Physical Organic Chemistry of Fischer Carbene Complexes
  • What are Fischer Carbene Complexes?
  • Current and Future Projects
  • References

    What Are Fischer Carbene Complexes?

    Fischer carbene complexes are compounds of the general structure 1. M is a transition metal, L are ligands while X and Y represent a variety of groups including alkyl, aryl, vinyl, alkynyl, amino, alkoxy, alkylthio, halo, and others; typically X or Y is a pi-donor such as alkoxy, amino, or alkylthio which provides stabilization of the electron deficient carbon. Some examples of Fischer carbene complexes that have been or are being studied in our laboratory include 2-8.

    Fischer carbene complexes have, over the last 30 years, become a major focus of many synthetic groups and the chemistry of these complexes has developed into an important branch of organometallic chemistry. On the other hand, until recently there have been only a few kinetic and/or thermodynamic studies of even the simplest reactions of these complexes. A few years ago our group started a research program aimed at studying the kinetics and thermodynamics of reactions of carbene complexes such as 2-8 with nucleophiles and bases. For example, many of these complexes are quite acidic (2, 4, 6, and 8) with pK a values in 50% aqueous MeCN between 4 (for 8) and 14.4 (for 4). The high acidity derives from the strong stabilization of the conjugate base by delocalization of the negative charge into the metal ligands, e.g. 6b-.

    Fischer carbene complexes are also highly electrophilic and prone to nucleophilic addition and substitution, e.g. eq 1.

    We have recently shown that in some cases the tetrahedral intermediate is directly observable which allows a direct kinetic determination of all the steps (k1, k-1, k2) of the reaction, e.g. in the reaction of eq 1 with thiolate ions (Ref. 13). Most of our past work has been summarized in a recent review (Ref. 12).

     

    Current and Future Projects

    Our current emphasis is in two areas. (1) Kinetics of proton transfers from acidic Fischer carbene complexes with the objective of determining intrinsic barriers and characterizing transition state structures (see The Big Picture and Proton Transfers) by means of structure-reactivity relationships (Brønsted aCH and bB). Examples include the reactions of 6, 8, 9-11 and others with amines, aryloxide ion, and carboxylate ions.

    (2) Kinetics of nucleophilic addition/substitution of carbene complexes with an emphasis on systems where the tetrahedral intermediate is directly observable because these systems allow again a determination of intrinsic barriers and a characterization of transition structure of reactions with poorly understood mechanisms. Examples include the reactions of 3 (also the Cr derivative) with MeO-, of 3 and 6 with amines and thiolate ions, and intramolecular reactions such as eq 2 with M = Cr and W, X = O or S, Y = O or S.

    Apart from filling a glaring gap in what is known about the reactivity of Fischer carbene complexes, including properties relevant to synthetic applications (e.g. pKa values), these studies should contribute to our understanding of the factors that affect intrinsic barriers of reactions in general. The metal moieties in 2-8 are more complex than the typical organic pi-acceptors and their interaction with pi-donor groups such as EtO, OMe, SMe, etc. leads to effects on intrinsic barriers and transition state structures (imbalance?) that are still poorly understood. More on this under Proton Transfers. Our rate studies will be supplemented by computational investigations, primarily involving ab initio methods.

    References

    1. J. R. Gandler and C. F. Bernasconi, "Proton Transfer from Methoxymethylcarbene(penta-carbonyl)chromium(0) in Aqueous Solution," Organometallics, 8, 2282-2284 (1989).
    2. C. F. Bernasconi and M. W. Stronach, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 2. Kinetics and Mechanism of Reactions of [Methoxy(phenyl)carbene]pentacarbonylchromium(0) with Primary Aliphatic Amines in Aqueous Acetonitrile," J. Am. Chem. Soc. 115, 1341-1346 (1993).
      3.
    3. C. F. Bernasconi and W. Sun, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 3. Thermodynamic and Kinetic Acidities of (CO)5 Cr=C(OCH3 )CH3 and (CO)5 Cr=C(OCH3 )CH2 Ph in Aqueous Acetonitrile," J. Am. Chem. Soc. 115, 12526-12532 (1993).
    4. C. F. Bernasconi, F. X. Flores, J. R. Gandler, and A. E. Leyes "Physical Organic Chemistry of Transition Metal Carbene Complexes. 4. Kinetics and Equilibria of Methoxide Ion Addition to [Methoxy(phenyl)carbene]pentacarbonylchromium(0)," Organometallics, 13, 2186-2193 (1994).
      5.
    5. C. F. Bernasconi, F. X. Flores and W. Sun, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 5. Kinetics and Mechanism of Hydrolysis of (CO)5 Cr=C(OCH3 )CH3 and (CO)5 Cr=C(OCH2 CH3 )CH3 in Aqueous Acetonitrile," J. Am. Chem. Soc. 117, 4875-4880 (1995).
    6. C. F. Bernasconi and W. Sun, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 6. Kinetics and Mechanism of the Thermal and Photochemical Hydrolysis of (CO)5 M=C(OMe)CH2 Ph (M = Cr and W) in Aqueous Acetonitrile," Organometallics 14, 5615-5621 (1995).
    7. C. F. Bernasconi, F. X. Flores and K. W. Kittredge, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 7. Kinetics of Hydrolysis of (CO)5 M=C(OR)Ph (M = Cr and W; R = Me and Et) and (CO) 5Cr=C(OMe)CH=CHPh in Aqueous Acetonitrile," J. Am. Chem. Soc. 119, 2103-2110 (1997).
    8. C. F. Bernasconi and W. Sun, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 8. Kinetic and Thermodynamic Acidities of (Alkoxyalkylcarbene)pentacarbonyl Complexes of Cr, Mo and W in Aqueous Acetonitrile. Dependence on Metal, Alkyl Group and Alkoxy Group," Organometallics 16, 1926-1932 (1997).
    9. C. F. Bernasconi and A. E. Leyes, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 9. Thermodynamic and Kinetic Acidity of (2-Oxacyclopentylidene)pentacarbonylchromium(0) in Aqueous Acetonitrile," J. Am. Chem. Soc. 119, 5169-5175 (1997).
      10.
    10. C. F. Bernasconi, W. Sun, L. Garcia-Río, K. Yan and K. W. Kittredge, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 10. Opposing Effects of a-Alkyl Groups on the The rmodynamic and Kinetic Acidities of (CO)5Cr=C(OMe)CHR¢R¢¢-Type Fischer Carbene Complexes i n Aqueous Acetonitrile. Analogy to the Nitroalkane Anomaly," J. Am. Chem. Soc. 119, 5583-5590 (1997).
    11. C. F. Bernasconi and A. E. Leyes, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 11. Kinetics and Mechanism of the Hydrolysis of (2-Oxacyclopentylidene)pentacarbonylchromium(0) in Aqueous Actetonitrile," J. Chem. Soc., Perkin Trans. 2, 1641-1647 (1997).
    12. C. F. Bernasconi, "Developing the Physical Organic Chemistry of Fischer Carbene Complexes," Chem. Soc. Rev. 26, 299-307 (1997).
      13.
    13. C. F. Bernasconi, F. X. Flores and K. W. Kittredge, "Physical Organic Chemistry of Transition Metal Carbene. 12. Spectroscopic Detection and Kinetic Characteristics of the Intermediate in the Nucleophilic Substitution Reaction of (CO)5Cr=C(OCH3)Ph with Thiolate Ion in Aqueous Acetonitrile," J. Am. Chem. Soc. 120, 7983-7984 (1998).
    14. C. F. Bernasconi, A. E. Leyes and L. García-Río, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 13. Kinetics of Proton Transfer from (5-Methyl-2-oxacyclopentylidene)pentacarbonylchromium(0) and Hydrolysis of Its Conjugate Anion in Aqueous Acetonitrile," Organometallics, in press.
    15. C. F. Bernasconi, A. E. Leyes. M. L. Ragains, Y. Shi, H. Wang, and W. D. Wulff, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 14. Thermodynamic Acidity Measurements of Fischer Carbene Complexes in Acetonitrile," J. Am . Chem. Soc. 120, 8632-8639 (1998).
    16. 16. C. F. Bernasconi and M. Ali, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 15. Kinetic and Thermodynamic Acidities of (Methylthiomethoxycarbene)pentacarbonyl complexes of Chromium and Tungsten in Aqueous Acetonitrile," J. Am. Chem. Soc. 121, 3039-3045 (1999).
    17. 17. C. F. Bernasconi, K. W. Kittredge, and F. X. Flores, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 16. Reactions of (CO)5M=C(OR)Ph (M = Cr or W; R = Me or Et) with Thiolate Ions in Aqueous Acetonitrile. Complete Kinetic Dissection of the Two-Step Mechanism," J. Am. Chem. Soc., 121, 6630-6639 (1999).
    18. 18. C. F. Bernasconi and M. Ali, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 17. Kinetics of the Reactions of (Arylthioalkoxycarbene)pentacarbonyl Complexes of Chromium(0) and Tungsten(0) with Thiolate Ions in Aqueous Acetonitrile. pKa Values of the Metal-Protonated Tetrahedral Adducts Formed between Carbene Complexes and Thiolate Ion," J. Am. Chem. Soc. 121, 11384-11394 (1999).
    19. 19. C. F. Bernasconi, M. Ali and F. Lu, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 18. Kinetics of Reversible Cyclization of the Fischer Carbene Complexes (CO)5M=C(SCH2CH2OH)Ph and (CO)5M=C(OCH2CH2OH)Ph (M = Cr and W) in Aqueous Acetonitrile," J. Am. Chem. Soc. 122, 1352-1359 (2000).
    20. 20. C. F. Bernasconi and L. Garc’a-R’o, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 19. Kinetics of Reversible Alkoxide Ion Addition to Substituted (Methoxyphenylcarbene)pentacarbonylchromium(0) and (Methoxyphenylcarbene)pentacarbonyltungsten(0) in Methanol and Aqueous Acetonitrile," J. Am. Chem. Soc. 122, 3821-3829 (2000).
    21. 21. C. F. Bernasconi, C. Whitesell, and R. A. Johnson, "Physical Organic Chemistry of Transition Metal Carbene Complexes. 20. Kinetics and Mechanism of Reactions of (CO)5M=C(OMe)C6H4X (M = Cr and W) with Primary Aliphatic Amines in Aqueous Acetonitrile. Substituent Effects," Tetrahedron, 56, 4917-4924 (2000).
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