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solutions. The presence of several such acylil radicals was confirmed by spectrophotometric, susceptibility, and electron spin resonance (e.s,r,)4-6measurements. -0 In this communication some results are presented on 0.H the photoformation and photosensitized formation of 3 such radicals in two sterically hindered acyloins, i.e., mately three parts of 4 to one of 5 whereas cis-2-decaisobutyroin (R = (CH3)2CH-) and pivaloin (R = lone produced approximately two parts of 5 to one of 6. (CHJ3C-). The sensitization results are of particular interest, since they represent a case in which radicals due to both the sensitizing and sensitized molecules have been observed simultaneously. Moreover, these results may provide additional insight into the reaction mechanism of sensitizers with alcohols in basic solutions. Ei 'H Osn Typical solutions used in the present work consisted 4 5 6 of 0.5 acyloin solutions and 0.5 M KaOH in alcoholU H = 6.49, 6.49, U H = 6.96, 12.63, U H = 6.38, 7 1 4 , water ( 7 0 : 3 0 ) . Such a solution of butyroin readily 12.34, and 13.29 gauss 12.98, and 12.98 gauss and 12.63 gauss exhibited the five-line e.s.r. spectrum shown in Fig. 1 The formation of 5 from both cis- and trans-2-decalone (upper). The lines have the approximate intensity indicates t h a t either an initially formed radical anion ratio of 1 : 4 : 6 : 4 : 1, and are due to the butyril radical or an oxidation intermediate, possibly the 1,%-diketone, (R = C3H7-). Only the four equivalent a-protons underwent epimerization a t C-9. Upon standing 5 interact with the uncoupled electron, which is delocalized decayed more rapidly than 4 or 6 so t h a t after 3 hr. only over the two carbonyl groups. The proton hyperfine 4 remained in the oxidation product of trans-2-decalone, splittings and g-value were found to be 13.5 Mc. and The spectrum of 4 was a 1 : 2 : 3 : 4 : 3 : 2 :1 heptet while 2.005, respectively. the spectrum of 5 was a 1 : 1 : 2 : 2 : 1 : 1 sextet. The specAttempts to form radicals in solutions of the sterically trum assigned to 6 contained 13 lines and is consistent hindered isobutyroin and pivaloin by simply mixing with four magnetically nonequivalent protons. This the reactants were unsuccessful. Hoyever, when these observation demands t h a t 6, a derivative of cis-decalin, solutions were exposed to light (3130 A , ) in the n T* has a frozen conformation in regard to the spectrometer absorption bands of the acyloins, the respective radicals frequency of -9,500 Mc. 'sec. formed readily. Selective light irradiation was acThe results so far obtained suggest that e.s.r. speccomplished with a Bausch and Lomb monochromator troscopy, particularly when used in conjunction with equipped with a 200-w. mercury lamp. The isobutyril oxidation of ketones in basic s o l ~ t i o n is , ~a~ versatile ~ radical thus formed exhibited the spectrum of Fig. 1 and unique analytical technique for structural assign(lower). The three lines with the intensity ratio of ments in rigid alicyclic systems as well as a powerful 1 : 2 : 1 are due to the two a-protons (hyperfine coupling tool for conformational analysis in nonrigid alicyclics. l = 6.0 Mc., g-value = 2.005). In some spectra addi(9) I t has been established b y ultraviolet spectroscopy t h a t t h e major tional incompletely resolved lines (coupling = 0.4Mc.) products of base-catalyzed oxidation of 3-keto steroids with cis- a n d (runswere detected in the main lines, suggesting a weak interA , B ring junctions are action with the twelve /I-protons. The spectrum due to the pivalil radical (not shown here) consisted of 11 lines (out of possible 19) due to the interaction of 18 @-protons (coupling = 0.8 hlc., g = 2.005). Further n amplification revealed the missing proton lines as well OH as some C13 satellites. respectively [ B Camerino, B. Patelli, a n d R. Sciaky, Tetvahedron Letters, Titration of the acyloins with sodium phenolindo554 (1961jj: phenol, which oxidizes specifically 1,2-dienols,' revealed GLEXh. RUSSELL DEPARTMESTOF CHEMISTRY ERACH R . TALATY a similar pattern. Deaerated butyroin solutions reIOWASTATE UXIVERSITY duced dilute solutions of this reagent in the absence of hMES, IOlVA light, indicating the dienol character of butyroin in RECEIVED . ~ C G U S T 31, 1964 basic solution. Isobutyroin and pivaloin, on the other hand, reduced tpe phenolindophenol only upon excitation with 3130-A. light. The results thus far suggest, Photoformation and Photosensitized Formation therefore, that dienolization in sterically hindered of the Isobutyril and Pivalil Anion Radicals acyloins may be accomplished oia an excited state (singlet or triplet) of their anions The oxidation of acyloins (R-CH(0H)-CO-R) to their respective diketones (R-CO-CO-R) in aerated The addition of a sensitizer, such as benzophenone, basic solutions was studied some 30 years ago by 1Veissgreatly enhanced the production of the isobutyril and berger and co-workers They concluded that the pivalil radicals. Moreover, radic?ls were now formed oxidation proceeds via a dienolization step and that the a t wave lengths other than 3130 A. Thus, the strong acylil anion radical was present in some of the reaction ( 2 ) I. Michaelisand E S. Fetcher, J . A m . Chem. S O L 6, 9 , 1246 ( 1 9 H i )
-
R-C-C-R ( I ! A Weissberger, E . Strasser, H . hlainz. a n d W . Schwarze. A n n . , 478, 112 (1930); A . Weissberger, Bel., 66B, 1815 (1932)
(3) J 1. Ihrig and R . G Caldwell, t b t d . , 78, 2097 (1056) ( 4 ) K Dehl and G K . Fraenkel. J , Chem P h y s . . 39, 1793 (1963) ( 5 ) G. A . Russell and E . T . S t r o m . J . A m Chem. Soc , 8 6 , 744 I 1964) (6) H . C Heller. unpublished results. ( 7 ) B Eistert in Houben-Weyl, ">lethoden der Organischen Chemie." Vol. 11. E p. 395.
h l u l l e r . Ed , 4 t h Ed
Georg Thieme Verlag. S t u t t g a r t . 19.52.
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Dec. 5 , 1964
4 0-protons 7.64 Mc 4 m-proton8 2.68 MC
2 p - protons 9.69 MC
' Fig. 1,-Electron
IOMc
'
spin resonance spectra of butyril (upper) and isobutyril (lower) anion radicals.
e.s.r. signals of the isobutyril radical in Fig. 2 (lower) were produced in an isobutyroin solution containing 0.01 M benzophenone, exposed to 3660 A., the n + ?r* band of benzophenone. Note t h a t Fig. 1 (lower) and Fig. 2 (lower) were obtained with the same spectrometer settings of microwave power and amplification, and t h a t Fig. 2 shows even CI3 lines of the radical. Clearly, there is an order of magnitude increase in the radical concentration in the sensitized run. Comparison of the upper and lower curves of Fig. 2 shows t h a t the benzophenone ketyl anion radical (C6&)2C=6 is also present in the sensitized experiment. T h e spectrum of the ketyl radical (Fig. 2 , upper) was observed in an alcohol-water solution of 0.01 M benzophenone and 0.5 h ' NaOH exposed to 3660-A. light. This radical has been previously observed.8 Two tentative mechanisms may be suggested for the photosensitized results : (1) Triplet-triplet energy transfer from the excited benzophenone to the isobutyroin anion, followed by the enolization and oxidation of the latter. A singletsinglet transfer can be ruled out, since the singlet state of the former (26,000 ~ m . - ~ is )lower ~ than t h a t of the latter (centered a t 34,500 cm.-l). T h e position of the triplet of isobutyroin is unknown but may be assumed to be close to that of acetone, i.e., approximately 24,500 cm. - I . l o Hence, energy transfer from the benzophenone triplet (24,250 C ~ . - I ) ~is conceivable. In the framework of this mechanism, the formation of the ketyl radical would be an independent process, just as i t is in the solution devoid of acyloin. (2) The effective transfer of an electron from the acyloin anion to the excited, electrophilic benzophenone molecule (probably triplet), resulting in the reduction of the latter and the oxidation of the former without an enolization step. This over-all reaction could proceed either through an actual electron transfer, yielding ( 8 ) P . B. Ayscough a n d K . Wilson, J . Chem S o c . , 5412 (1963). T h e excitation energy is n o t specified in t h i s work. (5) V . I-. Ermolaev, Sovirf P h r s . - C s p . ( E n g l . Transl.), 6,333 (1963). (10) G S.H a m m o n d , S J . T u r r o . a n d P. A . Leermakers, J . Phys. C h e m . ,
66, 1144 (1962).
Cl3
1
\ IJjI/$&&/+w :6
4 2
cycy)
1 CH protons 6.0 Mc
-
-
-
(CH,),CHI
IO Yo
+:e, :o m n
C-C-CHlCHsk
I
Fig. 2.-Electron spin resonance spectra of the benzophenone ketyl (upper and lower) and isobutyril anion (lower) radicals; arrow indicates position of center line of the ketyl radical.
00
Ii iI
directly (C6&)2C=O, R-C-C-R, a hydrogen-atom transfer step." (C6H&C-
and water, or via In the latter case,
OH and the isobutyril radical would form
first, and the former would then give (C6H&C=6 and HzO in basic solution. Further work on these phenomena, including the use of other sensitizers, is now in progress, (11) J. Pi. P i t t s , K . L. Letsinger, R. P . T a y l o r , J . .M. P a t t e r s o n , G . Recktenwald, a n d R . B. M a r t i n , J . A m . C h e m . SOL.,81, 1068 (1959); W.M. Moore, G. S. H a m m o n d , a n d R. P. Foss, ;bid., 85, 2789 (1961). (12) I n some former publications, C. Heller was used.
SCIENTIFIC LABORATORY HASAXC. H E L L E R ' ~ FORD MOTORCOMPASY DEARBORN, MICHIGAN RECEIVED SEPTEMBER 26, 1964
_______ Structure in the Ionization Efficiency Curves of Ar2+ by Pulsed Mass Spectrometry
Sir: Previous observations of the ionization e fficiency (using continuous ion repulsion) curves of Ar2+ (1) J. A. Hornbeck a n d J . P. Molnar, Phrs. Reu., 84,621 (1951). (2) J. S . Dahler, J. L. Franklin, M. S. B. M u n s o n , and F H. Field, J . C h e m . Phys., 3 6 , 3332 (1962). (3) W.K a u l , U. L a u t e r b a c h , a n d I
