A new maturity indicator of sedimentary organic matter based on thermal fission of allylic bond in porphyrins

Article abstract of DOI:10.1246/cl.2001.1174

A 3-ethyl-4-methylpyrrole unit in etioporphyrin was shown to be converted to a 3,4-dimethylpyrrole unit by the action of heat, which was proved to be catalyzed by Na-montmorillonite. Time courses of the reaction could be followed by analyzing 2-ethyl-3-me

Full text of DOI:10.1246/cl.2001.1174

1174
Chemistry Letters 2001
A New Maturity Indicator of Sedimentary Organic Matter
Based on Thermal Fission of Allylic Bond in Porphyrins
Shinya Nomoto,* Masaki Kozono, Hajime Mita, and Akira Shimoyama*
Department of Chemistry, University of Tsukuba, Tsukuba 305-8571
(Received July 17, 2001; CL-010668)
A 3-ethyl-4-methylpyrrole unit in etioporphyrin was shown
Etioporphyrin (0.19 mg) was heated at 250 to 350 °C, and
in the presence of Na-montmorillonite (20 mg) at 150 to 350 °C
up to 72 h in a degassed sealed tube. Heating with the clay was
performed, since clay minerals are well known to have played
important roles on maturation of organic matter in sediments.12
The products were oxidized, according to the method of
Furlong and Carpenter,6 with 10% CrO3 in 25% H2SO4 (1 mL)
at 0 °C for 2 h and then at room temperature for 2 h to afford a
mixture of maleimides, which was extracted with benzene and
analyzed by GC–MS. Identification and quantification of
maleimides were performed by comparison of GC–MS data
with those of authentic samples.
to be converted to a 3,4-dimethylpyrrole unit by the action of
heat, which was proved to be catalyzed by Na-montmorillonite.
Time courses of the reaction could be followed by analyzing 2-
ethyl-3-methyl- and 2,3-dimethylmaleimide after chromic acid
oxidation of the porphyrin. The ratio of these maleimides was
proposed as a new maturity indicator for sedimentary organic
matter.
Investigations which had related the chemical properties or
features of compounds found in sediments to their thermal his-
tories have gathered general interest. The ages of fossils or
ancient sediments were estimated by a molecular clock based
on their contents of D-amino acids.1 The carbon preference
index that describes the odd-over-even carbon number prefer-
ence of n-alkanes found in crude oils or sediments can provide
information on the levels of maturity, which represents their
time-temperature histories.2 It is therefore important to know
organic transformation of natural products incorporated into
sedimentary organic matter.
Since Treibs discovered porphyrins in sediments in 1934,3
much attention has been denoted to these biomarker compounds
that can be linked to chlorophylls.4–11 The sedimentary por-
phyrins are usually complicated mixtures with various sub-
stituents on their pyrrole rings as a result of diagenetic modifi-
cation of precursor chlorophylls. The substitution patterns can
be conveniently determined after decomposition to maleimides
by chromic acid oxidation.8 In the oxidation products of crude
oils and sediments previously reported, the predominant
maleimide was always 2-ethyl-3-methylmaleimide (1), which
can be attributed to the common presence of a 3-ethyl-4-
methylpyrrole ring in many chlorophylls.7,10,11 2,3-
Dimethylmaleimide (2) was usually found as the second most
abundant and is considered to be a diagenetic product because
of the lack of the corresponding chlorophyll.7,8,10,11 In the pres-
ent study, we confirmed, for the first time, thermal transforma-
tion from an ethylmethyl- into a dimethylpyrrole moiety of por-
phyrins by heating experiments of etioporphyrin. The 1 to 2
ratio can therefore be used as a new maturity indicator of sedi-
mentary organics.
Figure 1 shows typical gas chromatograms obtained by the
heating experiment. The unheated substrate yielded only 1
after oxidation in 10 runs, corresponding 10 to 20% of the ini-
tial etioporphyrin. After heating, etioporphyrin, which is solu-
ble in methanol, afforded products that were insoluble in
methanol. Although heating seemed to cause serious degrada-
tion or polymerization of etioporphyrin, both of maleimides 1
and 2 were successfully detected after oxidation of the prod-
ucts, indicating that the alkylpyrrole moieties survived to some
extent after heating. The total yields of 1 and 2 on the basis of
etioporphyrin were 10 to 20% in the reactions at 150 and 200
°C, and 0.3 to 10% in those at 250 to 350 °C. 2-
Methylmaleimide was formed only in a trace amount, and 2-
ethylmaleimide and maleimide were not found in all the reac-
tions performed. These facts indicate that the selective thermal
cleavage of an allylic C–C bond in the ethylmethylpyrrole moi-
ety of etioporphyrin was the predominant reaction that can be
observed after oxidation of the heating products.
This result may explain the fact that maleimide and 2-
methyl- and 2-ethylmaleimide had been found in crude oils or
sediments in considerably smaller amounts than those of 1 and
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1175
that the clay acts as a catalyst in the C–C bond fission. Clay
minerals are known to be an acidic catalyst,12 and often used for
catalytic thermal cracking of hydrocarbons.16,17 The detailed
mechanism of C–C bond fission on the clay is still not clear.
The present study demonstrated that transformation from
an ethylmethyl- into a dimethylpyrrole moiety in porphyrins
proceeds by the action of heat, and that the progress of the reac-
tion can be observed as a ratio of 1 to 2. Therefore, it was
expected that the ratio (1/(1 + 2)) can be used as a new maturity
indicator of sedimentary organic matter, since the maleimides
could have been oxidatively extracted from sediments. The
ratio of 1 to 2 in oxidation products of two different series of
natural sediments (Neogene and Cretaceous/Tertiary sediments)
has been found to be in agreement with the thermal maturity of
the sediments (Figure 4), which was evidenced by other maturi-
ty indicators.14,18 The ratio of 1 to 2 provides a new type of
maturity indicator, which can be obtained from insoluble stable
organic geopolymers in sediments.
References and Notes
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the lower bond energy of the allylic C–C bond, compared to
those of the other bonds that exist in etioporphyrin.
The ratio of 1 to 2, expressed as 1/(1 + 2), obtained in heat-
ing of etioporphyrin without and with the clay is plotted against
the reaction time in Figures 2a and 2b, respectively. The
decreasing trends of the ratio with time were observed,
although the data were somewhat scattered. The clay mineral
accelerated the transformation as seen in Figure 2, and the
accelerating effect was obvious at the early stage of the reac-
tion. This result is in agreement with our previous studies indi-
cating the deactivation of Na-montmorillonite within 48h in
reactions above 250 °C.13,14 In order to evaluate kinetically the
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