Olefin insertion, irreversible

Lewis bases also accelerate the PKR by supposedly acting as weak ligands to promote the dissociation of CO and to stabilize intermediates. However, density functional theory (DFT) calculations described by Gimbert demonstrate no acceleration for the loss of CO in the presence of a Lewis base. Instead, the calculations suggest that the Lewis base stabilizes the cobaltacycle 1-4. This stabilization effectively makes the olefin insertion irreversible. Dimethylsufoxide, amines such as... 

Effects of GO and H2 partial pressures on the reaction rate and selectivity of asymmetric hydroformylation of 1-hexene and styrene are examined using (7 ,A)-BINAPHOS-Rh catalyst system. For both substrates, high GO partial pressure tends to retard the reaction the partial pressure of H2 hardly affects the reaction rate (Phz -5 MPa). In most cases, the regio- and enantioselectivities are independent of H2 and GO pressure. Deuterioformylation experiments clearly demonstrate the irreversibility of the olefin-insertion step at total pressures of 2-10MPa (D2/G0=I/I). This fact proves that the regio- and enantioselectivity of the present hydroformylation should be controlled by the olefin-insertion step. Herrmann reported the theoretical calculation of the olefin coordination step, explaining selectivity obtained with (i ,A)-BINAPHOS/Rh system for the hydroformylation of styrene. 

In their studies the authors consider two possible scenarios for the formation of linear and branched aldehydes assuming an irreversible olefin insertion step. In the first scenario, the olefin insertion proceeds through a single intermediate, and the lineanbranched product distribution is determined by the partitioning between the two forms (linear or branched) of the transition state (kinetic discrimination). In the second scenario, the linear... 

Carbon monoxide insertion in a palladium-carbon bond is a fairly common reaction [21]. Under polymerization conditions, CO insertion is thought to be rapid and reversible. Olefin insertion in a palladium-carbon bond is a less common reaction, but recent studies involving cationic palladium-diphosphine and -bipyridyl complexes have shown that olefin insertion also, particularly in palladium-acyl bonds, appears to be a facile reaction [22], Nevertheless, it is likely that olefin insertion is the slowest (rate-determining) and irreversible step vide infra) in polyketone formation. 

In summary, chain propagation involves alternating reversible carbon monoxide insertion in Pd-alkyl species and irreversible insertion of the olefin in the resulting Pd-acyl intermediates. The overall exothermicity of the polymerization is caused predominantly by the olefin insertion step. Internal coordination of the chain-end s carbonyl group of the intermediate Pd-alkyl species, together with CO/olefin competition, prevents double olefin insertion, and thermodynamics prevent double CO insertions. The architecture of the copolymer thus assists in its own formation, achieving a perfect chemoselectivity to alternating polyketone. 

In the case that the olefin insertion into a Rh-H bond is irreversible, the enantio-face of a prochiral olefin is discriminated during this step. Thus, the structure of the Rh complex at the transition state becomes important. Here, we introduce an example of a theoretical approach most recently reported by Herrmaim and his coworkers [77,78]. The two structures shown in Fig. 2 as TSI and TSII illustrate the possible transition states of olefin insertion into RhH(CO) (R,S)-BINAPHOS. Density functional theory calculations on model rhodium complexes bearing... 

Although a model was proposed in which olefin insertion occurred to place the metal on the terminus of the alkene ( 1,2-addition ) [10, 26], based upon subsequent mechanistic and synthetic studies of the hydrosilylation reaction of styrenes (see below), this model would appear to be incorrect [27]. Thus an irreversible, stereochemically determinant 2,1-insertion probably initiates the reaction, with subsequent o-bond metathesis completing the process. Most remarkable is the fact that, if correct, this model demands that the olefin insertion takes place to orient the highly hindered metal center at a tertiary carbon center, and that apparently little, if any, P-hydride elimination occurs from the resultant organometallic. 

The olefin insertion into the syn-q -allyl-Ni" bond of 2, 2 gives rise to the q q, A- rans,-decatrienyl-Ni" isomers of 5 and bis(q )-allyl,A-frans,-dodecatrienediyl-Ni" isomers of 11, respectively, in an exergonic, irreversible process. These isomers, where the allylic groups preferably adopt the mode, are the thermodynamically favorable forms of the decatrienyl-Ni" and dodecatrienediyl-Ni" complexes, which furthermore represent the active precursor species for their decomposition into Cjo- and Ci2-olefins, respectively. Butadiene insertion, although kinetically disfavored (see above), is thermodynamically favorable when compared with ethylene insertion. This leads to strongly stabilized bis(q ),A-trans,-dodecatrienediyl-Ni" species, which act as a thermodynamic sink in the catalytic reaction course, and hence is well suited for experimental isolation and characterization [3a, 6b, 26,27]. 

As shown in the mechanism depicted in (Scheme 18), the Pd-catalyzed silyl-carbocyclization of 1,6-dienes involves a reversible insertion of an olefin moiety into the [Pd]-Si bond (E-II to E-III). However, the coordination of the second olefin moiety to the Pd metal (forming E-IV) would fill the coordination site required for the j8-silyl-elimination and would therefore render the C—Si bond formation irreversible, which leads to the irreversible carbometalation to jdeld E-V. Accordingly, when the chiral Pd(pyridine-oxazoline) complex is used as the catalyst, the enantioselectivity should be determined at the first olefin insertion step forming -silylalkyl-[Pd] complex E-III. 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

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