Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors

Article abstract of DOI:10.1039/c1ob06381b

Metal-free transfer hydrogenation of polarized olefins (RR′ CCEE′: R, R′ = H or organyl, E, E′ = CN or CO₂Me) using amine borane adducts RR′NH-BH₃ (R = R′ = H, AB; R = Me, R′ = H, MAB; R = ^tBu, R′ = H, tBAB; R = R′ = Me, DMAB) as hydrogen donors, were studied by means of in situ NMR spectroscopy. Deuterium kinetic isotope effects and the traced hydroboration intermediate revealed that the double H transfer process occurred regio-specifically in two steps with hydride before proton transfer characteristics. Studies on substituent effects and Hammett correlation indicated that the rate determining step of the H_N transfer is in agreement with a concerted transition state. The very reactive intermediate [NH₂BH₂] generated from AB was trapped by addition of cyclohexene into the reaction mixture forming Cy₂BNH₂. The final product borazine (BHNH)₃ is assumed to be formed by dehydrocoupling of [NH₂BH₂] or its solvent stabilized derivative [NH₂BH₂]-(solvent), rather than by dehydrogenation of cyclotriborazane (BH₂NH ₂)₃ which is the trimerization product of [NH ₂BH₂].

Full text of DOI:10.1039/c1ob06381b

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PAPER
Synthetic and mechanistic studies of metal-free transfer hydrogenations
applying polarized olefins as hydrogen acceptors and amine borane adducts as
hydrogen donors†‡
Xianghua Yang, Thomas Fox and Heinz Berke*
Received 12th August 2011, Accepted 11th October 2011
DOI: 10.1039/c1ob06381b
Metal-free transfer hydrogenation of polarized olefins (RR¢C CEE¢: R, R¢ = H or organyl, E, E¢ = CN
or CO2Me) using amine borane adducts RR¢NH–BH3 (R = R¢ = H, AB; R = Me, R¢ = H, MAB; R =
tBu, R¢ = H, tBAB; R = R¢ = Me, DMAB) as hydrogen donors, were studied by means of in situ NMR
spectroscopy. Deuterium kinetic isotope effects and the traced hydroboration intermediate revealed that
the double H transfer process occurred regio-specifically in two steps with hydride before proton
transfer characteristics. Studies on substituent effects and Hammett correlation indicated that the rate
determining step of the HN transfer is in agreement with a concerted transition state. The very reactive
intermediate [NH2 BH2] generated from AB was trapped by addition of cyclohexene into the reaction
mixture forming Cy2BNH2. The final product borazine (BHNH)3 is assumed to be formed by
dehydrocoupling of [NH2 BH2] or its solvent stabilized derivative [NH2 BH2]–(solvent), rather than
by dehydrogenation of cyclotriborazane (BH2NH2)3 which is the trimerization product of [NH2 BH2].
A great deal of hydrogen-rich molecules have been applied
as hydrogen donors in hydrogen transfer reactions, such as
Introduction
hydrazine, dihydronaphthalene, dihydroanthracene, formic acid,4
alcohols, esters,2 and also ammonia borane (AB) and related
amine boranes.5 In recent years, amine borane adducts have
attracted additional attention for their very high volumetric and
gravimetric hydrogen storage density as potentially safe and stable
hydrogen storage materials.6 Intensive studies were carried out on
the dehydrogenation and the hydrogenation (regeneration) of these
compounds, which remains as a practically unsolved problem.7
Therefore, we became also interested in the use of such compounds
as in situ hydrogen sources for the transfer hydrogenation of
unsaturated compounds exploring new reaction pathways in the
hope to create more reversible dehydrogenation processes of AB.
The catalytic reaction of AB in the presence of cyclohexene was
reported to result in a hydroboration reaction with formation of
Cy2BNH2 when using rhodium8 or platinum9 catalysts. On the
other hand, olefins including cyclohexene were seen to be transfer
hydrogenated by AB or dimethylamine borane (DMAB) using Rh
colloid or nitrosyl Re(I) complexes.5 Since the reaction course was
found to be quite sensitive to the type of the catalyst, it might
change completely if the substrate, solvent or other conditions
are adjusted. Considering the strongly polarized hydridic HB and
protic HN atoms in the amine borane molecules, the pathway
might be switched from stepwise transfers to concerted double-
H transfers as depicted in Scheme 1.
Transfer hydrogenations are hydrogenation reactions that involve
a double H transfer from hydrogen donors to hydrogen acceptors
containing unsaturated bonds, such as C C, C O and C
N
bonds,1 which can principally be carried out in a metal-free
fashion (Scheme 1), but can also be catalyzed by transition metal
complexes. The metal mediated reactions play an important role in
modern organic synthesis and industrial processes.2 Given suitable
hydrogen donor and acceptor molecules, transfer hydrogenations
offer in the metal-free form an eco-friendly alternative to present
hydrogenation methods.3 They are expected to be operable on
a metal-free base especially when polar hydrogen donors and
acceptors are applied.1
Scheme 1 Metal-free double H transfer as a concerted elementary
process. X, Y, X¢ and Y¢: main group element fragments.
Institute of Inorganic Chemistry, University of Zu¨rich, Winterthurerstr. 190,
8057, Zu¨rich, Switzerland. E-mail: hberke@aci.uzh.ch; Fax: 0041 4463
56802; Tel: 0041 4463 54681
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ob06381b
We have previously reported that aromatic imines, bearing a
polarized C N double bond, can be directly hydrogenated by AB
via a ‘polarity matched’ concerted double-H transfer process.10
‡ We are grateful to the Swiss National Science Foundation and the
University of Zurich for financial support.
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Table 1 Reactions of 2-cyclohexylidenemalononitrile (1a) with various amine borane adducts in different solventsa
Entry
R, R¢
Solvent
Temp.
Timeb
Dehydrocoupling compoundsc
1
2
R = R¢ = H
R = R¢ = H
R = R¢ = H
R = R¢ = H
R = R¢ = H
R = R¢ = H
R = R¢ = H
R = R¢ = H
R = R¢ = H
R = H, R¢ = Me
R = H, R¢ = tBu
R = R¢ = Me
THF-D8
CD3CN
Tol-D8
C6D6
CD2Cl2
CDCl3
C2D6SO
no solvent
CD3OD
CD3CN
CD3CN
CD3CN
r.t./0 ◦
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
C
< 10 min/1 h
1 h
16 h
20 h
2 d
5 d
> 5 d
< 10 min
< 10 min
15 h
3 d/5 h
5 d/20 h
30.71 (d, J = 133 Hz), (BHNH)3
(BHNH)3
(BHNH)3
(BHNH)3
(BHNH)3
(BHNH)3
(BHNH)3
(BHNH)3
12.26 (s), B(OCD3)3
33.42 (d, J = 154 Hz), (BHNMe)3
28.78 (d, J = 124 Hz), (BHNtBu)3
8.23 (t, J = 112 Hz),
(BH2NMe2)2 + 31.15 (d, J = 131
Hz), BH(NMe2)2
3d
4d
5d
6d
7e
8
9
10
11
12f
r.t.
r.t./60◦C
r.t./60◦C
1
a The reactions were carried out in a NMR tube and monitored by in situ H and 11B NMR spectroscopy (1 : 1 molar ratio of 1a to AB or 1 : 2 molar
ratio of 1a to other amine boranes). b Required reaction times for full conversion of 1a to 2a. c The structure of the dehydrocoupling products of the
amine boranes were determined by comparison of the resonances (in ppm) in the 11B NMR spectrum to literature values; a small amount of unidentified
insoluble white solid was also observed. d The solubility of AB in toluene, benzene, dichloromethane or chloroform is very low. e Less than 10% of the
olefin was hydrogenated after 5 days at room temperature. f The ratio of the two dehydrocoupling compounds is 9 : 1 by integrations in the 11B NMR
spectra.
Ketones and aldehydes can also be transfer hydrogenated by AB,
but a protic solvent such as methanol was required, since otherwise
in an aprotic solvent, like THF, hydroboration would take place.11
Furthermore, a step-by-step HB before HN transfer from AB to
polarized olefins was found to proceed under mild conditions in
THF or acetonitrile.12 In this paper, we report a detailed study of
the metal-free transfer hydrogenation and its mechanism of polar
olefins with amine borane adducts.
Compound 1a is a liquid at room temperature and was found
to dissolve AB and to induce solvent-free transfer hydrogenation
forming 2a and BZ at room temperature nearly instantaneously
(Table 1, entry 7).
When the protic solvent methanol was applied as the reaction
media, transfer hydrogenation of 1a occurred also very fast. How-
ever, a completely different reaction took place in this case, only
trimethyl borate was observed as the alcoholysis product of AB
(Table 1, entry 8). The reaction proceeded along a hydroboration
process, related to the reactions of AB with carbonyl compounds
in methanol.11
Amine borane adducts other than AB, such as N-methyl amine
borane (MAB), N-tert-butyl amine borane (tBAB) and N,N-
dimethyl amine borane (DMAB), were then employed to react with
1a in acetonitrile at room temperature. Transfer hydrogenation
reactions similar to those with AB took place, but at much lower
rates. Among these substituted amine boranes, MAB displayed
superior reactivity towards 1a with formation of the borazine
analog (BHNMe)3 as the dehydrocoupling product (Table 1, entry
9). The more sterically hindered mono-substituted amine borane
adduct tBAB required a much longer time to complete the hy-
drogen transfer (entry 10), while the di-substituted DMAB turned
out to react slowest, with the four-membered ring (BH2NMe2)2
as the main dehydrocoupling product together with about 10% of
BH(NMe2)214 (entry 11). Naturally, an increase of the temperature
to 60 ◦C accelerated the reaction rates significantly (entries 10 and
11).
Results and discussion
1. Solvent dependence and scope of amine borane adducts
2-Cyclohexylidenemalononitrile (1a) was selected as the model
olefin and AB as a model amine borane adduct to test the general
propensity of such compounds for hydrogen transfer reactions in a
variety of solvents (Table 1). The reaction in THF was completed
within 10 min at room temperature (Table 1, entry 1). When
the conditions were adjusted by changing the solvent to higher
polarity, like acetonitrile or by decreasing the temperature to 10 ◦C,
1 h was required to bring the hydrogen transfer to completion
(entries 1–2). Other aprotic solvents were also tried. In toluene,
benzene, dichloromethane (DCM), chloroform and DMSO the
reactions were slow, partially due to also poor solubility of AB
in these solvents (reasonable solubility in DMSO). The reaction
rates in less polar solvents, toluene, benzene and DCM (entries
3–5), were higher than in the more polar solvents, like chloroform
and DMSO (entries 6–7). Such a preference for apolar solvent
supports the idea of a non-polar transition state.13 In all these
reactions, borazine ((BHNH)3, BZ) was the main dehydrocoupling
product of AB.
2. Range of the applied polar olefins
Based on the suitability of polar solvents in the transfer hydro-
genations, acetonitrile was selected as the preferred solvent to test
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the substrate dependence of the double H transfer reaction with
AB as the model amine borane adduct (THF was the best solvent
for the reaction, but it was too fast for study of the reaction course
at room temperature). A series of activated olefins was employed
to test the range of the substituent tolerance.
Most of the olefins with geminal electron-withdrawing (EWD)
groups on one side of the CC double bond and H, alkyl or aromatic
substituents on the other side could be successfully transfer
hydrogenated by AB (Table 2, entries 1–7). In cases where there
were two strongly polarized vinyl groups attached to a methylene
group, deprotonation of the alkylic H took place yielding a stable
anion (2h, entry 8). Otherwise, transfer hydrogenation of diolefinic
substrates tended to occur primarily at the more strongly polarized
double bond (2i and 2j, entries 9–10). A completely different type
of reaction takes place when the olefin is stabilized by a p push-pull
interaction, such as in 1k, or depolarized by two geminal strong
EWD groups on the other side of the double bond as in 1l, a
complex mixture of compounds other than those originating from
hydrogenation or hydroboration resulted (entries 11–12). Olefins
with only one EWD group on the double bond were found to
be much more reluctant to undergo transfer hydrogenation, even
Fig. 1 Kinetic conversion chart of the reactions of 0.5 mmol 1a with
0.1 mmol AB, AB(D), A(D)B or A(D)B(D) pursued by in situ 11B NMR
spectroscopy in acetonitrile at room temperature with 2 min intervals.
Concentrations were taken from the intensities of AB of the 11B NMR
spectra and filled to the first order reactions. The ꢀ stands for reactions
with AB, for AB(D),
deuterium kinetic isotope effects: kAB/kAB(D) = 1.00, kA(D)B/kA(D)B(D) = 1.04,
AB/kA(D)B = 1.55, kAB(D)/kA(D)B(D) = 1.62.
for A(D)B and
for A(D)B(D). Simulated
◦
when heated to 60 C for a prolonged reaction time (entries 13–
14). The dehydrogenated Hantzsch ester could not at all be directly
hydrogenated by AB (1o, entry 15).
k
3. Regio-specificity of the H transfers and deuterium kinetic
isotope effects12
The regio-specificity and deuterium kinetic isotope effect (DKIE)
were exemplarily investigated via the reactions of 1a with AB
using the selectively deuterated AB derivatives (NH3BD3 (AB(D)),
ND3BH3 (A(D)B) and ND3BD3 (A(D)B(D)).
Tracing the regioselectivity of the deuterium addition, it was
found that the protic HN/DN and the hydridic HB/DB end up
at the CCN atom of the double bond and the cyclohexylidene
C atom, respectively. According to the kinetic conversion chart
obtained by in situ 11B NMR analyses for the reactions under
pseudo-first order conditions (Fig. 1), the DKIE values were
calculated from the simulated rate constants (k). Nearly no DKIE
could be observed for deuterium labeling on the boron atoms
(Fig. 1, kAB/kAB(D) = 1, kA(D)B/kA(D)B(D) = 1.04) and normal DKIEs
were obtained with deuterium on the nitrogen atoms (Fig. 1,
Scheme 2 Transfer hydrogenation of AB with 1d derivatives with different
substituents in the para-positions (1 : 1 molar ratio) carried out in
acetonitrile at room temperature. The trend in reaction times is comparable
with the sequence of the substituent constants.
A sequence of reaction rates 1s > 1r > 1q > 1d > 1p was
derived (Scheme 2), closely correlating with the sequence of the
substituents constants17 –CN > –NO2 > –Cl > –H > –OMe.
Nevertheless, the solubility of the -NO2 and -CN substituted
olefins were not good in acetonitrile, but did not show much
influence on the reaction rates.
k
AB/kA(D)B = 1.55, kAB(D)/kA(D)B(D) = 1.62). Assuming that the HB
transfer constitutes the rate determining step (RDS), this was
expected to render DKIE values π 1. A DKIE of 1 is expected to
indicate that the double H transfer is stepwise and the HB transfer
would be fast occurring before or after the rate determining step.15
log (k/k0) = log k - log k0 = r*s
(1)
(2)
4. p-Aryl substituent effects of the polar olefins and Hammett
correlations
log k = r*s + log k0
Electronic substituent effects are usually studied by the effect
of para-substituted phenyl groups at the crucial C atoms of the
substrate.16 Since the reaction of AB with the phenyl substituted
olefin 1c in CD3CN was too fast to be measured at ambient
conditions and the reaction with 1e was too slow, 1d was chosen for
a model study of substituent effects (Table 2, entries 3–5). Several
para-substituted derivatives of 1d were used to react with AB at
room temperature in CD3CN in a 1 : 1 molar ratio (Scheme 2).
Where k and k0 are the rate constants of the reactions with
substituted and unsubstituted substrates, respectively; s is the
substituent constant and r is the reaction constant.
Based on the Hammett eqn 1,17 the reaction constant r was
obtained as the slope of the s vs. log k plot (eqn (2)). The Hammett
correlation was plotted with 1d, 1p, 1q, 1r and 1s (Fig. 2) and a
r = 1.02 was obtained. The positive r value indicated increased
electron density in the aromatic ring18 suggesting that negative
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Table 2 Transfer hydrogenation of various polarized olefins with AB (1 : 1 molar ratio) in CD3CN
Entry
1
Substrates
Abbreviations
Temp.
r.t.
Time
1 h
Yield (%)a
99 (86)
1a/2a
2
3
4
1b/2b
1c/2c
1d/2d
r.t.
r.t.
r.t.
2 h
99
99
99
10 min
30 min
5
6
1e/2e
1f/2f
r.t.
r.t.
3 d
98
99
10 h
7
1g/2g
r.t.
r.t.
30 min
99
8
1h/2h
1i/2i
1 d
1 d
0b
9c
60 ◦
C
86
10c
1j/2j
r.t.
2 d
1.5 h
85
82
60 ◦
C
C
11d
12d
13
1k
1l
r.t.
5 d
1 d
Complex mixture
Complex mixture
60 ◦
r.t.
10 min
1m
r.t.
2 d
2 d
Trace
50
60 ◦
C
C
14
1n
r.t.
5 d
5 d
n.r.
Trace
60 ◦
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Table 2 (Contd.)
Entry
15
Substrates
Abbreviations
Temp.
60 ◦
Time
5 d
Yield (%)a
n.r.
1o
C
a The reactions were monitored by in situ 1H NMR spectroscopy and the yields of the hydrogenated products were determined by integrations of the GC
peaks within the GC-MS analyses (in brackets is the isolated yield). b Deprotonation of 1h took place exclusively, yielding the anionic product dicyano(1-
(dicyanomethylene)-1H-inden-3-yl)methanide 2h with 99% yield. c Hydrogenation reactions occurred mainly with the double bond substituted by two
–CN groups, the percentages adding up to 100% were attributed to products with both double bonds hydrogenated. d Neither transfer hydrogenation nor
hydroboration was observed, the resulting mixture displayed a very broad complex peak ranging from 1.5 to 6.5 ppm in the 1H NMR spectra.
To determine whether the final dehydrocoupling product BZ
was formed via dehydrogenation of cyclotriborazane (CTB), a
reaction of 1a with CTB (1 : 2 molar ratio) was carried out in
THF. After 2 days at room temperature 1a was still not completely
hydrogenated (Scheme 4).
Scheme 4 Hydrogenation of 1a with CTB in a 1 : 2 molar ratio in THF
at room temperature.
This reaction proceeded much slower than the reaction of 1a
Fig. 2 Hammett correlation for the substituent effect in the transfer
hydrogenation of polarized olefins with AB. The rate constants k were
obtained from kinetic pursuits of the 11B NMR spectra of the reactions
of 0.1 mmol AB with 0.5 mmol of the p-substituted 1d derivatives in
acetonitrile. The values of the substituent constant s were taken from
literature. ꢀ Represents the –OMe, –H, –Cl, -NO2 and –CN group,
respectively. The slope of the kinetic curve was r = 1.02.
with AB which was finished within 10 min in THF at room
temperature (Table 1, entry 1). Since the formation of BZ via the
dehydrogenation of CTB was so slow, we anticipated the existence
of another pathway leading to BZ or polyborazylene (PBZ), for
instance the direct dehydrocoupling of [NH2 BH2] or probably a
solvent-stabilized [NH2 BH2]–(solvent) pathway (Scheme 5).
charge is generated in the transition state of the RDS. Since the
transfer of the protic HN is participating in the RDS of these
reactions andtheattack of aH+ ion would generate positive charge,
it seemed reasonable to assume that the reaction passes over a
concerted transition state.
Scheme 5 Possible formation pathway of BZ (PBZ) from direct dehydro-
coupling of a solvent-stabilized [NH2 BH2]–(solvent).
5. The dehydrocoupling pathway of AB
In an attempt to trap boron based intermediates, cyclohexene (2
eq. to the amount of AB) was added into the reaction mixture
of 1a with AB in THF. Cy2BNH2 was observed at a much faster
rate than the direct reaction of cyclohexene with AB (Scheme 3).
As reported in the literature, such an observation indicated the
intermediacy of [NH2 BH2] along the reaction path.7a,8,9
6. Reactivity of the hydroboration intermediate
According to 13C and 11B NMR spectra,12 the intermediate 3a
is formed via hydroboration of 1a by AB (Scheme 6). A triplet
appeared for 3a at around -14 ppm at temperatures below -40 ◦C
in THF or acetonitrile.
Scheme 3 Transfer hydrogenation of 1a with AB in the presence of excess
cyclohexene in THF at room temperature.
Scheme 6 Trapping of the hydroboration intermediate (3a) by in situ
NMR spectroscopy of the reaction of 1a with AB at low temperature.
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In another reaction batch using 1a in excess, AB was completely
transformed into 3a (together with some BZ) at -40 ◦C in THF-
D8 requiring 7 days. The subsequent reaction step of 3a was then
pursued at room temperature by in situ 11B NMR spectroscopy.
It was found that 3a was rapidly converted to BZ and PBZ, with
CTB to be initially detected in tiny amounts as a shoulder to the
signal of 3a. CTB finally remained as the only –BH2– containing
species remaining until the end of the reaction (Fig. 3).
Scheme 7 Mechanistic scheme for the transfer hydrogenation of polarized
olefins with ammonia borane.
Conclusions
Fig. 3 In situ 11B NMR spectra in THF-D8 showing the decomposition
of 3a at room temperature. Initially 3a was formed by keeping a NMR
sample of 1a with AB (3 : 1 molar ratio) at -40 ◦C over a week; by that
time AB was completely consumed with concomitant formation of a small
amount of BZ.
In conclusion, the transfer hydrogenation of a series of activated
olefins, equipped with geminal electron-withdrawing groups on
one side of the double bond and H or organyl on the other
side, with amine borane adducts as hydrogen donors could be
accomplished metal-free under mild conditions. The reaction
constant obtained from Hammett correlations indicated that
negative charge is accumulated in the transition state. Together
with deuterium kinetic isotope studies and explorations on the
hydroboration intermediate, the actual reaction course could be
established: primary hydridic HB transfer occurs rapidly from the
borane part to the olefin before the RDS, then the transfer of the
protic HN occurs as the RDS, presumably via a concerted cyclic
transition state. The routes to the dehydrocoupling products of
ammonia borane were rationalized onthe basis that the main prod-
uct borazine was probably formed directly from dehydrocoupling
of [NH2 BH2] or of its solvent stabilized adduct [NH2 BH2]–
(solvent), rather than by dehydrogenation of cyclotriborazane
formed by trimerization of [NH2 BH2].
This further substantiated that CTB reacts very slowly with 1a.
Nevertheless, if the reaction was carried out at low temperature
with a 1 : 2 molar ratio of 1a to AB until 3a had completely
been formed (no free olefin detectable in the mixture) and
then was warmed up to room temperature, CTB was the only
dehydrocoupling product of AB to be detected. Since in the room
temperature reactions of 1a with AB only –BH– species from BZ
and PBZ were observed, they are expected to originate from direct
dehydrocoupling of [NH2 BH2] after elimination from 3a rather
than from dehydrogenation of CTB.
7. Mechanism of the double H transfer reaction
Based on the acquired mechanistic facts, a two-step hydrogen
transfer mechanism is proposed for the reactions of polar olefins
with AB (Scheme 7). The first step is the HB transfer forming
a hydroboration intermediate 3, presumably via the concerted
transition state 4. Subsequently HN transfer occurred as the RDS,
which might be transferred in an intra-molecular mode as shown
in 5 of Scheme 7, releasing the reactive aminoborane species
[NH2 BH2] presumablyin asolventstabilized form[NH2 BH2]–
(solvent). [NH2 BH2] or [NH2 BH2]–(solvent) is then dehydro-
genated subsequently in a fast reaction by another equivalent of
1 generating BZ and PBZ as the final dehydrocoupling products,
presumably via simultaneous double H transfer as shown in 6
(Scheme 7). The cyclic trimer CTB can be formed by direct
cyclization of [NH2 BH2], but only in a side reaction in small
amount or when the dehydrogenation of [NH2 BH2] is slowed by
applying low temperature and/or low concentrations of the olefin.
Such a mechanistic scheme is anticipated to be applicable to other
amine boranes.
Experimental
All of the hydrogen-transfer reactions were carried out at least
twice with reported reproducible results.
All the manipulations were carried out under a nitrogen
atmosphere using Schlenk techniques or a dry-box (Model MB-
150B-G). Reagent grade benzene, toluene, hexane, diethyl ether,
and THF were dried and distilled from sodium benzophenone
ketyl prior to use. Acetonitrile was distilled from CaH2, and
chloroform was dried by P2O5. All the amine borane adducts
and olefins 1l–1o are commercially available and purified prior
to use. Olefins 1a–1e were synthesized as described in our previous
communication.12 NMR spectra were measured on a Varian
1
Mercury spectrometer at 200 MHz for H, Varian Gemini-2000
spectrometer at 300.1 MHz for H, 96.3 MHz for 11B{ H} and
1
1
75.5 MHz for 13C{ H} and on a Bruker-DRX-500 spectrometer
1
at 500.2 MHz for H, 107 MHz for 11B{ H} and 125.8 MHz for
1
1
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1
1
13C{ H}. Chemical shifts for H and 13C are given in parts per
million relative to TMS and those for 11B relative to Et2O·BF3.
GC-MS was measured on a Varian 450 GC (Saturn 2000 GC-
MS/MS).
monium acetate (0.25 g, 3.2 mmol) dissolved in absolute ethanol
(6 mL) was heated at reflux for 30 min. After cooling to room
temperature, water (5 mL) was added and the solution acidified
with concentrated hydrochloric acid. The brown precipitate was
filtered off and washed with water. Recrystallisation from glacial
acetic acid afforded compound 1h (0.35 g, 45%) as a brown yellow
crystal. 2,2¢-(1H-Indene-1,3(2H)-diylidene)dimalononitrile:25 dH
(ppm; 300 MHz; CD3CN) 4.31 (s, 2H, -CH2-), 7.93–9.96 (m, 2H),
8.53–8.56 (m, 2H); dC (ppm; 75.4 MHz; CD3CN) 42.94 (-CH2-),
79.10 ( C(CN)2), 113.07 (-CN), 113.29 (-CN), 127.33, 137.03,
141.70, 167.92 (-CH C(CN)2).
Olefins 1f, 1g, 1j and 1p–1t were prepared via the following
method with nearly 100% yield: To a stirred solution of 0.01 mol
of aldehyde in 0.01 mol of malononitrile (or methylcyanoacetate)
was added 3 g of alumina Merck 90. The reaction was exothermic.
After the reaction was cooled down, the product was extracted
with dichloromethane (2 ¥ 20 ml).19 The solvent was then removed,
leaving a product with appropriate purity.
2-(3,4-Dihydronaphthalen-1(2H)-ylidene)malononitrile (1f),20
white solid: dH (ppm; 300 MHz; CD3CN) 1.89–2.14 (m, 2H,
-CH2-), 2.85 (t, J = 6 Hz, 2H, -CH2-), 2.98 (t, J = 6 Hz, 2H, -CH2-),
7.33–7.38 (m, 2H), 7.50–7.55 (m, 1H), 8.12 (d, J = 8 Hz, 1H); dC
(ppm; 75.4 MHz; CD3CN) 22.64, 30.12, 33.68, 80.45 ( C–(CN)2),
114.57 ( C–(CN)2), 115.35 ( C–(CN)2), 127.36, 128.63, 130.38,
131.10, 134.53, 143.80, 174.39 (C C–(CN)2).
Olefin 2-(3-ethoxy-1H-inden-1-ylidene)malononitrile (1i) was
prepared via the following method:25 A solution of indane-l,3-
dione (0.4 g, 2.74 mmol), malononitrile (0.45 g, 6.8 mmol) and
ammonium acetate (60 mg, 0.78 mmol) dissolved in absolute
ethanol (20 mL) was heated at reflux for 24 h. The solvent
was removed in vacuo, leaving a purple solid from which a red
product was extracted with boiling light petroleum (b.p. 40–
60 ◦C). Recrystallisation of the red product in light petroleum
afforded compound 1i (10 mg, 10%) as a bright red powder.
2-(3-Ethoxy-1H-inden-1-ylidene)malononitrile (1i):25 dH (ppm;
300 MHz; CD3CN) 1.45 (t, J = 7 Hz, 3H, -Me), 4.34 (q, J = 7
Hz, 2H, -OCH2-), 5.75 (s, 1H), 7.28–7.31 (m, 1H), 7.37–7.46 (m,
2H), 7.98–8.00 (m, 1H); dC (ppm; 75.4 MHz; CD3CN) 14.48 (-Me),
69.93 (-OCH2-), 70.08 ( C(CN)2), 97.75 (-CH C-OEt), 114.77
(-CN), 121.00, 124.89, 131.32, 133.37, 138.65, 166.84 (-CH C-
OEt), 173.46 (-CH C(CN)2).
Olefin cyclohepta-2,4,6-trienylidene malononitrile (1k) was pre-
pared via the following method:26 An oven-dried 50 mL three-
necked round-bottomed flask, equipped with a nitrogen gas
inlet, magnetic stirring bar, dropping funnel, thermometer, and
condenser, under an inert nitrogen atmosphere, was placed in an
ice bath and charged, while stirring, with 10 mL of pyridine and
0.9 g (5 mmol) of cycloheptatrienylium tetrafluoroborate at 0 ◦C.
Bromomalononitrile (0.73 g, 5 mmol) is added drop by drop within
a 10 min period, and the mixture is vigorously stirred at that
temperature for 1 h. The ice bath is replaced by a water bath, the
temperature is gradually raised to 40 ◦C, and the stirring mixture
is kept at that temperature for 5 h. The solution was acidified
with 1 M HCl solution and the organic phase was extracted with
chloroform. The final product 1k was obtained as red powder by
column chromatography eluted with hexane : Ethyl ester = 1 : 5
(yield: 60%). Cyclohepta-2,4,6-trienylidene malono-nitrile (1k):
2-(Naphthalen-1-ylmethylene)malononitrile (1g),21 yellow
solid: dH (ppm; 300 MHz; CD3CN) 7.60–7.73 (m, 3H), 7.98–8.19
(m, 4H), 8.90 (s, 1H); dC (ppm; 75.4 MHz; CD3CN) 86.58
(
C–(CN)2), 113.96 ( C–(CN)2), 114.85 ( C–(CN)2), 124.24,
126.34, 128.23, 129.13, 129.16, 129.33, 130.04, 131.93, 134.45,
135.22, 160.36 (C C–(CN)2).
(E)-2-(3-Phenylallylidene)malononitrile (1j),21 Yellow solid: dH
(ppm; 300 MHz; CD3CN) 7.18–7.23 (m, 1H), 7.25–7.31 (m, 1H),
7.39–7.49 (m, 3H), 7.65–7.70 (m, 2H), 7.81–7.87 (m, 1H); dC
(ppm; 75.4 MHz; CD3CN) 83.06 ( C–(CN)2), 112.94 ( C–
(CN)2), 114.93 ( C–(CN)2), 123.46, 130.04, 130.19, 132.75,
135.35, 151.53, 162.18 (C C–(CN)2).
(E)-Methyl 2-cyano-3-(4-methoxyphenyl)acrylate (1p),22 white
solid: dH (ppm; 300 MHz; C6D6) 3.84 (s, 3H, -Me), 3.87 (s, 3H,
-Me), 7.06 (d, J = 9 Hz, 2H), 8.00 (d, J = 9 Hz, 2H), 8.19 (s, 1H,
-CH C(CN)(CO2Me)); dC (ppm; 75.4 MHz; C6D6) 53.77 (-Me),
56.49 (-Me), 100.06 ( C(CN)(CO2Me)), 115.78, 117.21 (-CN),
125.28, 134.49, 155.33 (-CH C(CN)(CO2Me)), 164.32, 164.88
(-CO2Me).
(E)-Methyl 3-(4-chlorophenyl)-2-cyanoacrylate (1q),23 white
solid: dH (ppm; 300 MHz; CD3CN) 3.87 (s, 3H, -Me), 7.54
(d, J = 8 Hz, 2H), 7.96 (d, J = 8 Hz, 2H), 8.25 (s,
1H, -CH C(CN)(CO2Me)); dC (ppm; 75.4 MHz; CD3CN)
54.11 (-Me), 104.54 ( C(CN)(CO2Me)), 116.31 (-CN), 130.46,
131.34, 133.26, 139.66, 154.40 (-CH C(CN)(CO2Me)), 163.52
(-CO2Me).
d
H (ppm; 300 MHz; THF-D8) 6.96–7.11 (m, 4H), 7.32–7.37 (d*m,
(E)-Methyl 2-cyano-3-(4-nitrophenyl)acrylate (1r),24 milk-
yellow solid: dH (ppm; 300 MHz; CDCl3) 3.99 (s, 3H, -Me), 8.12–
8.15 (m, 2H), 8.32–8.37 (m, 3H); dC (ppm; 75.4 MHz; CDCl3)
53.85 (-Me), 106.87 ( C(CN)(CO2Me)), 114.44 (-CN), 124.29,
131.51, 136.75, 149.72, 151.95 (-CH C(CN)(CO2Me)), 161.86
(-CO2Me).
2 H, J = 12 Hz); dC (ppm; 300 MHz; THF-D8) 115.10 (-CN),
135.76, 138.47, 140.15, 164.47 (-CH C(CN)2).
General procedure for the transfer hydrogenation reactions
In a glove-box, a 0.5 mm Young NMR tube was charged with
olefin (0.1 mmol), amine-borane adducts (0.1 mmol) and dry
THF-D8 or acetonitrile-D3 (0.6 mL). The tube was sealed with
a screw cap and then stored at room temperature or the required
(E)-Methyl-2-cyano-3-(4-cyanophenyl)acrylate (1s),24 white
solid: dH (ppm; 300 MHz; CDCl3) 3.98 (s, 3H, -Me), 7.78–
7.81 (m, 2H), 8.05–8.08 (m, 2H), 8.26 (s, 1H); dC (ppm;
75.4 MHz; CDCl3) 53.80 (-Me), 106.26 ( C(CN)(CO2Me)),
114.54, 116.05 (-CN), 117.64 (-CN), 131.00, 132.82, 135.09, 152.47
(-CH C(CN)(CO2Me)), 161.97 (-CO2Me).
Olefin 2,2¢-(1H-indene-1,3(2H)-diylidene)dimalononitrile (1h)
was prepared via the following method:25 A solution of indane-1,3-
dione (0.48 g, 3.2 mmol), malononitrile (0.54 g, 8.2 mmol) and am-
1
temperature after shaking. The reaction was monitored by H
and 11B NMR spectroscopy every several minutes (depends on
the reaction rate). The typical resonance of the starting materials
decreased, and a new signal of the saturated products gradually
appeared. The disappearance of the starting olefin indicates that
the transformation has completed, 13C and 11B NMR spectra were
858 | Org. Biomol. Chem., 2012, 10, 852–860
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then recorded. Finally the reaction mixture was diluted for GC-
MS analysis. Samples for low-temperature NMR spectroscopy
were prepared at room temperature using chilled solvents, and im-
mediately put into a freezer until required for NMR measurement.
Samples for kinetic study were prepared with pseudo-first order
conditions with the 0.1 mmol AB and the olefin in great excess
(0.5 mmol).
The reaction of 1a with AB was also carried out in large scale
using a Young Schlenk tube: In a glove-box, a 50 mL Young
Schlenk tube was charged firstly with a magnetic stirring bar and
30 mg AB (1 mmol), then 10 ml dry THF and finally 0.144 mL 1a
(1 mmol). Then the tube was sealed with a screw cap and stirred
for 10 min. As inspected by GC-MS, the olefin was completely
hydrogenated. After that, all the volatile species (THF, BZ) were
removed from the tube by applying high vacuum, the remaining
residue was further purified by distillation. Finally 2a was obtained
as a colorless liquid in 86% yield (127 mg).
123.68, 125.93, 126.69, 127.16, 128.81, 129.38, 129.46, 130.82,
131.74, 134.36; MS (m/z) = 206.
Dicyano(1-(dicyanomethylene)-1H -inden-3-yl)methanide
(2h),34 purple solid: dH (ppm; 300 MHz; CD3CN) 5.73 (s, 1 H,
-
-CH C(C(CN)2 )), 7.32–7.36 (m, 2H), 7.92–7.96 (m, 2H); dC
-
(ppm; 75.4 MHz; CD3CN) 104.19 (-CH C(C(CN)2 )), 119.11 (-
CN), 119.27 (-CN), 122.68, 124.93, 126.38, 130.65, 139.46, 159.75
(-C C(CN)2); GC retention time: 13.582 min; (-)MS (m/z) =
241.
2-(3-Ethoxy-1H-inden-1-yl)malononitrile (2i), purple solid: dH
(ppm; 300 MHz; CD3CN) 1.67–1.81 (m, 1 H), 1.85–2.07 (m, 2H),
2.20–2.30 (m, 1H), 2.72–2.89 (m, 2H), 3.56–3.62 (m, 1H, -CH-),
4.63 (d, J = 5.5 Hz, 1H, -CH(CN)2), 7.16–7.30 (m, 4 H, CH-);
dC (ppm; 75.4 MHz; CD3CN) 14.69, 27.58, 45.67, 66.81, 98.24,
119.66 (-CN), 124.39, 126.48, 128.10, 128.85, 129.66, 161.35; GC
retention time: 9.849 min; MS (m/z) = 224.
2-Cinnamylmalononitrile (2j),27,29 white solid: dH (ppm;
300 MHz; CD3CN) 2.92–2.97 (m, 2H, -CH2-), 4.28–4.32 (m, 1H,
-CH(CN)2), 6.24–6.35 (m, 1H, CH-), 6.73–6.78 (m, 1H, CH-
), 7.26–7.51 (m, 5H); dC (ppm; 75.4 MHz; CD3CN) 24.41, 34.41,
114.45 (-CN), 122.62, 127.39, 129.12, 129.31, 129.68, 136.65; GC
retention time: 8.973 min;MS (m/z) = 182.
All the hydrogenated products were described earlier. The
obtained NMR data match literature values.
2-Cyclohexylmalononitrile (2a),27 colorless liquid, dH (ppm;
300 MHz; THF-D8) 1.15–1.44 (m, 5 H, -CH-), 1.69–2.06 (m, 6
H, -CH-), 4.25 (d, J = 5 Hz, 1 H, -CH-); dC (ppm; 300 MHz; THF-
D8) 26.34, 26.41, 29.57, 30.76, 40.06, 113.64 (-CN); GC retention
time: 5.145 min; MS (m/z) = 149.
Methyl 2-cyano-3-(4-methoxyphenyl)propanoate (2p),20,27,30
white solid: dH (ppm; 300 MHz; CD3CN) 3.14–3.28 (m, 2H,
-CH2-), 3.74 (s, 3H, -CH3), 4.01–4.08 (m, 1H, -CH(CN)2), 7.31–
7.37 (m, 5H); dC (ppm; 75.4 MHz; CD3CN) 35.99, 40.32, 54.07,
117.62 (-CN), 128.53, 129.67, 130.15, 137.11, 167.47 (-CO2Me);
GC retention time: 9.387 min; MS (m/z) = 219.
Methyl 2-cyano-2-cyclohexylacetate (2b),28 colorless liquid: dH
(ppm; 300 MHz; CD3CN) 1.08–1.35 (m, 5 H, -CH-), 1.63–1.82 (m,
5 H, -CH-), 1.94–2.07 (m, 1 H, -CH-), 3.64 (d, J = 5 Hz, 1 H, -CH-),
3.74 (s, 3 H, -Me); dC (ppm; 75.4 MHz; CD3CN) 26.27, 26.34,
26.51, 30.00, 31.62, 39.43, 45.11, 53.83, 117.10 (-CN), 167.60
(-C O); MS (m/z) = 181.
Methyl 3-(4-chlorophenyl)-2-cyanopropanoate (2q),29,30 white
solid: dH (ppm; 300 MHz; CD3CN) 3.17–3.33 (m, 2H, -CH2-), 3.79
(s, 3H, -CH3), 4.06–4.11 (m, 1H, -CH(CN)2), 7.29–7.44 (m, 4H);
dC (ppm; 75.4 MHz; CD3CN) 34.67, 39.56, 53.60, 116.90 (-CN),
129.08, 131.35, 133.35, 135.43, 166.69 (-CO2Me); GC retention
time: 9.092 min; MS (m/z) = 223.
2-benzylmalononitrile (2c),20,27–29 slightly yellow solid: dH (ppm;
300 MHz; THF-D8) 3.31 (d, J = 7 Hz, 2 H, -CH2-), 4.59 (t, J =
7 Hz, 1 H, -CH(CN)2), 7.32–7.38 (m, 5 H, -CH ); dC (ppm;
75.4 MHz; THF-D8) 25.25, 36.91, 114.19 (-CN), 128.90, 129.61,
130.19, 135.65; GC retention time: 5.807 min; MS (m/z) = 156.
Methyl 2-cyano-3-phenylpropanoate (2d),20,28,30 white solid: dH
(ppm; 300 MHz; CD3CN) 3.10–3.32 (m, 2 H, -CH2-), 3.74 (s, 3
H, -Me), 4.01–4.13 (m, 1 H, -CH-), 7.25–7.41 (m, 5 H, -CH );
dC (ppm; 75.4 MHz; CD3CN) 35.47, 39.80, 53.53, 117.07 (-CN),
127.96, 129.11, 129.59, 136.55, 166.89 (-C O); GC retention time:
7.770 min; MS (m/z) = 189.
Methyl 2-cyano-3-(4-nitrophenyl)propanoate (2r),20,29,35 white
solid: dH (ppm; 300 MHz; CD3CN) 3.31–3.47 (m, 2H, -CH2-),
3.80 (s, 3H, -CH3), 4.16–4.21 (m, 1H, -CH(CN)2), 7.56–7.59 (d,
J = 10 Hz, 2H), 8.22–8.25 (d, J = 10 Hz, 2H); dC (ppm; 75.4 MHz;
CD3CN) 34.82, 39.00, 53.73, 116.69 (-CN), 124.11, 130.85, 144.20,
166.45 (-CO2Me); GC retention time: 10.799 min; MS (m/z) = 235.
Methyl 2-cyano-3-(4-cyanophenyl)propanoate (2s),36 white
solid: dH (ppm; 300 MHz; CD3CN) 3.25–3.43 (m, 2H), 3.79
(s, 3H, -CH3), 4.11–4.17 (m, 1H, -CH(CN)2), 7.49–7.53 (m,
2H), 7.74–7.78 (m, 2H); dC (ppm; 75.4 MHz; CD3CN) 35.15,
39.05, 53.70, 111.61, 116.71 (-CN), 118.96, 130.61, 132.92, 142.14,
166.48 (-CO2Me); GC retention time: 9.989 min; MS (m/z) =
214.
Dimethyl 2-benzylmalonate (2e),20,31 colorless liquid: dH (ppm;
300 MHz; CD3CN) 3.13–3.17 (d, J = 8 Hz, 2 H, -CH2-), 3.63 (s, 6
H, -Me), 3.71 (t, J = 8 Hz, 1 H, -CH-), 7.09–7.36 (m, 5 H, -CH );
dC (ppm; 75.4 MHz; CD3CN) 35.30, 53.04, 54.35, 127.70, 129.46,
129.76, 139.03, 170.12 (-C O); GC retention time: 8.059 min; MS
(m/z) = 223.
2-(1,2,3,4-Tetrahydronaphthalen-1-yl)malononitrile
(2f),20,32
white solid: dH (ppm; 300 MHz; CD3CN) 1.67–1.81 (m, 1
H), 1.85–2.07 (m, 2H), 2.20–2.30 (m, 1H), 2.72–2.89 (m, 2H),
3.56–3.62 (m, 1H, -CH-), 4.63 (d, J = 5.5 Hz, 1H, -CH(CN)2),
7.16–7.30 (m, 4 H, CH-); dC (ppm; 75.4 MHz; CD3CN) 20.99,
27.74, 29.78, 30.44, 39.44, 114.10 (-CN), 114.30 (-CN), 127.21,
128.66, 128.74, 130.71, 133.73, 139.52; GC retention time: 9.461
min MS (m/z) = 196.
Notes and references
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©