Francesco de Angelis 4 AuthorId : Author. Evelina Colacino 3 AuthorId : Author. Clarence Charnay 1 AuthorId : Author. Hide details. Abstract : The ring-closing metathesis RCM of diethyl diallylmalonate in glycerol micellar conditions was studied using microwave irradiation.
The micellization of different cationic surfactants in glycerol was first investigated. The results show the superiority of micellar catalysis in glycerol for a RCM reaction compared to glycerol alone, limiting byproduct formation. In comparison with the classical solution syntheses, the method here described allows safer reaction conditions, less hazardous chemical syntheses, and use of renewable feedstocks. The practical workup, separation, and purification operations minimize the use of materials. Keywords : Gemini surfactants Glycerol Micellar catalysis Ring-closing metathesis Microwaves Critical micellar concentration Gemini surfactants.
The absolute values of strongly correlated rate constants cannot be obtained from fitting. Multiple data sets were imported into Berkeley Madonna and fitted simultaneously batch fitted to the Adjiman model, on the basis that fitting a number of the data sets in Table 2 would yield a set of rate constants that could describe a wider range of conditions. Unfortunately, a fit that described all data sets at once could not be achieved through this batch fitting approach. The rate constants corresponding to the best fit are in Table 3. We refer to the maximum difference between experimental and simulated concentrations during the reaction.
At this point, it is clear that the flexibility of the model with respect to diene concentration is a major limitation; at a give diene concentration, precatalyst concentration can be varied with good fits obtaining vide infra.
Different best fits can be found for each data set; either these are not global but local minima, or the model simply fails to describe a range of conditions. Attention then turned to constraining the value of k 1 in the fitting, reflecting the importance of the precatalyst initiation event. Values of 1. These modest differences in initiation rate render chloroform- d a cost-effective solvent for the study of RCM.
Though the 14e complex 9 is an accepted intermediate on the energy surfaces for metathesis pathways, the barriers to alkene coordination or phosphane recoordination are very small. The absolute value of K for the initiation therefore derives from the ca. The values of k 3 , which represent the rate of a bimolecular decomposition of active catalyst, also varied considerably with solvent.
The decomposition pathway characterized by Hong et al. At this point, the limitations appear to follow from the way in which the model is constructed that is, the differential equations used to set it up. The literature model was also constructed in Micromath Scientist, which provides a number of useful statistics with which to evaluate and compare reaction models, in order to evaluate how flexible the model was with a fixed value for k 1.
The confidence limits for k 3 are so wide and encompass zero as to suggest that its value has very little bearing on the reaction, as would be expected for a bimolecular process occurring under very dilute conditions. Attempts at fitting multiple data sets using Scientist were unsuccessful; fitting the three data sets which had been examined separately yielded a good fit for the mM reaction but underestimated the rates of the and 50 mM reactions by ca.
It is now useful to examine the model in more detail, understand more fully the importance of k 1 and identify the scope and utility of the model in its current form. When spectra were recorded over a 20 ppm chemical shift range, the change in concentration of 1 was seen to be modest during the course of the reaction. Capture of 16 by phosphane is slow because the concentrations of 16 and phosphane are very low; substrates, ethene and products, all competitive and reversibly binding ligands, are all present at much higher concentrations.
The continued decrease in the concentration of precatalyst 1 even after the reaction has finished is due to reaction with ethene, which is known to yield a low energy metallocyclobutane after reaction with two molecules of ethene.
Adjiman and Taylor present a simulation of active catalyst concentration which shows rapid and complete precatalyst dissociation followed by active catalyst decomposition at various rates in each solvent, 10 which is not consistent with experimental observations. Plot of simulated versus measured [ 1 ] after ca. It can be concluded that the rate constants in Table 4 represent the behavior of the precatalyst more effectively, but still do not completely account for its behavior in RCM reactions.
Various values of k —1 , k 2 and k —2 can still be obtained, depending on which data set is fitted, presumably due to the large number of local minima for the fitting method. Further changes to the model are required before a wide range of concentrations can be described. Attempts were therefore made to fit multiple data sets within a narrower range of concentrations and explore the concentration range over which the approximations and simplifications of the model hold.
Rate constants were obtained through data fitting that describe all experiments with — mM 7 Table 6 , entry 1; data sets 1 to 6 in Table 2 and a second set that describe all experiments with 50— mM 7 Table 6 , entry 2; data sets 7 to 12 in Table 2 ; k 1 and k 3 were fixed in the fitting. Rate constants in Table 6 describe experiments at a mM, using entry 1, and b 50 mM, using entry 2, relatively well, even when the precatalyst concentration varies.
There are clear differences between the rate constants required to fit the different concentration regimes, with higher k —1 and k 2 values required to successfully model the lower concentration reactions. The different value of k —1 is required due to the incorrect modeling of the 14e species methylidene 16 does not capture phosphane reversibly and the absence of ethene from the reaction model means that [ethene] is effectively embedded in k —2. However, the rate constants successfully fit different precatalyst loadings and substrate concentrations within these ranges and are therefore useful for predicting the effect of changing these parameters, for example, to predict the lowest loading of 1 that will effect complete reaction within a given time frame.
The extraction of rate constants for different substrates was the original aim of this project, so attention turned to assessing different substrates in RCM reactions quantitatively from reactions conducted at the same initial diene concentration. The values of k 1 1.
The model is therefore useful for deriving quantitative conclusions about the relative rate of RCM of different substrates. This is in stark contrast to heptadiene which produces oligomeric material even at concentrations as low as 10 —2 M, 7 further highlighting the need for a quantitative understanding of the effects of substrate structure on RCM rate and efficiency; trends in RCM rate are not always the same as those in RCM efficiency.
Many studies have been conducted to understand the effects of precatalyst on reaction rate and outcome, 17 , 30 so the ability of the model to predict the effect of changing the precatalyst was investigated. Precatalyst 2 generates the same active species as 1 after the first turnover, so two competition reactions in duplicate in chloroform- d with either 0.
Recently, the timing of events involved in the initiation of Grubbs-Hoveyda second generation precatalyst 2 has been re-evaluated. Vorfalt et al.
Modifying the way in which the initiation behavior is modeled for 2 may therefore be appropriate; an interchange mechanism, eqs 7 — 12 was therefore explored. The model was set up to allow simulation of RCM of binary mixtures of dienes 11 and 13 with 2 , and to see how the model behaved for data obtained with 1. Values of k 1 obtained from initiation kinetics were fixed in the simulations. It does not appear that the same model can be used for both precatalysts, which may suggest strongly that there are significant mechanistic differences between the processes involving 1 and 2.
Nevertheless, these initial successes for the simulation for the interchange mechanism are extremely promising. We have found that the kinetic model published by Adjiman and Taylor significantly simplifies the canonical mechanism for RCM at some cost. Specifically, the treatment of the ruthenium species is not detailed enough, the precatalyst initiation rates obtained in the original paper through data fitting are inconsistent with literature knowledge, the prediction of active catalyst concentration is incorrect, and ethene is not considered at all.
Archive ouverte HAL - Ring closing metathesis reaction in glycerol under microwave activation
Consequently the model does not describe even a modest range of substrate concentrations well. Unfortunately, it is not possible to use a common model for accurate simulation with precatalysts 1 and 2 , consistent with recent findings on the mechanism of initiation of 2.
- Redox-Switchable Catalyst for Ring-Closing Metathesis.
- Ruthenium Olefin Metathesis Catalysts Featuring a Labile.
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These studies show the need for the development of a more robust modeling approach for RCM if kinetic data are to be interpreted successfully and fully. We are currently working on further development and elaboration of these models for applications over wider concentration ranges and with different precatalysts. Both instruments possess temperature control units which maintained the samples at K throughout.
All kinetics experiments were conducted in NMR tubes fitted with pierced caps. Solutions for kinetics experiments were prepared using methods similar to those reported previously. All solutions were prepared in dry volumetric glassware and carefully transferred using dry gastight syringes.
Redox-switchable ring-closing metathesis: catalyst deisgn, synthesis, and study
The internal standard 1,3,5-trimethoxybenzene and precatalysts 1 and 2 were purchased from Sigma-Aldrich and used as supplied. A small ca. NMR spectra were then acquired periodically until after the RCM reaction had finished; a D 1 setting of 35 s was used 5 times the largest T 1 measured: 7 s, see Supporting Information. Spectra were processed using proprietary software.
We thank Dr. John Parkinson and Mr. Gavin Bain for Karl Fischer titrimetric measurements. Kinetic data, sample spectra, rate constants and simulated profiles from data-fitting. National Center for Biotechnology Information , U. The Journal of Organic Chemistry. J Org Chem.