Ruthenium carbene complexes initiator

NHC-bearing ruthenium carbene complexes are used to design latent and switch-able initiators. In this research field emphasis is given to the development of initiators, which are inactive at room temperature in the presence of the corresponding monomer and can be activated upon a proper stimulus such as heat [130] or light [131]. Once activated, a high polymerisation activity is desired which is provided by NHC co-ligands [132]. 

A proposed reaction pathway is shown in Scheme 7.29, where either the aromatic carbon or oxygen atom of naphthol may work as a nucleophile. Thus, the first step is the nucleophilic attack of the carbon atom of 1 -position of 2-naphthol on the C. atom of an allenylidene complex A to give a vinylidene complex B, which is then transformed into an alkenyl complex C by nucleophilic attack of the oxygen atom of a hydroxy group upon the Co, atom of B. Another possibility is the nucleophilic attack ofthe oxygen of 2-naphthol upon the Co, atom of the complex A. In this case, the initial attack of the naphthol oxygen results in the formation of a ruthenium-carbene complex, which subsequently leads to the complex B via the Claisen rearrangement of the carbene complex. 

Furthermore, ruthenium-carbene complexes are highly tunable, well-defined, single-site, homogeneous catalysts. These characteristics provide the ability to access all catalytically active sites and thus to influence catalyst initiation, propagation, and stability properties. The relatively simple ligand environment of Ru-2, Ru-4, and... 

A series of so-called Grubbs ruthenium—carbene complexes (Ru-12) can mediate living radical polymerization of MMA and styrene to afford controlled polymers with narrow MWDs (MJMn 1.2).63 66 The polymerization apparently proceeds via a radical mechanism, as suggested by the inhibition with galvinoxyl. For example, a novel ruthenium—carbene complex (Ru-13) carries a bromoisobutyrate group and can thus not only initiate but also catalyze living radical polymerization of MMA without an initiator.67... 

Thus, reaction of Cl2Ru(PPh3)3 or < 2 11(1 1 13)3 with 2,2-diphenylcyclopropene in benzene or methylene chloride yields the desired ruthenium carbene complex in quantitative yield. Typical alkylidene resonances for ll and are observed at =17.94 and 288.9 ppm (both in C, )(,). Despite a ratio of kj/kp <1, the compound was reported to be an efficient initiator for the polymerization of NBE. The rather low activity of the bis(triphenylphosphine) derivative for other cyclic olefins than NBE such as bicyclo[3.2.0]hept-6-ene or trans-cyclooctene was successfully enhanced by phosphine exchange with more basic analogues such as tricyclohexyl-phosphine and tri-i-propylphosphine (Scheme 5.12) [164]. 

With the work by Grubbs et al. [27] and Herrmann et al. [28], the use of ruthenium carbene complexes as homogeneous catalysts for the ROMP (Ring-Opening Metathesis Polymerization) of olefins was estabhshed (see Section 2.4.4.3). The development of catalysts that can catalyze hving polymerization in water was an important goal to achieve, especially for applications in biomedicine. In this context, two water-soluble ruthenium carbene complexes (3 and 4) have been reported that act as initiators for the living polymerization of water-soluble monomers in a quick and quantitative manner [29]. 

Treatment of an alkyne/alkene mixture with ruthenium carbene complexes results in the formation of diene derivatives without the evolution of byproducts this process is known as enyne cross-metathesis (Scheme 22). An intramolecular version of this reaction has also been demonstrated, sometimes referred to as enyne RCM. The yield of this reaction is frequently higher when ethylene is added to the reaction mixture. The preferred regiochemistry is opposite for enyne cross-metathesis and enyne RCM. The complex mechanistic pathways of Scheme 22 have been employed to account for the observed products of the enyne RCM reaction. Several experiments have shown that initial reaction is at the alkene and not the alkyne. The regiochemistry of enyne RCM can be attributed to the inability to form highly strained intermediate B from intermediate carbene complex A in the alkene-first mechanism. Enyne metathesis is a thermodynamically favorable process, and thus is not a subject to the equilibrium constraints facing alkene cross-metathesis and RCM. In a simple bond energy analysis, the 7r-bond of an alkyne is... 

Scheme 25) was observed when cyclic alkenes (e.g., 214) were treated with ruthenium carbene complex 18 in the presence of terminal alkynes (e.g., 215). A mechanism involving initial ROM, followed by alkyne insertion of the intermediate carbene complex, followed by ROM from intermediate 217, was proposed. In order to account for the unexpectedly high yield (the yield is higher than the anticipated E Z selectivity in the formation of 217) of the process, a second source of the observed product involving metathesis of an additional mole of cyclopentene from intermediate 217 was suggested. 

Because the key to control of molecular weight distribution depends on the relative rates for initiation and propagation, studies to control these relative rates have been conducted. Studies with the Grubbs-type ruthenium carbene complexes have shown that the ruthenium benzylidene complexes undergo faster initiation than vinyl alkylidene... 

Although intermolecular enyne metathesis is the simplest to envision, intramolecular enyne metathesis was the major focus of the initial work. Two representative intramolecular enyne meta theses are shown in Equations 21.44 and 21.45. The reaction in Equation 21.44 shows the value of this chemistry to form heterocycles. The reaction in Equation 21.45 shows how enyne metathesis can be used in combination with olefin metathesis to form bicyclic products. The initial enyne metathesis process in Equation 21.45 terminates in a ruthenium carbene complex. The carbene complex is then trapped by the remaining olefin in a [2+2] and retro-[2+2] cycloaddition sequence to generate the bicyclic organic product and a ruthenium carbene complex that re-enters the catalytic cycle by reaction witii the yne diene. 

In conclusion, it is clear that the two novel classes of ruthenium carbene complexes, the arylthio substituted ruthenium carbenes (chap. 2.3) and the 2-pyridylethanyl substituted carbenes (this chapter) are excellent initiators for the solvent-free polymerization of DCPD. It is possible to perform so-called "one-shot-full-cure" polymerizations in a preheated mold with polymerization times of less than 1 minute. Desired geltimes and inititation temperatures can be fine tuned by changing the substitution pattern on the pending ligands. Experiments of using these novel catalysts in other applications are currently in progress. 

The Grubbs ruthenium-carbene complexes (2 and 3) exhibit high reactivity (albeit lower than the best molybdenum- and tungsten-based catalysts) in a variety of metathesis processes while showing a remarkable tolerance towards many different organic functionalities. These initiators are stable for weeks and reactions can be carried out in the presence of air and humidity or even in water. 

The Grubbs pyridine solvates are the fastest initiators of alkene metathesis and are valuable as synthetic intermediates to prepare other ruthenium carbene complexes. In particular, the 18-electron pyridine solvates 4a,b are very fast initiators that were developed to catalyze difficult alkene metatheses (e.g., the cross metathesis of acrylonitrile) [6]. The rates of initiation for several complexes are provided in Table 9.9. The pyridine solvate 4a has been found to initiate about 105 times faster than the parent Grubbs complex 2 and at least 100 times faster than the second-generation triphenylphosphine variant 26. When compared with the Hoveyda-Blechert complex 3a, 4a initiated about 100 times faster (c entry 3 vs. entry 5). The bromopyridine solvate 4b exceeded all of these in its initiation rate it was at least 20 times more reactive than 4a. 

Modification of the Piers ruthenium carbene complex was performed through anion exchange as well as different phosphine substituents to evaluate their effects on initiation and stabihty. For RCM performed at 0"C, these complexes were found to exhibit similar initiation rates, indicating that the anion is non-coordinating and plays no role in the initiation step [40]. 

Ivin, K. J. Kenwright, A. M. Khosravi, E. Hamilton, J. G Ring-opening metathesis polymerization of 7-methylbicyclo[2.2.1]hepta-2,5-diene initiated by well-defined molybdenum and ruthenium carbene complexes. J. Organomet. Chem. 2000, 606, 37-48. 

Vinyl ethers are known to react with ruthenium carbene complexes to give the so-called Fischer carbenes which show greatly reduced olefin metathesis activity [13]. Therefore, ethyl vinyl ether is typically use din the nonfunctional termination of most living ROMP reactions using ruthenium initiators. Figure 3.2. When substituted vinyl ethers are employed, functional groups or even complex molecules can be transferred onto the polymer chain end in one step. This type of functional termination reaction works only for ruthenium carbene complexes because titanium, molybdenum, and tungsten carbenes are tolerant toward vinyl ethers. 

The terminal cross metathesis (CM) reaction, as depicted in Figure 3.5, is probably the most straightforward synthetic method to introduce complex molecular fragments or functional groups to a ROMP polymer chain end. The propagating ruthenium carbene complex typically reacts with an acyclic olefin in a CM reaction. The newly generated carbene complex is still metathesis-active, and can in principle undergo secondary metathesis reactions or initiate the polymerization of the residual monomer. 

In a sacrificial synthesis, cyclic monomers containing a cleavable group are incorporated into a block copolymer structure, as shown in Figure 3.7 [47]. A propagating ruthenium carbene complex is used as a macro initiator for the polymerization of the cleavable cyclic monomer to form a diblock copolymer. The polymer block composed of the cleavable monomer can be broken down into low molecular weight fragments (sacrificed), leaving just one functionality at the chain end of the first polymer block. 

First synthetic attempts with compound 11 using catalyst [Ru]-VII [17] delivered the undesired bicyclic structure 12, which results from the initial attack of the ruthenium carbene complex at the terminal olefin via intermediate 13 (Scheme 11.5). To direct the initiation of the catalytic cycle into the other double bond, through intermediate 14, Honda et al. decided to introduce steric hindrance using a disub-stituted olefin in the alkyl chain. Their results showed, however, that the resulting compound was not reactive enough to undergo metathesis under the chosen reaction conditions. With the use of a more electron-rich allyl ether moiety such as in 15, it was possible to isolate the desired product in 74% yield and successfully complete the synthesis of securinine. Ruthenium complexes [Ru]-I and [Ru]-II were... 

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