Journal of the American Chemical Society
Page 6 of 7
(10) Cook, A. G. Enamines: Synthesis, Structure and Reactions; 2nd
ed.; CRC Press: New York, New York, USA, 1987.
ments, product isotope effects, and other studies using subꢀ
strates of varying electron richness, basicity, and coordinative
capability. The results are all consistent with a mechanism of
initial concerted transfer of e− and H+ (H atom) from
SmI2(H2O)n to the enamine. There are strong arguments
against stepwise PCET mechanisms of initial electron transfer
or proton transfer, and there is no indication of a mechanism
involving coordination of the enamine substrate. A thermoꢀ
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(11) (a) Cukier, R. I.; Nocera, D. G. Annu. Rev. Phys. Chem. 1998,
49, 337; (b) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev.
2010, 110, 6961; (c) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.;
Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.;
McCafferty, D. G.; Meyer, T. J. Chem. Rev. 2012, 112, 4016.
(12) Schepp, N. P.; Johnston, L. J. J. Am. Chem. Soc. 1996, 118,
2872.
(13) (a) Based on ∆pKa values of 3.07, and 2.48 for cyclopentꢀ1ꢀenꢀ
1ꢀyl, cyclohexꢀ1ꢀenꢀ1ꢀyl, and cycloheptꢀ1ꢀenꢀ1ꢀyl derivatives; (b)
Cook, A. G.; Absi, M. L.; Bowden, V. K. J. Org. Chem. 1995, 60,
3169.
2+
chemical analysis indicates that the O–H bond in Sm(H2O)n
(aq), and by implication SmI2(H2O)n, is incredibly weak. This
O–H bond has a bond dissociation free energy of only 26 kcal
molꢀ1, being the weakest O–H bond for any stable material.
This BDE analysis therefore shows why SmI2(H2O)n can reꢀ
duce such recalcitrant substrates, but also raises questions
about the origin of the larger kinetic barriers for other subꢀ
strates. This study thus provides a more detailed understanding
of the important and varied chemistry of SmI2(H2O)n and
opens the door to new applications of this reagent as a hydroꢀ
gen atom donor.
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(14) Donghi, D.; Beringhelli, T.; D'Alfonso, G.; Mondini, M. Chem.
Eur. J. 2006, 12, 1016.
(15) The aqueous pKa of Sm(H2O)n2+ is not known, but the acidity of
‘hard’ aquo ions is well known to vary as Z2/r, where Z is the charge
on the ion and r is the radius. Comparing dications, Sm2+ is likely
very similar in size to Sr2+ (1.32 Å) since neighboring Eu2+ has r =
1.31 Å. Sr2+ has a pKa of 13.3, almost an order of magnitude weaker
than TFE. Principles and values from: (a) Wulfsberg, G. Principles of
Descriptive Inorganic Chemistry, Brooks/Cole, Belmont, CA, 1987,
pp. 24ꢀ30 and inside back cover.
(16) We thank the reviewer for suggesting a version of this
experiment.
ASSOCIATED CONTENT
(17) (a) Teprovich, J. A.; Antharjanam, P. K. S.; Prasad, E.;
Pesciotta, E. N.; Flowers, R. A. Eur. J. Inorg. Chem. 2008, 2008,
5015; (b) Yacovan, A.; Hoz, S.; Bilkis, I. J. Am. Chem. Soc. 1996,
118, 261; (c) Sadasivam, D. V.; Teprovich, J. A.; Procter, D. J.;
Flowers, R. A. Org. Lett. 2010, 12, 4140.
(18) Assuming a rateꢀlimiting transfer of a hydrogen atom from
SmI2(H2O)n to a carbon atom of the substrate (see below), the rate
constants should parallel the strengths of the C–H bonds formed.
Preliminary estimates are that these fall in the order 1p < stilbene <
anthracene. The dependence of rate constants on driving force in
SmI2(H2O)n reactions will be analyzed in more detail in future
publications.
Supporting Information. Preparative procedures, substrate and
product spectral data, and kinetic data and thermochemical analꢀ
yses are included in the supporting information. This material is
AUTHOR INFORMATION
Corresponding Author
orcid.org/0000ꢀ0002ꢀ3943ꢀ5250
Funding Sources
(19) Mayer, J. M. Acc. Chem. Res. 2011, 44, 36.
(20) M. Prochazka, V. K., M. Palecek and K. Pecka Collection
Czechoslov, Chem. Commun. 1970, 35, 3813.
This work was supported by U.S. NIH grant 5R01GM050422.
(21) Luo, Y. R. Comprehensive Handbook of Chemical Bond
Energies; CRC Press: Boca Raton, Florida, USA, 2007.
(22) (a) Bezdek, M. J.; Guo, S.; Chirik, P. J. Science 2016, 354, 730;
(b) Spiegel, D. A.; Wiberg, K. B.; Schacherer, L. N.; Medeiros, M.
R.; Wood, J. L. J. Am. Chem. Soc. 2005, 127, 12513; (c) Campaña,
A. G.; Estévez, R. E.; Fuentes, N.; Robles, R.; Cuerva, J. M.; Buñuel,
E.; Cárdenas, D.; Oltra, J. E. Org. Lett. 2007, 9, 2195.
(23) Chciuk, T. V.; Li, A. M.; VazquezꢀLopez, A.; Anderson, W. R.;
Flowers, R. A. Org. Lett. 2017, 19, 290.
(24) The value of –1.55 V vs. NHE for the standard reduction
potential of Sm3+/2+ appears in a number of reviews of standard
electrode potentials and of the thermochemistry of lanthanide
elements, including (a) Morss, L. R. Chem. Rev. 1976, 76, 827. (b)
Bratsch, S. G. J. Phys. Chem. Ref. Data 1989, 18, 1ꢀ21. It appears to
be relatively independent of the nature of the anions present: Timnick,
A.; Glockler, G. J. Am. Chem. Soc. 1948, 70, 1347–1350.
(25) Mohapatra, P. K.; Khopkar, P. K. Polyhedron 1989, 8, 2071.
(26) Shannon, R. Acta Crystallographica Section A 1976, 32, 751.
(27) Shi, S.; Szostak, R.; Szostak, M. Org. Biomol. Chem. 2016, 14,
9151.
ACKNOWLEDGMENT
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