Olefin 7r-complexes

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

Type II may deactivate simply by reverting back to a tr-alkyl (I), followed by termination via a Ziegler route, namely halogen abstraction from a metal, or rearrangement to a 7r-olefin complex according to Eq. (15) ... 

The results do not prove that in the reaction conditions used the alkyl formation is not reversible, but only that, if it is reversible, the carbon monoxide insertion on both diastereomeric rhodium-alkyls must be much faster than the rhodium-alkyls decomposition. Restricting this analysis of the asymmetric induction phenomena to the rhodium-alkyl complexes formation, two 7r-olefin complexes are possible for each diastereomer of the catalytic rhodium complex (see Scheme 11). The induction can take place in the 7r-olefin complexes formation (I — II(S) or I — II(R)) or in the equilibrium between the diastereomeric 7r-olefin complexes (II(r) and... 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

The model can be identified with the diastereoisomeric 7r-olefin complexes, and thus AG° should, at least qualitatively, control the sign of the asymmetric induction. In this case, if AGhi(S) > AG ii(R), AG° must be larger than AAG. If AG°nasymmetric induction will correspond... 

We first established that hydrocarbonylation reactions occur with cis-stereochemistry (29, 16) and that asymmetric induction occurs before or during the formation of the metal alkyl intermediate (5, 6). This means that is either during the 7r-olefin complex formation between catalyst and substrate or during the insertion of the 7r-complexed olefin into the M-H bond. Therefore, the model should focus on the interactions between the substrate double bond and the catalytically active metal atom of the catalyst. 

The fact that a model for the transition state controlling asymmetric induction based on steric interactions allows us to correctly predict the type of prevailing regio- and stereoisomer for about 85% of the asymmetric hydrocarbonylation experiments (including hydroformylation and hydrocarbalkoxylation) is an indication that asymmetric induction in these catalytic reactions is based mainly on steric interactions. The data obtained so far do not allow us to establish whether the more stable or the less stable 7r-olefin complex intermediate is the one that reacts preferentially. However, the regularities that we observed indicate that the kinetic features are the same, at least in most of the experiments. 

The 13C spectra of some (3-methoxyalkyl mercuric chlorides have been reported.175 The results indicate that (Hg-C) ( 1600—1800 Hz) varies in direct proportion to the increase in electron density at the mercury-carbon bond, and that an increase in mercury-carbon alkyl substitution, an effect which is in direct contrast to that observed for platinum 7r-olefin complexes. 

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and /i-H transfer processes, leading to tantalumalkylidene and tantaliun 7r-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl hgands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

During the reaction, palladium metal precipitation was observed as would be expected in the vinylation reaction (Reaction 1). During product isolation, water is added to the reaction system. In most of the reactions run at 25°C. this addition resulted in further precipitation of palladium metal from the brown solution, probably owing to decomposition of the trace of 7r-olefin complex of palladium (II) present. However, the acetic acid solutions of products obtained from 100°C. reactions containing chloride were bright yellow and did not precipitate more palladium when water was added. This color is typical of 7r-allylpalladium chloride complexes and indeed di-/ix-chloro-di-7r-(methyl-3-ethylallyl) dipalladium (II) could be isolated from the reaction mixture. Formation of these complexes, 7r-olefin or 7r-allyl would, of course, result in decreased yields of vinylation products. 

A common pathway in palladium-catalyzed oxidation reactions is that the 7r-olefin complex formed reacts with a nucleophile, either external or coordinated, and the new organometallic intermediate may then undergo a number of different reactions (Scheme l) (i) an intramolecular hydride shift leads to ketone formation (ii) a )6-elimination results in the formation of a vinyl functionalized olefin (iii) an oxidative cleavage of the palladium-carbon bond produces a 1,2-functionalized olefin and (iv) an insertion reaction, exemplified by insertion of an olefin, leads to formation of a new palladium-carbon bond, which may be cleaved according to one of the previous processes ()6-elimination or oxidative cleavage). In all cases palladium has removed 2 electrons from the organic molecule, which becomes oxidized. These electrons, which end up on Pd(0), are in turn transferred to the oxidant and Pd(II) is regenerated, in this way a palladium(II)-catalyzed oxidation is realized. 

The current understanding of the nature of the metal-( 72-H-X) bonding interaction for various types of cr-complexes is based on the traditional Dewar-Chatt-Duncanson model for the well-known 7r-olefin complexes which emphasizes both ligand-to-metal a bonding and metal(d7r)-to-ligand(a )... 

This is the rate-determining step involving heterolytic splitting of the hydrogen molecule and formation of an hydridoruthenium(II) complex. The next step involves rearrangement of the hydrido-7r-olefin complex to a (T-alkyl complex via insertion of the olefin into the metal hydride bond. Finally, electrophilic attack occurs on the metal-bonded carbon... 

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