Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,800
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Journal of the American Chemical Society
1.0 1.5 2.0 2.5
Figure 5. Comparison of observed rate constants for the intermolecular
HAT reaction of 1. tBupy with CHD and CHD-d8. The ratio of slopes is
105 + 28. Data obtained in toluene-d8 at -51(1) oC with 51 mM Fe and
0.26 M tBupy (for CHD) or 25 mM Fe and 0.20 M tBupy (for CHD-d8).
necessity for tBupy implicates 1- tBupy as the reactive form of the
With the first-order dependence on [CHD], one expects the
rate law in eq 4, where kinter is the second-order rate constant for
the elementary HAT step. The factor of 2 is added because CHD
contains two weak C-H bonds and thus requires 2 equiv of
1- tBupy to form benzene. This analysis assumes that the second
C-H bond reacts with a second molecule of 1*tBupy much
faster than the first HAT, which is reasonable because the C-H
BDE of cyclohexadienyl radical (22 kcal/mol) is significantly
lower than the C-H BDE in cyclohexadiene (77 kcal/mol).46
2kinter [CHD] [1. tBupy]
kinter [indene] o
Since the reaction was monitored at low temperature where
Keq for tBupy association is large (Keq = 250 20 M-l), the
weak-binding approximation used to generate eq 3 cannot be
used here. Thus, the full expression for [ 1- tBupy] in terms of
Keq, [1 ]o, and [tBupy]o must be used, which gives the rate law
in eq 5.
d[1 tBupy] = 2kinter[CHD] 0 [tBupy]0 eq
S+ [tBupy]o + q -4o[tBupy]o /2
However, the rate law is greatly simplified in the saturation
regime ([tBupy] > 0.05 M), where there is a zeroth-order
dependence on [tBupy] (eq 6).
[2dt = 2kinter [CHD]0 (6)
Under pseudo-first-order conditions (saturated in [tBupy] and
with excess CHD), kobs = 2kinter[CHD], and the elementary
second-order rate constant for the HAT step (knter = (9.1 0.9) X
10-3 M-1 s-1) is calculated from the slope of Figure 4a.
1,1,3-Trideuteroindene (indene-d3) was also studied to obtain
a KIE. Comparing the values of kobs for the reactions with indene
and indene-d3 (Supporting Information Figure S-10) gives a KIE
ofkH/kD = 80 12, similar to the KIE for the reaction of 1-tBupy
with CHD. The rate constants for the elementary HAT step
(kinter) for all substrates are summarized in Table 3. Conversion to
the expected products benzene, p-xylene, and naphthalene was
confirmed by GC/MS (92%, 99%, and 69%, respectively).47
Activation Parameters for Intermolecular HAT. The rate
constants kintra and kinter (for the reaction with CHD) were
determined over the temperature range of 10-50 oC, and it was
possible to determine the activation parameters of the HAT step
by plotting ln(k/T) versus 1/T (Eyring plot, Figure 6) and ln(k)
versus 1/T (Arrhenius plot, Supporting Information Figure S-1).
Activation parameters for the intramolecular HAT reaction
from the diketiminate ligand are AH*intra = 14.6(5) kcal/mol,
ASintra = -18(2) cal/molK, In(Aintra) = 22(1) s -, and
Ea,intra = 15.2(5) kcal/mol. Activation parameters for the inter-
molecular HAT from CHD are AHinter = 14.4(8) kcal/mol,
AS inter = -9(3) cal/mol K, ln(Ainter) = 26(1) M-1 s- , and
Ea,inter = 15.0(8) kcal/mol. At 298 K, the activation barriers are
AG intra,298 = 20(1) kcal/mol and AG inter,298 = 15(2) kcal/mol,
Computations. To help understand the role of tBupy in the
HAT reactions of 1- tBupy, and to understand the steric depen-
dence of the reaction, computations were performed on the
amido products (2 and 2- tBupy), imido starting materials (1 and
dx.doi.org/10.1021/ja2005303 IJ. Am. Chem. Soc. 2011, 133, 9796-9811
H/D Kinetic Isotope Effect for the Reaction of 1 - tBupy and
CHD. To probe the hypothesis that the rate-limiting step of the
mechanism is hydrogen atom transfer, we measured the inter-
molecular kinetic isotope effect (KIE) of the reaction with CHD
by extending the kinetic study to CHD-d8 (Figure 5). The 1H
NMR method was identical to that described above for CHD-h8,
although much higher concentrations of CHD-d8 were necessary
to speed up the reaction to a practical rate at -51 OC. The
calculated value kH/kD = 105 28 is a very large primary KIE,
which clearly supports the contention that HAT from CHD is
the rate-limiting step in the transformation.
HAT from Larger Substrates. To explore the substrate
dependence of the HAT reactivity of 1- tBupy, we chose a range
of substrates with C-H bonds that are weak enough to react
prior to intramolecular attack on the supporting ligand. The
kinetics of the reactions of 1 tBupy with indene, 1,4-dihydro-
naphthalene (DHN), 9,10-dihydroanthracene (DHA), 1,4-di-
methyl-1,4-cyclohexadiene (Me2CHD), and 1,2,4,5-tetramethyl-
1,4-cyclohexadiene (Me4CHD) were each measured in experi-
ments analogous to those for CHD. No reaction was observed
with either Me4CHD or DHA at -51 OC with [Fe] = 30 mM and
[substrate] = 1.2 M (<2% decrease in [1 tBupy] in 3 h). In each
of the other cases, a linear correlation of kobs with [substrate] was
observed (Supporting Information Figures S-2-S-7). In each
case, the reaction was performed with at least 0.2 M tBupy to
ensure saturation in [tBupy]. Thus, for the reactions with
substituted cyclohexadienes, the rate law is identical to eq 6.
For the reaction with indene, the rate law is given in eq 7, which
follows eq 6, except the factor of 2 is omitted since each indene
molecule supplies a single hydrogen atom.
d[1Bupy] kinter [indene] [1- tBupy]
Il -- ' . --- -- r I I
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Cowley, Ryan E.; Eckert, Nathan A.; Vaddadi, Sridhar; Figg, Travis M.; Cundari, Thomas R., 1964- & Holland, Patrick L. Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex, article, May 12, 2011; [Washington, D.C.]. (digital.library.unt.edu/ark:/67531/metadc107786/m1/5/: accessed May 25, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.