Initiation, of metathesis

The reverse reaction, extrusion of a methylene from a metal-alkyl complex to give a species of the type M(H)(C2H4) is also known, but it is far from being systematic this is the a-H elimination (see section 5.1 and Chap. 18.5) that sometimes plays an important role in the initiation of metathesis reactions (Chap. 15). 

In 2008, Blechert et al. reported the total synthesis of e/jt-lepadin F (89) and G (90) by a tandem ene-yne-ene RCM [74]. Lepadins are members of marine alkaloids with decahydroquinoline framework. As a key step in their synthesis (Fig. 26) they planned to construct the decahydroquinoline core skeleton by a selective tandem ene-yne-ene RCM of the dienyne precursor (91). Craiceivably, two different reaction pathways could be expected (1) initiation of metathesis may occur at the terminal double bond followed by two consecutive RCMs to afford the desired 6/6 bicycle (92) or (2) initiation may occur on the disubstituted alkene followed by tandem RCMs to produce the undesired 5/7 bicycle (93). Considering the preference of initiation on monosubstituted double bond as well as the directing effect of free hydroxyl group, pathway (1) may be more favored. Gratilyingly, treatment of dienyne (91) with 10 mol% Gmbbs I catalyst smoothly provided the desired 6/6 bicycle (92) in 90% yield. 

In comparison with the previous examples, Metz and coworkers developed a similar strategy applying selective tandem ene-yne-ene RCM to assemble a 5/7 bicyclic framework [75]. In their total synthesis of sesquiterpene natural products (—)-clavukerin A (94) and (—)-isoclavukerin A (95) (Fig. 27), treatment of dienyne precursors (96) and (97) with phosphane-free Hoveyda-Blechert catalyst under an ethylene atmosphere cleanly afforded ( )-clavukerin A (94) and ( )-isoclavukerin A (95) in 53% and 55% yields, respectively. The high selectivity could also be explained by the preferential initiation of metathesis on the less substituted olefin in this tandem sequence. Similarly, Metz and coworkers utilized this tandem reaction in a more complex system to construct efficiently a 5/61116 tetracyclic skeleton and... 

The efficiency of the new catalysts 7a and 7b was examined in some metathesis transformations. Interestingly, both catalysts can only very slowly initiate typical metathesis reactions when tested at room temperature. At a catalyst loading of 5 mol%, chelating sulfur catalysts 7a and 7b achieved 22% and 51% conversion of diallylamide (3d), respectively, after heating to 80 C for 24 h. Catalyst 7a converts... 

Evidence for a methylene-metal-initiating species was provided by detection of propylene early in the course of metathesis of 2,8-decadiene with Me4Sn/WCl6, in addition to normal metathesis products ... 

As pointed out in an earlier section, Katz and co-workers demonstrated that Casey s complex, (CO)5W=CPh2, could also be employed to initiate the metathesis of a-olefins (26) as well as the polymerization of certain cycloolefins (27, 63). 

The exact nature of the alkylidenes formed on various oxide surfaces is still uncertain, as is the nature of the alkylidenes responsible for the often observed metathesis activity. Mo(N)(CH2CMe3)3 also has been employed as a precursor to a surface-bound species believed to be of the type Mo(NH)(CHCMe3)(CH2CMe3) (Osurf) [115]. Although the alkylidene carbon atom could not be observed in solid state NMR spectra, which is typical of surface supported alkylidenes, reaction with acetone to give 2,4,4-trimethylpent-2-ene quantitatively confirmed the presence of the reactive neopentylidene complex. Such species would initiate various metathesis reactions when prepared on partially dehydroxylated silica. 

In turn, the propensity of 1 to respond to steric hindrance can be used to control the site of initiation of an RCM reaction in a polyene substrate (Scheme 9) [20]. Thus, dienyne 25 reacts with the catalyst regioselectively at the least substituted site the evolving ruthenium carbene 26 undergoes a subsequent enyne metathesis leading to a new carbene 27, which is finally trapped by the disubsti-tuted olefin to afford the bicyclo[4.4.0]decadiene product 28. By simply reversing the substitution pattern of the double bonds, the complementary bicyclo [5.3.0] compound 32 is formed exclusively, because the cyclization cascade is then triggered at the other end of the substrate. Note that in both examples tri-substituted olefins are obtained by means of a ruthenium based metathesis catalyst [20] ... 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Scheme 2 Termination of living polymers. A Wittig-type reaction for Mo-based initiators, B metathesis with ethyl vinyl ether for Ru-derived initiators...

Since its discovery more than 50 years ago, olefin metathesis has evolved from its origins in binary and ternary mixtures of the Ziegler-Natta type into a research area dominated by well-defined molecular catalysts. Surveys of developments up to 1993 were presented in COMC (1982) and COMC (1995). Major advances in ROMP over the last 10 years include the development of modular, stereoselective group 6 initiators, and easily handled, functional-group tolerant ruthenium initiators. The capacity to tailor polymer functionality, chain length, and microstructure has expanded applications in materials science, to the point where ROMP now constitutes one of the most powerful methods available for the molecular-level design of macromolecular materials. In addition to an excellent and comprehensive text on olefin metathesis, a three-volume handbook s has recently appeared, of which the third volume focuses specifically on applications of metathesis in polymer synthesis. 

In a two-component composition, the base paste contains the monomer and the catalyst paste contains the catalyst, which after mixing of the catalyst paste with the base paste, initiates the metathesis reaction of the olefinic substrate. 

To examine the possibility of a more efficient catalytic olefin metathesis, we prepared chiral Mo-based catalysts, 4a and 4b [10]. This approach was not without precedence related chiral Mo complexes were initially synthesized in 1993 and were used to promote polymer synthesis [6]. We judged that these biphen-based complexes would be able to initiate olefin metathesis with high levels of asymmetric induction due to their rigidity and steric attributes. Chiral complexes 4a and 4b are orange solids, stable indefinitely when kept under inert atmosphere. 

Over the past 15 years the understanding of the mechanism of these reactions has been greatly enhanced through the preparation of metal carbene complexes, particularly of Mo, W and Ru, that are both electronically unsaturated (<18e) and coordinatively unsaturated (usually <6 ligands), and which can act directly as initiators of olefin metathesis reactions. The intermediate metallacyclobutane complexes can also occasionally be observed. Furthermore, certain metallacyclobutane complexes can be used as initiators. 

Most of these metal-oxygen intermediates are expected to be reactive toward organic substrates and electrode surfaces. Hence, the presence of metal cations enhances the electron stoichiometry for the reduction of 02, but frequently passivates the electrode surface. Thus, the formation of (H20)4Znn(02) on the surface of a platinum electrode probably initiates a metathesis reaction ... 

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