Metal–Pyrazole Bifunction in Half‐Sandwich CN Chelate Iridium Complexes: Pyrazole–Pyrazolato Interconversion and Application to Catalytic Intramolecular Hydroamination of Aminoalkene

Abstract

Metal-ligand bifunctional molecular catalysts, in which the ligand cooperates in the substrate binding and activation through secondary interactions, such as hydrogen bonding and proton transfer, have attracted increased attention in synthetic organic chemistry. Among the most successful examples are the Noyoris hydrogenation and Noyori and Ikariyas transfer-hydrogenation catalysts; these catalysts have chelate amine ligands to provide a protic NH group at the a-position to the metal (Scheme 1a,b). [1, 2] The nitrogen ligand works as a hydrogen-bond donor to the substrate in the protonated amine form, and as an internal Brønsted base in the deprotonated amido form, to realize efficient catalytic reactions . Both amine and amido complexes are isolable in some cases, allowing for detailed mechanistic investigations. Shvos catalyst [3] and Milsteins pincer-type complexes [4] (Scheme 1c,d) represent an additional class of bifunctional catalysts, in which an operationally protic group is situated at a place more distant from the metal. As an extension of our extensive studies on the bifunctional chelate amine complexes, such as Scheme 1b,e, [2, 5,6] we recently synthesized a chelating N-heterocyclic carbene (NHC) complex, 1, bearing a protic b-NH group and revealed facile C À O bond cleavage of allyl alcohol, which was ascribed to the metal-NH bifunction.[7] Still, bifunctional catalysis based on the b-NH group in ligated N-heteroaromatics remains in its infancy if compared with the a-NH analogues. For example, full characterization of interconvertible couples linked with the reversible addition of H 2 or pronucleophiles has rarely been achieved for the protic NHC complexes [7][8][9] and for the related, and more accessible, pyrazole-based chelate complexes. [10][11][12] We report here the C À N chelating protic pyrazole complex 2, which is isoelectronic with 1, as a new entry of b-NH-based bifunctional catalysts (Scheme 1g). The welldefined deprotonation-reprotonation network involving 2, as well as catalytic hydroamination with 2 is described.[13]The chloro-pyrazole complex 2 was readily obtained by acetate-promoted cyclometalation [6, 14] of 3,5-diphenylpyrazole [Eq. (1)

Chiral Sulfur Functional Groups as Definers of the Chirality at the Metal in Ir and Rh Half‐Sandwich Complexes: A Combined CD/X‐ray Study

“…To promote the CÀHa ctivation reactioni nacontrolled manner, carbene 5bb was submitted to the standard reaction conditions (NaOAc/DCE, rt, overnight;S cheme 5). [6,14] The corresponding insertionp roduct 6bb waso btained in 97 %y ield as an enantiopure diastereoisomer.S pectroscopic data for compound 6bb weren early identical to those of the metallacycle 6bc,w hichs uggests that the diastereoisomer of each compound have the same configurationa tt he metal center. To ensure the S absolute configuration at the newly created Ir III chiral center,w ed etermined the structure of complex 6bb by X-ray diffraction analysis ( Figure 1, right).…”

Synthesis of Saturated Heterocycles via Metal-Catalyzed Alkene Hydroamination or Hydroalkoxylation Reactions

“…In 2010, Ikariya and coworkers reported the use of a neutral Ir-pyrazolato complex 70 in combination with a strong base (KOt-Bu) to catalyze the hydroamination of substrate 64a [56]. It is proposed that the base serves to deprotonate the pyrazole ligand from the chloride complex to afford an Ir-pyrazolato complex in which the ligand assists nucleophilic attack of amine onto a coordinated olefin (Scheme 31).…”

Cyclometalated Half-Sandwich Iridium Complex for Catalytic Hydrogenation of Imines and Quinolines

“…(iii) The redox property and solubility of these transition metal complexes can be tuned by introducing various types of substituents to cyclopentadienyl ring . Thus, exploring the synthesis and application of half-sandwich cyclometalated complexes has become one of the most active and exciting areas of organometallic chemistry, because of the useful catalytic reactivity that these ligands impart on complexes. − …”