Reductive activation of arenes: XIV.Effect of medium on the products ratio in the alkylation of nitrobenzene radical anion sodium salt

Article abstract of DOI:10.1023/A:1013969901048

The product of nitrobenzene reduction by 1 equiv of sodium reacted with tert-butyl iodide in liquid ammonia and its mixtures with tetrahydrofuran and hexamethylphosphoramide to afford a mixture of compounds alkylated either by the functional group (N-alkylation) or at the benzene ring (C-alkylation). The ratio of N-and C-alkylation products decreases in the solvents series NH₃-THF (5.7) > NH₃ (1.0) > NH₃-HMPA (0.6).

Full text of DOI:10.1023/A:1013969901048

Russian Journal of Organic Chemistry, Vol. 37, No. 12, 2001, pp. 1719 1722. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 12, 2001,
pp. 1800 1803.
Original Russian Text Copyright
2001 by Selivanov, Shteingarts.
Reductive Activation of Arenes: XIV.* Effect of Medium
on the Products Ratio in the Alkylation
of Nitrobenzene Radical Anion Sodium Salt**
B. A. Selivanov and V. D. Shteingarts
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences, Novosibirsk,
630090 Russia
Received January 9, 2001
Abstract The product of nitrobenzene reduction by 1 equiv of sodium reacted with tert-butyl iodide in
liquid ammonia and its mixtures with tetrahydrofuran and hexamethylphosphoramide to afford a mixture of
compounds alkylated either by the functional group (N-alkylation) or at the benzene ring (C-alkylation). The
ratio of N- and C-alkylation products decreases in the solvents series NH3 THF (5.7) > NH3 (1.0) >
NH3 HMPA (0.6).
It was shown formerly [2, 3] that the reaction
between alkali metal salts of nitrobenzene radical-
anion (I) with tert-butyl iodide in liquid ammonia
furnished products of nitrobenzene reductive alkyla-
tion both at nitrogen (N-alkylation products) and at
the para-position of the benzene ring (C-alkylation
products (see scheme).
increasing ion association of A and B salts in the
liquid ammonia along the above series. A significant
factor that to a great extent governs the character of
ion association of radical-anion salts in solution is the
nature of the environment. Since the excess electron
density in the nitrobenzene radical-anion is located
mostly on the nitro group the latter is the principal
center taking part in hydrogen bonds with the solvent
[4, 5 7] (specific solvation) and in association with
the counterion [4, 5, 8]. Therewith the strengthening
of both interactions results in still greater increase of
the electron density on the nitro group [5, 6, 8 10].
In evaluation of the solvation influence on the N/C
ratio it should be taken into account that the structures
A and B simulating the transition states of the N- and
The ratio of N- and C-alkylation products grows
with the increasing radius of the metal cation in the
series Li < Na < K < Rb < C s [3]. In the frame-
work of the late transition state model where the
transition states of the competing alkylation directions
may be represented by the corresponding intermedi-
ates this fact may be rationalized as originating from
Scheme.
*
For communication XIII see [1].
The study was carried out under financial support of the Russian Foundation of the Basic Research (grant no. 99-03-33111) and
of Ministry of Industry and Science of the Russian Federation (grant no. 9.3.01).
**
1070-4280/01/3712-1719$25.00 2001 MAIK Nauka/Interperiodica

1720
SELIVANOV, SHTEINGARTS
Yields of reaction products (mmol,a GLC) from nitrobenzene radical-anion sodium salt (I) and tert-butyl iodideb
Nitro
benzene
Azo-
Azoxy-
Yield of
Ratio
N/C
Solvent
II
III
IV
V
benzene benzene alkylation products
NH3 THF
NH3 [2]
NH3 HMPA
9.55
7.96
8.20
0.06
0.16
0.07
0.11
0.68
0.31
0.02
0.83
0.65
0.01
0.02
0.04
0.02
0.01
0.08
0.01
0.14
0.20
1.69
1.07
5.67
1.00
0.55
a
Into the reaction was brought 10 mmol of nitrobenzene, reduced with 5 mmol of sodium metal, and 10 mmol of tert-butyl iodide.
Concentration of reduced form 0.05 mol l .
Avereged results from 2 3 experiments. Deviations from the average do not exceed 15%.
1
b
C-alkylation respectively sharply differ since in the
former one the negative charge is virtually totally
localized on oxygen atoms whereas in the latter the
charge is considerably delocalized. Analogously to
the above this difference suggests with respect to the
nitrobenzene radical-anion that the increase both in
the specific solvation and ion association should
stabilize stronger the A structure, and consequently
in the products of tert-butylation of the sodium salt I
should grow the fraction of the N-alkylation products.
Thus establishing the character of solvent effect on
the N/C ratio would give a possibility to prove the
above statements on the nature of the transition states
of the transformations under consideration. This was
the target of the present study.
instance, in going from NH3 to the mixture NH3
THF the overall yield of the alkylation products is
8 times reduced, and in going to the mixture NH3
HMPA only 1.5 times. In the former case it was in
agreement with the previously found fact that in THF
occurred no alkylation [11]. It is apparently due
mostly to the lower solubility of the nitrobenzene
radical-anion salt I and the stronger ion association.
In keeping with the assumption on the reaction
mechanism developed in [2, 3] these processes should
reduce the rate of dissociative electron transfer from
the radical-anion to the alkyl iodide. It should be
noted that similar although not so pronounced reduc-
tion of the overall yield of the alkylation products
was formerly observed [3] in carrying out the tert-
butylation of the nitrobenzene radical-anion sodium
salt in the presence of excess sodium iodide which
also increased the ion association of the radical-anion
salt (cf. [5]). Basically some decrease in the overall
yield of alkylation products in the mixture of
ammonia with THF may be ascribed to the consump-
tion of the tert-butyl radical in reaction with THF
(cf. [12]). However hardly it is the main reason of
the decrease in the yield of the alkylation products.
Firstly, the rate constant of alkyl radicals reaction
with THF is considerably smaller (by 5 6 orders of
magnitude) than the rate constant of their recombina-
tion with radical-anions of the aromatic compounds
[12, 13]. Secondly, THF is the most commonly used
solvent for reactions of radical-anions of aromatic
compounds with alkyl halides following the SET
mechanism. In the course of these reactions form
alkyl radicals (cf., for example, [11, 13]) but the yield
of the alkylation product is nearly quantitative. For
instance the yields of tert-butylation products of
nitrobenzene radical-anion salt I in HMPA and its
mixture with THF are similar [11].
We investigated the reaction of salt I with tert-
butyl iodide in the binary mixtures (1: 1 by volume)
of the liquid ammonia and tetrahydrofuran (THF) and
hexamethylphosphoramide (HMPA) at 33 C. The
selection of components for the mixtures was aimed
at the widest possible variation of the solvating
properties of the environment accounting also for the
necessity to use the solvents where salt I could be
generated by reduction of its precursor with alkali
metal [11].
The reduction of nitrobenzene with sodium was
carried out in the indicated binary mixtures with
ammonia with the use of double excess of the pre-
cursor with respect to the alkali metal.
As seen from the table, in all cases formed
compounds II V and alongside these compounds
were also revealed nitrobenzene, azoxybenzene, and
at reaction in the THF ammonia mixture also a little
azobenzene.
The data in the table evidence that the nature of the
solvent significantly affects, firstly, the overall yield
of the alkylation products, and, secondly, the ratio
N/C = (II+ III)/(IV+ V) that characterizes the
competition of the two tert-butylation directions. For
The slight decrease in the overall yield of alkyla-
tion products in going from ammonia to its mixture
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 12 2001

REDUCTIVE ACTIVATION OF ARENES: XIV.
1721
with HMPA may be due to increased viscosity of the
medium and therewith to reduced recombination rate
of the tert-butyl radical and nitrobenzene radical-
anion. Besides the possibly decreased ion association
caused by HMPA does not favor the recombination
of the alkyl radical arising from the dissociative
electron transfer with nitrobenzene radical-anion
to yield intermediates (A) and (B) (cf. [14]); in the
course of this recombination the degree of localization
of the negative charge grows as compared to that in
the initial nitrobenzene radical-anion.
stationary phase CKTFT-50 or SKTFV-803 (15%)
on the carrier Chromosorb W, oven temperature
programmed from 70 to270 C at a rate 10 deg/min,
carrier gas helium, flow rate 0.6 l h , column
400 4 mm. Toluene was used as internal standard.
1
The liquid ammonia was purified by dissolving
sodium ( 2 2.5 g per 250 ml of liquid ammonia)
followed by distillation of the solvent into the reaction
vessel cooled to 70 C. The required amounts of
THF and HMPA dried by procedures [16] and [17]
respectively were preliminary charged into the
reactor.
Unlike the changes in the overall yield of the
alkylation products, the N/C ratio in going from
ammonia to its mixtures with THF and HMPA is
altered in the opposite directions: in the presence of
THF N/C grows from 1 to 5.7, and in the presence
of HMPA it decreases to 0.55. Within the framework
of the model of the late transition state this change in
the N/C value may be attributed to the following: The
intermediate of C-alkylation (B) is characterized by
considerably greater delocalization of the negative
charge and thus by lesser association of the counter-
ions than the intermediate of N-alkylation (A). At
THF addition causing decrease in the solvating power
of the medium and affecting primarily the metal
cation should favor due to the above key difference
the formation of the more associated intermediate (A)
and consequently N-alkylation. In going from NH3 to
its mixture with HMPA the opposite trend should be
observed. It should be noted that the N/C value
equal to 0.55 that characterizes the alkylation regio-
selectivity of nitrobenzene radical-anion salt I in the
mixture NH3 HMPA is close to the figure obtained at
tert-butylation in NH3 of the lithium salt of this
radical-anion [3]. Since the lithium salts in the liquid
ammonia possess the most loose ion pairs among the
alkali metal salts [15] it is presumable that in the
NH3 HMPA mixture the reactivity of salt I is due
mostly to the loose ion pairs or even to the free
radical-anion.
Nitrobenzene of pure grade was purified by
method [11]. tert-Butyl iodide was obtained by
procedure [18] and was purified just before the
experiment by passing through a layer of alumina.
The metallic sodium used was of the pure grade.
Reduction of nitrobenzene, tert-butylation of the
reduced form, and workup of the reaction mixtures
was performed as described in [2]. We used the
sodium salt of the reduced form of nitrobenzene in
1
0.05 mol l concentration.
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