Transfer hydrogenation of imines with ammonia-borane: A concerted double-hydrogen-transfer reaction

Article abstract of DOI:10.1002/anie.200906302

(Chemical Equation Presented) Direct transfer hydrogenation of imines was observed with ammonia-borane, which proceeded under mild conditions without the help of a catalyst. The results of deuterium kinetic isotope effects, Hammett correlations, and DFT calculations all support a concerted double-hydrogen- transfer mechanism (see scheme).

Full text of DOI:10.1002/anie.200906302

Communications
DOI: 10.1002/anie.200906302
Hydrogen Transfer
Transfer Hydrogenation of Imines with Ammonia–Borane:
A Concerted Double-Hydrogen-Transfer Reaction**
Xianghua Yang, Lili Zhao, Thomas Fox, Zhi-Xiang Wang, and Heinz Berke*
Ammonia–borane (H3N-BH3, AB) is considered a feasible
material for chemical hydrogen storage owing to its ideally
very high storage capacity (19.6 weight% H) and thus has
attracted much attention.[1] Dehydrogenations of AB were
accomplished either thermally or by transition metal catal-
ysis.[2] Considering AB as a significantly polarized molecule,
we reasoned that it could be dehydrogenated by direct
reaction with a similarly polarized unsaturated compound by
the rarely explored reaction mode of double H transfer
(Scheme 1).
yl substrates to alcohols is thought to proceed along such a
reaction path, and it can be carried out at room temperature.[7]
The mechanistically related reductions of olefins and acety-
lenes with diimine proceed even below room temperature.[8]
The ruthenium- or iridium-catalyzed multistep transfer
hydrogenation reactions provide a powerful basis for the
preparation of primary or secondary amines.[9] Furthermore,
Noyori- and Shvo-type transfer hydrogenations are widely
applied in organic chemistry.[10] All these catalytic reactions
are thought to proceed in their essential H2 transfer step as
bifunctional activation processes with double H transfers. In
recent studies, it was demonstrated that rhenium catalysts can
promote multistep transfer hydrogenations of olefins using
AB and dimethylamine borane as a polar hydrogen donor.[11]
Herein, we investigate the direct reaction between imines and
AB, and reach the conclusion that concerted polar double H
transfers are involved.
Scheme 1. Double H transfer as an elementary process. X, Y, X’, and Y’
are main group element or transition metal fragments.
N-benzylidene aniline (1a), as the simplest aromatic
Schiff-base derivative, was selected as the reference com-
pound for the reaction with AB. To avoid decomposition of
the Lewis pair AB, only temperatures below 608C and
concentrations lower than 0.2m were applied.[12] For the
reaction of 1a with AB, very practical rates were obtained at
608C (Table 1, entries 1, 5, and 6). The transfer hydrogenation
was found to proceed even at room temperature, but then
much more slowly (Table 1, entry 4). As the thermal dehy-
drogenation of AB was nearly indetectable at room temper-
ature, we can exclude the possible reaction pathway via
thermal dehydrogenation of AB followed by imine hydro-
genation. With the progression of the transfer hydrogenation,
a trace amount of H2 was always obtained. This side reaction
can be considered as evidence for the generation of BH2NH2,
which is presumed to be able to catalyze the dehydrogenation
of AB.[13,14] However, it was only a minor competition
reaction here, because 1 equiv of AB can hydrogenate
2 equiv of 1a nearly quantitatively if given enough time
(Table 1, entry 6). A scrambling experiment with AB and its
doubly deuterated isotopologue A(D)B(D) was also carried
out to see whether AB would decompose at such conditions.
No reaction was noticed over several hours at 608C and
several days at room temperature; therefore, transfer hydro-
genation via Lewis acid (BH3) or base mediation (NH3) can
be discounted.
In certain cases, double H transfers are the crucial step of
multistep transition-metal-catalyzed transfer hydrogena-
tions,[3,4] with separate processes for hydrogenation and
dehydrogenation. In contrast to these catalyses, the true
double H transfer reaction is an all-in-one transfer hydro-
genation, with concerted hydrogenation and dehydrogenation
reactions merged into one elementary step.
Such elementary double H transfer processes are rare to
date. DFT calculations suggest that the prototypical homo-
polar reaction course of H2 exchange between ethane and
ethylene could principally proceed along the lines of
Scheme 1.[3,5] Despite its principal character of a (4+2)
symmetry-allowed concerted process, it was calculated to
possess a very high barrier, thus making it difficult to realize
this reaction. However, polar reagents are expected to have
much lower barriers. For example, the aluminum-complex-
catalyzed Meerwein–Ponndorf–Verley reduction[6] of carbon-
[*] X. Yang, Dr. T. Fox, Prof. Dr. H. Berke
Institute of Inorganic Chemistry, University of Zꢀrich
Winterthurstrasse 190, 8057 Zꢀrich (Switzerland)
Fax: (+41)1-635-6802
E-mail: hberke@aci.uzh.ch
L. Zhao, Prof. Dr. Z.-X. Wang
College of Chemistry and Chemical Engineering
Graduate University of Chinese Academy of Sciences (China)
After this initial test on the double H transfer to
benzylidene aniline, an extended selection of imines was
probed with a 1:1 ratio of imine to AB by applying various
conditions given in Table 1 (entries 7–21). The reaction times
for various para-substituted N-benzylidene anilines were
found to vary between 0.5 h and several days. Alkyl imines
turned out to be much slower (Table 1, entries 7–10), and in
[**] This work is financially supported by the Swiss National Science
Foundation and the University of Zꢀrich. The computational study
is supported by the Chinese Academy of Sciences. We thank
Prof. Dr. R. Tom Baker (CCRI, Ottawa) for fruitful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906302.
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Table 1: Reactions of AB with various imines in THF.[a]
Entry R/R’ of imine or amine
T [8C]
t
Yield [%][b]
1
2
3
60
50
40
RT
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
7 h
14 h
2 d
5 d
3 h
24 h
3 d
4 d
3 d
1 d
14 h
8 h
5 h
3 h
1 d
99 (>99)[c]
(>99)
(>99)
1a/b: Ph/Ph
4
(84[d])
5[e]
(>99)
6[f]
(93[g])
7
8
9
2a/b: Ph/Bn[h]
3a/b: Ph/tBu
4a/b: Cy/tBu
5a/b: Ph/CHPh2
6a/b: Ph2/H
7a/b: p-MeOC6H4/Ph
8a/b: p-ClC6H4/Ph
9a/b: p-O2NC6H4/Ph
10a/b: Ph/p-C6H4OMe
11a/b: Ph/p-C6H4Cl
12a/b: Ph/p-C6H4NO2
13a/b: p-ClC6H4/p-C6H4OMe
14a/b: p-ClC6H4/p-ClC6H4
15a/b: p-MeOC6H4/p-C6H4Cl
16a/b: p-C6H4OMe/p-C6H4OMe
99 (>99)
45 (46)
97 (99)
10
11
12
13
14
15
16
17
18
19
20
21
99 (>99)
98 (>99)
99 (>99)
99 (>99)
99 (>99)
99 (>99)
99 (>99)
99 (>99)
98 (>99)
99 (>99)
99 (>99)
99 (>99)
Scheme 2. Proposed overall reaction route.
4 h
0.5 h
20 h
4 h
5 h
20 h
As illustrated in Scheme 2, the overall reaction rate from
AB to BZ and PBZ may be described by k1, k2, and k3, which
are potentially the double H transfer steps. Under pseudo-
first-order conditions with an excess of 1a, k1 and k2 were
determined in reactions with AB and CTB, respectively. As
BCDB could not be prepared in pure form, k3 could not be
determined. BCDB was anticipated to be a comparable
hydrogen donor to CTB and therefore its reaction rate with
1a was assumed to be close to k2. In separate experiments
under comparable conditions, k2 was six times faster than k1;
therefore, k1 of the AB dehydrogenation is rate-determining.
In conjunction with the kinetically silent consumption of 1a
via the reactions with CTB and BCDB, the order of the
reaction is one-half in 1a and one in AB, as established by
curve-fitting in appropriate kinetic experiments (Supporting
Information, S4).
These facts would be in agreement with a mechanistic
picture featuring the k1, k2, and k3 steps as double H transfers,
which poses the important question as to whether these polar
transfers are proceeding in a concerted or stepwise path-
way.[3,4] We tried to gain insights by DFT calculations using
the Gaussian 03 program[15] (Supporting Information, S5).
The quantum mechanics calculations carried out at the
M05-2X/6-311 + + G** level indicate the double-H-transfer
pathway for the reaction of AB with 1a.[16] A thermodynami-
cally and kinetically feasible concerted reaction path resulted
(Scheme 3). Two six-membered cyclic double-H-transfer
transition states (TSI and TSII) were approached with
[a] 0.1 mmol AB react with 0.1 mmol imine, unless stated otherwise.
[b] GC-MS yield (NMR yields in brackets) based on the initial amount of
imine. [c] Yield of isolated product 92%. [d] 100% could be obtained
with a longer reaction time. [e] Amount of AB: 0.2 mmol. [f] Amount of
1a: 0.2 mmol. [g] Better results can be obtained with longer reaction
times. [h] Bn=benzyl.
particular N-(tert-butyl)benzylimine (Table 1, entry 8). It will
be shown below that this behavior is due to the electron-
donating rather than the stereochemical properties of the tert-
butyl group.
Although the imines are all uniquely transformed to
amines, many dehydrogenation products of AB were
observed. According to the 11B NMR spectra (see the
Supporting Information), AB was converted into B-(cyclo-
diborazanyl)aminoborohydride (BCDB), borazine (BZ),
poly(borazylene) (PBZ), and cyclotriborazane (CTB). After
long reaction times (2 days at 608C with 0.1 mmol AB and
0.5 mmol 1a), both BCDB and CTB had disappeared
completely, thus indicating they are intermediates on the
way from AB to BZ and PBZ.
Other possible products, such as [NH3BH2NH3]BH4 and
cyclodiborazane, were not observed even at room temper-
ature. The reaction course of AB and 1a was further studied
tracing the boron-containing products. First, BZ and CTB
were employed to react with 1a. Although BZ turned out to
be stable towards 1a, CTB was found to hydrogenate 1a
much faster than AB, and the dehydrogenation product was
almost exclusively BZ. Although clearly identified in the
11B NMR spectrum, we were not able to separate these boron-
containing products. In accordance with earlier conclusions
on other AB dehydrogenations, the appearance of CTB and
BCDB witnesses the primary intermediacy of the very
=
different AB orientations with respect to the imine C N
ꢀ
bond. As expected, the reaction path via TSI with B H···C
ꢀ
and N H···N transfers is energetically highly favored over the
ꢀ
ꢀ
alternative TSII with N H···C and B H···N transfers (by ca.
23 kcalmolꢀ1). In TSI, the transferred H atoms on the boron
and nitrogen of AB have opposite polarization with respect to
the attacked carbon and nitrogen atoms, respectively. From a
charge point of view, this naturally copes with the low energy
pathway, whilst the opposite orientation in TSII would have
the transferred hydrogen atoms attacked at evenly charged
centers, thus disfavoring the pathway. Moreover, an initial
=
reactive [H2B NH2] molecule, which decayed in fast subse-
quent reaction steps. It remains an open question as to
=
whether CTB was formed by direct cyclization of [H2B NH2]
ꢀ
(path A, Scheme 2) or via isomerization of BCDB (path B,
N H···N hydrogen bonding in the reaction path via TSI
would help to pre-organize TSI in the right orientation but
Scheme 2).[12,14]
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Scheme 3. DFT calculations for the reaction of AB with benzylidene
aniline 1a leading to aminoborane and N-benzylaniline 1b. The bond
lengths [ꢁ] were obtained at the M05-2X/6-311++G**level. The DH
and DG values (kcalmolꢀ1) in THF at 298 K and 1 atm were corrected
with the M05-2X/6-311++G** gas-phase harmonic frequencies.
Figure 1. Conversion chart of the reaction of 0.5 mmol benzylidene
aniline 1a with 0.1 mmol AB, AB(D), A(D)B or A(D)B(D) followed by
11B NMR in [D8]THF at 608C, determined by intensities of AB on the
11B NMR spectra with 15 min intervals. Black squares: reactions with
AB, red circles: A(D)B(D), green triangles: AB(D), blue triangles:
A(D)B. Simulated reaction constants: kAB =0.00350, kAB(D) =0.00404,
not TSII. Stepwise reaction paths were also considered with
the assumption that the relative energies of the first H
transfer is the energetically decisive step.[17] All such possible
reaction channels are predicted to be endergonic. The most
energetically favored was 44.9 kcalmolꢀ1 higher in free
energy than the reactants (Supporting Information, S5),
which is 16.9 kcalmolꢀ1 higher than the transition state
(TSI). This undoubtedly verifies the favorable concerted
double-H-transfer mechanism. Note that the concerted
reaction path for the hydrogen transfer between ethane and
ethylene has been computed to be more favorable than the
stepwise pathway.[3] The concerted pathway was reconfirmed
by the MP2/6-311 + + G** calculations, which gave similar
geometric and energetic results (Supporting Information, S5).
To further support the understanding of the regioselec-
tivity of this reaction suggested by the calculations, deuterium
labeling studies were carried out. Deuterated ABꢀs (BD3NH3
(AB(D)), BH3ND3 (A(D)B)and BD3ND3 (A(D)B(D))) were
employed to react with 1a. The in situ 1H and 13C NMR
spectra proved stereocontrolled reaction courses. A product
k
A(D)B =0.00181, kA(D)B(D) =0.00251.
based on 11B NMR (Figure 1), the rate constants k were
simulated and the DKIE values obtained [Eq. (1)].
k
AB=kABðDÞ ¼ 0:87, kAB=kAðDÞB ¼ 1:93, kAB=kAðDÞBðDÞ ¼ 1:39
ð1Þ
For the reaction of 1a with AB(D), an inverse DKIE
[Eq. (1), DKIE = 0.87] was obtained, which speaks for the
formation of a stronger bond with a steeper potential energy
curve in the presumably late transition state.[18] Together with
the given size of the isotope effect, this effect could be
rationalized on the basis of the transfer of the hydridic HB
ꢀ
atom having a flatter ground state potential curve. The H C
bond developed in the late transition state is expected to
cause a steeper potential energy curve.
In the case of the reaction with A(D)B, a normal DKIE
[Eq. (1), DKIE = 1.93] was observed, which indicates that
=
ꢀ
deuterated at the carbon end of the C N bond was obtained
breakage of the N H bond participates in the rate-determin-
from an experiment with AB(D) and 1b and showed a triplet
(JC,D = 20 Hz) for the CHD group in the 13C NMR spectrum at
the same chemical shift as the CH2 group of 1b. A doublet at
d = 4.23 ppm in the 2H NMR spectrum further confirmed the
presence of the CHD unit. In the related experiment with
A(D)B and the imine, absence of the N–H signal in the
1H NMR spectrum and appearance of a N–D signal in the
2H NMR spectrum indicated the exclusive formation of an
ing step and that both the broken and the installed bonds have
approximately the same strengths.[18] For the double DKIE
reaction of A(D)B(D) with the imine, a DKIE smaller than
A(D)B was observed [Eq. (1), DKIE = 1.39]. This value
would constitute an average with approximate cancellation of
the kinetic effects of the two monodeuterated cases. The
calculated DKIE for AB(D) and A(D)B are 0.82 and 1.84,
respectively, which is in good agreement with the experimen-
tal values (0.87 and 1.93, respectively). However, the pre-
dicted DKIE value of A(D)B(D) (2.80) is somewhat higher
than the experimental value (1.39), which may be due to the
use of a harmonic model for the frequency calculations. The
averaging of the experimental double DKIE values of double
H transfers is presently not fully understood on a quantum
mechanical basis of the single DKIE values.[19] According to
the sizes of the single DKIE values, we may conclude that
both H transfers participate in the rate-determining step. This
participation makes a concerted mechanism quite plausible,
because if it proceeds via a stepwise mechanism, the reaction
ꢀ
N D bond. Both of these results are supportive of the TSI
geometry of Scheme 3. In the case of A(D)B(D), both the CH
and N positions of the amine were fully deuterated, as verified
by 2H NMR spectroscopy.
Further to these deuterium labeling studies, primary
deuterium kinetic isotope effects (DKIE) were investigated.
Considering the manifold dehydrogenation products of AB,
all the reactions were carried out with in situ NMR measure-
ments under approximate pseudo-first-order conditions for
AB with the imines in excess (0.5 mmol of 1a reacting with
0.1 mmol of AB). According to the kinetic conversion chart
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rate should at least not be affected by one of the deuterium
labeling reactions.
Keywords: ammonia–borane · density functional calculations ·
hydrogen transfer · imines
.
The Hammett correlation was then also checked for the
reactions of AB with the para-H-, methoxy-, chloro-, or nitro-
substituted N-benzylidene anilines 1a–12a. The initial rate
constants k were calculated from the conversion charts.
Hammett correlations (Figure 2) from plotting logk versus
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Figure 2. Hammett plots for the reactions of 0.1 mmol AB with
0.5 mmol para-substituted diaryl imines extracted from kinetic
11B NMR spectra in [D8]THF at 608C. Left, with a 1=1.61: reactions
with the para substitutent at the aniline side; right, with 1=0.69: para
substitution at the benzylidene side. Black: p-MeO, red: p-H, green:
p-Cl, blue: p-NO2 groups.
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ꢀ
+
ꢀ
ꢀ
ꢀ
ꢀ
B H C side and that of the N H N side should be of
different signs, which would result in one positive 1 and the
other negative.
In conclusion, metal-free transfer hydrogenations of
imines with ammonia–borane were observed to proceed
under mild conditions. Based on DKIE determinations,
Hammett correlations and ab initio calculations, the reaction
courses are proposed to follow the profile of concerted double
H transfers. The adaptability and generality of this type of
reaction applying to other unsaturated compounds with the
goal of the development of respective catalytic reactions is
currently under investigation in our group.
Received: November 9, 2009
Published online: February 19, 2010
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Communications
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