John S. Swenton
The Ohio State Universitv Columbus, Ohio 43210 '
I I
Photochemistry of Organic 11, Carbony/ Compounds
The photochemistry of carhonyl systems has been one of the most intensively investigated areas in photochemistry. In the abstract which follows no attempt will he made to cover the chemical literature comprehensively. Rather, the examples given were selected with the hope of illustrating the more basic photochemical transformations. The referencing in many cases is to leading articles and is not meant t o suggest any chronological order for the contributions of various research groups. Electronics of the Simple Carbonyl Group
Before consideration of the photochemistry of ketones, it seems instructive to briefly and qualitatively examine the electronic states and make-up of the simple carhonyl linkage. The ground state carhonyl group consists of a sigma framework made up of cylindrically symmetric sigma orbitals strongly localized between the carbon and oxygen atoms, a rr-system delocalized over the carbon and.oxygen atoms, and p, and sphybrid orbitals a t oxygen. In conjugated carhonyl compounds the sigma, p,, and s p orbitals of the carbonyl group are .retained, the major difference in electronic make-up heing that the a-system now extends over more atoms (Fig. 1). U-BOND
Figure 1.
Electronic make-up of the carbonyl group.
Simple ketones show several regions of absorption in the ultraviolet. The first absorption band is the very weak singlet-triplet transition near 4000A0(s,, 5 The weak transition occurring near 2800 A is the wellknown n-rr* singlet transition. This absorption appears responsible for most of the photochemical processes in saturated ketone photolysis. I n addition t o these weak absorptions there are two moderately intense bands occurring about 1950 and 1750 .&, originally assigned to n-a* transitions (1). Udvarhaai and ElSayed (2) recently suggested that the first band might be due to an n-8c.o transition and the second to an n - 6 c c promotion. The highest epergy band is a very intense absorption around 1500 A and corresponds t o a s-a* excitation. Figure 2 shows qualitatively the molecular orbitals of formaldehyde and the different electronic transitions available.
Figure 2.
Orbitals of formaldehyde.
The promotion of an n-electron to the rr-system (n-?r*) in formaldehyde has a pronounced effect on the geometry of the molecule. The carbon-oxygen distance changes from 1.21 A in the ground state to 1.32 A m the n-a* singlet state (5). I n agreement with the weakening of the C-0 bond in the excited state, the carhonyl stretching frequency decreases from its ground state value of 1745 cm-I (5.73.~)to 1180 cm-I (8.47~)in the excited state (8). Interestingly, both the excited triplet and singlet states of fo~maldehydeare of pyramidal structure. The out-of-plane angle is between 20-27' for the excited singlet and 35' for the triplet state (4). The effect of the promotion of the n-electron largely localized on oxygen t o the antihonding MO would also be expected to affect the dipole moment of the carhonyl group. Measurement of the dipole moment of formaldehyde in the excited state shows that it has decreased from 2.34 D in the ground state t o 1.48 D in the n-a* singlet (6). Thus in the simplest carbonyl compound, excitation has not only raised the energy content of the molecule hut has also had a profound influence on its geometry and charge distribution. In conjugated ketones both the n-a* and the a-a* singlet absorption hands shift to longer wavelengths, the magnitude of the rr-r* shift being larger than that for the n-rr* transition. For example, the rr-a* singlet absorption maximum of 2-cyclohexenone occurs around 230 mp, while the n-rr* hand centers around 320 mp, the exact position of both absorptions heing solvent dependent. While the energy difference between n-a* and a-rr* singlet states is appreciably large for most conjugated ketones (i.e., 2-cyclohexenones and cross-conjugated dienones), the energy gap becomes much smaller for the respective triplet states. Thus appropriately substituted unsaturated ketones may possess the r-a* configuration in the lowest triplet Volume 46, Number 4, April 1969
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217
state. This point mill be discussed in relation to the photochemistry of benzophenones and butyrophenones. General Remarks an Carbonyl Phofochemisfry
A large number of ketone and aldehyde photochemical reactions may be viewed within the framework of two general photochemical processes, the Norrish Type I cleavage and the Norrish Type I1 split.
The products obtained from the photolysis of a given ketone will then be partially determined by the relative rates of these basic processes. These rates in turn are dependent upon the phase, structure, and multiplicity of the excited ketone undergoing reaction. Aldehydes
The gas phase photochemistry of simple aldehydes shows three major dissociative processes (6) : alkyl-acyl fission yielding an alkyl radical and HCO, loss of CO yielding an alkane, and a Norrish Type I1 split yielding an olefin and en01 of a lower aldehyde. These processes are illustrated in eqn. (5) for n-butyraldehyde.
* The type I cleavage involves photochemical bond rupture ar to the carbonyl group to afford initially an acyl and an alkyl radical. The type I1 split occurs with ketones and aldehydes possessing r-hydrogens and results in formation of a lower ketone or aldehyde and an olefin. Thus the formation of alkanes, ketenes, and unsaturated aldehydes from photolysis of cyclic ketones can be easily rationalized by considering an initial type I process followed by decarbonylation and hydrogen transfer reactions. n
As is evident by exan$ation of the quantum yield data a t 3130 A and 2537 A, the alkyl-formyl fission assumes importance a t all wavelengths while the extrusion of CO becomes more important a t the shorter wavelengths. Process (5c) occurs of course only for those aldehydes having y-hydrogens. The data suggest that this process is important a t all wavelengths. The Norrish Type I1 split leads initially to an enol, which subsequently rearranges to the carbonyl tautomer (7). In the case of n-butyraldehyde the type I1 process arises from the excited triplet.
"-r /CHJ CH
I
R
The fact that these processes may be to some degree concerted does not affect the utility of this qualitative approach. Likewise cyclobutanol formation in certain ketone photolyses may be viewed as arising from a biradical formed by initial r-hydrogen abstraction. I
OH Cyclobutanol Formation
218
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Journal of Chemical Educafion
+
(4)
A photochemical process of aldehydes involving dissociation into an acyl radical (RC=O) and hydrogen atom is not commonly observed although such a cleavage has been found in flash photolysis investigations on acetaldehyde (8). The Norrish Type I and Type I1
The extent of type I cleavage for ketones is quite dependent upon the substitution a t the or-carbon and may occur from either the excited triplet or singlet state (9). Thus for a diary1 ketone such as benzophenone, the type I cleavage assumes no importance since such a process would afford the unstable phenyl radical. If the ketone is unsymmetrically substituted, preference is shown for cleavage of the weakest carbon-carbon bond. Thus methyl ethyl ketone shows apredominant cleava~e to yield an acetyl radical and ethyl radical a t 3130 A;
however, this selectivity diminishes as the energy input increases as shown below (10). The ultimate photochemical products in such gaseous systems are largely determined by secondary radical processes.
The observed lack of stereospecificity rules out a concerted decarbonylation mechanism for this system and strongly supports the intermediacy of diradicals whose rate of rotation is faster than ring closure.
Although yields of type I cleavage products are typically lower in inert solvents than in the vapor phase (possibly due to rapid cage recombination of the geminate radical pair), sufficientlystable alkyl radicals may be efficiently produced in solution. Thus Yang and Feit ( 3 ~ )have found that t-hutyl alkyl ketones undergo efficient type I cleavage even though a type I1 split is possible.
+=.03 (Type I1 products)
+
(8)
H
I
cp = .57(Type I products)
This is in dramatic contrast to straight chain aliphatic ketones with y-hydrogens which undergo predominantly type I1 processes. Quenching studies in this system have established that the type I process occurs from both singlet and triplet while the type I1 arises predominantly from the singlet excited state. Calvert and Nicol (11) in an extensive study of competing type I and type I1 processes in a series of n-propyl alkyl ketones in the gas phase found that *I increases and @11decreases as the alkyl group is changed from methyl through 1-butyl. The increasing amount of type I cleavage is probably related to the decreasing strength of the acyl-carbon bond in proceeding along the series. An especially informative investigation in establishing the presence of rotational equilibrium in a gas phase deearbonylation reaction has been carried out by Alumbaugh, Pritchard, and Rickborn (12). Photolysis of either cis- or trans-2,6-dimethylcyclohexanonegave the same major products in the same product ratios.
The question of the multiplicity of the excited state of aliphatic ketones in type I1 processes has been complicated by different results recorded by different investigators. Thus gas phase photoelimination from 2hexanone a t 3130 A was unaffected by even 560 mm of oxygen ({Sa). I n a gas phase study of methoxy acetone a t 3130 A, Srinivasan found that the type I1 split was unaffected by biacetyl, nitric oxide, or oxygen (1Sb). n
n
n
Since triplet quenchers did not affect these processes, the reaction appeared to derive from the singlet manifold. On the other hand, Ausloos and coworkers found a progressive decrease in the quantum yield for the type I1 process in 2-pentanone with the addition of increasing amounts of oxygen (14a,b). More recently Ausloosand Rebbert (15) have shown that 2-pentanone undergoes photoelimination and cyclobutanol formation from its triplet state. This conclusion is supported by the fact that biacetyl quenches phosphorescence, elimination, and cyclobutanol formation from 2-pentanone without quenching its fluorescence. The apparently conflicting results recorded in the case of 2-pentanone and 2-hexanone stems from the fact that both singlet and triplet excited states may be responsible for the type I1 split. Thus quenching data on the type I1 split of 2-pentanone, 2-hexanone, and 2-octanone in solution indicates that two species are reactants (16, 17). The type I1 process for these compounds is quenched by low concentrations of piperylene but the quenching effect levels off a t piperylene concentrations around 0.5M. As is evident from the Table 1 below, 2-pentanone reacts largely from the triplet state in agreement with the vapor phase results of Ausloos and coworkers (14, 15), while 2-hexanone reacts an appreciable portion of the time from the singlet state (Table 1). More recently a stereoelectronic requirement for the Volume 46, Number 4, April 1969
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219
Table 1.
Quantum Yields for Disappearance of Ketone
Ketone
&verdl
+singlet
+triplet
2-pentanone 2-hexsnone
0.44 0.60
0.05 0.21
0.39 0.29
type I1 cleavage has been noted. The irradiation of cis-2-n-propyl-4-t-butylcyclohexanoneleads to 4-t-butylcyclohexanone as the major product, while the transderivative yields the ris-isomer as the major product
character of the solvent is shown below for the photoreduction of benzophenone (19). The low efficiency of photoreduction of benzophenone and also acetophenone in benzene can be used to advantage when sensitization experiments are being performed. Photoreduction of Benzophenone in Va'arimsSolvents Solvent Disappearance .02 water hen~ene .05 toluene .45 hexane 1.0 1.0 ethanol isopropyl alcohol .8-2.0
*
Effect of Structure on Photoreduction of Benzophenones and Acetophenones
Investigations concerning the ability of excited triplet states of ketones to undergo photoreduction have been especially meaningful for development of reactivity indires fnr n-r* vnrms T-r* states. Table 2 comoares Selected Properties of Aromatic Ketones
Table 2.
Compound Benzophenones 4-H 4,4-dimethyl 4-ohloro
(18). The difference in reactivity has been attributed to the unfavorable geometric relationship of the y-hydrogen and the "n-electron" on oxygen. In the cis-isomer t,he y-hydrogen can form a six-member transition state for the type I1 split in which the C-H bond axis is directed toward the odd electron center on oxygen. Such a transition state would be of much higher energy for the trans-compound, thus or-cleavage followed by recombination to the more stable cis-isomer occurs.
unfavorable geometry for type I1
Photoreduction
One of the longest known reactions of a photoexcited ketone or aldehyde is the ability to undergo photoreduction in suitable hydrogen donating solvents. This process occurs for most aliphatic ketones and aldehydes under suitable conditions. For instance, irradiation of acetone in cyclohexane yields 2-propanol, pinacol, acetonylacetone, and cyclohexyldimethylcarbinol. An illustration of the dependence of the quantum yield of reduction on the hydrogen donating 220
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Journal of Chemical Educafion
PhotoTriplet reduced in energy in A prox ethanol- r f k ~ ether glass (m sec)
~~~~~~
j
fli*ali
Yes Yes Yes Yes Yes No No Yea (very slowly) No No
some aromatic carbonyl compounds with respect to photoreduction, triplet energy, and low temperature phosphorescence lifetime. Most aromatic ketones may be placed in one of two categories. One group comprises those which can be photoreduced, have high triplet energies (67-74 kcal/mole), and short triplet lifetimes a t low temperature. Hammond (20a) has suggested that these ketones have the n-s* (Table 2) configuration for the lowest triplet state. The second category comprises ketones which are much less disposed toward photoreduction, have triplet energies in the range 55-61 kcal/mole, and relatively long triplet lifetimes. These ketones are thought to possess a s-a* configuration for the lowest triplet state. The higher efficiency in photoreduction processes by the n-r* state may arise from the highly localized n-electron on oxygen. I n *-seathe unpaired electrons are more delocalized, and thus hydrogen abstraction is more endothermic.
7 n-r'
highly reactive center in hydrogen abstraction
odd electrons delocalized over r-system
r-r*-
However, in the presence of the good hydrogen donor, tri-n-butylstannane, ketones that are not reduced in alcoholic solvents may be photoreduced (ZOa,D). Similar observations have been noted by Baum, Wan, and Pitts in the type I1 processes in butyrophenoues (21). Here hutyrophenone itself affords ethylene with a quantum yield of 0.42, whereas the p-hydroxy and p-amino-substituted butyrophenones give no detectable yield of ethylene (.P = 0.00). It appears that in this case, too, the change in reactivity is due to a change in the triplet from n-s* to r-s* in going from butyrophenone to the p-substituted compounds. Inherent in the interpretation of the change in character of the lowest triplet upon substitution is the fact that the energy difference between n-r*3 and r-7P3 is small. Thus it might be expected that for certain molecules both states might be appreciably populated or solvents could invert the levels of the states. Recently Yang and Murov have found that 1-indanone shows two different phosphorescence emissions differing in lifetime as well as wavelength (22). One emission has a lifetime of 0.5 msec while the second has a much longer-lived emission of 150 msec. These workers have assigned the shorter-lived emission to that from the n-r* triplet while the longer-lived variety was inferred to arise from a mixed n-s* and s-s* state. I n a study of solvent perturbation of triplet levels of acetophenone, Lamola has recently identified the lowest r-s* triplet of acetophenone. Thus in going from a hydrocarbon glass to very polar media (ethylene glycol-water, 85% phosphoric acid, formamide, or glacial acetic acid) the characteristic short-lived n-n* phosphorescence is replaced by a long-lived (1000 msec) phosphorescence (25). Results such as this suggest that much more interesting work remains to be done concerning solvent effects on emission spectra. Non-Conjugated Unsaturated Ketones
The major reaction pathways available to p,y-unsaturated ketones are (a) Norrish Type I cleavage followed by secondary radical reactions, (b) cyclobutanol formation, (c) decarhonylations in those instances where transformation to a stable organic product is possible, and (d) cis-trans isomerization about the double bond. In one of the earlier studies of p,y-unsaturated ketone photolysis Yang and Thap investigated the photochemist,ry of isomesityl oxide ($4).
result from competing type I and y-hydrogen abstraction processes.
In ring systems where the group remains attached to the carbonyl system, combination a t the alternative allylic position occurs (85u,b). Thus in the photolysis of 3-cyclooctenone a photoequilibrium is established hetween the 3-cyclooctenone and Bvinylcyclohexanone (256). I n an irreversible fashion the system is transformed to the unsaturated aldehyde and ketene (trapped with methanol).
The hydrogen transfer processes leading to aldehyde and ketene are analogous to processes established in the photolysis of saturated cyclic ketones (25c). A rather complete referencing of analogous photochemical processes in p,y-unsaturated systems is given in references (25a,b). An interesting p,y-unsaturated system which affords an unusual transformation has been studied by Williams and Ziffer ( 2 6 ~ ) . Irradiation of the p,y-unsaturated ketone in 1-butyl alcohol afforded a 60% yield of the tricyclic ketone shown in eqn. (18).
The authors' proposed mechanism involves type I cleavage followed by radical cyclization. An alternative possibility might involve bonding from the carbonyl carbon to the p-carbon followed by alkoxy radical expulsion and ring closure as depicted below.
The quantum yield for disappearance of ketone was 0.33 indicating a relatively favorable pathway for decomposition. I n fact, the photolysis of acyclic p,y-unsaturated ketones serves as a preparative route to methylene cyclobutanols. The products appear t o
This reaction has been extended to the steroid series where it can be potentially useful in the synthesis of A-NOR-5 a-steroids. Volume 46, Number 4, April 1969
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The photodecarbonylation of ketones, a well-known process in the gas phase, has recently been observed for a number of systems in solution (27). One structural feature which facilitates solution phase decarbonylation of ketones is substitution a t the a and a' carbons of the ketone with groups which are radical stabilizing or else permit transformation of the carbonyl compound to a stable organic molecule. As the examples below illustrate, the stabilizing group(s) a t the m,a' position may be either the vinyl, tetraalkyl, cyclopropyl, or benzo.
Non-conjugated unsaturated ketones a priori might exhibit triplet energy transfer from the carbonyl group to the non-conjugated double bond even though such a process seems slightly exothermic. Thus Morrison found that trans4-hexen-2-one undergoes photochemical trans-cis isomerization along with other photochemirnl proresses (29a). 0
Although the isomerization of a @,-punsaturateddouble bond constitutes an ambiguous case for intramolecular energy transfer for a variety of reasons, the reaction also occurs for y,6-unsaturated ketones (29b). Since the exciting light was such that only the n-T* transition of the carbonyl group was excited, the observed isomerization of the double bond must have arisen from some interaction with the excited carbonyl. Triplet energy transfer from the carbonyl group to the double bond is a reasonable but not demanding interpretation. Isomerization of the double bond in trans-l-phenyl-2butene can also be effected by irradiation of the aromatic ring (296).
H\
In an extensive study of structure-reactivity relat,ions in vapor phase ketone photolysis, Hess and Pitts noted a dramatic change in the quantum yield of carbon monoxide as the position of the double bond or ryclopropane ring rhanged (28).
C=C
/CH,
v In this case isomerization would be the result of triplet energy transfer from the aromatic triplet state to the isolated double bond. Photochemistry of Cyclic Enones Dimerizafions and Cycloaddifionr
The utilization of photochemical cycloaddition reactions has already proven of value in synthetic organic rhemistry (SO). Much of the interest in this chemistry was generated from the dimerization reactions of cyclopentenones and cyclohexenones. Irradiation of cyclopentenone either neat or in solvent gives two dimers, the proportion of which is a function of solvent and concentration (,?I, 32). Here again molecules substituted so as to stabilize the hypothetical species of eqn. (29) have available a smooth and effirient pathway to decarbonylation. 0
-f
-0
C
-
products
(29)
The presence of two formally non-conjugated chromophores in a small molecule affords the opportunity of observing intramolecular energy transfer. 222
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Journal of Chemical Education
Similarly, cyclohexenones also dimerize to give mixtures of the head-to-head and head-to-tail products (33, 34).
The reaction is not limited to self-dimerization as cyclopentenone and cyclohexenone also add to olefins, acetylenes, and allenes to afford four-membered rings (55). The reaction is illustrated for cyclohexenone and isobutylene.
26% +
Thus, irradiation of 4,4-dimethyl-2-cyclohexenonein benzene with 1,l-dimethoxyethylene yielded the products shown in eqn. (35). From studies utilizing di-t-hutyl nitroxide as quencher, it was found that whereas the trans-adduct and oxetane were quenched at nearly the same rate, the formation of the cis-adduct was quenched substantially faster. Chapman has proposed. that two triplets are involved which may differ in electronic configuration, geometry, structure, or some combination of these features. intramolecular
Rearrangements
In irradiations carried out in dilute t-butyl alcohol solution, bimolecular reactions such as reduction, solvent addition, and dimerization are minimized (59). Thus it is possible to observe intramolecular rearrangements in these systems. The importance of solvent in modifying the nature of the products is reflected in the chemistry of 4,4-dimethyl-2-cyclohexenone. In t-hutyl alcohol, irradiation leads to a mixture of the bicyclic ketone and the cyclopentenone eqn. (36).
6.5% other products
Whereas cyclohexenone yields trans-fused products, only cis-fused products are obtained from cyclopentenones. It has been established from relative rate studies that electron rich olefins generally react faster t,han electron deficient olefins. \ In the last several years mechanistic scrutiny of these major reactions has led to some interesting results, the interHowever, irradiation in the more polar solvent acetic pretation of which is not altogether clear. The eviacid leads to the products shown in eqn. (37) (40). dence now available suggests that these cycloadditions are largely triplet reactions; however, more than one type of triplet may be involved. In a study of the photodimerization of cyclohexenone Hammond and coworkers (54) located the triplet energy of cyclohexenone a t 61 ? 1 kcal/mole. Furthermore, the dimerization OAC could be effected by sensitizers having as low a triplet (major) energy as naphthalene (ET = 61 kcal/mole). However, in the case of naphthalene sensitization this may I n a mechanistic study on the photorearrangements he singlet energy transfer as it has been shown that 2of enones to bicyclic ketones, Zimmerman and cocyclohexenones and dienones are good quenchers of workers (41) studied the reactions in eqns. (38) and naphthalene's fluorescence at room temperature (56). (39). Furthermore, naphthalene is moderately efficient in A ~ s i i . b * i ~ v , ~ b e ~ ~ . h h w e .d,.ljer?nws, c+k~6 kx a i d & of enones to ketones, Zimmerman and cocyclohexenones and dienones are good quenchers of workers (41) studied the reactions in eqns. (38) and --c)lf\n!snhf irdnq-ue-~~~~rom~*~wrprnIte~~~L~f;l*=.W8 vestigating the cycloaddition of cyclopentenone to 0 cyclohexene conclude that a higher triplet state (T2) of cyclopentenone is the reactant in this system (57). Renzophenone (ET = 69 kcal/mole) does not sensitize the reaction even though (a) benzophenone phosphorOne fact immediately apparent from the quantum yield escence is quenched by cyclopentenone, (h) cyclopentenone quenches benzophenone photoreduction in data in t-butyl alcohol shown above is that these reacisopropyl alcohol, and (c) the triplet energy of cyclotions are quite inefficient. From quenching and sensipentenone is located at 62 kcal/mole (98b). On the other tization studies it was shown that the triplet excited state is responsible for these rearrangements. The hand, sensitizers having a triplet energy greater than 73 kcal/mole are effective in sensitization of the reaction. rate of rearrangement of triplet enone to product (or The most recent investigation to appear reports that some precursor of product) was found to be about lo5 the cycloaddition of a cyclohexenone to 1,l-dimethoxysec-'. Studies on the optically active forms of the ethane proceeds via two triplet mechanisms (58). enone in reaction (40) have shown that the photo-
bait
u
w
\--,
-
Volume 46, Number 4, April
1969
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223
chemical reaction proceeds wit) retention of optical activity (42).
tion pathways lead to the same products, C14 labeling was carried out to show that the Type A process predominates t o the extent of 98.6% while the phenyl migration route is followed only 1.4y0of the time.
An especially informative system in probing the electronic character of the &carbon in enone photochemistry has been 4,4-diary1 substituted 2-cyclohexenones. In 1964 Zimmerman and Wilson (45) reported that 4,4-diphenylcyclohexenone did not undergo the usual cyclohexenone phototransformation (compare reaction (36)) but rather gave products in which phenyl migration had occured. Cross-Conjugated Cyclohexadienones
The discovery opened the possibility of probing the electronic character of the 8-carbon by studying migratory aptitudes of different substituted aryl groups. In one of the few published 'studies on photochemical migratory aptitudes, Zimmerman and coworkers (44) have found that the migratory aptitude of p-cyanophenyl versus phenyl is 14:l while that observed for p-anisyl versus phenyl is 10-15:l.
The photochemistry of cyclohexadienones has been one of the most active areas in organic photochemistry for the past ten years. Since numerous reviews on these photochemical rearrangements are available, this abstract will deal with only a few selected systems (@a-d). Part of the difficulty in understanding the photochemistry of these systems arises from the marked propensity of initially formed photoproducts toward further reaction. Thus products isolated in some instances have arisen from several consecutive photochemical reactions. Two systems which have been exhaustively studied are the4,4-diphenylcyclohexadienone(36) and4-methyl4-trichloromethylcyclohexadieuone (47, 48). Irradiation of the 4,4-diphenylcyclohexadienpne in dioxanewater affords initially a bicyclic ketone which under direct irradiation conditions is transformed to two phenols and a photoacid (36, 18). 0
0
(14 parts)
0
0
0
The ratio of the stereoisomeric ketones is dependent upon the length of the irradiation. It appears as though the photoproduct initially formed has the two aryl groups trans, the cis arising from fnrther irradiation of the trans. This, or course, has no effect on the results as the migratory aptitudes are time independent. From these results the workers conclude that the representation of the excited state of enones in this
+
reaction as 3C-C=C-C-Qr is a poor guide for predicting reactivity. It is proposed that a representation of the excited state as :C-C=C-0' is much preferred. In an attempt t o assess the relative rates of phenyl migration versus the normal rearrangement in cyclohexenone systems, the photochemistry of 4,5-diphenylcyclohexenone was investigated (45). Since both reac224
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Journol of Chemicol Education
The quantum yield for rearrangement of the dienone is interestingly over 200 times larger than the similar rearrangement observed for 2-cyclohexenones. The reaction is efficiently sensitized by acetophenone (+ = 0.81) and product formation can be completely quenched if the reaction is run in piperylene. From the lack of quenching at low quencher concentrations, it was concluded that the triplet state of the dienone must rearrange with a rate constant greater than 2 X lo1' sec-'. Note here, too, that this rate of rearrangement is a t least four powers of ten faster than the cyclohexenone system discussed earlier. In a subsequent investigation it has been shown that reaction (46) proceeds via excited states of different multiplicity.
(49) and a recent study on migratory aptitudes (62). In a-study of the migratory aptitudes in the 6-phenyl(i-p-cyanophenylbicyclo[3.1.0]hex-3-en-2-one system it was found that only phenyl migration occurred.
+
Thus the major portion of the acid product arises from the singlet state while the phenols arise exclusively from the excited triplet state. From the quantum yield data for reactions (45) and (46) it can be established that the direct phenyl migration from 4,4diphenylcyclohexadienone to yield 3,4-diphenylphenol does not occur.
+
acid (51)
This is striking in its own right, but especially so since p-cyano phenyl preferentially migrates in the 2-cyclohexenonc systems discussed earlier. The migratory aptitudes observed here arc highly suggestive of migration to an electron deficient system such as the zwitterionic intermediate of eqn. (50). Recently a photochemical study has been made of 4,4-dimethylcyclohexadienone in polar solvent, nonpolar solvent, and the gas phase (55). I t was hoped that such a study might afford evidence on the importance of solvation for the charge separated intermediates discussed in the previous paragraphs. The results are summarized below. Gas Phase
0
d k
0
0
cpI
One of the major points of interest in cyclohexadienone and bicyclic [3.1.0]hex-3-en-2-one photochemistry has been the representation of intermediates leading to the final products. Zimmerman has considered that a zwitterionic species leads to the observed rearrangement in the case of the dienone (60).
Cyclohexane
A supporting piece of evidence is that the hypothetical intermediate generated from the action of base on the bromoketone below affords the hicyclic ketone in 74% yield (51).
In the case of the rearrangement of 6,6-diphenylbicyclo[3.1.0]hex-3-en-2-one to the 3,4- and 2,3-diphenylphenols the zwitterion shown below has been suggested as an intermediate.
I t is interesting to note that rearrangement of the dienone to the bicyclic ketone occurs in all phases. This suggests an intermediate not involving such a charge separation as to preclude its formation in the gas phase or non-polar solvent. On the other hand, rearrangement of the bicyclic ketone to phenols occurs only in polar solvent. This result conforms nicely with Zimmerman's proposal of a discrete zwitterionic intermediate. Literature Cited
This proposal is supported by a kinetic analysis of the changing phenol composition with increasing acidity
( 1 ) MCMURRAY, H . L., J. Chem.P h p , 9,231, 241 (1941). A., AND EPSAYED,M. A., J . Chem. Phys., 42, ( 2 ) ~JDVARHAZI, 3335 (1965). (3) BRAND,3. C.D., J . Chem.Soc., 858 (1956). D. G., ( 4 ) For a review see BRAND,J. C. D., AND WIIL~AMSON. tn "Advances in Physical Organic Chemistry" (Edilor: GOLD,V.), Academic Press, New York, 1963, p. 365. D . E., AND KLEMPERER, W.,J . Chem.Phys.,40, (5) FREEMAN, 604 (1964).
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