Organic reductive elimination

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. 

The postulated steps that constitute the Suzuki coupling process are shown in Scheme 25. After oxidative addition of the organic halide to the palladium(o) catalyst, it is presumed that a metathetical displacement of the halide substituent in the palladium(ii) complex A by ethoxide ion (or hydroxide ion) takes place to give an alkoxo-palladium(ff) complex B. The latter complex then reacts with the alkenylborane, generating the diorganopalladium complex C. Finally, reductive elimination of C furnishes the cross-coupling product (D) and regenerates the palladium(o) catalyst. 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

Some reactions of PtMe4L2 systems do not involve reductive elimination thus reaction of PtMe4(NN) (NN = phen, bipy) with organic acids yielding PtMe3A(NN) (A = formate, acetate, benzoate, salicylate) is first order in both reactants [201]. 

The transmetallation reaction involves the transfer of the organic group from an organometallic species to a Pd(II) species and produces a trails Pd(II) species. Isomerization from the trans arrangement to a cis one is necessary prior to the reductive elimination step. Reductive elimination yields the coupled product and regenerates the transition metal catalyst. Because the reductive elimination is very fast, competing reactions leading to by-products are usually not observed. 

Biocatalysts have received great attention in these last few years. Due to their capacity to perform asymmetric transformations under mild conditions [78], they have been useful tools for synthesizing optically active organic molecules. They promote a variety of chemical transformations, including the syntheses of esters and amides and oxidations, reductions, eliminations and carbon carbon forming. Little is known about biocatalyst-promoted Diels Alder reactions. 

These are called cross-coupling reactions and usually involve three basic steps oxidative addition, transmetallation, and reductive elimination. In the transmetallation step an organic group is transferred from the organometallic reagent to palladium. 

Silane reduces the palladium acetate in 119 to the palladium hydride 120, which undergoes reductive elimination to provide the organic product and the catalytic Pd(II) species. This mechanistic hypothesis was supported by the use of EtsSiD as the reductant product was formed with D incorporation at only the methyl group [70]. This reaction is best performed with a Pd(0) precatalyst in the presence of acetic acid and 10 eq. of silane, which suppresses the competitive cycloisomerization reaction [70]. 

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

During the cross-couplings to form C—N, C—O, C—S, and C—P bonds, the arylpalladium halide complexes are converted to arylpalladium amide, alkoxide, thiolate, and phosphide complexes. Examples of each type of complex have now been isolated, and the reductive elimination of the organic products has been studied. Although the reductive elimination to form carbon-hydrogen and carbon-carbon bonds is common, reductive elimination to form carbon-heteroatom bonds has been studied only recently. This reductive elimination chemistry has been reviewed.23... 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

A similar reaction was not observed for 4-pentene-l-ol. It should be noted that a Pt(III) dimer complex is released after the reaction in Eq. (18), which is in contrast to the release of olefin and a Pt(II) dimer complex in aqueous solution by reductive elimination (Eq. (16)). The difference of such reactivity depending on the alkyl and the solvent would be caused by the difference of the electron density of the a-carbon atom and the dipole structure along the Pt-Pt bond in the solvents of different polarities. In aprotic organic solvent, the electron distribution along the Pt-Pt bond would be less polar, i.e., close to... 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

The reductive elimination/oxidative addition is of practical importance in catalytic cycles, for example the rhodium/methyl iodide catalysed carbonylation of methanol. In organic synthesis the palladium or nickel catalysed cross-coupling presents a very common example involving oxidative addition and reductive elimination. A simplified scheme is shown in Figure 2.19 [17],... 

The reaction starts with the oxidative addition of an aryl halide (Cl, Br or I) to palladium zero. The next step is the insertion of an alkene into the palladium carbon bond just formed. The third step is (3-hydride elimination giving the organic product and a palladium hydrido halide. The latter reductively eliminates HX, which reacts with base to give a salt (Figure 13.15). 

The transfer of iodine to the organic substrate represents a formal reductive elimination at tellurium(lV) to give tellurium(ll) as well as oxidation of the alkene. In a series of diaryltellurium(lV) diiodides, iodination of organic substrates is accelerated by electron-withdrawing substituents and is slowed by electron-donating substituents, which is consistent with the substituent effects one would expect for... 

The intrinsic instability of organocopper] 11) compounds is most probably associated with the redox properties of copper. Decomposition of organocopper] 11) compounds can occur by two different routes (i) formation of an organocopper]I) compound and an organic radical R" that can undergo further reactions, which formally represents a one-electron reduction process, and (ii) direct formation of R-R and Cu]0), which is formally a two-electron reduction process (reductive elimination cf Eqns. 1 and 2 in Scheme 1.3). 

Vinylation or arylation of alkenes with the aid of a palladium catalysts is known as the Heck reaction. The reaction is thought to proceed through the oxidative addition of an organic halide, RX onto a zero-valent [PdL2] species followed by coordination of the olefin, migratory insertion of R, reductive elimination of the coupled product and dehydrohalogenation of the intermediate [HPdXL2] (Scheme 6.1). 

The C-C coupling reaction between RMgX and R X is considered to proceed though an Ni(R)(R )Lm intermediate, and acceleration of the reductive elimination of R-R by coordination with olefinic or aromatic R X to Ni(R)(R )Lm is necessitated for a smooth catalytic reaction [15,16]. On these bases Ni-pro-moted dehalogenative polycondensation of dihalo organic compounds is suited to the preparation of 7i-conjugated aromatic and olefinic polymers. 

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

Coupling of organometallic reagents with hahdes in a carbon monoxide atmosphere leads to ketones by incorporation of a carbonylation step.147 148 These reactions involved a migration of one of the organic subsituents to the carbonyl carbon, followed by reductive elimination. These reactions can be carried out with stannanes149 or boronic acids150 151 as the nucleophilic component. 

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