Reductive elimination theory

Cu(0) species. Alternatively, the Cu(n) species may first undergo oxidation by an external oxidant (or internal redox process) to a Cu(m) intermediate, and then undergo reductive elimination to provide the product and a Cu(i) species. Re-oxidation to Cu(n) would then, in theory, complete the catalytic cycle, but in practice, most reactions of this type have been performed with stoichiometric amounts of the copper reagent. 

The discussion by Braterman and Cross of reductive elimination from square planar or octahedral complexes is a special case of frontier orbital theory. A transition metal L MR, R2 is taken to lose grcwps R, R2 in a concerted step to give L M + Rj - R2. By the usual book-keeping convention, the electrons in the initial M—R a bonds are assigned to the R groups but this is of course a mere convention of naming and does not affect the argument. The in-phase combination of metal-carbon a bonds correlates... 

Potassium hexamethylsilazide is used as non-nucleophilic base to deprotonate 24 next to the sulfone. In the (Sylvestre) Julia olefmation the resulting C-nucleophile adds to the aldehyde and, in theory, the following steps of the mode of action yield the desired double bond -selectively by reductive elimination. In this case the Z-configured product was obtained as the major isomer. 

Reductive elimination and oxidative addition are ubiquitous reaction steps in many TM-catalyzed processes. A recent study by Beste and Frenking (82) may serve as example for the general finding that relative energies of TM complexes with different coordination numbers may be subject to systematic errors at the DFT level of theory. Table 16 shows calculated energies at the CCSD(T)/n level and at B3LYP using three different basis sets, II-IV, for platinum complexes... 

Puddephatt etal. [41] have studied the C-H or C-C bond activation in the alkane complexes [PtMe(CH4)L2] or [PtMe(CHjCH3)L2] (L = NH3 or PH3) as well as the reductive elimination of methane or ethane from the five-coordinate model complexes [PtHMe2L2] or [PtMesLi], respectively, by carrying out extended Hiickel molecular orbital calculations and density functional theory. The oxidative addition and reductive elimination reactions occur by a concerted mechanism, probably with a pinched trigonal-bipyramidal complex on the... 

Subsequently, Green investigated the oxidahve addition of aryl chlorides to palladium complexes derived from NHCs and their role in catalytic aminations [206], by using density functional theory (DFT) methods. Here, the most likely transition-state model for oxidative addition is the monocarbene-palladium complex (NHC)Pd(ArCl). For the amination of chlorobenzene, coordination of the T-shaped oxidative addition product by the amine occurs with a rearrangement, placing the amine in a ds-position to the aryl group, a prerequisite for reductive elimination [2]. 

While the mechanism of many of the transformations described herein has not been investigated in detail, there are several examples where the reductive elimination pathway has been thoroughly evaluated via experiment and/or theory. Ultimately, a molecular-level mechanistic imderstanding of these processes should provide insights into the factors that control the relative and absolute rates of C-X bond-formation (and other fundamental organometallic transformations) at Pd. This, in turn, will serve as critical data to inform the rational design of new catalytic transformations via the Pd manifold. 

Combined experimental and computational studies have sought to probe the intimate mechanism for the formation of C2-methylated imidazolium salts from [Pd (Me)(NHC)]-type complexes. These studies illustrated that the reactions were unlikely to occur via a migratory process, and that a concerted reductive elimination was energetically far more favourable. " In further DFT (density functional theory) studies. 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

This led in turn to the idea to also use combinations of different metals (e.g., Co/Rh, Co/Pt, Co/Fe, Co/Mo, Rh/Fe, Rh/Mn, Rh/Re, Rh/W, Rh/Mo) with the aim of creating synergy effects [2]. In the last decade, especially G arland and coworkers accumulated much evidence through spectroscopic measurements and density functional theory (DFT) calculations that in rhodium-catalyzed hydroformylation of non-isomerizable olefins (cyclopentene or 3,3-dimethylbut-l-ene), carbonyl complexes, which are less active in hydroformylation, such as HMn(CO)5 or FIRe(CO)5 [3], can support the reductive elimination of the aldehyde from... 

The octahedral dipyridyl Rh(III) complexes 9a-c (Scheme 2) were shown to be stable in solution for extended periods of time. Attempts to promote the back reaction (reductive elimination) by the addition of hard ligands such as triethylamine, dimethyl sulfoxide, or NA -dimethylaminopyridine were unsuccessful. This observation was rationalized in terms of hard/soft acid base theory. The resulting soft Rh(I) species would be unstable when coordinated to hard ligands. 

Catalytic mechanisms are proposed that invoke both aurophilic di-gold(I) intermediates and covalently bonded di-gold(II) entities. The proposed intermediates are semi-supported or unsupported, according to the definition of Figure 11.16. Geometries and energies of putative intermediates are calculated within density-functional theory the computations support a bimetallic pathway. Reductive elimination, induced by an arylboronic acid, proceeds with retention of stereochemistry at carbon. 

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