Alawode et al.
JOCArticle
Conclusions
Note that the syntheses of 1a,b require TMSN3 and Me2SO4;
extreme caution must be taken during the handling of these
reagents because of their hazardous nature.
We have reported a clean photoconversion of 1-methyl-4-
phenyl-1H-tetrazole-5(4H)-thione (1a) and 1-(3-methoxy-
phenyl)-4-methyl-1H-tetrazole-5(4H)-thione (1b) to the cor-
responding photostable carbodiimides 5a,b. Analogous to
the 5-oxo derivatives of tetrazoles, the photodecomposition
5-thio derivatives occurs via the intermediacy of a 1,3-biradical
7a,b, which is believed to be in its triplet spin multiplicity. We
find no evidence for an alternative pathway involving a
carbene, wherein desulfurization occurs prior to loss of
dinitrogen. We were further interested in identifying the
nature of precursor that leads to this biradical. The photo-
sensitization and triplet quenching experiments argue against
the involvement of a triplet excited state. The obvious alternative
mechanistic pathway that could lead to the formation of a 1,3-
triplet biradical is a diradicaloid species generated directly from
the singlet excited state of 1a,b after the expulsion of dinitrogen.
Once formed, this diradicaloid species could be envisioned to
undergo intersystem crossing to generate the triplet biradical 7a,
b which then would undergo desulfurization to form 5a,b.
Overall, these studies indicate that 1a and 1b are highly
promising lead compounds for industrial, agricultural, and
medicinal applications. The search for derivatives with
improved quantum yields that retain the clean photochemistry
and end-product photostability of 1a,b would be of considerable
interest. Our mechanistic studies suggest that derivatives should
be sought that favor dinitrogen dissociation in the excited state,
since this appears to be the favored pathway in the photode-
composition of these compounds.
1-(3-Methoxyphenyl)-4-methyl-1H-tetrazole-5(4H)-thione (1b).
P2S5 (1.3 g, 5.8 mmol) was added to a solution of 4b (0.5 g,
2.4 mmol) in dry toluene (15 mL). The mixture was refluxed at
110 °C until the starting material disappeared. The reaction mixture
after filtration was concentrated under reduced pressure. Purifica-
tion by column chromatography (SiO2, hexane/ethyl acetate, 93:7)
gave 1b (0.42 g, 78% yield) as a white solid: Rf 0.64 (70:30 hexane/
ethyl acetate); mp 52-54 °C; FTIR (ATR) 1600, 1590, 1492, 1440,
1
1356, 1324, 1238, 1195, 1160, 1065, 1022, 865, 839, 782, 681. H
NMR (400 MHz, CD3CN): δ 7.53 (s, 1H), 7.51-7.49 (m, 1H),
7.46-7.43 (m, 1H), 7.13-7.10 (m, 1H), 3.89 (s, 3H), 3.85 (s, 3H).
13C NMR (400 MHz, CD3CN): δ 165.0, 161.0, 137.0, 131.2, 117.2,
116.2, 111.0, 56.4, 35.5. HRMS: exact mass calculated for (M þ Hþ)
C9H11N4OSþ=223.0654, found 223.0648; (M þ Naþ) C9H10-
N4OSNaþ = 245.0473, found 245.0468.
Spectroscopic Data. 1-Phenyl-1H-tetrazol-5(4H)-one (3a)18,19
.
White solid (1.67 g, 82%). H NMR (400 MHz, DMSO-d6):
1
δ 7.85 (d, J = 7.68 Hz, 2H), 7.56 (t, J = 7.60 Hz, 2H), 7.42 (t, J =
7.51 Hz, 1H). 13C NMR (400 MHz, DMSO-d6): δ 150.2, 134.1,
129.4, 127.5, 119.5.
1-Methyl-4-phenyl-1H-tetrazol-5(4H)-one (4a)19. Crystalline
solid (0.30 g, 91%). 1H NMR (400 MHz, DMSO-d6): δ 7.85 (d,
J = 8.27 Hz, 2H), 7.58 (t, J = 7.03 Hz, 2H), 7.44 (t, J = 7.40 Hz,
1H), 3.62 (s, 3H, CH3). 13C NMR (400 MHz, DMSO-d6):
δ 148.8, 134.2, 129.5, 127.7, 119.4, 31.2.
4-Methyl-1-phenyl-1H-tetrazole-5(4H)-thione (1a)19. Yellow
solid (2.30 g, 53%). 1H NMR (400 MHz, CD3CN): δ 7.89-7.85
(m, 2H), 7.63-7.55 (m, 3H), 3.89 (s, 3H). 13C NMR (400 MHz,
CD3CN): δ 165.1, 136.2, 130.8, 130.3, 125.3, 35.5.
1-(3-Methoxyphenyl)-1H-tetrazol-5(4H)-one (3b)20. Pale white
solid (0.52 g, 81% yield). 1H NMR (400 MHz, DMSO-d6):
7.47-7.41 (m, 3H), 6.98 (d, J = 4 Hz, 1H), 3.80 (s, 3H). 13C
NMR (400 MHz, DMSO-d6): 159.8, 150.2, 135.3, 130.4, 113.0,
111.3, 105.0, 55.4.
Experimental Section
General Procedures. Thin layer chromatography was carried
out on 250 μm silica gel plates, and UV light was used as a
visualizing agent. Standard column chromatography was per-
formed using 63-200 μm silica gel. 1H and 13C NMR spectra for
the structural characterization of the compounds were recorded
on 400 MHz NMR spectrometer. The carrier frequencies were
399.75 MHz (1H) and 100.53 MHz (13C). The number of scans
used was 64 for 1H NMR spectra and for 13C, ranging from 3-5 K
depending on the sample concentration. Both the 1H and 13C
spectra were recorded with longer relaxation time (10 s). Chem-
ical shifts and the coupling constants are reported in parts per
million and Hertz, respectively. All the quantitative analyses of
the photolyzed reaction mixtures were performed by NMR
spectroscopy with 1,4-dioxane as an internal standard.29 These
experiments were carried out on a 500 MHz NMR spectrometer
equipped with a 3 mm triple resonance inverse detection pulse
1-Methyl-4-(3-methoxyphenyl)-1H-tetrazol-5(4H)-one (4b)20.
Crystalline solid (0.50 g, 90%). 1H NMR (400 MHz, DMSO-d6):
7.49 - 7.43 (m, 3H), 7.15 (d, 1H), 3.81 (s, 3H), 3.61 (s, 3H). 13
C
NMR (400 MHz, DMSO-d6): 159.8, 148.7, 135.3, 130.6, 113.2,
111.3, 105.0, 55.5, 31.3.
The synthesis of the authentic samples is described below.
1-Methyl-3-phenylthiourea (8a)30. Prepared by a slight mod-
ification of the previously reported procedure. To a solution of
aniline (1.5 g, 16.1 mmol) in methanol (75 mL) was added methyl-
isocyanate (1.3 g, 17.7 mmol), and the reaction mixture was
stirred at 65 °C for 24-30 h. After the completion of the reaction,
solvent was removed by distillation under reduced pressure to
obtain crude product. Recrystallization in ethanol yielded 8a
(2.46 g, 92% yield) as white solid: Rf 0.29 (60:40 hexane/ethyl
acetate); mp 110-112 °C; FTIR (ATR) 3259, 3155, 2989, 2937,
1515, 1490, 1287, 1246, 1210, 1026, 1001, 723, 689, 640, 602. 1H
NMR (400 MHz, CD3CN): δ 8.12 (s, 1H), 7.41-7.37 (m, 1H),
7.29-7.22 (m, 3H), 6.56(s, 1H), 2.97 (d, J = 8 Hz, 3H). 13C NMR
(400 MHz, CD3CN): δ 182.8, 138.7, 130.4, 127.0, 126.0, 32.1.
1
field gradient probe operating at 499.848 MHz for H. The
spectra were an accumulation of 64 individual scans. The photo-
products were assigned by comparison of their chemical shift values
to that of authentic samples. The infrared frequencies are reported
in cm-1. High resolution mass spectra were acquired on a quadru-
pole/time-of-flight mass spectrometer. The samples were prepared
in methanol/acetonitrile (containing 0.1% formic acid in some
cases) and were introduced by continuous infusion into the electro-
spray ionization (ESI) source at a rate of 30 μL/min. TOF scans
were carried out in positive ionization mode. In most cases, both
[M þ H]þ and [M þ Na]þ ions were detectable for each species. All
irradiations were carried out in a Rayonet reactor at 254 nm and
broad band 300 nm UV light. Monochromatic 313 nm was obtained
by using broad band 300 nm UV lamps and by filtering the radiation
through a solution of 0.002 M K2CrO4 in 5% Na2CO3.
HRMS: exact mass calculated for (M þ Hþ) C8H11N2Sþ
167.0643, found 167.0637; (M þ Naþ) C8H10N2SNaþ
189.0462, found 189.0457.
=
=
1-(3-Methoxyphenyl)-3-methylthiourea (8b)31. Prepared as
previously reported.31 White solid (2.1 g, 87% yield); mp 98-
100 °C. 1H NMR(400 MHz, CD3CN): δ 8.06 (s, 1H), 7.29 (t, 1H),
6.88-6.78 (m, 3H), 6.62 (s, 1H), 3.78 (s, 3H), 2.97 (d, J = 8 Hz,
3H). 13C NMR (400 MHz, CD3CN): δ 182.7, 161.5, 139.7, 131.3,
(30) Ambati, N. B.; Anand, V.; Hanumanthu, P. Synth. Commun. 1997,
27, 1487.
(31) Vassilev, G. N.; Vassilev, N. G. Oxid. Commun. 2002, 25, 608.
(29) Peterson, J. J. Chem. Educ. 1992, 69, 843.
J. Org. Chem. Vol. 76, No. 1, 2011 221