Journal of the American Chemical Society
Communication
methane to give Ph3COH,25 suggesting that the tertiary carbon
radical may favor hydroxylation. However, [FeIV(O)(Cl)-
(PyTACN)]+ reacts with other C−H bonds (e.g., cyclo-
hexane), but still no halogenated products were observed. In
contrast, some halogenation has been seen for cyclohexane and
toluene with a non-heme FeIV(O)(X) (X = Cl, Br) oxidant.24
Reaction of Ph3C• with a low-coordinate FeIII(NHAr)(Cl)
complex disfavored halogenation, leading to amination
instead.47 The overall importance of the nature of the carbon
radical remains to be determined.
Complexes 3 and 4 are analogous to the key ferric
intermediates in non-heme iron halogenases and related
catalysts. Future plans include computational studies and
other mechanistic investigations of these rebound reactions to
understand the key factors that control the rebound selectivity
in both enzymes and models.
Figure 5. Displacement ellipsoid plot (50% probability level) for 5 at
110(2) K. Hydrogen atoms (except for N−H) have been omitted for
clarity.
raphy (TLC), and then the products were isolated by silica gel
chromatography in good yields (75% for Y = Cl and 84% for Y
= OMe). Although it is not possible to determine which of the
Cl ligands is transferred, it is clear from these results that
halogen transfer occurs easily in the absence of a terminal
hydroxide (Scheme 4). This result provides the first example,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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sı
X-ray crystallographic data for 1 (CIF)
X-ray crystallographic data for 2 (CIF)
X-ray crystallographic data for 3 (CIF)
X-ray crystallographic data for 4 (CIF)
Scheme 4. Reaction of 5 with Triphenylmethyl Radical
Derivatives
X-ray crystallographic data for 5 (CIF)
57
Synthesis, 1H NMR spectra, Fe Mossbauer data, UV−
̈
vis spectra, EI-MS spectra, Tables S1−S13, and Figures
AUTHOR INFORMATION
Corresponding Author
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to our knowledge, of the direct reaction of an isolated FeIII(Cl)
complex with a carbon radical to give FeII and a new C−Cl
bond, a process that mimics halogen rebound.
David P. Goldberg − Department of Chemistry, The Johns
Hopkins University, Baltimore, Maryland 21218, United
Our findings indicate that the reactions of tertiary carbon
radicals with 3 and 4 give hydroxylated products with no
evidence of halogenation, although the analogous dichloro
complex 5 is competent to transfer the halogen. The lack of
halogen transfer for 3 and 4 contrasts with what is seen for
both enzymes and halogenase models and thus deserves
further comment. The selectivity seen for 3 and 4 is in line
with thermodynamic expectations, which are supported by
calculations on both enzymes and models that uniformly
predict hydroxylation to be thermodynamically favored over
halogenation by at least 15 kcal/mol.14,21,22,26 However, the
same computational studies also predict that the reaction
barriers to halogenation are lower, leading to kinetically
favored halogenated products. A range of factors have been
offered as contributing to the lower kinetic barriers for
halogenation, including substrate orientation,16,23 relocation of
the positioning of the OH group on the metal,5,19,20 relative
ordering of frontier molecular orbitals,14 relative spin densities
on OH versus Cl,15 and steric effects.28 It has also been
suggested that H-bonds between the OH group of the putative
FeIII(OH)(Cl) intermediate and Arg254/Glu102/water mole-
cules in SyrB2 raise the hydroxylation barrier.14,19
Authors
Vishal Yadav − Department of Chemistry, The Johns Hopkins
University, Baltimore, Maryland 21218, United States;
Rodolfo J. Rodriguez − Department of Chemistry, The Johns
Hopkins University, Baltimore, Maryland 21218, United
Maxime A. Siegler − Department of Chemistry, The Johns
Hopkins University, Baltimore, Maryland 21218, United
Complete contact information is available at:
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The NIH (GM119374 to D.P.G.) is gratefully acknowledged
for financial support. Computer time was provided by the
Maryland Advanced Research Computing Center (MARCC).
The lack of halogenation seen for 3 and 4 occurs even
though the OH ligand is held in place and deactivated toward
the incoming carbon radical by two tight H-bonds. This
inherent reactivity may be a result of the nature of complexes 3
and 4 or may be related to the nature of the radical. It was
shown that [FeIV(O)(Cl)(PyTACN)]+ reacts with triphenyl-
REFERENCES
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20702.
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J. Am. Chem. Soc. XXXX, XXX, XXX−XXX