A series of ruthenium olefin metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands with varying degrees of backbone and N-aryl substitution have been prepared. These complexes show greater resistance to decomposition through C-H activation of the N-aryl group, resulting in increased catalyst lifetimes. This work has utilized robotic technology to examine the activity and stability of each catalyst in metathesis, providing insights into the relationship between ligand architecture and enhanced efficiency. The development of this robotic methodology has also shown that, under optimized conditions, catalyst loadings as low as 25 ppm can lead to 100% conversion in the ringclosing metathesis of diethyl diallylmalonate.

A process for the preparation of symmetric and unsymmetric imidazolinium chlorides that involves reaction of a formamidine with dichloroethane and a base (a) is described. This method makes it possible to obtain numerous imidazolinium chlorides under solvent-free reaction conditions and in excellent yields with purification by simple filtration. Alternatively, symmetric imidazolinium chlorides can be prepared directly in moderate yields from substituted anilines by utilizing half of the formamidine intermediate as sacrificial base (b).Since the first isolation of a stable N-heterocyclic carbene (NHC) by Arduengo, 1 their use as ligands in organometallic complexes has become routine. NHCs, as neutral, two-electron donors with little π-accepting character, have replaced phosphines in a variety of applications. 2 Particularly, the use of NHCs as ligands in ruthenium-based olefin metathesis has allowed for great gains in both activity and stability. 3 There is also increasing interest in the use of NHCs as nucleophilic reagents and organocatalysts, with wide application in reactions such as the benzoin condensation, among others. 4NHCs are often prepared in situ via the deprotonation of their corresponding imidazol(in)ium salts (eq 1). 5 Therefore, facile and high-yielding methods for the synthesis of imidazol(in)ium salts are of great interest. The synthesis of unsaturated imidazolium salts, previously optimized by Arduengo et al., involves a one-pot procedure from glyoxal, substituted aniline, formaldehyde, and acid starting materials. 6 Saturated imidazolinium salts, however, are prepared from the reaction of triethyl orthoformate with the corresponding diamine. 7 This approach suffers several drawbacks: the preparation of the diamine generally includes either a palladium C-N coupling or a condensation and reduction sequence (Scheme 1); 8 moreover, purification of the unstable diamine sometimes requires careful chromatography. Unsymmetric imidazolinium salts are especially challenging synthetic targets due to the introduction of the differing substituents. 9Recently, Bertrand et al. developed an alternative ret-rosynthetic disconnection and prepared a range of five-, six-, and seven-membered imidazolinium salts from the addition of "dielectrophiles" to lithiated formamidines. 10 For example, 1,3-dimesitylimidazolinium lithium sulfate was prepared in high yield with 1,3,2-dioxathiolane-2,2-dioxide as the dielectrophile (eq 2).Following Bertrand's report, we reasoned that imidazolinium chlorides could be more easily prepared directly from the reaction of formamidines with dichloroethane (DCE) in the presence of a base. Formamidines are ideal precursors for the preparation of imidazolinium chlorides because they are generally prepared in a one-step solvent-free reaction from materials already utilized in imidazolinium salt synthesis, namely anilines and triethylorthoformate.Herein, we report this new synthetic strategy for the preparation of imidazolinium chlorides under solvent-free reaction conditions and ...

The relative TONs of productive and nonproductive metathesis reactions of diethyl diallylmalonate are compared for eight different ruthenium-based catalysts. Nonproductive cross metathesis is proposed to involve a chain-carrying ruthenium methylidene. A second morechallenging substrate (dimethyl allylmethylallylmalonate) that forms a trisubstituted olefin product is used to further delineate the effect of catalyst structure on the relative efficiencies of these processes. A steric model is proposed to explain the observed trends.The widespread application of olefin metathesis in various fields of chemical synthesis has fueled the continued search for transition metal catalysts that exhibit high reactivity, selectivity, and stability.1 Studies have described the effect on reactivity upon modifying every ligand of ruthenium-based catalysts. Generally, activity is reported as yield or turnover number (TON) for the reaction of a substrate of interest, which does not account for nonproductive metathesis events.The role of nonproductive metathesis in the cross metathesis (CM) of terminal olefins has been studied in detail for early hetero-and homogeneous molybdenum and tungsten catalysts.2 In general, the rate of degenerate metathesis greatly exceeds that of the productive metathesis reactions and evidence suggests the chain-carrying intermediate is a metal alkylidene (M=CHR), not a methylidene (M=CH 2 ). Thus, the TON determined from the amount of product formed is less than the total number of metathesis events that the catalyst has accomplished. An efficient catalyst must perform many turnovers and be selective for productive pathways. Although degenerate reactions do not result in a net change in concentration of the catalyst or the substrate, they can provide additional opportunities for catalyst decomposition, and therefore can decrease efficiency. Recently, Hoveyda and Schrock reported that degenerate processes in asymmetric ring-closing metathesis (RCM) reactions are both prevalent and key to achieving high levels of enantioselectivity.3While measuring conversion of substrate is a common and straightforward method of assessing catalyst activity in olefin metathesis, and other catalytic reactions, degenerate Diethyl diallylmalonate (9) and allylmethallylmalonate (15) have become benchmark substrates for evaluation of olefin metathesis catalysts in RCM.5 Besides the productive pathway, there are at least two potential nonproductive pathways. The first begins with a ruthenium methylidene (10), and forms a β-substituted metallacycle (11); breakdown of this intermediate regenerates the starting material, but exchanges the methylene termini (eq 1). Alternatively, methylene exchange could take place via a α,α-disubstituted metallacycle (13) by coordination of a substrate molecule to ruthenium alkylidene 12 (eq 2).(1)To investigate these productive and nonproductive pathways, diethyl d 2 -diethyldiallylmalonate (9-d 2 ) was prepared and subjected to catalysts 1-8. The conversion to cyclopentene 14 was monitore...

A series of ruthenium catalysts have been screened under ring closing metathesis (RCM) conditions to produce five-, six-, and seven-membered carbamate-protected cyclic amines. Many of these catalysts demonstrated excellent RCM activity and yields with as low as 500 ppm catalyst loadings. RCM of the five-membered carbamate-series could be run neat, the six-membered carbamate-series could be run at 1.0 M concentrations and the seven-membered carbamate-series worked best at 0.2 M to 0.05 M concentrations.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.