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Relevant academic research and scientific papers

Gold(III) NHC Complexes for Catalyzing Dihydroalkoxylation and Hydroamination Reactions

A gold(III) complex of an N-heterocyclic carbene based hemilabile ligand with two pendant pyrazole arms (1,3-bis((1H-pyrazol-3-yl)methyl)-2,3-dihydro-1H-imidazole, LH) was synthesized. Complex [LAu(III)Cl3] is an excellent catalyst for promoting dihydroalkoxylation at room temperature, even catalyzing this reaction at 0 °C. [LAu(III)Cl3] is one of the most efficient catalysts reported to date for the spirocyclization of alkynyl diols. Furthermore, [LAu(III)Cl3] catalyzed intra- and intermolecular hydroamination reactions, achieving good to excellent conversions. [LAu(III)Cl3] is a more efficient catalyst than a gold(I) analogue, [LAu(I)Cl]. The dependence of the quantity of weakly coordinating anion [BArF4]- ((3,5-trifluoromethyl)phenyl borate) present on catalysis efficiency was probed for the dihydroalkoxylation reaction. X-ray diffraction analysis of single crystals demonstrated the solid-state structure of gold complexes [LAu(III)Cl3] and [LAu(I)Cl], which displayed the expected square-planar and linear coordination geometries, respectively.

Highly efficient Rh(i) and Ir(i) single and dual metal catalysed dihydroalkoxylation reactions of alkyne diols

A highly efficient rhodium(i) and iridium(i) catalysed dihydroalkoxylation reaction of alkyne diols is employed here for the synthesis of spiroketals and a fused bicyclic ketal. The two metal catalysts show differential selectivity and efficiency for either the cyclisation of the 5-exo or 6-endo-membered rings. For the first time, a dual metal (Rh and Ir) catalyst system is effectively utilised for the formation of the 5,6-spiroketals, more efficiently than the single metal catalysts. The two different metals create a dual activation pathway to enhance the 5- and 6-membered ring closure as compared with the equivalent single catalysts.

Cooperativity in bimetallic dihydroalkoxylation catalysts built on aromatic scaffolds: Significant rate enhancements with a rigid anthracene scaffold

This work describes investigations into metal-catalyzed sequential reactions using a series of single metal and bimetallic Rh(I) and/or Ir(I) pyrazolyl complexes. Monometallic complexes with bis(1-pyrazolyl)methane (bpm) ligands [M(CO)2(bpm)]BArF 4 (1), bimetallic complexes [M2(CO)2(Lscaffold)][BArF 4]2 (2-4) where M = Rh(I) or Ir(I) bearing bitopic ligands Lscaffold = bis(1-pyrazolyl)methane-derived ligands, p-C 6H4[CH(pz)2]2 (Lp), m-C6H4[CH(pz)2]2 (Lm), and anthracene-bridged 1,8-C14H8[CH(pz)2] 2 (LAnt), [M2(CO)4(L p)]-[BArF 4]2 (2), [M 2(CO)4(Lm)][BArF 4] 2 (3), and [M2(CO)4(LAnt)][BAr F 4]2 (4) were used as catalysts. The efficiency of the complexes as catalysts was tested for the dihydroalkoxylation of a series of alkyne diol substrates, 2-(6-hydroxyhex-1-ynyl)benzyl alcohol (5), 1-methyl-3-heptyne-1,7-diol (6), 2-(5-hydroxypent-1-ynyl)benzyl alcohol (7), and 2-(4-hydroxybut-1-ynyl)benzyl alcohol (8), forming spiroketals. All complexes tested were highly effective catalysts for the intramolecular dihydroalkoxylation reaction. The homobimetallic complexes 2-4 showed significant enhancement in activity and selectivity relative to the single metal catalysts (1). The order of catalytic activity of the bimetallic complexes was found to be [M2(CO)4(LAnt)][BArF 4]2 > [M2(CO)4(L m)][BArF 4]2 > [M 2(CO)4(Lp)][BArF 4] 2 for all substrates, and the bimetallic cooperativity index was established for each reaction.

Novel spiroacetal synthesis via hydroboration of alkynediols

Alkynediols were transformed to the corresponding spiroacetals by the sequence of hydroboration with disiamylborane, oxidation of the resulting alkenylboron intermediate with alkaline hydrogen peroxide, and catalytic treatment with p-TsOH. By this transformation, 6-(2-hydroxymethyl)phenyl-3-methyl-5-hexyn-1-ol was converted to spiro[3,4-dihydro-1H-2-benzopyran-3,2′-[4′]methyl[3′,4′, 5′,6′]tetrahydro-2′H-pyran] as a single diastereomer.

The advantages of covalently attaching organometallic catalysts to a carbon black support: Recyclable Rh(I) complexes that deliver enhanced conversion and product selectivity

Pure carbon black (CB) was covalently attached to a bidentate nitrogen coordination motif with a carbon-carbon bond by spontaneous reaction with an in situ generated ligand precursor. The functionalized support was treated with [Rh(CO)2(μ-Cl)]2 to form a heterogeneous carbon-based support covalently linked to a well defined Rh(i) coordination complex. The hybrid material was characterized using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Infrared (IR) spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS). The CB-supported Rh(i) catalyst was active in both hydroamination and dihydroalkoxylation reactions achieving turnover numbers approaching 1000 and was readily recycled. The selectivity of an intramolecular dihydroalkoxylation reaction was significantly improved by covalently anchoring the catalyst to the CB surface.

Bimetallic complexes for enhancing catalyst efficiency: Probing the relationship between activity and intermetallic distance

A series of new homoditopic ligands (14-17) containing two bis(pyrazol-1-yl)methane moieties connected to either flexible (1,6-bis(bis(pyrazol-1-yl)methyl)hexane, L6C (14); 1,7-bis(bis(pyrazol-1-yl)methyl)heptane, L7C (15)) or rigid scaffolds (4,5-bis(bis(pyrazol-1-yl)methyl)-9,9-dimethylxanthene, LXan (16); 4,6-bis(bis(pyrazol-1-yl)methyl)dibenzofuran, LDib (17)) were synthesized. A series of bimetallic rhodium(I) complexes [Rh2(CO) 4(LX)][BArF4]2 (X = Xan (8), Dib (9), Fc ((1,1′-bis(bis(pyrazol-1-yl)methyl)ferrocene) (10)), 6C (11), 7C (12)) and [Rh2(COD)2(LX)][BAr F4]2 (COD = 1,5-cyclooctadiene, X = 6C (21), 7C (22)) as well as the monometallic complexes [Rh(CO)2(L Ph)][BArF4] (7, LPh = α,α-bis(pyrazol-1-yl)toluene) and [Rh(COD)(LPh)][BAr F4] (20) were synthesized. The solid-state structures of 8, 10, 16, 17, and 21 were determined using single-crystal X-ray diffraction analysis. The catalytic activity of complexes 7-12 was established for the dihydroalkoxylation of the alkynediols 2-(5-hydroxypent-1-ynyl)benzyl alcohol (I) and 2-(4-hydroxybut-1-ynyl)benzyl alcohol (II). The rigid bimetallic scaffolds LXan and LDib were found to yield the most active catalysts, 8 and 9, respectively, with 9 achieving a reaction rate 5-6 times faster than the monometallic complex 7 for the dihydroalkoxylation of I. Density functional theory calculations were used to examine the intermetallic Rh···Rh distances in 8 and 9, and these were compared with those of three other related bimetallic catalysts reported previously. The calculations showed all these species to be very flexible at minimal energetic cost, both in terms of the Rh···Rh distance and in being able to access a range of different conformations. No clear correlation between Rh···Rh distance and catalytic activity was established here, which suggests that the observed experimental correlation between catalyst structure and activity may derive from the structures of key reaction intermediates.

Simple and reactive Ir(i) N-heterocyclic carbene complexes for alkyne activation

Two simple unsymmetrical monometallic Ir(i) complexes with an N-heterocyclic carbene ligand and an analogous bimetallic Ir(i) complex were synthesised. These complexes were found to be extremely active catalysts for a range of C-X (X = N or O) and Si-N bond forming reactions involving alkyne and imine activation for dihydroalkoxylation, hydroamination and hydrosilylation reactions. These catalysts exhibited reaction rates far exceeding those of other Rh(i) and Ir(i) complexes previously reported. In addition, a small change to the ligand design (phenyl vs. mesityl) substantially affected both the reactivity and product selectivity of the catalyst. The Ir(i) complex bearing a mesitylene wingtip provided unprecedented regioselectivity in the dihydroalkoxylation reaction and a new kinetic product from the typical hydrosilylation protocol of 2-benyzlpyrroline to produce an N-silylaminoalkene. Our mechanistic studies indicated that this transformation proceeded via a dehydrogenative coupling mechanism.

Ruthenium-Catalyzed Intramolecular Double Hydroalkoxylation of Internal Alkynes

Intramolecular double hydroalkoxylation of internal alkynes could be achieved using a Grubbs-type ruthenium carbene complex or its modified species to deliver a series of bridged- and spiroacetal derivatives in moderate to good yields. This study represen

Highly versatile heteroditopic ligand scaffolds for accommodating group 8, 9 & 11 heterobimetallic complexes

Two highly versatile xanthene scaffolds containing pairs of heteroditopic ligands were found to be capable of accommodating a range of transition metal ions, including Au(i), Ir(i), Ir(iii), Rh(i), and Ru(ii) to generate an array of heterobimetallic complexes. The metal complexes were fully characterised and proved to be stable in the solid and solution state, with no observed metal-metal scrambling. Heterobimetallic complexes containing the Rh(i)/Ir(i) combinations were tested as catalysts for the two-step dihydroalkoxylation reaction of alkynediols and sequential hydroamination/hydrosilylation reaction of alkynamines.

Development of tethered dual catalysts: synergy between photo- And transition metal catalysts for enhanced catalysis

While dual photocatalysis-transition metal catalysis strategies are extensively reported, the majority of systems feature two separate catalysts, limiting the potential for synergistic interactions between the catalytic centres. In this work we synthesise