Steric factors in liquid-phase oxidation of olefins with iso structure

Article abstract of DOI:10.1023/A:1012359819642

Products of liquid-phase oxidation of 2-methyl-2-butene and 2-methyl-2-pentene were studied by NMR and IR spectroscopy. Steric hindrances in liquid-phase oxidation of olefins with iso structure were shown to arise from branching of the alkyl group at the

Full text of DOI:10.1023/A:1012359819642

Russian Journal of Organic Chemistry, Vol. 37, No. 3, 2001, pp. 318 320. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 3, 2001,
pp. 347 349.
Original Russian Text Copyright
2001 by A. Voronenkov, Simonov, Yablonskii, Koshel’, V. Voronenkov.
Steric Factors in Liquid-Phase Oxidation of Olefins
with iso Structure
A. V. Voronenkov, N. A. Simonov, O. P. Yablonskii,
S. G. Koshel’, and V. V. Voronenkov
Yaroslavl State Technical University, Moskovskii pr. 88, Yaroslavl, 150023 Russia
Yarsintez Research Institute Public Corporation, Yaroslavl, Russia
Received March 4, 1999
Abstract Products of liquid-phase oxidation of 2-methyl-2-butene and 2-methyl-2-pentene were studied
by NMR and IR spectroscopy. Steric hindrances in liquid-phase oxidation of olefins with iso structure were
shown to arise from branching of the alkyl group at the -carbon atom.
NMR and IR study of products of liquid-phase
oxidation (hydroperoxides) of a mixture of cis- and
trans-4-methyl-2-pentenes at 70 C showed that only
the trans isomer is oxidized. The cis isomer remains
almost unchanged [1]. The reason is steric repulsion
of the alkyl groups in cis-4-methyl-2-pentene, which
disrupts , -conjugation between the tertiary -C H
bond and the double bond. As a result, formation of
stable (planar) allyl type radical is hindered [2]. This
conclusion is supported by spatial models of the
hydrocarbon and the radical (Scheme 1).
that its formation is hindered (Scheme 3). Therefore,
only one of the two tertiary alcohols corresponding to
Scheme 2.
Scheme 1.
Scheme 3.
Steric hindrances in substituted cis-allyl radical
make the formation of the corresponding oxidation
products impossible, as follows from the results of
our study of products of low-temperature oxidation
of 2-methyl-2-pentene. By oxidation of 2-methyl-2-
pentene at 70 C we obtained unsaturated hydroper-
oxides and unsaturated alcohols [3] which correspond
to two canonical structures B and C of intermediate
allyl type radical A (Scheme 2).
Tertiary radical C can exist as two isomers: cis and
trans. However, the cis configuration (E) is charac-
terized by steric repulsion between alkyl groups, so
1070-4280/01/3703-0318$25.00 2001 MAIK Nauka/Interperiodica

STERIC FACTORS IN LIQUID-PHASE OXIDATION OF OLEFINS
319
Scheme 4.
two isomeric allyl type radicals should be formed
preferentially, namely trans-2-methyl-3-penten-2-ol.
This assumption was proved by us experimentally
using NMR and IR spectroscopy.
However, the fact that the tertiary allyl type radical
has trans configuration which is free of steric repul-
sion was not taken into account. Thus, steric factors
in the oxidation of olefins with iso structure become
determining when the hydrocarbon chain is branched
at the -carbon atom.
No steric effect was observed in the oxidation of
olefins with iso structure which lack branching at
the -carbon atom. The major products of oxidation
of 2-methyl-2-butene at 60 C were the corresponding
secondary and tertiary hydroperoxides at a ratio of
2:1 (Scheme 4). Presumably, abstraction of hydrogen
from the three methyl groups in 2-methyl-2-butene
occurs with similar probabilities. Other conditions
being equal, the ratio of hydroperoxides depends on
the contributions of canonical structures to inter-
mediate allyl type radical. Scheme 4 shows that the
statistical probability for oxidation of secondary
radical is twice as large as that for tertiary radical.
EXPERIMENTAL
The IR spectra were recorded on a UR-10 instru-
1
ment. The H NMR spectra were obtained on a Tesla
BS-467C spectrometer in CCl4. The chemical shifts
were measured relative to hexamethyldisiloxane as
internal reference. The reaction mixture was analyzed
by gas liquid chromatography on a Chrom-5 instru-
ment equipped with a flame-ionization detector;
carrier gas nitrogen.
The structures of 2-methyl-2-pentene and 2-methyl-
2-butene resemble the structure of elementary unit in
natural rubber. When studying the mechanism of
oxidation of natural rubber, the formation of tertiary
allyl type radical was presumed (Scheme 5) [4].
2-Methyl-2-butene was oxidized with an oxygen
nitrogen mixture at 60 C (20 atm) in a glass reactor.
The reaction time was 8 h (until the oxidate contained
0.8 to 1.0 wt % of hydroperoxides). Unreacted
2-methyl-2-butene was distilled off under reduced
pressure at a temperature not exceeding 15 C. The
resulting product contained more than 50 wt % of
hydroperoxide. Its amount was determined by iodo-
Scheme 5.
1
metric titration. H NMR spectrum, , ppm: 1.15,
1.60, 4.30, 4.85, 5.10, 5.90, 9.60.
2-Methyl-2-pentene was oxidized with atmospheric
oxygen at 60 C until the oxidate contained 10
15 wt % of hydroperoxide.
Products of oxidation of 2-methyl-2-pentene were
reduced with sodium sulfite. Subsequent rectification
gave two alcohols: 2-methyl-3-penten-2-ol and
2-methyl-2-penten-4-ol.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

320
VORONENKOV et al.
Methyl-3-penten-2-ol. bp 121.5 122 C, d420
2. Nonhebel, D.C. and Walton, J.C., Free-Radical
Chemistry, Cambridge: Cambridge Univ., 1974.
0.8343, n2D0 1.4290. IR spectrum, , cm : 970 ( C H
1
1
in trans-HC CH), 1678 ( C C, trans). H NMR
3. Kryukov, S.I., Yablonskii, O.P., Voronenkov, V.V.,
and Simanov, N.A., Zh. Org. Khim., 1971, vol. 7,
no. 4, pp. 629 632.
spectrum, , ppm: 1.15, 1.55, 3.93, 5.40.
REFERENCES
4. Belyaev, V.A., Yablonskii, O.P., Voronenkov, V.V.,
and Pokrovskaya, Z.N., Neftekhimiya, 1973, vol. 13,
no. 5, pp. 715 717.
1. Voronenkov, V.V. and Musabekov, Yu.S., Nefte-
khimiya, 1968, vol. 8, no. 5, pp. 713 717.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001