Binding olefin

A model consistent with the stereochemical outcome of the reaction and with the role of the halide has been proposed (Fig. 10). This model suggests side-on olefin binding and reorganization of the halide ligands. In such geometry, a steric interaction between the unbound olefin and apical halide may justify the dramatic increase in enantioselectivity observed upon changing the halide from cr to r. 

The complex Ni[(S2C2(CF3)2)]2 (392) is able to bind light olefins selectively and reversibly.1081 According to Scheme 4, the reaction of olefins with (392) can be controlled electrochemically, where the oxidation state-dependent binding and release of olefins is fast on the electrochemical timescale. Olefin binding is supposed to occur via the ligand S-donors. 

Figure 1 Remote olefin binding in transition state.

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

The change in the nature of the adsorption with increasing coverage (dissociative followed by associative) has been explained by a statistical consideration of the reaction mechanism shown above120). Associative adsorption is expected to occur at vacant sites for which all adjacent olefin binding sites are occupied by earlier dissociation products (or carbon monoxide, as shown by Fig. 6b), because dissociative adsorption (formation of vinyl and hydride species, followed by hydride migration to another alkene) requires two adjacent vacant sites. 

Figure 13.10. A typical olefin polymerization cycle, consisting of olefin binding followed by M—C...

C02 to the metal center directly parallels that of metal - olefin binding (36). For the olefin insertion reaction, activation parameters support a concerted reaction process as depicted above with a relatively low enthalpy of activation, i.e., simultaneous bond breaking and bond making, and a negative activation entropy (37). Thus far, no studies determining the corresponding activation parameters for the C02 insertion process have been reported. 

Gillespie, A.M. and White, D.P. (2001) Understanding the steric control of stereoselective olefin binding in cyclopentadienyl complexes of rhenium an application of de novo ligand design. Organometallics, 20, 5149. 

Once the [3+2] mechanism was accepted as the operative one, to investigate the transition states associated with the formation of the osmate ester in a chiral system, one must take into account all of the different ways that an olefin can approach the catalyst. These different paths were classified according to the criteria shown in Fig. 12. The olefin binds to an axial and to an equatorial oxygen, providing three different families of reaction paths labeled as regions A, B and C. When the olefin has one substituent, it can be placed in four different orientations , which are labeled as I, II, III, IV. The... 

Three distinct propagating species, each having different equilibrium olefin binding constants and insertion rate constants, are present. They are Cp2ZrH(olefin)+, Cp2ZrR (olefin)+ and Cp2ZrR(olefin)+. 

For dienes such as norbornadiene and butadiene, a 1 1 adduct with the Ni(S2C2R2)2 (R = Ph, CF3) is formed. The adduct was originally proposed to have the structure of (12) (62). Subsequent study showed that only 13 is formed (2,3-dimethylbutadiene is used to illustrate the structures) (69, 71). Crystal structure of the adduct between 1,3-cyclohexadiene and Pd(sdt)2 was unambiguously established in later work by Clark et al. (73) (14). The olefin binds to the sulfur atoms across the ligands. 

Prior to 1992, it was generally accepted that inversion of chirahty in olefin binding would require complete dissociation of the olefin in an intermolecular process unless... 

The reason that the minor reactive intermediate leads to the major product is due to the large rate constant for hydrogenation (k ) associated with the S cycle compared to the R cycle. Clearly, the conventional lock and key analogy for the origin of enantioselectivity does not apply for this case since the selectivity is determined by kinetics of hydrogenation instead of thermodynamics of olefin binding. 

Molecular mechanics modeling of the asymmetric hydrogenation must begin with the mechanism of the reaction. When the prochiral olefin binds to the catalyst containing chiral bidentate phosphine, two possible diastereomers result one with the re face and one with the si face of the olefin coordinated to the metal (Fig. 3). Work in the Halpern and Brown laboratories has shown that the observed enantiomeric product cannot result from the diastereomer observed in solution (17-20). Thus, the minor diastereomer, which cannot be observed, must be responsible for the dominant chiral product. Any molecular mechanics model of the asymmetric hydrogenation reaction must explain how the minor diastereomer reacts faster than the major. 

In molecular mechanics a chemical bond is considered to be composed of two spheres attached by a spring. Modeling of M-olefin systems presents a simple problem Where do we anchor the metal (Strictly speaking, the metal should be anchored to the center of the olefin C=C bond, but there is no atom at the C=C centroid to anchor the metal.) One approach is to bond the metal to both carbon atoms in the olefin. This creates a metallocycle, which is not a realistic model for olefin binding. An alternate approach is to define a pseudoatom (an atom with... 

Figure 13 Formation of branched versus internal olefins from the insertion of 1-butene into the growing polymer chain. Notice that the branched/inter-nal ratio depends on how the olefin binds to the metal. (Redrawn from Ref. 87.)...

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