A new method for the generation of stable phenoxyl radicals by the reaction of [(Me3Si)2N]2E (E = Ge, Sn) with sterically hindered phenols

Article abstract of DOI:10.1023/A:1021341627253

An unusual reaction of diaminogermylene and diaminostannylene with sterically hindered phenols which leads to the formation of stable phenoxyl radicals in high concentrations was found. The reaction mechanism was proposed. Amides [(Me₃Si)₂N]₂E (E = Ge, Sn) were investigated electrochemically.

Full text of DOI:10.1023/A:1021341627253

Russian Chemical Bulletin, International Edition, Vol. 51, No. 10, pp. 1961—1962, October, 2002
1961
A new method for the generation of stable phenoxyl radicals
by the reaction of [(Me3Si)2N]2E (E = Ge, Sn)
with sterically hindered phenols
I. S. Orlov,a А. А. Moiseeva,b К. P. Butin,b M. P. Egorov a
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: (095) 135 5328. Eꢀmail: mpe@ioc.ac.ru
bMoscow State University, Department of Chemistry,
Leninskie Gory, 119899 Moscow, Russian Federation.
Fax: (095) 932 8846. Eꢀmail: butin@org.chem.msu.ru
An unusual reaction of diaminogermylene and diaminostannylene with sterically hindered
phenols which leads to the formation of stable phenoxyl radicals in high concentrations was
found. The reaction mechanism was proposed. Amides [(Me3Si)2N]2E (E = Ge, Sn) were
investigated electrochemically.
Key words: diaminogermylene, diaminostannylene, sterically hindered phenols, deꢀ
protonation reaction, ESR.
Divalent germanium and tin amides occupy a particuꢀ
lar place among carbene analogs. This is due to the presꢀ
ence of two reaction centers, viz., a twoꢀcoordinated gerꢀ
manium or tin atom and labile polar bonds Ge(Sn)—N,
which can easily be cleaved by sufficiently acidic reagents
(for example, NHꢀ and OHꢀ acids). Thus, depending on
the structure of reagents, GeII and SnII amides can unꢀ
dergo reactions typical of both carbene analogs (insertion
into σꢀbonds, addition to multiple bonds, complexation
reaction)1 and GeIV and SnIV amides playing a role of
metallation agents2—4 and leading to new germylenes
(stannylenes). For the metallation reactions, stable moꢀ
nomeric carbene analogs of the type [(Me3Si)2N]2E (E =
Ge (1a), Sn (1b)) were found to be the most convenient.
Particularly, reaction of amides 1 with sterically hindered
phenols ArOH (2) afford stable compounds (ArO)2E (3)2.
For the first time it was found that this metallation reacꢀ
tion (20 °C, C6H6) is accompanied by the formation
of high concentrations of phenoxyl radicals ArO• (4)
(Scheme 1).
The stable radicals 4 were identified by ESR spectrosꢀ
copy. ESR spectra of radicals 4 are identical to those
described in the literature5. The reaction of stannylene 1b
with phenols 2 leads to the formation of radicals 4 in
substantially higher concentrations (up to 2 orders of magꢀ
nitude) than the reaction of 2 with germylene 1a. The
formation of radicals 4 is not connected with the decomꢀ
position reaction of carbene analogs 3, which are the
main reaction products of 1 with 2. Indeed, according to
1
the H NMR data carbene analogs 3 are stable in a soluꢀ
1
tion at room temperature. Besides, H NMR monitoring
of the reaction of 1 with 2 did not reveal the formation of
Scheme 1
E = Ge (1a), Sn (1b);
,
R = But (2a), C(Me)=NOH (2b), Me (2c), CN (2d), Pt(PPh3)Cl (2e)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1808—1809, October, 2002.
1066ꢀ5285/02/5110ꢀ1961 $27.00 © 2002 Plenum Publishing Corporation

1962
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 10, October, 2002
Orlov et al.
carbon rod (diameter 1.8 mm), background electrolyte was
0.1 М solution of Bu4NСlO4 in ТHF, the reference electrode
was Ag/AgCl/KCl (sat.); THF was distilled over benzophenone
ketyl in an atmosphere of dry Ar directly to an electrochemical
cell. For SEꢀESRꢀexperiments, the electrochemical cell designed
by A. B. Ilyasov and Yu. M. Kargin was used9. This was a Pyrex
tube (diameter 5 mm) with the working electrode as a Pt helix
(diameter of the Pt wire 0.3 mm), an auxiliary Pt electrode
(diameter of the Pt wire 0.3 mm), and an Ag wire (diameter
0.4 mm) as a reference electrode. The cell was supplied with a
vacuum valve.
any intermediate compounds (for example, products of
insertion reaction of 1 into H—O bond of phenols 2),
whose decomposition could lead to radicals 4.
In order to estimate a possibility of the formation of
radicals 4 by a redox reaction of carbene analogs 1 with
phenols 2, we carried out electrochemical investigation of
amides 1. It was shown that both compounds are reduced
irreversibly at high cathode potentials (—2.83 (1a) and
–2.45 V (1b)). Therefore, the pathway of formation of
phenoxyl radicals 4 by the oxidation of phenols 2 by
carbene analogs 1 can be completely excluded. Comꢀ
pounds 1 are oxidized irreversibly at rather low anodic
potentials, 0.72 (1a) and 1.34 V (1b).
In the simultaneous electrochemical — ESR investiꢀ
gations (SEꢀESR) of the reduction and the oxidation of
compounds 1 we did not manage to observe any paramagꢀ
netic species either at room temperature or at lower temꢀ
perature (200 К), which suggests the short lifeꢀtimes of
the initial radical ions of the carbene analog 1.
The ESR and NMR spectra were recorded at 20 °C on Bruker
EMX 6ꢀ1 and Bruker AMꢀ200 instruments, respectively.
Reaction of 1 with phenols 2. Germylene 1a (61 mg,
0.156 mmol) was placed in an NMR tube and 0.5 mL of dry
C6D6 was added. Phenol 2а (35 mg, 0.134 mmol) was added in
three equal portions to the solution of 1а. After addition of the
phenol, the solution assumes a green colour. Broadened signals
1
of (Me3Si)2NH and phenolate 4а appear in the H NMR specꢀ
trum of the reaction mixture and a signal for phenoxyl radical 4a
appears in the ESR spectrum. Other reactions of amides 1 with
phenols 2 were carried out similarly.
Hence, the formation of phenoxyl radicals 4 is not
connected with either the decomposition of stable carbene
analogs 3 or other intermediates or with the redox reacꢀ
tions of 1 with 2. The most likely way of the formation
of radicals 4 could be hydrogen abstraction from the
OH group of phenols 2 under the action of amides 1
(Scheme 1, reaction (1)), which proceeds simultaneously
with the main process, viz., metallation of phenols
(Scheme 1, reaction (2)). Obviously, the other product of
the reaction (1), namely the elementꢀcentered radical 5,
is labile and its ESRꢀsignal cannot be observed against a
background of the intense signal of stable radical 4.
The hydrogen abstraction can occur directly or result
from the fragmentation of an intermediate donorꢀaccepꢀ
tor complex between 1 and 2. It should be noted that the
formal abstraction of two hydrogen atoms from cycloꢀ
1,3ꢀdiene with the formation of benzene by tin (II) amide
is documented6.
SEꢀESR investigation of 1. Bu4NСlO4 (136 mg) and
germylene 1а (stannylene 1b) (20 mg) were placed into an elecꢀ
trochemical cell for SEꢀESR in an atmosphere of dry Ar. THF
(2 mL) was distilled from Ph2CO/Na directly to the cell. The
cell was degassed and evacuated. ESR spectra were recorded in
the intervals of potentials from 0 to –3.0 V and from 0 to +3.0 V.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos. 00ꢀ15ꢀ
97387, 02ꢀ03ꢀ32148), INTAS (grant 97ꢀ30344), and the
Ministry of Industry, Science, and Technology of the
Russian Federation.
References
1. W. P. Neumann, Chem. Rev., 1991, 91, 311.
2. B. Cetinkaya, I. Gumrukcu, M. F. Lappert, J. L. Atwood,
R. D. Rogers, and M. J. Zawarotke, J. Am. Chem. Soc., 1980,
102, 2088.
3. M. Veith, A. Rammo, S. Faber, and B. Schillo, Pure Appl.
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4. J. Barrau and G. Rima, Coord. Chem. Rev., 1998,
178—180, 593.
5. V. V. Ershov, G. А. Nikiforov, and А. А. Volodkin, Sterically
Hindered Phenols, Khimiya, Moscow, 1972, 351 pp (in
Russian).
The reaction (1) is apparently specific for carbene anaꢀ
logs 1. Our attempts to perform it with gemylenes and
stannylenes of other structures (GeMe2, GeCl2•dioxane,
GeCl2•PPh3, GeI2, SnCl2, SnCl2•dioxane, SnI2,
[Sn(NEt2)2]x) were unsuccessful. The reaction (1) is the
first example of the reaction where carbene analogs with
Group 14 elements abstract hydrogen from the reagent.
6. M. Veith and F. Tollner, J. Organomet. Chem., 1983, 246, 219.
7. A. J. Gordon and R. A. Ford, The Chemist´s Companion, Wiley
Interscience Publication, New York—London—Sydney—
Toronto, 1972.
8. М. J. S. Gynane, D. H. Harris, M. F. Lappert, P. P. Power,
P. Riviere, and M. RiviereꢀBaudet, J. Chem. Soc., Dalton
Trans., 1977, 2004.
Experimental
All reactions and measurements were carried out in an atꢀ
mosphere of dry Ar. Benzene, deuterobenzene, and THF were
dried and degassed according to standard procedures7. Stable
germylene 1a and stannylene 1b were synthesised according to
the published procedures8. Specimens of phenols 2 were kindly
provided by E. P. Milaeva.
9. A. V. Il´yasov and Y. M. Kargin, Magn. Res. Rev., 1993,
16, 135.
Electrochemical investigations were carried out using a
potentiostate PIꢀ50ꢀ1.1. The working electrode was a glassyꢀ
Received April 29, 2002