concentration in RCM reactions using the most active of these new catalysts ultimately led to the ring closing of a variety of substrates at very low catalyst loadings.

The twin structure of La2Ti2O7: X-ray and transmission electron microscopy studies

La 2 Ti 2 O 7 , M r =485.613, monoclinic, P2 1 , a=7.812 (2), b=5.5440 (7), c=13.010 (2) A, β=98.66 (1) o , V=557.0 (4) A 3 , Z=4, T=298 K, D x =5.790 Mg m -3 , λ(Mo Kα)=0.71073 A, μ=17.86 mm -1 , F(000)=856. The crystals are twinned. The twin I and twin II intensities are superimposed in reciprocal-lattice layers with h even. The a * and b * axes of the twin components run antiparallel. The angle between a * twinI and a * twinII is 17. 12 o