A Practical and Highly Active Ruthenium-Based Catalyst that Effects the Cross Metathesis of Acrylonitrile

Abstract

The N-heterocyclic carbene-coordinated ruthenium benzylidene complex [(H 2 IMes)(PCy 3 )(Cl) 2 Ru¼CHPh] (1) is a highly active catalyst for a wide variety of olefin-metathesis reactions, [1] including those with sterically demanding [2] and electron-deficient olefins (Scheme 1). [3] In spite of recent advances, there are several processes that remain challenging, such as olefin cross metathesis (CM) with directly functionalized olefins. [4] For example, acrylonitrile CM has only been successful with Schrock©s arylimido molybdenum alkylidene catalyst [5] and the ether-tethered ruthenium alkylidene derivative [(H 2 IMes)(Cl) 2 Ru¼CH(o-iPrOC 6 H 4 )] (4). [6,7] Phosphane-ligated ruthenium catalysts have given poor results for this transformation, [3a, 5c, 6c, 8, 9] except for one report of efficient CM between purified acrylonitrile and 1-decene mediated by 1. [10] We have determined that dissociation rates of ligands are related to catalyst efficiency during CM with acrylonitrile. On this basis, we have developed a new, highly efficient ruthenium complex to perform acrylonitrile CM with unpurified acrylonitrile; this catalyst is the fastest initiator of any ruthenium-based catalyst reported to date. [11] Previous studies have shown that precatalysts of the type [L 2 X 2 Ru ¼ CHR] initiate by dissociating one L-type ligand before entering the catalytic cycle (Scheme 2); [12] in complexes 1±3, L is a phosphane (PR 3 ), and in complex 4, L is a tethered ether ligand (iPrO). Importantly, complexes 1±4 all provide the same propagating species (A and B) after a single turnover. [13] If either A or B is trapped by L, dissociation of L must occur before catalysis can continue. The relative affinity of A and/or B for the olefin in preference to L (i.e., favoring propagation) controls how long these species remain in the catalytic cycle. Consequently, the differences in activity between catalysts 1±4 depend on their rates of initiation and rebinding of L, both of which can be tuned by the nature of L. [12b, 14±16] Complexes 1±4 are all active for a variety of metathesis processes, such as CM, ring-closing metathesis (RCM), and ring-opening-metathesis polymerization (ROMP). However, the situation involving acrylonitrile CM is more complex; CM between acrylonitrile and allylbenzene proceeds efficiently with 4 (68 % yield) but not with 1±3 (21 %, 35 %, and 36 % yields, respectively; Table 1). [17] Clearly, this difference cannot simply be an issue of precatalyst initiation; 2 and 4 have roughly the same initiation rates and initiation is faster with 3 than 1, 2, or 4 Table 2. The difference is also not a result of COMMUNICATIONS

Effects of Grafting Density on Block Polymer Self-Assembly: From Linear to Bottlebrush

“…Grubbs' secondgeneration catalyst [(H2IMes)(PCy3)(Cl)2Ru=CHPh] was provided by Materia, and G3 was prepared according to the reported precedure. 66 CH2Cl2 was dried by passing through an activated 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 ...…”

Recent advances in ring-opening metathesis polymerization, and application to synthesis of functional materials

“…Thus, 13, the Grubbs third-generation catalyst, has been developed as a new class of Ru-based metathesis catalysts. 36 Complex 13 contains a strongly ligating NHC and weakly coordinating brominated pyridines, and features high activity in ROMP, based on the labile nature of the pyridine ligand. 37 It catalyzes living ROMP of norbornene and its derivatives to give polymers with small PDIs (o1.10) due to high initiation rates, and is used for preparation of block copolymers.…”

Stereocomplex Formation of Densely Grafted Brush Polymers

“…All other solvents and chemicals were used without further purification unless otherwise stated. 1 H NMR spectra were recorded at room temperature on a Varian Inova 500 (at 500 MHz). The NMR spectra were analyzed on MestReNova software and are reported relative to CDCl 3 (δ 7.26).…”

“…Then the initiator/catalyst solution was added rapidly to the monomer solution and the whole solution allowed to stir for 10 min before being precipitated into MeOH. 1 …”