Perfectly green organocatalysis: Quaternary ammonium base triggered cyanosilylation of aldehydes

Article abstract of DOI:10.1002/cjoc.201200598

Quaternary ammonium bases, such as aqueous (CH₃)₄NOH, were found to be an extraordinarily efficient catalyst for cyanosilylation of aldehydes. The addition reaction of trimethylsilyl cyanide (TMSCN) to equivalent aldehydes could proceed smoothly with turnover frequency (TOF) up to 3000000 h^-1 and in near 100% yield under solvent-free conditions. These organic catalysts also tolerated various aldehydes including aromatic, aliphatic and α,β-unsaturated aldehydes. This process perfectly conforms to the features of green chemistry: no waste regarding side-products and unconverted reactants, solvent-free, excellent catalytic activity, and no requirement for separation.

Full text of DOI:10.1002/cjoc.201200598

FULL PAPER
DOI: 10.1002/cjoc.201200598
Perfectly Green Organocatalysis: Quaternary Ammonium Base
Triggered Cyanosilylation of Aldehydes†
Wen, Yeqian(温叶倩)
Liang, Mengwei(梁梦伟)
Wang, Yiming*(王忆铭)
Ren, Weimin(任伟民)
Lü, Xiaobing*(吕小兵)
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, Liaoning 116024, China
Quaternary ammonium bases, such as aqueous (CH3)4NOH, were found to be an extraordinarily efficient cata-
lyst for cyanosilylation of aldehydes. The addition reaction of trimethylsilyl cyanide (TMSCN) to equivalent alde-
hydes could proceed smoothly with turnover frequency (TOF) up to 3000000 h－ and in near 100% yield under
1
solvent-free conditions. These organic catalysts also tolerated various aldehydes including aromatic, aliphatic and
α,β-unsaturated aldehydes. This process perfectly conforms to the features of green chemistry: no waste regarding
side-products and unconverted reactants, solvent-free, excellent catalytic activity, and no requirement for separa-
tion.
Keywords organocatalysis, green chemistry, cyanosilylation, quaternary ammonium base, solvent-free
Introduction
for target compounds with a 100% yield without any
production of waste.[10] The process should be eco-
nomical, safe, energy-efficient, environmentally benign,
and easily capable of being scaled up.[11] Herein, we
report a highly efficient organocatalysis: quaternary
ammonium base triggered cyanosilylation of aldehydes
at mild conditions (Figure 1). This process perfectly
conforms to the features of green chemistry: no waste
regarding side-products and unconverted reactants, sol-
vent-free, excellent catalytic activity with a TOF up to
3 000 000 h , and no requirement for separation. Fur-
thermore, two chiral quaternary ammonium bases I and
II derived from cinchonine were also synthesized for
investigating the effect of the cation on the reaction
(Scheme 1).
In the past decades, we have witnessed notable ad-
vances in organic synthesis. The resultant chemicals
concern almost every aspect of our lives and signifi-
cantly improve the quality of the lives of the billions of
individuals who now inhabit the planet. However, the
manufacturing processes of these useful chemicals usu-
ally need the use of a large amount of organic solvents,
and also produce undesired byproducts. These sub-
stances, as well as the inactive catalysts and uncon-
verted reactants have substantial negative effects on
human health and the environment. In fact, the greatest
release of hazardous waste to the environment is the
chemical industry.[1] Unfortunately, it wasn’t until
1990s that the environmental impact of chemical sub-
stances had been fully recognized as a serious prob-
lem.[2]
－
1
Along with the increasing concern about the global
problem of pollution, ‘Green chemistry’, which suggests
developing green chemical methods for pollution pre-
vention rather than end-of-pipe control,[3] was proposed
and rapidly become a significant area of organic syn-
thetic chemistry.[4] Along with the conception of ‘Green
chemistry’, several other topics, such as atomic eco-
nomic reaction,[5] green solvent,[6] phase transfer cataly-
sis,[7] ionic liquid,[8] and water-based reaction,[9] were
also put forward. An ideal reaction is expected to pro-
ceed efficiently under solvent-free condition with very
low catalyst loading, short reaction time and selective
Figure 1 (CH3)4NOH triggered cyanosilylation of aldehydes
with equimolar TMSCN.
Experimental
General information
1H, and 13C NMR spectra were recorded on a Varian
*
E-mail: chemwym@dlut.edu.cn; lxb-1999@163.com; Tel.: 0086-0411-84986255; Fax: 0086-0411-84986256
Received June 19, 2012; accepted August 1, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200598 or from the author.
Dedicated to the 80th Anniversary of Chinese Chemical Society.
†
Chin. J. Chem. 2012, 30, 2109—2114
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Wen et al.
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Scheme 1
N
N
N
HO
HO
HO
CH3I
Ag2O/H2O
OH
I
N
N
N
I
III
N
N
N
HO
HO
HO
PhCH2Cl
Cl
OH
Ag2O/H2O
Ph
Ph
N
N
N
IV
II
INOVA-400 MHz type (1H, 400 MHz; 13C, 100 MHz)
spectrometer. Their peak frequencies were referenced
versus an internal standard (TMS) shift at δ 0 for H
low solid (0.50 g, 76%). 1H NMR (400 MHz, CD3OD) δ:
8.93 (d, J＝4.8 Hz, 1H), 8.20 (d, J＝8.4 Hz, 1H), 8.11
(d, J＝8.4 Hz, 1H), 7.90 (s, 1H), 7.83—7.87 (m, 1H),
7.77—7.81 (m, 1H), 6.36 (s, 1H), 6.02—6.11 (m, 1H),
5.27—5.34 (m, 2H), 4.47—4.52 (m, 1H), 3.74—3.83
(m, 2H), 3.54—3.65 (m, 2H), 3.47 (s, 3H), 2.84—2.90
(m, 1H), 2.38—2.44 (m, 1H), 2.02—2.06 (m, 2H), 1.88
—1.93 (m, 1H), 1.02—1.07 (m, 1H).
1
NMR and against the solvent, chloroform-d at δ 77.0 for
13C NMR, respectively. High Resolution Mass Spec-
trometry (HRMS) was performed on a Micromass
Q-Tof (Micromass, Wythenshawe, UK) mass spec-
trometer equipped with an orthogonal electrospray
source (Z-spray). Fourier Transform Infrared spectros-
copy (FTIR) was performed on Nicolet NEXUS FT-IR
spectrophotometer equipped with a liquid cell (Harrick
Scientific Corporation). All aldehydes were freshly dis-
tilled before use. (CH3)4OH and (n-Bu)4OH were
bought from Sinipharm Chemical Reagent Co., Ltd.
Compound I A mixture of compound III (0.22 g,
0.50 mmol), H2O (9 μL, 0.50 mmol) and Ag2O (0.09 g,
0.38 mmol) in methanol (50 mL) in a 100 mL flask
wrapped in aluminum foil was stirred for 24 h at room
temperature. Then the mixture was filtered to remove
solid, and the filtrate was collected and the solvent was
removed. The residue was purified by column chroma-
tography on aluminum oxide (neutral) column, eluting
with dichloromethane/methanol (V∶V＝20∶1) to af-
ford compound I as a white solid (0.15 g, 92%). m.p.
256—259 ℃; 1H NMR (400 MHz, D2O) δ: 8.63 (d, J＝
4.8 Hz, 1H), 7.84 (d, J＝7.6 Hz, 1H), 7.79 (d, J＝7.6 Hz,
1H), 7.60—7.63 (m, 2H), 7.49—7.53 (m, 1H), 6.16 (s,
1H), 5.77—5.86 (m, 1H), 5.09—5.10 (m, 1H), 5.06—
5.07 (m, 1H), 4.14—4.20 (m, 1H), 3.41—3.45 (m, 2H),
3.28—3.36 (m, 2H), 3.22 (s, 3H), 2.60—2.67 (m, 1H),
2.01—2.07 (m, 1H), 1.78—1.79 (m, 2H), 1.52—1.57
(m, 1H), 0.68—0.72 (m, 1H); 13C NMR (100 MHz, D2O)
δ: 150.1, 146.8, 145.9, 145.8, 136.8, 130.5, 128.9, 128.1,
124.5, 122.7, 119.8, 117.3, 66.7, 65.8, 61.5, 59.8, 48.6,
37.6, 26.6, 23.9, 20.1. HRMS calcd for C20H25N2O
[M−OH]＋ 309.1967, found 309.1975.
Compound IV Benzyl chloride (1.2 mL, 10.5
mmol) was added to a solution of cinchonine (2.0 g, 7.0
mmol) in ethanol (50 mL), DMF (60 mL) and chloro-
form (2.0 mL), and the mixture was stirred at 100 ℃
for 10 h. After cooling to room temperature, the mixture
was added with Et2O and pink solid precipitated. The
precipitate was filtered and washed with additional Et2O
to give the product IV (2.8 g, 95%). 1H NMR (400 MHz,
CD3OD) δ: 8.96 (d, J＝4.8 Hz, 1H), 8.30 (d, J＝8.0 Hz,
1H), 8.13 (d, J＝7.6 Hz, 1H), 7.97 (d, J＝4.8 Hz, 1H),
Representative procedure for the cyanosilylation of
aldehydes
A 150-mL two-necked flask was equipped with a
Teflon-coated magnetic stirring bar. The flask was
charged with freshly distilled benzaldehyde (0.125 mol),
and 1% (CH3)4NOH aq. (2.3 μL, 2.5×10−7 mol, 2×
10−6 equiv.), respectively. Then TMSCN (0.125 mol, 1
equiv.) was added into the mixture with constant-pres-
sure drop funnel at ambient temperature, and the exo-
thermic reaction immediately began. After stirring for
20 min, the solution was changed into colorless. NMR
analyses confirmed the complete transformation of all
substrates to the corresponding cyanosilylation product,
2-phenyl-2-trimethylsilyloxyacetonitrile.
Synthesis of quaternary ammonium bases derived
from cinchonine
Compound III To a suspension of cinchonine
(0.44 g, 1.50 mmol) in methanol (50 mL) in a 100 mL
flask wrapped in aluminum foil, CH3I (0.14 mL, 2.25
mmol) was added. The resulting mixture stirred for 24 h
at room temperature before removing the solvent. The
crude product was purified by column chromatography
on silica gel, eluting with dichloromethane/methanol
(V∶V＝10∶1) to afford compound III as a light yel-
2110
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 2109—2114

Perfectly Green Organocatalysis: Quaternary Ammonium Base Triggered Cyanosilylation of Aldehydes
7.85—7.90 (m, 1H), 7.79—7.83 (m, 1H), 7.72—7.74
(m, 2H), 7.59—7.61 (m, 3H), 6.64 (s, 1H), 6.03—6.12
(m, 1H), 5.26—5.31 (m, 2H), 5.01—5.08 (m, 2H),
4.40—4.46 (m, 1H), 4.02 (t, J＝10.0 Hz, 1H), 3.88 (t,
J＝10.4 Hz, 1H), 3.64 (t, J＝11.2 Hz, 1H), 3.07—3.15
(m, 1H), 2.60—2.67 (m, 1H), 2.47—2.53 (m, 1H), 1.96
(s, 1H), 1.81—1.89 (m, 2H), 1.08—1.13 (m, 1H).
Compound II The synthesis of compound II was
the same procedure of compound I. m.p. 258—260 ℃;
1H NMR (400 MHz, CD3OD) δ: 8.96 (d, J＝4.8 Hz,
1H), 8.30 (d, J＝8.0 Hz, 1H), 8.13 (d, J＝7.6 Hz, 1H),
7.97 (d, J＝4.4 Hz, 1H), 7.87 (t, J＝7.2 Hz, 1H), 7.81 (t,
J＝6.8 Hz, 1H), 7.72—7.74 (m, 2H), 7.59—7.61 (m,
3H), 6.64 (s, 1H), 6.03—6.11 (m, 1H), 5.26—5.31 (m,
2H), 5.01—5.08 (m, 2H), 4.40—4.45 (m, 1H), 4.02 (t,
J＝9.6 Hz, 1H), 3.85—3.91 (m, 1H), 3.64 (t, J＝11.2
Hz, 1H), 3.07—3.14 (m, 1H), 2.60—2.67 (m, 1H), 2.47
—2.53 (m, 1H), 1.96 (s, 1H), 1.81—1.89 (m, 2H), 1.05
—1.12 (m, 1H); 13C NMR (100 MHz, CD3OD) δ: 151.0,
148.7, 147.4, 137.7, 134.9, 131.7, 131.1, 130.4, 130.2,
129.1, 128.8, 126.1, 124.4, 121.2, 117.7, 69.0, 66.9,
64.96, 58.1, 55.8, 38.9, 28.5, 24.7, 22.3; HRMS (ESI)
calcd for C26H29N2O [M−OH] ＋ 385.2280, found
385.2290.
light, the reaction proceeded within 10 min at a sub-
strate-to-catalyst molar ratio as high as 10000 under
solvent-free condition. There was no influence on reac-
tion rate and yield even using equivalent TMSCN and
benzaldehyde. Considering the hydrophilic property of
quaternary ammonium base, we tried to use 25%
(CH3)4NOH aqueous solution as catalyst for this reac-
tion. It was found that the simple catalyst system was
still effective under the same conditions, indicating that
the (CH3)4NOH-catalyzed cyanosilylation reaction was
not sensitive to moisture. The high efficiency of the
simple catalyst was also observed in the system with a
ratio of substrate-to-catalyst to 500000∶1 (Table 1,
Entry 1). The reaction was finished within 20 min and
－
1
the TOF reached up to 1500000 h . To the best of our
knowledge, this system represents the most efficient
catalyst known to date for this reaction. Such a high
efficiency is comparable with oxynitrilase.[33] Notably,
the system proved to be easily capable of being scaled
up. In the presence of 0.36 μL (0.001 mmol) 25%
(CH3)4NOH aqueous solution, 53.0 g (500 mmol) ben-
zaldehyde and 66.6 mL (500 mmol) TMSCN were
completely converted into the corresponding cyano-
hydrin trimethylsilyl ether with 100% selectivity within
1
20 min. The H NMR spectrum of the resultant product
without any purification is very clear and has no differ-
ence with that of its pure form (Figure 2). Such a low
catalyst concentration and no waste regarding byprod-
ucts and unconverted reactants make purification proc-
ess unnecessary. From an economical as well as an en-
vironmental standpoint, this reaction process perfectly
embodies economical, resource- and energy-efficient,
and environmentally benign characters.
Results and Discussion
Cyanohydrins are versatile synthetic intermediates in
organic chemistry, which can be transformed into a va-
riety of valuable functional compounds, such as
α-hydroxy acids, α-hydroxy aldehydes, β-hydroxy
amines, and α-amino acid derivatives.[12] The addition
reaction of trimethylsilyl cyanide (TMSCN) to carbonyl
compounds has been well-known as one of the most
effective methods for synthesizing cyanohydrins.[13]
Numerous catalyst systems, including Lewis acids,[14]
Lewis bases,[15-20] N-heterocyclic carbenes,[21] quater-
nary ammonium or phosphonium salts,[22-25] metal com-
plexes,[26] inorganic salts,[27] and bifunctional cata-
lysts,[28-30] have been developed for this transformation.
Notably, solvent-free cyanosilylation of aldehydes was
realized in some systems.[31]
The present study started with an accidental discov-
ery that when tetrahydrofuran (THF) stored in sodium
was used as solvent, the cyanosilylation of benzalde-
hyde could finish in 1 h without any external catalyst,[32]
while no reaction was observed in the systems with the
use of untreated solvent or distilled THF. It is tenta-
tively ascribed to the trace amount of sodium hydroxide
generated from sodium with the adventurous water in
THF. Further studies found that various inorganic bases
including LiOH, NaOH and KOH dissolved in THF
could effectively catalyze this reaction. Detailed opti-
mization experiments found that organic quaternary
ammonium bases were shown to be the most effective.
Subsequently, we adopted the simplest quaternary am-
monium base (CH3)4NOH instead of inorganic base as
catalyst to make sure the function of anion. To our de-
Figure 2 1H NMR spectra of benzaldehyde, TMSCN, and the
untreated reaction mixture of benzaldehyde with equivalent
TMSCN using (CH3)4NOH as catalyst.
The scope with respect to aldehyde substrates was
then explored with 0.0002—0.002 mol% (CH3)4NOH
loading under solvent-free condition. As shown in Table
1, various aromatic, aliphatic, and α,β-unsaturated al-
Chin. J. Chem. 2012, 30, 2109—2114
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Wen et al.
FULL PAPER
dehydes smoothly underwent the addition reaction and
were converted into the corresponding cyanohydrin
trimethylsilyl ether with 100% selectivity in excellent
yields. The catalyst was particularly effective for ali-
phatic aldehydes. For example, when isobutylaldehyde
or cyclohexanecarboxaldehyde was used as substrate
(S/C＝500000), the reaction could completed within 10
min and the TOF value is up to 3000000 h－1 (Entries 10
and 11). In the system regarding α,β-unsaturated alde-
hyde, only the 1,2-adduct was produced absolutely (En-
try 14). The products of the reaction could be used for
synthesizing agrochemicals, flavorings, and fragrances,
for example, the cyanosilylation product of 3-phenoxy-
benzaldehyde is an intermediate for industrial produc-
tion of pyrethroid insecticides (Entry 15). Unfortunately,
upon replacing aldehydes with a ketone such as aceto-
phenone, the cyanosilylation proceeded in a very low
rate. With 1 mol% catalyst loading, the prolonged reac-
tion time of 48 h gave a yield of 88%.
Similarly, anhydrous or aqueous (n-Bu)4NOH also
exhibited excellent activity for this reaction. For exam-
ple, with 3-phenylpropanal as substrate, the cyanosilyla-
tion reaction was completely finished within 5 min un-
der the same condition as (CH3)4NOH (Table 1, Entry
16). For a comparison purpose, we also conducted con-
trol experiments using chiral quaternary ammonium
bases I and II derived from cinchonine as catalysts for
the cyanosilylation of benzaldehyde. Unfortunately,
only racemic product was obtained with a 97% conver-
sion after 15 min (S/C＝100000/1), suggesting that the
chiral property of the cation has no effect on the nu-
cleophilic attack process.
Table 1 (CH3)4NOH aq.-catalyzed cyanosilylation of alde-
hydesa
TMS
O
O
(CH3)4NOH aq. (0.0002 - 0.002 mol%)
solvent-free, r.t.
+ TMSCN
R
H
R
CN
Run
1
Aldehyde
S/C t/min Conv.b/% TOF/h－
1
CHO
500000 20
100
1500000
H3C
F
CHO
2
3
4
5
50000 10
500000 60
100000 15
100
97
300000
490000
CHO
Cl
CHO
CHO
99
400000
F3C
100000
5
100
1,200,000
CHO
OCH3
6
7
8
100000 20
100000 20
98
97
290,000
290,000
CHO
Cl
CHO
CHO
100000
5
100
1,200,000
F
Because of its easier weighing than (CH3)4NOH, we
chose (n-Bu)4NOH as model quaternary ammonium
base for investigating the catalytic mechanism. Fourier
Transform Infrared spectroscopy (FTIR) experiments
were studied for the interaction of TMSCN and
(n-Bu)4NOH. There is a strong absorption band at 2188
O
9
500000 20
500000 10
100
100
1,500,000
3,000,000
10
CHO
CHO
cm－ derived from C≡N in the FTIR spectrum of
1
TMSCN. After 0.2 equiv. of (n-Bu)4NOH was added
11
500000 10
100000 10
99
3000000
into TMSCN, a new band at 2045 cm－ rapidly ap-
1
peared, along with the decrease in intensity of the
－
1
original band at 2188 cm . The complete disappear-
ance of the band at 2188 cm－1 was observed at the addi-
tion of equivalent (n-Bu)4NOH (Figure 3). Prior to our
study, Wang and Tian[24] reported the activation of
TMSCN by benzyltriphenylphosphonium chloride by
FTIR method. They found the appearance of a new cya-
nide stretching band at 2254 cm－1 in the IR spectrum of
the 1∶1 mixture, which was different from that for
12
13
100
100
600000
CHO
CHO
100000
5
1200000
CHO
14
15
100000 30
30000 20
99
99
200000
89000
CHO
CHO
－
－
1
1
TMSCN (2190 cm ) and TMSN＝C (2088 cm ).
Moreover, when benzaldehyde was introduced into the
mixture of (n-Bu)4NOH and TMSCN with 1∶1 ratio,
PhO
the absorption peak at 2045 cm－ speedily disappeared,
1
16c
100000
5
100
1200000
occurring with the formation of the cyanosilylatio
product (Figure 4). These results indicate a transition
state of pentacoordinated alkylsilicate originated from
the coordination of OH－ to silicon atom. Indeed, nu-
a All reactions were conducted with a scale of 0.025 mol alde-
hydes; n(aldehyde)∶n(TMSCN)＝1∶1. Conversion was de-
termined by 1H NMR. c (n-Bu)4NOH aq. as catalyst.
b
2112
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Chin. J. Chem. 2012, 30, 2109—2114

Perfectly Green Organocatalysis: Quaternary Ammonium Base Triggered Cyanosilylation of Aldehydes
cleophilic substitution at silicon in R3SiX compound-
scan be activated by nucleophiles such as RCO－2 , RO－,
N-oxide, resulting in the formation of hypervalent sili-
con intermediates with penta- or hexacoordination
states.[34-36] It is generally known that pentacoordinate
allylsilicates are more reactive than tetracoordinate sili-
con, since σ-π conjugation results in the enhanced nu-
cleophilicity of the γ-carbon of the allylsilicates.[37,38]
Scheme 2 Possible mechanism of (CH3)4NOH-catalyzed cya-
nosilylation of aldehydes
OTMS
TMSCN
(CH3)4N+OH-
Ph
CN
cm-1
cm-1
ν(C≡N) = 2188
ν(C≡N) = 2239
-
-
OH
Si
CN
OH
Si
(CH3)4N+
(CH3)4N+
cm-1
ν(C≡N) = 2045
O
A
H
Ph
C
-
OH
PhCHO
(CH3)4N+
Si
CN
O
H
Ph
B
Conclusions
In summary, we have developed an extraordinarily
efficient and simple organocatalyst for cyanosilylation
of various aldehydes under extremely mild conditions.
Aqueous (CH3)4NOH triggered addition reaction of
trimethylsilyl cyanide (TMSCN) to equivalent alde-
hydes which could proceed smoothly in a very low
catalyst loading, with a turnover frequency (TOF) up to
Figure 3 FTIR spectra of the mixture of TMSCN and
(n-Bu)4NOH in different ratios.
3000000 h－ and in near 100% yield under solvent-free
1
conditions. This reaction process perfectly conforms to
the feature of green chemistry: no waste regarding
side-products and unconverted reactants, solvent-free,
100% product selectivity, and no requirement for sepa-
ration. Furthermore, it represents a rare example of per-
fect organocatalysis with very low catalyst loading and
excellent activity/selectivity.[39,40] Although chiral qua-
ternary ammonium bases I and II derived from cin-
chonine as catalysts also exhibited excellent activities
for the cyanosilylation of benzaldehyde, the chiral
property of the cation has no effect on the nucleophilic
attack process and only racemic product was obtained.
Figure 4 The changes of absorption peak strength of activated
C≡N with addition of benzaldehyde to the mixture of TMSCN
and (n-Bu)4NOH in FTIR spectra. Procedure: (i) (n-Bu)4NOH
(0.52 g, 2.0 mmol) and TMSCN (0.2 g, 2.0 mmol) were dissolved
in 0.4 mL THF, (ii) then half of PhCHO (0.1 mL, 1.0 mmol) was
added, (iii) finally the other half PhCHO (0.1 mL, 1.0 mmol) was
added into the solution.
Acknowledgement
This work is supported by National Natural Science
Foundation of China (NSFC, No. U1162101), and Na-
tional Basic Research Program of China (973, No.
2009CB825300).
On the basis of the above observations, a possible
mechanism was proposed as shown in Scheme 2. Firstly,
OH－ anion coordinates to the Si atom to form the more
reactive pentacoordinate silicate (A). Then the activated
CN－ attacks the carbonyl group of an aldehyde, fol-
lowed with the generation of the desired product and
release of quaternary ammonium base compound. In the
catalytic cycle, the cation of quaternary ammonium base
perhaps stabilizes the more reactive pentacoordinate
silicate through Coulombic interaction, and does not
participate in the attack of the nucleophile.
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