Alkenes by reductive elimination

Johnson CR, Kirchhoff RA (1979) Synthesis of Alkenes by Reductive Elimination of 6-Hydroxysulfoximines. J Am Chem Soc 101 3602... 

A variety of a-substituted ketones (a-halo, -hydroxy, -methoxy and -amino) have been converted into alkenes by reductive elimination. Obviously, this approach, depends not only on the feasibility of a-functionalization but also on the ability to introduce the a-substituent regioselectively. a-Halo ketones, which are readily prepared from the starting ketone," have been subjected to reductive elimination conditions to afford the desired alkenic products. For example, in the preparation of ( )-eriolanin (Scheme 5)," the protected dihydroxy keto ester is treated sequentially with LDA and Br to afford the a-bromo... 

Phenylthio)methyl metal (metal = Li or MgCl) adds to the carbonyl group of ketones and aldehydes Benzoyl derivatives of these adducts are converted to alkenes by reductive elimination... 

In this chapter we deal with the tegio- and stereo-controlled synthesis of alkenes by reductive elimination of 1,2-disubstituted alkanes. Where appropriate the related 1,4-elimination reactions leading to conjugated dienes will also be considered. The principal criterion for inclusion here is synthetic utility, and reactions such as the Julia alkenation are given prominence because they incorporate a reductive elimination as a key step in a sequence which is connective. Consequently such reductive eliminations should not be considered in isolation, but as an integral part of a sequence which achieves regio- and stereo-con-... 

The full paper on the conversion of 1,2-diols into alkenes by reduction-elimination of appropriate 1,3,2-dioxaphospholans has appeared (see Organophosphorus Chemistry , Vol. 8, p. 123). 2 method was examined with particular regard to the conversion of cyclodecane- and cyclododecane-1,2-diols into the cycloalkenes, as mixtures of cis- and tmns-isomers, and consists of the treatment of the cyclic ethyl esters or AW-dimethylamides with lithium in liquid ammonia or with Ti-THF. The kinetics of the methanolysis of thiophosphoryl chlorides have been studied. The rather unusual isomerization of 2-hydroxyethyl phosphorothioates (70) yields the 2-mercaptoethyl isomers after several days at room temperature instead of the expected thiono-esters. This observation should be compared with those reported in ref. 83. 

Synthesis of Alkenes by Reductive Elimination. The treatment of 2-halo-3-hydroxy esters and amides with samarium iodide leads to the corresponding di- or trisubstituted E)-a,p-unsaturated derivatives in high yields and diastereoselectivities (eqs 39 and 40). The precursors are readily accessible by condensation of the lithium enolate of a-haloesters or amides. If the substrate contains y,i5-unsaturation, the /3,y-unsaturated ester is generated in the process (eq 41). 

Phenylthio)methyl metal (metal s Li or MgCl) adds to the carbonyl group of ketones and aldehydes 5-12,13 Benzoyl derivatives of these adducts are converted to alkenes by reductive elimination with Li-NHs, TiCU-Zn" or Ti. Transformation of (3) into an alkene via its phosphoric ester has also been reported (equation 3)."... 

The most useful reaction of Pd is a catalytic reaction, which can be carried out with only a small amount of expensive Pd compounds. The catalytic cycle for the Pd(0) catalyst, which is understood by the combination of the aforementioned reactions, is possible by reductive elimination to generate Pd(0), The Pd(0) thus generated undergoes oxidative addition and starts another catalytic cycle. A Pd(0) catalytic species is also regenerated by /3-elimination to form Pd—H which is followed by the insertion of the alkene to start the new catalytic cycle. These relationships can be expressed as shown. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

The unsaturated c.vo-enol lactone 17 is obtained by the coupling of propargylic acetate with 4-pentynoic acid in the presence of KBr using tri(2-furyl)-phosphine (TFP) as a ligand. The reaction is explained by the oxypalladation of the triple bond of 4-pentynoic acid with the ailenyipailadium and the carbox-ylate as shown by 16, followed by reductive elimination to afford the lactone 17. The ( -alkene bond is formed because the oxypalladation is tnins addition[8]. 

J. K. Kochi, D. M. Singleton u. L. J. Andrews, Alkenes from Halides and Epoxides by Reductive Eliminations with Cr(lI)-Complexes, Tetrahedron 24, 3503 (1968). 

The borostannylation of an enyne has also been reported by Tanaka to proceed in a high yield (Scheme 71).273 The mechanism of this cyclization has not been investigated in detail, but the insertion of the alkyne takes place preferentially into the Pd-B bond over the Pd-Sn bond. Then, the addition of the vinylpalladium 279 to the alkene moiety followed by reductive elimination furnished the cycloadduct 278. However, Tanaka does not exclude a palladacycle intermediate. Similarly, a borylsilylative carbocyclization has also been reported by Tanaka.274... 

The reagent PhI=NTs can be drawn in the resonance form Phi-NTs, where its resembalnce to CIO-becomes clear. Moreover, the issues of the square planar coordination sphere of the Mn(salen) complex don t exist with Cu(II), so a very simple mechanism can be drawn coordination of the N of the reagent to Cu(II), displacement of Phi by a lone pair on Cu to give a Cu(IV)=NTs reagent, [2 + 2] addition to the alkene, and reductive elimination. 

The difference between this catalytic system and Wilkinson s catalyst lies in the sequence of the oxidative addition and the alkene complexation. As mentioned above, for the cationic catalysts the intermediate alkene (enamide) complex has been spectroscopically observed. Subsequently oxidative addition of H2 and insertion of the alkene occurs, followed by reductive elimination of the hydrogenation product. 

While the reductive elimination is a major pathway for the deactivation of catalytically active NHC complexes [127, 128], it can also be utilized for selective transformations. Cavell et al. [135] described an interesting combination of oxidative addition and reductive elimination for the preparation of C2-alkylated imida-zohum salts. The in situ generated nickel catalyst [Ni(PPh3)2] oxidatively added the C2-H bond of an imidazolium salt to form a Ni hydrido complex. This complex reacts under alkene insertion into the Ni-H bond followed by reductive elimination of the 2-alkylimidazolium salt 39 (Fig. 14). Treatment of N-alkenyl functionalized azolium salts with [NiL2] (L = carbene or phosphine) resulted in the formation of five- and six-membered ring-fused azolium (type 40) and thiazolium salts [136, 137]. 

The first peroxide oxygen atom is transferred to the alkene by a peroxymetalation pathway involving coordination of both Oz and alkene to the metal in (71a) or (71b), followed by the formation of a peroxometallacycle (72a) or (72b) which decomposes to give the methyl ketone and the rhodium-oxo (73a) or -hydroxo (73b) species. The second hydroxo oxygen atom is transferred via an internal cis hydroxymetalation mechanism (Wacker type), which produces a second mole of ketone and regenerates the initial Rh111 hydride or the Rh1 species by reductive elimination of the latter. Two plausible alternative versions of the peroxymetalation pathway can be envisaged133 and these are described below. 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

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