Karl W. Törnroos

Simple and Highly Z-Selective Ruthenium-Based Olefin Metathesis Catalyst

A one-step substitution of a single chloride anion of the Grubbs-Hoveyda second-generation catalyst with a 2,4,6-triphenylbenzenethiolate ligand resulted in an active olefin metathesis catalyst with remarkable Z selectivity, reaching 96% in metathesis homocoupling of terminal olefins. High turnover numbers (up to 2000 for homocoupling of 1-octene) were obtained along with sustained appreciable Z selectivity (>85%). Apart from the Z selectivity, many properties of the new catalyst, such as robustness toward oxygen and water as well as a tendency to isomerize substrates and react with internal olefin products, resemble those of the parent catalyst.

Syntheses, crystal structures and magnetic properties of copper(II) polynuclear and dinuclear compounds with 2,3-bis(2-pyridyl)pyrazine (dpp) and pseudohalide as ligands

Abstract The preparation, crystal structures and magnetic properties of four heteroleptic copper(II) complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) and azide, cyanate or thiocyanate as ligands are reported, [Cu(dpp)(N3)2]n (1), [Cu(dpp)(NCO)2]n (2), [Cu(dpp)(NCS)2]2 (3) and [Cu(H2O)(dpp)(NCS)2]2·2H2O (4). Compounds 1 and 2 are isomorphous, triclinic, space group P1, and consist of mononuclear building blocks featuring copper atoms with close to square planar coordination geometries. The mononuclear units are, however, associated into chains through weak axial Cu–N bonds formed by end-on asymmetrically bridging azido/cyanato groups and by pyridyl nitrogen atoms. Taking these contacts into account, copper may be described as elongated octahedral. Compound 3 is monoclinic, space group P21/c. The mononuclear building blocks are similar to those in 1 and 2, but in this case the association between these units is such as to form dinuclear molecules through end-to-end bridging thiocyanate with weak axial CuS bonds, yielding a square pyramidal environment of copper. Compound 4 is triclinic, space group P1, and features a mononuclear unit which contains an apically coordinated water molecule. This leads to a significantly weaker intermolecular Cu⋯S contact as compared to that found in 3, but an association into dinuclear units is structurally and magnetically evident. The shortest Cu⋯Cu separations occur across the azido, cyanato or thiocyanato bridges, respectively, which are coordinated equatorially to one unit and axially to the neighbouring unit; 3.568(1) (1), 3.547(1) (2), 5.432(1) (3) and 5.371(1) A (4). Variable temperature magnetic susceptibility measurements on compounds 1, 2 and 4 reveal weak antiferromagnetic coupling across the azido, cyanato and thiocyanato bridges, respectively (J-values of −2.3(1) (1), −1.0(1) (2) and −1.0(1) (4) cm−1).

A Rare‐Earth Metal Variant of the Tebbe Reagent

Tebbe reagent [Cp2Ti{(m-CH2)(m-Cl)Al(CH3)2}] (A ; Cp = cyclopentadienyl) belongs to the most enigmatic organometallic compounds. Its successful synthesis, resulting from the careful investigation of the reaction of [Cp2TiCl2] with two equivalents Al(CH3)3, was triggered by important discoveries in two fundamentally different areas of homogeneous catalysis. Indeed, the initial studies of methane (and methylidene) formation from [Cp2TiCl2]/Al(CH3)3 mixtures were conducted in the context of Ziegler–Natta polymerization catalysis, but the methylene unit was structurally characterized by X-ray crystallography for the first time in tantalum alkylidene complexes, such as [Cp2Ta(CH2)(CH3)], [3] and tungsten methylene compounds, related to proposed catalysts for olefin metathesis. Although catalytically active in olefin metathesis, the Tebbe reagent is currently used for efficient carbonyl methylenation reactions. In his initial studies, Tebbe also commented on the synthesis of the structurally similar all-methyl derivative [Cp2Ti{(m-CH2)(m-CH3)Al(CH3)2}] (B) from the labile [Cp2Ti(CH3)2] and Al(CH3)3, suggesting [Cp2Ti(CH3)2{Al(CH3)3}] (B ) as a stabilized intermediate. Although the structures of the bis(neopentyl) derivative [Cp2Ti{(m-CH2)(m-Cl)Al(CH2C(CH3)3)2}] [7] and a zirconium analogue [Cp2Zr{(m-CHCH2C(CH3)3)(m-Cl)Al(CH2CH(CH3)2)2}] [8] have been reported, there are no X-ray structures of the Tebbe reagent nor of discrete metallacycles of the type [M(m-CH2)(m-R)Al(CH3)2] (R = Cl, CH3). [9, 10] Previous studies from our laboratories on rare-earthmetal(III) tetramethylaluminate complexes [LxLn{Al(CH3)4}y] (y = 1, 2, 3; x + y = 3, L = monovalent ancillary ligand, Ln = lanthanides and Sc, Y, La) as polymerization catalysts 12] led to the isolation of Ln clusters with methylene, 14] methine, and carbide functionalities. We also found that complex [Cp*3Y3(m-Cl)3(m3-Cl)(m3-CH2)(thf)3] (Cp* = C5(CH3)5) displayed Tebbe-like reactivity. [13]

Synthesis and Stability of Homoleptic Metal(III) Tetramethylaluminates

Whereas a number of homoleptic metal(III) tetramethylaluminates M(AlMe(4))(3) of the rare earth metals have proven accessible, the stability of these compounds varies strongly among the metals, with some even escaping preparation altogether. The differences in stability may seem puzzling given that this class of metals usually is considered to be relatively uniform with respect to properties. On the basis of quantum chemically obtained relative energies and atomic and molecular descriptors of homoleptic tris(tetramethylaluminate) and related compounds of rare earth metals, transition metals, p-block metals, and actinides, multivariate modeling has identified the importance of ionic metal-methylaluminate bonding and small steric repulsion between the methylaluminate ligands for obtaining stable homoleptic compounds. Low electronegativity and a sufficiently large ionic radius are thus essential properties for the central metal atom. Whereas scandium and many transition metals are too small and too electronegative for this task, all lanthanides and actinides covered in this study are predicted to give homoleptic compounds stable toward loss of trimethylaluminum, the expected main decomposition reaction. Three of the predicted lanthanide-based compounds Ln(AlMe(4))(3) (Ln = Ce, Tm, Yb) have been prepared and fully characterized in the present work, in addition to Ln(OCH(2)tBu)(3)(AlMe(3))(3) (Ln = Sc, Nd) and [Eu(AlEt(4))(2)](n). At ambient temperature, donor-free hexane solutions of Ln(AlMe(4))(3) of the Ln(3+)/Ln(2+) redox-active metal centers display enhanced reduction to [Ln(AlMe(4))(2)](n) with decreasing negative redox potential, in the order Eu ≫ Yb ≫ Sm. Whereas Eu(AlMe(4))(3) could not be identified, Yb(AlMe(4))(3) turned out to be isolable in low yield. All attempts to prepare the putative Sc(AlMe(4))(3), featuring the smallest rare earth metal center, failed.

Half‐Sandwich Bis(tetramethylaluminate) Complexes of the Rare‐Earth Metals: Synthesis, Structural Chemistry, and Performance in Isoprene Polymerization

The protonolysis reaction of [Ln(AlMe(4))(3)] with various substituted cyclopentadienyl derivatives HCp(R) gives access to a series of half-sandwich complexes [Ln(AlMe(4))(2)(Cp(R))]. Whereas bis(tetramethylaluminate) complexes with [1,3-(Me(3)Si)(2)C(5)H(3)] and [C(5)Me(4)SiMe(3)] ancillary ligands form easily at ambient temperature for the entire Ln(III) cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4-(Me(3)C)(3)C(5)H(3)] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X-ray structure analyses of seven representative complexes of the type [Ln(AlMe(4))(2)(Cp(R))] reveal a similar distinct [AlMe(4)] coordination (one eta(2), one bent eta(2)). Treatment with Me(2)AlCl leads to [AlMe(4)] --> [Cl] exchange and, depending on the Al/Ln ratio and the Cp(R) ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe(4))(mu-Cl)(Cp(R))}(2)] and [{Ln(mu-Cl)(2)(Cp(R))}(n)], respectively, have been identified. Complexes [{Y(AlMe(4))(mu-Cl)(C(5)Me(4)SiMe(3))}(2)] and [{Nd(AlMe(4))(mu-Cl){1,2,4-(Me(3)C)(3)C(5)H(2)}}(2)] have been characterized by X-ray structure analysis. All of the chlorinated half-sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe(4))(2)(Cp(R))] with boron-containing cocatalysts, such as [Ph(3)C][B(C(6)F(5))(4)], [PhNMe(2)H][B(C(6)F(5))(4)], or B(C(6)F(5))(3), produces initiators for the fabrication of trans-1,4-polyisoprene. The choice of rare-earth metal cation size, Cp(R) ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe(4))(2)(C(5)Me(4)SiMe(3))]/B(C(6)F(5))(3) (trans-1,4 content: 95.6 %, M(w)/M(n)=1.26) and [La(AlMe(4))(2)(C(5)Me(5))]/B(C(6)F(5))(3) (trans-1,4 content: 99.5 %, M(w)/M(n)=1.18).

Na2PdH2, a hydride with a novel linear [PdH2] complex

Abstract The ternary hydride Na2PdH2 was synthesized by the reaction of sodium hydride with palladium in a hydrogen atmosphere at 370 °C. The structure was derived from X-ray investigations on powdered samples and on a single crystal as well as from neutron diffraction experiments on the deuterated compound. Na2PdH2 crystallizes in the tetragonal space group I4/mmm and is isotypic with Na2HgO2. The atomic arrangement is characterized by a novel linear [PdH2] complex.

Elusive Trimethyllanthanum: Snapshots of Extensive Methyl Group Degradation in LaAl Heterobimetallic Complexes

Reactions of [La(AlMe4)3] and [Y(AlMe4)3] with PMe3 show that the phosphine can cleave Ln--CH3--Al linkages, separating Me3Al(PMe3). PMe3 (3 mol equiv) reacts with [Y(AlMe4)3] to give [(YMe3)n] contaminated with by-products containing phosphorus and aluminum. The La-based analog, [(LaMe3)n], is not formed selectively from the reaction of [La(AlMe4)3] with PMe3 or Et2O, which rather yields insoluble La/Al heterobimetallic products. Three multi-nuclear La-based clusters were obtained from a reaction of [La(AlMe4)3] with PMe3 (1 equiv) and identified by X-ray structure analyses. Each cluster exhibits extensive methyl group degradation and contains methylene, methine, or carbide moieties. [La4Al8(CH)4(CH2)2(CH3)20(PMe3)] has a [La4(CH)4] cuboid core supported by AlMe3, Me2AlCH2AlMe2, and PMe3 ligands. [La4Al8(C)(CH)2(CH2)2(CH3)22(toluene)] also contains a cuboid core, [La3Al(C)(CH)2(CH2)], which includes one exo cubic lanthanum atom, and is supported by AlMe3, Me3AlCH2AlMe2, (AlMe4)-, and toluene ligands. The lanthanum atoms in [La5Al9(CH)6(CH3)30] are arranged in a trigonal bipyramidal fashion with (CH) functionalities capping each face. The [La5(CH)6]3- core is formally balanced by three AlMe2 + moieties and is additionally supported by six AlMe3 ligands. The unit cell contains two independent La5 clusters, one with pseudo-C3h and the other with pseudo-D3 symmetry, as well as two molecules of the separation co-product Me3Al(PMe3).

Synthesis and characterization of novel Pd(II) complexes with chelating and non-chelating heterocyclic iminocarbene ligands

The imidazolium salts [3-R1-1-{2-Ar-imino)-2-R2-ethyl}imidazolium] chloride (C-N; Ar = 2,6-iPr2C6H3; R1/R2 = Me/Me (a), Me/Ph (b), Ph/Me (c), 2,4,6-Me3C6H2 (d), 2,6-iPr2C6H3 (e)) react with Ag(2)O to give Ag(I) iminocarbene complexes (C-N)AgCl (4a-e) in which the iminocarbene ligand is bonded to Ag via the imidazoline-2-ylidene carbon atom. The solid-state structures of 4b and 4d were determined by X-ray crystallography and revealed the presence of monomeric (carbene)AgCl units with Z and E configurations at the imine C=N bonds, respectively. Carbene transfer to Pd occurs when compounds 4b-e are treated with (COD)PdCl2 to yield bis(carbene) complexes (C-N)2PdCl2 (6b-e) containing two kappa1-C bonded iminocarbene moieties. NMR spectroscopic data indicated a trans coordination geometry at Pd. This conclusion was supported by an X-ray structure determination of 6b which clearly demonstrated the non-chelating nature of the iminocarbene ligand system. EXSY 1H NMR spectroscopy suggests that the non-chelating structures undergo E/Z isomerization at the imine C[double bond, length as m-dash]N double bonds in solution. The preparative results contrast our earlier report that the reaction between 4a and (COD)PdCl2 results in a chelating kappa2-C,N bonded iminocarbene complex (C-N)PdCl2. The coordination mode and dynamic behavior of the iminocarbene ligand systems have been found to be dramatically affected by changes in the substitution pattern of the ligand system. Sterically unencumbered systems (a) favor the formation of kappa2-C,N chelate structures containing one iminocarbene moiety per metal upon coordination at Pd(II); these complexes were demonstrated to engage in reversible, solvent-mediated chelate ring-opening reactions. Sterically encumbered systems (b-e) form non-chelating kappa1-C iminocarbene Pd(II) complexes containing two iminocarbene ligands per metal. Transannular repulsions across the chelate ring are believed to be the origin of these structural differences.

N-{2-(4-Methoxyphenyltelluro)ethyl}morpholine (L1) and bis{2-(N-morpholino)ethyl}telluride (L2): synthesis and complexation with palladium(II) and mercury(II). Crystal structures of trans-[PdCl2(L1)2] and trans-[PdCl2(L2)2]

Abstract The reactions of 4-(2-chloroethyl)morpholine hydrochloride with ArTe− and Te2−, generated in situ (under N2 atmosphere) have resulted in N-{2-(4-methoxyphenyltelluro)ethyl}morpholine (L1) and bis{2-(N-morpholino)ethyl}telluride (L2), respectively, which are first tellurated derivatives of morpholine. 1H- and 13C{1H}-NMR spectra of L1 are as expected but HETCOR experiments are used to assign the overlapping signals of CH2Te and CH2N in 1H-NMR spectrum of L2. The complexes of stoichiometries [PdCl2(L1/L2)2] (1/3) and [HgBr2(L1/L2)]2 (2/4) are synthesized. The NMR (1H and 13C{1H}) spectra of all the four complexes have CH2Te and ArCTe signals deshielded with respect to those of free L1/L2, indicating that the two ligands coordinate through Te only. The trans Pd-complexes 1 and 3 are characterized structurally and their PdTe bond lengths (average) are 2.596 and 2.600 A, respectively. The Pd-Cl bonds in 1 are marginally shorter (average 2.312 A) in comparison to those of 3 (average 2.325 A). The geometry of palladium is square planar. The TeC(aryl) is shorter than TeC(alkyl). The dimeric mercury complexes 2 and 4 appear to be formed through the formation two bromo bridges between Hg atoms.

Synthesis of Stereodefined Piperidines from Aziridines and Their Transformation into Conformationally Constrained Amino Acids, Amino Alcohols and 2,7-Diazabicyclo[3.3.1]nonanes

2-(2-Cyano-2-phenylethyl)aziridines were converted into novel cis- and trans-2-chloromethyl-4-phenylpiperidine-4-carbonitriles via alkylation with 1-bromo-2-chloroethane followed by microwave-assisted 6-exo-tet cyclization and regiospecific ring opening. The latter piperidines were used as eligible substrates for the synthesis of stereodefined 2-chloromethyl-, 2-hydroxymethyl-, and 2-carboxymethyl-4-phenylpiperidine-4-carboxylic acids, 2-hydroxymethyl-4-phenylpiperidine-4-carbonitriles, 3-hydroxy-5-phenylazepane-5-carbonitriles, and 5-phenyl-2,7-diazabicyclo[3.3.1]nonanes.