First examples of carbene-catalyzed allylation of benzaldehyde with allyltrichlorosilane

Article abstract of DOI:10.1007/s13738-014-0582-8

Abstract We report here first examples of carbene-catalyzed allylation of benzaldehyde. N-Heterocyclic carbenes applied here, 9a and 10a, were derived from imidazolinium zwitterions, and contained sulfonate and sulfamate substituents, respectively. Different reaction conditions such as temperature, organic solvents, additive and bases were used to optimize the reaction. Sulfonate substituted NHC 9a is found more efficient organocatalyst for the allyltion of benzaldehyde (greater than 99 % yield at lower temperature) than sulfamate-based NHC 10a (a yield of no more than 15 %). These results are justified on the basis of philicity descriptors of the NHCs and their corresponding allyldichlorosilane complexes. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13738-014-0582-8

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0582-8
ORIGINAL PAPER
First examples of carbene‑catalyzed allylation of benzaldehyde
with allyltrichlorosilane
Sobia Tabassum · Mazhar Amjad Gilani ·
Khurshid Ayub · René Wilhelm
Received: 22 May 2014 / Accepted: 22 December 2014
© Iranian Chemical Society 2015
Abstract We report here first examples of carbene-cata-
lyzed allylation of benzaldehyde. N-Heterocyclic carbenes
applied here, 9a and 10a, were derived from imidazolinium
zwitterions, and contained sulfonate and sulfamate sub-
stituents, respectively. Different reaction conditions such as
temperature, organic solvents, additive and bases were used
to optimize the reaction. Sulfonate substituted NHC 9a is
found more efficient organocatalyst for the allyltion of ben-
zaldehyde (greater than 99 % yield at lower temperature)
than sulfamate-based NHC 10a (a yield of no more than
15 %). These results are justified on the basis of philicity
descriptors of the NHCs and their corresponding allyldi-
chlorosilane complexes.
Graphical abstract
Keywords N-Heterocyclic carbenes · Imidazolinium
salts · Zwitterions · Organocatalysis · Allyltrichlorosilane
Introduction
Organocatalysts provide simple operational facility with
low level of toxicity compared to metal-based catalysts
[1–3]. In this context, carbenes have emerged as efficient
organocatalysts and ligands. In carbene family, N-heter-
ocyclic carbenes (NHCs) are Lewis-base organocatalysts
and offer alternative ways for conventional reactions. For
example, NHCs render electrophiles such as carbonyl and
Michael systems to acyl anion equivalent and homoeno-
lates, respectively, via umpolung [4–7]. There is a rapid
progress in the field of NHC catalysis because NHCs offer
efficiency and new reactivity with a range of reactions, such
as umpolung reactions of aldehydes [8], conjugate umpol-
ung [8], benzoin condensation [9], Stetter reaction [10],
cycloaddition reactions [11, 12], ring opening reactions
[13], aza-Morita–Baylis–Hillman reactions [14], addition
of silylated nucleophiles to electrophiles [15], transesterifi-
cation [16], and polymerization [17].
S. Tabassum (*)
Interdisciplinary Research Center in Biomedical Materials,
COMSATS Institute of Information Technology, Defence Road,
Off Raiwind Road, Lahore, Pakistan
e-mail: sobiatabassum@ciitlahore.edu.pk
M. A. Gilani
Department of Chemical Engineering, COMSATS Institute
of Information Technology, Defence Road, Off Raiwind Road,
Lahore, Pakistan
e-mail: mazhargilani@ciitlahore.edu.pk
K. Ayub
Department of Chemistry, College of Science, King Faisal
University, Al-Ahsa 31982, Kingdom of Saudi Arabia
K. Ayub
Department of Chemistry, COMSATS Institute of Information
Technology, Abbottabad 22060, Pakistan
The allylation of carbonyl compounds leads to a car-
bon–carbon bond formation and the product of the reac-
tion is homoallylic alcohol, an important intermediate/
synthon in organic synthesis [18, 19]. In addition to
R. Wilhelm
Department of Chemistry, University of Paderborn,
Warburgerstrasse 100, 33098 Paderborn, Germany
1 3

J IRAN CHEM SOC
Table 1 Allylation
of benzaldehyde with
Entry
Solvent
Basea
T (°C)
Time (h)
Zwitterion
Yield (%)
N-heterocyclic carbenes derived
from 10 mol % zwitterion
1
THF
NaH
rt
48
48
65
48
48
48
24
24
24
24
9
10
10
9
57
Traces
15
2
THF
NaH
rt
3
THF
NaH
rt
4
CH2Cl2
Toluene
THF
NaH
rt
56
5
KHMDS
KHMDS
NaH
rt
9
45
6
rt
9
56
7b
8b
9b
10b
CH2Cl2
THF
rt
9
75
a
9 mol % base was used to
NaH
rt
9
60
generate NHCs
Toluene
CH2Cl2
NaH
rt
9
73
b
Diisopropylethylamine was
NaH
−60
9
99
used as an activator
different metal-based allylating agents, allyltrichlorosi-
lane has been proven as an effective allylating agent. The
success of the reagent lies in the fact that it can react with
Lewis-base catalysts and a hypervalent silicon complex
is formed. This silicon complex can efficiently react with
different electrophiles and corresponding allylated prod-
ucts are formed [20]. A pioneer work has been done by
Kobayashi et al. who reported the allylation to electro-
philes catalyzed by a Lewis-base, N,N-dimethylforma-
mide [21, 22]. After that, a variety of Lewis-basic mol-
ecules have been reported. Examples include dinitrones
[23], ureas [24], phosphoramides [25], formamides [26–
28], N-oxides [29–31] and phosphine oxides [32, 33].
In the ongoing search for new classes of organocatalysts
in the allylation of aldehydes, we report here the allyla-
tion reactions catalyzed by nucleophilic NHCs derived
from the imidazolinium zwitterions. According to our
knowledge there is no report about exploitation of NHCs
as organocatalysts for the allylation of aldehydes. A com-
parison of NHCs derived from imidazolinium zwitterions
having sulfonate and sulfamate substituents is described.
The philicity descriptors such as electrophilic/nucleo-
philic power calculated by theoretical calculations are
used to explain the efficiency of sulfonate substituted imi-
dazolinium NHC.
General procedure for allylation of benzaldehyde using
allyltrichlorosilane in the presence of carbene
1‑Phenylbut‑3‑en‑1‑ol
A stirred solution of imidazolinium zwitterion (10 mol %)
was treated with 9 mol % base for 0.5–1.5 h at rt to −60 °C
according to the entries in the Table 1. Benzaldehyde
(0.05 mL, 0.49 mmol, 1 eq) and the additive diisopropyl-
ethylamine (0.41 mL, 2.35 mmol, 5.0 eq) were added. Then
allyltrichlorosilane (0.105 mL, 0.735 mmol, 1.5 eq) was
added dropwise. The reaction was stirred for a fixed time
as mentioned in the Table 1. The reaction was carefully
quenched with 1 mL of 10 % NaOH and extracted three
times with ethyl acetate. The organic layer was washed
with 15 mL of 5 % HCl, NaHCO3 and brine, dried over
Na2SO4 and concentrated. For the purification of crude
product, flash column chromatography (EtOAc/petrol ether
15:85) was performed to yield 1-phenylbut-3-en-1-ol as
an oil. The spectral data were consistent with the literature
[32].
1H-NMR (CDCl3, 400 MHz): δ ppm = 7.38–7.27 (m,
5H), 5.82 (ddt, J = 17.2, 10.0, 7.2 Hz, 1H), 5.17 (dd,
J = 17.2, 1.2 Hz, 1H), 5.15 (dd, J = 10.4, 1.2 Hz, 1H),
4.74 (dt, J = 6.4, 2.4 Hz, 1H), 2.58–2.6 (m, 2H), 2.06
(d, J = 2.8 Hz, 1H). 13C-NMR (CDCl3, 100 MHz): δ
ppm = 143.9, 134.5, 128.5, 127.6, 125.9, 118.5, 73.5, 44.1.
Experimental
Theoretical calculation
General remarks
All calculations were performed with Gaussian 09 [36].
The geometries of the structures were optimized by using
a density functional theory (DFT) at hybrid functional
(B3LYP) [37] with a basis set 6-31G(d). The method and
basis set chosen provided a nice balance between cost and
accuracy. The geometries of the optimized structures were
plotted with Marvin View [38]. The stationary points were
confirmed as true minima through frequency calculations
(no imaginary frequency). Geometries and intermolecular
NMR spectra were measured on a Bruker AMX 400
(400 MHz) and a Bruker AC 200F (200 MHz) in deuter-
ated solvents as mentioned. Flash column chromatogra-
phy [34] was performed on Sorbisil C-60. The reactions
were monitored by TLC plates of Merck Silica gel 60
F254. The zwitterions 9 and 10 were synthesized and
characterized according to our already reported protocol
[35].
1 3

J IRAN CHEM SOC
,
,
, rt,
1 ArCHO H
16 h
2O
.
.
5
(
)
3C
for AcOH MeO
H
R1
R1
R1
R1
,
1 h
C
H
R1
R1
,
, rt,
reflux,
2 NaBH MeOH
16 h
3CN
4
or
H2N
NH2
H2N HN R2
N
N
R2
(
)
6
for MeO 2CHNMe2
o
-
-
,
(
)
Tol Br Pd
120 ° 1 h
OAc
C
,
BINAP NaOtBu 2
,
H
1
1
R2 CH Mes
1
1
,
5 R1
2Mes
CH
R
R
Ph
H
=
=
,
3
4
91
%
96
%
R
R
H
=
=
=
91
%
80
%
R2
=
=
=
=
2
Tol
1
2
2
1
2
o
-
,
o
-
Tol
Ph R
=
,
6
R
Ph R
O
S
O
O
N
S
7
8
O
O
O
Ph
Ph
O3S
Mes
Mes
N
N
N
N
N
SO3
95% 10
Base
81% 9
Base
Ph
N
Ph
N
O3S
N
N
..
..
N
SO3
10a
9a
Scheme 1 Synthesis of imidazolinium sulfonate/sulfamate zwitterions
interaction energies such as energy of highest occupied
molecular orbital (HOMO), energy of lowest unoccupied
molecular orbital (LUMO), band gap, electronic properties
(Mulliken and NPA charge analysis), chemical hardness
(η), electronic chemical potential (µ), global softness (S),
local softness (Sk), Fukui function, global nucleophilicity
(N), local nucleophilicity (NK), global electrophilicity (ω)
and local electrophilicity (ωk) for carbenes and carbene-
allyldichlorosilane complexes were calculated at DFT/
B3LYP/6-31G(d) level. The negative of HOMO and LUMO
values were estimated as ionization potential (I) and elec-
tron affinity (A), respectively [37, 39]. For the estimation of
Fukui functions, energy calculations of the N − 1 and N + 1
electronic species were done using unrestricted open shell
method at the optimized geometry of the N-electron species
at DFT/B3LYP/6-31G (d) level. The condensed Fukui func-
tions were calculated by using Mulliken as well as natural
population analysis (NPA) [40].
respectively, were synthesized according to our reported
method. (Scheme 1) [35]. First step in the synthesis of imi-
dazolinium zwitterions is the mono-substitution of diamines
with mesityl aldehyde or with toluene bromide. The imi-
dazolines were prepared by the reaction of monosubtituted
diamines 3 and 4 with an electrophile such as trimethyl-
orthoformate [41] and dimethoxy-N,N-dimethylformamide,
respectively [42, 43], under neat reaction conditions. The
imidazoline 5 was quaternized with 1,3-propanesultone 7
and the imidazoline 6 with cyclic sulphamidate 8. The com-
pound 7 was commercially available, whereas 8 was synthe-
sized by a modified method of Avenoza et al. [44] from eas-
ily available, commercial, and cheap prolinol.
The NHCs 9a and 10a were generated in situ from their
corresponding imidazolinium zwitterions 9 and 10, respec-
tively, and were applied as organocatalysts in the allylation
of benzaldehyde (Scheme 2). In the initial experiments,
sodium hydride was used as a base for deprotonation of
the imidazolinium salts and the reaction was performed in
THF. The alkyl sulfonate substituted NHC 9a gave encour-
aging results with a yield of 51 %. The traces of the prod-
uct were obtained when alkyl sulfamate substituted NHC
10a generated in situ was applied as catalyst and the max-
imum yield was obtained to 15 % after a longer reaction
Results and discussion
Two imidazolinium zwitterions, 9 and 10, functional-
ized with alkyl sulphonate and sulphamate substituents,
1 3

J IRAN CHEM SOC
Scheme 2 Allylation of benzal-
dehyde with 10 mol % NHC
OH
O
SiCl3
10 mol% imidazolinium zwitterion
9 mol % base
H
Scheme 3 Mechanism of
NHC-catalyzed allylation
R
R
N
N
R
R
Cl
SiCl3
Cl
Cl
Si
11
O
Ar
H
R
R
OH
N
N
R
R
Ar
R
R
Work up
N
N
Cl
R
R
H
OSiCl3
SiCl2
Ar
Ar
O
12
dual activation of
aldehyde and allylating
agent
time of 65 h (Table 1, entries 2 and 3). Further optimiza-
tion was performed with the NHC derived from the zwit-
terion 9. The yields were comparable when a different base,
KHMDS, was used in toluene and THF solvents, respec-
tively (Table 1, entries 5 and 6).
allyltrichlorosilane and its valence shell is expanded. As
a result, one of the chlorine silicon bonds is ionized and
a positively charged reactive hypervalent silicon complex
is formed, which in turn acts as a Lewis acid activator for
aldehydes [46, 47]. In our experiments NHCs act as Lewis
base organocatalysts, therefore the same mechanism could
be rationalized (Scheme 3) which was supported by the
subsequent theoretical investigations.
We present here a theoretical explanation for high effi-
ciency of the organocatalyst 9a over 10a in catalyzing
the allylation of benzaldehyde. To understand chemical
reactivity, electron density distribution and electrostatic
interactions provide useful information for the explana-
tion of nucleophilic and electrophilic attack. In a catalytic
cycle the nucleophilic–electrophilic interactions govern
the outcome of the reaction. Before going into the details
of theoretical explanation of the mechanism, an introduc-
tion of the basic reactivity descriptors is given. Accord-
ing to the Koopmans’ theorem for closed-shell molecules,
negative energy values of the highest occupied molecu-
lar orbital (EHOMO) and the lowest unoccupied molecular
orbital (ELUMO) express ionization potential (I) and elec-
tron affinity (A), respectively [48]. When the values of I
and E are known, one can determine the absolute hardness
(η), softness (S), and chemical potential (μ): (Hardness
In the allylation reaction it is well established that dif-
ferent catalyst activators can improve the catalytic activity
and reduce the reaction time [32, 45]. Diisopropylethyl-
amine was shown to produce excellent results in the allyla-
tion of aldehydes when used in combination with tetrabu-
tylammonium iodide [33]. Therefore, we have chosen
diisopropylethylamine to evaluate as a catalyst activator. It
was observed that the activator shortened the reaction time
to 24 h and the yields were improved to 75 % in dichlo-
romethane and 60 % in THF, respectively, (Table 1, entries
7 and 8). As the solvent dichloromethane was giving the
higher yield therefore further optimization was done in it
and at a lower temperature of −60 °C, a quantitative yield
of the product was obtained. (Table 1, entry 10). A control
reaction in the presence of diisopropylethylamine without
NHC did not proceed.
The well accepted mechanism for the addition of orga-
nosilicon reagents to aldehydes involves the activation
of carbon–silicon bond by Lewis-base organocatalysts.
Lewis base donates its charge density to silicon atom of
1 3

J IRAN CHEM SOC
η = ELUMO − EHOMO, softness S = 1/2η, chemical poten-
tial μ = ELUMO + EHOMO/2). Νucleophilicity index N is
defined as N = EHOMO(nu) − EHOMO(TCE) and tetracyanoeth-
ylene (TCE) is used as reference for nucleophilicity scale
[49]. Similar to chemical hardness and chemical potential,
the concept of electrophilicity (ω) has been introduced by
Parr et al. [50]. This reactivity descriptor calculates the
stabilization in energy when the system gets an additional
electronic charge ΔN from the environment. The electro-
philicity is defined as ω = μ2/2η. In the past few years, the
local variants of the global reactivity parameters have been
introduced which indicate the most reactive sites intramo-
lecularly and intermolecularly. These local variants are cal-
culated from Fukui function and global parameters [51].
The Fukui functions are used to identify the local reactivity
sites (electrophilic or nucleophilic) of molecules in a reac-
tion where the electron transfer is involved. Single-point
N + 1 and N – 1 calculations were carried out for an N
electrons system. The electronic population for an atom k
in the molecules has been calculated from natural popula-
tion analysis (NPA) or Mullikan population analysis (MPA)
[52]. The equations showing a condensed form of Fukui
functions for an atom k in a molecule for nucleophilic,
electrophilic and radical attacks are outlined as:
ꢀ
ꢁ
fk+ = qk(N + 1) − qk(N) for nucleophilic attack
ꢀ
ꢁ
fk− = qk(N) − qk(N − 1) for electrophilic attack
ꢀ
ꢁ
fE0 = 1/ q (N + 1) − q (N − 1) for radical attack
2
k
k
where qk is the electronic population of atom k of the mol-
ecule under investigation.
The global philicity index is the sum of philicity over all
atoms, but site selectivity is more important to understand a
chemical reaction. Although global philicity values execute
Table 2 Ionization potential (I), electron affinity (A), chemical hard-
ness (η), electronic chemical potential (µ), global softness (S), global
nucleophilicity (N), Fukui function (f−) and local nucleophilicity (NK)
for carbenes generated from corresponding zwitterions obtained at
the level of BLYP/6-31G(d)
Carbene
I (eV)
A (eV)
η (eV)
μ (eV)
S (eV)
N (eV)
MPA1
NPA1
f−
Nk (eV)
f−
Nk (eV)
9a
2.346
2.163
−2.205
−1.943
4.551
4.107
−0.070
−0.109
9.103
8.213
6.775
6.958
0.118
0.804
0.334
0.093
0.056
0.632
0.390
10a
0.0479
1
Fukui functions, local nucleophilicities were calculated and compared at the carbenic carbon atoms of the corresponding carbenes
Fig. 1 Optimized geometries of
the carbenes and their allyldi-
cholorosilane complexes
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J IRAN CHEM SOC
Table 3 Ionization potential (I), electron affinity (A), chemical hard-
ness (η), electronic chemical potential (µ), global softness (S), global
electrophilicity (ω), Fukui function (f+), local electrophilicity (ωk),
and local softness (Sk) for carbene-allyldichlorosilane complexes
obtained at BLYP/6-31G(d) level
NHC-silane complex I (eV) A (eV) η (eV) μ (eV) S (eV) ω (eV) MPAa
NPAa
ωk (eV) Sk (eV) f+
f+
ωk (eV) Sk (eV)
9a-silane complex
10a-silane complex
6.24
5.59
2.22
1.45
4.02
4.14
4.23
3.52
92.1
89.4
2.228
1.499
0.123 0.275
0.069 0.104
11.38
6.204
0.253 0.191
0.086 0.099
7.903
5.910
a
Fukui functions, local electrophilicities and softness were calculated and compared at silicon atoms of the corresponding carbene-allyldichlo-
rosilane complexes
reliable results yet there are examples in the literature where
local indices (i.e., ωkα) are more reliable compared to global
philicity (ω) [53–55]. Moreover, local philicity expresses
combination of information obtained from global to local
parameters such as global electrophilicity index, global
nucleophilicity index, Fukui function, local softness, or
global softness, etc. [56, 57]. Local philicity can be defined
as ωkα = f ωkα; α = +, −, 0. Condensed local nucleophilic-
ity index can be expressed as Nk = Nfk− [58]. Local softness
describes the reactivity of a specific atom in a molecule and
can be defined as skα = Sfkα; α = +, −, 0 [59].
In the proposed mechanism, first step is the nucleophilic
attack of carbene generated in situ to allyltrichlorosilane
reagent. The more nucleophilic the carbene is, greater is
the chance for activated silyl formation and the product in
turn. Local nucleophilicity has proven itself a better crite-
rion for explaining the greater nucleophilic power of a cer-
tain site in a molecule or between the molecules. It is evi-
dent by comparing the values of Fukui function and local
nucleophilicity at the carbenic carbons of the correspond-
ing carbenes that the carbene from the zwitterion 9 is more
nucleophilic (Table 2). This greater nucleophilicity favors
the formation of activated silyl reagent that takes part in the
second step. The results are same either calculated from the
Mullikan population analysis (MPA) or from the natural
population analysis (NPA) (Fig. 1).
The second step involves the activation of the starting
material by Lewis acid–base type of interactions. The sili-
con of the activated allyl reagent acts as Lewis acid for the
carbonyl group of benzaldehyde. Again, local electrophilic-
ity and local softness values were calculated at the silicon
atom of the proposed Lewis acids made from the NHCs 9a
and 10a, respectively. The silicon of 9a-allyldichlorosilane
complex shows greater value of Fukui function, local elec-
trophilicity and local softness as compared to the 10a-allyl-
dichlorosilane complex. Hence it is clear that the 9a-allyl-
dichlorosilane complex acts effectively as Lewis acid and
thus provides higher yield (Table 3).
reaction. This proposition is also supported by the fact that
starting material was recovered after workup in this case. It is
pertinent to mention that unlike some of the results reported
in the literature [60] the values of Fukui function, local phi-
licity and local softness showed the same trend.
Conclusion
The role of N-heterocyclic carbenes as organocatalysts is
presented for the first time in the allylation of benzalde-
hyde. It was found that NHC 9a generated from the imi-
dazolinium zwitterion having alkylsulfonate substituent
proved superior catalyst compared to sulfamate substi-
tuted 10a. A quantitative yield of homoallylic alcohol was
obtained at low temperature with NHC 9a. The observed
reactivity patterns were justified on the basis of reactivity
descriptors calculated from density functional theory.
Acknowledgments We acknowledge Higher Education Commis-
sion of Pakistan for the financial support.
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