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Deactivation of ru-benzylidene grubbs catalysts active in olefin metathesis

As well as this general difference in behaviour towards alkenes, first- and second-generation catalyst systems will react differently with some substitution patterns found in substrates. Type I alkenes react and dimerise quickly, but dimers are consumable; a number of alkenes have been charged to CM reactions as the dimer. Type II alkenes react and dimerise more slowly, and the corresponding dimers are consumed slowly.

Categorising each CM partner for a given reaction allows the outcome to be predicted. The category into which a given motif falls is a function of the pre-catalyst, with different behaviour observed for molybdenum complexes, and for first- and second-generation ruthenium complexes.

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Notably, the reactivity of second-generation complexes varies considerably, depending on the nature of the dissociating ligand and on the identity of the NHC ligand. The degree of thermodynamic control in a metathesis reaction will also depend on the pre-catalyst system; the issue of thermodynamic control has been reviewed recently by Fogg et al.

  • Scheme 10 A prototypical cross-metathesis reaction;
  • Categorising each CM partner for a given reaction allows the outcome to be predicted;
  • Users should refer to the original published version of the material for the full abstract;
  • The degree of thermodynamic control in a metathesis reaction will also depend on the pre-catalyst system; the issue of thermodynamic control has been reviewed recently by Fogg et al;
  • No warranty is given about the accuracy of the copy.

This was demonstrated experimentally by Percy and co-workers, who showed that the RCM of 1,8-nonadiene using different loadings of G2 and the ROMP of cycloheptene in the presence of ethene led to the same equilibrium mixture. While RCM to form smaller ca. Scheme 10 A prototypical cross-metathesis reaction.

Cavallo explored the early stages of metathesis with a series of complexes, from initiation to MCB formation, in one of the first DFT studies of metathesis that considered untruncated structures Scheme 11. In addition, the steric pressure exerted by the S IMes ligands destabilised the phosphine-bound and olefin-bound intermediates, leading to more energetically favourable metathesis but not increased initiation rate. Scheme 11 Structures explored by Cavallo.

For second-generation complexes, the energy difference between reactive and unreactive conformers was smaller than for first-generation complexes, explaining the observed reactivity differences. Attempts to optimise the structure of the reactive conformer led to MCB formation. A later study evaluated the bonding in active ruthenium carbene complexes to rationalise the effect of the NHC on their stabilisation. This is, of course, an interaction that is not available in first generation complexes.

Metathesis catalysts can react with alkynes and allenes, as well as alkenes, allowing elaborate cascade reactions. Sohn and Ihee used time-dependent fluorescence quenching studies to explore the affinity of metathesis catalysts for alkenes, allenes and alkynes.

During enyne metathesis, G1 favoured reaction with the alkene terminus first. Understanding this order of selectivity is important in the design of cascade metathesis reactions.

  1. Type I alkenes react and dimerise quickly, but dimers are consumable; a number of alkenes have been charged to CM reactions as the dimer. Scheme 10 A prototypical cross-metathesis reaction.
  2. The highly flexible NHC scaffold has allowed for a wide variety of complexes to be prepared; for example, N-alkyl-N-aryl-bearing NHCs have found application as selective ethenolysis catalysts, 93 and in the preparation of small cyclic oligomers instead of long-chain polymers.
  3. Metathesis catalysts can react with alkynes and allenes, as well as alkenes, allowing elaborate cascade reactions. Users should refer to the original published version of the material for the full abstract.
  4. Type I alkenes react and dimerise quickly, but dimers are consumable; a number of alkenes have been charged to CM reactions as the dimer. However, metathesis catalysts can be difficult to quench; careful treatment of samples is necessary to deactivate the catalyst, as concentration and analysis by GC while the catalyst is still active will lead to an effective increase in the reaction concentration and misleading data on the degree of conversion and oligomerisation.

The use of NHCs as ligands for metathesis pre-catalysts has allowed access to a vast range of interesting catalyst structural motifs, each with different reactivity. More favourable MCB formation in particular leads to an enhancement of catalytic performance. The highly flexible NHC scaffold has allowed for a wide variety of complexes to be prepared; for example, N-alkyl-N-aryl-bearing NHCs have found application as selective ethenolysis catalysts, 93 and in the preparation of small cyclic oligomers instead of long-chain polymers.

CM reactions seek the selective coupling of two different alkene partners, and therefore the challenge lies in this selectivity. In contrast, the RCM reactions of dienes can be complicated by competing CM processes to produce dimers or polymers. An understanding of the kinetics and thermodynamics of these processes can help mitigate the impact of these deleterious reactions on RCM.

Deactivation of Ru-benzylidene Grubbs catalysts active in olefin metathesis.

While the concept of effective molarity EM 95,96 Scheme 12 has been applied widely in the study of acid- and base-catalysed nucleophilic ring-closing chemistry, there are relatively few examples of this concept in metathesis chemistry. The authors also proposed that oligomeric material is an intermediate in the preparation of medium 7, 8 and 9-membered rings.

  • The degree of thermodynamic control in a metathesis reaction will also depend on the pre-catalyst system; the issue of thermodynamic control has been reviewed recently by Fogg et al;
  • Cavallo explored the early stages of metathesis with a series of complexes, from initiation to MCB formation, in one of the first DFT studies of metathesis that considered untruncated structures Scheme 11;
  • The category into which a given motif falls is a function of the pre-catalyst, with different behaviour observed for molybdenum complexes, and for first- and second-generation ruthenium complexes;
  • The category into which a given motif falls is a function of the pre-catalyst, with different behaviour observed for molybdenum complexes, and for first- and second-generation ruthenium complexes;
  • CM reactions seek the selective coupling of two different alkene partners, and therefore the challenge lies in this selectivity;
  • Scheme 13 RCM of simple prototypical diene substrates.

However, metathesis catalysts can be difficult to quench; careful treatment of samples is necessary to deactivate the catalyst, as concentration and analysis by GC while the catalyst is still active will lead to an effective increase in the reaction concentration and misleading data on the degree of conversion and oligomerisation.

Instead, slow oligomerisation competed with ring-closing. Scheme 12 Effective molarity to measure cyclisation efficiency. Scheme 13 RCM of simple prototypical diene substrates.

Importantly, it was shown that the reaction outcomes were predictable, using thermodynamic data available in the literature 98,101 or from DFT calculations Table 3. In this manner, the optimal initial reaction concentration can be selected on the basis of straightforward calculations, rather than by expensive and time-consuming trial and error.

References

The ratio of intra- to intermolecular products also depends on the degree of thermodynamic control. In the example above, the ring-opening of strained cycloalkenes is typically fast, allowing the reaction to reach a thermodynamic end point within hours.

For reactions where product ring-opening is slow, the final ratio of cyclic product to oligomer will depend predominantly on the rate of the formation of each species.

In contrast, where reactions are under thermodynamic control, the same equilibrium position will be achieved regardless of whether this position is approached from the substrate or from the product. Table 3 Calculated versus measured thermodynamic effective molarities for simple cycloalkene compounds Ring size.

  • Deactivation of Ru-benzylidene Grubbs catalysts active in olefin metathesis;
  • Importantly, it was shown that the reaction outcomes were predictable, using thermodynamic data available in the literature 98,101 or from DFT calculations Table 3;
  • Instead, slow oligomerisation competed with ring-closing.