Reductive activation of arenes: XVII. Effect of methyl substituent in anion-radicals of tolunitriles on the mechanism of their reaction with alkyl bromides in liquid ammonia

Full text of DOI:10.1023/B:RUJO.0000043725.48757.14

Russian Journal of Organic Chemistry, Vol. 40, No. 5, 2004, pp. 747-750. Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 5, 2004,
pp. 781-784.
Original Russian Text Copyright Ó 2004 by Vaganova, Shteingarts.
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SHORT
COMMUNICATIONS
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Reductive Activation of Arenes: XVII.*
Effect of Methyl Substituent in Anion-radicals of Tolunitriles
on the Mechanism of Their Reaction
with Alkyl Bromides in Liquid Ammonia
T.A. Vaganova1 and V.D. Shteingarts1,2
1Vorozhtsov Nobosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
Novosibirsk, 630090 Russia
e-mail: shtein@nioch.nsc.ru
2 Novosibirsk State University, Novosibirsk, Russia
Received November 18, 2003
Earlier studies of factors governing the concurrent
SN and ET processes in reactions of arenecarbonitriles
anion-radicals with alkyl halides established that the
reaction of benzonitrile anion-radical (Ia) with primary
alkyl halides in liquid ammonia furnished products cor-
responding to ipso-alkylation with respect to cyano group.
Inasmuch as with a model reagent, cyclo-propylmethyl
bromide, no alkyl fragment isomerisation in the course of
the reaction was observed, the reaction was assigned SN
mechanism [2]. Introduction of an additional electron-
attracting substituent , a second cyano group, into the
ortho- or para-position of the anion-radical crucially alters
the mechanism and associated with it orientation of
alkylation: in reactions of terephthalonitrile and phthalo-
nitrile anion-radicals with alkyl halides due to an electron
transfer followed by recombination of the anion-radical
with an alkyl radical occurs substitution of both cyano
group and hydrogen atom.. Therefore among the products
of reaction between the above anion-radicals and
cyclopropylmethyl bromide are present compounds with
isomerized alkyl fragment [3–5]. Incorporation of a
methyl group into the meta- or para-position of benzo-
nitrile anion-radical did not affect the orientation of alkyla-
tion in reactions of m-tolunitrile (Ib) [6] and p-tolunitrile
(Ic) [1] with butyl bromide in liquid ammonia: the resulting
products were butylarenes IIb, c and cyanobutylcyclo-
hexadienes IIIb, c originating from cycnohexadienyl
anion IV as the common precursor (see scheme).
Besides with nitrile Ic due to its high CH-acidity
formed a product of cyanobenzyl anion V alkylation, p-
amylbenzonitrile (VI).
A similar orientation in reaction of nitrile Ia anion-
radical with cyclopropylmethyl bromide affording
benzylcyclopropane (VIIa) and in alkylation of anion-
radicals from nitriles Ib, c with butyl bromide furnishing
1-butyl-substituted compounds IIb, c and IIIb, c is an
indirect evidence of the fact that introducing a methyl
group into meta- or para-position of the arenecarbonitrile
anion-radical does not result in a replacement of SN
mechanism with ET one. On the other hand, on the force
of electrochemical data it was formerly inferred that
tolunitrile anion radicals in reactions with primary alkyl
halides showed both nucleophilic (SN mechanism) and
electron-donating (ET mechanism) reactivity [7]. Yet
these data do not allow a conclusion on the alkylation
orientation inherent to each of concurrent mechanisms.
In extension of these investigations we report here on
the study of reaction of anion-radicals generated by
sodium action on nitriles Ib, c in liquid ammonia (Na–
–2
ArCN ratio 9:10, concentration £10 mol l-1, –33°C), with
cyclopropylmethyl bromide applied as a reagent for testing
a mechanism. It was established that the reaction of nitrile
Ib anion-radical gave rise to a product of cyano group
replacement, m-methylbenzylcyclopropane (VIIb)
isolated in ~35% yield (maximal possible yield 50%), and
no compounds with isomerized alkyl fragment were
detected. Taking into account the estimates of
* For communication XVI see [1].
1070-4280/04/4005-0747Ó2004 MAIK “Nauka/Interperiodica”

748
VAGANOVA, SHTEINGARTS
Scheme.
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I, X = H (a), m-CH3 (b), p-CH3 (c).
cyclopropylmethyl bromide in going from nitriles Ia, b to
Ic and, secondly, in reaction of nitrile Ic anion-radical
with alkyl bromide in going from butyl bromide to
cyclopropylmethyl bromide. In the first case the change
in the mechanism is likely caused by the growing electron-
attracting ability of the anion-radicals in nitrile series Ia–
c (cf., for instance, the corresponding values of –E°
measured in DMF against standard calomel electrode:
2.23, 2.26, 2.34 [8]). In the second case the reason may
be the easier reduction of cyclopropylmethyl bromide
compared to butyl bromide due to overlapping of the
p-orbital of the lone electron with the quasi-p-system of
the cyclopropyl ring [9] in the cyclopropylmethyl radical.
The latter arising by electron transfer from the nitrile anion-
radical to cyclopropylmethyl bromide transforms further
along two concurrent routes: by reaction with the initial
anion-radical, and by isomerization into a 3-butenyl radical
[10] that in its turn also reacts with the anion-radical. Yet
in contrast to analogous reaction of terephthalonitrile
anion-radical [3, 4] in this case the alkyl radicals do not
undergo recombination with the nitrile Ic anion-radical
but are reduced therewith giving the corresponding alkyl
anions. This difference may originate from either higher
isomerization extent expected at realization of the ET
mechanism [2–5] the above experimental data support
the assumption [6] that the anion-radical of nitrile Ib reacts
with the primary alkyl bromides as nucleophile.
In contrast the reaction of nitrile Ic anion-radical with
cyclopropylmethyl bromide gave the ipso-alkylation
product, p-methylbenzylcyclopropane (VIIc) in a low yield
(£3%), and the main products were p-(cyclopropyl-
ethyl)benzonitrile (VIn) (11%) and p-(4-pentenyl)-
benzonitrile (VIi) (15%).At a 5-fold increase in the anion-
radical concentration the overall yield of nitriles VIn, i
did not change, but the ratio VIn/VIi grew from 0.7 to
1.7. As mentioned before, in reaction of nitrile Ic anion-
radical with butyl bromide the alkylation at the methyl
group providing nitrile VI occured simultaneously with
formation of diene IIIc generated due to protonation by
the substrate of anion IV followed by alkylation of arising
therewith cyanbenzyl anion V. Unlike that in the reaction
mixture obtained from nitrile Ic and cyclopropylmethyl
bromide diene of type III was lacking. The data obtained
apparently suggest that the process takes another route
and indicate that probably the SN mechanism gives way
to ET, firstly, in the reaction of anion-radical with
RUSSIAN JOURNALOF ORGANIC CHEMISTRY Vol. 40 No. 5 2004

REDUCTIVEACTIVATION OFARENES: XVII.
749
electron-donating capacity of nitrile Ic anion-radical
compared to anion-radicals of dinitriles or the presence
of a methyl group in its para-positin where the highest
spin density is located [11]. The alkyl anions get protonated
either by the initial nitrile Ic or more probably by ammonia
producing amide anion. In both cases finally arises a
cyclobenzyl anion V that on alkylation affords VIn, i.
anion-radical was stronder suppressed by electron-
withdrawing substituent than electron-donating activity.
The effect of methyl group discovered in this study
suggests that the introduction of an electron-donating
substituent increased stronger the activity of
arenecarbonitrile anion-radical as one-electron reductant
than as nucleophile.
The procedures for generating and alkylating anion-
radical are similar to those described in [1, 5]. The
cyanobenzyl anion was obtained by adding nitrile Ic to a
dispersion of sodium amide in liquid ammonia.
We demonstrated by special experiment that anion V
generated by treating nitrile Ic with sodium amide actually
reacted with the cyclopropylmethyl bromide furnishing
compounds VIn, i, and the ratio VIn/VIi increased from
–2
Characteristics of individual compounds are as follows:
17 to 65 at growing reagents concentration from 10 to
–1
10 mol l-1. The presence of product VIi containing an
1
p-(Cyclopropylethyl)benzonitrile (VIn). H NMR
isomerized alkyl fragment indicates that reaction of anion
V with cyclopropylmethyl bromide involves at least
partially an electron transfer resulting in formation of
cyclopropylmethyl radical that undergoes fast
isomerization, and thus reaction occurs by SRN
mechanism. Examples of concurrent occurrence with the
SN2 mechanism of similar processes initiated by solvated
electron are known for reactions with carbaniuons of a
number of benzyl and alkyl halides activated by electron-
attracting substituent [12]. The difference in the ratio VIn/
VIi obtained in reaction of cyclopropylmethyl bromide
with a specially prepared anion V and nitrile Ic anion-
radical is presumably due to the conciderably lower
concentration of anion V in the latter case and
consequently with a higher degree of cyclopropyl radical
isomerization.
spectrum (CDCl3), d, ppm: –0.02–0.06 m (2H), 0.37–
0.47 m (2H), 0.53–0.73 m (1H, cyclo-C3H5), 1.51 q (2H,
C2H2, J 8 Hz), 2.75 t (2H, C1H2, J 8 Hz), 7.27 d (2H, H3,5,
Jo 8 Hz), 7.51 d (2H, H2,6). Found: M+ 171.10472.
C12H13N.. Calculated: M 171.10479.
p-(4-Pentenyl)benzonitrile (VIi). 1H NMR spec-
trum (CDCl3), d, ppm: 1.73 m (2H, C2H2), 2.08 m (2H,
C3H2), 2.67 t (2H, C1H2, J 8 Hz), 4.94–5.07 m (2H,
C5H2), 5.70–5.90 m (1H, C4H), 7.28 d (2H, H3,5, JO 8
Hz), 7.52 d (2H, H2,6). Found: M+ 171.10472. C12H13N.
Calculated: M 171.10479.
m-Methylbenzylcyclopropane (VIIb). 1H NMR
spectrum (CDCl3), d, ppm: 0.17–0.23 m (2H), 0.46–0.53
m (2H), 0.86–0.96 m (1H, cyclo-C3H5), 2.33 s (3H, CH3),
2.50 d (2H, CH2, J 8 Hz), 6.98–7.22 m (4H, H2,4,5,6).
Found: M+ 146.10966. C11H14.. Calculated: M 146.10954.
p-Methylbenzylcyclopropane (VIIc). 1H NMR
spectrum (CDCl3), d, ppm: 0.14–0.20 m (2H), 0.45–0.52
m (2H), 0.81–0.91 m (1H, cyclo-C3H5); 2.31 s (3H, CH3),
2.48 d (2H, CH2, J 8 Hz), 7.09, 7.13 AB-system (4H,
H2,3,5,6). Found: M+ 146.10963. C11H14. Calculated: M
146.10954.
1H NMR spectra were registered on spectrometer
Bruker WP-200SY, high resolution mass spectra were
measured on Finnigan MAT-8200 instrument.
Thus the results obtained evidence firstly in favor of
mechanism SN in the previously studied [1, 6] reaction of
nitriles Ib, c anion-radicals with butyl bromide, and
secondly reveal the difference in effects of methyl group
from the meta- and para-positions of benzonitrile anion-
radical on its reactivity due apparently to localization in
the para-position of the main part of the electron density
from the lone electron. Whereas in goimg from benzonitrile
to m-tolunitrile the ratio of anion-radical reactivities with
respect to SN and ET mechanisms does not apparently
significantly alter and it behaves as nucleophile in
reactions with alkyl bromides, in going to p-tolunitrile the
electron-donating activity of anion-radical relative its
nucleophilic activity increases and thus in reaction with
cyclopropylmethyl bromide possessing higher electron
affinity than that of n-alkyl bromides the SN mechanism
is replaced by ET one. It was formerly shown that a
similar change in mechanism occurred in going from
benzonitrile to dicyanoarenes [3–5], and therefore a
conclusion was done that the nucleophilic reactivity of
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