Ricardo Castarlenas

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.

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.

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 CH Bond Functionalization of 2‐(2‐Thienyl)pyridine by a Rhodium N‐Heterocyclic Carbene Catalyst

[Rh(μ‐Cl)(H)2(IPr)]2 (IPr=1,3‐bis‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) catalyzes the selective functionalization of 2‐(2‐thienyl)pyridine efficiently with a range of alkenes and internal alkynes. A catalytic cycle is proposed on the basis of the identification of key reaction intermediates and the study of their reactivity by NMR spectroscopy. Theoretical calculations at the DFT level support that the reaction proceeds by initial κ1N coordination of 2‐(2‐thienyl)pyridine followed by the loss of H2 to afford the active catalyst. Subsequently, cyclometalation of 2‐(2‐thienyl)pyridine, coordination of the unsaturated substrate (alkyne or alkene) at the vacant position trans to the hydride, and reductive elimination of the thiophene moiety occur. Finally, cyclometalation of the thiophene moiety renders the hydride cis to the unsaturated substrate, which leads to migratory insertion into the RhH bond and subsequent reductive elimination of the functionalized product.

Nazarov Type Cyclization on an Osmium-Dienylcarbene Complex

The Nazarov reaction is an acid-catalyzed 4pi-electrocyclic ring closure of dienylketones, which affords cyclopentenones. This type of cyclization has been increasing in interest over the years, due to the importance of the construction of five-membered rings in the synthesis of natural products. However, one potential problem is that the carbonyl group necessary for the cyclization to occur may not be required in the final synthetic target and can sometimes be difficult to remove or modify. One possible solution is to design analogous reactions which do not suffer the carbonyl dependence.

gem-Selective Cross-Dimerization and Cross-Trimerization of Alkynes with Silylacetylenes Promoted by a Rhodium–Pyridine–N-Heterocyclic Carbene Catalyst

The gem‐selective cross‐dimerization and ‐trimerization of silylacetylenes with alkynes through CH activation using a rhodium(I)–pyridine–N‐heterocyclic carbene catalyst have been developed. This reaction is applied to various aliphatic or aromatic terminal alkynes, internal alkynes, and gem‐1,3‐disubsituted enynes to afford the corresponding enynes and dienynes with high regio‐ and stereoselectivities and in good isolated yields (up to 91 %).

CH activation of methyl vinyl ketone in Ir(acac){η2-CH2CHC(O)CH3}(PCy3)

Abstract The cyclooctene complex Ir(acac)(cyclooctene)(PCy 3 ) ( 1 ) reacts with methyl vinyl ketone to give Ir(acac){ η 2 -CH 2 CHC(O)CH 3 }(PCy 3 ) ( 2 ) and cyclooctene. In benzene at 70°C, complex 2 affords by means of an intramolecular CH activation process the thermodynamically favored alkenyl derivative I r(acac)H{(Z)-CHCHC(O )CH 3 }(PCy 3 ) ( 3 ), which was isolated as a mixture of the isomers 3a (PCy 3 trans to carbonyl group of alkenyl ligand) and 3b (PCy 3 trans to acac). Isomers 3a and 3b do not react with tricyclohexylphosphine. However, the complex Ir(acac)H{( E )-CHCHC(O)CH 3 }(PCy 3 ) 2 ( 4 ) can be obtained by treatment of 2 with the phosphine in benzene at 70°C. Complex 2 also reacts with HBF 4 at -78°C, to give the five-coordinate hydrido derivative [Ir(acac)H{ η 2 -CH 2 CHC(O)CH 3 }(PCy 3 )]BF 4 ( 5 ). In solution, complex 5 is only stable at temperatures lower than −40°C. At room temperature and in the presence of acetonitrile, it evolves into the alkyl compound [I r(acac){CH 2 CH 2 C(O )CH 3 }(NCCH 3 )(PCy 3 )]BF 4 ( 6 ), as a result from the selective anti -Markovnikov insertion of the carboncarbon bond of the activated olefin into the IrH bond of the 5 .