I3C Hyperfine Structure of Tropone Radical Anion
a planar system, 17 has a dihedral anglelg of 38' and a substantially reduced electron affinity. More important in the present context, the radical anion of 17 would be expected to be more basic than 1, a prediction born out by the difference in behavior between 1 and 17 observed here. It has been observed that 1 is reduced 2.8 times faster than 17 by sodium-ammonia at -33 O C 2 0 This, and the difference observed here in the relative basicity of their radical anions, clearly indicates a much greater electron affinity for 1 than for 17.
References and Notes Because of the magnitude of the body of literature in this area only key reviews and recent lead references are cited. M. Szwarc, frog. Phys. Org. Chem., 6,323 (1968);T. A. Ward, G. Levin, and M. Szwarc, J . Am. Chem. Soc., 97,258 (1975);M. Szwarc, Acc. Chem. Res., 2,87 (1969);G.Levin, B. Lundgren, M. Mohammed, and M. Szwarc, J. Am. Chem. Soc., 98, 1461 (1976);K. Shimada and M. Szwarc, ibid., 97,332 (1975);G. Levin, B. E. Holloway, and M. Szwarc, ibid., 98,5707 (1976);G. Levin and M. Szwarc, ibid., 98,4211 (1976); C.-T. Ho, R. T. Conlin, and P. P. Gaspar, ibid., 96, 8109 (1974);0.P. de Marguez, J. Giulianelli, T. C. Wallace, and D. H. Eargle, Jr., J . Org. Chem., 41,739 (1976); Ya. P. Stradyn' and R. A. Gavar, "Progress in Electrochemistry of Organic Compounds", A. M. Frumkin and A. B. Ershler, Ed., Plenum Press, New York, N.Y., 1971;R. H. Schuler and P. Neta, J. Am. Chem. Soc., 97,912(1975); J. F. Garst, Acc. Chem. Res., 4,400 (1971);"Free Radicals", Vol. I, J. K. Kochi, Ed., Wiley-Interscience, New York, N.Y., 1973. K. Suga, S.Watanabe, and K. Kamma, Can. J . Chem., 45,933 (1967);K. Suga and S. Watanabe, Buii. Chem. SOC.Jpn., 40,1257 (1967);K. Suga, S.Watanabe, T. Watanabe, and M. Kuniyoshi, J . Appi. Chem., 19,318(1969);S. Watanabe and K. Suga, Aust. J . Chem., 24, 1301 (1971);K. Suga, S.Watanabe, and T. Fujita, /bid.,
25, 1583 (1972). J. Brossas, C. P. Pinazzi, and F. Clouet, J. Polym. Sci., Polym. Chem. Ed., 11, 1517 (1973). D. H. Levy and R. J. Myers, J . Chem. fhys., 44, 4177 (1966). D. Y. Myers, R. R. Grabbe, and P. D. Gardner, Tetrahedron Lett.,
533 (1973). J. Jacobus and J. F. Eastham, J . Chem. Soc., Chem. Commun.,
138 (1969).
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978 1125
(7) 0. Levin, C. Sutphen, and M. Szwarc, J. Am. Chem. Soc., 94,2652 (1972),and references cited therein; S. Bank and S. P. Thomas, Tetrahedron Lett., 305 (1973). (8) See, for example, R. G. Harvey, P. P. Fu, and P. W. Rabideau, J. Org. Chem., 41,2706 (1976),and reference cited therein; P. Gans, J. B. Gill, and M. Griffin, J. Am. Chem. SOC.,98,4661 (1976);J. 8. Gill and B. M. Lowe, J. Chem. Soc.,Dalton Trans., 1959 (1972). (9)D. Valentine, N. J. Turro, Jr., and G. S.Hammond, J . Am. Chem. SOC., 86,5202 (1964). (10) Reductive carbon-carbon cleavage where strain is released and the radical and anion fragments are stabilized by delocalization seems now to be a predictable reaction, B. R. Ortiz de Montellano, B. A. Loving, T. C. Shields, and P. D. Gardner, J . Am. Chem. SOC.,89, 3365 (1967);P. H. Ruehle, T. K. Dobbs, L. L. Ansell, R. van der Helm, and E. J. Eisenbraun, J . Org. Chem., 42, 1098 (1977). (11) Z.Csuros, P. Caluwe, and M. Szwarc, J. Am. Chem. Soc.,95,6171 (1973),and references cited thereln. (12) 0 . Levin, J. Jagur-Grodzlnski, and M. Szwarc, J. Am. Chem. Soc.,
92,2269 (1970). (13)The coupling of alkyl radicals with metal naphthalenide has been estimated to occur at a rate of the order 2 X log M-' s-', J. F. Oarst, "Free Radlcals", Voi. I,J. K. Kochi, Ed., Wiley, New York, N.Y. 1973, Chapter 9. (14)A. J. Birch, J . Chem. Soc., 1551 (1950). (15)D. Y. Myers, 0. G. Stroebel, B. R. Ortiz de Montellano, and P. D. Gardner, J . Am. Chem. Soc., 95,5832 (1973).The coupling of radicals with anions to produce adduct radical anions seems now to be a general reaction when the adduct radical anion has sufficient stabllity. The reader is referred to the following publications and references cited therein: D. Y. Myers, G. 0. Stroebel, B. Ortiz de Montellano, and P. D. Gardner, J. Am. Chem. Soc.,96,1981 (1974); G. A. Russell and W. C. Danen, ibid., 90, 347 (1968);N. Kornblum, Angew. Chem., Int. Ed. Engl., 14,734 (1975);S.Hoz and J. F. Bunnett, J . Am. Chem. Soc., 99, 4690 (1977). Alternative mechanisms for the formation of 7 involving a carbocation Intermediate or radical-radical coupling appear untenable considering the reaction conditions used and the moieties being coupled. (16) R. B. Bates, L. M. Kroposki, and D. E. Potter, J . Org. Chem., 37,
560 (1972). (17) Approximately 5% conversion was observed after 12 h at 30 OC. (16) N. L. Alllnger and J. T. Sprague, J. Am. Chem. Soc., 95,3893(1973); R. B. Turner, 6. J. Mallon, M. Tichy, W. von E. Doering, W. R. Roth, and G. Schroeder, ibid., 95,8605 (1973). (19) M. Traetteberg, Acta Chem. Scand., 24,2285 (1970). (20)R. R. Grabbe and P. D. Gardner, manuscript in preparation.
Electron Spin Resonance Studies in the Tropone Series. 1. Carbon-13 Hyperfine Structure of the Tropone Radical Anion P. Furderer and F. Gerson" Physikalisch-Chemisches Institut der Universitat Basel, Klingelbergstrasse 80, 4056 Basel, Switzerland (Received September 1, 1977)
The coupling constants, ac,,, of 13C isotopes in natural abundance have been determined for all positions, I* = 1 to 7 , of the tropone radical anion: acl = (-)8.32; ~ c 2 , 7= (+)12.33; aC3,6 = (-16.02; and a ~= (+)4.54 ~ , ~G. Their assignment is based on relationships connecting the a-spin populations pp at the centers p with the values ac,, and with the widths, AH,of the 13Csatellite lines. The radical anion of 2,7-diisopropyl-4,5-dimethyltropone has also been investigated; its hyperfine data confirm the assignment made previously for the proton coupling constants of the parent radical anion.
Introduction Several years ago, Ikegami and Setul first reported the proton coupling constants for the radical anion of tropone (1). Their data were subsequently confirmed by Russell and Blankespoor2 who reduced 1 under different conditions. Russell and Stevenson3 also obtained the corresponding values for the radical anion of 2,7-dimethyltropone (2) and thus provided experimental support for the assignment of the largest coupling constant to the protons in the 2,7 positions of 1--. The present paper completes these results by an investigation of the radical anion of 2,7-diisopropyl-4,5-dimethyltropone (3), and, in 0022-3654/78/2082-1125$01 .OO/O
particular, by a reexamination of the ESR spectrum of 1-. under higher amplification. 0
6ws
2 : R =CH,, R'=H 3: R = CH(CH,),, R'=CH,
R'
R'
Experimental Section Tropone (1) was synthesized according to the procedure derivof Radlick? while its 2,7-diisopropyl-4,5-dimethyl 0 1978 American Chemical Society
1126
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978
P. Furderer and F. Gerson
0 aH4,5
I
I
I
Figure 1. ESR spectrum of the radical anion of tropone (1): solvent, DMF; counterion, Et4N+; temperature, -60 "C. On the left, the low-field end of the spectrum is reproduced at greater amplification which allows the 13C satellite lines, A, 6,C, and D, to be detected. On the right, the outermost high-field hyperfine component is shown at 4-20 O C ; the improved resolution achieved at this temperature enables one to observe the coupling constant a H3,6.
TABLE I: Proton Coupling Constants aJ.Jp (G) for the Radical Anions of Tropone (l), and Its 2,7-Dimethyl- (2) and 2,7-Diisopropyl-4,5-dimethyl (3) Derivatives Position
3-.Q
1-9
p
+2O"C
-60°C
2-.b
- 40 "C
2,7 3,6 4,5
8.67
8.75 (O.lO)f 5.10
8.76' 0.48 4.77
2.44,d0.17e 0.65 5.53'
0.10 5.08
'This work; experimental error: k0.02 G. Reference 3;temperature not specified. ' Six methyl p protons. Two methine p protons; value markedly temperature dependent (cf. text). e Twelve methyl y protons. f Splitting unresolved. ative (3) was kindly supplied by Professor Noyoria5 The radical anions 1-. and 3-. were generated at -60 OC by electrolytic reduction of the respective neutral compounds in N,N-dimethylformamide (DMF) with tetraethylammonium (Et,N+) perchlorate as the supporting salt. Use was made of the cylindrical cell constructed in this laboratory.6 Spectral resolution greatly improved when the temperature was raised from -60 to +20 "C, but at the same time the stability of the radical anions rapidly decreased.
Results Tropone (1). Figure 1 shows the ESR spectrum of the radical anion l-. at -60 "C. At this temperature the small splitting of 0.10 G is unresolved; it becomes visible on
warming the solution to +20 "C (Figure 1, right). The proton coupling constants, uHp, observed for 1; at -60 and +20 "C are given in Table I. They agree essentially with those reported by the other authors.lI2 Their previous assignment is confirmed by comparison of the values aHa for l-. with the analogous data for the radical anions of 2,7-dimethyl- (2)3and 2,7-diisopropyl-4,5-dimethyltropone (3) (see below). These data are also included in Table I. Amplification of the spectrum of 1--reveals the presence of satellite lines stemming from 13C isotopes in natural abundance. The four lines, labeled A, B, C, and D, which occur at the low-field end (Figure 1,left), are characterized in Table I1 by their widths, AH, their relative integrated intensities, and the coupling constants, acp,associated with them. It is noteworthy that the same values AH also hold for the four corresponding I3C satellite lines a t the high-field end of the spectrum. The assignment of one 13Ccoupling constant, 8.32 G, to the single center p = 1follows from the relative integrated intensity of the pertinent line B. This intensity is diagnostic of one carbon site, whereas those of the remaining lines A, C, and D, are each in accord with two equivalent sites for the 13C isotopes (cf. Table 11). The values ac , which belong to these three lines, 12.33, 6.02 and 4.54 respectively, are assigned by arguments presented in the Discussion. 2,7-Diisopropy1-4,5-dimethyltropone (3). The ESR spectrum of 3 - e at -40 "C is shown in Figure 2. Also reproduced is a derivative curve which was computed with the use of the proton coupling constants aH,, listed in Table
TABLE 11: Characteristic Data for the 13CSatellite Lines in the ESR Spectrum of the Radical Anion of Tropone (1) Satellite Assignment line Line width' Integrated No. of equiv Coupling to position (Figure 1) AH,G intensity,b % sites constant acp,G I.( 1.0 2 12.33 297 A 0.30 0.5 1 8.32 1 B 0.16 1.1 2 6.02 396 C 0.15 1.1 2 4.54 495 D 0.21 *0.01 50.1 50.05
'Peak-to-peak distance. main spectrum.
Relative to the integrated intensity (100%)of the corresponding hyperfine component in the
6,
‘% Hyperfine Structure of Tropone Radical Anion
The Journal of phvsical Chemistry, Vol. 82, No. 10, 1978 1127
Figure 2. ESR spectrum of the radical anion of 2.7diisopropyl-4,5dimethyltropone(3): (top) experimental spectrum; solvent, DMF; counterion, Et,N+; temperature, -40 ‘G (bonom)spectrum simulated by means of the proton coupling constants listed in Table I; line shape, Lorentrian; line width, 0.07 G.
TABLE 111: “Experimental” and Calculated =-Spin Populations p,, in the Radical Anion of Tropone (1) Position “Best” Expt“ Calcd * valuese
,,
2,7
3,6
0.336 0.004
4,5
0.196
1 0 =FOP
+0.352 - 0.001 ~0.183 -0.035
+0.336 -0.004
-0.021
+0.196 -0,035 -0.021
+1.000
+1.000
Absolute values obtained by means of eq 1from the coupling constants ( I H ~in Table I (l-.,a t -60 %); IQl = 26 G. b Computed by the McLachlan procedure; m o = 01 + 1.2p;pco = 1.560; A = 1.0. e See text. a
I. The assignment is straightforward for the values arising from the sets of six (5.53 G) and twelve equivalent protons (0.17 G). Of the two remaining coupling constants (2.44 and 0.65 G ) ,each due to a pair of equivalent protons, the larger one can readily he assigned to the methine 0 protons,’ because its temperature dependence (2.38 G at -60 “C and 2.48 G a t -30 “C) and its ratio (ca. 0.3) to the analogous value for 2.. (8.76 G) are indicative of such protons in isopropyl substituents.8 Discussion a-Spin Distribution in the Radical Anion of Tropone (1). Table I11 gives the “experimental” a-spin populations lp,l which were obtained for the carbon centers fi = 2 to 7 in 1; hy making use of the coupling constants aHp(Table I; -60 “ C ) and the McConnell equations an,, = QP,,
(1)
with IQI = 26 G. This value of IQl is an average of 27.4 and 24.6 G which are the total spectral spreads of the tropylium radicalloand its dianion,” respectively. In addition, values p,, were computed by the McLachlan procedure (A = 1.0).’2 Owing to the vertical nodal plane passing through the C=O group in the singly occupied HMO of 1-., the calculated a-spin populations pr are rather insensitive to variations of the heteroatom parameters a. and pco in the ranges la + PI < la01 < 101 + 1.501 and IBI < 10~01 < 1201.
Table 111 lists the values p,, which resulted from the use of a. = a + 1.20 and pco = 1.560, as proposed hy Vincow and Fraenkel.I3 What may he regarded to be the most reliable representation of the a-spin distribution in 1-. is a combination of the experimental values for the centers hearing protons ( p = 2 to 7) with those calculated for the centers void of protons (fi = 1and 0). This combination yields the a-spin populations p,, which are denoted in Table 111 as the “best” values. The signs adopted therein for the ~ , p4,5 accord with the experimental numbers p2.7, P ~ , and prediction hy theory. Since Q < 0 (eq l),they require that = the large coupling constants of the protons in 1-., aH2,7 8.75 and aH4,5= 5.10 G, must he negative, whereas the small one, uH3,6= 0.10 G, should have a positive sign. The “best” values pp will be subsequently related both to the widths, AH, of the I3C satellite lines and to the associated coupling constants, ac,, and thus will serve as a hasis for the assignment made in Table 11. Widths, AH, of I3C Satellite Lines. The widths AH are to a large extent determined by g and hyperfine anisotropies. The contribution of AH of these anisotropies is, in general, expressed by
(AH)aniso= A + BM,
+ CM,’
where MI (= +‘I2or -‘I2)is the spin quantum number of the I3C nucleus. The coefficients A, B, and C, which are dependent on the products of anisotropy tensors, have been considered in detail el~ewhere.’~ In the spectrum of one and the same radical, the term BM, (= +‘J2B or -lJ2B)gives rise to varying widths of those I3C satellite lines which are placed symmetrically about the center of the spectrum, i.e., are associated with an equal coupling constant ac,, whereas the term CM? (= Ij4C) accounts only for different widths of lines due to I3C isotopes with nonequal values of aCr The finding that the corresponding lines a t both spectral peripheries of .1 have almost identical widths (see Results) points therefore to a rather small value of the coefficient B.I5 In contrast, the marked differences in AH, observed for the lines A, B, C, and D (Figure 1 and Table 11) at the same field-end of the spectrum, indicate that the value of C is quite considerable. Since C depends on the square of the I3C hyperfine an-
1128
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978
P. Furderer and F. Gerson
TABLE IV: Observed a n d Calculated I 3 C C o u p l i n g Constants acu. (G) for t h e R a d i c a l Anion of T r o p o n e (1)
stead of eq 4. An analogous modification of eq 5 would lead to
acl = (35.7 G)p1 - (11.6 G)(p2 + ~ (25.1 G)Po
Position
2,l 396 435 1
Expt
Calcd
P
+12.50a - 7.54a t 4.31a -9.66‘
a E q u a t i o n 4. E q u a t i o n 7.
+13.42& - 6.33& t 5.34b -8.52d
E q u a t i o n 6.
(tp2.33 (-)6.02 (t )4.54 (-)8.32
‘E q u a t i o n 5.
isotropy tensor, it is essentially determined by the square of the a-spin population pr a t the carbon center p in question. Inspection of Tables I1 and I11 affords the following sequences of increasing values AH and p:, respectively: C 5 B < D < A and p = 3,6 < 1 < 4,5 < 2,7. The assignment resulting from the correlation between the two sets of values is just the one presented in Table 11. It will be further substantiated in the next section. Calculation of 13CCoupling Constants. The coupling constants ac can be estimated by the Karplus-Fraenkel relationshipf6
acp = (8, +
3 3 u Z , Q ~ ~ u ) + ~ Pu = 1 Q X , C P ~
(3)
where pr and pv are the a-spin populations at the site w of the I3C isotope and at the adjacent atoms X,, respectively. In the case of the proton bearing centers p = 2,7,3,6, and 4,5 of l-., eq 3 reduces to
acp = (35.6 G ) p p - (13.9 G ) ( p u+ p u t )
(4)
if the standard valued6 Sc = -12.7, Qcc, = +14.4, QCH= +19.5, and QC~C = -13.9 G are adopted for the spin polarization parameters Sc, Qcx,, and QxVc. For p = l, on the other hand, the expression acl = (32.7 G ) P -~ (13.9 G)(pZ
+~ 7 ) (5)
(25.1 G)Po
is appropriate, since QCH has now to be replaced by Qco = +16.6 G and an additional parameter Qoc = -25.1 G must be i n t r 0 d ~ c e d . l ~ Insertion of the “best” a-spin populations pp (Table 111) into eq 4 and 5 leads to the coupling constants acplisted in Table IV where they are compared with their experimental counterparts. For the sake of completeness, Table IV also includes the values ~c2,7,aC3,6, and a ~calculated ~ , ~ according to the formula
acp = (38.6 G)pW - (11.6 G ) ( p v +
put)
(6)
which was more recently suggested by Strom et a1.18 in-
7 ) -
(7)
and result in the value given likewise in Table IV. It is interesting to note that the coupling constants ~c2,7 ~ , which a positive sign is anticipated, are more and U C ~ , for satisfactorily reproduced by eq 4 than by eq 6, whereas eq 6 and 7 work better than eq 4 and 5 with the values aC3,6 and acl, respectively, which are expected to have a negative sign. On the whole, agreement between theory and experiment is very satisfactory for all four 13C coupling constants under consideration. This agreement not only definitely confirms the assignment of the observed values acr, but also leaves no doubt that the signs predicted for them by theory are correct.
Acknowledgment. We thank Professor R. Noyori of the Nagoya University for a sample of 3. This work was supported by the Swiss National Science Foundation (Project 2.523.76). References and Notes Y. Ikegaml and S.Seto, Bull. Chem. SOC.Jpn., 41, 2225 (1968); see also Y. Ikegami, H. Watanabe, and S.Seto, ibki, 45,1976 (1972). G. A. Russell and R. L. Blankespoor, Tetrahedron Lett., 47, 4573 (1971). G.A. Russell and G. R. Stevenson, J. Am. Chem. SOC., 93, 2432 (1971). P. Radiick, J . Org. Chem., 29, 960 (1964). R. Noyori, S.Makino, and H. Takaya, J. Am. Chem. SOC., 93, 1272 (1971). F. Gerson, H. Ohya-Nishiguchi, and Ch. Wydler, Angew. Chem., 88, 617 (1976); Angew. Chem., Int. Ed. Engl., 15, 552 (1976); F. Gerson and H. Ohya-Nishiguchi, to be published. In ESR spectroscopy, protons separated from a T center by 0, 1, 2...sp3-hybridized carbon atoms are denoted a,6, 7 , .. . See, e.g., G. A. Russell and R. D. Stephens, J. Phys. Chem., 70, 1320 (1966); F. Gerson, J. Heinzer, and M. Stavaux, Helv. Chim. Acta, 56, 1845 (1973). H. M. McConnell, J. Chem. Phys., 24, 632 (1956). G. Vincow, M. L. Morrell, W. V. Volland, H. J. Dauben, Jr., and F. R. Hunter, J. Am. Chem. Soc., 87, 3527 (1965). N. L. Bauld and M. S.Brown, J. Am. Chem. Soc., 87, 4390 (1965). A. D. McLachlan, Mol. Phys. 3, 233 (1960). G. Vincow and G. K. Fraenkei, J. Chem. Phys., 34, 1333 (1961). See, e.g., G.K. Fraenkel, J. phys. Chem.,71, 139 (1967); A. Hudson and G. R. Luckhurst, Chem. Rev., 69, 191 (1969). The coefficient B contains the product of both g and hyperfine anisotropy tensors. Undoubtedly, it is the relative insignificance of the ganisotropy which causes B to be small in the case of 1;. This insignificance Is conslstent with the low m p l n population at the oxygen atom (po = -0.021) and with the only slight deviation of the gfactor (2.0027 rt 0.0001) from the free electron value. M. KarDlus and G. K. Fraenkel. J. Chem. Phvs.. 35. 1312 (1961). P. D. Sullivan, J. R. Bolton, and W. E. Geige;, Jr., j . Am. C h e i . SOC.,92, 4176 (1970). 901T.(1972). E. Strom, G. R. Underwood, and D. Jurkowitz, Mol. Phys., 24,
