The palladacycle-catalyzed asymmetric PH addition of 4-oxo-enamides has been developed, which provides efficient access to phosphinocarboxamides and their analogues. Solvent-mediated reversal of stereoselectivity (ee from +96 % to −92 %) was observed, and the underlying mechanism that allows facile access to both enantiomers of the product by judicious choice of solvents is revealed.

Two structurally isomeric substituted N,N-dimethylethanamines have been prepared. Treatment of the 2,4-di-tert-butylphenyl isomer with Pd^II ions generated the ortho-metalated complexes. On the other hand, treatment of the 2,5-di-tert-butylphenyl-substituted amine resulted in the unexpected chemoselective cleavage of one of the three N–C bonds, thus generating the corresponding secondary amine. The N-demethylation process could be catalyzed at room temperature by palladium(II) catalysts such as PdCl₂ or Pd(OAc)₂. Furthermore, treatment with a stoichiometric amount of Pd^II ions gave a metal complex in which both secondary amines were bound to Pd in an N-monodentate fashion. When triethylamine was introduced, one of the N-ethyl groups in NEt₃ was cleaved, and an unexpected heteroamine complex was produced. The products generated were isolated and characterized by X-ray crystallography. Mechanistic insights into the cyclometalation and C–N cleavage observed are discussed.

The first asymmetric phospha-Michael addition of diarylphosphines to N-enoyl phthalimides has been developed in the presence of a chiral palladacycle catalyst. A library of free chiral tertiary phosphine adducts were directly obtained with excellent yields and enantioselectivities. Products can be subsequently functionalized to afford β-phosphinoamides, the direct preparation of which from cinnamides has been notoriously challenging.

An enantioselective Michael addition reaction of diphenylphosphine to substituted alkenylisoxazoles has been developed. The reaction proceeds efficiently under mild conditions with high yields (up to 99%) and moderate to excellent enantioselectivities (up to 92%) thus providing a hitherto unavailable direct access to a library of chiral tertiary phosphine-functionalized isoxazoles.

Phenylbis(phenylethynyl)phosphane PhP(C≡CPh)₂ coordinates regiospecifically to the α-methyl-chiral ortho-platinated and -palladated naphthylamine units at the positions trans to the nitrogen donors. The PPt coordination bond is kinetically inert, whereas the PPd bond is labile. Upon heating of these phosphane complexes at 70 °C, one of the C≡C bonds in the coordinated PhP(C≡CPh)₂ was activated towards an intermolecular Pd–C bond insertion reaction with an external ortho-palladated naphthylamine ring. No intramolecular insertion reaction occurred. In contrast to its palladium analogue, the ortho-platinated ring is not reactive towards coordinated PhP(C≡CPh)₂, although it can promote the Pd–C bond insertion reaction. However, despite the high kinetic stability of the PPt coordination, the organoplatinum unit is a noticeably weaker activator than its organopalladium counterpart. The chirality of the reacting ortho-metallated naphthylamine ligand exhibited high stereochemical influence on the formation of the new stereogenic phosphorus center during the course of these C–C bond-formation reactions. The coordination chemistry and the absolute stereochemistry of the dimetallic products were determined by single-crystal X-ray crystallographic analysis.

Palladium and platinum complexes containing a sulfur-functionalised N-heterocyclic carbene (S-NHC) chelate ligand have been synthesised. The absolute conformations of these novel organometallic S-NHC chelates were determined by X-ray structural analyses and solution-phase 2D ¹H–¹H ROESY NMR spectroscopy. The structural studies revealed that the phenyl substituents on the stereogenic carbon atoms invariably take up the axial positions on the Pd-C-S coordination plane to afford a skewed five-membered ring structure. All of the chiral complexes are structurally rigid and stereochemically locked in a chiral ring conformation that is either (R_s,S,R)-λ or (S_s,R,R)-δ in both the solid state and solution.

An efficient methodology involving the predominant formation of CC bonds is described for the first direct synthesis of 2-allylanilines from allylic alcohols via a one-pot tandem allylic amination/allylation protocol catalyzed by a palladacycle under mild conditions without the requirement for additional activators.

A series of chiral ortho-platinated and -palladated complexes derived from N,N,α-trimethylbenzylamine has been prepared. The dialkynylphosphane PhP(C≡CMe)₂ coordinated regiospecifically as a PM monodentate ligand to these cyclometallated units in the positions trans to the nitrogen donors of the metallacycles. Upon introduction of selected chirality on the benzylamine unit and subsequent monitoring by using NMR spectroscopy, the PPt bond was found to be thermodynamically stable and kinetically inert. The PPd bond must be kinetically labile since the palladium complexes readily undergo a facile ligand redistribution process. Nevertheless, both ortho-metallated units activated one of the C≡C bonds towards an intermolecular Pd–C bond insertion reaction into another cyclopalladated ring. No intramolecular insertion reaction occurred within the same molecule. Coordination of PhP(C≡CMe)₂ to the ortho-palladated unit also protected the organopalladium ring from intermolecular attack by other coordinated dialkynylphosphanes. The Pt–C bonds in cycloplatinated rings are not reactive toward the insertion reaction, although they are good reaction activators for coordinated dialkynylphosphanes.

Cyclopalladated and cycloplatinated complexes, which incorporated the N,N-dimethylbenzylamine and N,N-dimethylnaphthylamine motifs, were successfully employed to promote a series of intermolecular monoinsertion reactions of diphenyl-1-propynylarsane, Ph₂AsC≡CMe, into the Pd–C bond of the chiral α-methyl N,N-dimethylbenzylamine palladacycles. The precursor complexes were prepared by means of the coordination of the Ph₂AsC≡CMe moiety onto the metal center trans to the benzylamine– or naphthylamine–N donor atom of the cyclometallated complexes. Subsequently a series of monoinsertion reactions were carried out between these precursor complexes and the enantiomerically pure N,N-dimethyl benzylamine palladacycles. These insertion reactions showed high regioselectivity under mild conditions and a variety of homo- and heterobimetallic arsenic functionalized complexes were formed. The coordination chemistry and the absolute stereochemistry of the monoinsertion products were determined by X-ray crystallography.

A novel amine ligand, 1-(2,5-dimethylphenyl)-N,N,2,2-tetramethylpropan-1-amine, was synthesized in six steps from commercially available p-xylene. Direct ortho-palladation of this amine ligand proceeded readily to form the racemic dimeric complex. The palladacycle structure and ring conformations of its triphenylphosphane derivative were thoroughly investigated by X-ray structural analysis in the solid state and 2D ¹H–¹H ROESY NMR spectroscopy in solution. This racemic palladacycle was efficiently resolved through the formation of its (S)-prolinato derivatives and an efficient separation of the resulting diastereomeric complexes was achieved by slow crystallization with the use of different solvent systems. The structure and absolute configuration of the two optically resolved palladium complexes were determined by X-ray diffraction. Both the (R,R) and (S,S)-di-μ-chlorido dimeric palladium complexes could be obtained chemoselectively by treating the corresponding prolinato derivatives with 1 M hydrochloric acid. The asymmetric hydrophosphanation reaction between diphenylphosphane and diethyl acetylenedicarboxylate was promoted by this newly synthesized palladacycle and resulted in the formation of a single isomer according to the ³¹P NMR spectrum. The amine auxiliary could be subsequently removed chemoselectively from the palladium center by treatment with concentrated HCl. An optically pure C₂-symmetrical diphosphane ligand containing two ester functional groups at the two chiral carbon stereogenic centers was prepared by ligand displacement.

A novel chiral six-membered bidentate pyridine-N-heterocyclic carbene palladacycle was prepared via optical resolution of the racemic palladacycle. This is the first successful demonstration of the synthesis of a chiral carbene palladacycle via fractional crystallization of its chiral amino acid derivatives. Upon formation of (R_C,S_C) and (S_C,S_C)-phenylalanate derivatives, the (R_C,S_C)-phenylalanate diastereomer crystallized out spontaneously. Optically active (R)- and (S)-dichloropalladacycles can then be achieved by subsequent cleavage of the chiral amino acid auxiliary in the presence of aqueous HCl from their respective diastereomers. The absolute configurations of both the (R)- and (S)-palladacycles were determined by X-ray diffraction studies.

The cover picture shows a novel chiral palladacycle, which was readily prepared from p-xylene and then resolved through the separation of its (S)-prolinate diastereomeric derivatives. The catalytic ability of the newly synthesized palladacycle was demonstrated in the preparation of a new diester-substituted diphosphane ligand by an asymmetric hydrophosphanation reaction, which proceeded with excellent selectivity. Details are discussed in the article by P.-H. Leung et al. on p. 4427 ff. The key molecule is depicted as superimposed over the image of the Chinese Heritage Centre building at the Nanyang Technological University, Singapore, and is meant to represent the contributions of Asian authors/readers to the European Journal of Inorganic Chemistry.

The cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylphosphane sulfide was promoted by chiral palladium and platinum complexes containingortho-metalated (S)-[1-(dimethylamino)ethyl]naphthalene. They exhibited similar stereoselectivity; the palladium cycloadducts could not be separated via column chromatography and fractional crystallization, however, the corresponding platinum complexes could be successfully converted into their enantiomerically pure counterpart. A formal arsanylidene elimination reaction was observed on the liberated free As/P=S bidentate ligand.

A novel palladacycle was designed and prepared by direct cyclopalladation of the ligand 1-(3,6-dimethylnaphthalen-1-yl)-N,N-dimethylethanamine, which was synthesized by a multistep sequence starting from 2,7-dimethylnaphthalene. The palladacycle structure and ring conformations of its triphenylphosphane derivative was investigated by X-ray structural analysis in the solid state and 2D ¹H–¹H ROESY NMR spectroscopy in solution. The ortho-palladated ring is stereochemically rigid, and the five-membered ring conformation is locked by repulsive interactions between the methyl group on the prochiral carbon atom and its neighboring naphthylene proton. The racemic cyclopalladated complex could be efficiently resolved through the formation of its (S)-prolinato derivatives, and efficient separation of the resulting diastereomeric complexes was achieved by simple silica-gel chromatography. The structures and absolute configurations of the two optically resolved palladium complexes were determined by X-ray crystallography. Both the (R,R)- and (S,S)-di-μ-chloride dimeric palladium complexes could be obtained chemoselectively by treating the corresponding prolinato derivatives with 1 M hydrochloric acid. Despite the severe interchelate steric constraints within this new organopalladium complex, the bulky monodentate ligand 3,4-dimethyl-1-phenylphosphole (dmpp) was coordinated regiospecifically to the ortho-palladated amine unit trans to the NMe₂ group. In comparison to its naphthylamine analogue, the new palladacycle showed a much higher stereoselectivity in the chiral template-promoted asymmetric Diels–Alder reaction of coordinated dmpp and N,N-dimethylacrylamide.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The asymmetric hydroarsanation reactions between diphenylarsane and (E)-1-phenyl-3-(pyridin-2-yl)-2-propenone and (E)-1-methyl-3-(pyridin-2-yl)-2-propenoate have been achieved by use of the organopalladium complex containing ortho-metalated (R)-[1-(dimethylamino)ethyl]naphthalene as the chiral reaction template in high regio- and stereoselectivities under mild conditions. Hydroarsanation of (E)-1-phenyl-3-(pyridin-2-yl)-2-propenone with diphenylarsane generated two stereoisomeric products in the ratio of 3:1 as five-membered As–N bidentate chelates on the chiral naphthylamine palladium template. Using the same chiral metal template, the corresponding hydroarsanation reaction with (E)-1-methyl-3-(pyridin-2-yl)-2-propenoate gave only one product as a six-membered As–N bidentate chelate. The naphthylamine auxiliary could be removed chemoselectively by treatment with concentrated hydrochloric acid to form the corresponding optically pure neutral complexes. Subsequent ligands displacement from the palladium using aqueous potassium cyanide generated the optically pure keto- and ester-functionalized chiral pyridylarsane ligands. The absolute configuration and the coordination properties of the pyridylarsanes have been established by single-crystal X-ray analysis.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Enantiomerically pure, alcohol-functionalized diphosphane ligands carrying one phosphorus and three carbon stereogenic centers were generated from the Diels–Alder reactions of phosphane-functionalized terminal alkenols [3-(diphenylphosphanyl)but-3-en-1-ol and 2-(diphenylphosphanyl)prop-2-en-1-ol] with 3,4-dimethyl-1-phenyl-1H-phosphole. The reactions were promoted and controlled by the organoplatinum complex containing ortho-metalated (R)-[1-(dimethylamino)ethyl]naphthalene, and both cycloadditions showed excellent regio- and stereoselectivity under mild conditions with only one enantiomer being formed. The products were isolated in high yield and were characterized by single-crystal X-ray diffraction analysis. Their structures in solution were analyzed by 2D ¹H–¹H ROESY NMR spectroscopy. Subsequent decomplexation and repreparation of the products proved the optical purity of the alcohol-functionalized, chiral diphosphanes formed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A novel chiral palladacycle containing ortho-metallated (±)-1-(3,6-dimethylnaphthalen-2-yl)-N,N-dimethylethylaminewas designed and synthesized by a multi-step synthesis by using 2,7-dimethylnaphthalene as the starting material. The structure and palladacycle ring conformation of the triphenylphosphane derivative was investigated by X-ray structural analysis in the solid state and by 2D ROESY NMR spectroscopy in solution. Through a designed intramolecular steric interaction, the five-membered naphthylamine chelate can be locked into a fixed conformation, both in the solid state and in solution. The racemic cyclopalladated complex could be efficiently resolved through the formation of its (S)-prolinato and (S)-alaninato derivatives. The structure and absolute configuration of the two optically resolved palladium complexes were determined by X-ray diffraction. Both the (R,R) and (S,S)-di-μ-chlorido dimeric palladium complexes containing the resolved amine ligand was obtained chemoselectively by treating the corresponding prolinato and alaninato derivatives with 1 M hydrochloric acid. Despite the severe interchelate steric constraint within these new organopalladium complexes, the bulky monodentate ligand 3,4-dimethyl-1-phenylphosphole (dmpp) was able to split the chlorido bridges regiospecifically in the position trans to the NMe₂ group. With the new palladacycle used as the chiral template, the stereoselectivity of Diels–Alder cycloaddition between dmpp and ethyl vinyl ketone significantly improves relative to that of other analogue complexes.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)