Chemistry and catalysis advances in organometallic chemistry and catalysis
Download 11.05 Mb. Pdf ko'rish
|
|
Figure 4.11 Calculated Gibbs free energies (in kcal/mol; relative to free (PCP)Ir and methyl tosylate) for the reaction of (PCP)Ir with methyl tosylate. 2.9, whereas a rate-determining step involving H-migration is predicted to yield a significantly greater KIE of 6.7. Comparing these values to our experimentally determined KIE of 2.4, we clearly see much better agreement with rate-determining α-OTs
migration. 4.3 CLEAVAGE AND OXIDATIVE ADDITION OF C–F BONDS Having made headway into facile C–O bond cleavage via initial C–H bond activation, we next sought to extend the same strategy to the activation of C–F bonds. Although oxidative addition of C–Cl, C–Br, and C–I bonds to metal centers is well studied [66], there is no precedent for oxidative addition of unactivated C(sp 3 )–F bonds. Thus, C(sp 3 )–F oxidative addition is even less well precedented than C(sp 3 )–O oxidative addition; perhaps, this is not surprising in the light of the greater bond dissociation energies of C(sp 3 )–F bonds (e.g., the C–F BDE of fluoromethane is 110 kcal/mol while the anisole C(sp 3 )–O BDE is only 65 kcal/mol) [67]. Note, however, there are numerous reported examples of C–F oxidative addition (and reductive elimination), generally involving fluorine bound to sp 2 -hybridized vinyl or aryl carbon atoms (Fig. 4.12) [68–81]. 52 ACTIVATION OF C–O AND C–F BONDS BY PINCER–IRIDIUM COMPLEXES F 6
Ni F L L F 5 N NH 2 F F F F F W(CO)
3 (PrCN)
3 N NH F F F F W CO OC CO F Rh Cp*
Me 3 P + F 6 hv Rh Cp* Me 4 P F F 6 − C 2 H 4 Reference 74 Reference 68 References 69,70 65 %
3 PrCN Figure 4.12 Some notable examples of oxidative addition of C sp 2
Ir(NBE) P
B u
P t B u 2 + CH
3 F 50 °C p-xylene-d 10 NBE Ir P
B u
P t B u 2 CH 3 F Scheme 4.20 Ir(NBE)
P t Bu 2 P t Bu 2 + F R R 60 °C p-xylene-d 10 NBE Ir P P t Bu
Bu F
R R = CH
3 R = CF
3 Scheme 4.21 Nevertheless, the C(sp 3 )–OR cleavage chemistry indicated that the tendency toward oxidative addition correlated with the electron-withdrawing ability of the R group. This suggested that C(sp 3 )–F addition would be even more favorable than any of the C(sp 3 )–OR additions. Indeed, (PCP)Ir(NBE) was found to react readily with fluoromethane at 50 ◦ C to yield a single major species, the oxidative addition product (PCP)Ir(CH 3 )(F) (Scheme 4.20) [33]. Likewise, benzyl fluoride derivatives [containing either methyl or trifluoromethyl substituents meta to the –CH 2 F group to prevent C(sp 2 )–H activation] also react readily at 60 ◦ C to afford (PCP)Ir(F)(CH 2 Ar ) complexes (Scheme 4.21). In the case of the trifluoromethyl-substituted substrate, a clean conversion to the product in 95% yield is observed, while the methyl substituted substrate exhibits a lower yield (70%) with significant amounts of unidentified side products. As in the case of the C–O addition reactions, a KIE was measured to probe the reaction pathway. (PCP)Ir was reacted with a fivefold excess of 3,5-bis(trifluoromethyl)benzylfluoride and its deuterated (CD 2 F) analog at 60 ◦ C (Fig. 4.13). The KIE (k
2 F /k CD 2 F ) was determined to be 2.7, indicating that the C–F addition proceeds via C–H addition. Analogously to the C–O additions, we propose that the reaction proceeds via the initial C-H activation to yield the five-coordinate CLEAVAGE AND OXIDATIVE ADDITION OF C–F BONDS 53 Ir(NBE)
P t Bu 2 P t Bu 2 + F F 3 C CF 3 F F 3 C CF 3 D D Ir P P t Bu
Bu
Bu
Bu F
3 CF 3 Ir P P F C CF 3 CF 3 + D 2 NBE Figure 4.13 Competition kinetic isotope effect experiment for the addition of 3,5-bis(trifluoromethyl)benzylfluoride and its deuterated analog to (PCP)Ir(NBE). Ir(NBE)
P t Bu 2 P t Bu 2 + CH 3 F Ir P
Bu 2
t Bu 2 H CH 2 F Ir P t Bu 2 P H CH 2 F
Bu Ir
t Bu 2 P t Bu 2 CH 3 F C–H addition α-fluorine elimination 1,2 H-migration
Mechanism for the net oxidative addition of CH 3 -F to (PCP)Ir. Ir(NBE) P
Bu 2
t Bu 2 + CF 3 H Ir P
Bu 2
t Bu 2 H CF 3 Ir P
Bu 2
H CF 2 F t Bu 2 Ir P
Bu 2
t Bu 2 CF 2 HF C-H addition α-fluorine elimination H-F Elimination Figure 4.15 Mechanism for the reaction of (PCP)Ir(NBE) and CHF 3 to yield the difluorocarbene complex, (PCP)IrCF 2 . Ir(III) intermediate, (PCP)Ir(H)(CH 2 F ), followed by α-F elimination to yield a methylidene intermediate, and then hydride migration from iridium to the carbene to afford the overall oxidative addition product (Fig. 4.14). In an effort to observe the proposed intermediates (or analogs thereof), we investigated the reaction of trifluoromethane [33] with the hope that additional fluoro groups might stabilize either of the two intermediates shown in Fig. 4.14 [82–84]. Indeed, at moderately low temperature ( −10
◦ C), an intermediate species was observed that is apparently the C–H addition intermediate (Fig. 4.15), most characteristically evidenced by a broad hydride triplet at −45.5 ppm (this high upfield shift is consistent with a square pyramidal complex where the hydride is trans to a vacant coordination site). On warming to room temperature, conversion of this intermediate to a major new product characterized as the four-coordinate square planar difluorocarbene, (PCP)Ir(CF 2 ), occurred, along with a minor product corresponding to (PCP)Ir(CO) and an additional unidentified product. The four-coordinate carbene product would result from HF elimination from the putative six-coordinate (PCP)Ir(CF 2 )(H)(F) complexes (Fig. 4.15). The reaction of four-coordinate metal difluorocarbene complexes with adventitious water has been previously shown to yield corresponding metal carbonyls [83–85]. The unidentified product revealed spectroscopic features strongly indicative of metal bifluoride complexes presumably resulting from reaction with free H–F [86]. The pathway leading to net C–F oxidative addition via initial C–H activation is supported by DFT calculations (Fig. 4.16). Direct oxidative addition of the C–F bond of fluoromethane was calculated to have an activation barrier of 31.1 kcal/mol relative to free fluoromethane and (PCP)Ir [or 37.5 kcal/mol relative to (PCP)Ir(NBE)], corresponding to the three-centered TS for C–F cleavage. In contrast, the pathway leading to net C–F oxidative addition through C–H activation has a calculated barrier of 16.5 kcal/mol relative to free fluoromethane and (PCP)Ir, or 22.9 kcal/mol relative to (PCP)Ir(NBE), with α-elimination of fluorine being the rate-determining step. This calculated value of G = is fully consistent with the experimentally determined (approximate) reaction rate. 54 ACTIVATION OF C–O AND C–F BONDS BY PINCER–IRIDIUM COMPLEXES P
Bu 2 P t Bu 2 Ir P
Bu 2
t Bu 2 Ir H CH 2 F CH 3 F +
P t Bu 2 P t Bu 2 Ir H CH 2 F 5.7 0.1 (0.0)
−1.9 P
Bu 2
t Bu 2 Ir H CH 2 F 16.5 P t Bu 2 P t Bu 2 Ir H F CH 2 3.9 11.8 P
Bu 2
t Bu 2 Ir H F CH 2 −20.4 P t Bu 2 P t Bu 2 Ir CH 3 F P
Bu 2
t Bu 2 Ir CH 3 F 31.1
P t Bu 2 P t Bu 2 Ir H CH 2 F −6.4 P t Bu 2 P t Bu 2 Ir(NBE) CH 3 F + Figure 4.16 Calculated Gibbs free energies (in kcal/mol; relative to free (PCP)Ir and CH 3 F) for the reaction of (PCP)Ir and CH 3 F. Ir(NBE) P t Bu 2 P t Bu 2 + CH 3 CH 2 F 25 °C Ir P t Bu 2 P t Bu 2 Ir P t Bu 2 P t Bu 2 H F + Scheme 4.22 Ir(NBE)
PH t Bu 2 P t Bu 2 + 80 °C F Ir P t Bu 2 P t Bu 2 H F +
Scheme 4.23 As discussed earlier, alkyl oxygenates with β-C–H bonds undergo 1,2-H–O elimination as opposed to C–O addition. The analogous behavior is observed with alkyl fluorides. Thus, the reaction of (PCP)Ir(NBE) with fluoroethane results in rapid formation of an equimolar mixture of (PCP)Ir(H)(F) and (PCP)Ir(ethylene) (Scheme 4.22). Reaction of (PCP)Ir(NBE) with 2-fluoropropane, followed by heating to 80 ◦ C, resulted in quantitative conversion to (PCP)Ir(H)(F) (Scheme 4.23). The mechanism for the reaction of alkyl fluorides bearing a β-hydrogen atom, in analogy with the oxygenates, likely proceeds via initial C–H oxidative addition to give (PCP)Ir(H)(fluoroalkyl), which then undergoes β-F elimination to yield ACKNOWLEDGMENTS 55 Ir(NBE)
P t Bu 2 P t Bu 2 + R F Ir P t Bu 2 P t Bu 2 H F R Ir P t Bu 2 P H F t Bu R Ir P t Bu 2 P t Bu 2 H F (PCP)Ir(NBE) NBE
Ir P t Bu 2 P t Bu 2 Figure 4.17 Mechanism for the reaction of (PCP)Ir(NBE) with fluoroethane and 2-fluoropropane. (PCP)Ir(H)(F)(olefin) (Fig. 4.17). Presumably, ethylene is lost from this six-coordinate species to afford the hydrido fluoride product; the free ethylene then displaces NBE from (PCP)Ir(NBE) to form the (PCP)Ir(ethylene). For 2-fluoropropane, the same general mechanism applies, but the propylene formed from β-fluorine elimination binds less strongly than ethylene to (PCP)Ir, thus eventually allowing quantitative conversion to (PCP)Ir(H)(F).
We have found that typically unreactive C(sp 3 )–O and C(sp 3 )–F bonds, including those of methyl aryl ethers (with electron- poor aryl groups), methyl esters, methyl tosylate, and methyl or benzyl fluoride, can undergo relatively unprecedented oxidative additions of the C(sp 3 )–X bond to the 14-electron fragment (PCP)Ir (X = OR, including OAr, OAc, or OTs, or X = F). Perhaps even more surprisingly, the reactions are found to proceed not via direct oxidative addition of the C–X bond (which is computationally predicted to have a prohibitively high energy barrier), but rather through the initial activation of a C–H bond positioned α to the O or F atom. In the case of alkyl oxygenates or fluorides in which the alkyl groups have H atoms in the β-position, reaction with (PCP)Ir also results in C–X bond cleavage, but of a very different type, namely, 1,2-dehydro-oxygenation or 1,2-dehydrofluorination. In this case, the reaction appears to proceed via oxidative addition of a β-C–H bond, followed by β-C–X migration, to give (PCP)IrHX and olefin. While C–H addition has been an intensively studied field of chemistry for several decades now, this has almost invariably been with an eye toward “functionalizing the C–H bond” in some manner. This work indicates that C–H addition can represent a route toward activating a different bond in the molecule, in this case a C–X bond located either α or β to the C–H bond that undergoes addition. By the principle of microscopic reversibility, these reaction mechanisms suggest new routes for the formation of C(sp 3 )–X
bonds. Perhaps most intriguing, the mechanism elucidated for C–X oxidative addition implies that its microscopic reverse, C–X reductive elimination, must occur, for these systems, via an initial α-H migration from alkyl group to the metal center to generate the key alkylidene intermediate. The generality of this with respect to systems beyond (PCP)Ir(alkyl)X remains to be determined, but given that reductive elimination from late metal systems often proceeds via five-coordinate d 6 complexes, this mechanism may well be more broadly applicable. Likewise, the mechanism of the 1,2-H–X elimination reactions may offer insight applicable to the design of potential catalysts for 1,2-H–X addition reactions. ACKNOWLEDGMENTS The studies that are the focus of this chapter were conducted, with great thought, skill, and dedication, by Jongwook Choi, Sabuj Kundu, Yuriy Choliy, David Y. Wang, Xiawei Zhang, and Thomas J. Emge. This material is based on work supported by the National Science Foundation under Grant CHE #1112456 to A.S.G and K.K.-J. J.H. thanks the NSF IGERT program (Renewable and Sustainable Fuel Solutions for the 21st Century) for a Graduate Training Fellowship.
56 ACTIVATION OF C–O AND C–F BONDS BY PINCER–IRIDIUM COMPLEXES REFERENCES 1. Albrecht, M.; van Koten, G. Angew. Chem., Intl. Ed. 2001, 40 , 3750. 2. The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C., Eds.; Elsevier: Amsterdam, 2007. 3. van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103 , 1759. 4. Leis, W.; Mayer, H. A.; Kaska, W. C. Coord. Chem. Rev. 2008, 252 , 1787. 5. Benito-Garagorri, D.; Kirchner, K. Acc. Chem. Res. 2008, 41 , 201. 6. Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111 , 1761. 7. Haibach, M. C.; Kundu, S.; Brookhart, M.; Goldman, A. S. Acc. Chem. Res. 2012, 45 , 947. 8. Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2000, 122 , 11017. 9. Zhang, X.; Kanzelberger, M.; Emge, T. J.; Goldman, A. S. J. Am. Chem. Soc. 2004, 126 , 13192. 10. Ghosh, R.; Zhang, X.; Achord, P.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2007, 129 , 853. 11. Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem. Commun. 1996, 2083. 12. Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121 , 4086. 13. Krogh-Jespersen, K.; Czerw, M.; Summa, N.; Renkema, K. B.; Achord, P. D.; Goldman, A. S. J. Am. Chem. Soc. 2002, 124 , 11404. 14. Krogh-Jespersen, K.; Czerw, M.; Kanzelberger, M.; Goldman, A. S. J. Chem. Inf. Comput. Sci. 2001, 41 , 56. 15. Renkema, K. B.; Kissin, Y. V.; Goldman, A. S. J. Am. Chem. Soc. 2003, 125 , 7770. 16. Goldman, A. S.; Renkema, K. B.; Czerw, M.; Krogh-Jespersen, K. In Activation and Functionalization of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.; ACS Symposium Series, Vol. 885 ; American Chemical Society: Washington, DC, 2004; p 198. 17. Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2004, 126 , 13044. 18. G¨ottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126 , 1804 19. Kundu, S.; Choliy, Y.; Zhuo, G.; Ahuja, R.; Emge, T. J.; Warmuth, R.; Brookhart, M.; Krogh-Jespersen, K.; Goldman, A. S.
20. Ahuja, R.; Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.; Brookhart, M.; Goldman, A. S. Nature Chem. 2011, 3 , 167. 21. Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science 2006, 312 , 257. 22. Huang, Z.; Rolfe, E.; Carson, E. C.; Brookhart, M.; Goldman, A. S.; El-Khalafy, S. H.; MacArthur, A. H. R. Adv. Synth. Catal. 2010, 352 , 125. 23. Bailey, B. C.; Schrock, R. R.; Kundu, S.; Goldman, A. S.; Huang, Z.; Brookhart, M. Organometallics 2009, 28 , 355. 24. Santana, R. C.; Do, P. T.; Santikunaporn, M.; Alvarez, W. E.; Taylor, J. D.; Sughrue, E. L.; Resasco, D. E. Fuel 2006, 85 , 643. 25. Morales-Morales, D.; Lee, D. W.; Wang, Z.; Jensen, C. M. Organometallics 2001, 20 , 1144. 26. Morales-Morales, D.; Redon, R.; Wang, Z.; Lee, D. W.; Yung, C.; Magnuson, K.; Jensen, C. M. Can. J. Chem. 2001, 79 , 823. 27. Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao, J.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125 , 13644. 28. Zhao, J.; Goldman, A. S.; Hartwig, J. F. Science 2005, 307 , 1080. 29. Morgan, E.; MacLean, D. F.; McDonald, R.; Turculet, L. J. Am. Chem. Soc. 2009, 131 , 14234. 30. Ghosh, R.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2008, 130 , 11317. 31. Choi, J.; Choliy, Y.; Zhang, X.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2009, 131 , 15627. 32. Kundu, S.; Choi, J.; Wang, D. Y.; Choliy, Y.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc., 2013, 135 , 5127. 33. Choi, J.; Wang, D. Y.; Kundu, S.; Choliy, Y.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. Science 2011, 332 , 1545. 34. Ittel, S. D.; Tolman, C. A.; English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1978, 100 , 7577. 35. Tolman, C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. J. Am. Chem. Soc. 1979, 101 , 1742. 36. van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Vigalok, A.; Milstein, D. Angew. Chem., Intl. 1997, 36 , 625. 37. van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 1998, 120 , 6531. 38. For an example of catalytic cleavage of C(sp 2 )-O bonds, and a lead reference for such chemistry, see: Sergeev, A. G.; Hartwig, J. F. Science 2011, 332 , 439. 39. Ueno, S.; Mizushima, E.; Chatani, N.; Kakiuchi, F. J. Am. Chem. Soc. 2006, 128 , 16516. 40. Yamamoto, T.; Akimoto, M.; Yamamoto, A. Chem. Lett. 1983, 1725. 41. Yamamoto, T.; Akimoto, M.; Saito, O.; Yamamoto, A. Organometallics 1986, 5 , 1559. 42. Trovitch, R. J.; Lobkovsky, E.; Bouwkamp, M. W.; Chirik, P. J. Organometallics 2008, 27 , 6264. 43. Lara, P.; Paneque, M.; Poveda, M. L.; Salazar, V.; Santos, L. L.; Carmona, E. J. Am. Chem. Soc. 2006, 128 , 3512. 44. Conejero, S.; Paneque, M.; Poveda, M. L.; Santos, L. L.; Carmona, E. Acc. Chem. Res. 2010, 43 , 572.
|
