524
of orbital (and state) correlations are just as real in there seems to be no entirely conclusive experimental excited states as they are in ground states. Even evidence to support such predictions. However, if the deactivation step leading to products actually Becker, et a / . ,1 2 , 1 3 have reported that the fluorescence proceeds through the V-0 m i n i m ~ m that , ~ minimum quantum yield of several compounds with benzene cannot be reached if such a barrier intervenes, unless or naphthalene chromophores which transform photosufficient extra energy is available. chemically into quinoid derivatives decreases strongly We wish to propose that the requirement of "extra with decreasing wavelength of exciting light, presumenergy" in photochemical reactions i n dense media ably because the photochemical process competes more may be much more common than is currently realized, efficiently, l 4 Experimentally, it may give rise to (a) teniperatureWe are presently extending our calculations to new dependent quantum yields, (b) wavelength-dependent systems and searching for experimental evidence to quantum yields, (c) triplet reactions requiring a senverify our predictions. We hope that this communicasitizer of disproportionately high T1 energy, (d) twotion will stimulate additional interest in this area. photon reactions (So hv-. s, TI h v + T, + prodAcknowledgment. I am very grateful to Professor uct), depending on the height of the barrier i n v o l ~ e d . ~ - ~ F. E. Harris, University of Utah, who generously supSo far, our calculations revealed two general cases ported part of this work using funds from NSF Grant where such phenomena are predicted for monomoNO. GP-11170. lecular electrocyclic reactions: first, those in which the orbital correlation resembles that in Figure 1 in (12) R . S. Bccker and J. Michl, J . Amer. Chrm. Soc., 88, 5931 (1966). that it is not the highest occupied molecular orbital (13) R . S . Becker, E. Dolaii, and D. E. Balke, J . Chem. Phys., 50,239 (1969). of the reactant, but one of the more bonding ones, (14) Another explanation of these results would be wavelength which becomes antibonding during the reaction (or dependent rate of intersystem crossing followed by a reaction from the similarly for the lowest free MO of the reactant or triplet state. Bccker, e t a l . , consider this unlikely for systems such as the chronieiic?.l~ However, in other molecules, particularly those with both). Typically, such reactions involve a drastic transrelatively lo\v-lying ii-r* ctnfi's, this may be an important source of formation of the chromophore, and barriers are present wavelcngth dcpcndcncc. both in S1 and TI. In addition to I + 11, another Josef Michl example for which such behavior is predicted is 111 + Depcrrft?ieiifof Physics atid Deparfmeiif of Chemistry 1V.l" In the second case the orbital "crossover" is Uiiiwrsify of Utah, Salt Lake City, Utah 84112 normal, but S1 is not derived from the lowest energy Receiwd Noaember 10, 1970 configuration, 1 + -1. This situation arises conimonly in aromatic chromophores as a result of strong configuration interaction between singlet 2 + - 1 and Oxidation by Metal Salts. V1I.l Syntheses 1 -+ -2 excitations. According to our calculations, Based on the Selective Oxidation of such lowest singlet state (2 -+ -1, 1 -t -2, or 'Lb in Organic Free Radicals Platt's notation) typically correlates with a highly excited state of products. It is only the second singlet Sir: state, lLa, which is of 1 + -1 character, that correlates We wish to report a novel reaction of enolizable smoothly with the first excited state of the product. ketones with olefins which takes place in the presence of The magnitude of the expected barrier for the reaction metal oxidants such as manganese(II1) and cerium(1V) from Sl depends on the separation between 'Lb and acetates leading to the formation of y-keto esters. This lLa. It is usually much smaller than the barrier found reaction, which depends c n the selective ability of for cases such as I + IT. For triplet states, the 1 -,- 1 (3La) state is invariably below 1 + -2, 2 + -1 (3Lb), higher kalent metal ions to oxidize organic free radicals, was in turn used to estimate the relative strengths of the and no barrier results." Examples of such reactions various metal oxidants, Cu(II), Ce(IV), and Mn(II1). are transformations of benzene and naphthalene chroAs a representative example, the reaction of acetone mophores into quinoid ones, such as V + VI, where with octene-1 in the presence of manganic acetate under disrotatory opening from S1 should require "activation a nitrogen atmosphere was studied in detail. A soluenergy" (conrotatory opening is forbidden much more of octene-1 (7.5 nimol), acetone (200 mmol), and tion strongly yet and lacks the V-0 minimum). So far, manganic acetate (15 mmol) in glacial acetic acid (40 ml) containing 10% potassium acetate was heated at 85" until the brown manganic color had disappeared. (7) When several competing reactions can occur, different product composition uould be expected as wavelength of the exciting light or Three major products were isolated after work-up : temperature is changed. the saturated ketone undecanone-2 (I, 1.13 niniol); an (8) The barriers for the processes A* B and B* A may bc of unsaturated ketone? CllH200 (11, 0.54 mmol), bp unequal magnitudes; e . g . , the former may require two photons while the latter already proceeds from SI a t elevated temperatures. 79-81" (1.5 nini); and a keto acetate3 C13H2i03(111, (9) Professor A. Weller suggested t o us that two-photon reactions 1.78 mmol), bp 112" (1.0 mm). The relative yields of might also proceed in solutioii ria triplet-triplet annihilation (TI + TI the three products could be changed by modifying the S, product). (IO) During the preparation of this manuscript, Professor J. Meinreaction conditions. The same products were obwald kindly communicated to us that he and his collaborators have
+
-
+
-+
--f
-+
-+
indeed found I11 t o behave as prcdictedl (submitted for publication). ( 1 1) We believe that differences between singlet and triplet reactivities can be quite generally understood on the basis of differences in the shapes of potential energy surfaces of SI and TI due either t o differences in configuration intcraction, such as those discussed presently, or, more comnionly, to the presence of V - 0 minima in the SI but not TI surface3 (which in turn has minima corresponding t o biradical structures). A more comprehensive communication on this topic is under preparation.
Journal of the American Chemical Society
1 93:2
(1) Preceding papcr by R. M. Dessau, S. Shih, and E. I . Heiba, J . Amer. Chcm. Soc., 92, 412 (1970). (2) Probably a mixture of noiiconjugated unsaturated ketones having ir absorption at 1727 em-' and a complex nmr multiplet in the T 4.54.8 region. (3) The ir spectrum showed two carbonyl absorptions at 1742 and 1728 cm-1, and the nnir spectrum was consistent with the assigned structure.
January 27, 1971
525
,
5 ,
1
,
I
I
,
,
I
OAc
I11
I1
tained when ceric acetate was used in place of manganic acetate. The formation of these products can best be explained by the mechanism shown below. In this mechanism, the initial step involves the oxidation of the ketone by either manganic or ceric ion leading to the a-keto r a d i ~ a l . ~This , ~ radical rapidly adds to the olefin forming a secondary alkyl radical which can either abstract hydrogen atoms from the solvent forming the saturated ketone I or be oxidized by the higher valent metal ion to the unsaturated ketone I1 and the keto acetate 111. 0
II
CH,CCH
Mn (HI) -Ior Ce (IV)
-
0
I
CH,CCH,.
+
Mn(I1) or Ce (111)
0
I1
CH,CCH,'
+
H+
0
+ C6H&H=CH2
1I
4
[ACETONE]
Figure 1. Effect of ketone concentration on product ratio: A, [Mn(III)]< = 8.34 X 10-2M, T = 45"; B, [Mn(III)]l = 8.34 X M , T = 70"; C, [Mn(IIt)], = 1.67 X 10-'M, T = 70".
adducts from olefin^,^ and the addition of free radicals to aromatic hydrocarbons. lo Applying a steady-state kinetic treatment to the mechanistic scheme shown above, one can derive the following equation, which predicts a linear relationship
CsH12CH-CH2CH2CCHS
d[I1 $11
0
I
CGHj,CH=CHCH,CCH,
I1
0
II
CsHj3CHCH&HzCCH,C
I
OAc
I11
The success of this reaction can be attributed to the very selective oxidation of organic radicals by metal ions. The initially formed a-keto radical is not easily oxidized due to the electron-withdrawing character of the carbonyl group,fi whereas the secondary alkyl radical formed by addition to the olefin is quite readily oxidized by the metal ion.' The rapid oxidation of this adduct radical also accounts for the absence of significant polymerization during the course of the reaction. Other examples of syntheses based on the selective oxidation of carbon radicals include the Meerwein reaction,* the formation of lactones and alkyl ester (4) R. VanHelden and E. C. Kooyman, Recl. Trac. Cbim. Pays-Bas, 80, 57 (1961); H . J. den Hertog, Jr., and E. C. Kooyman, J . Cutal., 6 , 357 (1966). ( 5 ) G. A . Russell and J. Lokensgard, J . Amer. Chem. Soc., 89, 5059 ( 19 67). (6) J. I
