Spin chemistry of organometallic compounds: Part 1. Interaction of N-bromohexamethyldisilazane with allyltriethylstannane

Article abstract of DOI:10.1016/S0022-328X(01)00812-9

It has been shown, on the basis of the analysis of CIDNP effects, that allene and (Me₃Si)₂NH - major products of the photoinduced reaction of Et₃SnCH₂CH=CH₂ and (Me₃Si)₂NBr - result from a radical pair comprised of ·CH₂CHBrCH₂SnEt₃ and ·N(SiMe₃)₂ free radicals. Allene is formed through the β-cleavage of the short-lived homolytic substitution product Et₃SnCH₂CBr=CH₂.

Full text of DOI:10.1016/S0022-328X(01)00812-9

Journal of Organometallic Chemistry 636 (2001) 12–16
www.elsevier.com/locate/jorganchem
Spin chemistry of organometallic compounds
Part 1. Interaction of N-bromohexamethyldisilazane with
allyltriethylstannane
a,
a
b
Marc B. Taraban *, Alexander I. Kruppa , Vladimir I. Rakhlin ,
b
a
b
Stanislav I. Grigor’ev , Olga S. Volkova , Rudolph G. Mirskov ,
a
Tatyana V. Leshina
a
Institute of Chemical Kinetics and Combustion, Institutskaya Street 3, 630090 No6osibirsk 90, Russia
b
Irkutsk Institute of Chemistry, 664033 Irkutsk 33, Russia
Received 11 January 2001; accepted 30 March 2001
Abstract
It has been shown, on the basis of the analysis of CIDNP effects, that allene and (Me Si) NH — major products of the
3
2
’
photoinduced reaction of Et SnCH CHꢀCH and (Me Si) NBr — result from a radical pair comprised of CH CHBrCH SnEt
3
2
2
3
2
2
2
3
’
and N(SiMe3)2 free radicals. Allene is formed through the b-cleavage of the short-lived homolytic substitution product
Et SnCH CBrꢀCH . © 2001 Elsevier Science B.V. All rights reserved.
3
2
2
Keywords: Homolytic substitution; Free radicals; b-Cleavage; Radical pairs; CIDNP
1
. Introduction
the examples of these unusual transformations, the
photoinitiated reaction of allyltriethylstannane,
Et SnCH CHꢀCH , with N-bromohexamethyldisila-
Photoinduced reactions of the homolytic addition of
3
2
2
various reagents to multiple bonds of unsaturated or-
ganic derivatives of Group 14 elements, particularly to
corresponding tin derivatives, are of great importance
as synthetic routes to a number of rather complex
alycyclic and heterocyclic compounds [1]. The analysis
of the basic peculiarities of these processes allows to
conclude that otherwise almost inaccessible unusual
products could also be formed in the reactions of
homolytic substitution of hydrogen atom in the organic
substituent of organotin species; in particular, if the
hydrogen atom is substituted by halogen. A major
challenge is to select a proper reagent capable of having
regiospecific interaction with the substrate which would
proceed solely via free-radical mechanism, since the
ionic reactions usually lead to the elimination of an
organic substituent. The present study considers one of
zane, (Me Si) NBr, leading to the high yield of allene
3 2
CH ꢀCꢀCH2 (ca. 92%). The bromination reagent
2
(
Me Si) NBr has been selected on the basis of reference
3 2
data [2] demonstrating that its interaction with olefins
results in the products of allylic bromination. It has
been suggested [2] that the reaction proceeds via free-
radical mechanism, and this forms the grounds to as-
sume that the reaction of (Me Si) NBr with
3
2
Et SnCH CHꢀCH will also follow the allylic bromina-
3
2
2
tion mechanism.
If the process under study indeed follows a free-radi-
cal pathway, i.e. involves spin selective reaction stages,
it would be possible to elucidate the mechanism of
allene formation by means of the CIDNP method. This
is the main goal of the present paper.
It has been shown earlier [3] that the application of
spin chemistry methods to the investigations of pho-
toinduced reactions of organometallic compounds al-
lows to derive important data on the elementary
mechanisms of these processes.
*
Corresponding author. Tel.: +7-3832-331405; fax: +7-3832-
3
0
42350.
E-mail address: taraban@ns.kinetics.nsc.ru (M.B. Taraban).
022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00812-9

M.B. Taraban et al. / Journal of Organometallic Chemistry 636 (2001) 12–16
13
2. Experimental
analysis of the reaction mixture revealed no traces of
allene.
Standard techniques were used to synthesize allyltri-
ethylstannane, Et SnCH CHꢀCH [4], and N-bromo-
3
2
2
hexamethyldisilazane, (Me Si) NBr [5]. Their physical
3
2
3. Results and discussion
constants matched with the reference data, and the
purity grade was not lower than 99.5% (determined by
GLC).
The photolysis of the reaction mixture of
Et SnCH CHꢀCH and (Me Si) NBr directly in the
3
2
2
3
2
probe of the NMR spectrometer has allowed the detec-
1
2
.1. Reaction of Et SnCH CHꢀCH with (Me Si ) NBr
3
2
2
3
2
tion of H-CIDNP effects (Fig. 1) of the methyl protons
of (Me Si) NBr and (Me Si) NH, and the methylene
3
2
3
2
A mixture of 13 g (0.052 M) of Et SnCH CHꢀCH
3
2
2
protons of allene. The spectrum also demonstrates the
polarization effects in the region of olefin protons of
the initial Et SnCH CHꢀCH .
and 12.6 g (0.052 M) of (Me Si) NBr was subjected to
3
2
UV irradiation (UV lamp DRT-375) during 10 h in a
sealed vacuum tube. The tube was opened and the
allene collected (2 g, ca. 92%).
3
2
2
Thus, the following equation describes the brutto-re-
action of Et SnCH CHꢀCH with (Me Si) NBr:
3
2
2
3
2
−
1
IR data (KBr) of the gaseous product (cm ): 1870,
910 (wCꢀCꢀC); 950 (lꢀCH2); 1440 (kꢀCH2); 3000, 3090,
120 (wꢀCH2). The observed frequencies correspond to
hw
Et SnCH CHꢀCH +BrN(SiMe ) “Et SnBr
1
3
3
2
2
3 2
3
+HN(SiMe ) +H CꢀCꢀCH
3
2
2
2
the reference spectrum of allene [6].
GLC analysis of the liquid fraction using reference
substances has shown that its basic components include
hexamethyldisilazane, (Me Si) NH, and bromotriethyl-
According to the data published earlier [2], the pri-
mary act of the process is the homolytic decomposition
of (Me Si) NBr (generation stages, Scheme 1). Note
3
2
3
2
stannane, Et SnBr, in nearly equimolar proportion. The
that Et SnCH CHꢀCH is stable under UV irradiation
3
3
2
2
products were separated by fractional distillation; their
physical constants matched with the reference data.
After the photolysis of Et SnCH CHꢀCH the com-
and does not form the photocleavage products in the
absence of initiators, in this case, (Me Si) NBr. Further
3
2
reaction stages include the interactions of Et SnCH -
3
2
2
3
2
’
position of the products was analyzed using a GLC
Tsvet-500’ (heat conductivity detector, He, stainless
CHꢀCH2 with the (Me Si) N radical and bromine
3
2
‘
atom. Previous CIDNP studies [3] of the photoinduced
steel column, 4 mm×3 m, 10% PMS-1000 on a Chro-
maton N-AW-HMDS, 0.2–0.25 mm grain).
H-CIDNP spectra were detected by means of a
JEOL JNM FX90Q high-resolution NMR spectrometer
interaction of Et SnCH CHꢀCH with BrCCl3 have
3
2
2
shown that bromine adds to the least hydrogenated
1
carbon atom of Et SnCH CHꢀCH . It has been shown
3
2
2
in Ref. [3] that the brominated product with the
bromine atom in b-position to triethyltin substituent
undergoes fast b-cleavage — only polarized signals of
this product were detected and no traces could be seen
in the NMR spectrum after photolysis. In the present
case, CIDNP effects of the allene protons suggest that
similar to the main product of the photolysis of
(
operating at 90 MHz frequency) equipped with a
special in-house device for the irradiation of the sam-
ples directly in the probe of the spectrometer. The
irradiation was carried out in standard Pyrex 5 mm
NMR tubes using the full light of a DRSH-1000 high-
pressure mercury lamp (1 kW). A thermal filter was
used to prevent the heating of the sample.
Et SnCH CHꢀCH
with BrCCl3 (trichlorobutene
3
2
2
CH ꢀCHCH CCl [3]), CH ꢀCꢀCH results from the
2
2
3
2
2
2
.2. Reaction of Et GeCH CHꢀCH with (Me Si ) NBr
b-cleavage of the brominated product Et SnCH C-
3
2
2
3
2
3
2
BrꢀCH (I, Scheme 1). Compound I originates from the
2
’
disproportionation of the radical pair of CH CHBr-
2
A mixture of 20 g (0.099 M) of allyltriethylgermane
and 23.9 g (0.099 M) of (Me Si) NBr was subjected to
’
CH SnEt and (Me Si) N radicals; another product of
3
2
2
3
3
2
UV irradiation (UV lamp DRT-375) during 10 h in a
sealed vacuum tube. The tube was then opened; how-
ever, the analysis of the reaction mixture revealed no
traces of allene.
this pair is polarized (Me Si) NH, since according to
3 2
’
2
Ref. [2] the (Me Si) N radical readily abstracts hydro-
3
gen from the hydrocarbon radical and does not enter
the recombination reaction.
The analysis of the observed CIDNP effects, which
form the basis of Scheme 1, has been carried out
2
.3. Reaction of Et SiCH CHꢀCH with (Me Si ) NBr
3
2
2
3
2
’
assuming that the g-factor of the (Me Si) N radical is
3
2
A mixture of 10 g (0.038 M) of allyltriethylsilane and
6 g (0.066 M) of (Me Si) NBr was subjected to UV
irradiation (UV lamp DRT-375) during 10 h in a sealed
vacuum tube. The tube was then opened; however, the
greater than that of its carbon-centered partner in the
radical pair, and the HFI constants of Me protons in
1
3
2
’
(Me Si) N radical are positive. The bromosubstituted
3 2
’
CH CHBrCH SnEt radical has the following HFI
2
2
3

14
M.B. Taraban et al. / Journal of Organometallic Chemistry 636 (2001) 12–16
constants: A(a-CH )B0, A(CHBr)\0, A(CH Snꢁ)
there are several unassigned CIDNP signals in the
region of the protons of the initial compound
Et SnCH CHꢀCH (Fig. 1), which could be attributed
to the transformations of this radical. However, both
NMR and chromatographic analysis of the reaction
mixture have not identified any products that could be
associated with the above allylic radical. Therefore, it
would be quite reasonable to refrain from speculations
concerning the pathways of generation and decay of
2
2
B0 [7].
The results of the analysis of CIDNP effects are in
agreement with the above assumption on the reaction
mechanism. On the other hand, it has been demon-
strated in Ref. [8] that compounds with bromine atom
3
2
2
in b-position to Alk Si and Alk Ge groups (in contrast
3
3
to their tin analogs) are quite stable in solution, and the
reaction of b-cleavage is observed only during the at-
tempt to separate them. If the above assumptions on
the mechanism of CH ꢀCꢀCH formation are correct
Et SnCHCHꢀCH2.
:
3
2
2
Let us again emphasize the unusual character of the
observed results. It is a common belief that radicals
usually attack the terminal g-carbon atom of the allylic
substituent. Indeed, CIDNP effects detected in the pho-
tolysis of Et SnCH CHꢀCH in the presence of BrCCl
(
Scheme 1), the high yield of allene should not be
observed in the photoreactions of allyltriethylsilane and
germane with N-bromohexamethyldisilazane. Our ex-
-
periments have shown that the mixture after the reac-
3
2
2
3
tion does not contain CH ꢀCꢀCH at all (see Section
2
2
[3] unambiguously point to the formation of the radi-
2). Instead, the resulting mixture contains a complex set
cal-adduct Et SnCH CHCH CCl3 resulting from the
:
3
2
2
of products, whose analysis is not a part of the present
study.
’
addition of the CCl radical to the terminal g-carbon.
3
In the case under study, we do not observe the products
that could stem from the analogous addition of the
’
(Me Si) N radical to the allylic g-carbon or CIDNP
3 2
Another polarized product, initial N-bromohexam-
ethyldisilazane, could result in the reaction of chemical
’
exchange between the polarized (Me Si) N radical with
3
2
effects that could be ascribed to certain unstable para-
or diamagnetic intermediates formed through the said
the initial (Me Si) NBr (chemical exchange, Scheme 1).
3
2
Earlier research [2] of the photolysis of (Me Si) NBr
3
2
’
addition. Instead, the bromine Br attacks the least
in the presence of olefins also suggested that in the
presence of readily abstractable allylic hydrogen, the
hydrogenated b-carbon atom of allylstannane and the
’
:
(
Me Si) N radical is capable of producing the allylic
resulting Et SnCH CHBrCH radical becomes the pre-
3 2 2
3
2
type radicals; in the present case, this will correspond to
the formation of the Et SnCHCHꢀCH radical. Indeed,
cursor of the unusual products of the reaction of ho-
molytic substitution.
:
3
2
1
Fig. 1. 90 MHz H-NMR spectra of the reaction mixture of Et SnCH CHꢀCH with (Me Si) NBr in deuterobenzene. Bottom spectrum, the initial
3
2
2
3
2
reaction mixture; upper spectrum, CIDNP effects detected 20 s after the UV irradiation. Spectra show polarized signals corresponding to main
reaction products.

M.B. Taraban et al. / Journal of Organometallic Chemistry 636 (2001) 12–16
15
Scheme 1. Radical stages of the photolysis of allyltriethylstannane, Et SnCH CHꢀCH , with N-bromohexamethyldisilazane, (Me Si) NBr (A,
3
2
2
3
2
absorption; E, emission; F, free uncorrelated radical pair).
4
. Conclusions
‘Spin Chemistry of Organometallic Compounds’ and
the Russian Foundation for Basic Research (Project
No. 00-03-33048) is gratefully acknowledged. We thank
Dr Takayuki Uno and Mr Shin-ichi Otsu (Analytical
Instruments Division, JEOL Ltd.) who help us to main-
tain the JEOL NMR Spectrometer in running
condition.
Thus, the analysis of CIDNP effects observed during
the photolysis of Et SnCH CHꢀCH in the presence of
3
2
2
(Me Si) NBr as well as the quantitative yield of one of
3 2
the main products of this process, allene CH ꢀCꢀCH ,
shows that the b-cleavage pathway of the bromosubsti-
tuted allylic tin derivative Et SnCH CBrꢀCH with
bromine in b-position with respect to tin atom (Scheme
1
toinitiated reaction of Et SnCH CHꢀCH with bro-
motrichloromethane BrCCl [3] the present case does
2
2
3
2
2
) is prevalent. In contrast to the earlier studied pho-
References
3
2
2
3
[
1] V.I. Rakhlin, R.G. Mirskov, M.G. Voronkov, Zh. Org. Khim. 32
1996) 807 (in Russian).
not demonstrate the formation of a radical-adduct
which could be expected to result from the addition of
(
[2] B.P. Roberts, C. Wilson, J. Chem. Soc. Chem. Commun. 17
(1978) 752.
’
(Me Si) N radical to Et SnCH CHꢀCH .
3
2
3
2
2
[
3] T.V. Leshina, R.Z. Sagdeev, N.E. Polyakov, V.I. Valyaev, M.B.
Taraban, V.I. Rakhlin, R.G. Mirskov, S.Kh. Khangazheev, M.G.
Voronkov, J. Organomet. Chem. 259 (1983) 295.
[
4] W.J. Jones, W.J. Davies, S.T. Bowden, C. Edwards, V.E. Davis,
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Acknowledgements
[
[
5] N. Wiberg, F. Raschig, Angew. Chem. 77 (1965) 130.
6] K. Nakanishi, Infrared Absorption Spectroscopy, Nankado Co.
Ltd, Tokyo, 1962.
Financial support of the Integration Programme of
the Siberian Division of Russian Academy of Sciences

16
M.B. Taraban et al. / Journal of Organometallic Chemistry 636 (2001) 12–16
’
slightly increase this value. The g-factor of CH CHBrCH SnEt
2 2 3
[
7] According to reference data from: H. Fischer, K.-H. Hellwege
Eds.), Landolt–Boernstein New Series. Numerical Data and
(
is 52.003, analogous species could be found in the reference book.
[8] T.V. Leshina, V.I. Valyaev, M.B. Taraban, V.I. Maryasova, V.I.
Rakhlin, S.Kh. Khangazheev, R.G. Mirskov, M.G. Voronkov, J.
Organomet. Chem. 299 (1986) 271.
Functional Relationship in Science and Technology, Springer,
’
Berlin, 1979, Parts B–D. The g-factor of R N radicals amounts
2
to 2.0040, and the presence of the attached Me Si groups will
3