Catalysis of aldehyde and imine silylcyanation by platinum and palladium NCN-pincer complexes

Higher fluorescence in platinum(iv) orthometallated complexes of perylene imine compared with their platinum(ii) or palladium(ii) analogues

The reaction of 3-perylenylmethylen-4'-ethylaniline () with [Pt2Me4(μ-SMe2)2] (and subsequent addition of PPh3) or with [Pt2(η(3)-C4H7)2(μ-Cl)2] produced cyclometallated Pt(II) complexes [Pt(C^N)Me(PPh3)] () and, respectively, [Pt2(C^N)2(μ-Cl)2] () (HC^N = 3-C20H11CH[double bond, length as m-dash]NC6H4-p-C2H5), with Pt bound to the ortho site of the perylenyl fragment. From the mononuclear complexes [Pt(C^N)L2] (L2 = acac (); S2COMe (); S2CNEt2 () are easily formed. Oxidative addition of methyl iodide to the square-planar Pt(II) complexes , , and gave the corresponding cyclometallated Pt(IV) compounds [Pt(C^N)L2MeI] , and . The X-ray structures of , , and show that the perylenyl fragment remains essentially flat in and and slightly twisted in . Comparison of the optical properties of these Pt(II) complexes with those reported for similar Pd(II) derivatives reveals that the change of metal exerts a notable influence on the UV-vis spectra. In solution at room temperature, all the Pt complexes exhibit fluorescence associated with the perylene fragment with low emission quantum yields for the Pt(II) complexes (<1%) and remarkably higher emission values for the Pt(IV) complexes: up to 29%, with emission lifetimes of 1-5 ns. Time-dependent density functional theory (TD-DFT) calculations were performed on the perylene imine and on representative complexes [M(C^N)(acac)] (M = Pd, Pt) and [Pt(C^N)(acac)MeI] to analyse the absorption spectra. These calculations support a perylene-dominated intraligand π-π*emissive state based on the HOMO and LUMO orbitals of the perylene chromophore, and a ligand-to-ligand charge-transfer (more intense for the Pt(II) complex) that explains the observed influence of the metal on the absorption properties.

Kinetics and mechanism of the racemic addition of trimethylsilyl cyanide to aldehydes catalysed by Lewis bases

The mechanism by which four Lewis bases, triethylamine, tetrabutylammonium thiocyanate, tetrabutylammonium azide and tetrabutylammonium cyanide, catalyse the addition of trimethylsilyl cyanide to aldehydes is studied by a combination of kinetic and spectroscopic methods. The reactions can exhibit first or second order kinetics corresponding to three different reaction mechanisms. Spectroscopic evidence for the formation of hypervalent silicon species is obtained for reaction between all of the tetrabutylammonium salts and trimethylsilyl cyanide. The reactions are accelerated by the presence of water in the reaction mixture, an effect which is due to a change in the reaction mechanism from Lewis to Brønsted base catalysis. Tetrabutylammonium thiocyanate is shown to be an excellent catalyst for the synthesis of cyanohydrin trimethylsilyl ethers on a preparative scale.

Asymmetric Strecker synthesis of α-arylglycines

A practically simple three-component Strecker reaction for the asymmetric synthesis of enantiopure α-arylglycines has been developed. Addition of a range of aryl-aldehydes to a solution of sodium cyanide and (S)-1-(4-methoxyphenyl)ethylamine affords highly crystalline (S,S)-α-aminonitriles that are easily obtained in diastereomerically pure form. Heating the resultant (S,S)-α-aminonitriles in 6 M aqueous HCl at reflux resulted in cleavage of their chiral auxiliary fragments and concomitant hydrolysis of their nitrile groups to afford enantiopure (S)-α-arylglycines. The enantiopurities of these (S)-α-arylglycines were determined via derivatization of their corresponding methyl esters with 2-formylphenylboronic acid and (S)-BINOL, followed by (1)H NMR spectroscopic analysis of the resultant mixtures of diastereomeric iminoboronate esters.

Supramolecular Catalysis of Strecker Reaction in Water under Neutral Conditions in the Presence of β-Cyclodextrin

An environmentally benign and highly efficient procedure for the nucleophilic addition of trimethylsilyl cyanide to imines (Strecker reaction) has been developed under biomimetic conditions in water in the presence of beta-cyclodextrin to afford alpha-aminonitriles in quantitative yields. The use of cyclodextrin precludes the use of either acid or base, and the catalyst can be recycled a number of times without loss in activity.

Cobalt(II) chloride catalyzed one-pot synthesis of alpha-aminonitriles

A simple and efficient method has been developed for the synthesis of alpha-aminonitriles by a one-pot three- component condensation of aldehydes, amines, and potassium cyanide in acetonitrile in the presence of a catalytic amount of CoCl2 at room temperature.

NCN Pincer Palladium Complexes: Their Preparation via a Ligand Introduction Route and Their Catalytic Properties

A wide range of NCN pincer palladium complexes, [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chloropalladium (alkyl = n-butyl, benzyl, cyclohexyl, tert-butyl, adamantyl, phenyl, 4-methoxyphenyl), were readily prepared from trans-(4-tert-butyl-2,6-diformylphenyl)chlorobis(triphenylphosphine)palladium via dehydrative introduction of the corresponding alkylimino ligand groups (ligand introduction route) in excellent yields (71-98%). NMR studies on this route for forming pincer complexes revealed the intermediacy of [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chlorobis(triphenylphosphine)palladium which is in equilibrium with the corresponding NCN pincer complexes via coordination/dissociation of the intramolecular imino groups and triphenylphosphine ligands. A series of chiral NCN pincer complexes bearing pyrroloimidazolone units as the trans-chelating donor groups, [4-tert-butyl-2,6-bis{(3R,7aS)-2-phenylhexahydro-1H-pyrrolo[1,2-c]imidazol-1-on-3-yl}phenyl]chloropalladium, were also prepared from the same precursor via condensation with proline anilides in high yields. The catalytic properties of the NCN imino and the NCN pyrroloimidazolone pincer palladium complexes were examined in the Heck reaction and the asymmetric Michael reaction to demonstrate their high catalytic activity and high enantioselectivity.