6343
spectrum is in clear contrast to those of tridehydro[26]ann~lene,~ tridehydr0[30]annulene,~and pentadehydr0[30]annulene,~in which all the protons appear as a very broad band at -7 2.0-4.5. Acknowledgments. We are indebted to the Science Research Council and to the Royal Society for generous financial support. * Address correspondence to this author. R. M. McQuilkin, F. Sondheimer* Chemistry Department, University College London W.C.1, England Received August 10, 1970
Ketonization of Gaseous Enol Ions'
Sir : In the mass spectra of aliphatic ketones abundant ions can be formed in both the keto and enol forms, but
sented to show that the keto tautomer is of higher energy, and that the enol ion does not ketonize prior to f r a g m e n t a t i ~ n . ~We report evidence from the metastable ion abundances of labeled compounds that the major decomposition pathways of three simple enol ions appear to involve initial ketonization. In ion cyclotron resonance studies3t4 of C 3 H 6 0 + ions no appreciable tautomerism between the keto and enol forms was observed despite the relatively long (millisecond) ion lifetimes involved. However, we find that the predominant product, C2H30+,arising from decomposition of the enolic C3H,0+ (1) in the mass spectrometer appears to be formed through rearrangement to an unstable keto intermediate, 2, not through the simple cleavage 1 --t 12 (Table I).7 Direct losses of CH2" and HYfrom 1 to form CH3CO+ (7), especially in greater abundance than C2H2"HYO+ (8 or 12), as observed, appear improbable. However, formation of 7 and 8 through the common intermediate 2 is consistent with the identity (izO.2 eV) found for the
-
i*
b," +OH*
+
CH,C=O+
.cH,"H*
H3C
13
+
CH,CH,.
CH,=C=OH 15
+
+
*CH1
16
1
- 1+*
CH,CH,C=O+
19
+
CH,'
CHSCH,
CH3CH2.
+
CHB
CH3CE O f 20
18
interconversion between such tautomers has not been reported. 2-4 Considerable evidence3 has been pre(1) (a) Metastable Ion Characteristics. XVI. Paper XV: I . Howe and F. W. McLafferty, J . Amer. Chem. Soc., 92, 3797 (1970); (b) reported in part at the International Mass Spectroscopy Conference, Kyoto, Japan, Sept 1969, Preprints, p 217. (2) H . Budzikiewicz, C. Djerassi, and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, San Francisco, Calif., 1967, p 18. (3) J. Diekman, I. K . MacLeod, C. Djerassi, and J. D. Baldeschwieler, J . Amer. Chem. Soc., 91, 2069 (1969), and references therein. (4) G . Eadon, J. Diekman, and C. Djerassi, ibid., 91, 3986 (1969); submitted for publication. We are indebted to Professor Djerassi for preprints of these manuscripts.
appearance potentials of 7 and 8. Also 1 --t 12 is a simple cleavage reaction, and so should be favored over the rearrangement 1 ---t 2 in decomposition of the higher energy normal ions;8 the data of Table I are
-
+
( 5 ) For example, the reactions 6 11b,3v4and 14 --c 15 17.6 (6) W. Carpenter, A. M. Duffield, and C. Djerassi, J . Amer. Chem. Soc., 90, 160 (1968).
(7) The full papers will include data on nine different labeling patterns of enolic C3(H,D)sO+ ions and six of Cd(H,D)80+ ions. Except where noted, the normal spectra indicated that the isotopic enolic ions were formed in good purity. (8) F. W. McLafferty and R. B. Fairweather, J . Amer. Chem. Soc., 90, 5915 (1968).
Communications to the Editor
6344 Table I. Decomoosition of CdH.D)&I+ Ions Relative metastable" (normal)* ion abundances----CzH80+ CzHzDO' CzHDzO+ CzD30'
7 -
Precursor molecule
Ion
OH
I I
'OH
I:.
CDjC-CHz
I
CD*-CLCH~
CHzCHz OD
I
+OD
1;.
CDaC-CHz
CD,CC ' H,
1 1
CHzCHz OD
I I 1
'OD
I(.
CHaC-CDz
CH,C'-CD,
CDzCHz ' 0H
I:.
CDaC(=O)C4Hed
CDJC"CH2
54 zt 4
22 f 3
20 32 3
100
54 i 4
19 =k 3
18 f 5
100
+OH
I;.
CD,C%H,
a Determined by the Barber-Elliot-Major defocusing technique. * Hydrocarbon species eliminated by high resolution. Error limits indicate the reproducibility of the results. Contained 10-15 CD2HC(=O)CHDC3Hi due t o a-a' exchange on storage. e Contained 10-15 CD3CH2CHDCOCHDC3Hidue to a-a' exchange on storage.
z
z
Table 11. Decomposition of C,(H,D),O. Precursor molecule
Ions
Ion
OD
CaHjO+
Relative metastable (normal) ion abundancesO C ~ H I D O ' CsHaD20+ GHzD,O+ CzH3OC CzHzDO'
C,H3D,0f
+OD
1 !.
I
CHaCHzC-CHz
I
+
0.6 i 0.2 (14)
CH~CHZC-CH~
1
67 i 5
100
2.1 r 0.2
(97)
(100)
(25)
1 . 2 I- 0 . 2 (60)
100 (100)
CHzCH2 OH
+OH
I CDaCHzC-CHz I 1
1'.
+
0.4 0 . 2 0 . 8 i 0 . 2 70 i 4 (
