Four-center intermediate

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

As proposed in the case of molecular complexes of early transition metals, the mechanism of such C-H bond activation is expected to be a <7 bond metathesis process with four-center intermediates but this has not yet been substantiated by any theoretical approach. 

Kinetic studies show that hydrolysis of 1-organyl- and 1-alkoxysilatranes in neutral aqueous solutions is a first-order reaction catalyzed by the formed tris(2-hydroxyalkyl)amine13 294. As a rule, electron release and steric effects of the substituent X hinder the reaction. However, the hydrolytic stability of 1-methylsilatrane is just below that of 1-chloromethylsilatrane294. Successive introduction of methyl groups into the 3, 7 and 10 sites of the silatrane skeleton13,294 and substitution with ethyl group on C-459 retard sharply the hydrolysis rate. It was proposed294 that nucleophilic attack at silicon by water proceeds via formation of the four-centered intermediate 57 (equation 56). 

The semipolar Sb—S bond may easily allow formation of trimethylanti-mony halide thioalkoxide (XII) via a four-centered intermediate (XI). 

Although the heterolytic process here is formally a concerted ionic splitting of H2 as often illustrated by a four-center intermediate with partial charges, the mechanism does not have to involve such charge localization. In other words, the two electrons originally present in the H H bond do not necessarily both go into the newly-formed M H bond while a bare proton transfers onto L or, at the opposite extreme, an external base. The term a-bond metathesis is thus actually a better description and may comprise more transition states than the simple four-center intermediate shown above, e.g., initial transient coordination of H2 to the metal cis to L and dissociation of transiently bound H- L as the final step. Examples of this type of activation will be given in this Section. 

The reaction of trimethylantimony sulfide with acyl halide proceeds via a four centered intermediate yielding halo trimethylantimony thiocarboxylates335 ... 

A2 4 A-i accounts for the possible interrelation between the frequency factors of two monomolecular reactions the formation of a complex four-centere intermediate during the insertion reaction compared with simple olefin desorption The better agreement between the calculated A and experimental Ap values corresponds to the case for which the rate-determining step involves olefin coordination. 

In addition, the (=Si-0)2Tani-H center catalyzed the metathesis of alkanes, an unprecedented process affording higher and lower homologs. For example, in the metathesis of propane to ethane and butanes, formation of four-centered intermediates was postulated (Scheme 51).708 In the first step, the C—11 bond is activated to yield (=Si-0)2Ta I-Prn and (=Si-0)2Tam-Pr ... 

Information for other organometallic compounds is scarce. The corresponding methyl and ethyl derivatives of mercury, tin, and silicon were shown to undergo redistribution to yield random equilibrium mixtures of all possible metal alkyls. Rapid intermolecular exchange of the methyl groups in mixtures of trimethylaluminum-dimethylcadmium and dimethylzinc-dimethylcadmium was also demonstrated by NMR. The redistribution of mercury derivatives was found to be much slower. Further data are also available for thallium. - The four-center intermediate 154 was suggested to explain the observations [Eq. (6.149)] ... 

We have previously shown that the mononuclear zirconium hydride complexes 1 activate, under very mild conditions, the C-H bond of alkanes, including methane [7], The mechanism involves a four center intermediate, as proposed earlier for electrophilic activation of C-H bonds by group 3, 4 and lanthanides d° complexes [8], Given the similarities of the energies of dissociation of C-H and Si-H bonds, it is not surprising at ail that activation of Si-H bonds occurs with 1. Reactions of H/D exchange, followed by in situ IR spectroscopy, reveal that all types of silanes are activated, i.e. primary, secondary and even tertiary silanes [9],... 

The rate depends on the coordinating nature of the solvent, the more coordinating, the slower (despite the opposite trend for the self-exchange of CdMej, which can be explained, assuming a four-centered intermediate, by the competition of solvent and exchanging group for the vacant orbitals of the metal. 

Phenyl-substituted cyclopolyphosphines, cyclo-(PhP) , are known for n = 3, 4, 5 and 6. There is increasing evidence that cyclo-(PhP)5 is the thermodynamically most stable form. Both cyclo-(PhP)3 and cyclo-(PhP)4 spontaneously rearrange to cyclo-(PhP)5 by a mechanism that may involve four-centered intermediates. For the cyclo-(PhP)3 to cyclo-(PhP)j conversion the proposed scheme is ... 

The latter conversion occurs faster in THF than in aromatic hydrocarbons, suggesting that the proposed four-centered intermediate involves charge-separated stages of development stabilized by the more polar solvent . 

The hydroformylation reaction in which hydrogen is transferred to an acyl ligand provides another potential example of a four-center intermediate. DFT... 

Isotopic mixing experiments with the isomeric compounds methylacetylene (propyne) and allene showed that a four-center intermediate is important in many of their condensation reactions. The intermediate complex (C6H4D4) loses preferentially C2H2D2... 

Takao and co-workers [487] propose a mechanism involving a four-center intermediate having olefin and dioxygen in the coordination sphere. Benzaldehyde and a... 

The commonly accepted mechanism for the Homer-Wadsworth-Emmons reaction is as depicted in Scheme 1.6. Here, reaction of the phosphonate stabilized carbanion with an aldehyde forms the oxyanion intermediates 4 under reversible conditions. Rapid decomposition of 4, via the four-centered intermediates 5, then affords alkenes 6. 

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