Olefins coordinated

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

A MOF constructed from rhodium paddlewheel clusters linked to porphyrinic ligands already discussed in Section 4.3.1.1 shows an interesting synergetic behavior when the porphyrinic rings are loaded with metals like Cu , Ni , or Pd . In the hydrogenation of olefins, the hydride species at the rhodium center is transferred to the coordinated olefin adsorbed on a metal ion in the center of the porphyrin ring to form an alkyl species, and next this alkyl species reacts with a hydride species activated at the rhodium center to form the alkane [81]. 

A family of cyclopentadiene(Cp)-containing iron-olefin complexes has been pioneered by Jonas [13-15]. The complexes 38-40 (Scheme 7) can be obtained in a large scale from ferrocene 37 under reducing conditions in the presence of suitable coordinating olefins. Complex 38 is a highly air-sensitive, crystalline material, whereas complexes 39 and 40 are more robust due to their cyclooctadiene (cod)... 

Gault and coworkers [ 149] have observed that the distribution of products obtained by hydrogenolysis and isomerization of methylcyclopentane was the same as those obtained with hexane. They proposed two competing mechanisms a selective mechanism implying an a, a, p, j0-tetra-adsorbed species and a non-selective mechanism implying coordinated olefin and bis-carbene intermediates (Scheme 38). 

Alkylidene-methylene coupling affording a coordinated olefin... 

A transition-metal-based olefin polymerization catalyst is generally comprised of a metal, ligand(s), a growing polymer chain, a coordinated olefin, and a cocatalyst (activator), as depicted in Fig. 5. 

An FI catalyst normally assumes a C2-symmetric trans-O, ds-N, and d.s-Cl configuration as the predominant isomer. In addition, DFT calculations suggest that a catalytically active species derived from an FI catalyst favors a C2-symmetric configuration with a trans-O, cis-N, and d.v-polymer chain/coordinated olefin arrangement. Thus, FI catalysts have been targeted as catalysts capable of producing iPP via a site-control mechanism. 

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

Synthesis of 63 and 64 supports the olefin oxidation mechanisms in Fig. 16. These mechanisms have several important and noteworthy points about Ptm chemistry (1) olefins coordinate to Ptm at the axial position, which is contrasted to the -coordination of olefins perpendicular to the square-planar coordination plane of Ptn. Olefin coordination to Pt(III) should also be contrasted to the fact that olefins do not coordinate to Pt(IV). (2) Platinum111 is strongly electron-withdrawing, and the coordinated olefins receive nucleophilic attack. (3) The alkyl ce-carbon on the Ptm undergoes nucleophilic attack in aqueous solution, whereas in aprotic solvent, aldhyde (and possibly also ketone in other cases) is produced by reductive elimination. 

In this proposed process, p-hydride elimination first yields a putative hydride olefin rc-complex. Rotation of the -coordinated olefin moiety about its co-ordination axis, followed by reinsertion produces a secondary carbon unit and therefore a branching point. Consecutive repetitions of this process allows the metal center to migrate down the polymer chain, thus producing longer chain branches. Chain termination occurs via monomer assisted p-hydrogen elimination, either in a fully concerted fashion as illustrated in Figure 2b or in a multistep associative mechanism as implicated by Johnson1 et al. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

Ligand 19 performs excellently with the wide variety of l,l -disubstituted olefins reported. Substrates 61a-m are efficiently reduced at 1 bar of hydrogen in high enantioselectivity with very little dependence on the bulk of the alkyl substituents. Strongly coordinating olefins such as 611 and 61m tyqrically perform poorly in iridium-catalyzed hydrogenations, but reduction with 19 clearly breaks this rule and the substrates are reduced in excellent selectivity and yield. 

Chelate complexes could only be prepared in the case of platinum(II) as the metal ion, while the group V atom alone acted as a donor toward palladium(II) and mercury(II). The coordinated olefin in the chelate complexes was found to be readily displaced by monodentate ligands such as tertiary arsines, -toluidine and the thiocyanate ion. It was suggested by these workers that chelation would take place more readily if the olefinic phosphine or arsine were subject to greater steric restrictions than was the pentenyl ligand. 

The coordinated olefin in PtBr2 (ap) is also displaced by excess ligand, ap, to give PtBr2 (ap)2, but this is a general reaction. 

Kouwenhoven (6) sjmthesised two bis-olefinic ligands related to the simple pent-4-enyl compounds mentioned above. These were of the general formula PhD[(CH2)aCH =CH2]2 (D = P, As) and formed platinum(II) and mercury(II) complexes. The platinum complexes LPtCla (L = PhD[(CH2)3 CH=CH2]2) were found to be monomeric species containing, as shown by their i. r. spectra, one coordinated and one free double bond. The two mercury complexes [LHgClaJs did not contain coordinated olefinic groups. 

As the chelate selenide complexes reacted with -toluidine mainly by displacing the coordinated olefinic group from the metal, Goodall assumed that they were more stable than the sulphur analogues. 

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