Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron (III) Complex Page: 9,799
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Journal of the American Chemical Society
parameters Keq, [ 1 ]o, and [tBupy]o.
d[l1tBupy] k Keq o [tBupy]o (3)
dt - kintra i7 K e B q j (3)
By fitting eq 3 to the data in Figure 2, the elementary rate
constant is calculated to be kintra = 5.1(1) x 102 s-1 for the
Table 2. Pyridine Substituent Effects on the Intramolecular
HAT Rate Constant"
kintra (S-1) at 40 C
NMe2 1.5(2) x 101
tBu 5.1(1) x 10-2
Ph 3.1(2) x 10-2
CF3 1.3(2) x 10-2
a Data obtained at 40(1) OC in C6D6 with  = 77 mM and [pyridine] =
-0.8 -0.6 -0.4 -0.2
0 0.2 0.4 0.6
Figure 3. Hammett plot of para-substituted pyridines for the intramo-
lecular HAT reaction (ref 44). Data obtained at 40(1) OC in C7D8s. The
slope of the dashed line gives p = -0.77(4).
reaction of 1 and tBupy. The pseudo-first-order rate constants kobs
were also determined using several para-substituted pyridines (40 OC,
[Fe] = 26 mM, [pyridine] = 0.26 M). Assuming that the rate law in
eq 3 applies to each reaction, it is possible to calculate the rate
constants kintra for the HAT reaction using each para-substituted
pyridine (Table 2), using the independently determined values of Keq
(Table 1). The Hammett plot of the elementary rate constant kintra as
a function of op shows that more electron-donating pyridine substit-
uents (Figure 3) lead to faster HAT,44 with a p value of -0.77(4).
Intermolecular HAT Reaction with 1,4-Cyclohexadiene.
Addition of certain reagents with weak C-H bonds such as
CHD prevented intramolecular attack on the diketiminate ligand,
in favor of an intermolecular reaction. For example, the product
of reaction between 1-tBupy and CHD was LMeFe(NHAd)-
(tBupy) (2. tBupy), observed in 91% yield by 1H NMR spectros-
copy. Hydrocarbons with stronger C-H bonds (e.g., toluene,
with a benzylic C-H BDE ~89 kcal/mol4s) did not react with
1l tBupy prior to intramolecular HAT reaction. Therefore, the
intermolecular HAT reactivity of 1 tBupy is limited to substrates
that react more quickly than the intramolecular reaction.
The rate of the reaction of 1 tBupy with CHD was chosen for
detailed kinetic study by 1H NMR spectroscopy. The intermo-
lecular reaction with CHD is much faster than the intramolecular
HAT reaction discussed above, so the kinetic experiments were
performed at -51(1) oC, a temperature at which the intramo-
lecular HAT reaction is not observed. The disappearance of
1-tBupy and concomitant appearance of 2- tBupy were mon-
itored by integration against a capillary integration standard.42 In
each case, the spectroscopically observed yield of 2- tBupy was
>90%. Under pseudo-first-order conditions ( = 51 mM,
[tBupy] = 257 mM) at -51(1) OC in toluene-ds, the reaction
rate had a linear dependence on [CHD] between 0.13 and 0.79 M
(Figure 4a), suggesting that the rate-limiting step is attack on the
hydrocarbon. Similar to the intramolecular reaction, the intermole-
cular HAT rate is dependent on the presence of tBupy. If tBupy was
omitted, no reaction was observed in 3 h at -51 OC (<5% decrease
of the integration of 1, and no 2 was detected). Since tBupy binds
more strongly at -50 OC (Keq = 250 20 M-1) than at 40 OC
(Keq = 2.5 0.1 M-l), the rate of reaction with CHD saturates at a
lower concentration of [tBupy] (Figure 4b) than the intramolecular
HAT reaction. As with the intramolecular HAT reaction, the
0.20 0.40 0.60 0.80
U.U I I I I
0.0 0.050 0.10 0.15
Figure 4. Observed rate of intermolecular HAT as a function of (a) 1,4-cyclohexadiene (CHD) concentration at constant [tBupy] = 257 mM and
(b) tBupy concentration at constant [CHD] = 150 mM. Data obtained in toluene-d8 at -51(1) oC. The dashed line in panel b represents the fit to eq 5.
dx.doi.org/10.1021/ja2005303 IJ. Am. Chem. Soc. 2011, 133, 9796-9811
p = -0.77(4) CF3
I I I I I I I
I1 + tBupy < 1-tBupy
I I I I
1 + tBupy < > 1-tBupy
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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/m1/4/: accessed May 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.