Photoredox chemistry in the synthesis of 2-aminoazoles implicated in prebiotic nucleic acid synthesis

Article abstract of DOI:10.1039/d0cc05752e

Prebiotically plausible ferrocyanide-ferricyanide photoredox cycling oxidatively converts thiourea to cyanamide, whilst HCN is reductively homologated to intermediates which either react directly with the cyanamide giving 2-aminoazoles, or have the potential to do so upon loss of HCN from the system. Thiourea itself is produced by heating ammonium thiocyanate, a product of the reaction of HCN and hydrogen sulfide under UV irradiation. This journal is

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DOI: 10.1039/D0CC05752E
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Photoredox chemistry in the synthesis of 2-aminoazoles
implicated in prebiotic nucleic acid synthesis
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
Ziwei Liu , Long-Fei Wu , Andrew D. Bond , and John D. Sutherland*
DOI: 10.1039/x0xx00000x
II
Prebiotically plausible ferrocyanide-ferricyanide photoredox ferrocyanide (Ca
2
[Fe (CN)
6
]) could have been a source of
1
4
cycling oxidatively converts thiourea to cyanamide, whilst HCN is cyanamide 1 through salt hydrolysis . However, the formation
II
15
reductively homologated to intermediates which either react of Ca
directly with the cyanamide giving 2-aminoazoles, or have the modelling suggesting that under a CO
potential to do so upon loss of HCN from the system. Thiourea calcium ions would precipitate as the carbonate leaving
itself is produced by heating ammonium thiocyanate, a product of ferrocyanide to precipitate as its sodium salt. Thus, unless CO
2
[Fe (CN)
6
] on early Earth has been challenged recently ,
2
-rich atmosphere,
2
the reaction of HCN and hydrogen sulfide under UV irradiation.
was transiently removed from Earth’s atmosphere by
reduction following collision with an iron- and nickel-rich
impactor, for example, the presence of calcium cyanamide
seems unlikely.
Cyanamide 1 has been reported as a product of the
decomposition of the thiourea oxidation product, formamidine
Cyanamide 1 is pivotal to the synthesis of both 2-aminooxazole
2
(2-AO) and 2-aminoimidazole 3 (2-AI) through condensation
with glycolaldehyde 4. 2-AO 2 is formed in the absence of
added ammonium salts and a mixture of 2 and 2-AI 3 is formed
1, 2
when such salts are added . Nitrile photoredox chemistry
was previously reported to generate simple sugars, including
16, 17
disulfide under non-acidic conditions
. We reasoned that
III
3-
thiourea 5 might be oxidized by ferricyanide [Fe (CN)
6
] in a
3
-6
glycolaldehyde 4 and glyceraldehyde . 2-AO 2 reacts with
glyceraldehyde to generate predominantly arabino- and ribo-
configured pentose aminooxazolines, the former of which is a
5, 6
photoredox cycle also involving ferrocyanide . Here we show
that cyanamide 1 can be generated by irradiating thiourea 5 in
the presence of a catalytic amount of ferrocyanide in aqueous
solution through photoredox coupling to cyanide reduction.
We further show how this can lead to the generation of 2-AO 2
and 2-AI 3. Lastly, we outline a synthesis and purification of
1, 2
precursor of pyrimidine ribonucleotides
and the latter of
7
which is a precursor of both pyrimidine ribonucleosides and
8
,
9
purine
deoxyribonucleosides
.
5'-Phosphoro-2-
and transiently
aminoimidazolides derived from 2-AI
3
18
thiourea 5 including a plausible prebiotic crystallization step .
A mixture of C-labelled thiourea 5 (20 mM) with potassium
activated nucleotides participate in non-enzymatic RNA
replication chemistry via the intermediacy of 5'-5'-
imidazolium-bridged dinucleotides10, 11.
13
ferricyanide (40 mM) in phosphate buffer (pH = 8, 500 mM)
was incubated at 23 °C. After 3 days, cyanamide 1 was
Despite the importance of cyanamide 1 in potential prebiotic
chemistry there have been few attempts at investigating its
provenance on early Earth. An attempt to synthesize 1 by
subjecting mixtures of cyanide and ammonia to UV irradiation
gave low yields of dicyanamide from which the intermediacy of
13
detected as the major product by quantitative C-NMR
spectroscopy (45 % yield, Fig. 1). Meanwhile, another mixture
of cyanamide 1 (50 mM), glycolaldehyde 4 (50 mM) in
phosphate buffer (pH = 8, 200 mM) was incubated at 23 °C for
1
2
9 hours. 2-AO 2 and its hydrate 6 were observed in yields of
9 % and 65 %, respectively (Fig. S1. ESI). Thus, conditions for
1
2
cyanamide 1 was inferred, but no 1 was detected . Recently
the synthesis of cyanamide 1 in low yield by carefully dosed
radiolysis of a solution containing hydrogen cyanide and high
concentrations of chloride (5.1 M) and ammonium (0.1 M) ions
the synthesis of cyanamide 1 from thiourea 5 are consistent
with those required for the reaction of 1 with glycolaldehyde 4
although the majority of the product of the latter reaction is in
the form of its hydrate.
1
3
was reported . It has also been proposed that calcium
cyanamide generated by thermal decomposition of calcium
a. MRC Laboratory of Molecular Biology, Cambridge Biomedical Campus,
Cambridge, UK. Email: johns@mrc-lmb.cam.ac.uk
b.
Department of Chemistry, University of Cambridge, Cambridge, UK.
Electronic Supplementary Information (ESI) available: CCDC-2024098. See
DOI: 10.1039/x0xx00000x
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83, 4
. The formation of glyceronitrile 10 in 27% yield suggested
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DOI: 10.1039/D0CC05752E
that cyanide was outcompeting cyanamide 1 in reaction with
glycolaldehyde 4. As cyanohydrin formation is reversible, this
did not overly concern us at this point, indeed we could see a
benefit to sequestering glycolaldehyde 4 as the cyanohydrin 10,
thereby delaying the synthesis of 2-AO 2 from 4 and
cyanamide 1, because of the reported photolability of 2-AO 2 .
We thus investigated whether the precursors to 2-AO 2 could
be produced from thiourea 5 and HCN 7 directly. A mixture of
2
0
1
3
C-labelled thiourea 5 (50 mM), potassium ferrocyanide (5
mM) and potassium cyanide (100 mM) in phosphate buffer
pH = 7 or 8, 200 mM) was irradiated for 6 hours. Glycolonitrile
1
3
13
Fig. 1. C-NMR Spectra of a mixture of C-labelled thiourea 5 (20 mM) and
potassium ferricyanide (40 mM) in 500 mM phosphate buffer (pH = 8, in 10%
O in H O) after 3 days.
(
D
2
2
8
9
(24 % yield at pH 7 and 16 % yield at pH 8), aminoacetonitrile
(28 % yield at pH 7 and 24 % yield at pH 8), glyceronitrile 10
Previously, we reported that hydrated electrons produced by
II
4-
(12 % yield at pH 7 and 11 % yield at pH 8), and 2-AI 3 (4 % at
the known photoionization of ferrocyanide [Fe (CN)
6
] drive
1
both pH values) were observed by H-NMR spectroscopy, and
the reductive homologation of hydrogen cyanide 7 (HCN) to
1
3
13
C-labelled cyanamide
1
was observed by
C-NMR
the simple carbohydrates, glycolaldehyde
4
and
5
spectroscopy (Fig. S3, Fig. S4, Fig. S5, Table S1, ESI). 2-AO 2 was
not observed directly, but the liberation of glycolaldehyde 4
from glyceronitrile 10 thereby allowing reaction with
cyanamide 1 giving 2-AO 2 (and its hydrate 6) by wet-dry
cycling was reported recently . Thus, what has been termed a
continuous reaction network from HCN 7 and thiourea 5 is
possible through photoredox cycling. However, we note that
further progression from 2-AO
deoxyribonucleosides almost certainly requires discontinuity in
the synthesis and we have shown that such discontinuities can
be accomplished by flow chemistry which mimics a fluvial
glyceraldehyde in a Kiliani-Fischer-type process . In this earlier
work, the ferricyanide resulting from the photoionization of
ferrocyanide was reduced back to the latter with bisulfite
enabling photoredox cycling. We now wondered if such
ferrocyanide-ferricyanide photoredox cycling could couple the
oxidation of thiourea 5 with the reduction of glycolonitrile 8,
an intermediate in the Kiliani-Fischer-type process (Scheme 1,
Stage 2). This would generate cyanamide 1 and glycolaldehyde
1
3
1
3
2
to ribo- and
4
simultaneously, potentially allowing subsequent formation
of 2-AO 2 in situ. Were this to work, it might then be possible
to couple the oxidation of thiourea 5 with the multi-step
reductive homologation of HCN 7 to glycolaldehyde 4.
6
scenario on early Earth . Furthermore, exploitation of the
conglomerate crystallization of a later intermediate in the
nucleoside synthesis is most easily achieved by flow as it
allows plausible separation of crystalline material from mother
liquor. Such crystallization removes by-products, salts and
excess reagents and effectively resets the dial regarding yield
in the sense that low yields mainly impact multistep prebiotic
synthesis in a negative way because of associated lowering of
product purity.
Because ferrocyanide and ferricyanide undergo photoaquation
(in addition to the photoionization of ferrocyanide) at the 254
nm wavelength we employ in our exploratory prebiotic
1
9
photochemistry experiments
and the photoaquation
products interfere with NMR analysis, cyanide was added to
increase the rate of the back reactions which reverse
photoaquation and thus shift photostationary equilibria in
favour of fully cyanated complexes. We realized that this
addition would mean that we would not be able to say
whether any 2-AO 2 resulted from the direct reduction of
glycolonitrile 8, or the reductive homologation of HCN 7, but
we reasoned that by having 8 present in addition to 7, we
would maximize our chances of producing and detecting 2-AO
We considered it noteworthy that 2-AI 3 observed in our
experiments (Fig. S3, Fig. S4. ESI) is an essential compound in
the nonenzymatic copying of oligoribonucleotides as reported
1
0
by Szostak and co-workers . To further investigate the route
by which 2-AI 3 is produced by photoredox chemistry, a
1
3
1
3
mixture of aminoacetonitrile 9 (50 mM), C-labelled thiourea
(50 mM), potassium ferrocyanide (5 mM) and potassium
cyanide (30 mM) in phosphate buffer (pH = 7 or 8, 200 mM)
2
. Accordingly, a mixture of glycolonitrile 8 (50 mM), C-
5
labelled thiourea 5 (50 mM), potassium ferrocyanide (5 mM),
and potassium cyanide (30 mM) in phosphate buffer (pH = 8,
1
3
was irradiated for 14 hours. 2- C-2-AI 3 (50 % yield at pH 7
2
00 mM) was subjected to UV irradiation. After 7 hours
1
1
and 20 % yield at pH 8) was observed by H-NMR spectroscopy.
irradiation, 2-AO 2 was observed in 4 % yield by H-NMR
spectroscopy, which suggests that the transformation of
thiourea 5 to cyanamide 1 is coupled to the synthesis of
glycolaldehyde 4 in this photoredox system. Other HCN
reductive homologation products, aminoacetonitrile 9 (8 %
yield) and glyceronitrile 10 (27 % yield) were observed as well
(Fig. S6, Table S1, ESI). This indicates that in our system, 2-AI 3
is formed from the reduction of aminoacetonitrile 9 to the
imine of aminoacetaldehyde followed by addition of
cyanamide 1. Thus, a synthesis of 2-AI 3 that does not require
high concentrations of ammonium ions has been uncovered.
The foregoing results show that thiourea 5 is an effective
precursor of cyanamide 1 through ferrocyanide-ferricyanide
photoredox cycling and so it is reasonable to ask how thiourea
(Fig.S2, Table S1, ESI, Scheme 1). The formation of
aminoacetonitrile indicated that was reduced to
9
7
methanimine, at least some of which was trapped by addition
of cyanide rather than undergoing hydrolysis to formaldehyde,
trapping of which by cyanide addition, generates glycolonitrile
5
could have been produced prebiotically on early Earth.
According to the cyanosulfidic chemistry scenario we have
2
| J. Name., 2012, 00, 1-3
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S atoms) upon
we studied ferrocyanide-ferricyanide
photoredox chemistry in the presence of ammonium
previously described14, 21, it is conceivable that a buffered photodissociation (to cyanide anion and 3P
cyanide solution corroded metal sulfides (including traces of irradiation
copper sulfide) releasing hydrosulfide (HS ) into solution
J
View Article Online
2
8,
29
DOI: 10.1039/D0CC05752E
,
-
22, 23
.
Irradiation of such a mixture results in photoredox cycling with thiocyanate 11. A solution containing ammonium thiocyanate
-
reduction of HCN 7 and oxidation of HS , but instead of 11 (20 mM), thiourea 5 (80 mM), KCN (100 mM), potassium
products with S-S bonds accumulating, cyanide cleaves them ferrocyanide (5 mM) in phosphate buffer (pH = 6, 200 mM)
as they are formed generating thiocyanate. Meanwhile, was irradiated with 254 nm light. After 19 hours, glycolonitrile
hydrolysis of imines resulting from nitrile reduction in the 8 (36 %), aminoacetonitrile 9 (13 %), glyceronitrile 10 (47 %)
1
reductive homologation of HCN 7 generates ammonium ions and 2-AI 3 (4 %) were observed by H-NMR spectroscopy (Fig.
as counter ions to the thiocyanate. Absent cyanometallate S9). The results indicated that ammonium thiocyanate 11 did
photoredox cycling, the irradiation of buffered solutions not adversely affect the ferrocyanide-ferricyanide photoredox
-
containing HCN
7
and HS still results in reductive system.
homologation, albeit more slowly. In this case, photoionization In summary, a direct link of cyanamide 1 to cyanosulfidic
of HS- generates hydrated electrons and thiocyanate is chemistry via thiourea 5 was developed by geologically
.
- .
ultimately produced from the resultant HS / S radicals and plausible crystallization (Scheme 1). Furthermore, a catalytic
HCN 7 (either via the transient formation of S-S bonds and photoredox cycling system was established which starts from
.
- .
cleavage thereof by cyanide, or by addition of HS / S to HCN 7 thiourea 5 and HCN 7 or glycolonitrile 8. Homologation
.
- .
followed by hydrogen atom abstraction by a second HS / S ). To products of 7/reduction products of 8 react with in situ
assess thiocyanate production in a cyanometallate-free system, generated cyanamide 1 to form 2-aminoimidazole 3 and 2-
a solution containing cyanide (50 mM) and hydrosulfide (50 aminooxazole 2. These findings reinforce the cyanosulfidic
mM) at pH = 8 was irradiated with 254 nm light for 1 hour to scenario in which HCN 7 is central to the synthesis of protein,
give thiocyanate (70 % yield) as the major product according to lipid, and nucleic acid building blocks.
quantitative 13C-NMR spectroscopy (Fig. S7. ESI). Previously,
we focused on the homologation products of HCN 7 in the
Stage 1: Formation of thiourea
various systems we have investigated and regarded the
ammonium thiocyanate 11 as waste, but a central tenet of
synthetic systems chemistry is that there is ideally no waste
and so we surveyed the literature for reactions of 11. More
than 150 years ago in a classic study on par with Wöhler’s
synthesis of urea, it was reported that heating ammonium
thiocyanate 11 in the dry state at 170 °C gave a 3:1 equilibrium
S
HCN/CN + FeS
[FeII(CN)6]4-
x2
+
HS
NH4SCN +
H2N
NH2
7
h

11
5
(1:4, inclusion complex)
2
e-aq.
+ 6H+
HS SH
HS + e aq.
-
crystallization
HS + H
O
+
+
SCN
S

NH4SCN
NH4SCN
H2N
NH2
NH4
11
11
5
H
H
(3:1)
2
4
mixture of ammonium thiocyanate 11 and thiourea 5 . The
simplicity of this procedure appealed to us because it would
correspond to a geochemical scenario in which a body of water
that contained ammonium thiocyanate 11 was geothermally
heated to dryness at first and then beyond. In this early study
it was further reported that if the 3:1 mixture of ammonium
thiocyanate 11 and thiourea 5 is dissolved in water with
heating and cooled, fine, long, silky crystals are deposited
which upon recrystallization give thiourea. A follow-on study
reported that the first formed fine, long, silky crystals are co-
crystals of ammonium thiocyanate 11 and thiourea 5 now in a
Stage 2: Photoredox reactions
NH2
NH
e-aq.
NH
2e-aq.+ 2H+
2
2
HCN
HCN
HCN
H2N
N
2H+
H
H
N
NH CN 1
+
7
9
8
-
H2O
3
Generation of cyanamide
and active reducing agent
NH3 H2O
S
NH2
O
H2N
NH2
NH2CN
O
N
HO
HO
5
1
N
-
H
H
2
S + 2H+
2e aq.+ 2H+
NH
II
4-
[
Fe (CN)6]
[
FeIII(CN)6]3-
CN
NH2
HCN
HCN
-H2O
HO
HO
h

NH2
O
e-aq.
NH3 H2O
2
5
1
:3 ratio . We repeated the crystallization process to further
probe this, suspecting that these earlier authors had obtained
crystals of thiourea inclusion complex, species first
N
CN
OH
NH2CN
1
O
HO
HO
10
4
6
a
Scheme 1. Schematic representation of the systems chemistry network
2
6
characterized as such in the mid 1940’s . When a 3:1 mixture producing thiourea 5, reductive homologation of HCN 7, and the formation of 2-
AO 2 and 2-AI 3 by photoredox chemistry.
of ammonium thiocyanate 11 and thiourea 5 in aqueous
solution was subject to crystallization conditions, white
needles were obtained. X-ray crystallographic analysis showed
Conflicts of interest
them to be of an inclusion complex with a 1:4 ratio of
2
7
ammonium thiocyanate 11 and thiourea 5 (Fig. S8) . If this 1:4 There are no conflicts to declare.
ratio inclusion complex was recrystallized from water,
hexagonal crystals, of pure thiourea 5, were formed.
As the inclusion complex requires only one crystallization to Notes and references
form, a 1:4 mixture of ammonium thiocyanate 11 and thiourea
5
seems more prebiotically plausible than pure thiourea 5.
1
M. W. Powner, B. Gerland and J. D. Sutherland, Nature 2009,
459, 239-242.
Given that thiocyanate can undergo photoionization, or
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–
+
7 Crystal data. (CH
4 2 4 4
N S) ·NCS ·NH , M = 380.61,
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3
=
94.7245(13)°, U = 1813.93(13) Å , T = 180(2) K, space
1
group P2 /c (no.14), Z = 4, 14100 reflections measured, 3171
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