Transition-metal derivatives reductive elimination

Transition metal-catalyzed allylic alkylation is generally considered to involve mechanistically four fundamental steps as shown in Scheme 1 coordination, oxidative addition, ligand exchange, and reductive elimination. A key step of the catalytic cycle is an initial formation of a (7r-allyl)metal complex and its reactivity. The soft carbon-centered nucleophiles, defined as those derived from conjugate acids whose pAj, < 25, usually attack the allyl ligand from the opposite side... 

Another arylation method, in the case of nitrogen heterocycles, does not need a halogenated derivative but a heterocycle activated by triflic anhydride260,261 (reaction 22). Simple aryl halides usually do not react with phosphines and special methods therefore have to be used for their arylation. The most widely used is the complex salt method , in which an aryl halide is heated with a phosphine in the presence of a transition metal such as nickel (II)2e (reaction 23). The catalytic cycle probably takes place by means of a reduced nickel(I) complex, generated in situ from the starting nickel(II) salt this nickel(I) species could undergo an oxidative addition of the aryl halide to yield a transient nickel(III) adduct, which after the reductive elimination of the aryphosphonium affords the recovery of the first active-nickel(I) complex (reaction 24). 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

The electron-releasing phosphine promotes oxidative addition of the bromo derivative to Pd(0) and, because of its bulkiness, readily generates free coordination sites by dissociation. Ethylene coordination and insertion then occur, followed by reductive elimination, triethylamine acting as a base to neutralize hydrogen bromide. As in most cases of transition metal-catalyzed reactions the fine details of the mechanism are still under investigation. Thus recent studies by Amatore s group suggest that the palladium(O) species formed by reduction of palladium acetate is an anionic acetato complex. 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

Ru-catalyzed Suzuki-type cross-coupling reactions of aniline derivatives and organoboronates have been achieved via unreactive aryl C—N bond cleavage (Equation 11.48) [109]. The proposed reaction pathway is a sequence of oxidative addition of an unreactive aryl C—N bond to the late transition metal complex, followed by transmetalation between the Ru-NR2 species and organoboronates, and reductive elimination. 

Aspects of the reaction chemistry of transition-metal silyl derivatives have been extensively reviewed6"14. This section will focus primarily on reactivity in which M-Si bonds actively participate. Note that reactions involving oxidative additions and reductive eliminations of Si-X bonds with transition-metal centers are also considered separately in Sections II, III, IV and V. 

The products are versatile auxiliaries not only for enantioselective deprotonation and elimination (Section C.), but are also valuable chiral ligands for complex hydrides in the enantioselective reduction of ketones (Section D.1.4.5.)- They are also applied in enolate reactions (Section D.l.5.2.1., D.1.5.2.4.). transition-metal-catalyzed Michael additions (Section D.l.5.8.), 1,3-dipolar cycloadditions (Section D.l.6.1.2.1.), and additions ofGrignard reagents (Section D.l.3.1.4.2.5.). (5 )-2-(Phenylaminomethyl)pyrrolidine has found most application and is also commercially available. Several methods exist for the preparation of such compounds. Two typical procedures for the synthesis of (.S)-2-(l-pyrrolidinylmcthyl)pyrrolidine are presented here. The methodology can be readily extended to other amides and alkylamino derivatives of proline. 

Likewise when two alkyne molecules coordinate to a transition metal such as Co(I) with subsequent coupling of the C-C bond, oxidative cyclization takes place to give a metallacyclopentadiene. Further reaction of another alkyne molecule with the metallacyclopentadiene followed by reductive elimination liberates benzene derivatives. Thus cyclotrimerization of three alkyne molecules catalyzed by a cobalt complex [40,41] can be performed. If a nitrile is used as the second component, pyridine derivatives are obtained catalytically as shown in Scheme 1.13 [42]. The catalytic cyclotrimerization and cyclodimerization of alkynes and conjugated enynes have found extensive applications in synthesis of complex cyclic compounds such as steroid derivatives [43]. 

Another approach of using transmetallation process is to transfer the organic moiety bound with early transition metal complexes to late transition metal complexes such as nickel to utilize the reactivity of the diorganonickel complexes to undergo reductive elimination. Various benzene and pyridine derivatives have been prepared by the methodology [100]. 

We extended the study of C-C reductive elimination reactions to other members of late transition metals in order to find possible alternatives to Pd/Pt complexes for catalytic coupling reactions. The calculations were performed only for the corresponding phosphine complexes, for which experimental precedents for the reaction were reported. In the case of Rh and Ir derivatives, an extra o-bonded ligand has to be added to maintain correct oxidation state, because Rh and Ir compounds are rarely known [48]. The chloride has been chosen for this purpose. Thus the model compounds studied are 19 [Rh (CH=CH2)2(PH3)3Cl], 20 [Ir (CH=CH2)2(PH3)3Cl],21 [Ru (CH=CH2)2(PH3)3],and22 [Os (CH=CH2)2-... 

The last step, the release of acetaldehyde, can be interpreted as a reductive elimination to give the hydrate of acetaldehyde (Eqs. (9.19) and (9.20)) where Eq. (9.19) represents merely the completion of the complex ligation sphere at the central Pd by a solvent molecule. A reductive elimination is a common reaction of group 8 metal compounds. For this case, it has been first proposed in [20]. Keith etal. [21] derived such a pathway but chloride assisted from theoretical considerations. The barrier heights of the transition states for other pathways, for example, P-hydride elimination, were found to be too high. The route according to Eq. (9.20) would also be valid for chloride-free Pd compounds. 

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