On the properties of the anions derived from α-deprotonation of α-(o-carboran-1-yl)- and α-ferrocenyl-1-alkylbenzotriazoles

Article abstract of DOI:10.1007/s11172-012-0268-2

The synthesis of 1-[(2-R-o-carboran-1-yl)methyl]benzotriazoles (R = Me, Ph) is described. α-Deprotonation of these compounds followed by reactions of the generated carbanions with MeI give the corresponding α-methylated products. Even at room temperature,

Full text of DOI:10.1007/s11172-012-0268-2

Russian Chemical Bulletin, International Edition, Vol. 61, No. 10, pp. 1933—1942, October, 2012
1933
On the properties of the anions derived from ꢀdeprotonation of
ꢀ(oꢀcarboranꢀ1ꢀyl)ꢀ and ꢀferrocenylꢀ1ꢀalkylbenzotriazoles
S. K. Moiseev, M. A. Cherevatskaya, T. A. Verbitskaya, I. V. Glukhov, A. S. Peregudov, and V. N. Kalinin
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 6549. Eꢀmail: vkalin@ineos.ac.ru
The synthesis of 1ꢀ[(2ꢀRꢀoꢀcarboranꢀ1ꢀyl)methyl]benzotriazoles (R = Me, Ph) is described.
ꢀDeprotonation of these compounds followed by reactions of the generated carbanions with
MeI give the corresponding ꢀmethylated products. Even at room temperature, they exist in
solution as an equilibrium mixture of two conformers due to steric crowding around the C atom.
In contrast, the anions generated by deprotonation of ꢀferrocenyl derivatives of 1ꢀmethylꢀ and
1ꢀethylbenzotriazoles are ambident and are attacked by electrophiles at both the C and
N(3) atoms.
Key words: benzotriazole, stabilized carbanions, ꢀcarboranyl carbanions, ꢀferrocenyl
carbanions.
The Nꢀbenzotriazolyl (Bt) group has found extensive
synthetic use as an auxiliary structural fragment for furꢀ
ther modification of organic molecules. This group can
relatively easily be introduced into a substrate molecule to
give 1ꢀ or 2ꢀsubstituted benzotriazoles (Bt1 and Bt2 derivꢀ
atives, respectively) with very similar chemical behavior
or their mixture (usually with a Bt1 derivative as the major
component) and can easily be removed after the desired
modification has been effected.1 For synthetic purposes,
the Bt fragment is most frequently used to facilitate the
deprotonation and nucleophilic substitution at the ꢀposiꢀ
tion of Nꢀalkylbenzotriazoles containing various substituꢀ
ents in the alkyl chain (this is due to the strong electronꢀ
withdrawing effect of the Bt group) and to modify a subꢀ
strate molecule via nucleophilic substitution for or elimiꢀ
nation of the Bt group itself.1
stituent as an electronꢀdonating group.3 In addition,
Fc (a typical representative of transition metal ꢀcomꢀ
plexes) and especially R´Cb are very bulky substituents,
which makes the possibility of ꢀfunctionalization of
benzotriazole derivatives of the types 1 and 2 relevant to
the derivatives of other transition metal ꢀcomplexes,
icosahedral boranes, and carboranes.
Scheme 1
It is of interest the possibility of using this approach for
the preparation of derivatives of carborane or transition
metal ꢀcomplexes via functionalization of the side chain.
The choice of methods for chemical modification of such
compounds, in contrast to common organic compounds,
is often limited because of possible decomposition of the
organometallic fragment under the reaction conditions.
In the present work, we describe the behavior of 1ꢀ[1ꢀ(oꢀ
carboranꢀ1ꢀyl)alkyl]benzotriazoles (1) and 1ꢀ[1ꢀ(ferroꢀ
cenyl)alkyl]benzotriazoles (2) on the ꢀdeprotonation as
well as the behavior of the resulting carbanions 3 and 4 in
reactions with electrophiles with the aim of obtaining the
corresponding ꢀsubstituted derivatives 5 and 6 (Scheme 1).
We employed the R´Cb substituent as a group with proꢀ
nounced electronꢀwithdrawing properties2 and the Fc subꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1918—1927, October, 2012.
1066ꢀ5285/12/6110ꢀ1933 © 2012 Springer Science+Business Media, Inc.

1934
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
Moiseev et al.
Results and Discussion
dynamic acidity), being inferior in this aspect to the Ph
group.3 Therefore, Fcꢀcontaining carbanions 4 should be
expected to be less stable (especially for tertiary anions)
but more nucleophilic than R´Cbꢀcontaining anions 3.
(ꢀCarboranylalkyl)benzotriazoles. Compound 1a was
obtained by a reaction of Bt1CH2Cl (7)25 with the sodium
derivative of oꢀcarborane, MeCbNa, prepared by reaction
of MeCbH with NaNH2 in liquid NH3 (Scheme 2). With
this synthetic approach, the low yield of compound 1a
(50%) counting on the alkyl chloride 7 used is due to the
higher CHꢀacidity of product 1a, which, once being
formed, is in situ deprotonated by the base present in the
reaction mixture (MeCbNa).
The resulting anion 3a is not able to repeated alkylaꢀ
tion with alkyl chloride 7 (see below). So, a half of the
MeCbNa used is withdrawn from the reaction as MeCbH.
The yield of carboranylmethylbenzotriazole 1 (R = H)
can be increased by using a twofold excess of an organoꢀ
metallic carborane derivative. For instance, the yield of
compound 1c obtained from chloride 7 and PhCbLi used
in a ratio of 1 : 2 was 66% (see Scheme 2).
The fundamental possibility of the formation of benꢀ
zotriazole derivatives sterically crowded at the C atom is
suggested by the existence (in the form of Bt1 and Bt2
isomers or their mixture) of such compounds as, e.g.,
BtCPh3, its 5,6ꢀdimethyl analog (the positions 5 and
6 refer to Bt),4 Bt3CH,5,6 (2ꢀHOC6H4)C(Bt1)(Me)ꢀ
[CH(OH)Ph],7 and (pyrrolꢀ1ꢀyl)C(Bt)(SiMe3)2.8 Howꢀ
ever, steric crowding has natural limits. For instance, the
steric crowding in Bt3CH does not hinder its deprotonaꢀ
tion with BunLi and subsequent reactions of the generated
carbanion with electrophiles E+ (nꢀalkyl halides, acyl
chlorides, thiocyanates, aldehydes, CS2, and ClCOOEt)
to give the expected products Bt3CE. However, this carbaꢀ
nion is inert to secondary alkyl halides and ketones, which
is attributed to steric effects.6 There are some other expeꢀ
rimental facts in evidence of the effect of steric crowding
around the C atom on the possibility and regiochemical
outcome of reactions of ꢀcarbanions generated from
benzotriazole derivatives.9—13
Although the auxiliary Bt group has been already used
in the synthesis of oꢀcarborane derivatives,14 Btꢀcontainꢀ
ing carboranes of the type 1 have not, to the best of our
knowledge, been documented hitherto. In contrast, Fcꢀ
containing derivatives 2 (R = H,15,16 Me,15—17 Et,18 Ph,15
and FcCH=CH (see Ref. 15)) and other compounds15,16,18
have been synthesized earlier and used as ferrocenylalkylꢀ
ating reagents16,19 for the preparation of Fcꢀcontaining
derivatives with putative antitumor activity.16 In fact, the
ferrocenylalkylation reaction is the nucleophilic substituꢀ
tion of Bt group in compounds 2. The properties of carbaꢀ
nions 4 have not been disclosed so far.
The 1ꢀoꢀcarboranyl group is a pronounced electron
acceptor.2,20 For this reason, as with R´CbCH2Ph,21,22
one should expect the enhanced acidity of the ꢀH atom
in compounds 1 along with higher stability of carbanꢀ
ions 3 arising on their deprotonation.
Our attempt to prepare ꢀmethyl substituted comꢀ
pound 1d by the similar procedure from BtCH(Me)Cl (see
Ref. 26) and PhCbLi failed. Apparently, the steric hinꢀ
drances make it difficult for the bulky carboranyl nucleoꢀ
phile to approach the C atom under the SN2 reaction
conditions (see below).
For this reason, ꢀmethylated derivatives 1b,d, which
are identical with compounds 5 (R = H; R´ = Me, Ph;
E = Me), were obtained according to Scheme 1: deprotoꢀ
nation of the methylene group in compounds 1a,c with
BunLi was followed by reactions of the generated carbanꢀ
ions 3a,c with MeI as the electrophile E+. The formation
of these carbanions is specifically suggested by appearance
of the yellowꢀorange color of the reaction mixtures after
adding of BunLi to solutions of compounds 1a,c in THF.22
Compounds 1a—d are Bt1 derivatives (1H NMR data).
Unfortunately, this approach was ineffective for the
synthesis of other substitution products 5 from ꢀmethyꢀ
lated derivatives 1b,d and E+. The yellowꢀorange color of
the reaction mixtures prepared by adding BunLi to soluꢀ
tions of compounds 1b,d in THF indicated the formation
of carbanions 3b,d. However, subsequent addition of an
appropriate electrophile and workup of the reaction mixꢀ
The kinetic CH acidity of compounds 2 should be lowꢀ
er than that of compounds 1 because of the positive inducꢀ
tive effect of the Fc substituent,23,24 which is not the case
of R´Cb. In contrast to the wellꢀknown ability of Fc to
stabilize ꢀcarbocations, this substituent poorly stabilizes
ꢀcarbanions (i.e., only slightly increases the thermoꢀ
Scheme 2

Anions derived from substituted 1ꢀalkylbenzotriazoles Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
1935
ture (see Experimental) always gave the starting compound
1b or 1d as the only carboraneꢀcontaining component
(1H NMR data).
Bt1 group is broadened and is virtually invisible against the
background signals. The signal for the H(6) proton of the
Bt1 group and the doublet for the oꢀprotons of the Ph
group are also broadened (however, the broadening of the
latter signal can be attributed to the magnetic nonequivaꢀ
lence of these protons because of the presence of an asymꢀ
metric center in the molecule). In the 13C NMR spectrum
of compound 1d, the signals for the C and nearby carbon
nuclei (CH3, CCb—Ph, C(7), C(7a), and C(3a)) are broadꢀ
ened as well.
We studied a temperature dependence of the 1H NMR
spectrum of compound 1d in a range from 213 to 293 K
(Fig. 1). According to the results obtained, this structure
at low temperatures exists as a mixture of two conformers
which undergo interconversions upon heating. The specꢀ
tral pattern changes dramatically with an increase in the
temperature. Two doublets for the Me protons at  1.85
and 1.93 (213 K) merge into one doublet at  1.87 (283 K).
Two doublets at  8.06 and 8.09 for the H(4) proton of the
Bt group also coalesce into one doublet.
Similar changes are observed for the signal from the
H(7) proton of the Bt fragment. At room temperature, this
signal is virtually invisible against the background signals.
At 273 K, its highꢀfield component appears as a wide sinꢀ
glet at  6.32, which is split into a distinct doublet ( 6.27)
at 233 K. On further cooling, this signal keeps on shifting
upfield to  6.25 at 213 K. The lowꢀfield component of the
signal for the H(7) proton was not identified because of its
overlap with the signals for the other aromatic protons.
The data obtained suggest that structure 1d at low temꢀ
peratures exists in chloroform as an equilibrium mixture
of two conformers in a ratio of 56 : 44, which undergo
slow interconversions. Using dynamic 1H NMR line shape
analysis (see Experimental), we determined the kinetic
and thermodynamic parameters of these interconversions.
The exchange rate constants for the conformers at difꢀ
ferent temperatures were calculated from their average lifeꢀ
times  estimated when comparing the experimental and
simulated spectra.
Since the great steric crowding around the C atom in
carbanions 3b,d precludes introduction of the fourth subꢀ
stituent into this position, we tried less sterically hindered
carbanions 3a,c in reactions with electrophiles. We found
that the redꢀorange color due to anions 3a,c generated by
metalation of the starting oꢀcarborane derivatives graduꢀ
ally disappeared in all the reactions. However, no target
substitution products 5 (R = H) were detected by 1H NMR
spectroscopy. Reactions of carbanions 3a,c with I2 gave
complex mixtures of unidentified products. 4ꢀMethoxyꢀ
benzaldehyde was reduced with anions 3a,c to the correꢀ
sponding alcohol.
No substitution products were detected in the reaction
mixtures obtained by addition of the complex CuBr•PPh3
(0.5—1.0 equiv.) to carbanions 3a,c at –40 C followed by
treatment with AcCl or 4ꢀmethoxybenzoyl chloride.
One can conclude that carbanions 3 generated from
carboraneꢀcontaining benzotriazoles 1 do not exhibit their
nucleophilic properties in reactions with electrophiles, exꢀ
cept for the mixtures 1a,c + MeI. However, these carbanꢀ
ions can sometimes act as reducing agents, which is evident
from the formation of 4ꢀmethoxybenzyl alcohol in the
reactions of carbanions 3a,c with 4ꢀmethoxybenzaldehyde.
Obviously, this is due to steric overcrowding around the
C atom bound to two bulky organic groups (benzotriꢀ
azolyl and carborane). As a result, the methyl group is
proved to be the bulkiest one to substitute for the hydrogen
atom at C. Even the presence of this relatively small
substituent in compounds 1b,d makes steric crowding
around the C atom insurmountable for attempted addiꢀ
tion of electrophiles to the corresponding carbanions 3b,d.
This overcrowding results in both the limited conformaꢀ
tional flexibility of molecules 1b,d in solution and, obviꢀ
ously, their structural features in crystals. The latter fact is
confirmed by Xꢀray diffraction data for compound 1d
(see below).
The conformational behavior of molecules 1b,d in soluꢀ
tion was studied by 1H and 13C NMR spectroscopy.
Preliminary correlation experiments (H,HꢀCOSY, NOESY,
HMQC, and HMBC) were used to assign all the signals in
the 1H and 13C NMR spectra of these compounds.
According to 1H and 13C NMR data, compounds 1b,d
exist in CDCl3 as mixtures of conformers in dynamic equiꢀ
librium even at room temperature. The interconversions
of their conformers are relatively rapid on the NMR time
scale. This is evident from the very wide ( ~0.3—0.4 ppm)
signal with totally lost fine structure for the CH proton in
the spectra of these compounds. In addition, the 1H NMR
spectrum of compound 1b show broadened signals for the
H(7) proton of the Bt1 group and for the methyl group of
the carborane fragment. In the 1H NMR spectrum of comꢀ
pound 1d, the signal at  ~6.8 for the H(7) proton of the
The signals for the CH group of the CH—Me fragment
were chosen as indicating signals since they are two wellꢀ
resolved quartets below 233 K. The intensity ratio of the
quartets corresponds to the center population ratio PA : PB =
= 0.56 : 0.44. A temperature rise results in widening of
both the quartets so that they lose their fine structures at
253 K, being two mere broadened signals. On further heatꢀ
ing, these signals keep on widening and coalesce at 283 K.
With a further increase in the temperature, the resulting
singlet becomes narrower; at 308 K, its width is 45 Hz.
It should be emphasized that the signals at low tempeꢀ
ratures (213—253 K) are spaced at a nearly constant disꢀ
tance of 260 Hz. The ratio of the signal intensities also
varies only slightly with the temperature. This enables us
to use, in the line shape analysis, the same parameters at
both nearꢀcoalescence and higher temperatures.

1936
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
Moiseev et al.
233 K
253 K
273 K
PhCb—CH2—Bt
5.2 5.0 4.8

1.98 1.89

293 K
8.2
7.8 7.6 7.4 7.2 7.0
6.6
6.2
5.8
5.4
5.0
4.6
4.2
3.8
3.4
3.0
2.6
2.2
1.8

Fig. 1. The 1H NMR spectra of compound 1d (contaminated with compound 1c) in a temperature range from 233 to 293 K (CDCl3,
600 MHz).
Data for the temperature dependence of the rate conꢀ
stant k (k = 1/ for a firstꢀorder process) of the conformaꢀ
tional change in compound 1d are given below.
greater steric crowding compared to the groundꢀstate
conformers.
According to Xꢀray diffraction data for compound 1d
at 120 K, the conformation of its molecule in the crystal is
typical of C,Cꢀdisubstituted oꢀcarboranes containing one
aryl substituent (Fig. 2). For instance, the phenyl ring is
nearly perpendicular to the CCb—CCb bond: the absolute
value of the torsion angle [C(1)—C(2)—C(21)—C(26)] is
93.7(8) (see Fig. 2). The relative position of the bulky
substituent at the C(1) atom is fairly expected: the H(13A)
atom is the nearest neighbor to the phenyl ring attached to
the C(2) atom, which minimizes the steric crowding in
the structure. For instance, the shortest distance between
the H(13A) atom and the phenyl ring is 2.707 Å
(H(13A)...C(21)). For the same reasons, the benzotriazole
fragment tends, in turn, to be coplanar with the phenyl
ring; the angle between their planes is 31.9.
The aforementioned steric crowding results in lengꢀ
thening of the CCb—CCb bond to 1.705(9) Å, which is
comparable with a known range of bond lengths in diꢀ
substituted oꢀcarboranes.
Although the reflectance of the crystal examined
is relatively weak (the number of observed reflections up
to  = 52 is 39%, see Experimental), it is safe to say
that the crystal at 120 K shows no pronounced therꢀ
mal motion of groups of atoms that could indicate the
T
k/s–1
5263
3225
1667
1149
1971
1800
T
k/s–1
526
333
217
167
133
308
298
288
283
280
278
273
268
265
258
253
Based on them, we obtained the following correlation
equations:
log(1/) = (–2.425 0.087)/T + (11.62 0.31)
(R = –0.994, SD = 0.059),
[log(1/)]T = (–2.300 0.087)/T + (8.72 0.32)
(R = –0.994, SD = 0.059).
Using these equations, we calculated the thermodyꢀ
namic parameters: Ea = 11.1 0.4 kcal mol–1, H
= 10.5 0.4 kcal mol–1, S = –7 2 eu, and G
=
=
298
298
= 12.6 0.4 kcal mol–1
.
Note a slightly negative entropy of activation for this
conformational change; this suggests that the transition
state in the dynamic process is characterized by somewhat

Anions derived from substituted 1ꢀalkylbenzotriazoles Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
1937
C(17)
ducts but also considerable amounts of the starting 1ꢀalkylꢀ
benzotriazole and BtH.27 In contrast, the deprotonation
of 1ꢀ(arylmethyl)benzotriazoles28 and 1ꢀ(hetarylmethyl)ꢀ
benzotriazoles8 is selective at the ꢀposition. Taking into
account that the Fc group, as mentioned above, is inferior
to Ph in the stabilization of the ꢀcarbanionic center, one
can expect that deprotonation of compounds 2 will be
more selective than that of 1ꢀalkylbenzotriazoles and that
the Bt1 group will contribute more to the stabilization of
anions 4 compared to the stabilization of ꢀanions geneꢀ
rated by deprotonation of 1ꢀ(arylmethyl)benzotriazoles
and 1ꢀ(hetarylmethyl)benzotriazoles.
C(16)
C(20)
C(15)
C(18)
N(3)
N(2)
C(19)
C(14)
N(1)
C(24)
C(25)
C(13)
C(23)
C(22)
C(1)
B(6)
B(3)
B(4)
C(26)
C(21)
Compounds 2a,b were obtained from BtH and the
carbinols FcCH2OH and FcCH(OH)Me, respectively, in
the presence of HBF4 as described earlier.16
B(5)
B(9)
C(2)
B(7)
Compounds 2 are fairly strong CH acids and can readily
be deprotonated under the action of BunLi in THF in
a temperature range from 20 to –70 C. This is evident
from immediate coloration of the reaction mixtures turnꢀ
ing very intense dark blue or dark brown upon the addition
of BunLi to solutions of compounds 2a and 2b. Dark blue
coloration has also been observed in the ꢀdeprotonation
of 1ꢀ(arylmethyl)benzotriazoles.28 Anions 4a,b are very
different in thermal stability. When a solution of anion 4a
is kept at 20 C for 15 min and then treated with water, the
starting compound 2a is completely recovered (1H NMR
data). Under the same conditions, anion 4b is completely
transformed into anil FeC(Me)=NPh (8) via known1 (for
benzotriazolyl ꢀcarbanions) elimination of a N2 moleꢀ
cule (Scheme 3).
B(11)
B(10)
B(8)
B(12)
Fig. 2. Molecular structure 1d.
coexistence of different conformers or their interconꢀ
versions.
The NMR spectra provide another piece of evidence
that the bulkiness of the substituent R in compounds 1a—d
is a decisive factor for the formation of a steric barrier to
the conformational flexibility of their molecules and for
the possibility of reactions of carbanions 3a—d with elecꢀ
trophiles. It is well seen in Fig. 1 that the singlet for the
CH2 group of compound 1c ( 4.90) contained as a minor
impurity in the test sample of compound 1d remains unꢀ
changed throughout the temperature range studied, in conꢀ
trast to the signals for compound 1d. Moreover, the
1H NMR spectrum of compound 1b containing a much
smaller Me (instead of Ph) group in the oꢀcarboranyl fragꢀ
ment also shows a very wide signal for the CH proton. In
addition, the latter fact disproves the formation of the
Scheme 3
1
conformers detected by H NMR spectroscopy via rotaꢀ
tion of the substituent R´ about the CCb—CR´ bond since
the rotation of the Me group (for R´ = Me) can hardly give
rise to considerable energy barriers. Two other alternatives
that could be responsible for the formation of relatively
stable conformers of compounds 1b,d include hindered
rotation about the C—CCb and C—CBt bonds. The latter
seems to be preferred because the Bt1 group is more asymꢀ
metric (with respect to the above rotation) than CbR´
(especially for R´ = Me).
(ꢀFerrocenylalkyl)benzotriazoles. Metalation of
1ꢀalkylbenzotriazoles is known to proceed nonselectively:
along with ꢀdeprotonation, the metalation occurs at difꢀ
ferent positions of the aromatic ring of the Bt1 group. In
addition, reactions of the thus generated anions with an
electrophile give not only electrophilic substitution proꢀ
1
The H NMR spectrum of product 8 is identical with
the spectrum of an independently prepared sample of
anil 8. However, compound 2b subjected to metalaꢀ
tion—hydrolysis at –70 C for 15 min is recovered intact.
Therefore, in contrast to perfectly stable (at room temperaꢀ
ture) carbanions 3 containing the electronꢀwithdrawing
carboranyl substituent, anions 4 containing the electronꢀ
donating Fc substituent differ in thermal stability: seꢀ
condary anion 4a keeps stable at 20 C, while tertiary anion
4b is stable only at –70 C, undergoing rapid decomposiꢀ
tion to anil 8 at room temperature. In this aspect, anion 4b

1938
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
Moiseev et al.
Scheme 4
is similar to anions derived from 1ꢀ(arylmethyl)benzotriꢀ
azoles via ꢀdeprotonation, which decompose slowly
above –20 C and rapidly at 0—5 C;28 anion 4a is appreꢀ
ciably more stable. For this reason, subsequent generation
of anion 4b and its reactions with electrophiles were carꢀ
ried out at –70 C.
spectroscopy. In certain experiments, some products were
isolated in the individual state by column chromatography
on SiO2 (200×25 mm) with light petroleum—AcOEt
(10 : 1  5 : 1) as an eluent. In each case, the reaction
products mainly included four types of compounds (2, 6,
9, and 10; see Scheme 5) or some of them. Other Fcꢀ or
Btꢀcontaining compounds sometimes detected in the reacꢀ
tion mixtures were not identified, except for FcCH=CH2
formed in negligible amounts in the reactions of comꢀ
pound 4b with MeI and ClCOOEt.
The fact that secondary anion 4a is much more stable
than tertiary anion 4b suggests that this factor is substanꢀ
tially more important for the thermal stability of (ꢀferroꢀ
cenylalkyl)benzotriazolyl anions compared to (ꢀcarboꢀ
ranylalkyl)benzotriazolyl ones. In other words, for a benzoꢀ
triazolyl ꢀcarbanion additionally stabilized by an elecꢀ
tronꢀwithdrawing ꢀsubstituent (Cb), it does not matter
whether this anion is secondary or tertiary. This distincꢀ
tion becomes crucial when the carbanion is already destaꢀ
bilized by an electronꢀdonating ꢀsubstituent (Fc). In the
latter case, the negative charge of the carbanion center is
partially delocalized over the Bt group as well, which will
increase the contribution from the resonance structure 4X
(Scheme 4) to anions 4 (i.e., anions 4 become ambident,
in agreement with structure 4X containing two nucleoꢀ
philic sites (C and N(3)).
Scheme 5
Anions 4a and 4b generated by addition of BunLi to
compounds 2a and 2b at 20 and –70 C, respectively, were
kept for 10 min and treated at the same temperatures with
various electrophiles E+ (MeI, ClCOOEt, ClCOOMe,
AcCl, CH2=CHCH2Br, 4ꢀMeOC6H4CHO, PhCH2Cl,
and PhNCO) to obtain substitution products 6 (see
Scheme 1). The final reaction mixtures were hydrolyzed
with water, the products were extracted with diethyl ether,
and the extracts were concentrated. The products were
identified, and their ratios were determined, by 1H NMR
2, 4, 9: R = H (a), Me (b); 6, 10: for R and E, see Table 1.
The final reaction mixture almost always contained
the starting (ferrocenylalkyl)benzotriazole 2 and carbonyl
compound 9, the former being often the major product.
Substituted benzotriazole 10 was never detected in the
reactions with alkyl halides, being produced only by the
most reactive electrophiles (ClCOOEt, ClCOOMe, and
AcCl). That is why products 10 were actually acylated

Anions derived from substituted 1ꢀalkylbenzotriazoles Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
Table 1. Range and ratio of the reaction products obtained from anions 4a,b and electrophiles E+ (see Scheme 5)
1939
Anion
E+
Molar ratio of the products
2
6
9
10
4a
4b
4a
4b
4a
4a
4b
MeI
MeI
H2C=CHCH2Br
H2C=CHCH2Br
PhCH2Cl
2a
2b
1.0 (2a)
2b
1.4 (2a)
2a
10.0 (2b)
—
—
—
—
—
—
2.1
—
—
1.3 (9a)
—
1.0 (9a)
—
—
—
—
—
—
—
ClCOOMe
ClCOOMe
0.8 (9b)
2.0
(E = COOMe)
0.6
(E = COOEt)
—
(R = Me, E = COOMe)
—
4a
4b
4b
4b
ClCOOEt
ClCOOEta
ClCOOEtb
ClCOOEtc
1.0 (2a)
—
10.0 (2b)
10.0 (2b)
10.0 (2b)
6.0
0.2 (9b)
—
(R = Me, E = COOEt)
25.0
—
(R = Me, E = COOEt)
—
0.2 (9b)
1.2
(E = COOEt)
2.4 (E = Ac)
4.0 (E = Ac)
4a
4b
4a
4b
4a
AcCl
AcCl
PhNCO
1.5 (2a)
2.0 (2b)
10.0 (2a)
20.0 (2b)
2a
—
—
—
—
—
1.0 (9a)
1.0 (9b)
1.3 (9a)
1.0 (9b)
—
d
—
—
d
PhNCO
4ꢀMeOC6H4CHO
—
a Ethyl chloroformate was added to a dark brown solution 20 s after the addition of BunLi.
b Ethyl chloroformate was added to a dark brown solution 10 min after the addition of BunLi.
c Ethyl chloroformate was added to an orange solution 30 min after the addition of BunLi.
d The reaction mixture also contains a number of unidentified Btꢀ and Fcꢀcontaining products.
benzotriazoles Bt1COR´ (R´ = Me, OMe, or OEt). The
target products 6 were obtained only in the reactions of
compound 4b (but not 4a!) with chloroformates: R = Me;
E = COOMe and COOEt. Products 2a,b and 9a,b were
identified by comparing the 1H NMR spectra of the reacꢀ
tion mixtures with the spectra of their authentic samples
obtained in alternative ways. The 1H NMR spectra of acylꢀ
benzotriazoles 10 were identical with published data for
these compounds (E = Ac,29 COOMe,30 and COOEt (see
Ref. 30)). The composition and ratio of the products deꢀ
rived from anions 4 and various electrophiles are given in
Table 1.
The tabulated data suggests a number of parallel and
competing processes (rather than only one) leading to the
reaction products. An electrophilic attack on the C atom
of anion 4b yields substitution products 6. It is also obviꢀ
ous that compounds 10 will be produced only when the
reagent attacks the alternative nucleophilic N(3) site of
the anions rather than neutral compounds 2a,b since the
latter, as demonstrated in special experiments, are inert to
ClCOOEt, ClCOOMe, and AcCl at room temperature for
a long period of time in the absence of a base. The formaꢀ
tion of carbonyl compounds 9 can result only from cleaꢀ
vage of the C—Bt bond. The presence of the C—O bond
in products 9 suggests hydrolysis of the reaction mixture
because water is the only oxygenꢀcontaining compound
(except for THF) in the reaction of, e.g., anion 4a with
H2C=CHCH2Br or PhCH2Cl. The fact that the reactions
studied always produce carbonyl compounds 9a,b is eviꢀ
dence in favor of the unified mechanism of their formaꢀ
tion, despite a wide variety of the used electrophiles. Moreꢀ
over, it is obvious that the formation of the C—O bond
cannot result from an electrophilic attack on the C atom;
this suggests that the reaction mixtures contain species in
which the C atom is not a carbanionic center. The reacꢀ
tions of anions 4a,b with PhNCO gave complex mixtures
of products that were separated by chromatography only
in part, with the compounds being isolated and identified
were the starting compounds 2a,b and carbonyl derivaꢀ
tives 9a,b. The reaction of anion 4a with 4ꢀMeOC6H4CHO
gave no products containing the pꢀanisyl fragment; only
the unreacted starting aldehyde was detected.
Note that the range of products derived from anion 4b
and ClCOOEt depends on the time interval between the
deprotonation of compound 2b with BunLi and the addiꢀ
tion of the electrophile. When ClCOOEt was added alꢀ
most immediately (with a 20ꢀsecond delay) after the addiꢀ
tion of BunLi, the hydrolyzed reaction mixture contained
only the starting compound 2b (major component) and the
target ꢀsubstitution product 6 (R = Me, E = COOEt).

1940
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
Moiseev et al.
When the delay was 10 min, the ratio of compounds 2b
and 6 was inverted, the latter being the major product. It
should be emphasized that products 9 and 10 were absent
(or present only in trace amounts) in both cases. However,
with a longer delay before adding the electrophile (30 min),
the reaction yielded products 9b and 10 (E = COOEt) but
no compound 6.
will be protonated during hydrolysis to give the starting
compounds 2a,b.
However, it still remains unclear why the reactions of
anions 4a,b with MeI, CH2=CHCH2Br, and PhCH2Cl
give no Cꢀ and Nꢀalkylated analogs of acylated products 6
and 10. A plausible explanation is that the above electroꢀ
philes are weaker than AcCl, ClCOOMe, and ClCOOEt;
so the Fcꢀcontaining carbanion has enough time to reꢀ
arrange itself into anion 11 and acts as a oneꢀelecꢀ
tron donor (rather than a nucleophile) reducing the reꢀ
agents, as in the reactions of Cbꢀcontaining anions 3a,c
with 4ꢀMeOC6H4CHO (see above).
The intensely colored solutions of anions 4a and 4b
generated and kept at 20 and –70 C, respectively, graduꢀ
ally turned orangeꢀbrown (usually within 30 min). Hyꢀ
drolysis of anion 4a with water recovered the starting comꢀ
pound 2a, no matter whether the hydrolysis was carried
out immediately (i.e., for an intensely colored solution of
the anion) or after its fading. Likewise, when anion 4b
generated at –70 C was treated with an electrophile and
then was warmed to room temperature for ~1 h (this periꢀ
od of time is quite enough for the thermolysis of the anion)
and hydrolyzed (see Experimental), the final reaction mixꢀ
ture never contained anil 8, the thermolysis product from
anion 4b. Therefore, the latter is absent from the reaction
mixture before its having warmed to the thermolysis temꢀ
perature of anion 4b. However, when compound 2b was
deprotonated at 20 C with immediate addition of an elecꢀ
trophile and then the reaction mixture was kept for 1 h and
hydrolyzed, anil 8 was always detected among the reacꢀ
tion products.
Experimental
All reactions were carried out in anhydrous solvents under
dry argon. Diethyl ether and THF were refluxed over sodium
benzophenone ketyl and then distilled. Electrophilic reagents
were distilled before use.
1H and 13C NMR spectra were recorded on Avanceꢀ300TM
(300.13 (1H) and 75.47 MHz (13C)) and Avanceꢀ600TM specꢀ
trometers (600.22 (1H) and 150.925 MHz (13C)) (Bruker) in
CDCl3. Chemical shifts in the 1H NMR spectra were measured
with reference to the residual signal of the solvent (H 7.27) and
converted to Me4Si. Chemical shifts in the 13C NMR spectra
were measured with reference to the signal of the solvent (C 77)
and converted to Me4Si.
The facts cited above can be interpreted as follows.
Apparently, Fcꢀsubstituted anions 4a,b in solution are
gradually and irreversibly transformed into more stable
species. This is evident from the fading of their intense
blue or dark brown solutions to an orangeꢀbrown color as
well as from the absence of compound 6 among the reacꢀ
tion products obtained by adding ClCOOEt to anion 4b
after its initial intense color has already disappeared. The
formation of noticeable amounts of product 10 (rather
than the target compound 6) in this case suggests that the
new anionic species is no longer nucleophilic at the C
atom, yet being capable of reacting with electrophiles at
the position corresponding to N(3) in anions 4. We believe
that such properties can be characteristic of structure 11
(see Scheme 4).
Thus, the deprotonation of compounds 2a,b produces
the anions, which can most accurately be described as
a superposition of resonance structures 4a,b and 4X. They
react with electrophiles at the C atom to give products 6,
as illustrated with the reactions of compound 4b with
chloroformates. A possible competing electrophilic attack
on the N(3) atom becomes more clear if the anion is deꢀ
picted as structure 4X. Anions 4 can undergo an irreversꢀ
ible transformation into anion 11. In this case, the reacꢀ
tion with RCOCl occurs only at the site corresponding to
N(3) in anions 4. Hydrolysis of acylated derivative 12 ultiꢀ
mately yields products 9 and 10. It should be emphasized
that all the aforementioned anionic species (4a,b, 4X,
and 11), if not consumed in reactions with electrophiles,
The thermodynamic parameters of the conformational
change were evaluated using dynamic NMR spectroscopy.31 In
the line shape analysis, theoretical spectra were simulated with
the Spinworks 3.1.7 program.32
Thinꢀlayer chromatography was carried out on Sorbfil UV
254 plates containing a luminescent indicator ( = 254 nm).
Spots were visualized under UV light.
1ꢀ[(2ꢀMethylꢀoꢀcarboranꢀ1ꢀyl)methyl]benzotriazole (1a).
A solution of 1ꢀmethylꢀoꢀcarborane (10.00 g, 0.063 mol) in diꢀ
ethyl ether (60 mL) was added dropwise at –40 C to a stirred
suspension of NaNH2 prepared from metallic Na (1.50 g,
0.065 mol) in liquid NH3 (150 mL) at –40 C. After 30 min,
a solution of 1ꢀ(chloromethyl)benzotriazole (11.10 g, 0.066 mol)
in THF (40 mL) was added dropwise. The reaction mixture was
stirred at –40 C for 1 h and then the cooling bath was removed.
After the ammonia was completely evaporated, water (50 mL),
methanol (30 mL), and NaCl (5.0 g) were added to the mixture.
The organic layer was separated and the product from the aqueꢀ
ous layer was extracted with CHCl3 (3×40 mL). The combined
organic phases were dried with anhydrous MgSO4 and concenꢀ
trated. The residue was recrystallized from heptane—EtOAc.
The yield of compound 1a was 8.56 g (47%), colorless crystals,
m.p. 164—166 C. Found (%): C, 41.69; H, 6.74; B, 37.02;
N, 14.53. C10H19B10N3. Calculated (%): C, 41.50; H, 6.57;
B, 37.37; N, 14.53. 1H NMR (300 MHz), : 1.00 —3.30 (m, 10 H,
B10H10); 2.42 (s, 3 H, Me); 5.35 (s, 2 H, CH2); 7.43 + 7.58
(m + m, 1 H + 2 H, H(5) + H(6) + H(7)); 8.09 (d, 1 H, H(4)).
1ꢀ[(2ꢀPhenylꢀoꢀcarboranꢀ1ꢀyl)methyl]benzotriazole (1c).
A 1.34 M solution of BunLi (36.98 mL, 49.55 mmol) in hexane
was added through a dropping funnel at –10 C for 1 min to
a solution of 1ꢀphenylꢀoꢀcarborane (10.00 g, 45.45 mmol) in
diethyl ether (60 mL). The resulting yellow mixture was stirred

Anions derived from substituted 1ꢀalkylbenzotriazoles Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
1941
at ~20 C for 10 min, transferred by argon pressurization to
a dropping funnel, and added dropwise for 10 min to a stirred
solution of compound 7 (3.77 g, 22.51 mmol) in THF (40 mL).
The reaction mixture turned brown. After 60 min, water (80 mL)
was added and the organic layer was separated. The product
from the aqueous layer was extracted with ethyl acetate (2×20 mL).
The organic phases were combined, dried with anhydrous
Na2SO4, and concentrated. The residue was refluxed in hexane
(50 mL) for 2 min and the hot suspension was filtered through
a glass filter. The solid product was washed on the filter with
hexane (15 mL) and recrystallized twice from ethyl acetate. The
yield of compound 1c was 5.20 g (66%), colorless crystals.
Found (%): C, 52.57; H, 5.91; B, 30.73; N, 11.84. C15H21B10N3.
Calculated (%): C, 51.26; H, 6.02; B, 30.76; N, 11.96. 1H NMR
(600 MHz), : 1.50—3.50 (m, 10 H, B10H10); 4.90 (s, 2 H, CH2);
10 min, and an appropriate electrophile (1 equiv.) was added
(E+ = MeI, EtI, H2C=CHCH2Br, PhCH2Cl, Me2CHCHO,
PhCHO, 4ꢀMeOC6H4CHO, Me2CO, Ph2CO, AcCl, ClCOOMe,
and 4ꢀMeOC6H4COCl). In the case of MeI, the amount of the
electrophile was 3 equiv. The reaction mixture was kept at ~20 C
for 24 h (with MeI, for 72 h) and hydrolyzed with water (20 mL).
The product was extracted with diethyl ether (2×20 mL). The
organic extracts were combined, dried with anhydrous Na2SO4,
and concentrated. The residue was dried in a vacuum desiccator
for 12 h. The reaction products were identified, and their ratios
were determined, from 1H NMR spectra.
Reactions of anion 4a generated from 1ꢀ(ferrocenylmethyl)ꢀ
benzotriazole (2a) with electrophiles E+ (general procedure).
A 0.89 M solution of BunLi (0.71 mL, 0.63 mmol) in hexane was
added to a solution of compound 2a (0.20 g, 0.63 mmol) in THF
(10 mL). The resulting dark blue mixture was stirred for 10 min,
whereupon an electrophile (1 equiv.) was added (E+ = MeI,
H2C=CHCH2Br, PhCH2Cl, ClCOOMe, ClCOOEt, AcCl,
PhNCO, and 4ꢀMeOC6H4CHO). After 1 h, the reaction mixꢀ
ture was hydrolyzed with water (20 mL) and the product was
extracted with diethyl ether (2×20 mL). The organic extracts
were combined, dried with anhydrous Na2SO4, and concentratꢀ
ed. The residue was dried in a vacuum desiccator for 12 h. The
reaction products were identified, and their ratios were deterꢀ
mined, from 1H NMR spectra.
Reactions of anion 4b generated from 1ꢀ[1ꢀ(ferrocenyl)ꢀ
ethyl]benzotriazole (2b) with electrophiles E+ (general procedure).
A 0.89 M solution of BunLi (0.67 mL, 0.60 mmol) in hexane was
added at –70 C to a solution of compound 2b (0.20 g, 0.60 mmol)
in THF (10 mL). The resulting dark brown mixture was stirred
for 10 min, whereupon an electrophile (1 equiv.) was added
(E+ = MeI, H2C=CHCH2Br, ClCOOMe, ClCOOEt, AcCl, and
PhNCO). The reaction mixture was stirred on the cooling bath,
allowing it to naturally warm to room temperature (~1 h), and
hydrolyzed with water (20 mL). The product was extracted with
diethyl ether (2×20 mL). The organic extracts were combined,
dried with anhydrous Na2SO4, and concentrated. The residue
was dried in a vacuum desiccator for 12 h. The reaction products
were identified, and their ratios were determined, from 1H NMR
spectra.
7.24 (d, 1 H, H(7), J6,7 = 7.7 Hz); 7.38 (ddd, 1 H, H(5), J4,5
= J5,6 = 8.3 Hz, J5,7 = 0.9 Hz); 7.49 (ddd, 1 H, H(6), J4,6
=
=
4
4
= 0.8 Hz); 7.52 (dd, 2 H, 2 mꢀH (Ph)); 7.57 (t, 1 H, pꢀH (Ph),
J = 7.3 Hz); 7.82 (d, 2 H, 2 oꢀH (Ph), J = 7.5 Hz); 8.05 (d, 1 H,
H(4)). 13C NMR, : 49.53 (CH2); 78.28, 84.69 (2 CCb); 109.04
(C(7)); 120.40 (C(4)); 124.39 (C(5)); 128.27 (C(6)); 129.47 (2 C,
mꢀPh); 129.71 (Cipso, Ph); 131.52 (pꢀPh); 131.79 (2 C, oꢀPh);
133.25 (C(7a)); 145.53 (C(3a)).
1ꢀ[1ꢀ(2ꢀPhenylꢀoꢀcarboranꢀ1ꢀyl)ethyl]benzotriazole (1d).
A 0.80 M solution of BunLi (24.93 mL, 19.94 mmol) in hexane
was added at 0 C for 1 min to a solution of compound 1c (7.00 g,
19.94 mmol) in THF (100 mL). The resulting yellowꢀorange
mixture was stirred at ~20 C for 10 min. Then MeI (3.69 mL,
8.41 g, 59.24 mmol) was added. The reaction mixture was kept at
~20 C for 72 h and treated with water (10 mL). The product was
extracted with diethyl ether (2×30 mL). The organic extracts
were combined, dried with anhydrous Na2SO4, and concentratꢀ
ed. The residue was repeatedly recrystallized from AcOEt. The
yield of compound 1d was 2.39 g (33%), colorless crystals. Found
(%): C, 52.28; H, 6.28; B, 29.72; N, 11.29. C16H23B10N3. Calꢀ
culated (%): C, 52.58; H, 6.34; B, 29.58; N, 11.50. 1H NMR
(600 MHz), : 1.50—3.50 (m, 10 H, B10H10); 1.87 (d, 3 H, Me,
J = 7.3 Hz); 4.70—5.25 (br.s, 1 H, CHCH3); 6.00—7.20 (vbr.s,
1 H, H(7)); 7.31—7.38 (m, 2 H, H(5) + H(6)); 7.49 (m, 2 H,
2 mꢀH (Ph)); 7.58 (t, 1 H, pꢀH (Ph)); 7.71 (d, 2 H, 2 oꢀH (Ph));
8.01 (d, 1 H, H(4)). 13C NMR, : 21.93 (vbr, Me); 55.52 (vbr,
CHMe); 84.02 (CCb—CH); 85.43 (br, CCb—Ph); 109.79 (vbr,
C(7)); 120.36 (C(4)); 124.24 (C(5)); 127.71 (C(6)); 129.58 (2 Cm
(Ph)); 129.90 (Cipso (Ph)); 131.58 (Cp (Ph)); 131.64 (2 Co (Ph));
132.26 (br, C(7a)); 145.69 (br, C(3a)).
1ꢀ[1ꢀ(2ꢀMethylꢀoꢀcarboranꢀ1ꢀyl)ethyl]benzotriazole (1b) was
obtained from compound 1a (0.20 g) and MeI as described above
for the synthesis of 1d. The crude product was recrystallized
from AcOEt to give a 2.1 : 1 mixture of compounds 1b and 1a.
Product 1b was identified using 1H NMR data (300 MHz),
: 1.00—3.30 (m, 10 H, B10H10); 2.11 (d, 3 H, CHCH3); 2.37
(br.s, 3 H, Cb—Me); 5.60—5.90 (vbr.s, 1 H, CHMe); 7.41 + 7.53
(m + m, 1 H + 1 H, H(5) + H(6)); 7.60—7.75 (vbr.s, 1 H, H(7));
8.09 (d, 1 H, H(4)).
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Received March 16, 2012;
in revised form September 17, 2012