2027
NOTES
Nuclear Magnetic Resonance Studies of 2-Pyridones, 2-Pyridthiones, and 2-Thioalkylpyridines
by W. E. Stewart and T. H. Siddall, I11 Savannah River Laboratory, E. I . du Pont de Nemours and Company, Aiken, South Carolina 29801 (Received November l S 3 1969)
The higher barriers to rotation around the C(X)-N bond in thionamides (X = S) as compared to the corresponding amides (X = 0) have been attributed to the greater contribution in the former case of resonance structure 2 to the ground state.2 Upon the basis of this X
X-
I
,C-N(
+-+
,C-N\I
Results and Discussion
+/
2
1
proposal, the ring n-electron delocalization, or aromat i ~ i t y , ~of" , 2-pyridthiones ~~ (3, X = s) should be greater than that of 2-pyridones (3, X = 0) because of the greater contributions of structures 4 and 5 to the ground state of 2-pyridthiones. This line of reasoning has been used to explain the much larger dipole moment of 2-pyridthiones as compared to the corresponding 2-p~ridones.~Jackman and coworkers, de-
3
4
teriomethyl-2-pyridone were prepared by the oxidation of the corresponding 1-methylpyridinium sulfate^.^ The 2-pyridthiones were prepared by the action of PZSS on the corresponding oxygen compoundsa8 The 2-thioalkylpyridines were prepared by alkylation of the potassium salt of 2-pyridthiones in ethanolic ~ o l u t i o n . ~ All compounds mere purified by vacuum distillation, recrystallization, or both. The samples for nmr analysis consisted of 20 mt % of the compound in a specified solvent and were degassed and sealed under vacuum. The spectra were calibrated by the usual side-band method, using TMS as internal standards on a Varian A-60 or an HR-60 nmr spectrometer at probe temperature. The reported line positions are averages of four forward and four reverse sweeps. Average deviations were always less than 0.08 cps.
5
fining aromaticity as the ability to sustain a ring current, have concluded from a study of the chemical shifts of methyl protons attached to the 2-pyridone ringsa and from SCF calculation^^^ that 2-pyridones possess about 35% of the aromaticity of the benzene ring. Analyses of the pmr spectra (using computer program L A O C N ~ ~ )of several N-substituted 2-pyridones, 2pyridthiones, and 2-thioalkylpyridines show that the 2-pyridthiones do indeed exhibit greater delocalization as evidenced by the trends in the pmr parameters in the series pyridone + pyridthione + pyridine. Experimental Section The compounds in this study that were not commercially available were prepared by conventional means. 3a 4-t-Butyl-1-methyl-2-pyridone and l-deu-
The observed ring proton coupling constants and chemical shifts are listed in Table I. The values obtained for the pyridones and pyridthiones agree with previously reported first-order analyses for some of the c o m p o u n d ~ ~and ~ ~ ~with 0 an exact analysis reported for N-methyl-2-pyridone in CDCl,." The values for 2-thioalkylpyridine coupling constants are close to those reported for 2-methoxypyridine.l 1 and Coupling across the fixed double bonds CS-CS decreases, but coupling across the single bond c4-C~increases in going through the series pyridone + pyridthione + pyridine. The decreases in J34 and J56 indicate a decrease in the double-bond character of (1) The information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S. Atomic Energy Commission. (2) R. C. Neuman, Jr., D. N. Roark, and 17. Jonas, J . Amer. Chem. Sac., 89, 3412 (1967), and references therein. (3) (a) J. A . Edvidge and L. 14.Jackman, J . Chem. Soc., 859 (1961) : (b) G. G. Hall, 9. Hardisson, and L. AMiI. Jackson, Tetrahedron, 19, 101 (1963). (4) M. H. Krackov, C. M. Lee, and H. G. Mautner, J . Amer. Chem. Sac., 87, 892 (1965). (5) A revised version of LAOCOON I1 described by S. M. Castellano and A. A. Bothner-By, J . Chem. Phys., 41, 3863 (1964). (6) D. J . Cook, R. E. Bowen, P. Sorter, and E. Daniels, J . Org. Chem., 2 6 , 4949 (1961). (7) E. A. Prill and S. M. McElvain, Org. Syn., 2 , 419 (1943). (8) R. B. Wagner and H. D. Zook, "Synthetic Organic Chemistry," John Wiley and Sons, New York, N. Y., 1953, p 827. (9) J. Renalt, A n n . Chim. (Paris), 10, 135 (1955). (10) C. L. Bell, R. S. Egan, and L. Bauer, J . Heterocycl. C h e m , 2 , 420 (1965). (11) R. H. Cox and A . A. Bothner-By, J . Phys. Chem., 73, 2465 (1969). Volume 74, A'umber 9
April SO, 1970
NOTES
2028 Table I : Nmr Parameters of Ring Protons of Substituted 2-Pyridones (A, X 2-Pyridthiones (A, X = S), and 2-Thioalkylpyridines B
=
0),
B
R1
A Substituents
Solvent
Y aa
Y4
VS
Y6
Jarb
Jas
1.38 1.37 1.25 1.33 1.32 1.32 1.44 1.33
0.71 0.72 0.65 0.64 0.65 0.73 0.71
J36
Ja6
J ~ s
Jss
2-Pyridones
Ri =
CD3
DMSO CCla
Ri =
CH3
TFAc
Ri
Et
DMSO CHsOH
R1= 2-propyl Ri CH3, Re =
DMSO C6D6
385.57 379.95 449.68 382.89 392.13 389.84 383.21 384.53
CHad Ri = CH3, Ra = t-butyl
CDCls
392.64
=
C6D6
445.17 434.20 494.95 443.61 449.75 423.99 442,24 431.29
372.27 362.72 442.03 372.64 382.54 354.98 375.42 361.62
462.03 450.04 489.22 461.35 458.49 431.96 462.64
...
9.13 9.16 8.72 9.08 9.09 9.12 9.13 9.12
...
373.29
437.37
...
8.69 8.68 8.65
2.13 2.13 1.76 2.08 2.08 2.17 2.04
6.71 6.71 6.55 6.70 6.81 6.74 6.79
...
6.62 6.63 7.40 6.60 6.73 6.62 6.63 6.83
..,
...
2.16
0.53
...
...
7.06
0.79 0.75 0.67 0.55
7.10 6.98 6.86
1.67 1.72 1.62
...
1.42, 1.50 1.63 2.33
6.62 6.54 6.66 6.95
8.14 8.07 8.04
1.02 1.03 1.05
0.99 0.98 0.98
7.37 7.39 7.36
2-Pyridthiones
R1 = R1 = R1 = Ri
CHI
Et
2-propyl CH3, R4 t-butyl
DMSO DMSO DMSO DMSO
450.40 450.04 452.63 445.31
440.40 439.55 437.06
...
406.28 408.72 410.84 412.57
492.65 487.64 489.12 485.64
...
I
.
.
2-Thioalkylpyridines
Ri = CHI Ri Et Rl = 2-propyl a
DMSO
DMSO DMSO
I n HZfrom TMS at 60 MHz.
436.98 434.95 434.02
457.23 456.24 456.49
425.09 424.35 424.81
1.88 1.91 1.93
4.91 4.97 4.92
I n Hz.
the C3-C4 and the c5&6 bonds, but the increase in J45 indicates an increase in the C4-Cb double-bond character, as would be expected for increasing delocalization.12-'5 Of the four-bond coupling constants, Jaj is the same for the pyridthiones as for the pyridones, but J46 is about 0.45 cps less for the pyridthiones than for the pyridones. The fact that Jaj does not change but J4a does suggests that the change in J46 may be partly due to the increased positive charge on the nitrogen atom in the more polar pyridthiones. (Jaa increases from 1.79 in pyridine to 1.54 in K-protonated pyridine, but J 3 5 shows no significant decrease.)I6 Within a series, neither solvent nor nature of the N-alkyl or S-alkyl substituent has a significant effect upon the coupling constants; however, the remaining coupling constants change when a t-butyl group is substituted into the 4-position in N-methyl-2-pyridone arid N-methyl-2-pyridthione. The directions and magnitudes of the coupling constant changes are the same The Journal o j Physical Chemistry
507.56 506.81 507.21
as those for the corresponding benzene ring proton coupling constants when a t-butyl group is substituted. l7 I n the pyridones and pyridthiones with R1 = CH, and Re = H, coupling between the N-CH, protons couand Ha was large enough to obscure the pling, but was not large enough to be resolved. I n these cases it was necessary to use deuterium substitution or decoupling of the N-CH3 protons. With R1 = ethyl or 2-propyl, the only observable effect of the coupling was a slight broadening of the H6 1'ines. (12) N. Jonathan, S. Gordon, and B. P. Dailey, J . Chem. Phys., 36, 2443 (1962). (13) \V. B. Smith, W. H. Watson, and S. Chiranjeevi, J . Amer. Chem. Soc., 89, 1438 (1967). (14) H. Gtmther, Tetrahedron Lett., 2967 (1967). (15) S. Castellano and R. Kostelnik, J . Amcr. Chem. Soc., 90, 141 (1967). (16) J. B. Merry and J. H. Goldstein, ibid., 88, 5560 (1966). (17) S. Castellano and R. Kostelnik, Tetrahedron Lett., 5211 (1967).
NOTES
2029
The chemical shifts of the ring protons are quite dependent on concentration and solvent. Because the measurements were not extrapolated to infinite dilution, no attempts are made to rationalize the solvent shifts (with the exception of the benzene solvent shifts) or the shifts within a series. There are significant changes in the proton shifts in going from pyridones to pyridthiones, however. Protons Hs, Hs, and Hs are shifted downfield by about 64 Hz, 34 Hz, and 30 He, respectively, but Hd undergoes a slight upfield shift. Downfield shifts of the ring protons in the pyridthiones are consistent with increased ring current and the larger -C=X group anisotropy shifts to be expected in pyridthiones.18-21 The upfield shift of Hdis not easily explained. It could arise from increased charge density on Cd, but this is unlikely. Previous studies of the magnetic anisotropy effect in amides suggest that protons in the plane of the amide group are deshielded.22 Thus, if the amide group is replaced with the more anisotropic thionamide group,23the H4 resonance should shift downfield. Possible explanations of this presumably anomalous behavior are that H4 lies in a nodal region or that the anisotropic field of the thionamide group does not have the same shape as that of the amide group. The chemical shifts of the protons of the N-alkyl groups are given in Table 11. The 19-Hz downfield Table'II:
Proton Chemical Shifts of N-Substituents of
R 7-----Hz X
R
YCHa
0
CH3 CH3 CzHs CzHs !&propyl 2-propyl
215.0 233.7 81
S 0 S 0
S
80
from TMSvCHi
7
YCH
245 273
83.0 83.5
325 372
shift of the N-CHa protons in pyridthiones as compared to that in pyridones is consistent with a larger anisotropy in pyridthiones and with a freely rotating methyl group. The much larger downfield shifts of the a protons of the N-ethyl and N-isopropyl groups suggests that the preferred conformations in these cases are those with the p methyl groups oriented away from the carbonyl group, as in structure 6.
ox I
R- - -C
'R
" 6
(18) K. Nagarajan, 1683 (1967).
M. D. Nair, and P. M. Pillai, Tetrahedron, 23,
(19) P. L. Southwick, J. A. Fitzgerald, and G. F. Milliman, Tetrahedron Lett., 124:7 (1965). (20) R. Greenhalgh and M. A. Weinberger, Can. J . Chem., 43, 3340 (1965). (21) H. Booth and A. H. Bostock, Chem. Commun., 637 (1967). (22) R. F. C. Brown, L. Radom, S. Sternhell, and I. D. Rae, Can. J . Chem., 46, 2577 (1968), and references therein. (23) P. V. Demarco, D. Doddrell, and E. Wenkert, Chem. Commun., 1418 (1969).
The Dimerization of the Tetracyanoethylene Anion Radical
by Raymond Chang Department of Chemistry, Williams College, Williamstown, Massachusetts 02367 (Received December 8 , 1969)
In recent years the dimerization reaction of radicals in solution has been successfully studied by the electron spin resonance (esr) and optical techniques. For example, the Wurster blue perchlorate cation radical' and tetracyanoquinodimethan anion radical2 are known to form diamagnetic dimers in solution. Aromatic ketyl radicals3 and certain pyridinyl radicals14on the other hand, dimerize to form biradical species. We report here yet another radical dimerization reactionthat between the tetracyanoethylene (TCSE) anion radicals to form the diamagnetic dimer. The existence of this dimer has recently been postulated by Freed, et al., in their study of Heisenberg spin exchange processesO6 At room temperature the esr spectrum of the TCNE anion radical prepared by the reduction with sodium in 2-methyltetrahydrofuran shows the expected nine lines. The intensity of the lines increased with decreasing temperature, reaching a maximum at about -60". Below this temperature the integrated intensity decreased with decreasing temperature, in contrast to Curie's law. The line shape did not vary appreciably between -90 and - 140°, however, and the hyperfine splitting constant remained virtually unchanged throughout. Following Kawamori, et al.,' we write the equilibrium constant for the dimerization 2TCNE- ~2 (TCNE-)z as K = K O exp(-AH/RT) = (1 - a)/ (1) A . Kawamori, A. Honda, N. Joo, K. Suzuki, and Y . Ooshika, J . Chem. Phys., 44, 4363 (1966). (2) R. H. Boyd and W. D. Phillips, ibid., 43, 2927 (1965). (3) N. Hirota and S. I. Weissman, J . Amer. Chem. Soc., 86, 2538 (1964). (4) M. Itoh and E. M. Kosower, ibid., 89, 3655 (1967). (5) M. P. Eastman, R. G. Kooser, M. R. Das, and J. H. Freed, J . Chem. Phys., 51, 2690 (1969).
Volume 74, Number 9
April SO, 2970
