Metathesis termination step

The metathesis reaction is still occasionally referred to as disproportionation although it bears no resemblance to disproportionation as a chain-growth termination step. Rather, it is analogous to interchange in step-growth polymerization (reaction 10.7 in Section 10.2.2) except that the reactants are broken apart at their double bonds and the fragments reconnected by double bonds. 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

There are several mechanisms for termination of the growing polymer chain. One common chain termination step is jS-hydride elimination to give Cp2ZrH+ and the polymer with a terminal double bond. When the polymerization is carried out under H2, a cr-bond metathesis can take place to give the saturated polymer and Cp2ZrH +. ... 

Schrock and co-workers note that the chain mechanism is almost certainly correct, but major questions remain unanswered. They are conducting studies with alkyhdene complexes of niobium, tantalum, and tungsten, directed towards understanding in detail how and why metathesis catalysts work. From preliminary results they predict that the olefin co-ordinates to the metal before a metallocyclobutane complex can be formed, that rearrangement of metallocyclobutane is slow relative to the rate of metathesis, and that important chain-termination steps are rearrangement of metallocyclobutane intermediates and bimolecular decomposition of methylene complexes. For these systems, co-catalysts such as the alkyl-aluminium chlorides are not necessary the initial alkyl group on the metal... 

Mangohas raised an objection to the Chauvin mechanism. His analysis and calculation based on basic principles of thermodynamics indicates that more cyclopropane should be present in metathesis reactions than has been observed, i.e, at equilibrium 20% ethylene converts to cyclopropane. However, arguments that Mango s analysis is in error have been presented. Grubbs notes that the formation of cyclopropane is a chain-termination step and, since the initiation of metal carbenes is very slow compared to the catalytic reaction itself, the concentration of cyclopropane cannot be greater than the metal carbene. Grubbs concludes that the Chauvin mechanism is not inconsistent with thermodynamic calculations and remains as the mechanism most compatible with a large body of other experimental... 

The same neopentylidene-alkoxo complexes react with cis-2-pentene to give the two initial metathesis products (4,4-dimethyl-2-pentene and 5,5-dimethyl-3-hexene) and catalyze the metathesis of cis-2-pentene to 2-butenes and 3-hexenes. Furthermore, propylene and ethylene appear in the reaction medium as the catalyst deactivates. These latter olefins are formed by rearrangement of the ethylidene and propylidene intermediates, providing the mechanism for the metathesis chain termination step ... 

In the same way Ta(CHCMe3)(OCMe3)2 reacts with cis-2-pentene to give the initial and productive metathesis olefins but deactivates without formation of propylene and ethylene. In this case the chain termination steps must be a bimolecular decomposition of the alkylidene intermediates. Adding monodentate tertiary phosphine increases the lifetime of the complex, probably by a stabilizing effect of the alkylidene intermediates. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

A general mechanism for this reaction, including the metathesis, initiation by either heat or light, the propagation cycle, and the termination steps in the presence or absence of LiCl, is outlined below. 

Migrastatin (192) (Scheme 37) is a novel macrolide natural product that displays an inhibitory effect on the migration of human tumor cells. After an RCM-based synthesis of the 14-membered macrolide core of 192 [94], Danishefsky also achieved the first total synthesis of the natural compound [95], using the fully functionalized tetraene 191 as the metathesis precursor. Under the conditions shown in Scheme 37, the ring-closing step proceeded (E)-selectively with exclusive participation of the two terminal double bonds in 191, delivering only the ( , ,Z)-trienyl arrangement present in 192. 

Whereas Hegedus [335] and Danishefsky [336] were the first to discover a tandem Heck reaction from o-allyl-A -acryloylanilines leading to tricyclic pyrrolo[l,2-a]indoles or pyridino[l,2-a]indoles [336], it has been the fantastic work of Grigg to unleash the enormous potential of this chemistry. Grigg and his co-workers parlayed their Pd-catalyzed tandem polycyclization-anion capture sequence into a treasure trove of syntheses starting with IV-allyl-o-haloanilines [337-345], Diels-Alder and olefin metathesis reactions can be interwoven into the sequence or can serve as the culmination step, as can a wide variety of nucleophiles. An example of the transformation of 289 to 290 is shown below in which indole is the terminating nucleophile [340],... 

In parallel, since the first preparation of allenylidene-metal complexes in 1976, the formation of these carbon-rich complexes developed rapidly after the discovery, in 1982, that allenylidene-metal intermediates could be easily formed directly from terminal propargylic alcohols via vinylidene-metal intermediates. This decisive step has led to regioselective catalytic transformations of propargylic derivatives via carbon(l)-atom bond formation or alternately to propargylation. Due to their rearrangement into indenylidene complexes, metal-allenylidene complexes were also found to be catalyst precursors for olefin and enyne metathesis. 

Alkenes. At present alkene isomerization is an important step in the production of detergent alkylates (Shell higher olefin process see Sections 12.3 and 13.1.3).264 265 Ethylene oligomerization in the presence of a nickel(O) catalyst yields terminal olefins with a broad distribution range. C4-C6 and C2o+ alkenes, which are not suitable for direct alkylate production, are isomerized and subsequently undergo metathesis. Isomerization is presumably carried out over a MgO catalyst. 

Ring closure occurs during the nexi step, which takes the form of an olefin metathesis. The two terminal double bonds in 50 are coupled together with release of ethene (58), thereby closing the ring to 17. 

ADMET is a step growth polymerization in which all double bonds present can react in secondary metathesis events. However, olefin metathesis can be performed in a very selective manner by correct choice of the olefinic partner, and thus, the ADMET of a,co-dienes containing two different olefins (one of which has low homodimerization tendency) can lead to a head-to-tail ADMET polymerization. In this regard, terminal double bonds have been classified as Type I olefins (fast homodimerization) and acrylates as Type II (unlikely homodimerization), and it has been shown that CM reactions between Types I and II olefins take place with high CM selectivity [142], This has been applied in the ADMET of a monomer derived from 10-undecenol containing an acrylate and a terminal double bond (undec-10-en-l-yl acrylate) [143]. Thus, the ADMET of undec-10-en-l-yl acrylate in the presence of 0.5 mol% of C5 at 40°C provided a polymer with 97% of CM selectivity. The high selectivity of this reaction was used for the synthesis of block copolymers and star-shaped polymers using mono- and multifunctional acrylates as selective chain stoppers. 

Formally the reaction combines the ring opening of norbornene, RCM with the terminal double bond and CM with a second alkene. The domino metathesis involving an allylsilyl derivative has been used as a step in the synthesis of (-)-halosaline [69] and (-)-indolizidine [90]. 

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