Vapor phase phototransposition chemistry of dimethylpyrazines and dimethylpyrimidines

Article abstract of DOI:10.24820/ark.5550190.p010.267

Based on their phototransposition chemistry, the three dimethylpyrazines and four dimethylpyrimidines can be arranged into two groups. 2,5-Dimethylpyrazine, 2,5-dimethylpyrimidine, and 4,6-dimethylpyrimidine constitute a photochemical triad. Irradiation of any one member of the triad in the vapor phase results in the formation of the other two members. The other four isomers, 2,6-dimethylpyrazine, 2,3-dimethylpyrazine, 2,4- dimethylpyrimidine, and 4,5-dimethylpyrimidine constitute a photochemical tetrad. Irradiation of any one member results in the formation of the other three. In addition, 2,4-dimethylpyrimidine and 2,6-dimethylpyrazine also photoisomerize to 3,6-dimethylpyridazine. Irradiation of the last in the vapor state resulted in the four members of the tetrad.

Full text of DOI:10.24820/ark.5550190.p010.267

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Paper
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Organic Chemistry
Arkivoc 2017, part v, 216-228
Vapor phase phototransposition chemistry of dimethylpyrazines
and dimethylpyrimidines
James W. Pavlik,a* Tharinee Vongakorn,a and Naod Kebedeb
a Department of Chemistry and Biochemistry, Worcester Polytechnic Institute,
Worcester, Massachusetts 01609, USA
b Department of Chemistry, Edinboro University, Edinboro, Pennsylvania 16444, USA
E-mail: jwpavlik@wpi.edu
This paper is dedicated to Dr. Robert E. Connors (October 1, 1945 - July 16, 2015), friend and colleague, in
recognition of his many contributions to spectroscopy and photochemistry
Received 07-18-2017
Accepted 09-23-2017
Published on line 10-26-2017
Abstract
Based on their phototransposition chemistry, the three dimethylpyrazines and four dimethylpyrimidines can be
arranged into two groups. 2,5-Dimethylpyrazine, 2,5-dimethylpyrimidine, and 4,6-dimethylpyrimidine
constitute a photochemical triad. Irradiation of any one member of the triad in the vapor phase results in the
formation of the other two members. The other four isomers, 2,6-dimethylpyrazine, 2,3-dimethylpyrazine, 2,4-
dimethylpyrimidine, and 4,5-dimethylpyrimidine constitute a photochemical tetrad. Irradiation of any one
member results in the formation of the other three. In addition, 2,4-dimethylpyrimidine and 2,6-dimethyl-
pyrazine also photoisomerize to 3,6-dimethylpyridazine. Irradiation of the last in the vapor state resulted in the
four members of the tetrad.
CH3
H3C
N
h
h
N
N
H3C
N
N
N
CH3
CH3
CH3
h

h



h

h

h

h
h
h

h


h
CH3
N
CH3
N
H3C
N
N
CH3

h
h
H3C
H3C
h
h
N
N

N
H3C
N
CH3
Keywords: Dimethylpyrazines, dimethylpyrimidines, vapor phase, phototransposition, photoisomerization
DOI: https://doi.org/10.24820/ark.5550190.p010.267
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Introduction
Lahmani and coworkers reported over 40 years ago that pyrazine 1 undergoes vapor phase phototransposition
to pyrimidine 2.1-3 These same workers reported that methylpyrazine 3 and dimethylpyrazines 4 and 5 undergo
vapor phase photoisomerization to yield pyrimidines 6-12 (Figure 1).
Figure 1. Previously observed products from vapor phase phototranspositions of pyrazines 1, 3-5.
According to these workers, these photoisomerizations are 100 times more efficient upon direct *
excitation than upon direct n * excitation. Based on these few examples, it was concluded that “a compound
of benzvalene type structure is the most probable intermediate in these transformations.”
More recently, we used deuterium labeling to study the vapor phase phototransposition chemistry of
1
pyrazine 1.4 GLC and H NMR analyses of the product mixtures revealed that 2,5-dideuteriopyrazine 1-2,5-d2
and 2,6-dideuteriopyrazine 1-2,6-d2 each transposed to a mixture of two isomeric dideuteriopyrimidines and a
trace quantity of a dideuteriopyridazine (Figure 2). Although these transpositions cannot be explained by a
mechanistic path involving Dewar-diazaprismane intermediates, they are identical to those predicted by a
diazaprefulvene mechanism. A similar biradical structure, called prefulvene, was first suggested by Bryce-Smith
and Longuet-Higgins to be an intermediate in the photoisomerization of benzene.5
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Figure 2. Vapor phase phototransposition studies of deuterium labeled pyrazine 1.
Although the photoreactions of the dimethylpyrazines (Figure 1) have been known for over four decades,
no complete mechanistic scheme for the vapor phase phototransposition chemistry of dimethylpyrazines and
dimethylpyrimidines has appeared in the literature. To better understand the photochemistry of these
compounds we studied the vapor phase photochemistry of a complete set of dimethylpyrazines and
dimethylpyrimidines. These include the three possible dimethylpyrazines (4, 5 and 14) and the four possible
dimethylpyrimidines 9-12 as well as a dimethylpyridazine 15 (Figure 3).
Figure 3. Dimethyl-substituted diazines studied herein.
Results and Discussion
Each dimethylpyrazine (4, 5 or 14) and dimethylpyrimidine (9-12) vapor, with a total pressure of 1.0-1.5 Torr,
was irradiated for 5 minutes in a three-liter quartz vessel in a Rayonet reactor with light of 254 nm emitted from
2, 4, 6, or 8 low pressure Hg lamps. After each irradiation, the volatile materials were pumped out of the reaction
flask, condensed and dissolved either in a standard volume of ether for quantitative GLC analysis or in
deuteriochloroform for 1H NMR analysis. Products were identified by GLC retention times, by GLC co-injections
with authentic samples, and by comparison of the observed 1H NMR chemical shifts with the chemical shifts of
authentic samples of the various isomeric products.
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Figure 4. Primary products; positive numbers give products yields, negative numbers give consumption yields.
The photoreactions were studied under a wide range of reactant conversions to differentiate primary and
secondary products. Figure 4 shows the maximum quantity of primary photoproducts formed from each
dimethyldiazine. Inspection of these products revealed that the seven dimethyldiazines can be arranged into
two distinct groups: The first group contains 2,5-dimethylpyrazine 4, 2,5-dimethylpyrimidine 9, and 4,5-
dimethylpyrimidine 10, and constitutes a photochemical triad (Scheme 1); irradiation of any member of the
triad results in the formation of the other two members as the only isomerization products. The second group
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contains the other four isomers, 2,6-dimethylpyrazine 5, 2,3-dimethylpyrazine 14, 2,4-dimethylpyrimidine 12
and 4,5-dimethylpyrimidine 11, and constitutes a photochemical tetrad (Scheme 2).
Scheme 1. Photointerconversions within triad.
Scheme 2. Photointerconversions within tetrad.
Irradiation of any member of the tetrad led to the formation of the other three. In addition to
phototransposing to the other members of the tetrad, 2,4-dimethylpyrimidine 12 and 2,6-dimethylpyrazine 5
also photoisomerize to 3,6-dimethylpyridazine 15. Irradiation of the vapor of pure pyridazine 15 gave the four
members of the tetrad.
The vapor phase photochemistry of these dimethyldiazines thus resembles that of the dimethylpyridines.6
In the latter case, the six isomers consisted of two photochemical triads. Although leakage between the two
triads was observed, in the present study, no interconversion between the triad and the tetrad was detected.
As shown in Schemes 1 and 2, the observed reactions include pyrazine to pyrimidine, pyrimidine to pyrazine,
pyrimidine to pyrimidine, and one example each of a pyrimidine or a pyrazine to a pyridazine photo-
transposition. To the best of our knowledge, these latter two types of reactions have not been previously
reported.
Diazabenzvalene valence isomers were suggested as intermediates in the photoisomerizations of
dimethylpyrazines to dimethylpyrimidines.1-3 Although such an intermediate can account for pyrazine to
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pyrimidine interconversions, a benzvalene-type species could not account for pyrazine to pyrimidine or
pyrimidine to pyrazine interconversions.
The simplest mechanistic scheme for the triad to account for these observations is illustrated in Scheme 3.
Photochemical excitation is suggested to lead to intramolecular C2-C6 or C3-C5 bond formation in the case of a
pyrazine, or to C2-C4 or C2-C6 bonding in the case of a pyrimidine, leading to the formation of diazaprefulvene
intermediates BC-4, BC-9 or BC-10. Interconversion of these bicyclic intermediates via one 1,3-N shift for a
pyrazine to pyrimidine interconversion, i.e. 4  BC-4 BC-10  10 or 4  BC-4  BC-9  9 led to the
conversion of 4 to 10 or 9, respectively. The observed greater yield of 9 in this reaction reflects the greater
stability of BC-9 relative to BC-10.
Scheme 3. Mechanism for triad isomerization.
Irradiation of pyrimidines 9 or 10 led to their conversion into pyrazine 4 via one 1,3-N migration as in 9 
BC-9  BC-4  4 or 10  BC-10  BC-4  4 or into pyrimidines 9 or 10 via two nitrogen shifts as in 9  BC-9
 BC-4  BC-10 10. The greater yield of pyrazine 4 relative to pyrimidine 10 on irradiation of pyrimidine 9
indicated that the rate of rearomatization of BC-4  4 was faster than the rate of isomerization of BC-4 to BC-
10. The slow isomerization of BC-4 to BC-10 reflects the greater stability of BC-4 relative to BC-10. The observed
equal yields of diazines 4 and 9 on irradiation of pyrimidine 10 suggests that the rate of isomerization of BC-4
 BC-9 was more competitive with the rate of rearomatization of BC-4 to 4 than the rate of isomerization of
BC-4 to BC-10 was with the rearomatization of BC-4  4. Tentatively, this may be because BC-9 was more stable
than BC-10 due to their substitution patterns. Thus, interconversion of BC-9, BC-4, and BC-10 followed by
rearomatization allowed interconversion of the three members of the triad, but not to any other
dimethylpyrazine or dimethylpyrimidine. Furthermore, the relative stabilities of the three bicyclic isomers
account for the relative yields of photoproducts.
Irradiation of 4,5-dimethylpyrimidine 11, a member of the tetrad, to low reactant conversion, gave 3,4-
dimethylpyrimidine 12, 2,3-dimethylpyrazine 14, and 2,6-dimethylpyrazine 5, the other members of the tetrad
1
as primary products. The formation of these products was confirmed by H NMR and GLC. More prolonged
irradiation of pyrimidine 11 led to greater quantities of these primary products and to 3,6-dimethylpyridazine
15, which appears to be a secondary photoproduct, presumably formed by photoisomerization of 2,4-
dimethylpyrimidine 12. Indeed, irradiation of pure pyrimidine 12 gave diazines 14, 5, and 11, the other members
of the tetrad, and to pyridazine 15.
As shown in Scheme 4, photochemical excitation of pyrimidine 11 led to C2-C4 or C2-C6 bonding to afford
bicyclic intermediates BC-11’ and BC-11, respectively. Although both bicyclics are stabilized by methyl
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substitution at the end of the allyl system, BC-11 is expected to be less stable due to methyl substitution at the
bridgehead position. This suggests that the yields of products resulting from initial C2-C4 bonding, i.e. diazines
12 and 14, should predominate over product 5 that arises from initial C2-C6 bonding followed by the one N-
migration pathway, BC-11’  BC-5  5. Although all the product yields in this reaction are low, the observed
yields (Figure 4) are consistent with this suggestion. Scheme 4 shows, however, that the two N-migration
pathway (11  BC-11’  BC-5  BC-11’’  11’’) leads back to the reactant. To the extent that this energy
wasting pathway operates, the importance of C2-C6 bonding would be underestimated.
N
N
H3C
N
N
N
H3C
H3C
N
CH3
CH3
CH3
2,6-
N
BC-11
BC-14
BC-12
h
N
H3C
H3C
H3C
N
H3C
N
CH3
CH3
N
N
11
h
14
12
2,4-
N
N
N
N
H3C
H3C
CH3
H3C
N
CH3
N
CH3
5
BC-11'
BC-5
N
N
N
N
H3C
H3C
CH3
CH3
11"
BC-11"
Scheme 4. Photoconversions of 4,5-dimethylpyrimidine 11.
As shown in Scheme 5, irradiation of 2,4-dimethylpyrimidine 12 led to the formation of the other three
members of the tetrad, 5, 14, and 11, and also to dimethylpyridazine 15. According to Scheme 5 the two former
products, 14 and 11, are formed by initial C2-C4 bonding leading to BC-12B followed by the one N-migration
pathway BC-12A  BC-14  14 or the two N-migration pathway BC-12A  BC-14  BC-11  11. Accordingly,
the yields of 14 and 11 depends on the relative rates of the reactions BC-14  14 and BC-14 → BC-11. Since 11
was observed in greater yield, the isomerization BC-14  BC-11 must be faster than the rearomatization BC-14
 14. This may be due to the greater stability of BC-11 relative to BC-14.
Alternatively, C2-C6 bonding in the excited state of dimethylpyrimidine 12 leading to BC-12A is suggested
to yield pyrazine 5 via the pathway BC-12B  BC-5  5 or 15 via the pathway BC-12B  BC-15 15, both
involving one N-migration. Since diazines 5 and 15 were observed in equal yields, the rates of BC-12B  BC-5
and BC-12B  BC-15 would have to be equal. It is not clear why pyridazine formation is so significant in this
reaction.
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Scheme 5. Photoconversions of 2,4-dimethylpyrimidine 12.
Scheme 5 also shows that in addition to rearomatization, BC-5, formed from pyrazine 5, could isomerize to
BC-12’ which could rearomatize to pyrimidine 12’. The overall pathway thus leads back to the reactant with the
transposition of carbon atoms 2 and 4 and carbon atoms 5 and 6. To test this possibility, 2,4-dimethylpyrimidine-
1
6-d 12-6-d was synthesized and its vapor phase photochemistry examined. Before irradiation the H NMR
spectrum of pyrimidine 12-6-d from  6.5 to 9.0 ppm showed a sharp singlet at 6.95 due to the C-5 proton and
1
a very small doublet (J 5.1 Hz) at  8.45 due to a small amount of residual protons at ring position 6. H NMR
analysis after irradiation revealed that irradiation was accompanied by the formation of a singlet growing in at
the center of the doublet at  8.45. As shown in Scheme 6, this indicates that an isomer of pyrimidine 12-6-d
has formed which now has a proton at C6 and deuterium at C5. This shows that 2,4-dimethylpyrimidine-6-d (12-
6-d) has phototransposed to 2,4-dimethylpyrimidine-5-d (12-5-d) as demanded by the mechanism in Scheme 5.
Scheme 6. Deuterium scrambling in 2,4-dimethylpyrimidine-6-d (12-6d).
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1H NMR analysis also revealed that pyrimidine 12-6-d phototransposed to diazines 14-6-d, 5-3-d, 11-2-d and
12-5-d (Scheme 7).
Scheme 7. Phototranspositions of 2,4-dimethylpyrimidine-6-d (12-6-d).
In a diazaprefulvene intermediate of type A in Scheme 8, the migrating nitrogen can shift away from the
ring nitrogen to yield B, a precursor of a pyrazine, or toward the ring nitrogen to yield C, a precursor of a
pyridazine. Tentatively, the general absence of pyridazine products in these reactions can be attributed to a
much faster A  B than A  C isomerization; the latter N-shift requires trading a strong N-C bond for a
comparatively weak N-N bond.
Scheme 8. Isomerization pathways.
In addition of C3-C5 bonding to yield BC-5, 2,6-dimethylpyrazine 5 can undergo C2-C6 bonding to yield BC-
5A, a precursor of 4,5-dimethylpyrimidine 11 via the one N-walk mechanism 5  BC-5A  BC-11A  11. Since
BC-5 (Scheme 5) is more stable than BC-5A, product 5 is favored over 11.
Scheme 9. Phototransposition of 2,6-dimethylpyrazine 5.
As shown in Scheme 5, irradiation of 2,3-dimethylpyrazine 14, would yield only BC-14, which can isomerize
to BC-12A, the precursor of pyrimidine 12, or BC-11, the precursor of pyrimidine 11. Based on the relative
stability of BC-12A and BC-11, 11 is expected to be the major product. As can be seen in Figure 4, however, 2,4-
dimethylpyrimidne 12 is formed in higher yield.
The 00 bands for S1 and S2 absorption in pyrazine and pyrimidine vapor occur at 325 and 270 nm and at
321 and 249 nm, respectively. These absorptions correspond to S1 (n,*) and S2 (,*) states at 88 and 106 kcal
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mol-1 for pyrazine and at 89 and 115 kcal mol-1 for pyrimidine. Methyl substitution in both pyrazine and
pyrimidine significantly lowers the S2 energy levels, while the S1 levels are nearly unchanged.7,8 The energies of
the S1 (n,*) and S2 (,*) states in 2,3-dimethylpyrazine, for example, lie at 89 and 102 kcal mol-1 while these
states in 2,4-dimethylpyrimidine 12 lie at 89 and 109 kcal mol-1. These energies correspond to S0S2 (,*)
onsets in these UV absorption spectra at 278 nm and 260 nm, respectively. The S1 (n,*) and S2 (,*) energy
levels of other dimethylpyrazines and dimethylpyrimidines are expected to lie at similar energies.
Photochemical excitation of these dimethylpyrazines and dimethylpyrimidines with light of 254 nm is expected
to result in the population of S2 (,*) excited states with excess vibrational energy.
The non-radiative decay properties for aza- and diaza-aromatic compounds are similar to those of benzene.9
Thus, for these heteroarenes, particularly fast non-radiative decay channels have been observed and it has been
suggested that these non-radiation channels may be similar to “channel three” in benzene.10-15 In benzene, this
decay pathway, which originates in a vibrationally excited ,* state, results in the formation of the ground
state diradical perfulvene,16 the presumed precursor of benzvalene.
For pyridine, theoretical calculations predict that the S2 (,*) state crosses both the S1(n,*) and S0 states
along a concerted pathway leading to the ground-state azaprefulvene diradical,17 a species which has been
suggested to be the key intermediate in the phototransposition reactions of variously substituted pyridines.
Furthermore, femtosecond transient absorption spectroscopy has shown that in the condensed phase the S2
(,*) state of pyridine passes through a conical intersection to the ground-state of the azaprefulvene
diradical.18 This species reverts to pyridine in greater than 2 ns.18 Considering the observed phototransposition
reactions of deuterium,19 methyl,6 and cyano20 substituted pyridines, migration of the N atom around the
cyclopentenyl ring during the lifetime of the azaprefulvene diradical must be possible.
Theoretical calculations also predict that the S2 (,*) state of pyrazine crosses both the S1 (n,*) and S0
state along a concerted pathway, leading through a conical intersection to the ground-state diazaprefulvene
diradical,21 the suggested intermediate in these phototransposition reactions.
Su has used theoretical calculations to study this mechanistic possibility in dimethylpyrazine.22 According to
his study the Franck-Condon state of 2,6-dimethylpyrazine 5, for example, follows two pathways, leading to
conical intersections resulting in the formation of 4,5-dimethylpyrimidine 11 and 2,4-dimethylpyrimidine 12,
respectively. Based on the relative stabilities of the two conical intersections, Su predicts that 4,5-
dimethylpyrimidine 11 should be the major product whereas 2,4-dimethylpyrimidine should be the minor
product. Su states that this is consistent with the product distribution given by Lahami and Ivanoff.1-3 To the
best of our knowledge, these authors did not give the relative yields of these two products or even identify
pyrimidine 12 as a product.
The structure proposed by Su for the conical intersection leading to 4,5-dimethylpyrimidine 11 shows C2
and C6 moving closer together while the N moves out of the plane of the remaining 5-membered ring. The
structure resembles the higher energy bicyclic diradical species 5A resulting from C2-C6 bonding of pyrazine 5
suggested earlier. The conical intersection leading to 2,4-dimethylpyrimidine 12, however, appears to be
essentially planar with only a small amount of C3-C5 interaction. This suggests that the conical intersection is
reached earlier on the reaction coordinate leading to the C3-C5 bonded diradical BC-5. The conical intersection
then resembles the Franck-Condon species and is therefore of higher energy then the analogous conical
intersection leading to 4,5-dimethylpyrimidine 11.
The results predicted by the theoretical calculations do not agree with our experimental observations. Thus,
we observe that pyrimidine 12 is the major product formed in 17% yield, whereas pyrimidine 11 is the minor
product formed in 8% yield. These results agree with the relative stabilities of the precursor bicyclic diradicals.
Apparently, more research is required to determine the structures of the species in the conical intersection.23
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Conclusions
Based on their phototransposition chemistry the three dimethylpyrazines and four dimethylpyrimidines can be
arranged into two sets: The first, a triad, consisting of the photointerconverting group of 2,5-dimethylpyrazine,
2,5-dimethylpyrimidine, and 4,6-dimethylpyrimidine. Irradiation of any one member of the triad in the vapor
phase results in the formation of the other two members. The second, a tetrad, is made up of the four isomers,
2,6-dimethylpyrazine, 2,3-dimethylpyrazine, 2,4-dimethylpyrimidine, and 4,5-dimethylpyrimidine. Irradiation of
any one member in the vapor phase results in the formation of the other three. In addition, 2,4-
dimethylpyrimidine and 2,6-dimethylpyrazine also photoisomerize to 3,6-dimethylpyridazine. Irradiation of the
latter in the vapor state afforded the four members of the tetrad. This is the first report of phototransposition
of pyrimidine or a pyrazine to pyridazine. Evidence supporting the proposed mechanism was provided with
deuterium labeling studies.
These experimental data thus provide a complete mechanistic scheme for the vapor phase phototransposition
chemistry of dimethylpyrazines and dimethylpyrimidines.
Experimental Section
1
General. H and 13C NMR spectra were recorded at 400.1 and 100.6 MHz, respectively, in deuteriochloroform
1
on a Bruker FT-NMR system. H and 13C NMR chemical shifts were measured relative to internal
13
tetramethylsilane or chloroform, respectively. All C NMR spectra are proton decoupled. Mass spectra were
recorded with an HP-5970 mass selective detector interfaced to a HP-5880 capillary column gas chromatograph.
GLC analysis were performed on a PE-9000 FID instrument equipped with a 15 m × 3 mm Carbowax bonded
capillary column.
The three dimethylpyrazines and 4,6-dimethylpyrimidine were commercially available. 2,4- and 2,5-
Dimethylpyrimidine,24 4,5-dimethylpyrimidine,25 4,6-dimethylpyrimidine,26,27 3,6-dimethylpyridazine28 were
synthesized according to literature procedures.
2,4-Dimethylpyrimidine-6-d (12-6-d). To a suspension of 6-chloro-2,4-dimethylpyrimidine24 (0.40 g, 2.8 mmol),
sodium carbonate (1.50 g), 10% Pd-C (0.30 g) in methanol-d (20 mL) was passed D2 (g) for 4 hours. Kugelrohr
distillation (100 oC, 1 mmHg) gave the title compound 12-6-d as a colorless liquid (0.27 g, 87%); 1H NMR (CDCl3)
 2.50 (s, 3H), 2.60 (s, 3H) 7.40 (s, 1H), 8.40 (residual proton at C-6 (d, J 5.1 Hz); 13C NMR (CDCl3)  24.1 (CH3),
24.7 (CH3), 118.0 (C-5), 156.9 (C-6, t, J 26.8 Hz), 167.3 (C-4), 168.2 (C-2).
Irradiation procedure. The dimethylpyrazine or dimethylpyrimidine was placed in a Pyrex tube, attached to the
vacuum line, and subjected to three freeze-thaw cycles. The remaining material was then allowed to vaporize
into a quartz reaction flask (3 L) that had been evacuated overnight. The resulting pressure in the reaction flask
ranged from 1.0 to 1.5 Torr. The flask was irradiated for 5 min. in a Rayonet reaction equipped with either 2, 4,
6, 8, or 10 low-pressure 2537 Å Hg lamps.
The following Table summarizes the amounts of initial reactant and of final products.
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Table 1. Mass of reactants and recovered products
Reactant
Mass of reactant (mg)
Mass of products (mg)
2,3-dimethylpyrazine 14
2,6-dimethylpyrazine 5
2,5-dimethylpyrazine 4
2,4-dimethylpyrimidine 12
4,5-dimethylpyrimidine 11
2,5-dimethylpyrimidine 9
4,6-dimethylpyrimidine 10
30
32
25
30
22
22
26
16
20
12
26
12
12
12
Analysis procedure. After irradiation by the above procedure, the 3 L reaction flask was attached to the vacuum
line, and the volatile contents were recovered by pumping it out through a trap cooled in an acetone/dry ice
bath. The contents of the trap were weighed (Table 1) and dissolved in diethyl ether or deuteriochloroform for
1
GLC or H NMR analysis, respectively. Products were identified by comparison of the observed GLC retention
times for the observed chemical shifts with the known retention times or chemical shifts determined in this
laboratory.
References
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