Cyclic diene metathesis

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

It should be noted that when an R group, e.g. a methyl group, is present at the internal unsaturated carbon atom in a terminal diene molecule, as in the CH2=C(Me)—(CH2)x-C(Me)=CH2 monomer, the cyclic diene metathesis ceases [7,8]. The steric effect exerted by the R substituent can even be important at the a position to the double bond in the monomer. Sterically encumbering this position hinders polymer formation [9]. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Murakami and coworkers [267] described a combination of an intermolecular ene-yne metathesis followed by a disrotatory 6ir-electrocyclic reaction to give six-membered cyclic dienes. Thus, reaction of 6/3-98 and of styrene (6/3-99) in the presence of a ruthenium catalyst at 100 °C led to the condensed cyclohexadienes 6/3-100, in good yield (Scheme 6/3.29). 

Enyne metathesis can also be used with highly substituted substrates. Catherine Lievre of the Univcrsite de Picardie reports (J. Org. Chem. 2004,69, 3400) that enynes such as 11, readily prepared from carbohydrate precursors, are cyclized by the second generation Grubbs catalyst 2 to the enantiomerically-pure cyclic dienes, exemplified by 12. 

The formation of polymers containing [=CH(CH2)4CH=], units is possible through the ROMP of an appropriate cyclic diene, such as cycloocta-1,3-diene, or by a double-bond shift reaction of a polymer such as poly(l-pentenylene). Such units can be eliminated as cyclohexene so long as metathesis activity is present in the system360. The ROMP of 2,3-dihydropyran, initiated by Mo(CO)6/CBr4// v, has been reported361. 

With the advances in pro-catalyst design that have been witnessed over the last decade or so, the transition-metal-catalysed alkene metathesis reaction has now become a practical procedure that can be utilised by the chemist at the bench. Undeniably, this has added a new dimension to the repertoire of synthetic organic chemistry as it facilitates disconnections that, pre-metathesis, simply would not have been considered. Take, for example, a macro-cyclic amide where the normal disconnection would be at the amide. Now, with the ready reduction of alkenes to alkanes, a ring-closing diene metathesis (RCM), followed by hydrogenation, becomes an alternative disconnection. And, when one considers that any of the C—C linkages could be established in such a manner, the power of the RCM disconnection becomes obvious. 

Dienes are cyclized by intramolecular metathesis. In particular, cyclic alkenes 43 and ethylene are formed by the ring-closing metathesis of the a,co-diene 46. This is the reverse reaction of ethenolysis. Alkene metathesis is reversible, and usually an equilibrium mixture of alkenes is formed. However, the metathesis of a,co-dienes 46 generates ethylene as one product, which can be removed easily from reaction mixtures to afford cyclic compounds 43 nearly quantitatively. This is a most useful reaction, because from not only five to eight membered rings, but also macrocycles can be prepared by RCM under high-dilution conditions. However, it should be noted that RCM is an intramolecular reaction and competitive with acyclic diene metathesis polymerization (ADMET), which is intermolecular to form the polymer 47. In addition, the polymer 47 may be formed by ROMP of the cyclic compounds 43. 

Reaction of the carbene complex 148 with alkyne affords vinylcarbene 150 via metallacyclobutene 149. In the intramolecular reaction of enyne 152, catalysed by carbene complex 151, the triple bond is converted to vinylcarbene 153 which then reacts with the double bond to give the conjugated diene 154. Generation of 154 is expected by the formation and cleavage of cyclobutene 155 as a hypothetical intermediate. Based on this reaction, Ru-catalysed intramolecular metathesis of enyne 156 gave the N-containing cyclic diene 157, from which (—)-stemoamide (158) was synthesiszed. The reaction can be understood by assuming the formation of the hypothetical cyclobutene 159 from 156 [52],... 

This intramolecular reaction results in the formation of a cyclic system, and therefore it is called ring-closing metathesis (RCM). In this process a diene 36 is treated with a metal alkylidene 37. Two competing pathways are available via the intermediate metal alkylidene 38 A) RCM will occur to afford cyclic adducts 39 and B) intermolecular reaction can occur to form polymeric structures 40 (acyclic diene metathesis polymerization (ADMET)). The reaction is also complicated because of the possibility of ring-opening metathesis (ROM), the retro reaction of path A, and ring opening metathesis polymerization (ROMP) (path C).13... 

Among cyclic polyenes, cyclic dienes, trienes and tetraenes have been ring-open polymerised via the metathesis reaction. Representative of the cyclodienes most commonly used for polymerisation are 1,5-cyclooctadiene, norbornadiene (bicyclo[2.2.1]hept-2,5-diene) and dicyclopentadiene as mono-, bi- and tricyclic diolefins respectively. Cycloocta-1,5-diene metathesis polymerisation is another approach to the preparation of 1,4-polybutadiene ... 

Let us emphasise that the driving force for acyclic diene metathesis, which is a step-growth condensation polymerisation, is the release and removal of a small condensate molecule. The polycondensation is performed preferably under bulk conditions (no solvent used), since acyclic diene metathesis is thermally neutral and there is no need to remove the heat of the reaction, in contrast to exothermic cyclic olefin ring-opening metathesis polymerisation. 

In the ring-closing metathesis reaction, intramolecular metathesis closes a ring to form a small cyclic molecule with concurrent loss of a small molecule (ethylene). Conversely, in the case of the acyclic diene metathesis reaction, macromolecules are formed by successive intermolecular condensation of two olefinic molecules [1],... 

Ru-catalysed enyne metathesis offers a short approach to chiral derivatives of 3-vinyl-5,6-dihydro-2//-pyrans. Some epimerisation can occur at the pyranyl C atom at elevated temperatures (Scheme 3) <02T5627>. The bispropargyloxynorbomene derivative 6 undergoes a cascade of metathesis reactions in the presence of alkenes and Grubbs catalyst incorporating an enyne-RCM that leads to fused cyclic dienes. A dienophile can be added to the reaction mixture, resulting in Diels-Alder reactions and the formation of functionalised polycyclic products <02TL1561>. 

Unsaturated polymers can be produced by means of ring-opening metathesis polymerization (ROMP) of cyclic alkenes. These unique polymers can also be produced via intermolecular Acyclic Diene Metathesis (ADMET). Dienes can also react intramolecularly via Ring Closing Metathesis (RCM) to afford cyclic products. RCM is often applied to synthesis of compounds for fine chemical and pharmaceutical application. Generic examples of these reactions are shown in Figure 2. 

The tungsten alkylidyne complex (t-BuO)3W(C-t-Bu) [130-132] is capable of catalyzing alkyne metathesis polymerization of cyclic alkynes. In addition, it can be used in acyclic diene metathesis (ADIMET) [133, 134]. The synthesis of the corresponding Mo-compound Mo(CCH2SiMe3)(OAd)3 (Ad=adamantyl) has been reported by C.C. Cummins et al. [135]. Though the active species is not known yet, a mixture of Mo(CO)6 and 4-chlorophenol is nowadays used for purposes of convenience [136]. 

The next few stages to produce the metathesis substrate 78 are straightforward and involve the creation of no new chiral centres. The metathesis was very successful with the original Grubbs catalyst 85 and gave the cyclic diene 86. Now the two carbonyl groups and the last two chiral... 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

A number of recently reported synthetic methods include the formation of organic polymers with pendent cyclic or linear phosphazene side groups (reactions 7-9) and a process for the preparation of linear polymers in which phosphazene rings are linked together by organic oligomer chains using acyclic diene metathesis (ADMET) techniques (reaction 10). ... 

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

A large number of applications involve the synthesis of polymers by either ring-opening metathesis polymerization (ROMP), which transforms cyclic olefins into unsaturated polymers (Fig. 4.14) [46,47], or acyclic diene metathesis polymerization (ADMET), which converts acyclic olehns into polymers (Fig. 4.15)... 

The metathesis reaction may be used to produce a range of polymers by two processes (Scheme 10). Cyclic olefins with a threshold level of ring strain can be polymerized via ROMP chain process. Under appropriate conditions, certain acyclic dienes can be polymerized in a condensation process known as acyclic-diene metathesis polymerization, or ADMET. The elimination of a volatile olefin byproduct (usually ethylene) provides a driving force for the latter process, while alleviation of ring strain is the primary driver in ROMP. Because ADMET is a condensation... 

The chemist of metathesis polymerization has been applied to preparation of unsaturated polycarbonates. This is a case of an acyclic diene metathesis. It takes place when Lewis-acid free catalysts are employed. An example of one such catalyst is Mo[CHC(CH3)2Ph](A 2,6-C6H3-/-Pt2)[OCCH3(CF3)2]2. One interesting point about this process is that unconjugated dienes are polymerized to high molecular weight linear polymers without formation of any cyclic structures. 

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