Enyne Metathesis Catalyzed by Ruthenium Carbene Complexes
Ruthenium olefin metathesis catalysts with N-heterocyclic carbene ligands bearing N-naphthyl side chains.
Enyne Metathesis - Organic chemistry
The ethylene-alkyne metatheses are complicated. One might expect ethylene metathesis to proceed faster because ethylene is small and should rapidly bind to the vinyl carbene 133 and increase the rate of turnover. However, the change in rate law for alkyne 118 reveals a change in rate-determining step to step II, alkyne binding/cycloaddition. Once 133 is formed it reacts forward faster than step II (, above). Somewhat surprisingly, ethylene metatheses are slower than cross metathesis with 1-alkenes, even at relatively high ethylene pressures (60-100 psig). Though the rate laws are different for ethylene-118 vs. 1-hexene-118, when the two reactions are run under similar concentrations of reactants and catalyst, the ethylene metathesis is slower. The ethylene metathesis depends on a 14-electron LnRu=CH2 intermediate 131 (R=H). For the second generation Grubbs methylidene, coordination by tricyclohexylphosphine is strong and the dissociative loss of phosphine is slow and unfavorable. This suggests that the resting state has shifted from vinyl carbene complex 134 (1-hexene metathesis) to methylidene complex 130 (R =H). Using a slender alkyne changes the reaction kinetics. In this case, the kinetics show zero-order dependencies on ethylene and alkyne. Apparently, the more slender alkyne propargyl benzoate 129 produces vinyl carbene intermediate 133 which can be captured by phosphine to give resting state 134 (R=H). This could be true if the rate of phosphine release from the 16-electron complex 134 is rate-limiting. Consistent with this proposal, we found that the Ph3P version of the Grubbs complex 2 reacted 3.2 times faster (based on initial rates) in the ethylene metathesis of propargyl benzoate. Further experiments are needed to test these conclusions based on the limited set of rate data obtained so far.
This review provides an overview of ruthenium vinyl carbene reactivity as it relates to enyne metathesis. Methods for the synthesis of metathesis-active and metathesis-inactive complexes are also summarized. Some of the early hypotheses about vinyl carbene intermediates in enyne metatheses were tested in the arena of synthetic chemistry and subsequently led to mechanistic studies. In these two areas, studies from the author's labs are described. There are still many unresolved questions in enyne metathesis that trace back to vinyl carbene reactivity. Hopefully this review will stimulate further investigation into vinyl carbene reactivity which should further refine our understanding of catalytic enyne metathesis.
The intermolecular process is called Cross-Enyne Metathesis, ..
Enyne metathesis has emerged as an important synthetic method to construct conjugated dienes. The enyne metathesis joins an alkene and alkyne reactant together to produce a conjugated 1,3-diene by pi bond reorganization (). The intermolecular enyne metathesis, or ‘cross metathesis,’ is depicted in . This catalytic reaction is promoted by Grubbs' ruthenium carbenes, complexes 1-4 (). Though other late transition metal cycloisomerizations are known, the Grubbs' carbene-promoted reaction is mechanistically-distinct. The high degree of chemoselectivity displayed by carbenes 1-4 explains their widespread use in metathesis. The chemoselectivity for alkenes is known as functional group tolerance, a paramount concern for controlling reactivity as needed in complex molecule synthesis. Though many metal complexes will promote intramolecular enyne bond reorganization, the Grubbs carbenes are by far the most useful carbene catalysts in enyne metathesis because they are functional group tolerant and because they promote intermolecular enyne metathesis. Several reviews are available.[-]
Alkynes react with metal carbenes to produce vinyl carbene intermediates. A major difference between enyne and alkene metathesis is the presence of the alkyne in enyne metathesis. Alkene metathesis involves two types of carbene intermediate: the alkylidene and the methylidene. The alkylidene has the generic structure LnRu=CHR. The methylidene has the general structure LnRu=CH2. Reaction of either of these ruthenium carbenes with alkynes produces vinyl carbene complexes of ruthenium ().
we will focus on this family of catalysts
Vinyl carbenes are unique intermediates in enyne metathesis. The presence of these ruthenium carbenes distinguishes enyne metathesis as a catalytic process uniquely different from alkene metathesis. Accordingly, the mechanism of catalysis involves alkene coordination complexes of vinyl carbenes, and vinyl carbene reactivity will influence the rate of catalysis. Though mechanistic details of catalysis are slowly emerging (vide infra), fundamental reactivity studies on vinyl carbenes are lacking. The purpose of this review is to survey ruthenium vinyl carbene complexes and relate our understanding of their reactivity to our emerging mechanistic view of catalytic enyne metathesis. It is significant to note that most of the known reactivity of ruthenium vinyl carbenes comes from inferences based on their intermediacy in catalysis. For organic chemists, this means that reactivity is deduced from phenomenology when studying the scope of reaction or influence of reaction parameters. Typically this judgment is focused on product yield rather than reaction rates. For organometallic chemists, the structure of vinyl carbenes has been studied. However, the structural study is often not linked to catalytic activity. It is therefore useful to blend both viewpoints of ruthenium vinyl carbene reactivity from these two bodies of literature to generate a cogent view of their structure and reactivity.
The second revolution in ruthenium-catalyzed olefin metathesis was triggered by substituting one phosphine ligand in 1st generation catalysts with a more basic N-heterocyclic carbene (NHC). The activity of these 2nd generation (NHC-bearing) catalysts …
activity of ruthenium-based olefin metathesis catalysts.
This review provides an overview of ruthenium vinyl carbene ..
Ruthenium Olefin Metathesis Catalysts with N-Heterocyclic Carbene Ligands ..
The intermolecular enyne metathesis, or ‘cross ..
30/12/2017 · Ene–yne cross-metathesis with ruthenium carbene ..
Metal carbenes in enyne metathesis: Synthetic and mechanistic ..
Ruthenium carbenes ..
practical catalysts for alkene metathesis
Vinyl substitution has a significant influence on ruthenium carbene reactivity. The vinyl substituent is stabilizing to the electron-deficient carbene center. Presumably the vinyl carbenes are more stable due to conjugation by the vinyl group. Resonance helps to stabilize the electron-deficient carbene carbon, making it less electron-deficient and less reactive than a metal alkylidene (). This stabilizing effect is thought to modulate the reactivity of vinyl carbenes as compared to the alkylidenes encountered in alkene metathesis. The first report of cross enyne metathesis by Blechert interpreted that a slower enyne metathesis reflected the lower reactivity of the vinyl carbenes.
Enyne Metathesis (Enyne Bond Reorganization) - …
Ruthenium vinyl carbenes are significant in their own right because they are the intermediates in catalytic enyne metathesis. Little is known about their reactivity as compared to other metal carbenes. How significant is resonance delocalization? How does substitution influence reactivity? The answers to these questions are mechanistically complicated since there are intrinsic structural effects on stability and structural effects on kinetic reactivity. These may work in concert or oppose each other. For instance, the ethylene-alkyne metathesis can produce two possible vinyl carbene intermediates A and B. Both vinyl carbenes would be stabilized by resonance, however B experiences greater stabilization due to the additional alkyl substitution on the carbene carbon. Despite the expected electronic stabilization of carbene B, it might be more reactive than A due to instability of the cognate 16-electron complex. As a result, there may be a counteracting interplay between electronic and steric factors in vinyl carbene reactivity.
10/03/2004 · Enyne Metathesis (Enyne Bond ..
The organic perspective of ruthenium vinyl carbene reactivity is exemplified in the development of metathesis-based synthetic methodologies. In some cases, an interest in mechanism proved useful to reaction development. For example, my research group focused on isomerization studies as a key to achieving high yields in a tandem enyne metathesis for cyclohexadiene ring synthesis. In this case, the reaction development spawned a deeper interest in the enyne metathesis reaction mechanism by identifying shortcomings in the reaction and guided by hypotheses regarding vinyl carbene reactivity. Eventually, this led to our interest in conducting kinetic and mechanistic studies of enyne metathesis. The original studies focused on the behavior of vinyl carbene intermediates (by studying equilibration and electronic effects). We then pursued mechanistic studies in a collaborative project with Jerome Keister at Buffalo, and have begun to develop a mechanistic picture of catalysis. With this knowledge, my research group has returned to developing new enyne metathesis methodology and has employed mechanistic knowledge to design a useful cyclodiene synthesis based on vinyl carbene equilibration. A goal of metathesis research is high catalyst turnover in the typical functional group-rich environment for which the Grubbs carbenes are most useful. Mechanistic information will lead to a better understanding of substrate limitations and catalyst decomposition pathways. Moreover, carbene catalyst decomposition may be unique in enyne metathesis since it may emanate from vinyl carbene intermediates.
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