Thermodynamics of N-heterocyclic carbene dimerization: The balance of sterics and electronics

Organometallics

Volumn 27, Issue 12, 2008, Pages 2679-2681

  Poater, Albert ^a   Ragone, Francesco ^a   Giudice, Simona ^a   Costabile, Chiara ^a   Dorta, Reto ^b   Nolan, Steven P ^c   Cavallo, Luigi ^a  

^a UNIVERSITY OF SALERNO   (Italy)

^b UNIVERSITY OF ZURICH   (Switzerland)

^c INSTITUTE OF CHEMICAL RESEARCH OF CATALONIA ICIQ   (Spain)

Author keywords

[No Author keywords available]

Indexed keywords

ORGANOMETALLICS; PHOSPHORUS COMPOUNDS;

ANCILLARY LIGANDS; BALANCE (WEIGHTING); CHEMICAL ROUTES; N HETEROCYCLIC CARBENES (NHC); N-HETEROCYCLIC CARBENE (NHC); NEW PRODUCTS; NHC LIGANDS; ORGANOMETALLIC CHEMISTRY; PHOSPHINES (PH3); STERICS; TERTIARY PHOSPHINES;

LIGANDS;

EID: 47249106966     PISSN: 02767333     EISSN: None     Source Type: Journal    
DOI: 10.1021/om8001119     Document Type: Article

References (26)

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12. There is ample experimental evidence that NHCs derived from benzimidazolium and imidazolinium salts dimerize upon deprotonation. Dimerization of deprotonated imidazolidium salts seems more difficult. For a rare example, see: Taton, T. A.; Chen, P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1011.
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15. DFT have been calculated as the energy difference between the optimized geometries of the dimer NHC=CHN and of two monomers NHC See ref 7 and the Supporting Information for details.
16. The DFT calculations were performed with the Gaussian03 package, using the GGA BP86 functional, with the TZVP basis set of Ahlrichs on main group atoms, and the relativistic Stuttgart ECP basis set on Ir. Calculations were performed in the gas phase and in solution (the PCM solvation model has been used to simulate THF as the solvent). See the Supporting Information. This approach results in a dimerization enthalpy of-8.8 kcal/mol for 2c, versus an experimental value of-13.7 ±0.6 kcal/mol; see ref 5.
17. S-T has been calculated as the energy difference between the DFT-optimized geometries of the monomer NHC in the triplet and singlet electronic states, respectively. See ref 7 and the Supporting Information.
18. 2Cl complex. See ref 7 and the Supporting Information for details.
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21. Sometimes, the alkali-metal base used for deprotonation is not innocent and can coordinate to the nascent carbene; see:Alder, R. W.; Blake, M. E.; Bortolotti, C.; Bufali, S.; Butts, C. P.; Linehan, E.; Oliva, J. M.; Orpen, A. G.; Quayle, M. J. Chem. Commun. 1999, 241.
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23. As an example, too bulky NHC ligands are detrimental in the case of the ring-closing metathesis of tri- and tetrasubstituted substrates See: (a) Stewart, I. C.; Ung, T.; Pletnev, A. A.; Berlin, J. M.; Grubbs, R. H.; Schrodi, Y Org. Lett. 2007, 9, 1589.
24. (b) Berlin, J. M.; Campbell, K.; Ritter, T.; Funk, T. W.; Chlenov, A.; Grubbs, R. H. Org. Lett. 2007, 9, 1339, as well as in the case of ring-opening metathesis polymerizationSee:
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