The importance of the electronic and steric features of the ancillary ligands on the rate of cis–trans isomerization of olefins coordinated to palladium(0) centre. A study involving (Z)-1,2-ditosylethene as olefin model
Graphical abstract
With the aim of better understanding the mechanisms governing the spontaneous cis–trans isomerization of the olefins promoted by palladium(0) centers, we have synthesized several novel complexes bearing (Z)- or (E)-1,2-ditosylethene and a combination of other two spectator ligands including phosphines (PPh3 and 1,3,5-triaza-7-phosphaadamantane), isocyanides (i.e. 2,6-dimethylphenylisocyanide) and N-heterocyclic carbenes (NHCs).
Introduction
Owing to the importance of palladium(0) olefin complexes as catalysts or pre-catalysts in the field of the homo and hetero cross-coupling processes [1] and alkyne hydrogenation [1b] we have been often involved in the studies on the stability and the related reactivity of such compounds for long time.
In particular, we have determined: (i) the scale of stability imparted by the different olefins to the derived complexes and consequently new synthetic protocols have been proposed [2]; (ii) the nature of the fluxionality affecting the palladium olefin derivatives [3]; (iii) the reactivity of palladium(0) complexes as a function of the their spectator ligands [4] and of the coordinated olefin [2], [5].
In this latter case, we have sometimes noticed that the palladium(0) complexes stabilized by the strong electron-withdrawing (Z)-1,2-ditosylethene (cis-disulf) promoted a spontaneous reaction yielding the corresponding complex bearing the isomerized olefin (E)-1,2-ditosylethene (trans-disulf) which in all cases represents the energetically favored species [2], [5].
Owing to the great importance of olefin isomerization processes in chemistry [6] and biology [7] and the originality of the observed process which is usually promoted by external factors [5 and Refs. therein], we decided to extend our knowledge on this kind of systems and carry out a reasonably exhaustive study on the capability of the different spectator ligands in stabilizing the cis-disulf derivatives or inducing its isomerization. In this respect we decided to synthesize several palladium(0) complexes stabilized by the olefins 1,2-ditosylethene (E and Z) and a selected combination of spectator ligands i.e. phosphines (PPh3 and 1,3,5-triaza-7-phosphaadamantane), isocyanides (i.e. 2,6-dimethylphenylisocyanide) and N-heterocyclic carbenes (NHCs). Although the strong σ-donating property is a common feature of these ligands, their steric and π-accepting nature can be modulated by a careful choice of the substituents thereby providing some more information on the mechanism involved in the olefin isomerization. Our interpretation of the mechanism of isomerization is discussed in this paper whereas the synthesized and studied complexes are reported in the following Scheme 1.
Section snippets
Synthesis of the olefin complexes with two identical spectator ligands
Complexes 1a–5a are thermodynamically stable whereas in the case of the 1b–4b complexes the isomerization of the coordinated cis-disulf cannot be excluded a priori. Therefore, the preparation of the complexes 1b–4b was checked by NMR in order to establish the rate of the reaction, the work-up conditions and the temperature. However, 1b–4b complexes do not show particular propensity to fast isomerization and only after several hours is a partial olefin rearrangement observed. However, in order
Conclusion
The number of the synthesized and fully characterized palladium(0) complexes coordinating a chosen range of ancillary ligands with different electronic and steric characteristics and the comparative kinetic study allow a more exhaustive interpretation than that advanced before of the mechanism of spontaneous isomerization of the coordinated (Z)-1,2-ditosylethene into the coordinated (E)-1,2-ditosylethene. Thus, the conclusions drawn elsewhere [2], [5] have been partially updated and in
Solvents and reagents
The solvent CH2Cl2 and CHCl3 were distilled over CaH2, acetone was refluxed over 4 Å molecular sieves and distilled. All other solvents and chemicals were commercial grade products and used as purchased.
IR, NMR, UV–Vis measurements and elemental analysis
The IR, 1H, 13C and 31P NMR spectra were recorded on a Perkin-Elmer Spectrum One spectrophotometer and on a Bruker 300 or 400 Advance spectrometers, respectively. The elemental analysis of the synthesized complexes was carried out using an Elemental CHN “CUBO micro Vario” analyzer.
Synthesis of the complexes Sa–c
Complexes Sa,
References (25)
- et al.
Chem Rev.
(2003) Acta Crystallogr., Sect. D
(2010)Acta Crystallogr., Sect. A
(2015)- et al.
Acta Crystallogr., Sect. D
(2010) J. Appl. Crystallogr.
(2012)The PyMOL Molecular Graphics System
(2015)- et al.
Theor. Chem. Acc.
(2008) - et al.
J. Chem. Phys.
(1985) - et al.
J. Chem. Theory Comput.
(2008) - et al.
J. Chem. Phys.
(1997)
Essentials of Computational Chemistry
Introduction to Computational Chemistry
Gaussian 09
Cited by (8)
Atomically dispersed palladium catalyzes H/D exchange and isomerization of alkenes via reversible insertion and elimination
2021, Chem CatalysisCitation Excerpt :The high activity, selectivity, durability, and feasible recyclability of this atomically dispersed Pd catalyst outperforms many other homogeneous and heterogeneous metal catalysts in this reaction.18,62 As distinguished from enhanced HMI selectivity by a bulky ligand of organometallics under mild conditions,63–65 the selective H/D exchange and HMI induced by heterolytically dissociated H2 at the atomically dispersed metal sites shows incomparable advantages. This result not only extends the application of atomically dispersed catalysts, but also indicates that some of the reported “inactive” catalysts for alkene hydrogenation are worth being carefully revisited for HMI.
A critical review of palladium organometallic anticancer agents
2021, Cell Reports Physical ScienceCitation Excerpt :The only category of palladium(0) derivatives whose anticancer properties have been investigated so far is that of Pd(0)-η2-olefin complexes. The nature of the Pd-olefin bond, the reactivity of this organometallic fragment toward different organic substrates, the catalytic activity of the corresponding complexes, and their thermodynamic stability depending on the type of coordinated olefin have been extensively studied in the last several decades.82–91 However, with the exception of Pd2(dba)3 (dba, dibenzylideneacetone), the anticancer properties of Pd(0)-olefin complexes remained substantially unexplored until 2018.
Biological and Catalytic Applications of Pd(II)-Indenyl Complexes Bearing Phosphine and N-Heterocyclic Carbene Ligands
2023, European Journal of Inorganic ChemistrySynthesis, characterization, and anticancer activity of ferrocenyl complexes bearing different organopalladium fragments
2022, Applied Organometallic ChemistryAlternative Transformations of N-Heterocyclic Carbene Complexes of the Group 11 Metals in Transmetalation Reactions (A Review)
2021, Russian Journal of General Chemistry