The chemistry of cyclometalation : a comprehensive study on cycloiridated 1-arylalkylamine and imine complexes

As fundamental ligand frameworks within the field of cyclometalation, the 1-arylalkylamine and imine motifs have a widespread presence as cyclometalating ligands for an extensive number of transition metal elements. In the thesis, we investigate the outcome of cycloiridation of various 1-arylalkylamine and imine ligands with [IrCp*Cl2]2. A good balance of steric factors within the organic scaffold was found to be vital for the formation and stability of amino-derived iridacycles. While excessive steric hindrance would lead to numerous side reactions affording an assortment of byproducts, the lack of steric bulk within the amine ligands would lead to an internal oxidation to its imine functionality within the cyclometalated ring. Investigations into the failed cyclization procedure revealed a consistent competing pathway leading to either a strained ring system or an iminium species. The postulated intermediates can then readily undergo hydrolysis to provide a N-dealkylated amine moiety and a carbonyl compound. Optically-active cycloiridated complexes from successful direct ortho-metalation procedures were characterized for its conformational lock and evaluated for its efficiency in the catalytic asymmetric hydrogen transfer reaction. Lastly, we examine the crystallographic structures of the synthesized iridacycles to gain insights into their bonding.​Doctor of Philosophy (SPMS

Evaluation of palladacycles as a non-rhodium based alternative for the asymmetric conjugate 1,4-addition of arylboronic acids to α,β-unsaturated enones

The asymmetric conjugate 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds is an extremely versatile and widely used organic transformation. While the rhodium(I)-catalysed reaction has been thoroughly explored, the asymmetric palladium-catalysed protocol is far less developed and understood, particularly with acyclic enones as substrates. Herein, we report the systematic evaluation of a series of metallacycles for this reaction and the conjugate addition of arylboronic acids to a wide range of α,β-unsaturated enones, catalysed by an easily accessible and robust chiral phosphapalladacycle in high yields and enantioselectivities

Challenges in cyclometalation : steric effects leading to competing pathways and η1,η2-cyclometalated iridium(iii) complexes

The iridation of (R)-N,N-dimethyl-1-(1-naphthyl)ethylamine in the presence of a base afforded an assortment of products ranging from organic molecules to coordinated systems and cyclometalated complexes. The transformation affirmed the postulation where steric effects within the coordination sphere favor a β-hydride elimination-like decomposition pathway, competing alongside ortho-metalation, thus leading to iminium intermediates. The same procedure also generated an unprecedented carbocyclic η1,η2-cycloiridated species that could not be attained from the direct cyclometalation of its organic ligand

Asymmetric Catalytic 1,2‐Dihydrophosphination of Secondary 1,2‐Diphosphines – Direct Access to Free P

A 1,2-dihydrophosphination of bis(phenylphosphino)ethane with a wide range of activated olefins was achieved in a catalytic manner with enantiomeric and diastereomeric excess of up to >99% and 44% respectively. The protocol can be extended to selected substrates leading to free P- and C-chiral 1,2-diphosphines. The synthesized ligands can also undergo direct complexation onto palladium affording complexes with complete retention of stereo-integrity. (Figure presented.).Ministry of Education (MOE)Nanyang Technological UniversityThis work was supported by the Ministry of Education Academic Research Fund (AcRF-RG10/19-(S)).We are also grateful to Nanyang Technological University for Ph.D. scholarship to Teo R. H.

Synthesis, optical resolution, and stereochemical properties of a rationally designed chiral C–N palladacycle

A novel racemic tertiary amine, 1-(2,5-diisopropylphenyl)-N,N-dimethylethanamine, was synthesized from 2,5-diisopropylbenzaldehyde via a multistep approach in high overall yield. The ortho palladation of this ligand was found to be sensitive to the reaction conditions and the palladating reagents employed. The metal complexation process could thus generate a cyclopalladated complex in high yield, lead to an unexpected N-demethylated amine palladium(II) complex, or both. Both products have been isolated and characterized crystallographically in the solid state and spectroscopically in solution. The racemic cyclopalladated complex could be efficiently resolved via the formation of (S)-prolinato derivatives. The absolute stereochemistries of the resolved diastereomeric complexes were determined by single-crystal X-ray crystallography in the solid state and by 1H–1H rotating frame Overhauser effect (ROESY) NMR spectroscopy in solution. An evaluation of the sterically hindered resolved cyclopalladated units as chiral auxiliaries was conducted in the endo-cycloaddition reaction between 3,4-dimethyl-1-phenylphosphole (DMPP) and ethyl vinyl ketone. The two expected phosphanorbornene adducts were generated with moderate stereoselectivity.Accepted versio

Stereogenic Lock in 1‑Naphthylethanamine Complexes for Catalyst and Auxiliary Design: Structural and Reactivity Analysis for Cycloiridated Pseudotetrahedral Complexes

A series of optically active pseudo-tetrahedral five-membered cyclometalated 1-naphthylethanamine iridium­(III) complexes were prepared and characterized to analyze the efficacy of the stereogenic conformational lock in both solid and solution phases. The synthesis of the iridacycles was diastereoselective, and the compounds were found to be conformationally rigid. In comparison to its phenyl derivative, the structural lock prevented oxidation of the amine moiety within the five-membered organometallic ring during its synthesis. With up to three stereogenic centers in one of the naphthalene complexes, the stereochemistry of the metallacycle remained stable to both thermal and chemical changes. In terms of catalytic performance, the complexes displayed excellent activity for the asymmetric hydrogen transfer reaction, albeit with modest enantioselectivities

Synthesis, Optical Resolution, and Stereochemical Properties of a Rationally Designed Chiral C–N Palladacycle

A novel racemic tertiary amine, 1-(2,5-diisopropylphenyl)-<i>N</i>,<i>N</i>-dimethylethanamine, was synthesized from 2,5-diisopropylbenzaldehyde via a multistep approach in high overall yield. The ortho palladation of this ligand was found to be sensitive to the reaction conditions and the palladating reagents employed. The metal complexation process could thus generate a cyclopalladated complex in high yield, lead to an unexpected N-demethylated amine palladium­(II) complex, or both. Both products have been isolated and characterized crystallographically in the solid state and spectroscopically in solution. The racemic cyclopalladated complex could be efficiently resolved via the formation of (<i>S</i>)-prolinato derivatives. The absolute stereochemistries of the resolved diastereomeric complexes were determined by single-crystal X-ray crystallography in the solid state and by ¹H–¹H rotating frame Overhauser effect (ROESY) NMR spectroscopy in solution. An evaluation of the sterically hindered resolved cyclopalladated units as chiral auxiliaries was conducted in the endo-cycloaddition reaction between 3,4-dimethyl-1-phenylphosphole (DMPP) and ethyl vinyl ketone. The two expected phosphanorbornene adducts were generated with moderate stereoselectivity