Andrea Di Giuseppe

Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts.

Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(η(2)-olefin)](2) and RhCl(IPr)(py)(η(2)-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)(η(2)-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.

The emergence of transition-metal-mediated hydrothiolation of unsaturated carbon-carbon bonds: A mechanistic outlook

The hydrothiolation of unsaturated carbon-carbon bonds is a practical and atom-economical approach for the incorporation of sulfur into organic frameworks. In recent years, we have witnessed the development of a range of transition-metal-based catalytic systems for the control of the regio- and stereoselectivity. In this Minireview we highlight the mechanistic background behind this transformation so as to help the design of more specific and active organometallic hydrothiolation catalysts.

Pyridine‐Enhanced Head‐to‐Tail Dimerization of Terminal Alkynes by a Rhodium–N‐Heterocyclic‐Carbene Catalyst

A general regioselective rhodium-catalyzed head-to-tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh-N-heterocyclic-carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π-alkyne-coordinated Rh(I) species [RhCl(NHC)(η(2)-HC≡CCH2Ph)(py)] (3) and [RhCl(NHC){η(2)-C(tBu)≡C(E)CH=CHtBu}(py)] (4) and the Rh(III)-hydride-alkynyl species [RhClH{-C≡CSi(Me)3}(IPr)(py)2] (5). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C-H oxidative addition, alkyne insertion, and reductive elimination. A 2,1-hydrometalation of the alkyne is the more favorable pathway in accordance with a head-to-tail selectivity.

A New Access to 4 H‐Quinolizines from 2‐Vinylpyridine and Alkynes Promoted by Rhodium–N‐Heterocyclic‐Carbene Catalysts

Forging the lock that autolocks! Rh-NHC catalysts promote a new access to 4 H-quinolizine species from 2-vinylpyridine and terminal and internal alkynes through C-H activation and C-C coupling reactions (see figure). N-Bridgehead heterocycle formation is favored for internal- over terminal-substituted butadienylpyridine derivatives in a thermal 6π-electrocyclization process.

Hydride-rhodium(III)-N-heterocyclic carbene catalysts for vinyl-selective H/D exchange: a structure-activity study.

A series of neutral and cationic Rh(III) -hydride and Rh(III) -ethyl complexes bearing a NHC ligand has been synthesized and evaluated as catalyst precursors for H/D exchange of styrene using CD(3)OD as a deuterium source. Various ligands have been examined in order to understand how the stereoelectronic properties can modulate the catalytic activity. Most of these complexes proved to be very active and selective in the vinylic H/D exchange, without deuteration at the aromatic positions, displaying very high selectivity toward the β-positions. In particular, the cationic complex [RhClH(CH(3)CN)(3)(IPr)]CF(3)SO(3) showed excellent catalytic activity, reaching the maximum attainable degree of β-vinylic deuteration in only 20 min. By modulation of the catalyst structure, we obtained improved α/β selectivity. Thus, the catalyst [RhClH(κ(2)-O,N-C(9)H(6)NO)(SIPr)], bearing an 8-quinolinolate ligand and a bulky and strongly electron-donating SIPr as the NHC, showed total selectivity for the β-vinylic positions. This systematic study has shown that increased electron density and steric demand at the metal center can improve both the catalytic activity and selectivity. Complexes bearing ligands with very high steric hindrance, however, proved to be inactive.

Selective catalytic oxidation of olefins by novel oxovanadium(IV) complexes having different donor ligands covalently anchored on SBA-15: a comparative study

Using a post-synthetic grafting method, mesoporous SBA-15 was functionalized first with aminopropyl silane reagents such as (3-aminopropyl)trimethoxysilane (APTMS) or bis[3-(trimethoxysilyl)propyl]amine (BTMSPA) and then with chlorinated ligands such as 4-(2-chloroacetyl)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one (H-AP) or 5-chloromethyl-8-quinolinol hydrochloride (H-HQ). Vanadyl cations [(VO)2+] were then immobilized over functionalized silica samples in order to prepare new oxovanadium(IV) based “quasi-homogeneous” catalysts, namely Ia (Ia′), Ib (Ib′), and IIa. Alternatively, a preformed oxovanadium(IV) complex such as [VO(AP)2(H2O)] has been immobilized over a previously functionalized SBA-15-NH2 silica support, affording catalyst Ic. Elemental analysis (C, H, N), N2 adsorption–desorption isotherms, FT-IR, 29Si and 13C CP-MAS NMR, XPS, ICP-AES, SEM-EDX and TG-DTG techniques have been used for the full characterization of these materials. Their catalytic properties in the H2O2 promoted oxidation of conjugated olefins like styrene, α-methyl and β-methylstyrene were investigated, in terms of activity, selectivity and recyclability, and compared with that shown by more simple systems prepared by the direct grafting of the vanadyl cation onto SBA-15 (catalyst A) or SBA-15-NH2 (catalyst B). Generally, high activity, selectivity and recyclability (over the first three runs) were shown by all anchored catalysts, with the only exception of A and B systems. A full account of the obtained results, along with insights into the effects due to the different strategies employed for the functionalization of SBA-15 on the properties of final anchored catalysts, are reported.

Mechanistic insight into the pyridine enhanced α-selectivity in alkyne hydrothiolation catalysed by quinolinolate–rhodium(I)–N-heterocyclic carbene complexes

RhI–NHC–olefin complexes bearing a N,O-quinolinolate bidentate ligand have been prepared from [Rh(μ-OH)(NHC)(η2-olefin)]2 precursors (olefin = cyclooctene, ethylene). The disposition of the chelate ligand with regard to the carbene in the square planar derivatives is strongly influenced by the steric hindrance exerted by the coordinated olefin. These complexes efficiently catalyzed the addition of thiophenol to phenylacetylene with good selectivity to α-vinyl sulfides, which can be increased up to 97% by addition of pyridine. Several key intermediates have been detected including the η1-alkenyl species resulting from alkyne insertion into a Rh–H bond. DFT calculations on the mechanism support a hydrometallation pathway that entails the oxidative addition of thiol, 2,1-insertion of the alkyne into the Rh–H bond, and reductive elimination as the rate-determining step. Remarkably, coordination of pyridine to the β-alkenyl intermediate but not to the α-alkenyl, which results in a net stabilization, is the key for the Markovnikov selectivity.

Vinylidene-based polymers by Rh(I)-NHC catalyzed thiol–yne click polymerization: synthesis, characterization and post-polymerization modification

Complex RhCl(IPr)(pyridine)(η2-coe) (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) efficiently catalyzes the polyhydrothiolation of aromatic diynes with aliphatic dithiols to give vinylidene-based polymers (vinylidene content of 80%) with high molecular weights (Mw up to 199 000). These polymers have been shown to be chemically modifiable through hydrogenation and chemical oxidation processes.

Rhodium Catalysts for C–S Bond Formation

Sulfur-containing molecules are commonly found in chemical biology, organic synthesis, and materials chemistry. The preparation of these compounds through traditional methods usually required harsh reaction conditions. The use of transition-metal-based catalysts has allowed the development of more efficient and sustainable synthetic processes. Rhodium-catalyzed C–S bond formation through the reaction between sulfur sources such as S8, thiols, or disulfides with organic substrates such as alkynes, allenes, and aryl/alkyl halides is one of the most important methods in the synthesis of thioethers. Here, we summarize recent efforts in the reactions of cross coupling, C–H activation, metathesis, thiolation, carbothiolation, and hydrothiolation for the C–S bond formation catalyzed by rhodium complexes, particularly highlighting the synthetic and mechanistic aspects.

CCDC 1569683: Experimental Crystal Structure Determination

Related Article: Mert Olgun Karatas, Andrea Di Giuseppe, Vincenzo Passarelli, Bulent Alici, Jesus J. Perez-Torrente, Luis A. Oro, Ismail Ozdemir, Ricardo Castarlenas|2018|Organometallics|37|191|doi:10.1021/acs.organomet.7b00750