Kinetics Metathesis catalysts

The above studies are consistent with the hypothesis that the metathesis reaction itself brings about cis-trans isomerization (46). This hypothesis is further supported by the results of a kinetic study of the reactions of the three linear butenes on the metathesis catalyst Mo(CO)6-A1203 by Davie et al. (107), who concluded that cis-trans isomerization for their system is a bimolecular reaction. 

As was mentioned previously, certain disubstituted styrene ethers can be efficiently resolved through the Zr-catalyzed kinetic resolution. As illustrated in Eq. 7, optically pure cycloheptenyl ether 64c is obtained by the Zr-catalyzed process. The successful catalytic resolution makes the parent alcohol and the derived benzyl ether derivatives 64a and 64b accessible in the optically pure form as well. However, this approach cannot be successfully applied to all the substrates shown in Table 1. Lor example, under identical conditions, cyclopentenyl susbstrate 60b is recovered in only 52% ee after 60% conversion. Cycloheptenyl substrates shown in entry 4 undergo significant decomposition under the Zr-catalyzed carbomagnesation conditions. These observations indicate that future work should perhaps be directed towards the development of a chiral metathesis catalyst that effects the chromene formation and resolves the two styrene ether enantiomers simultaneously. 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

Hocker, H., Ring-opening Polymerisation of Cycloolefins by Means of Metathesis Catalysts Kinetic and Thermodynamic Effects on the Product Distribution , Makro-mol. Chem. Macromol. Symp., 6, 47-52 (1986). 

Wagener, K.B. Lehman, S.E. Comparison of the kinetics of acyclic diene metathesis promoted by Grubbs ruthenium olefin metathesis catalysts. Macromolecules 2002, 35, 48-53. 

Olefin metathesis has been extensively written on in both books and journals [1-10]. This chapter will focus on ADMET. Of particular interest are the issues of catalysis, mainly functional group tolerance, kinetics, and mechanistic details. The development of late-transition metal catalysts has enormously expanded the scope of ADMET, so particular attention will be given to the well-defined ruthenium-based olefin metathesis catalysts. Pertinent information pertaining to catalysts of Group VI metals will also be provided. Important procedural aspects of ADMET will be presented in conclusion. 

In order to give perspective on the kinetic versus thermodynamic balance, the cis-trans selectivity of some commonly utilized Ru metathesis catalysts is presented. A number of catalysts with modified ligands that result in a distinct stereochemical preference are then compared with these original catalysts and their reactivity discussed. Finally, the successful implementation of ligand-driven selectivity has led to three families of Ru-based metathesis catalysts that can perform Z-selective metathesis. For each of these catalyst families, a model for the origin of Z-selectivity, the role of ligands in influencing stereochemistry and trends in their reactivity are examined. 

As mentioned above in Section 1.25.5.2, rhodium and iridium pincer complexes have been used to catalytically dehydrogenate alkanes, giving terminal olefins as the kinetic products. In a recent report by Goldman and Brookhart, the iridium Pincer complexes were combined with Schrock s alkylidene metathesis catalyst... 

The pentacarbonylrhenium halides, first prepared by Hieber, are starting materials for the syntheses of many novel rhenium carbonyl compounds. Photochemical, vibrational, and kinetic " properties of these molecules have been studied. A rhenium carbonyl halide-alkyl aluminum halide system polymerizes acetylene and is a useful olefin-metathesis catalyst. " ... 

In a subsequent report, the CAAC-based metathesis catalysts were examined for selectivity in the formation of Z E olefins, as well as their activity for ethenolysis [12]. Both of these processes require kinetic selectivity to produce the thermodynamically less-favored Z and terminal olefins, respectively. It was discovered that the CAAC catalysts displayed improved conversion to the Z olefin EIZ= 1.5—2.5 after 70% conversion) for the cross metathesis of cis-l,4-diacetoxy-2-butene with allylbenzene, relative to that observed using the classical NHC- and phosphine-based systems ElZ = 3-4) at comparable conversion (Scheme 4.3). 

Previous studies on allenylidene-ruthenium complexes as alkene metathesis catalysts revealed that on thermal reaetion they produced a new active species that was also evidenced by kinetic studies and spectroscopic observations [44]. This species was identified arising from another observation the profitable influence of strong acid addition [11], Thus the RCM of A,A-diallyltosylamide led to a TOF of 10.5/h with complex 7a (80 C, 3 h, 70 %) and to a TOF of 53/h (room temperature, 1 h, 75 %) when five equivalent of TfOH or HBF4 were added to complex 7a. More drammatically, the ROMP of eyclooctene with 7a was achieved at room temperature in 15 h (TOF = 63/h), whereas only 1 min was necessary when five equivalent of TfOH were added to 7a (TOF=57.200/h) (Table 5). It was clear that strong acid addition promoted the generation of a new very active species. 

Herein, we report the syntheses of new metathesis catalysts Cl2-Ru-(py)2(3-phenylindenylidene) 4a-c. Kinetic studies enabling thorough evaluation of their stabilities and catalytic activities in RCM are also presented. 

Nonetheless, ADMET is a versatile technique that allows the incorporation of a wide variety of functional groups into the resultant polymers. Scheme 1.9 shows the catalytic cycle of ADMET, controlled by the metathesis catalyst, which can be either ruthenium- [76, 77] or molybdenum-based [78, 79]. While the kinetics are controlled by the catalyst (there is no reaction in its absence), it still follows the kinetic picture described in Section 1.3.2. This is because the catalyst is removed from the chain end after each successful alkene metathesis reaction (i.e., coupling) and the olefin with which it subsequently reacts is statistically random. 

Accordingly, considerable effort has been dedicated to the development of olefin metathesis catalysts exhibiting kinetic selectivity. As a result, a number of Z-selective tungsten-, molybdenum-, and ruthenium-based olefin metathesis catalysts have been recently developed (For Mo- and W-based Z-selective catalysts [24-41], For Ru-based Z-selective catalysts [42-45], For cyclometalated Ru-based Z-selective catalysts [46-58]). Many of these systems exhibit consistently high levels of activity and selectivity across a broad range of substrates. Herein, we will focus specifically on the cyclometalated ruthenium-based catalysts developed in our laboratory [46-58]. This chapter is intended to provide a comprehensive summary of the evolution of these cyclometalated ruthenium catalysts, from their initial serendipitous discovery to their recent applications in Z-selective olefin metathesis transformations. Current mechanistic hypotheses and limitations, as well as future directions, will also be discussed. 

The selectivity exhibited by an olefin metathesis catalyst for the production of -olefins or Z-olefins is a result of both kinetic and thermodynamic factors. Kinetic selectivity results from preferential formation of either syn- or anri-metallacyclo-butanes following olefin binding iy -metallacycles will undergo a cycloreversion to produce Z-olefins, whereas -olefins are derived from anfi-metallacycles. Thermodynamic selectivity arises as a result of secondary metathesis processes, in which the product olefins continue to react with the propagating catalyst. 

There are kinetic, spectroscopic, and computational studies that are fully consistent with the general mechanism of metathesis discussed earlier. One of the early evidences for the cleavage of the alkenes only at the double bonds came from isotope-labeling studies. A mixture of but-2-ene and perdeuterated but-2-ene on exposure to metathesis catalysts shows that the product but-2-ene is deuterated only at the 1,2 positions. 

In the other mechanism apart from the carbene there is no other intermediate. The product is formed by a concerted reaction between the carbene and the alkene. The transition state for the concerted mechanism is shown in Figure 7.10, and found to be consistent with computational and kinetic isotope effect studies. Note that had ML been a metathesis catalyst, other carbenes such as L M=CH2 or L M=CHR would be generated from the reaction of L M=CHR with RCH=CH2. 

---