Dependence of the Wittig reaction mechanism on the environment and on the substituents at the aldehyde group and at the phosphonium ylide (Articolo in rivista)

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  • Dependence of the Wittig reaction mechanism on the environment and on the substituents at the aldehyde group and at the phosphonium ylide (Articolo in rivista) (literal)
Anno
  • 2010-01-01T00:00:00+01:00 (literal)
Alternative label
  • Alagona* G., Ghio C. (2010)
    Dependence of the Wittig reaction mechanism on the environment and on the substituents at the aldehyde group and at the phosphonium ylide
    in International journal of quantum chemistry
    (literal)
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  • Alagona* G., Ghio C. (literal)
Pagina inizio
  • 765 (literal)
Pagina fine
  • 776 (literal)
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  • In THF both the concerted and stepwise mechanisms for the two models can be compared with those produced by sys. Again the Ph3 model is in better agreement with sys, although differences exist between both models and sys. In particular, in the reference system, the barriers for TS1 and TSb are so low that cannot be located, whereas they are appreciable in the models, especially for the free energy values in solution including ZPE and thermal corrections. Furthermore, using the Me3 model for the stepwise mechanism, a transition state (TS120) between Ba and OP1 can be located. The related free energy surface in THF studied using as leading parameters PC1C2O and P–C1 confirms that OP1 is on the pathway to TS2 eventually yielding the products. Conversely, using the Ph3 model, an alternative pathway directly connecting Bg (or B’g) to TS2 cannot be ruled out. In summary, the behavior of both models is occasionally similar to that of the bulky system. If necessary, the Ph3 model is preferable in THF as well. (literal)
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  • 110 (literal)
Rivista
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  • Published online 20 April 2009 in Wiley InterScience (www.interscience.wiley.com) - DOI 10.1002/qua.22207 - Additional Supporting Information may be found in the online version of this article. (literal)
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  • The concerted and stepwise mechanisms of the Wittig reaction have been investigated in vacuo at the B3P86/6-31G* level and in THF in the IEF-PCM/B3P86/6-31G* framework to elucidate the performance of two model systems as compared to the results recently obtained at the same levels for a bulky chiral aldehyde [(2S,3R)-2,4-dimethyl-3-pyrrol-1-yl-pentanal] reacting with the unsubstituted triphenylphosphonium ylide (this reference system is named sys). The models, named respectively Me3 and Ph3, consist of acetaldehyde and either the trimethylphosphonium or triphenylphosphonium ylide (Me3PCH2 or Ph3PCH2). A systematic analysis of the structures and stabilities of the various stationary points on the potential and free energy surfaces (at 298 K and 1 atm) has been carried out in vacuo. The reaction with the Me3 model occurs only with a concerted mechanism, whereas with the Ph3 model a gauche betaine-type intermediate is located in vacuo (Bg). This behavior is similar to that observed in sys, where however two betaine-type intermediates are obtained. The energy and free energy profiles of both models for the concerted mechanism are consistent with the sys ones, although the barriers are underestimated with the Me3 model. Therefore these results support the use of the realistic ylide (Ph3PCH2). (literal)
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  • ISI Web of Science (WOS) (literal)
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  • IPCF-CNR MML, Pisa (literal)
Titolo
  • Dependence of the Wittig reaction mechanism on the environment and on the substituents at the aldehyde group and at the phosphonium ylide (literal)
Abstract
  • The B3P86/6-31G* mechanistic results for the Wittig reaction of acetaldehyde, CH3CHO, in vacuo and in tetrahydrofuran (THF) solution in the IEF-PCM framework, with an unsubstituted trimethylphosphonium or triphenylphosphonium ylide, that is Me3PCH2 or Ph3PCH2, have been compared to those recently obtained at the same levels for the reaction of Ph3PCH2 with a bulky chiral aldehyde, (2S,3R)-2,4-dimethyl-3-pyrrol-1-yl-pentanal [TCA (submitted)], here named sys for short. The two model systems show distinct, but similar, behaviors that however differ from the large system one. In particular, betaine-type intermediates are not located in vacuo when Me3PCH2 is used, while only a gauche betaine is obtained using Ph3PCH2; the relevant barriers are anyway smaller than those found for sys. Conversely, in THF, the concerted and stepwise mechanisms are both represented and show TS1 / TSb barriers, that are negligibly small for sys. Thus, in contrast to assessed literature, models with methyl groups in place of phenyl rings and branched aldehyde chains show a different behavior from realistic systems and prevent inferring general rules from their use, suggesting to resort to Ph3PCH2 whose results in vacuo and in THF are closer to sys. (literal)
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