Labile Rhodium(I)–N-Heterocyclic Carbene Complexes

The neutral square-planar complexes Rh­(acac)­(IPr)­(η²<i>-</i>olefin) have been prepared from [Rh­(μ-Cl)­(IPr)­(η²<i>-</i>olefin)]₂ (IPr = 1,3-bis-(2,6-diisopropylphenyl)­imidazol-2-carbene; olefin = cyclooctene, ethylene) and sodium acetylacetonate (acac). Protonation of the acetylacetonato complexes with triflic acid opens the way to the formation of the putative bare [Rh-IPr]⁺ fragment that has been stabilized at low temperature by labile ligands such as triflate, cyclooctene, and acetonitrile to generate Rh­(OTf)­(IPr)­(η²<i>-</i>coe), [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf, and [Rh­(IPr)­(NCCH₃)₃]­OTf complexes. The derivative [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf was further characterized by an X-ray diffraction analysis

Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation

The new Rh–hydroxo dinuclear complexes stabilized by an N-heterocyclic carbene (NHC) ligand of type [Rh­(μ-OH)­(NHC)­(η²-olefin)]₂ (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward α-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex <b>3</b> has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including Rh^I-thiolato species Rh­(SCH₂Ph)­(IPr)­(η²-coe)­(py) (<b>6</b>) and Rh­(SCH₂Ph)­(IPr)­(η²-HCCCH₂Ph)­(py) (<b>7</b>), and the Rh^III-hydride-dithiolato derivative RhH­(SCH₂Ph)₂(IPr)­(py) (<b>8</b>) as the catalytically active species. Computational DFT studies reveal an operational mechanism consisting of sequential thiol deprotonation by the hydroxo ligand, subsequent S–H oxidative addition, alkyne insertion, and reductive elimination. The insertion step is rate-limiting with a 1,2 thiometalation of the alkyne as the more favorable pathway in accordance with the observed Markovnikov-type selectivity

Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation

The new Rh–hydroxo dinuclear complexes stabilized by an N-heterocyclic carbene (NHC) ligand of type [Rh­(μ-OH)­(NHC)­(η²-olefin)]₂ (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward α-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex <b>3</b> has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including Rh^I-thiolato species Rh­(SCH₂Ph)­(IPr)­(η²-coe)­(py) (<b>6</b>) and Rh­(SCH₂Ph)­(IPr)­(η²-HCCCH₂Ph)­(py) (<b>7</b>), and the Rh^III-hydride-dithiolato derivative RhH­(SCH₂Ph)₂(IPr)­(py) (<b>8</b>) as the catalytically active species. Computational DFT studies reveal an operational mechanism consisting of sequential thiol deprotonation by the hydroxo ligand, subsequent S–H oxidative addition, alkyne insertion, and reductive elimination. The insertion step is rate-limiting with a 1,2 thiometalation of the alkyne as the more favorable pathway in accordance with the observed Markovnikov-type selectivity

Labile Rhodium(I)–N-Heterocyclic Carbene Complexes

The neutral square-planar complexes Rh­(acac)­(IPr)­(η²<i>-</i>olefin) have been prepared from [Rh­(μ-Cl)­(IPr)­(η²<i>-</i>olefin)]₂ (IPr = 1,3-bis-(2,6-diisopropylphenyl)­imidazol-2-carbene; olefin = cyclooctene, ethylene) and sodium acetylacetonate (acac). Protonation of the acetylacetonato complexes with triflic acid opens the way to the formation of the putative bare [Rh-IPr]⁺ fragment that has been stabilized at low temperature by labile ligands such as triflate, cyclooctene, and acetonitrile to generate Rh­(OTf)­(IPr)­(η²<i>-</i>coe), [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf, and [Rh­(IPr)­(NCCH₃)₃]­OTf complexes. The derivative [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf was further characterized by an X-ray diffraction analysis

Pentacoordinated Rhodium(I) Complexes Supported by Coumarin-Functionalized <i>N</i>‑Heterocyclic Carbene Ligands

New coumarin-tethered benzimidazolium (BzICou^RHCl) and imidazolium (ICou^RHCl) salts have been prepared as precursors for coumarin–NHC rhodium­(I) complexes RhCl­(NHC)­(cod). Trigonal bypiramidal pentacoordinated bis-coumarin–NHC rhodium­(I) species, RhCl­(NHC)₂, can be obtained by heating rhodium-cod derivatives in the presence of coumarin–azolium salts and a base. These unusual species are stabilized by coordination of the unsaturated bond of both coumarin moieties by the same enantioface. The allyl substituent on doubly functionalized NHC competes for coordination with coumarin wingtips. DFT calculations upon coordination of the olefin moieties support the experimental results

Pentacoordinated Rhodium(I) Complexes Supported by Coumarin-Functionalized <i>N</i>‑Heterocyclic Carbene Ligands

New coumarin-tethered benzimidazolium (BzICou^RHCl) and imidazolium (ICou^RHCl) salts have been prepared as precursors for coumarin–NHC rhodium­(I) complexes RhCl­(NHC)­(cod). Trigonal bypiramidal pentacoordinated bis-coumarin–NHC rhodium­(I) species, RhCl­(NHC)₂, can be obtained by heating rhodium-cod derivatives in the presence of coumarin–azolium salts and a base. These unusual species are stabilized by coordination of the unsaturated bond of both coumarin moieties by the same enantioface. The allyl substituent on doubly functionalized NHC competes for coordination with coumarin wingtips. DFT calculations upon coordination of the olefin moieties support the experimental results

Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts

Rh–N-heterocyclic carbene compounds [Rh­(μ-Cl)­(IPr)­(η²-olefin)]₂ and RhCl­(IPr)­(py)­(η²-olefin) (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl­(IPr)­(py)­(η²-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate–hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate–alkyne disposition, favoring formation of 2,2-disubstituted metal–alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl–hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium–thiolate bond is the rate-determining step

Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts

Rh–N-heterocyclic carbene compounds [Rh­(μ-Cl)­(IPr)­(η²-olefin)]₂ and RhCl­(IPr)­(py)­(η²-olefin) (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl­(IPr)­(py)­(η²-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate–hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate–alkyne disposition, favoring formation of 2,2-disubstituted metal–alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl–hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium–thiolate bond is the rate-determining step

Rhodium(I)-N-Heterocyclic Carbene Catalyst for Selective Coupling of <i>N</i>‑Vinylpyrazoles with Alkynes via C–H Activation

The complex [Rh­(μ-Cl)­(<i>I</i>Pr)­(η²<i>-</i>coe)]₂ {<i>I</i>Pr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-carbene, coe = <i>cis</i>-cyclooctene} efficiently catalyzes the coupling of alkynes and <i>N</i>-vinylpyrazole via C–H activation, leading to Markovnikov-selective butadienylpyrazole derivatives under mild conditions. A straightforward approach to cross-conjugated acyclic trienes is also operative through a one-pot alkyne dimerization-hydrovinylation tandem reaction. The proposed mechanism involves C–H activation of vinylpyrazole directed by nitrogen coordination to the metallic center. Subsequent alkyne coordination, insertion, and reductive elimination steps lead to the coupling products. Several key intermediates participating in the catalytic cycle have been detected and characterized, including a κ-N, η²-CC coordinated vinylpyrazole complex and a Rh^III-hydride-alkenyl species resulting from the C–H activation of the vinylpyrazol

Design of Highly Selective Alkyne Hydrothiolation Rh^I‑NHC Catalysts: Carbonyl-Triggered Nonoxidative Mechanism

New Rh^I-IPr (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-carbene) complexes bearing an <i>N</i>,<i>O</i>-pyridine-2-methanolato (N-O) bidentate ligand have been prepared. The carbonyl complex Rh­(N-O)­(IPr)­(CO) efficiently catalyzes the hydrothiolation of a range of alkynes with high selectivity to α-vinyl sulfides. Reactivity studies and DFT calculations have revealed a new nonoxidative catalytic pathway, passing through Rh^I catalytic intermediates, which is driven by the interplay between the pyridine-2-methanolato and carbonyl ligands. The basic alkoxo ligand promotes the deprotonation of the thiol to generate the Rh^I active species, whereas the π-acceptor character of the carbonyl ligand hinders the oxidative addition process. In addition, the stereochemistry of the key thiolate-π-alkyne intermediate, which is determined by the electronic preference of the carbonyl ligand to coordinate cis to IPr, facilitates the rate-limiting alkyne thiometalation step