COMMUNICATIONS TO THE EDITOR
744
Vol. 86
for a legume with no other source of nitrogen. Table I summarizes experiments completed thus far. Acknowledgment.--The work has been supported by a summer grant from the Climax Molybdenum Company.
electron by the aromatic rings. It was surprising to discover that base and traces of oxygen convert many alicyclic ketones (method 1) and a-ketols (method 2) to stable semiquinones. These radical-anions can also be prepared by electron transfer between 1,2-diketones and anions derived from acyloinsZb(method 3) or by DEPARTMENT O F CHEMISTRY G. P. HAIGHT,J R . SWARTHMORE COLLEGE RONALD SCOTT electron transfer between 1,2-diketones and other SWARTHMORE, PENNSYLVANIA carbanions, such as [CsH&OCHCH3]- (method 4). RECEIVEDNOVEMBER 22, 1963 In a typical experiment, air-saturated solutions of a ketone (0.05 M ) and potassium t-butoxide (0.1 M) in dimethyl sulfoxide (SO%)-t-butyl alcohol (20%) Formation of Alicyclic Semiquinones. Conformational are mixed under nitrogen. In many cases an e.s.r. Analysis by Electron Spin Resonance1 spectrum can be detected immediately. Use of too Sir : much oxygen gives rise to other stable radicals in Paramagnetic intermediates in the basic oxidation of addition to the desired semiquinones. Oxidation of the acyloins is best performed using partially deoxygenated aromatic acyloins are recognized to be radical-anions of TABLE I CONFORMATIONS OF CYCLOALKANEDIONE RADICAL-ANIONS Method of prepn.
Radical-anion
axial
1 Cyclopentane-1,2-dione 1 4-t-Butylcyclohexane-1,2-dione Cyclohexane-1,2-dione 1, 2, 3, 4 1 Cycloheptane-1,2-dione 1 Cyclooctane-1,2-dione 1 Cyclononane-1,2-dione 1, 2 Cyclodecane-l,2-dione 1 Cyclododecane-l,2-dione 1 Cyclopentadecane- 1,2-dione 1, 4 Carnphorquinone 1 Tropane-1,'L-dione 1 l-Phenylpropane-l,2-dione l-Phenylbutane-1,2-dione 1 2 t-Butylphenylglyoxal 2 Di-t-butylglyoxal
equatorial
14.16" 13.10
6.55
6.70
1.97 3.33f
12.57 8.33 7.88 7.23 Umethyi-a amethyl-H amethyl-H
5.49 -0 -0 2.07
amethyl-H
I
+I3 (+43)" + 3 o r -50 ($30 or -60)' +I1 m-30 --30 +2 or -50
BP
Pb
17.8 13.8 13.86 6 . 7 o r 16.7 4 . 4 o r 13.3 13.0 11.1 10.5 7 . 2 or 1 7 . 5
0.30 .24 .24 . l l or .29 . 0 8 o r .23 .22 .19 .18 .12 or .30
= 2.66' = 5.95, &ingle-H = 1.go = 3.46
amethylem-H amethyl-H
&ial-H
( +27)c*d
9.82"
I
r-
(gauss)a-
-----Q=-H
3.42
= 0.17
0.31h
hyperfine splitting. Figure 1 shows the spectra of the oxidation products of (a) 2-hydroxycyclohexanone or cyclohexanone and (b) 4-t-butylcyclohexanone. The unsubstituted compound has four equivalent a-hydrogens with a hyperfine splitting constant (hfsc) of U H = 9.82 gauss, peak height ratios = 1:3.8:6:3.8:1. This cyclohexene derivative does not possess conformational stability in terms of the spectrometer frequency (-lo4 Mc./sec.). However, the substitution of a t-butyl group gives rise to a conformationally stable radicalof protons with
UH
=
13.10 and 6.55 gauss.
The
(2) (a) B.Venkataramen and G. K . Frankel, J . A m . Chcm. SOC., I T , 2707 Fig. 1.-E.s.r. spectra of cyclohexane-1,2-dione radical anions (1955); (b) G .A . Russell, E . G . Janzen, and E. T. Strom, i b i d . , 84, 4155 prepared by oxidation of ( a ) cyclohexanone (1962); (c) R . Dehl and G. K . Frankel, J Chem. P h y s . , 89, 1793 (1963). or 2-hydroxycyclohexanone and ( b ) 4-t-but~l-2-h~drox~cYClO- (3) Kindly supplied by Professor D . E. Applequist. (4) C . Heller and H. M. McConnell, J. Chcm. P h y s . , 84, 1535 (1960) hexanone in the presence of base.
COMMUNICATIONS TO THE EDITOR
Feb. 20. 1964
I
For the ethyl radical, B has a value of 58.5 gauss.' Solution of the equations, aaxia1.H = Bp COS2 eaxial, Uequat-H = B p COS2 eequat, and Oequat-H = Baxial-H f 120' (tetrahedral a-carbon atom) gives the results listed in Table I . 6 It is interesting that the value of Oaxial for the a-hydrogens of the 4-t-butylcyclohexane- 1,2-dione radical-anion is the same as found for cyclohexene by n.m.~-.~ Our results demonstrate the application of e.s.r. to conformational analysis of nonrigid systems and suggest the use of e.s.r. for structural determinations of rigid ketonic systems, such as the steroidal ketones. Work in this area is in progress.
745
were, respectively, 0.17 f 0.01, 0.07 f 0.01, 0.05 f 0.01, and 0.01 f 0.01. In addition, fluorescence was not detected in any of these solutions. However, chemical transformation does attend irradiation of azomethane in degassed solution and can be observed by ultraviolet absorption and by proton magnetic resonance spectroscopy. A solution of azomethane (I, to lo-' M ) in degassed DzO shows a . , ~ is single sharp resonance lying a t 6 $0.984 ~ . p . m and indefinitely stable in the dark a t room temperature. Upon irradiation of the solution a t room temperature, a new sharp singlet (11) appears 0.10 p.p.m. to high field from the resonance of I . By following the ratio of the height of the new resonance (11) to that of I as a function of irradiation time, it can be demonstrated that a photochemical equilibrium hv
I
I1 hu
is established. Experiments using D2O as solvent reveal no proton-deuteron exchange with the solvent while coming to and remaining a t photochemical equilibrium, This, together with the simplicity of the spectrum of 11, demands magnetic equivalence of all the protons of 11. A degassed aqueous sollition of I has an absorption (5) R . W. Fessenden and R . H . Schuler, J . Chem. P h y s . , 89, 2147 (1963). These authors determined Qp (CHa). Since c 0 s ~ 4 5(free ~ rotation) is * / n , B is maximum a t 343 mp ( E 25); irradiation with 363-mb twice their value of Q p (CHd light results in a shift of the apparent maximum to (6) Two values of Bp and two values of 0 (except when cos2 Oequat.H = 0 ) longer wave lengths with a concomitant increase in the are obtained Unrealistic solutions ( B p > 58.5/2) have not been listed optical density maximtlm(ADmax). Plots of ADma, (7) G. V . Smith and H . Kriloff, J . A m . Chem Soc., 86, 2016 (1963) These authors use a dihedral angle for axial hydrogens equivalent t o 90' vs. time of irradiation confirm the attainment of photo- e.. chemical equilibrium, the band undergoing a 70% in(8) (a) Alfred P . Sloan Foundation Fellow, 1959-1963; (b) National crease in ADmaxwith the establishment of the new Institutes of Health Predoctoral Fellow, 1962-1963. GLENA . RUSSELL& maximum a t 330 mp. DEPARTMENT OF CHEMISTRY E. THOMAS S T R O M ~ ~ From the photochemical steady-state areas of the IOWASTATEUNIVERSITY AMES, IOWA proton resonance peaks, KD,O= [II]/ [ I ] = 0.09 5 0.01, RECEIVED NOVEMBER 4, 1963 for 365-mp light. Under basic conditions I1 reacts (vide infra) a t least 100 times faster than I , so that it can be removed selectively from solution. This perPhotoisomerization of Azomethane mits an independent spectrophotometric determination Sir : of K , which, over a tenfold concentration range (5 x Although cis-trans isomerization is well-established M ) has a value of 0.10 f 0.01, in to 5 X for aromatic azo compounds,1,2i t has not been observed agreement with the proton resonance method. in the aliphatic series. We wish to report experiments Additionally, optical density measurements show in which the photochemical cis-trans isomerization of that methanol, ethanol, and ethyl ether permit attainazomethane and the isolation of pure cis-azomethane ment of equilibrium with little photolytic decomposihave been achieved. tion. I n methanol, the equilibrium is insensitive to CII3 temperature over the range -40 to f30". Indeed, I1 / is even formed on irradiation of an ether glass of I a t N=N CHFN-NHCH, -196". The production of I1 also can be observed in / CHI CH3 CHI carbon tetrachloride, toluene, and isooctane, although I I1 I11 photochemical equilibrium cannot be reached because Azomethane (I) which exists in the trans configuraof concomitant decomposition. ti~n,~ photolyzes -~ readily in the gas phase, yielding If a solid thin film of pure I is irradiated a t - 196", I1 nitrogen and two methyl radicals. In agreement with is formed. Unisomerized I together with the very previous work,6 the quantum yield ( 4 ~for ~ )the photosmall amounts of ethane and nitrogen formed by photolytic decomposition of gaseous azomethane (I) by 365lytic decomposition may be removed by vacuum distilmp light was found to be 1.0 f 0.1. In marked conlation a t -78". Further fractionation a t c a . -50" trast, the quantum yield decreases dramatically when provides I1 as a colorless liquid of m.p. - 53 to -49". azomethane is irradiated in solution. Thus, for care11, dissolved in D20, shows the expected single sharp M solutions in isooctane, fully degassed 5 X proton resonance a t 1.08p.p.m. to high field from HDO. ethanol, N,N-dimethylformamide, and water a t 25", Irradiation of this solution gives back I ; there is no the quantum yields, determined by nitrogen formation, dark reversion of I1 to I. The absorption spectrum of an aqueous solution of I1 has a maximum a t 353 mp ( e (1) G. Zimmerman, I,. Chow, and U. Paik, J . A m Chem. Soc.. 80, 3528 (1958); E. Fischer, i b i d . , 81,3249 (1960). 240).* The value, = 0.09, calculated from the ~
(2) c i s and trans isomers of difluorodiazine (NzFd are known: C . B. Colburn, F . A . Johnson, A . Kennedy, K . McCallum, L. C . Metzger, and C . 0 . Parker, i b i d . , 81, 6397 (1959); J . H . Noggle, J. D. Baldeschwieler, and C . B. Colburn, J . Chem. Phys., 87, 182 (19621. (3) W. West and R . B. Killingsworth, ; b i d . , 6, 1 (1938). (4) G. Herzberg, "Infrared and Raman Spectra," D. Van Nostrand Company, Inc., Princeton, PI'. J . , 1945, p. 357. ( 5 ) I . D. Brown and J. D . Dunitz, Acta C r y s f . , 18, 28 (1960). (6) E. W. R . Steacie, "Atomic and Free Radical Reactions," Vol. I , Reinhold Publishing Corp., New York, N . Y., 1954, p . 376.
(7) Chemical shifts are expressed in p . p . m . displacement (negative to low field, positive to high field) from the solvent H D O resonance. ( A dilute solution of azomethane in CClr shows its resonance at 3.67 i 0 02 p p m . t o low field from internal tetramethylsilane.) (8) The molar extinction coefficient and Xmax accord with values obtained from the literature for cyclic aliphatic azo compounds constrained t o a cis configuration: S . G. Cohen, R Zand, and C . Steel, J . A m . Chem S o r . , 8 8 , 2895 (1961); S. G. Cohen and R . Zand, ibid , 84, 586 (1962); C . G Overberger, J. P . Anselme, and J. R . Hall, ibid.,86, 2752 (1963).
