Cyclic (Alkyl)(amino)carbenes (CAACs) in Ruthenium Olefin Metathesis

IPr# - highly hindered, broadly applicable N-heterocyclic carbenes

IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represents the most important NHC (NHC = N-heterocyclic carbene) ligand throughout the field of homogeneous catalysis. Herein, we report the synthesis, catalytic activity, and full structural and electronic characterization of novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept, including IPr#, Np# and BIAN-IPr#. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420, enabling broad access of the academic and industrial researchers to new ligands for reaction optimization and screening. In particular, the synthesis of IPr# hinges upon cost-effective, modular alkylation of aniline, an industrial chemical that is available in bulk. The generality of this approach in ligand design is demonstrated through facile synthesis of BIAN-IPr# and Np#, two ligands that differ in steric properties and N-wingtip arrangement. The broad activity in various cross-coupling reactions in an array of N–C, O–C, C–Cl, C–Br, C–S and C–H bond cross-couplings is demonstrated. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(i), Rh(i) and Pd(ii) is presented. Given the tremendous importance of NHC ligands in homogenous catalysis, we expect that this new class of NHCs will find rapid and widespread application.

We report novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420.

Cyclic (Alkyl)(amino)carbene Ligands Enable Cu-Catalyzed Markovnikov Protoboration and Protosilylation of Terminal Alkynes: A Versatile Portal to Functionalized Alkenes*

Regioselective hydrofunctionalization of alkynes represents a straightforward route to access alkenyl boronate and silane building blocks. In previously reported catalytic systems, high selectivity is achieved with a limited scope of substrates and/or reagents, with general solutions lacking. Herein, we describe a selective copper-catalyzed Markovnikov hydrofunctionalization of terminal alkynes that is facilitated by strongly donating cyclic (alkyl)(amino)carbene (CAAC) ligands. Using this method, both alkyl- and aryl-substituted alkynes are coupled with a variety of boryl and silyl reagents with high α-selectivity. The reaction is scalable, and the products are versatile intermediates that can participate in various downstream transformations. Preliminary mechanistic experiments shed light on the role of CAAC ligands in this process.

Bimolecular Coupling in Olefin Metathesis: Correlating Structure and Decomposition for Leading and Emerging Ruthenium-Carbene Catalysts

Bimolecular catalyst decomposition is a fundamental, long-standing challenge in olefin metathesis. Emerging ruthenium-cyclic(alkyl)(amino)carbene (CAAC) catalysts, which enable breakthrough advances in productivity and general robustness, are now known to be extraordinarily susceptible to this pathway. The details of the process, however, have hitherto been obscure. The present study provides the first detailed mechanistic insights into the steric and electronic factors that govern bimolecular decomposition. Described is a combined experimental and theoretical study that probes decomposition of the key active species, RuCl2(L)(py)(═CH2) 1 (in which L is the N-heterocyclic carbene (NHC) H2IMes, or a CAAC ligand: the latter vary in the NAr group (NMes, N-2,6-Et2C6H3, or N-2-Me,6-iPrC6H3) and the substituents on the quaternary site flanking the carbene carbon (i.e., CMe2 or CMePh)). The transiently stabilized pyridine adducts 1 were isolated by cryogenic synthesis of the metallacyclobutanes, addition of pyridine, and precipitation. All are shown to decompose via second-order kinetics at -10 °C. The most vulnerable CAAC species, however, decompose more than 1000-fold faster than the H2IMes analogue. Computational studies reveal that the key factor underlying accelerated decomposition of the CAAC derivatives is their stronger trans influence, which weakens the Ru-py bond and increases the transient concentration of the 14-electron methylidene species, RuCl2(L)(═CH2) 2. Fast catalyst initiation, a major design goal in olefin metathesis, thus has the negative consequence of accelerating decomposition. Inhibiting bimolecular decomposition offers major opportunities to transform catalyst productivity and utility, and to realize the outstanding promise of olefin metathesis.

Routes to High-Performing Ruthenium-Iodide Catalysts for Olefin Metathesis: Ligand Lability Is Key to Efficient Halide Exchange

Clean, high-yielding routes are described to ruthenium-diiodide catalysts that were recently shown to enable high productivity in olefin metathesis. For the second-generation Grubbs and Hoveyda catalysts (GII: RuCl2(H2IMes)(PCy3)(=CHPh); HII: RuCl2(H2IMes)(=CHAr), Ar = C6H4-2-O i Pr), slow salt metathesis is shown to arise from the low lability of the ancillary PCy3 or ether ligands, which retards access to the four-coordinate intermediate required for efficient halide exchange. To exploit the lability of the first-generation catalysts, the diiodide complex RuI2(PCy3)(=CHAr) HI-I 2 was prepared by treating "Grubbs I" (RuCl2(PCy3)2(=CHPh), GI) with NaI, H2C=CHAr (1a), and a phosphine-scavenging Merrifield iodide (MF-I) resin. Subsequent installation of H2IMes or cyclic (alkyl)(amino)carbene (CAAC) ligands afforded the second-generation iodide catalysts in good to excellent yields. Given the incompatibility of the nitro group with a free carbene, the iodo-Grela catalyst RuI2(H2IMes)(=CHAr') (nG-I 2 : Ar' = C6H3-2-O i Pr-4-NO2) was instead accessed by sequential salt metathesis of GI with NaI, installation of H2IMes, and finally cross-metathesis with the nitrostyrenyl ether H2C=CHAr' (1b), with MF-I as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.

Ruthenium Complexes Featuring Unsymmetrical N-Heterocyclic Carbene Ligands-Useful Olefin Metathesis Catalysts for Special Tasks

This review describes a distinct class of ruthenium olefin metathesis catalysts featuring unsymmetrical N-heterocyclic carbene (uNHC) ligands, from its historical beginning to the present state of the art. Thanks to advantageous traits, such as pronounced thermodynamic stability, chemical latency, outstanding selectivity, and compatibility with green solvents, these catalysts led to good results in a number of specialized metathesis transformations. Therefore, while being a niche, the uNHC complexes can potentially be implemented in a number of industrial processes, such as valorization of Fischer-Tropsch olefin fractions, ethenolysis of renewable products, and modern pharmaceutical production.