Reductive electrocatalytic dehalogenation of nitrobenzyl halides: Nitrophilic or halophilic attack?

Article abstract of DOI:10.1007/s11172-005-0237-0

Electrochemical reduction of Cp₂TiCl₂ in the presence of benzylic halides results in their catalytic dehalogenation to form toluene derivatives. Possible schemes for the process were proposed on the basis of the results of electrochemical studies and digital simulation data. The catalytic scheme including the halophilic attack of the Ti^III complex to organic halide as a key step occurs for unsubstituted benzylic halides. In the case of nitro-substituted substrates, the reaction is strongly accelerated. In this case, an electron transfer from the reduced form of the catalyst to the NO₂ group of organic halide is possibly accompanied by the intramolecular charge transfer to the C-Hal bond, thus facilitating its cleavage. Thus, the nitro group in the starting benzylic halide acts as a "redox antenna," transferring an electron to the C-Hal bond. The proposed scheme is supported by semiempirical calculations of the geometry of molecular complexes with the Ti-Hal or Ti-NO₂ coordination.

Full text of DOI:10.1007/s11172-005-0237-0

Russian Chemical Bulletin, International Edition, Vol. 54, No. 1, pp. 201—210, January, 2005
201
Reductive electrocatalytic dehalogenation of nitrobenzyl halides:
nitrophilic or halophilic attack?
T. V. Magdesieva,aꢀ O. M. Nikitin, P. I. Dem´yanov, M. Graczyk, A. Vallat,
a
a
b
b
b
a
M. A. Vorotyntsev, and K. P. Butin
aDepartment of Chemistry, M. V. Lomonosov Moscow State University,
1
Leninskie Gory, 119992 Moscow, Russian Federation.
Eꢀmail: tvm@org.chem.msu.ru
b
University of Bourgogne, Dijon, France.*
Eꢀmail: MV@uꢀbourgogne.fr
Electrochemical reduction of Cp TiCl in the presence of benzylic halides results in their
2
2
catalytic dehalogenation to form toluene derivatives. Possible schemes for the process were
proposed on the basis of the results of electrochemical studies and digital simulation data. The
III
catalytic scheme including the halophilic attack of the Ti complex to organic halide as a key
step occurs for unsubstituted benzylic halides. In the case of nitroꢀsubstituted substrates, the
reaction is strongly accelerated. In this case, an electron transfer from the reduced form of the
catalyst to the NO2 group of organic halide is possibly accompanied by the intramolecular
charge transfer to the С—Hal bond, thus facilitating its cleavage. Thus, the nitro group in the
starting benzylic halide acts as a "redox antenna," transferring an electron to the C—Hal bond.
The proposed scheme is supported by semiempirical calculations of the geometry of molecular
complexes with the Ti—Hal or Ti—NO coordination.
2
Key words: titanocene dichloride, benzylic halides, reductive dehalogenation, electroꢀ
catalysis, semiempirical calculations.
Reactions of halogen substitution by hydrogen in orꢀ
ganic compounds are well known and can be effected
with various reagents and proceed via different mechaꢀ
nisms. A possible way is the reductive dehalogenation of
oxidative or reducing agents in the reaction mixture, etc.).
Therefore, it seems of special interest to develop new
electrochemically activated reactions involving different
types of transformations.
The purpose of the present work is to study the
electrocatalytic dehalogenation of compounds of the
III
1
benzylic halides (ArCH X) catalyzed by Ti complexes.
2
This reaction is of interest as a method for the selective
dehalogenation in polyfunctional compounds. It can be
used, in particular, as a synthesis of glycals from glycosyl
ArCH X series catalyzed by the reduced form of
2
5
bis(η ꢀcyclopentadienyltitanium) dichloride in solution.
halides.2 The Ti complexes used as catalysts are genꢀ
,3
III
erally synthesized from titanocene dichloride or other
available Ti complexes by the action of reducing agents,
Experimental
IV
e.g., metals (Zn, Mn, Al) or organometallic compounds
(
form of the catalyst is prepared by electrochemical reducꢀ
Instruments and electrodes. Cyclic voltammograms were reꢀ
corded on IPCꢀWin and Autolab PGSTAT30 potentiostats
Ecochemie, Netherlands) at stationary platinum electrodes 3.8
RMgX, RLi).1 However, in recent works, the active
,4
(
IV
5—8
and 0.8 mm in diameter in anhydrous organic solvents (THF,
MeCN) with Bun4NPF6 as a supporting electrolyte at 20 °C.
A special electrochemical cell connected by the threeꢀelectrode
tion of the Ti complex,
and this form favors the
5
reductive coupling of aldehydes to form pinacones and
6
—8
the selective reduction of the nitro group.
The recent
on the nature of active species produced by the
electrochemical reduction of Cp TiCl in solution show
–
1
scheme was used, and the potential sweep rate was 100 mV s
.
9
,10
data
Electrolysis was carried out on a Pꢀ5827M potentiostat in a twoꢀ
compartment electrolytic 10ꢀmL cell. The working electrode
was a graphite 1×1ꢀcm tissue mounted on a platinum wire. In all
cases, the counter electrode was platinum, and the reference
electrode was Ag/0.01 M AgNO3 in CH3CN (the standard poꢀ
2
2
that these species undergo the subsequent electrocatalytic
reactions with benzaldehyde or benzyl chloride.
It is known that electrocatalytic processes have several
substantial advantages over common catalysis (high seꢀ
lectivity and ecological safety, no necessity for chemical
+
tential vs. Fc/Fc in CH CN is –0.1 V). Peak potentials were
3
recalculated taking into account ohmic losses.
Electrolysis products were analyzed on a Varian GC 3400
chromatograph equipped with a FID and HP1 silicone column
(50 m × 0.32 mm) with a film thickness of 0.52 µm. The temꢀ
*
Université de Bourgogne, LSEOꢀUMR 5188 CNRS, Dijon,
France.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 196—205, January, 2005.
066ꢀ5285/05/5401ꢀ0201 © 2005 Springer Science+Business Media, Inc.
1

202
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
Magdesieva et al.
perature of the detector and injector was 310 °C, and the temꢀ
i/µA
perature regime was 40 °C (3 min) → 300 °C, 10 deg min–
1
.
Mass spectra were recorded on an MSꢀGC Finnigan MAT
SSQ 7000 instrument with an ionization energy of 70 eV equipped
with an HPꢀ5 capillary silicone column (30 m).
1
0
Purification of solvents. Acetonitrile (pure) was stirred for
1
2 h above CaH and distilled. Then it was refluxed above P O
2
2
5
1
for 2 h and distilled again, collecting a fraction with b.p.
–1
8
1—82 °C (760 Torr).
Tetrahydrofuran (pure) was stirred above KOH, distilled
–2
above LiAlH , and stored above sodium benzophenone ketyl.
4
A freshly distilled portion of the solvent was used in each exꢀ
periment.
–3
6
Argon was conveyed from a gas cylinder through a column
filled with anhydrone and saturated with solvent vapor in an
additional vessel. Dry argon saturated with solvent vapor was
passed through the cell to remove oxygen from an electrolyte
solution and for stirring.
Starting compounds. Titanocene dichloride (Acros), tetraꢀ
butylammonium hexafluorophosphate (Aldrich), benzyl chloꢀ
ride, benzyl bromide, and 4ꢀmethylbenzyl chloride (Merck) were
used. Other halides were synthesized according to known proceꢀ
dures.
–1.2
–1.0
–0.8
–0.6
E/V
Fig. 1. Cyclic voltammograms for Cp TiCl (1.1 mmol L–1) in
the presence of PhCH Cl: 0 (1), 10 (2), 20 (3), 50 (4), 80 (5),
and 120 mmol L (6). Experimental conditions: Pt, THF,
2
2
2
–1
–1
0
.1 M Bu NPF , 100 mV s .
4 6
Cyclic voltammetry and preparative electrolysis. The
voltammogram for Cp TiCl recorded at the Pt electrode
in THF against 0.1 М Bu NPF and its transforming obꢀ
tained upon the addition of benzyl chloride to the soluꢀ
tion are shown in Fig. 1. Additives of PhCH Cl increase
2
2
4
6
Preparative electrolysis (general procedure). Electrolysis was
carried out in the corresponding solvent in the potentiostatic
regime at the potential of Cp TiCl reduction. A solution of
2
2
2
–
1
–1
slightly the current of Cp TiCl reduction, whereas the
2 2
Cp TiCl (3 mmol L ) and organic halide (10 mmol L ) was
2
2
III
reverse peak of Ti complex reoxidation decreases, i.e.,
the reduced form of titanocene dichloride is consumed in
the presence of organic halide. A similar pattern only with
more pronounced changes in the voltammograms was
used for electrolysis. The electrolysis was monitored by a digital
voltmeter by changing the electrolysis current. After the end of
electrolysis (2 F mol–1 of organic halide were passed), the solꢀ
vent was evaporated in vacuo (~100 Torr), and the residue was
extracted with benzene or diethyl ether and studied by GLC or
GCꢀMS. The chemical and current yields were calculated in
respect of the starting organic halide.
observed for PhCH Br.
2
The presence of the nitro group in the aromatic ring
changes sharply the voltammetric pattern. Already in the
presence of an equimolar amount of 4ꢀNO C H CH Cl,
Semiempirical calculations were carried out by the PM3
method extended by including the parameters for all transition
metals of the I Group and some II and III Group metals. This
extended method (PM3(tm)) is included in the HyperChem 7.01
program package (HyperCube Inc., FL, USA). The geometry of
molecules was optimized with the established convergence graꢀ
2
6
4
2
III
the peak of Ti complex reoxidation disappears comꢀ
pletely, and an almost threefold increase in the cathodic
current is observed (Fig. 2). This indicates a fast chemical
III
reaction of the Ti complex in solution.
The results of potentiostatic electrolysis of the ArCH X
series in the presence of Cp TiCl at the potential of
–
1
dient not higher than 10 cal (Å mol) . The systems with the
close and open shells were calculated using the restricted and
unrestricted Hartree—Fock methods, respectively.
2
2
2
Cp TiCl reduction are presented in Table 1. The compoꢀ
2
2
Digital simulation of voltammograms in the framework of
different kinetic schemes was performed by the DIGISIM proꢀ
gram (Bioanalytical Systems) with an increment of 5 mV. The
ohmic resistance was 1 kOhm.
sition of the reaction mixtures obtained by electrolysis
was analyzed by GLC and GCꢀMS. In all cases, the main
reaction product was toluene or its derivative with the
respective substituent; the corresponding dibenzyls were
also formed in small amounts (<10%).
Results and Discussion
Electrolysis was carried out in the presence of apꢀ
proximately threefold (10 : 3) excess of benzyl halide over
Cp TiCl in the potentiostatic regime at the potential of
Benzyl chloride, benzyl bromide, and their derivaꢀ
tives containing the methyl or nitro group in the benzene
2
2
ring were chosen as objects of the study: PhCH Cl,
Cp TiCl reduction. The quantity of electricity passed
2 2
2
PhCH Br, 4ꢀMeC H CH Cl, 4ꢀNO C H CH Cl, and
was 2 F per mole of organic halide. In the case of
nitrobenzyl halides, a high conversion (up to 70% calcuꢀ
lated to the starting halide) is observed. This indicates
that the catalyst is permanently regenerated during the
reaction, i.e., a catalytic cycle occurs. For nonactivated
PhCH Cl, PhCH Br, and 4ꢀMeC H CH Cl, the reacꢀ
2
6
4
2
2
6
4
2
4
4
ꢀNO C H CH Br. Several aryl halides (4ꢀNO C H Cl,
2 6 4 2 2 6 4
ꢀNO C H Br, 4ꢀMeOC H Cl) were also studied for
2
6
4
6
4
comparison. The electrocatalytic reductive dehalogenaꢀ
tion of these compounds was studied in THF or CH CN
3
solutions by cyclic voltammetry (CV) and electrolysis
methods using semiempirical calculation data.
2
2
6
4
2
tion mixture contained a considerable amount of the startꢀ

Reductive dehalogenation of benzyl halides
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
203
p
i/µA
∆E = 0.3 V, vs. Ag/0.01 M AgNO in MeCN), which is
3
optimal, in this case, for efficient redox catalysis. Howꢀ
ever, the electrochemical gap for the Cp TiCl /PhCH Cl
1
0
1
2
3
2
2
2
1
p
p
pair is much broader (E
= –1.12 V, E
=
Cp2TiCl2
PhCH2Cl
p
–
2.39 V; ∆E = 1.27 V) that makes the mediator outerꢀ
sphere reduction is much less probable.
When considering the outerꢀsphere mediator reducꢀ
tion, we inevitably conclude that the probability of the
–
–
–
III
–
outerꢀsphere redox process involving the [Cp Ti Cl2]
2
radical anion as a mediator is low. It can be assumed that
the energies of an electron in this radical anion and in the
electrode at the potential of radical anion formation are
the same (because the free Gibbs energies of the initial
4
–
1.2
–1.0
–0.8
–0.6
E/V
(
Cp TiCl and the electron in the electrode) and final
2 2
–
Fig. 2. Cyclic voltammograms for Cp TiCl (0.92 mmol L–1) in
the presence of 4ꢀNO C H CH Cl: 0 (1), 1.1 (2), 2.2 (3), and
.3 mmol L (4). Experimental conditions: Pt, THF, 0.1 M
Bu NPF , 100 mV s
states (Cp TiCl ) should become equal after equilibraꢀ
tion). The probability of direct reduction at the "elecꢀ
trode" is much higher. The more so, any steric or other
2
2
2
2
2
6
4
2
–
1
3
–
1
.
hindrance for the electron transfer to ArCH X can hardly
4
6
2
be discussed. A gain in the rate is possible only due to the
elimination of twoꢀdimensional restrictions on going from
the heterogeneous electron transfer from the electrode to
homogeneous redox catalysis in solution, if the process is
ing halides. Aryl halides containing the 4ꢀNO group are
2
characterized by very low yields of the reductive dehaloꢀ
genation products, and 4ꢀMeOC H Cl is not dehaloꢀ
6
4
III
III
diffusionally controlled. As for Cp Ti Cl or [Cp Ti Cl] ,
genated at all (see Table 1).
2
2
2
their reducing ability, according to the data in Ref. 9, is
lower than that of the radical anion and, therefore, these
species can hardly pretend to be outerꢀsphere mediators.
The second opportunity is to perform an innerꢀsphere
electron transfer, i.e., the process during which an orꢀ
ganic substrate incorporates into the coordination sphere
of the metal to form a coordination bond with the latter.
Since the innerꢀsphere electron transfer rate is usually
several orders of magnitude higher than the outerꢀsphere
Mechanism of catalysis. The electrochemical reducꢀ
tion of Cp TiCl in solution produces radical anions
2
2
III
–
–
[
Cp Ti Cl ] , which dissociate in part to the Cl anꢀ
2 2
ion and Cp Ti Cl radical. The latter forms dimers
Cp Ti Cl]2.
III
2
III
9,10
III
[
All of the three Ti ꢀcontaining species
2
are equilibrated and can act as mediators of the outerꢀ
sphere electron transfer from the electrode to ArCH X to
2
form the reductive dehalogenation products. This route is
more probable for nitrobenzyl halides than for benzylic
halides containing no nitro group. This is indicated, in
particular, by the difference of potentials between the
reduction peaks of the mediator and nitrohalide (for exꢀ
1
1
transfer rate (at close ∆G) and the probability of the
outerꢀsphere electron transfer seems to be very low in the
case, then an innerꢀsphere electron transfer is possible for
III
p
= –1.12 V, E p
coordinately unsaturated Cp Ti Cl.
ample, E
= –1.42 V;
2
Cp2TiCl2
NO2C6H4CH2Cl
It is reasonable to assume that the coordinately unꢀ
saturated (if ignoring coordination with the solvent)
Table 1. Results of the potentiostatic electrolysis of Cp TiCl
2
2
III
–
1
Cp Ti Cl radical reacts first of the three species considꢀ
(
С0 = 3 mmol L , E = –1.15 V vs. Ag/AgNO in MeCN) in the
2
3
III
–
III
III
–
1
ered ([Cp Ti Cl ] , Cp Ti Cl, [Cp Ti Cl] ). The
2 2 2 2 2
Cp Ti Cl ] radical anion with the coordination numꢀ
presence of ArCH X or ArX (C = 10 mmol L
)
2
III
–
[
2 2
ber 8 cannot coordinate to an organic substrate, because
the coordination number 9 is rather improbable for Ti due
to its small atomic size. The same is valid for the eightꢀ
Compound
Solꢀ Additives
vent
Yield Current effiꢀ
(%)
ciency(%)
4
4
4
4
4
4
ꢀNO C H CH Cl THF
—
55—70
60—72
53—60
65—75
15—24
16—27
8—12
25—30
25—32
4—6
50
55
40
50
13
13
6
20
20
4
III
2
6
4
2
coordinate dimeric molecule [Cp Ti Cl] , whose Ti atꢀ
2
2
ꢀNO C H CH Cl THF Ph CH
2
6
4
2
3
—
—
oms are bonded through the chloride bridges.
ꢀNO C H CH Cl MeCN
2
6
4
2
In the case of simple benzylic halides, [Cp TiIIICl]
2
ꢀNO C H CH Br THF
2
6
4
2
can be coordinated only through the halogen atom, and
an organic molecule is reduced via the halogen atom
transfer (halophilic mechanism). However, the case of
nitrobenzyl halide (Scheme 1) provides the second possiꢀ
ꢀMeC H CH Cl
THF Ph CH
6
4
2
3
ꢀMeC H CH Cl MeCN
—
—
—
6
4
2
PhCH Cl
THF
THF
2
PhCH Br
2
PhCH Br
MeCN Ph CH
3
2
bility: coordination through the NO group. Then the
2
4
4
4
ꢀNO C H Cl
MeCN
MeCN
MeCN
—
—
—
2
6
4
longꢀrange electron transfer to the σ*ꢀC—Hal bond folꢀ
lowed by the cleavage of this bond becomes possible
ꢀNO C H Br
7—9
5
2
6
4
ꢀMeOC H Cl
0
0
6
4
(
nitrophilic mechanism).

204
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
Magdesieva et al.
Scheme 1
The catalytic cycle presented in Scheme 2 can be proꢀ
the reaction with the paramagnetic TiIII complex can
occurW
posed for an innerꢀsphere process with significant coordiꢀ
nation. This scheme takes into account a possibility of
III
ArCH2• + Cp TiCl → Cp TiCl(ArCH )
(2)
coordination of the Cp Ti Cl radical to both the starting
2
2
2
2
nitrobenzyl chloride and reaction product (nitrotoluene).
Scheme 2 can be applied to both halophilic and
nitrophilic reactions.
and a hydrogen atom can be eliminated from a solvent
molecule or other specially added substances (for example,
Ph CH, as in our case)
3
A solvent or special additives, for instance, Ph CH,
3
can be used as a source of H atoms.
ArCH2• + AH → ArCH + A .
•
(3)
3
Further transformations of the benzylic radicals that
formed can be described by different schemes. The benꢀ
zylic radicals can dimerize
Dimeric compounds can be formed via the following
scheme:
2
ArCH2• → (ArCH2)2,
(1)
Cp TiCl(ArCH ) + ArCH Cl → Cp TiCl + (ArCH ) . (4)
2 2 2 2 2 2 2
Scheme 2
AH is the source of H atoms in solution.

Reductive dehalogenation of benzyl halides
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
205
The further reduction of the benzyl radicals both at
the electrode
i/µA
0
ArCH2• + e → ArCH –
,
(5)
2
ArCH2– + H+ → ArCH3,
–1
–2
–3
–4
and in solution
1
ArCH2• + [Cp TiCl ] → Cp TiCl + ArCH2–,
–
(6)
2
2
2
2
ArCH2– + H+ → ArCH3.
cannot be excluded.
Analysis of the reduction potentials of the benzylic
6
1
2
radicals (EBn/Bn
–
=
–1.215
V (SCE, DMF),
–
1.2
–1.0
–0.8
–0.6
E/V
1
3
ENO2C6H4CH2/NO2C6H4CH2
–
= –0.45 V (SCE, DMF) ) inꢀ
Fig. 4. Digital simulation of the electrocatalytic reaction of
dicates that the substantial contribution from reactions (5)
and (6) can be expected only in the case of nitrobenzyl
halides.
Since the main electrolysis product is RH in our case,
reactions (1) and (4) affording dibenzyls can hardly conꢀ
tribute substantially to the overall process.
Cp TiCl2 with PhCH Cl according to Scheme 2 (PhCH Cl:
2
2
2
–
1
0
(1), 10 (2), 20 (3), 50 (4), 80 (5), and 120 mmol L (6)).
Calculated rate constants: k = 1000 s– , K = 0.001 mol L
1
–1
,
f1
1
kf2 = 10 L (mol s)– , K = 0.005, k = 1 s–1, K > 100).
1
2
f3
3
Step K in Scheme 2 takes into account a possibility of
the process for unsubstituted benzylic halides and nitroꢀ
4
the reaction product (nitrotoluene) to coordinate to the
benzyl chloride. Step K was ignored for unsubstituted
4
coordinately unsaturated Cp TiCl complex. To check this
benzylic halides, and Cp TiCl can coordinate only to the
2
2
hypothesis by the CV method, we studied mixtures of
halogen atom of the starting organic halide. For unꢀ
substituted benzylic halides, the CV curves calculated by
Scheme 2 agree well with the experimental curves (cf.
Figs. 1 and 4) at the following rate and equilibrium conꢀ
solutions of Cp TiCl and 4ꢀNO C H CH , viz., the
2
2
2
6
4
3
dehalogenation product of 4ꢀnitrobenzyl halides. The adꢀ
dition of 4ꢀnitrotoluene to titanocene dichloride does not
–
1
result in the catalytic increase in the current of Cp TiCl
stants indicated in Scheme 2: k = 1000 s , K1 =
2
2
f1
reduction; however, the peak of TiIII reoxidation disapꢀ
pears completely (Fig. 3). This suggests that the proposed
reaction of the Ti center with the nitro group of the reacꢀ
tion product (nitrotoluene), viz., K and k (Scheme 2),
0.001 mol L , k =10 L (mol s) , K = 0.005, k = 1 s–1,
–1
–1
f2
2
f3
K > 100 (step k1´ was not considered separately).
3
Scheme 2, being confirmed by the data of digital simuꢀ
lation of the voltammetric curves for unsubstituted
4
f4
is possible and can be a reason for the fast disappearance
of the peak of Ti reoxidation.
To verify the correspondence of the process under
study to Scheme 2, we performed the digital simulation of
ArCH X, assumes the halophilic attack of the reduced
2
III
form of titanocene dichloride to the organic substrate.
1
0
This assumption was also advanced earlier along with
possible structures of the intermediates with the Cl atom
as a bridge between the reactants.
i/µA
However, for 4ꢀNO C H CH Cl, digital simulation
2
6
4
2
by Scheme 2 ignoring the subsequent coordination of the
product formed (NO C H CH ) with Cp TiCl (step K
5
0
0
2
6
4
3
2
4
1
including a possible subsequent fast step, for example, the
rearrangement shown above) gave an explicit discrepꢀ
ancy: an increase in the cathodic current for Cp TiCl
2
2
2
and a decrease in the reverse peak of reoxidation of the
–
50
III
Ti complexes in solution are much faster in the experiꢀ
3
mental curve than in the theoretical one.
–
–
100
150
When we introduced the step of interaction of the
III
Cp Ti Cl radical with nitrotoluene (K and k ) into the
2
4
f4
calculation kinetic scheme, the digital simulation of the
process for 4ꢀNO C H CH Cl by complete Scheme 2
2
6
4
2
gave a good coincidence of the theoretical and experiꢀ
–
1.6
–0.8
0
0.8 E/V
mental curves (cf. Figs 5 and 2) at the following rate
–
1
–1
5
Fig. 3. Cyclic voltammograms for Cp TiCl (1 mmol L–1) (1)
and its mixture with 4ꢀNO C H Me (1.2 mmol L ) (2, 3).
Experimental conditions: Pt, THF, 0.1 M Bu NPF , 100 mV s
constants: k = 1000 s , K = 0.001 mol L , k = 4•10
f1
1
f2
2
2
L (mol s) , K = 1000, k > 100 s , K = 1000 mol L–1,
–
1
–1
–
1
2
6
4
2 f3 3
–
1
5
–1
6
–1
.
k = 4•10 L (mol s) , K = 10 L mol (as in the case
f4 4
4
6

206
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
Magdesieva et al.
i/µA
an organic substrate, thus accelerating the reaction subꢀ
stantially (Scheme 3).
a
Analogies to the proposed scheme are presented in
Refs 14 and 15 along with the proofs that the easily reducꢀ
0
1
2
3
4
5
1
i/µA
ible NO group can serve as an electron carrier to the
2
–
–
–
–
–
b
kinetically impeded part of the molecule. The coordinaꢀ
1
6
0
tion of the nitro group is additionally favored by the fact
–
that the nitroxyl anion substitutes readily the Cl anion in
–
–
–
1
2
3
the CpTiCl complex, i.e., coordination with the oxygen
3
atom is more preferential.
We also attempted to carry out reductive dehaloꢀ
genation for a series of halobenzenes under similar conꢀ
ditions. However, the reaction does not occur for
5
–
1.2 –1.0 –0.8 –0.6
E/V
4
ꢀMeOC H Cl, and although the replacement of the
6 4
methoxy group by the nitro group affords the reductive
dehalogenation product, its amount is very low (see
Table 1). Perhaps, in this case, the presence of the NO2
–
1.2
–1.0
–0.8
–0.6
–0.4 E/V
Fig. 5. a. Digital simulation of the electrocatalytic reaction of
Cp TiCl (0.92 mmol L ) with 4ꢀNO C H CH Cl according
to Scheme 2. b. Results obtained by the addition of small amounts
of 4ꢀNO C H CH Cl: 0 (1), 0.3 (2), 0.4 (3), 0.5 (4), and
.1 mmol L . Calculated rate constants: k = 1000 s , K =
f1 1
–
1
2
2
2
6
4
2
group makes it possible to transfer an electron (outerꢀ or
innerꢀsphere ET) from the reduced form of titanocene
dichloride to nitrophenyl halide, thus facilitating the elimiꢀ
nation of the halogen atom. However, the elimination of
halogen in the halobenzene radical anion is much more
difficult than that in the benzyl halide radical anion. This
is indicated by the data on the rate of dissociation of the
2
6
4
2
–
1
–1
1
0
–
1
5
–1
–1
.001 mol L , kf2 = 4•10 L (mol s) , K = 1000, k > 100 s
K = 1000 mol L , kf4 = 4.10 L (mol s) , K = 10 L mol–1.
,
2
–1
f3
6
–
1
5
3
4
of unsubstituted benzylic halides, the complexation step
with the starting 4ꢀnitrobenzyl halide (K ´ and k ´) was
not separately considered).
1
7
σꢀC—Hal bond in radical anions of the nitrophenyl and
1
f1
1
8
nitrobenzyl types to form the halide anion and organic
As shown in Scheme 2, for nitrobenzyl halides, coorꢀ
dination with the O atom of the nitro group can be an
alternative of the halophilic attack for the coordinately
radical. For 4ꢀNO C H CH Hal, logk = 3.9 (Cl) and
2
6
4
2
5.7 (Br), whereas for 4ꢀNO C H Hal, logk = –6.0 (Cl)
2
6
4
and –2.9 (Br). As can be seen from these data, the differꢀ
ence in stability of the phenylic and benzylic type radical
anions is very high. This results in the situation when even
unsaturated Cp TiCl complex. In our opinion, the reꢀ
2
action between the electrochemically reduced form
Cp TiCl and 4ꢀNO C H CH Cl proceeds, more probꢀ
the presence of the NO group in a halobenzene molecule
2
2
2
6
4
2
2
ably, as an innerꢀsphere electron transfer (ET) from the
Ti atom to the nitro group rather than the halophilic
affords no significant amount of the reductive dehaloꢀ
genation product.
III
attack. This innerꢀsphere ET is accompanied by the inꢀ
tramolecular electron transfer to the C—Cl bond, resultꢀ
ing in the elimination of the halide ion. In this case, the
Semiempirical calculations. We performed the quanꢀ
0
tumꢀchemical calculations of the d complexes of the
IV
+
[Cp Ti Cl] cation with pꢀnitrobenzyl chloride coorꢀ
2
NO group acts as a "redox antenna" facilitating the elecꢀ
tron transfer from the reduced form of the Ti complex to
dinated to the Ti atom through the oxygen atom of the
2
IV
+
nitro group viz., [Cp Ti (Cl)—ON(O)C H CH Clꢀp] ,
2 6 4 2
Scheme 3

Reductive dehalogenation of benzyl halides
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
207
and through the benzyl chlorine atom, viz.,
an insignificant structural reorganization of the molꢀ
IV
+
0
[
Cp Ti (Cl)ClCH C H NO ꢀp] , as well as the d comꢀ
ecule. The difference in the geometries of the NO ꢀ and
2
2
6
4
2
2
plex with paraꢀnitrotoluene coordinated through the
Clꢀcoordinated complexes is related, probably, to the difꢀ
ference in orbital Ti—donor atom interactions.
IV
+
nitro group, viz., [Cp Ti (Cl)—ON(O)C H Meꢀp] .
2
6
4
1
9
0
In addition, we also calculated the reduced forms of
these complexes, which are the uncharged radical species
According to the theoretical concepts, the d fragꢀ
IV
ment of the Cp Ti complex has three unoccupied orbitꢀ
2
1
III
of the d type: [Cp Ti (Cl)—ON(O)C H CH Clꢀp],
[
als lying in the interring plane of the fragment. When
ignoring a possibility of rotation of the Cp rings about the
normals passing through the Ti atom, due to which they
can exist in eclipsed or inhibited conformations, then the
2
6
4
2
III
III
Cp Ti (Cl)ClCH C H NO ꢀp], and [Cp Ti (Cl)—
2 2 6 4 2 2
ON(O)C H Meꢀp].
6
4
The geometry of the complexes coordinated to the
NO group differs from that of the complexes coordiꢀ
symmetry of the Cp Ti fragment is C , and the orbitals
2
2 2v
nated to the Cl atom (Fig. 6): in the first case, the benꢀ
zene ring of the "guest" (organic molecule) is unfolded
in such a way that it is projected on the horizontal
plane passing through the Ti atom between the Cp rings
can be classified according to this point group using desꢀ
ignations a and b. The unoccupied orbitals and the origin
of their shapes from the hybridization of the canonical
1
9
orbitals of the metal are presented in Fig. 7, a—c. The
orbitals of the a symmetry are hybrid, and the b orbital is
and C face, and in the second case, it is projected by
6
1
2
the edge.
nonꢀhybrid, i.e., an almost pure dxy orbital. The three
Note that the geometry of the cationic TiIV d0 comꢀ
plexes differs slightly from the geometry of the radical
lowest unoccupied orbitals obtained by the calculation of
IV 2+
0
the [Cp Ti ] d dication correspond to the theoretiꢀ
2
III
1
IV
III
Ti d complexes, i.e., the Ti → Ti reduction leads to
cally predicted orbitals (see Fig. 7, d—f ).
Calculated structures
a
b
c
IV
+
IV
+
IV
+
[
Cp Ti (Cl)ClCH C H NO ꢀp]
[Cp Ti (Cl)NO C H CH Clꢀp]
[Cp Ti (Cl)NO C H CH ꢀp]
2
2
6
4
2
2
2
6
4
2
2
2
6
4
3
d
e
f
III
III
III
Cp Ti (Cl)ClCH C H NO ꢀp
Cp Ti (Cl)NO C H CH Clꢀp
Cp Ti (Cl)NO C H CH ꢀp
2 2 6 4 3
2
2
6
4
2
2
2
6
4
2
Spin density
g
h
i
Fig. 6. Geometry calculated by the PM3(tm) method for the [Cp Ti Cl] and Cp TiIIICl molecular complexes with pꢀnitrobenzyl
IV
+
2
2
chloride for coordination at the chlorine atom and nitro group and with pꢀnitrotoluene for coordination at the nitro group (a—f ) and
III
the localization of the total spin density in the Cp Ti ClX complexes (g—i).
2

208
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
Magdesieva et al.
a
d
dz
2
d
2
2
s
2a1
x –y
2
a1 (LUMO+2)
b
e
d
2
2
dz
2
1a1
x –y
b2 (LUMO+1)
c
f
dxy
1
a1 (LUMO)
Fig. 7. Theoretically predicted (a—c) and calculated (d—f ) three lowest free orbitals of the Cp Ti2 cation.
+
2
The orbitals of the 2a and b symmetry are used for
For the "plane" orientation, a charge transfer is posꢀ
1
2
bonding the Lewis lone electron pairs of the Cl and O
sible between Cp Ti and nitrobenzyl halide "through
2
donating atoms (Fig. 8, a).
bonds" and "through space." This is favored by the close
arrangement of the benzene ring to the Ti atom. Accordꢀ
ing to the calculations, the distance between the Ti atom
and benzene ring is rather short: 3.29 Å for the
The 1а orbital in symmetry is suitable for bonding of
1
the p(π) orbital of a donor if this orbital is plane, i.e., lies
in the plane of the 1а orbital (Fig. 8, b).
1
IV
+
0
Coordination to the NO group uses the nonꢀhybrid
[Cp Ti (Cl)—ON(O)C H CH Clꢀp]
d
complex
2
2
6
4
III
2
2
p(O) orbital rather than π(O=N), because the energy of
and 3.24 Å for the [Cp Ti (Cl)—ON(O)C H Meꢀp]
2
6
4
1
the pure p orbital is higher than that of p(π) (Fig. 8, c).
In this case, in the preferential geometry the benzene
ring is projected to the interring plane by its planar face.
For coordination to the Cl atom, both p orbitals are
almost degenerate and, therefore, one can choose beꢀ
tween the "plane" and "edge" orientations. It is most likely
that an organic molecule prefers, in this case, the "edge"
orientation for steric reasons (Fig. 8, d).
d complex. This distance is much longer (3.8 Å) in the
[Cp Ti (Cl)ClCH C H NO ꢀp] complex coordinated
IV
+
2
2
6
4
2
to the chlorine atom, i.e., the charge transfer "through
space" is less probable.
On going from the [Cp Ti ClX] d0 cations to the
IV
+
2
III
0
1
reduced [Cp Ti ClX] d radicals for the complexes
2
coordinated to the NO group, we obtain no changes
2
in the orientation of the benzene ring (in both cases,

Reductive dehalogenation of benzyl halides
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
209
Fig. 8. Theoretical consideration of bonding in the Cp TiXCl molecules: (a) orbitals of the 2a and b symmetry used for bonding of
2
1
2
the Lewis lone electron pairs of the donating Cl and O atoms; (b) orbital 1a bonding the p(π)ꢀorbitals of the donor if this orbital is
1
plane; (c) nonꢀhybrid 2pꢀorbital of the O atom used for coordination at the NO group; and (d) edge orientation for coordination at
2
the Cl atom.
the "plane" orientation with respect to the interring
is coordinated to the chlorine atom, no charge transfer is
observed. For example, in the case of unsubstituted benꢀ
zylic halide, the reaction occurs as an innerꢀsphere Cl
atom transfer (see Scheme 2).
Cp Ti plane is observed). However, the Ti—ON(O) bond
2
1
in the d radical shortens (by approximately 0.3 Å)
compared to that in the d cation. The calculated Ti—O
bond lengths in [Cp Ti (Cl)—ON(O)C H CH Clꢀp]
and [Cp Ti (Cl)—ON(O)C H Meꢀp] are 2.32—2.33 Å,
0
IV
+
2
6
4
2
IV
+
2
6
4
*
*
*
whereas those in the corresponding radicals are
.02—2.03 Å. This implies that the charge transfer from
2
Thus, the results obtained in this work indicate that
the reductive dehalogenation of benzylic halides in the
presence of Cp TiCl gives rise to the corresponding toluꢀ
1
Cp Ti to NO C H R in the radical d complexes should
2
2
6
4
be more efficient. It should be mentioned for comparison
that the Xꢀray diffraction data2 for the compounds with
the coordination type of the Ti—O bond, for instance, for
2
2
0
ene derivatives. Possible schemes of the catalytic process
were proposed on the basis of electrochemical studies,
digital simulation data, and semiempirical calculations.
The catalytic scheme, whose key step is the halophilic
III
+
–
the [Cp Ti (EtOEt) ] [(C F ) BH] complexes, give
2
2
6 5 3
2
.26 Å. For the Cp Ti(ONO ) complex containing niꢀ
2 2 2
III
trate anions as ligands, the Ti—O bond is shorter and
attack of the Ti complex to the organic halide, takes
2
1
equal to 2.15 Å according to the Xꢀray diffraction data.
place, most likely, for unsubstituted benzylic halides. The
introduction of the nitro group into the aromatic ring
considerably accelerates the reaction. In this case, the
process can pass through the electron transfer from the
1
In the radical d complexes, the spin density is localꢀ
ized on the aromatic ligand if the latter is coordinated by
the nitro group (see Fig. 6). For coordination to the chloꢀ
rine atom, the spin density remains on the Ti atom of
the Cp Ti 3d fragment. Actually, the NO ꢀcoordinated
d complexes can be considered as complexes of the nitroꢀ
reduced form of the catalyst to the NO group of organic
2
1
halide and is accompanied by the intramolecular charge
transfer to the C—Hal bond, thus favoring its cleavage.
The nitro group in the starting benzyl halide acts, thereꢀ
fore, as a "redox antenna" transferring an electron to the
C—Hal bond.
2
2
1
IV
0
arene radical anion with the Cp Ti Cl d cation.
2
Thus, the semiempirical calculations show that in the
III
1
radical [Cp Ti (Cl)—ON(O)C H CH Clꢀp] d complex
2
6
4
2
the nitroarene ligand can act, in fact, as a "redox antenna"
withdrawing electrons from the reduced form of titanocene
dichloride. Due to vibrations, the symmetry of nitroarene
changes and the π and σ*(C—Cl) orbitals are mixed to
result in the C—Cl bond cleavage. When NO C H CH Cl
Other examples for changing the reaction mechanism
by the introduction of the nitro group as a substituent are
published. The classical example is the alkylation of the
nitroalkane anions. The ambidentate 2ꢀnitropropane anꢀ
ion reacts, as a rule, with different alkylating agents to the
2
6
4
2

2
10
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 1, January, 2005
Magdesieva et al.
oxygen atom. The only exception are the paraꢀnitrobenzyl
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2
The work was financially supported by the Russian
Foundation for Basic Research (Project 05ꢀ03ꢀ32759).
Received October 29, 2004;
in revised form December 28, 2004