Rapidly Initiating Ruthenium Olefin‐Metathesis Catalysts

Abstract

Olefin metathesis is arguably the most powerful carboncarbon bond breaking and making reaction in chemical synthesis.[1] Depending on the nature of the reacting partners, olefin metathesis can be used for ring-opening polymerization (ROMP), [2] to create advanced polymeric materials, [3] transformation of acyclic diene substrates into complex cyclic organic molecules (in ring-closing metathesis (RCM)) [4] or polymers (in acyclic diene metathesis (ADMET)) [5] or in cross metathesis (CM) to generate unsymmetrical olefins. [6,7] Although olefin metathesis is fully reversible, RCM, ADMET, and CM rely on the elimination of ethylene, the simplest olefin, as a thermodynamic driving force. Used by itself or in tandem with other synthetic transformations, [8,9] olefin metathesis is a versatile method for the modern synthetic chemist.It is generally acknowledged that a metal carbene species, {L n M=CRR'}, is required and that interaction with an olefin substrate leads to four-membered metallacyclobutane intermediates or transition states, {L n M(CRR') 3 }, by a 2+2 cycloaddition; cleavage of this intermediate in the opposite sense by which it was formed leads to olefin metathesis, creating a new carbon-carbon double bond and regenerating an active metal carbene. Metal carbenes are generally classified as being nucleophilic (electron rich) or electrophilic (electron poor) in character at the carbene carbon atom, but an effective olefin-metathesis catalyst exhibits behavior between these two extremes. Two carefully tuned classes of mediator have evolved into the catalysts of choice for olefin metathesis. Schrock catalysts [10,11] are molybdenum-or tungsten-based alkylidenes with a fairly specific ligand set designed to modulate the properties of the carbene (Figure 1). These catalysts display high activities and stabilities, but are sensitive to ambient air and moisture and are relatively intolerant of polar functionalities. The Grubbs-catalyst portfolio [12] consists of a variety of ruthenium-based systems of general formula [Cl 2 (L)(L')Ru = C(H)R] (compounds 1) which are significantly more functional-group tolerant, but do not exhibit the same levels of activity or longevity as the Schrock catalysts.

Assignment of pre-edge features in the Ru K-edge X-ray absorption spectra of organometallic ruthenium complexes

“…If, by contrast, the apical benzylidene ligand in 1a,b is replaced with a terminal carbide, as in 3a,b, the effect is a substantial increase in the pre-edge intensity. This effect corresponds with the much shorter M-C bond in the triplybonded carbides (3b: 1.650 Å [20]) as compared to the benzylidenes (1a: 1.838 Å [21], 1b: 1.835 Å [23], 4b: 1.873 Å [21]) and phosphonium alkylidenes (2b: 1.817 Å [19]). …”

“…4-nitropyridine was purchased from AlfaAesar and dried under vacuum. Compounds 2a, 2b, 3a, and 3b were synthesized as described elsewhere [19,20] and obtained from Warren E. Piers.…”

“…The square-pyramidal Grubbs complexes [17,18] (1a,b) as well as three related classes of compounds were examined (Table 1). Relative to the Grubbs complexes, the Piers phosphonium alkylidenes [19] (2a,b) have a phosphine transferred onto the alkylidene carbon while the Heppert carbides [20] (3a,b) contain a triple bonded carbide in place of the benzylidene. Two pyridine ligands replace one phosphine resulting in the octahedral Grubbs bispyridine complexes [21,22] (4-7a,b).…”

Improving Grubbs’ II type ruthenium catalysts by appropriately modifying the N-heterocyclic carbene ligand

“…15 To further assess the reactivity of complexes 2a-e, we turned our attention to the most challenging RCM reaction, the one leading to tetrasubstituted olefins ( Figure 5). Building upon recent reports 115 describing the beneficial effects of C 6 F 6 in these difficult RCM …”

Ring closing metathesis employing organometallic substrates and the templated synthesis of macrocycles

“…As well, olefin metathesis is now firmly entrenched in polymer chemistry [1,10]. In addition to the original catalysts, a number of research groups have actively ଝ A portion of this contribution was presented as a lecture at the 16th Interna- pursued the development of either derivative or novel systems that have opened new areas in this chemistry by improving activity, chemo-, regio-and stereo-selectivity as well as addressing important factors such as catalyst stability, ease of synthesis, recycling and functional group tolerance; [1][2][3][4][5][6][7][8][9][11][12][13][14][15][16] see Fig. 1 for a small selection of metathesis catalysts with some selected properties.…”