Jesús J. Pérez-Torrente

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.

Effective Fixation of CO2 by Iridium-Catalyzed Hydrosilylation†

Financial support from MINECO/FEDER (CTQ2010-15221, CSD2009-00050 CONSOLIDER-INGENIO 2010, CTQ2011-27593, “Ramon y Cajal” (P.J.S.M.) and “Juan de la Cierva” (M.I.) programs), and DGA/FSE (group E7), is acknowledged.

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.

An Alternative Mechanistic Paradigm for the β-Z Hydrosilylation of Terminal Alkynes: The Role of Acetone as a Silane Shuttle

The β-Z selectivity in the hydrosilylation of terminal alkynes has been hitherto explained by introduction of isomerisation steps in classical mechanisms. DFT calculations and experimental observations on the system [M(I)2{κ-C,C,O,O-(bis-NHC)}]BF4 (M=Ir (3a), Rh (3b); bis-NHC=methylenebis(N-2-methoxyethyl)imidazole-2-ylidene) support a new mechanism, alternative to classical postulations, based on an outer-sphere model. Heterolytic splitting of the silane molecule by the metal centre and acetone (solvent) affords a metal hydride and the oxocarbenium ion [R3Si-O(CH3)2](+), which reacts with the corresponding alkyne in solution to give the silylation product [R3Si-CH=C-R](+). Thus, acetone acts as a silane shuttle by transferring the silyl moiety from the silane to the alkyne. Finally, nucleophilic attack of the hydrido ligand over [R3Si-CH=C-R](+) affords selectively the β-(Z)-vinylsilane. The β-Z selectivity is explained on the grounds of the steric interaction between the silyl moiety and the ligand system resulting from the geometry of the approach that leads to β-(E)-vinylsilanes.

Mild and selective H/D exchange at the β position of aromatic α-olefins by N-heterocyclic carbene-hydride-rhodium catalysts

Financial support from the MICINN of Spain (project numbers CTQ2009-08089 and CTQ2010-15221), the ARAID Foundation under the program “jovenes investigadores”, and CONSOLIDER INGENIO-2010 program, under the projects MULTICAT (CSD2009-00050) and Factoria de Cristalizacion (CSD2006-0015) is gratefully acknowledged. R.C. thanks the CSIC and the European Social Fund for his Research Contract in the framework of the “Ramon y Cajal” program.

Rhodium complexes of the binucleating ligands pyridine-2-thiolate and benzothiazole-2-thiolate. Crystal structures of [{Rh(µ-SC5H4N)(CO)2}2] and [{Rh(µ-SC5H4N)(tfbb)}2]·Me2CO (tfbb = tetrafluorobenzobarrelene)

The binuclear complexes [(Rh)µ-SC5H4N)(diolefin)}2][SC5H4N = pyridine-2thiolate, diolefin = cycle-octa-1,5-diene (cod)(1), norborna-2,5-diene (nbd)(2), or tetrafluorobenzobarrelene (tetrafluorobenzo[5,6] bicyclo[2.2.2]octa-2,5,7-triene)(tfbb)(3)] are prepared by reaction of LiSC5H4N with the appropriate complex [{Rh(µ-Cl)(diolefin)}2] and show fluxional behaviour in solution associated with the bridging ligands. The related compounds [{Rh(µ-C7H4NS2)(diolefin)}2][C7H4NS2= benzothiazole-2-thiolate, diolefin = cod(5), nbd(6), or tfbb(7)] are prepared by a similar route. Carbonylation reactions of (1) and (5) give the tetracarbonyl complexes [{Rh(µ-L)(CO)2}2][L = SC5H4N (4) or C7H4NS2(8) respectively]. Triphenylphosphine replaces carbon monoxide stepwise in compound (4) yielding the mono-(10) and di-substituted (11) complexes whilst the disubstituted complex [{Rh(µ-C7H4NS2)(CO)(PPh3)}2](9) is obtained from compound (8). Methyl iodide adds to complexes (4) and (10) affording respectively the diacetyl complexes [{Rh(µ-SC5H4N)(COMe)I(CO)}2](12) and [{Rh)(µ-SC5H4N)(COMe)l}2(CO)(PPh3)](13). The molecular structures of (3) and (4) have been determined by X-ray analyses. Crystals of (3) are triclinic, space group P, with a= 13.772(7), b= 14.184(8), c= 9.738(4)A, α= 108.99(2), β= 75.65(3), γ= 106.04(3)°, and Z= 2. Crystals of (4) are orthorhombic, space group P21212 (no, 18), with a= 14.637(6), b= 6.734(4), c= 8.852(5)A, and Z= 2. Both complexes are binuclear with two pyridine-2-thiolate groups acting as bridges. In (4) both ligands, having a head-to-tail disposition, bridge the metal centres through the N and S atoms whereas in (3) one of the pyridine-2-thiolate ligands bridges through the S atom only. The square-planar environments of the Rh atoms are completed by two tfbb ligands [(3)] or four CO groups [(4)], with metal–metal separations of 3.028(2) and 2.941 (2)A respectively.

Oxidation and β-Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N-Heterocyclic Carbene Ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for CC bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)-cyclooctadiene complexes having a NHC ligand with a O- or N-functionalised wingtip efficiently catalysed the oxidation and β-alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH3 )(cod)(MeIm(2- methoxybenzyl))][BF4 ] (cod=1,5-cyclooctadiene, MeIm=1-methylimidazolyl) having a rigid O-functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF0 ) of 1283 h(-1) , and also in the β-alkylation of 2-propanol with butan-1-ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan-2-ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross-aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new CC bond that involves the reaction of an O-bound enolate generated in the basic medium with the electrophilic aldehyde.

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.

Trinuclear angular aggregates of rhodium: synthesis and crystal structures of [Rh3(µ3-SC5H4N)2(CO)6][ClO4](SC5H4N = pyridine-2-thiolate) and [Rh3(µ3-C7H4NS2)2(CO)2(PPh3)2(tfbb)][ClO4](C7H4NS2= benzothiazole-2-thiolate, tfbb = tetrafluorobenzobarrelene)

The binuclear complexes [{Rh(µ-L)L′2}2]{L = pyridine-2-thiolate (SC5H4N–) or benzothiazole-2-thiolate (C7H4NS2–); L′2= cyclo-octa-1,5-diene (cod), norborna-2,5-diene (nbd), tetrafluorobenzobarrelene (tetrafluorobenzo[5,6]bicyclo[2.2.2]octa-2,5,7-triene, tfbb), (CO)2, or (CO)(PPh3)} react with the appropriate species cis-[RhL′2(Me2CO)x]+ to give trinuclear aggregates [Rh3(µ3-L)2(L′2)3][ClO4]. A study of this reaction has led to the controlled synthesis of a single isomer of the complexes [Rh3(µ3-C7H4NS2)2(CO)2(PPh3)2L′2][ClO4]. The trinuclear complexes have been characterized by 1H, 31P n.m.r., and u.v.-visible spectroscopy and in the case of [Rh3(µ3-SC5H4N)2(CO)6][ClO4]·0.5CH2Cl2(4) and [Rh3(µ3-C7H4NS2)2(CO)2(PPh3)2(tfbb)][ClO4](12) by single-crystal X-ray diffraction methods. In both structures trinuclear cationic rhodium complexes are present, in which two pyridine-2-thiolate[(4)] or benzothiazole-2-thiolate[(12)] ligands, acting as triple bridges through the nitrogen and one sulphur atom, interact with all three metal atoms, which are in a bent arrangement. Carbonyl ligands [(4)] and carbonyl, PPh3, and tfbb (through the two double bonds) ligands [(12)] complete the slightly distorted square-planar co-ordination of the Rh atoms.

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.