Aluminum(III) complexes containing O,O chelating ligand

The stoichiometric reactions of trimethylaluminum with 2,6‐(MeOCH₂)₂C₆H₃OH (LH) revealed compounds L₃Al ( 1 ) and L₂AlMe ( 2 ). On the other hand reaction of 1 equiv. of LH with trimethylaluminum did not lead to the formation of complex LAlMe₂ ( 3 ), rather 2 together with Me₃Al were observed as a result of a disproportionation of 3 . Compounds 1 and 2 were characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy and in the case of 1 by X‐ray diffraction. Derivative 2 underwent transmetalation with Ph₃SnOH, giving LSnPh₃ ( 4 ) as the result of a migration of ligand L from the aluminum to the tin atom. The identity of 4 was established by elemental analysis, ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopy and ¹H, ¹¹⁹Sn HMBC experiments. The system 2 and B(C₆F₅)₃ in a 1:1 molar ratio was shown to be active in the polymerization of propylene oxide and ε‐caprolactone. Copyright © 2007 John Wiley & Sons, Ltd.     

Engineering a Cu‐MOF Nano‐Catalyst by using Post‐Synthetic Modification for the Preparation of 5‐Substituted 1H‐Tetrazoles

A new nano scale Cu‐MOF has been obtained via post‐synthetic metalation by immersing a Zn‐MOF as a template in DMF solutions of copper(II) salts. The Cu‐MOF serves as recyclable nano‐catalyst for the preparation of 5‐substituted 1H‐tetrazoles via [3 + 2] cycloaddition reaction of various nitriles and sodium azide in a green medium (PEG). The post‐synthetic metalated MOF were characterized by FT‐IR spectroscopy, powder X‐ray diffraction (PXRD), atomic absorption spectroscopy (AAS), and energy dispersive X‐ray spectroscopy (EDX) techniques. The morphology and size of the nano‐catalyst were determined by field emission scanning electron microscopy (FE‐SEM).     

Self-Assembly of Benzimidazole-Derived Tris-NHC Ligands and AgI-Ions to Hexanuclear Organometallic Cages and Their Unusual Transmetalation Chemistry

Multi-ligand self-assembly to attain the Ag^I-N-heterocyclic carbene (NHC)-built hexanuclear organometallic cages of composition [Ag₆( 3 a , b )₄](PF₆)₆ from the reaction of benzimidazole-derived tris(azolium) salts [H₃- 3 a , b ](PF₆)₃ with Ag₂O was achieved. The molecular structures of the cages were established by X-ray diffraction studies along with NMR and MS analyses. The existence of a single assembly in solution was supported by diffusion-ordered spectroscopy (DOSY) ¹H NMR spectra. Further, transmetalation reactions of these self-assembled complexes, [Ag₆( 3 a , b )₄](PF₆)₆, with Cu^I/Au^I-ions provided various coinage metal-NHC complexes having diverse molecular compositions, which included the first example of a hexanuclear Cu^I-dodecacarbene complex, [Cu₆( 3 b )₄](PF₆)₆.     

Modification of bidentate bis(N-heterocyclic imine) ligands for low-valent main group complexes

The synthesis of modified neutral bis-NHI (NHI is N-heterocyclic imine) ligands and their application for the stabilization of tetryliumylidenes are reported. The ligands’ scaffolding consists of either saturated or methylated imidazoline backbones, and the bridge alternated from flexible ethylene to more rigid o-phenylene. Transmetalation reactivity of the cationic Sn^II compounds was tested towards LiAlH₄ and IDipp→SiCl₂ [IDipp is 1,3-bis(2,6-diisopropyl- phenyl)imidazol-2-ylidene] affording the respective aluminium and silicon complexes.     

Transmetalation in the Suzuki–Miyaura Coupling: The Fork in the Trail

The Suzuki–Miyaura coupling is one of the few transition‐metal‐catalyzed C? C bond‐forming reactions that have been used in applications ranging from discovery chemistry to manufacturing processes. Although coupling proceeds through the generic three‐stage ‘oxidative addition, transmetalation, reductive elimination’ sequence, there are a number of features that differentiate the Suzuki–Miyaura process from other transition‐metal‐catalyzed cross‐couplings. Most of these features are centered around, or are a consequence of, activation of the boron reagent for transmetalation through one or both of two distinct pathways. This review focuses on the evidence that has been presented for this ‘fork in the trail′, and the potential to apply such mechanistic insight to the design of reaction conditions.     

Catalyst‐ and Solvent‐Dependent Stereodivergence in the Intramolecular Et2Zn/Pd0‐Promoted Carbonyl Propargylation: Mechanistic Implications

Carbonyl‐tethered propargylic benzoates undergo intramolecular carbonylpropargylation upon treatment with Et₂Zn in the presence of a catalytic amount of Pd⁰ with the formation of 2‐alkynylcyclopentanol products. A ligand/solvent effect on the cis/trans selectivity (referring to the relative positions of alkynyl and OH groups) of ring‐closure has been found. In a non‐coordinating solvent (benzene), increasing the electron‐donating ability of the phosphine ligand (while decreasing its dissociation ability) leads to an increased tendency towards the trans product. On the other hand, the combination of a coordinating solvent (THF) and PPh₃, an easily dissociated phosphine, results in the exclusive formation of cis products. Experimental and computational results are compatible with a divergent behavior of an allenylethylpalladium intermediate that partitions between competitive carbonyl‐addition and transmetalation pathways, each leading to a different diastereoisomer. These results also suggest that the dissociating ability of the phosphine regulates that behavior.     

Stereospecific and Chemoselective Copper‐Catalyzed Deaminative Silylation of Benzylic Ammonium Triflates

A method for the synthesis of benzylsilanes starting from the corresponding ammonium triflates is reported. Silyl boronic esters are employed as silicon pronucleophiles, and the reaction is catalyzed by copper(I) salts. Enantioenriched benzylic ammonium salts react stereospecifically through an S_N2‐type displacement of the ammonium group to afford α‐chiral silanes with inversion of the configuration. A cyclopropyl‐substituted substrate does not undergo ring opening, thus suggesting an ionic reaction mechanism with no benzyl radical intermediate.     

Reversible coordinative chain transfer polymerization of styrene by rare earth borohydrides,chlorides/dialkylmagnesium systems

The ability of various rare earth borohydride and chloride complexes/n‐butylethylmagnesium systems to operate styrene chain transfer polymerization in mild conditions has been assessed. Thirteen precatalysts have been considered: the rare earth trisborohydrides Ln(BH₄)₃(THF)_x (x = 3, Ln = Nd (1), La (2), Sm (3), x = 2, Ln = Y (4), Sc (5)), the rare earth chlorides LnCl₃(THF)_x (x = 3, Ln = Nd (6), La (7), Sm (8), Y (9), x = 2, Ln = Sc (10)), the mixed La(BH₄)₂Cl(THF)_2.6 (11) and the half‐lanthanidocenes Cp*Ln(BH₄)₂(THF)₂ (Ln = Nd (12), La (13)). Six systems were found to be active precatalysts for the polymerization of styrene. 1 , 2 , and 11 led to an efficient transmetalation of the growing polystyrene chain with the simultaneous occurrence of βH elimination, whereas 7 , 12 , and 13 led to catalyzed chain growth behavior. It is noteworthy that the catalyzed chain growth obtained with 12 and 13 occurs with significant stereoselectivity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 802–814, 2010