Formal reductive elimination

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 mechanism of the final step of C-0 bond formation by a formal reductive elimination was included as part of Chapter 11. In particular, two model systems have provided information on the mechanism of this reaction. First, the reaction orders, solvent effects, and electronic effects on the reductive elimination of methyl acetate and methyl aryl ethers from methylplatinum(lV) acetate and phenoxide complexes (Equation 18.16 and Scheme 18.4) indicated that these reductive eliminations occur by backside attack on a platinum methyl. Second, a study of the stereochemistry of the attack of water on a Pt(lV) alkyl showed that the formation of alcohol occurred with the inversion of configuration that reflects a backside attack. ... 

Many of the reactions shown herein involve a formal reductive elimination process between a tertiary amine function or a thioether unit and a vinylic carbon atom, a reaction that has been rarely encountered before... 

One of the most common methods employed for the generation of allylpalladium complexes involves oxidative addition of allylic electrophiles to Pd . This transformation has been explored by several groups, and has been the topic of recent reviews [69]. A representative example of this process was demonstrated in a recent total synthesis of (+ )-Biotin [70]. The key step in the synthesis was an intramolecular amination of 89, which provided bicydic urea derivative 90 in 77% yield (Eq. (1.40)). In contrast to the Pd"-catalyzed reactions of allylic acetates bearing pendant amines described above (Eq. (1.10)), which proceed via alkene aminopalladation, Pd -catalyzed reactions of these substrates occur via initial oxidative addition of the allylic acetate to provide an intermediate Jt-allylpalladium complex (e.g., 91). This intermediate is then captured by the pendant nudeophile (e.g., 91 to 90) in a formal reductive elimination process to generate the product and regenerate the Pd catalyst. Both the oxidative addition and the reductive elimination steps occur with inversion... 

Upon heating, loss of a third equivalent of carbon monoxide opens a coordination site to provide 1-2 and allows cobalt to complex the alkene as in 1-3. The alkene inserts into the least hindered cobalt-carbon bond to form the cobaltacycle 1-4. Carbon monoxide inserts into the new cobalt-carbon bond to give 1-5. Next, acyl migration from cobalt to carbon forms the final carbon-carbon bond as in 1-6. Finally, a formal reductive elimination of the spectator cobalt releases the product cyclopentenone P and cobalt-hexacarbonyl. The cobalt is then ready for another catalytic cycle. In practice the cobalt is most often used stoichiometrically. The alkyne cobalt complex is a stable species that can be purified and isolated by silica gel chromatography. Conditions for catalytic turnover of the cobalt have been reported. 

Although the reaction mechanism is ambiguous, coordination of oxovanadium(V) species to organoaluminums is considered to promote electron transfer or trans-metallation for the oxidative ligand coupling. This ttansformation is the first example for the formal reductive elimination on aluminnm. 

The intermediate 7.28 with two rj -allyl ligands can be observed by in situ NMR spectroscopy. As shown by reaction 7.2.4.1, conversion of 7.28 to 7.29 can be treated as a formal reductive elimination reaction where the product, 1,5-COD remains coordinated to nickel before being eliminated. Similarly, as shown by reaction 7.2.4.2, the formation of vinyl cyclohexene can also be explained satisfactorily by arrow pushing formabsm. 

In the alkanethiol case, the reaction may be considered formally as an oxidative addition of the S—H bond to the gold surface, followed by a reductive elimination of the hydrogen. When a clean gold surface is used, the proton probably ends as a molecule. Monolayers can be formed from the gas phase (241,255,256), in the complete absence of oxygen ... 

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

Sect. 2.1.1) and [3C+2S] cyclopentene derivatives. The product distribution can be controlled by choosing the appropriate reaction conditions [72]. Moreover, the cyclopentene derivatives are the exclusive products from the coupling of fi-pyrrolyl-substituted carbene complexes [72b,c] (Scheme 25). The crucial intermediate chromacyclobutane is formed in an initial step by a [2+2] cycloaddition. This chromacyclobutane rearranges to give the rf-complex when non-coordinating solvents are used. Finally, a reductive elimination leads to the formal [3C+2S] cyclopentene derivatives. 

Coupling of alkenylcarbene complexes and siloxy-substituted 1,3-dienes affords vinylcyclopentene derivatives through a formal [3C+2S] cycloaddition process. This unusual reaction is explained by an initial [4C+2S] cycloaddition of the electron-poor chromadiene system as the 471 component and the terminal double bond of the siloxydiene as the dienophile. The chromacyclohexene intermediate evolves by a reductive elimination of the metal fragment to generate the [3C+2S] cyclopentene derivatives [73] (Scheme 26). 

The reaction of methyl acrylate and acrylonitrile with pentacarbonyl[(iV,iV -di-methylamino)methylene] chromium generates trisubstituted cyclopentanes through a formal [2S+2S+1C] cycloaddition reaction, where two molecules of the olefin and one molecule of the carbene complex have been incorporated into the structure of the cyclopentane [17b] (Scheme 73). The mechanism of this reaction implies a double insertion of two molecules of the olefin into the carbene complex followed by a reductive elimination. 

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

The cyclo-oligomer products are formed in final reductive elimination steps commencing from the octadienediyl-Ni11 and dodecatrienediyl-Ni11 complexes for the C8- and Ci2-cyclo-oligomer production channels, respectively. Reductive elimination is accompanied with a formal electron redistribution between the nickel and the organyl moieties, which will be analyzed in Section 5.4. 

The thermodynamically favorable bis(r 3),A-cis/trans configuration 7b of the [Nin(dodecatrienediyl)] complex also represents the catalytically active species for reductive elimination. The new C-C a-bond is preferably established between the terminal unsubstituted carbons on two r 3-allylic groups (Fig. 9) giving rise to the formal 16e [Ni°(CDT)] product 8b, where CDT is coordinated to nickel by its three olefinic double bonds. 

In addition to the reaction shown in Scheme 53, some other related reactions that are thought to proceed via cyclic carbometallation have also been reported (Scheme 54). In the cyclization reaction of 2-ethenyl-2 -ethynylbiphenyl, both Cr and W carbyne complexes must undergo the same cyclic carbometallation as that shown in Scheme 53 to give the corresponding metallacyclohexadiene intermediates, but the final products obtained were different.256 Some tungsten-carbyne complexes have been shown to undergo a stepwise [2 + 2 + 2]-cyclization via formal cyclic carbometallation that can be followed by reductive elimination to produce cyclopentadiene-tungsten complexes.2... 

The third transformation, by far the most encountered process, is the /3-hydride elimination which is the major and the fastest process in many cases (Scheme 49). The /3-elimination is usually followed by the reductive elimination to give the cycloadduct and regenerate the active metal species. Depending on the regioselectivity of the elimination (Ha or Hb), two dienes, 1,3- and/or 1,4-diene, can be obtained. The products of the latter case formally correspond to Alder-ene adducts (see Chapter 10.12). 

The [4+ 4]-homolog of the [4 + 2]-Alder-ene reaction (Equation (48)) is thermally forbidden. However, in the presence of iron(m) 2,4-pentanedioate (Fe(acac)3) and 2,2 -bipyridine (bipy) ligand, Takacs57 found that triene 77 cyclizes to form cyclopentane 78 (Equation (49)), constituting an unprecedented formal [4 + 4]-ene cycloisomerization. The proposed mechanism for this transformation involves oxidative cyclization followed by /3-hydride elimination and reductive elimination to yield the cyclized product (Scheme 18). 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

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