Coordination chemistry of a novel phosphine-alkene ligand

1. Coordination chemistry of a novel phosphine-alkene ligand Luke Tuxworth 20thMarch 2013 EWPC10: Regensburg

2. What is a phosphine-alkene ligand? • A ligand that contains a phosphine and an alkene metal binding site • Why synthesise them? • Significant electronic asymmetry • Alkenesoften consider good π-acceptors • Phosphinesconsidered good σ-donors Lei Grützmacher A. Lei et al.Org. Lett., 2007, 9, 4571-4574. H. Grützmacheret al., Chem.-Eur. J., 2006, 12, 5849-5858.

3. The proposal • Exploit π-accepting nature of phosphine-alkene ligands. • Decrease in electron density on metal centre can promote reductive elimination (R.E.). • Slow R.E. has been shown to be a problem in some palladium-catalysed cross-coupling reactions. • Rate-limiting step

4. Novel phosphine-alkene ligand 31P (CDCl3) δ = 41.6 ppm P^= • Features: • Rigid structure imposed by norbornene-like moiety • Double bond in five-membered ring – favour alkene coordination W. Maisonet al., Org. Lett., 2006, 8, 1681-1684. V. Zunic et al., Org. Lett., 2002, 4, 3465-3468. L. Tuxworth and P. W. Dyer et al., Chem. Commun., 2012, 48, 10413-10415.

5. Studying the electronic character of the phosphine-alkene ligand

6. Probing the electronic character: 1JSeP • Poorly basic phosphorous centre, 1JSeP = 764 Hz. D. W. Allen and B. F. Taylor, J. Chem. Soc., Dalton Trans., 1982, 51-54. L. Tuxworth and P. W. Dyer et al., Unpublished results.

7. Probing electronic character: Cr(P^=)(CO)4 31P NMR(CDCl3) δ 131.7 ppm Yield = 44% IR (Nujol): 1884, 1915, 1952, 2017 cm-1 Cr(dppe)(CO)4: IR (Nujol): 1865, 1883, 1907, 2001 cm-1 J. A. Iggo and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1985, 1009-1013. L. Tuxworth and P. W. Dyer et al., Unpublished results.

8. Focus on the molecular structure P C1 Cr C2 • Phosphine and alkene have the same trans-influence L. Tuxworth and P. W. Dyer et al., Unpublished results.

9. Comparison of Cr-C bond lengths F. A. Cotton et al., ActaCryst. B, 1971, 27, 1899-1904. A. Weinkauf, ActaCryst.C, 1991, 47, 1087-1088. This work • Approximation of the donor/acceptor character of overall ligand.

10. Studying reductive elimination from palladium(II) Simple reductive elimination process

11. Reductive elimination of ethane 1:1 ligand:Pd 31P (d8-toluene) δ = 81.3 ppm 31P (d8-toluene) δ = 82.2 ppm L. Tuxworth and P. W. Dyer et al., Chem. Commun., 2012, 48, 10413-10415.

12. Reductive elimination of ethane 1:1 ligand:Pd 31P (d8-toluene) δ = 81.3 ppm 31P (d8-toluene) δ = 82.2 ppm • (P^=)PdMe2 thermally unstable • Calculated barrier to R.E. of ethane, ∆G‡ = 28.1 kcal mol-1* • cf. [PdMe2(dmpe)]; heating at 90 °C for 1 week required for ethane R.E. * B97D/SDD+f(Pd), 631G** (other atoms) level of theory G. van Kotenet al., Organometallics, 1989, 8, 2907-2917. L. Tuxworth, L. Estevez, K. Miqueu and P. W. Dyer et al., Unpublished results. L. Tuxworth and P. W. Dyer et al., Chem. Commun., 2012, 48, 10413-10415.

13. Molecular structures P P Pd P Pd C C C C C=C bond distance; 1.359(3) Å C=C bond distance; 1.401(2) Å L. Tuxworth and P. W. Dyer et al., Chem. Commun., 2012, 48, 10413-10415.

14. Accelerated reductive elimination of ethane 2:1 ligand:Pd Not observed • Rate of reaction significantly fasterthan in 1:1 case (5 d) • Barrier to R.E. reduced in 5-coordinate intermediate, (∆G‡ = 22.3 kcal mol-1)* * B97D/SDD+f(Pd), 631G** (other atoms) level of theory L. Tuxworth, L. Estevez, K. Miqueuand P. W. Dyer et al., Unpublished results. L. Tuxworth and P. W. Dyer et al., Chem. Commun., 2012, 48, 10413-10415.

15. Reaction profile 1:1 ligand:Pd Transition state 28.1 All values in kcal mol-1 L. Tuxworth, L. Estevez,K. Miqueu and P. W. Dyer et al., Unpublished results.

16. Reaction profile 1:1 ligand:Pdand 2:1 ligand:Pd Transition state Transition state 28.1 22.3 -5.2 Lower barrier pathway identified when excess ligand present All values in kcal mol-1 L. Tuxworth, L. Estevez,K. Miqueu and P. W. Dyer et al., Unpublished results.

17. Accelerated reductive elimination of ethane 1:1 ligand:Pd + 5 mol% PPh3

18. Studying reductive elimination from palladium(II) – harder systems • Can this ligand be applied to harder R.E. processes e.g. alkyl halide? • Simple case = Me/Cl R.E.

19. Reductive elimination of chloromethane? 31P (CDCl3) δ = 38.6 ppm • Formation of Pd(0) complex and phosphonium salt within seconds at -40 °C. • Addition of 3 equivalents of phosphine-alkene leaves no PdCl(Me)(COD) unreated. L. Tuxworth and P. W. Dyer et al., Chem. Commun., 2012, 48, 10413-10415.

20. What is the mechanism? • Direct MeCl R.E.? • Barrier to MeCl R.E. ∆G‡ = 48.9 kcal mol-1 * * B97D/SDD+f(Pd), 631G** (other atoms) level of theory L. Tuxworth, L. Estevez, K. Miqueuand P. W. Dyer et al., Unpublished results.

21. What is the mechanism? • R.E. of via 5-coordinate intermediate? • Proposed mechanism • Barrier to unassisted MeCl R.E. ∆G‡ = 33.2 kcal mol-1 * Unassisted pathway * B97D/SDD+f(Pd), 631G** (other atoms) level of theory L. Tuxworth, L. Estevez, K. Miqueuand P. W. Dyer et al., Unpublished results.

22. What is the mechanism? • R.E. of via 5-coordinate intermediate? • Proposed mechanism • Barrier to unassisted MeCl R.E. ∆G‡ = 33.2 kcal mol-1 * • Barrier to assisted MeCl R.E. ∆G‡ = 12.1 kcal mol-1 * Assisted pathway Unassisted pathway * B97D/SDD+f(Pd), 631G** (other atoms) level of theory L. Tuxworth, L. Estevez, K. Miqueuand P. W. Dyer et al., Unpublished results.

23. Conclusions • New phosphine-alkene ligand synthesised, can be considered electron-withdrawing. • Reductive elimination of ethane can be promoted in two ways. • Destabilisation of the palladium(II) 4-coordinate species, PdMe2(P^=) • Formation of a 5-coordinate intermediate • Reductive elimination of chloromethane has been shown

24. Acknowledgements PhoSciNet Dr P. W. Dyer • Dr A. S. Batsanov (Crystallography) • L. Estevez, J-M Sotiropoulos and K. Miqueu, Universitéde Pau (Computation) Lab 101 members past and present Durham University Analytical services Funding: Loan of palladium salts:

25. Thank you for listeningAny questions?