Communication
hydrogenation product
3 (autoinduced catalytic
cycle) as the acting Lewis base (Scheme 1). It can be
assumed that this mechanism is the basis of the au-
toinduced catalysis because both the starting materi-
al and the product are involved in the H activa-
2
[
14]
tion.
the H2 activation by the FLP comprising borane
and imine 2 or amine 3 as the step with highest
energy barrier making the heterolytic H -splitting
Quantum-mechanical calculations supported
1
2
rate-determining
for
the
overall
process
[
9,11]
(Scheme 1).
The transition state energies for those
activations were calculated both to be 16.5 kcal
À1 [9]
mol , although the amine should be a more
Figure 2. Eyring plots for the FLP-mediated hydrogenation of 2 (left) and deuterohydro-
genation (right) in the presence of the three boranes.
potent Lewis base for the H activation than the imi-
2
[
7a,b]
ne.
Consequently, the H -splitting by 1 is unselec-
2
tive with regard to the Lewis base resulting in the ab-
sence of the characteristic kinetic features of autoinduced cat-
alysis.
[
18]
a general autocatalytic reaction. The graphical analysis yield-
ed the values for both rate constants, k and k for the imine-
1
2
The reactivities of the three different boranes B(C F ) (1),
and the amine-mediated FLP-catalyzed hydrogenation of 2
and allowed the determination of the free activation parame-
ters. The results of the kinetic analysis are summarized in
Figure 2 and in Table 1 (see Supporting Information for details).
The reaction catalyzed by B(C F ) (1) displayed a 1.5–1.9 fold
6
5 3
B(2,4,6-F -C F ) (4), and B(2,6-F -C H ) (5) were studied in the
3
6
3 3
2
6
3 3
hydrogenation of 2 at 708C (Figure 1). The reaction with
6
5 3
rate increase for k (see Figure 2a) resulting in an apparent
2
first-order type reaction rate. Consequently, the free activation
enthalpies for both cycles are very similar ((21.4Æ13.6) kcal
À1
À1
mol and (20.1Æ23.7) kcalmol , Table 1, for detailed error
treatment see Ref. [19] and Supporting Information) and their
absolute values are within an acceptable agreement with the
À1 [9]
previously calculated ones (lit. 16.5 kcalmol ). This situation
changed significantly when the weaker Lewis acids 4 and 5
were used. The rate constants (k ) for the product-accelerated
2
Figure 1. Time versus yield plot for the FLP-catalyzed hydrogenation of N-
cycle were about one order of magnitude larger than for the
benzylidene-tert-butylamine (2) (5 mol% borane, CD
2
Cl
were determined by H NMR spectroscopy using hexamethylbenzene as in-
B(2,6-F -C H ) (5);
2
, 0.4m, 708C; yields
imine-catalyzed cycle (k ) (Figure 2a). The comparison of the
1
1
free reaction enthalpies of activation obtained from k and k
2
ternal standard; ^ B(C
B(2,6-F -C (5) and 5 mol% benzyl-tert-butylamine (3)).
F
5
)
(1); ~ B(2,4,6-F
-C
6
F
3
)
(4); &
&
1
6
3
3
3
2
6
3 3
6
H
3
)
3
show that the imine-mediated H activation by the less Lewis-
2
2
À1
À1
acidic boranes is 1.6 kcalmol and 1.8 kcalmol higher com-
pared to the amine-mediated H -splitting, thus triggering the
2
B(C F ) qualitatively proceeded with highest rates exhibiting
observed autoinduced catalysis (Table 1). The increased nega-
tive activation entropies for the boranes 4 and 5 compared to
1 may result from the formation of hydrogen-bonded com-
plexes in the transition state of the imine-reduction as pro-
6
5 3
the characteristic curve shape for a second-order type reac-
[
16]
tion. In contrast, the reaction profiles of the hydrogenations
catalyzed by the two weaker Lewis-acidic boranes 4 and 5 dis-
played sigmoidal curve shapes. This implies a significant in-
crease in rate during the course of the reaction. Addi-
[11]
posed by Privalov. Experimental evidence for such interac-
tion of 5 mol% of the reaction product 3 at the be-
ginning of the reaction using B(2,6-F -C H ) (5) as
2
6
3 3
Table 1. Activation parameters for the FLP-catalyzed hydrogenation of 2 (values in
Lewis acid resulted in the same dramatic rate in-
crease, which is a strong indication for autoinduced
catalysis. Kinetic data for the FLP-catalyzed hydroge-
nation of 2 using the three boranes were acquired at
three different temperatures and were analyzed ac-
parentheses correspond to the reaction with D
2
).
for k (standard catalytic cycle)
1
2
for k (autoinduced catalytic cycle)
°[a,b]
°[b]
°[c]
°[a,b]
°[b]
°[c]
DG
DH
DS
DG
DH
DS
[
d]
1
4
5
21.4Æ13.9 15.6Æ7.1 À19.1Æ21.1 20.1Æ23.7 19.6Æ14.2 À5.8Æ42.5.0
(
21.0Æ10.5) (12.9Æ5.4) (À25.7Æ16.0)
cording to the following criteria: i) the H and borane
24.1Æ3.5 8.5Æ1.9 À48.0Æ5.1
22.5Æ2.5 6.9Æ1.3
À49.7Æ3.7
2
[
17]
concentration in solution is constant,
and ii) the
(24.1Æ5.6) (6.0Æ3.0) (À55.1Æ8.2) (22.4Æ3.4) (5.2Æ1.8) (À52.0Æ4.9)
24.5Æ4.0
6.5Æ2.1
À55.1Æ5.9
22.7Æ2.3 5.8Æ1.2
À51.7Æ3.4
shift of the equilibrium of the FLP 3/4 and 3/5 to-
(
24.4Æ4.8) (5.8Æ2.6) (À56.7Æ7.1) (22.5Æ2.5) (5.7Æ1.3) (À51.3Æ3.7)
wards its H -activation products is slow (vide infra).
2
À1
À1
[
a] Calculated for 343 K. [b] In kcalmol . [c] In calmol K. [d] The reaction with D
2
was
These requirements allow a quantitative analysis of
the collected data conforming with the rate laws for
2
fitted to a first-order rate law, hence k could not be determined.
Chem. Eur. J. 2015, 21, 8056 – 8059
8057
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