Olefins alkene/alkyne reactions

Solutions of alkali metals in liquid ammonia have been developed as versatile reducing agents which effect reactions with organic compounds that are otherwise difficult or impossible/ Aromatic systems are reduced smoothly to cyclic mono- or di-olefins and alkynes are reduced stereospecifically to frani-alkenes (in contrast to Pd/H2 which gives cA-alkenes). 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

In a recent example, Mironov used the rational replacement of starting materials for oligomerization reactions to discover new MCRs in a systematic way [31]. A reaction library of six alkenes/alkynes, two isonitriles, two nitriles and isoquinoline was set up, giving (n1 — n)/2 different reactions products. A minimum peak height of 30% of the total reaction product was used as a criterion for identifying an efficient MCR. In this way, a novel MCR that yields pyrrolo[2,l-a]isoquinolin-1-ones from electron-deficient olefins, isonitriles and isoquinoline was found (Scheme 10.6). 

Until recently, intermolecular enyne metathesis received scant attention. Competing CM homodimerisation of the alkene, alkyne metathesis and polymerisation were issues of concern which hampered the development of the enyne CM reaction. The first report of a selective ruthenium-catalysed enyne CM reaction came from our laboratories [106]. Reaction of various terminal alkynes 61 with terminal olefins 62 gave 1,3-substituted diene products 63 in good-to-excellent yields (Scheme 18). It is interesting that in these and all enyne CM reactions subsequently reported, terminal alkynes are more reactive than internal analogues, and 1,2-substituted diene products are never formed thus, in terms of reactivity and selectivity enyne CM is the antithesis of enyne RCM. The mechanism of enyne CM is not well understood. It would appear that initial attack is at the alkyne however, one report has demonstrated initial attack at the alkene (substrate-dependent) is also possible, see Ref. [107]. 

Examples of the application of this chemistry to the preparation of cyclobutanones, cyclobutenones, and P-lactams are presented in the Table. The mesityl thiol ester has proven to be particularly effective in reactions with less ketenophilic alkenes, although with the more reactive ketenophiles nearly identical results are obtained using either the mesityl a-diazo thiol ester or the more readily available thiophenyl ester. In the case of readily available ketenophiles, the reaction is best conducted using excess alkene, alkyne, or imine, but in other cases the cycloaddition can be carried out with excess diazo thiol ester. The efficiency of the reaction with unactivated alkenes is especially notable, and compares favorably with results obtained previously employing dichloroketene. For example, addition of dichloroketene to methylenecyclohexane is reported to proceed in 55% yield," while up to 81% of the desired [2-1-2] cycloadduct is produced in the reaction of (mesitylthio)ketene with this olefin under our conditions. 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

Abstract Progress in the field of metal-catalyzed redox-neutral additions of oxygen nucleophiles (water, alcohols, carboxylic acids, and others) to alkenes, alkynes, and allenes between 2001 and 2009 is critically reviewed. Major advances in reaction chemistry include development of chiral Lewis acid catalyzed asymmetric oxa-Michael additions and Lewis-acid catalyzed hydro-alkoxylations of nonacti-vated olefins, as well as further development of Markovnikov-selective cationic gold complex-catalyzed additions of alcohols or water to alkynes and allenes. 

Iridium has been found to be a very robust late transition metal which can mediate or catalyze C—H bond activation reactions very efficiently. However, the highly enantioselective Ir-catalyzed C—H bond functionalization via a transient C—Ir species for the construction of C—C or C—X bonds only emerged recently. Mechanistically, the catalytic cycle starts with oxidative addition of the Ii catalyst to the inert C—H bonds (such as aromatic, olefinic, or aliphatic C—H bonds), which are usually assisted with an ortho directing group. Subsequently, the formed C—Ir species inserts into an unsaturated functionality such as alkene, alkyne, or imine, delivering a new C—Ir speeies. Finally, the reductive elimination releases the products and regenerates the Ir catalyst. 

Transition metal alkyne complexes also react with nucleophiles, in this case to generate CT-vinyl complexes. There are fewer stable alkyne complexes of higher oxidation state or cationic metals than olefin complexes. Because these types of alkyne complexes are most susceptible to nucleophilic attack, less information is available on tfiis reaction than on nucleophilic attack on coordinated alkenes. Nevertheless, reactions of several cationic alkyne complexes with nucleophiles have been reported, and a few examples are presented here. 

The [2+2] reactions of the zirconium-imido compounds with alkynes and alkenes occurs by a mechanism similar to that for the [2+2] reactions of carbenes with alkynes and alkenes. The alkene or alkyne first binds to an open coordination site at the metal, and this coordination is followed by conversion of the alkyne or alkene complex to the metallacyhc product (e.g. Equation 13.76). Thus, the [2+2] reaction requires a 16-electron intermediate to bind the olefin or alkyne, even though the metallacyHc product and the imido complex have the same overall electron count. In support of the coordination of alkyne or alkene, albeit weakly, to the d° metal center, the rate of the reaction of alkynes with the 18-electron zirconocene-imido compound containing bound pyridine-N-oxide was inhibited by added pyridine-N-oxide (Equation 13.76). ... 

The hydroboration of olefins is a classic reaction in organic synthesis. - Dialkylbo-ranes add rapidly to alkenes in the absence of catalyst. However, dialkoxyboranes, such as catecholborane and pinacolborane, add more slowly to olefins and alkynes. Thus, transition metal complexes could catalyze the addition of dialkoxyboranes to olefins and alkynes without interference from the background reaction. The potential to alter chemoselectivity, regioselectivity, enantioselectivity, and diastereoselectivity has led a munber of groups to develop metal-catalyzed versions of hydroboration. " Enantioselective hydroboration would alleviate the need to use boranes containing stoichiometric amounts of chiral substituents to generate optically active alkylboranes. 

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