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Metathesis of Alkyl Radicals and AlkenesЦa Novel Elementary Reaction of Alkyl Radicals.

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pretation is supported by the significantly lengthened Si-C
bonds (I99 pm compared with 187 pm for the single bond)
and the expanded angle at the tertiary C atoms (C-C-C
between 104.2 and 106.5", Si-C-C between 110.9 and
114.8"). The Si-Si distance in 1 corresponds to a Pauling
bond order of 0.45 and, as in the case of hexa-terr-butylcyclotrisilane,"' the sum of the bond orders for the Si atom in
1 (2.55) is considerably less than the expected value of
four.
The variability in the distance between Si atoms is remarkable. The distance for a bonding interaction found in
I is larger than that in fibrous SiOz (258 pm@'), although
the interaction in the latter case is repulsive. The extreme
case of the approach of Si atoms toward one another as a
consequence of their surrounding was recently found in
cyclosiloxanes (-R2Si-0-),."] In that case, the Si-Si distance is shorter (231 pm) than that of a Si-Si single bond.
+ CHz=CH-R'
RH + OR'
+O R + CH,-?H-R~
(d)
i
OR+ HR'
(b)
R-CH,-CHq
i
R-CH=CH,
RH
R-CHZ-CHY
+ CH,=CH-R'
+ Ho
(4
+
R-CH=CH2
+ CH,-?H-R'
(d)
known.''] The first experimental example of reaction (d) is
reported here.
Reaction (d) should be possible to observe as a reaction
competing with the addition (e) of an alkyl radical to the
C C double bond of an alkene
2 R~-0
C H - C H ~ - R ~+
Received: September 12, 1985;
supplemented: October 16. 1985 [Z 1461 IE]
German version: Angew. Chem. 98 (1986) 100
I R~-CH=CH-R~
[I]A. Schafer, M. Weidenbruch, K. Peters, H. G. von Schnering, Angew.
Chem. 96 (1984) 31 1: Angew. Cliem. Inf. Ed. Engl. 23 (1984) 302.
[2] Experimentalprocedure: Potassium (0.782 g, 20.0 mmol) and tri-tert-butyliodosilane (3.263 g, 10.0 mmol) [4] were allowed to react in refluxing heptane for 6 h. Nitrosyl tetrafluoroborate (0.771 g, 6.60 mmol) in 20 mL of
heptane was added to the tBu,SiK-containing reaction solution so
formed after it had been cooled to - 100°C. The reaction mixture was
warmed to room temperature and filtered to separate the colorless precipitate (KBF,). All volatiles were removed from the solvent at room temperature in high vacuum and the residue was dissolved in 30 m L of pentane. Colorless, crystalline 1 (0.58 g, 1.46 mmol, 44%) was obtained at
- 30°C.
[ 3 ] 1: 'H-NMR (C6D,): 6=1.379. rBu,SiX ( X = H , halogen): ' H - N M R
(C,D,,): 6= 1.12-1.16. 1: "C-NMR (C6Dn): 6=27.28 (C(CH,),), 34.63
(C(CH,),); ? S - N M R (C,D6): 6=28.7; MS (15 ev) m / z 341 ( M + - f B u ,
1 I%),
199 (tBu,Si', IS%), 115 (fBu3Si+-2C,H6, IOOYo). Me,Si-SiMe,:
Raman: bfSiLSi)=404 c m - ' ; longest wavelength UV absorption:
L2,,,=198
nm, E ca. SO00 (cf. E. Hengge, Top. Curr. Chem. 51 (1974)
52)
141 M. Weidenbruch, H. Pesel, W. Peter, R. Steichen, J. Urganomet. Chem.
141 (1977) 9.
[5] Structure dam: Four-circle diffractometer PZ,, room temperature, MoK,,,
graphite monochromator; 3061 observed reflections, 1742 with F>3cr(F);
Ibca, a= 1135.5(7), b=2944.8(13), c = 1594.8(7) pm, Z=X,p,,,,,=O.994
g
cm-'; R=0.063 for anisotropic refinement of the Si and C atoms (H
atoms isotropic). Further details of the crystal structure investigation are
available on request from the Fachinformationszentrum Energie, Physik,
Mathematik GmbH, D-7514 Eggenstein-Leopoldshafen 2, on quoting the
depository number CSD-51574, the names of the authors, and the journal
citation.
[6] A. Weiss, A. Weiss, Z. Anorg. Allg. Chem. 276 (1954) 93.
171 M. J. Fink, K. J. Haller, R. West, J. Michl, 1.Am. Chem. Soc. 106 (1984)
822.
Metathesis of Alkyl Radicals and Alkenesa Novel Elementary Reaction of Alkyl Radicals
By Jiirgen 0. Metzger*
A notable gap exists in the series of known elementary
reactions (a)-(c) of alkyl radicals in which a hydrogen
atom is transferred: the thermochemically possible transfer
(d) of a f$-hydrogenatom of an alkyl radical to an alkene
with the formation of a new alkyl radical and a new alkene.
The search for an example of the frequently postulated"'
reaction (d) appeared promising since corresponding reactions in analogous oxygen-containing systems are
[*J Priv.-Doz. Dr. J . 0. Metzger
Fachbereich Chemie der Universitat
Postfach 2503, D-2900 Oldenburg (FRG)
80
0 VCH Verlagsgesellschaft mhH. 0-6940 Weinheim, 1986
s
/ R2
o
RI-CH-CH
+
R1
=
o
CH-CH-~2
4
C-CeH11
'-'EH1l
a, R1
\
+
R2 = COOMe: b, R' = Ph; R2 = COOMe
As an example, the addition of cyclohexane to dimethyl
fumarate l a and to methyl cinnamate l b was studied. This
radical chain reaction (0can be carried out above 250°C
under mainly thermodynamic control with respect to the
addition (e).I3I
+
R'-CH=CH-R2
c-C6~,,
1
+
:R
/
-
CH-CH,-R~
R2
R~-cH~-c$
5
c-C6H11
+ R'-CH,-CH,-R~
7
Besides the addition products 5 (via 3) and 6 (via 4),
the dihydro product 7 is formed from l a and l b in a reaction that is first order with respect to alkene l b . This may
be readily explained by the exothermic reaction (d) and
subsequent stabilization of the radical 2 by H abstraction
from the solvent cyclohexane. The complementary product, cyclohexene, was identified in the reaction mixture
(7a :cyclohexene, ca. 1.2 : 1) as the dibromo adduct.
The experimental results (Table 1) are easily understandable. The metathesis (d), as a "closed-shell" reaction,
has a higher activation energy than the "open-shell'' reaction (b). Accordingly, the amount of the dihydro product 7
increases relative to the amounts of addition products 5
and 6 with increasing temperature (experiments 1-5). The
reaction rate of (d) is independent of the concentration of
H donor in contrast to that of (b). Therefore, the relative
amount of 7 increases with lower concentrations of cyclohexane; 7 can even become the main product (experiments 5-9). The addition of radical formers such as 2,3d i p h e n y l b ~ t a n e [increases
~]
the rate of formation of both
competitive products equally (experiment 4). Activated
0570-0833/86/0101-0080 $ 02.50/0
Angew. Chem. Int. Ed. Engl. 25 11986) No. I
Table I . Reactions of cyclohexane with dimethyl fumarate l a and methyl cinnamate I b [a]. 5 a = 6 a
No.
1 <'I
[c-C,H,~]
Ibl
250
360
390
390
400
400
400
400
400
5.76
5.56
8.13
5.13
5.00
3.68
2.44
0.90
0.40
T
I [d]
2
3
4
5
6
1
8
9
Conversion l a
[Sa]
['la]
[Cl
[cl
[Cl
> 99
> 99
> 99
11
78
15.6
3.1
5.3
6.4
22.9
14.7
11.8
-
-
-
70.1
65.5
62.9
40.7
14.1
6.2
7.1
7.6
9.1
1.1
11.3
9.2
8.3
4.5
I .8
-
> 99
> 99
97
79
49
5a/7a
Conversion I b
[cl
[5b]
[cl
[6b]
[Cl
[7b]
11
4.2
22.7
9.5
18.9
20.1
14.6
8.2
1.7
0.4
1.3
8.8
3.8
7.1
8.3
5.9
3.4
0.7
0.2
0.4
8.0
4.1
9.5
11.4
9.5
6.8
2.3
0.9
46
25 [el
56 1% fl
55
44
30
10
4
ZSb,6b/7b
ICI
13.7
3.9
2.8
2.8
2.5
2.2
1.1
1.0
0.7
[a] With the exception of No. I , the reactions were carried out in a high-pressure, high-temperature flow reactor [3,7] in a stainless steel capillary; reaction time 2-3
m i n ; molar ratio cyclohexane : 1 = 1000: I. The concentration of cyclohexane was adjusted by variation of the pressure. The addition products are stable under the
reaction conditions. Under the same conditions at 400"C, less than 1% of 5b decomposes to 7b. [b] mol/L. [c] Yields and conversion in mol-% based on unreacted
1. [d] Reaction in duran glass ampules; reaction time 10 min. Upon addition of activated charcoal ( I mg/mL reaction solution) as radical trap, only traces of the
products were formed. [el Reaction time 1 min. [fl Addition of 10 mol-Yo 2,3-diphenylbutane based on Ib.
charcoal as a radical trap inhibits the formation of both
products (experiment I). The kinetic chain length is estimated to be 100 from the amount of chain-terminating
product cyclohexylcyclohexane (ca. 1 mol-% based o n
reacted starting material la).
Alternative explanations for the formation of the dihydro product 7, such as the frequently postulated pericyclic
H transfer['] or the symproportionation of alkene and alkane to give two alkyl radicals (a),['."' are clearly ruled out
on the basis of the experimentally confirmed radical chain
reaction. A conceivable b-cleavage of the cyclohexyl radical (c) and addition of the H atom to the alkene 1 would
be a reaction of zero order with respect to 1 and thus is
also ruled out.
The unequivocal experimental proof of the metathesis of
alkyl radicals and alkenes (d) and the reaction conditions
required for it to occur show that chain transfer during polymerization""' via this reaction, which has been frequently
postulated, is unlikely. A thermal pericyclic hydrogenation
of alkenes with alkanesr5]is also kinetically unlikely.
Received: July 18, 1985;
revised: October 7, 1985 [Z 1395 IEj
German version: Angew. Chem. 98 (1986) 79
[ I ] a) P. Ehrlich, G . A. Mortimer, Adu. Polym. Sci. 7 (1970) 386; b) H. F.
Kauffmann, H . Harms, 0. F. Olaj, J. Po[ym. Sci. Polym. Chem. Ed. 20
(1982) 2943.
[21 H. D. Becker in S . Patai (Ed.): The Chemistry o f f h e Hydroxyl Group.
Interscience, London 1971, p. 835.
(31 J. 0. Metzger, Angew. Chem. 95 (1983) 914; Angew. Chem. Int. Ed. Engl.
22 (1983) 889; Angew Chem. Suppl. 1983. 1256.
(41 C. Ruchardt, H.-D. Beckhaus, Angew. Chem. 92 (1980) 417; Angew.
Chem Int. Ed. Engl 19 (1980) 429.
[ 5 ] D. F Feller, M. W. Schmidt, K. Ruedenberg, J. Am. Chem. Sac. 104
(1982) 960.
161 A. Heesing, W. Miillers, Chem. Ber. 113 (1980) 24.
[7] P. Koll, J. Metzger, Angew. Chem. 90 (1978) 802; Angew. Chem. I n f . Ed.
Engl. 17 (1978) 154.
A Stable Diozonide of a Conjugated Diene by
Ozonolysis of 2,3,4,5-Tetramethyl-2,4-hexadieneon
Polyethylene
By Karl Griesbaum* and Willi Volpp
Stable diozonides of conjugated dienes were hitherto
unknown. Admittedly, the diozonides of 2,3-dimethyl-1,3butadiene and of isoprene could be obtained by ozonolysis
["I
Prof. Dr. K. Griesbaum, DipLChem. W. Volpp
Engler- Bunte-lnstitut, Bereich Petrochemie der Universitat
Kaiserstrasse 12, D-7500 Karlsruhe (FRG)
Angew Chem Int Ed Engl 25 11986) No I
of the respective dienes in pentane, but they exploded violently upon attempted isolation."' Since ozonides are
known to be stabilized by methyl substituents, ozonolysis
of 1 in pentane was also attempted, but no ozonide was
obtained.['] This is concordant with the experience that tetrasubstituted olefins d o not furnish the corresponding
ozonides.['] We have recently found, however, that ozonolysis of olefins adsorbed on polyethylene may provide
ozonides from olefins for which other methods of ozonation
1
2
Ozonolysis of 1 on powdered polyethylene at -75°C
and subsequent chromatographic separation of the crude
product (water-cooled column 3 x 62 cm, 200 g of silica
gel; n-pentane/diethyl ether 20: 1, ca. 14 mL/min) afforded
a mixture of two ozonides of structure 2 , probably the
meso- and ruc-isomers, in ca. 43% yield. They could be isolated by HPLC separation (column 3.2 x 12 cm, Lichrosorb
Si 60, particle size 5 pm; n-pentane/diethyl ether 20 : I, ca.
7.5 mL/min), albeit not yet assigned stereochemically. Each
of the isomers gave a positive peroxide test, correct values
for the elemental analysis, a ( M + I)@ ion at m/z 235 in the
C I / M S analysis, two quartets (6= 1.30, 1.43, J=O.64 Hz,
and 6= 1.30, 1.40, J=0.61 Hz, respectively) and a singlet
(6= 1.52, each) with relative intensities of 1 : 1 : 1 in the ' H N M R spectrum (C,D,, TMS), three quartets (6= 20.16,
23.12, 24.94 and 6=20.13, 23.41, 24.93, respectively) and
two singlets (6= 108.98, 109.60 and 6= 108.88, 109.76, respectively) in the I3C-NMR spectrum (CDCI3, TMS), and
no carbonyl absorption in the IR spectrum. The two isomers show significant differences in their melting points
( S l T and < -6O"C, respectively). At -20°C each ozonide was stable for months, whereas at room temperature
decomposition was observed to take place after a few
hours. Reduction of a mixture of the isomers of structure 2
with LiAIH, gave a mixture of meso- and rac-2,3-butanediol as well as 2-propanol; 2 reacts with triphenylphosphane to give a mixture of acetone, biacetyl 5 , and acetic
anhydride.
It is known that 1 is cleaved by ozone to give 3, 4, and
5.".'l Consistent with the finding that carbonyl oxides d o
not undergo cycloadditions with non-activated ketones,["
0 VCH Verlagsges~llsehafimbH. 0-6940 Wernherm. 1986
0570-0833/86/0101-0081!3 02 50/0
81
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