J. Phys. Chem. 1993, 97, 13368-13374
13368
Rate Constants for Degenerate Hydrogen Atom Exchange between a-Hydroxy Radicals and Ketones Peter J. Wagner,' Yuanda Zhang, and Allen E. Puchalski Chemistry Department, Michigan State University, East Lansing, Michigan 48824 Received: June 22, 1993; In Final Form: August 13, 1993'
When a phenyl ketone is irradiated in the presence of 1-phenylethanol, the photoproducts include acetophenone pinacol, the pinacol from the other phenyl ketone, a mixed pinacol, and acetophenone, which is formed by irreversible hydrogen atom exchange between the hemipinacol radical of acetophenone and the other ketone. Rate constants for this hydrogen atom exchange between the hemipinacol radical of acetophenone and three other ketones were determined by measuring how acetophenone yields depend on the concentration of the other ketone. Comparable measurements were made for p-chloroacetophenoneformation by irradiating acetophenone in the presence of 1-@-chloropheny1)ethanol. As the starting ketone concentration increases, so does the amount of exchange relative to pinacol, while the pinacol content reflects decreasing amounts of the original alcohol. Exchange is measurable at ketone concentrations below 0.01 M and is complete by 0.1 M. Rate constants k,, of 3.7,6.3,4.2, and 8.6 X lo3 M-l s-l were deduced for hydrogen transfer to propiophenone, isobutyrophenone, p-methylacetophenone, and p-chloroacetophenone, respectively, based on a competing rate constant for radical coupling of 2 X lo9 M-1 s-l. Equilibrium constants for the hydrogen transfer were determined from the product ratios obtained by irradiating a mixture of two ketones with 2-propanol; from these k, values of 11.5,57, 13, and 1.8 X 103 M-1 s-l, respectively, were deduced for hydrogen transfer to acetophenone from the four other ketone hemipinacol radicals. These exchange rate constants depend more on the structure of the radical than on that of the ketone. Actual quantum yields for pinacol formation do not exceed 50%; this maximum quantum efficiency rises to 71% for 1-phenylethanol-0-d. From this inverse isotope effect it is concluded that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants.
The ability of 1-hydroxy radicals to reduce ground-state molecules by transfer of a hydrogen atom is well-known.' When the substrate is a ketone, the reaction is degenerate in the sense that both the reactants and products are a ketone and the hemipinacol radical of a(nother) ketone. Huyser and Neckers demonstrated that this hydrogen atom exchange occurs efficiently with thermally produced 1-hydroxyradicals.2 They also showed that ring substituents on acetophenones cause the equilibrium constant for the exchange to vary by a factor of 30 at 125 K.
+ R$-OH
+
+ Ph,C-OH R2C=0 (1) This exchange process was first uncovered as one of the most intriguing aspects of the much studied photoreduction of ketones. Early product studies3and later quantum yield measurements of photoreductions run in alcohol solvents indicated that transfer of a hydrogen atom from hydroxy radical to ground-state ketone must be faster than coupling of the initially formed pair of hydroxy radicals. Schuster and Karp have reviewed4 all of the early evidence for this process and reconfirmed a classic study of product yields in the photoreduction of benzophenone-dloby benzhydroldo, in which the pinacol product is almost entirely d20 at early stages of reaction. In the well-known case of benzophenone photoreduction in 2-propanol, essentially the only products at high ketone concentrations are acetone and benzpinacol.3 It was concluded that the equilibrium constant for hydrogen exchange must strongly favor the benzylic radicals so that quantum yields for benzpinacol and acetone formation approach unity. Even when equilibrium constants for hydrogen atom transfer between ketone and hydroxy radical are close to unity, as in the deuteration study by Schuster, the large concentration of starting ketone compared to that of radicals produces the same effect on equilibrium. Rubin then verified Testa's earlier observation that at high light intensities the quantum yield of benzpinacol formed in aliphatic alcohols drops appreciablyand yields of cross-coupled Ph2C=0
*Abstract published in Advance ACS Absrracfs, November 15, 1993.
0022-3654/93/2097- 13368$04.00/0
pinacols, which are negligible at low intensity, increase.'j They both concludedthat radical-radical reactions compete better with hydrogen exchange as the steady-state concentration of radicals increases.
PhpCO
-
HO OH
hv
+
RpCHOH
I
OH
0. ArKAr
t
PhpC-CPhp
+
2
R T R
OH
-ArAAr
-
ArAAr
+
RpC4
OH +
RAR
HO OH A r w A r Ar Ar
This exchange process has also been recognized by both Closs7 and RothE as being responsible for the CIDNP observed from cage-escaped hemipinacol radicals, such exchange occurring competitively with nuclear spin relaxation of the radicals. Actual rate constants for these hydrogen transfers have been measured only recently, although Closs was able to estimate from his CIDNP studies a value of 8 X 104 M-1 s-1 for benzaldehyde and its hemipinacol radical? Recognizing that the actual rates for hydrogen exchange and couplingof hemipinacol radicals must be competitive, and having measured rate constants for the coupling reaction in the 108-109 M-1 s-1 range (depending on solvent)? Steeland co-workerswereable tomeasurerateconstants for hydrogen transfer to benzophenone both from its own 0 1993 American Chemical Society
H Atom Exchange between OH Radicals and Ketones SCHEME I
OH
0 ' ArKR
+
Phi,,
K*
AH2
0
OH
A~K +phRA K
*AH
kx
-
OH
ArAR
+PhAMe
*KH
*AH
OH ArAR *K H
f
OH
o
0 +PhKbh A
OH
hemipinacol radical and from acetone hemipinacolradical of 1.5 X 104 and 3.5 X l o 4 M- I s-l, respectively.10 They derived these values by measuring pinacol yields as a function of benzophenone concentrationand by then applyingsteady-stateanalysis, knowing the rate constants for the major competingreactions. It is curious that the very exothermic transfer and the thermoneutral transfer have such similar rate constants. Demeter and co-workers independently measured a value of 7 X 104 M-1 s-1 for the rate constant of exchange between acetone hemipinacol radical and benzophenone by combiningflash measurements of couplingrate constants with concentration effects on benzophenone yields.I1 At the time we were engaged in similar ~ t u d i e s , ' ~inJ ~which we measured product quantum yields in the photoreduction of substituted acetophenonesby 1-phenylethanol. Like the Brandeis group, we recognized the necessity of measuring product yields at low conversion, including radical disproportionation products as well as radical coupling products. The near degeneracy of exchange in dur case, unlike that in the Brandeis study, allowed us to monitor formation of the oxidation product acetophenone, thus allowing us to compare a direct product of exchange with the cross-coupled pinacols that reflect exchange indirectly. We report here the rate constants and equilibrium constants that we have measured for exchange and their dependence on structure.
Kinetics Scheme I presents the full mechanism for radical production and product formation in the case of one ketone triplet state reacting with the alcohol of a second ketone so as to generate equal initial concentrations of two different hemipinacolradicals
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13369 AH and KH. In our studies A is always acetophenone and K some substituted acetophenone. In the absence of exchange, a 1:2: 1 ratio of pinacols is expected, and since AH radicals undergo only some 2% disproportionation relative to coupling14J5 (b = 0.98), only 2% as much A as acetophenone pinacol AH-AH. Both the presenceof excess A and pinacol product ratios favoring the KH component reflect varying amountsof exchange competing with radical-radical reactions. Exchange is effectively irreversible because of the very low concentration of product A compared to that of unreacted K. Therefore, eq 2 describes the quantum efficiency for formation of A. The intersystem crossing yield is known to be unity for these phenyl ketones. The factor ~ H T [ A Hrepresents ~] the quantum yield for radical production. The term in large parentheses represents the competition between exchange and radical-radical reactions. As just described, a and ,!3 values are expected to be close to unity, so eq 3 is an acceptable approximation. As described later, the results confirm this assumption. Moreover, we have measured the same value of 2 X lo9 M-l s-I for the second-order decay of both AH and PH (propiophenone hemipinacol) radicals by flash kinetics.16 Consequently, eq 14 is also a good approximation, where [AH + KH] represents the total hemipinacol radical concentration, readily calculated from quantum yields and light intensity (eq 6). Finally, standard double-reciprocal plots according to eq 5 provide values of k,,/k~.~[radicals]from the intercept/slopevalue. These studies provide k, but not k, values. The latter could be determined by merely switching ketone and alcohol and performing the same experiments. However, inasmuch as the equilibrium constant differs from unity, very low concentrations of one ketone must be used in order to prevent the faster of the two reversible exchanges from dominating radical coupling.
(4)
(
za[@KH2+ '(@(AH)2 + *AHKH + @(KH)2)1 kAA
)
1'2
(6)
Both light absorption and accurate analysis of low conversion product ratios become troublesome under such conditions. Therefore, we have simply measured the equilibrium constants separately by irradiating relatively high concentrations of two ketones in 2-propanol and measuring radical-radical product ratios. Under such conditions all the hemopinacol radicals have time to equilibrate before coupling. Exchange is fast,loJ1 and coupling of the hydroxy radicals is slowed by sol~ation.~
Experimental Section Chemicals. All ketones were available from earlier work and were either distilled or recrystallized before use.15 1-Phenyl-
13370 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993
TABLE I: Pinacol Ratios for Competitive Photoreduction of 0.1 M Acetophenone and 0.1 M Substituted Acetopbenone K with 1 M 2-PropmOl. K
(AH12 AH-KH ( K H h 'AH/'KH
pMe0-A 100 pMe-A
P m-Me-A
68.7 58.0 39.0 3.5
>50
28.7 36.4 52.0 22.5
2.6 5.6 9.0 74.0 100
4.9 3.2 1.86 0.17 50
m-CF3-A A-u-F~ 9 Run in benzene at room temperature. * Reference 2 at 125 K.
0.1 0.2
0.5 1 .o
0.17 0.17
0.116 0.104
TABLE III: Pinacol Ratios Obtained upon Irradiation of 0.1 M Acetopbenone and a Second Ketone K in Benzene Containing 1 M 2-Propanol K
'AH/'KHb 6.0 4.9 1.7 3.2 1.9 1.18 0.17 0.20 65% 0-deuterated. The corresponding p-chloro alcohol was prepared by reacting methyl magnesium iodide with p-chlorobenzaldehyde; the crude product was distilled. The spectroscopic properties of all reactants conformed to those reported. Benzene was commerical reagent grade purified by treatment with sulfuric acid and then distillation from phosphorus pentoxide. Acetonitrilewas distilled from base and permanganate. Reagent grade 2-propanol was used after simple distillation. Products. Symmetric pinacols were prepared by irradiation of a given ketone in excess deoxygenated 2-propanol and then, after removal of solvent, recrystallization. Reduction of each ketone with lithium aluminum hydride produced the l-arylalkanols. Cross-coupled pinacols were not prepared independently but were identified as the third product in GC analysisof reaction mixtures, with retention times always between those of the two homo-coupled pinacols. Techniques. Standard irradiation procedures were followed.15 All samples were prepared in volumetric glassware. Aliquots containing 2.8 mL were syringed into 13 X 100 mm Pyrex test tubes that were vacuum degassed by several freeze/pump/thaw cycles and then sealed. Samples were irradiated in parallel on a merry-go-round that rotated around a water-cooled Hanovia 450-W medium-pressure mercury arc. A basic potassium chromate solution surrounding the lamp isolated the 3 13-nm mercury emission. Valerophenone actinometer" samples were irradiated in parallel in order to monitor light intensity. GC analysis of product mixtures was performed on 15-m Megabore columns, DB-210 at 60-170° for the more volatile components and DB-1 at 175O for the pinacols. These columns do not separate diastereomers,thus simplifying analysis. Product concentrations were calculated after calibrating the flame ionization detector response with actual products; cross-coupled pinacol response was assumed to be the average of the two homo-coupled pinacols. Quantum yields were calculated from the measured total light flux correctedfor partial absorption in cases where optical densities were less than 2.
Re5ults Equilibrium constants at room temperature for reaction 1 were determined for several ketone pairs by irradiating (313 nm) a mixture of two ketones in benzene containing excess 2-propanol. Table I lists pinacol yields for several ketone pairs, and Table I1 lists actual quantum yields for all products formed from a mixture of acetophenoneA and propiophenoneP.12 Thesteady-state ratio
+ 2%
'AH/'KH' &*/&, 3.67 0.1 3.7 4.68 0.1 9.4 2.92* 0.05 8.8 1.50* 0.1 3.0 1.10* 0.01 3.3 0.41 0.02 0.20 0.67 0.03 0.22
*
([KHJz) + 95 (AH-KH).
TABLE Iv: Reaction of Acetophenone with 0.2 M l-Phenyl-l-propanol* [ALM @AHZ *P @(AH)Z @AH-PH @-(PH)z AH/PH 0.03 0.06 0.15 0.30
0.0075 0.20 0.0064 0.23 0.0053 0.0027
0.20 0.23 0.20 0.14
0.042 0.017 0.005 0.001
-0.002
9.8 27.5 80 280
Run in benzene at room temperature; total I = 0.008einstein L-I h-I.
TABLE V Reaction of Propiophenone with 0.5 M 1-Phenylethanol' [PI,M %H2 @A @(AH)z @AH-PH @(PH)Z PH/AH 0.052 0.145 0.26
0.0084 0.0092 0.0110
0.059 0.11 0.125
0.013
0.051 0.031 0.019
0.045 0.071 0.075
1.82 4.17 9-09
Run in benzene at room temperature; total I = 0.0080einstein L-* h-I.
TABLE VI: Irradiation of Propiophenone with 0.2 M 1-Phenylethanola [PI,M @A %HZ @(PH)Z @PH-AH @(AH)Z PH/AH 0.006 0.017 0.025 0.050 0.10 0.15
0.079 0.12 0.14 0.16 0.17 0.17
0.0028 0.023 0.036 0.0076 1.67 0.012 0.057 0.035 0.0062 3.4 0.013 0.073 0.034 0.0022 5.1 0.016 0.10 0.029 0.0011 7.9 0.020 0.11 0.018 13.6 0.017 0.10 0.014 16.5 a Run in benzene at room temperature; total I = 0.0046einstein L-1
h-1.
of the two hemipinacol radicals was extracted from product yields at low conversion for the three pinacols formed by radical coupling and the minor amounts of alcohols formed by radical disproportionation. No other productswereexpocted or observed. Since disproportionation accounts for only 2% of the products from A/P mixtures,14J5it was ignored in calculating the hemipinacol radical ratios in Table I. Table I11 lists similar results for the ketones whose product yields were also studied as a function of ketone concentration. Equilibrium constants in Tables I and I11 were calculated from the radical ratios as described by eq 7. Note that we now explicitly define k , as the rate conrtantfor hydrogen atom transfer from acetophenone hemipinacol radical to the other ketone. As much as 11% of the self-reaction of two isobutyrophenone hemipinacol radicals (iBH) is disproportionation (see below), so its inclusion has a small but measurable effect on the calculated equilibrium constant in Table 111.
_ k,, -- ['AHl[Kl kex
['KH][A]
(7)
Tables IV-IX report product ratios obtained by irradiation of varying concentrations of one ketone with a fixed concentration of the alcohol of a second ketone. Since quantum yields of hydrogen abstraction are not unity at the alcohol concentrations used, such quantum yields at 0.05 M ketone were measured as
H Atom Exchange between OH Radicals and Ketones TABLE W: Imdiatioa of Isobutyrophewne with 0.2 M I-Pbenyletbanol in Benzene [iBI,M @A OBH2 @(BH)Z OBH-AH AH)^ BH/AH 0.0071 0.096 0.013 0.048 0.050 0.012 2.15 0.0100 0.097 0.014 0.051 0.045 0.010 2.48 0.016 0.12 0.014 0.061 0.041 0.003 3.76 0.025 0.13 0.021 0.063 0.033 0.004 4.15 0.050 0.14 0.019 0.061 0.010 14.6 0 Run in benzene at room temperature; total I = 0.0040 einstein L-’
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13371
TABLE X Results of Plotting Reciprocal Quantum Yields of Acetophenone Formation ys Reciprocal Concentration of 1-Arylethanols in Benzene 0.05 M A/AH2 0.50 0.05 M iB/AHz 0.42 0.05 M MeA/AHZ 0.23 0.05 M A/ClAH2 0.36 a l/intercept. Intercept/slope.
0.25 10 0.18 3.9 0.14 7.6 0.17 4.5 O(0.2 M)/Ob.
0.50 0.43 0.60 0.47
h-1.
TABLE VIIk Irradiation of pMethylacetophenonewith 0.2 M I-Pbenyletbmol in Benzene [PIvM *A @ M H ~ @(MH)2 OYH-AH O(AH)Z MH/AH 0.0051 0.057 0.0024 0.024 0.053 0.013 1.30 0.0071 0.067 0.0032 0.039 0.036 0.0098 2.09 0.010 0.079 0.0038 0.055 0.014 0.0053 5.10 0.016 0.092 0.0050 0.076 0.016 0.0048 6.65 0.025 0.099 0.0063 0.079 0.011 13.5 0.051 0.110 0.0067 0.092 0.0073 22 0 Run in benzene at room temperature; total I = 0.0055 einstein L-1 h-I. TABLE Ix: Irradiation of Acetophenone with 0.2 M l-@Chlorophenyl)ethanol in Benzene [AI, M @CIA *AH2 *‘CIAH-AH *(AH)Z 0.008 0.084 0.038 0.05 1 0.010 0.098 0.032 0.047 0.013 0.1 1 0.028 0.049 0.0 16 0.12 0.0019 0.028 0.065 0.025 0.14 0.0027 0.023 0.079 0.17 0.0037 0.016 0.10 0.05 1 0.10 0.18 0.0054 0.01 1 0.13 a Run in benzene at room temperature; total I = 0.0026 einstein L-1 h-I.
10
-
8 -
1IQ
-
8 -
4 -
2
? -
IIQ
4
8
8
10
I*H21-’
F’igure 1. Dependence of acetophenone quantum yields on the concentration of 1-arylethanol for irradiation of 0.05 M ketones: ( 0 ) acetophenone with pchlorophenylethanol;(0)isobutyrophenone and (A) pmethylacetophenonewith phenylethanol.
a function of alcohol concentration. Standard double-reciprocal plots’s as shown in Figure 1, produce the results summarized in Table X. The intercepts provide limiting quantum yields when all triplet ketone reacts with alcohol; intercept/slope ratios provide kH/kd values. One column lists the quantum yield for formation of hemipinacol radicals a t the 0.20 M alcohol concentration used in Tables 111-VIII, and the final column represents the efficiency of triplet reaction at 0.2 M alcohol. Comparison of Tables X and VI-IX indicates that maximum
0
0
0
5 -
i 1
I 2
8 l/[alcoholl
4
8
Figure 2. Dependence of acetophenone pinacol quantum yield on
1-phenylethanolconcentration from irradiation of 0.1 M acetophenone in acetonitrile; ( 0 )PhCH(0H)Me; (0)PhCH(0D)Me.
TABLE XI: Comparison of Product Yields from Imdiation of Propiophenone with Either 1-Phenylethol or Acetophenone Pinecol in Benzene [reactants1- (MI [products] M) . . . _ . . P AH2 AH-AH A AH-AH AH-PH PH-PH 0.02 0.10 0.93 2.6 5.9 4.3 0.10 0.10 12.3 4.3 10.5 0.02 0.10 4.9 -2.1 1.5 0.10 0.10 6.1 -2.8 2.3 a All samples irradiated in parallel for 14 h, total I = 0.1 1 einstein L-I. quantum yields for product formation are significantly lower than unity. In general, A (or CIA) yields approach thesums of pinacol and alcohol yields a t high ketone concentrations where exchange is almost complete. There appears to be an inherent inefficiency in the reaction between triplet ketone and alcohol. Several experiments were directed toward determining whether reaction with the 0-H bond might be involved. Figure 2 compares the quantum efficiencies for photoreduction of acetophenone in acetonitrile by 1-phenylethanol and its 0-deuterated form.I9 Quantum yields are cu. 50% higher for the latter at all alcohol concentrations. Because of the reported ketone sensitized decomposition of pinacols,zO we checked the extent to which these ketones can react with the pinacol products. Table XI contains results from irradiation of propiophenone P with acetophenone pinacol (AH)2. The P is reduced to its pinacol, and the (AH)2 is oxidized to free A. The quantum efficiencies are only 5 4 % for acetophenone formation and 1.5-2% for propiophenone pinacol formation, compared to some 12% for pinacol formation from l-phenylethanol. The cross-coupled pinacol could not be measured in the presence of (AH)z reactant, but its probable yields are estimated from the A and (PH), concentrations. These two exchange products are both enhanced at higher P concentrations; but the much greater amount of A at low [PI from the pinacol relative
8 2
-
13372 The Journal of Physical Chemistry, Vol. 97, No. 50. 1993 18
Wagner et al.
TABLE XII: Kinetics Parameters for Exchange reactants slope intercept k,/ku[Z]'
16
P/AHz iB/AHZ MeA/AHZ A/CIAHz
14
a
12
0.043 0.024 0.046 0.050
5.5 7.1 8.3 5.1
128 296 180 102'
Intercept/slope; [Z] = [AH
+ KH].
lodl CDz
1@3
k, 10-3 kQ
1.3 0.25 3.7 1.1 0.21 6.3 1.5 0.18 4.2 0.74 0.20 8.6
In einstein L-I
11.5 57 13 1.8 5-l. c k*/
kdZ1.
'l@AP
and 6,produces the k,, values listed in the table. Multiplication of these by the equilibrium constants in Table I11 provides the k,, values listed. The large measured values for k , / k u [ Z ] togetherwith ketoneconcentrations>0.01 M vindicate thevalidity of dropping the last two terms from eq 2 in deriving eqs 3-5.
10
8.0
6.0
50
100
150
200
[Ketone]-'
Figure 3. Dependence of acetophenone quantum yields on ketone concentration: (0)m-propiophenone,(A)isobutyrophenone, ( 0 )pme thylacetophenone.
to phenylethanolindicates 80% direct formation of acetophenone with only 20% involving exchange.
Discussion Exchange Equilibrium Constants. As Table I indicates, the effects of substituents on the pinacol ratios are qualitatively the same in our photoinduced test as in the earlier thermal process. The two sets differ quantitatively in the expected fashion, with lower selectivitiesat the higher temperature. This parallel between the thermal and photochemical measurements is important because triplet energy transfer between phenyl alkyl ketones is fast enough" that triplet energy differences coupled with different kH values produce AH/KH ratios that are initially different. Interestingly, the observed product ratios for A/P and A/iB are close to the predicted initial ratios. However, p-chloroacetophenone triplet is less reactive than A triplet and of only slightly lower energy, yet its hemipinacol radical is favored. As evidenced by Tables VI-IX, exchange is nearly complete at high ketone concentrations. Moreover, 2-hydroxy-2-propylradical rapidly reduces ketones.lOJ1 Thus, we are confident that we have measured equilibrium rather than kinetic radical ratios. We conclude, in agreement with prior observation,*that the equilibriumconstantsare subject to the expected inductive effects, electron-withdrawingsubstituents having a destabilizing effect on the ketone and a stabilizing effect on the hemipinacolradical. That triplet n,r* excitation energies are lowered and reactivities enhanced by electron-withdrawingsubstituents for similar electronic reasons does not lessen the validity of these equilibrium measurements.
Exchange Rate Constants. Figure 3 plots the data in Table VI-VIII. As predicted by eq 5, the reciprocal plots are linear within experimentalerror, all four having correlation coefficients 20.998. TableXII summarizestheimportant kinetic parameters derived from these plots together with the steady-state concentrations and formationquantum yields of the hemipinacol radicals. Insertion of these values into eq 8, which is derived from eqs 5
We have made but one assumption, that the coupling rate constants for the different hemipinacol radicals are all the same, the 2 X 109 M-1 s-1 measured for AH and PH. This assumption is reasonablefor all but isobutyrophenone,which undergoesmore disproportionation than the less sterically congested radicals.14 Inasmuch as the assumed value of AB may be too high, the k , values for isobutyrophenonewould be too high. However, high disproportionation/coupling ratios do not necessarily mean lower total rate constants for radical-radical reactions?' Tables IV and V represent preliminary experiments1*and are included for their important qualitative information. In the acetophenone-propiophenone pair, exchange occursat much lower ketone concentrationfor A than for P, in accord with the measured equilibriumconstants. For P with AHz, exchange is less efficient at the higher light intensity and higher alcohol concentration of Table V than at the lower intensity and concentration of the more careful study in Table VI. This result confirms the mechanistic model, since the higher radical concentrations at higher intensity make coupling more competitive with exchange.6 The rate constants that we have derived are somewhat unusual in that all of the k,, values for both exergonic and endergonic reaction of acetophenonehemipinacol radical are within a factor of 2 of each other. The k,, values vary over a factor of 30, a factor of only 5 for the three exergonic reactions. It appears from this brief survey that the structure of the H-donor hemipinacol radical is more important than that of the receptor ketone in determining H atom exchange rate constants. Thus, k,, for exergonic reduction of CIA by AH radical is only twice as fast as for endergonic reduction of the worse electron acceptor MeA, while MeAH reduces A 7 times faster than does ClAH. These exchangereactionsare often considered coupled proton/ electron transfers. The small variations that we have measured in k,, values, together with the similar rate constants for acetone and benzophenone measured by others,"JJl provide little support for significant charge transfer from ketyl radical to ketone in the rate-determining step for these hydrogen atom transfers. In contrast, the low k, value for the hard-to-oxidize ClAH and values for MeAH and PH that are higher than the presumed value for AH + A of -4 X lo3 M-l s-l could reflect some charge transfer. It is not obvious why the equilibrium with P and IB favors the acetophenone hemipinacol radical as the size of the alkyl group increases, but most of the effect is determined by the value of k,. Inductive effects would not be expected to be so large; so we presume that subtle steric effects on molecularorientation are responsible. Radical Ratios. Figure 4 plots the dependenceof hemipinacol radical ratios, as derived from total product concentrations in Tables VI and VIII, on ketone concentration. The large decrease in the AH radical parallels the increase in acetophenoneproduct.
H Atom Exchange between OH Radicals and Ketones I
I
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13373
I
3 n* 0
OH
3r
el
OH
20
0 15
0
/
0
KHIAH
2
OH
PhA Me
10
0 S
1 diffusion 1v PhCH(0H)R
The sensitized disproportionation of propiophenone and acetophenone pinacol also represents transfer of an 0-bonded hydrogen to a triplet ketone. No mechanistic studies have been performed on this process to determine the extent of hydrogen and charge transfer.
0 0 . 0
3 0.04
0.08 [Ketone]
0.12
FIgwe4. Dependenceofhemipinacolradicalratioonketoneconcentration when irradiated with 0.2 M 1-phenylethanol: ( 0 )propiophenone, (0) pmethylacetophenone.
Unfortunately, these plots cannot be treated quantitatively. They should curve upward in a quadratic fashion. However, any incage recombination of the initially formed radical pair produces some AH-containing product even when most of the free AH is oxidized by ketone. Consequently, a certain unknown amount of the mixed pinacol should be ignored in calculating the radical ratio subject to exchange. There has been some confusion about the extent of in-cage coupling, since benzophenone undergoes some with 2-propanol22but not with ben~hydrol.~ However, the rate constants for coupling of AH and PH are considerablyhigher than those for couplingof benzophenone hemipinacol so some in-cage recombination is expected. Quantum Efficiency. As noted above, maximum product quantum yields extrapolated to infinite alcohol concentration are only some 50% in these reactions of substituted acetophenones. In the case of benzophenones, quantum yields approach unity, so there is some intrinsic inefficiency that afflicts hydrogen abstraction from aryl alcohols by acetophenones. Our study of 0-deuterated 1-phenylethanols19revealedthat thereis a significant isotope effect ( k H / k D = 3.1) on radiationless decay, the limiting quantum yield rising from 44% for 0-H to 71% for 0-D. At the same time k H / k D = 1.6 for the total k, for reaction of triplet A with the alcohol. In this scenario, only 44% of the total rate constant for attack on 1-phenylethanolrepresents abstraction of hydrogen from carbon; so the actual isotope effect for attack on oxygen is 3.0, similar to some primary isotope effects already reported for hydrogen abstraction by excited ketones. We have suggested that the quantum inefficiency directly measures abstraction of a hydrogen from oxygen, perhaps facilitated by some charge transfer from the phenyl ring to the triplet ketone.19 The resulting radical pair presumably disproportionates back to starting materials. Such disproportionation must occur largely within the initial cage, since free alkoxy radicals would react rapidly with more to generate another hemipinacol radical that would proceed to normal product formation. So much in-cage radical-radical reaction of triplet radical pairs is unusual. Two possible explanations come to mind: (1) the spin on oxygen promotes rapid spin relaxation in the radical pair, or (2) disproportionationcan produce acetophenone enol as its triplet. The latter process is exothermic because abstraction at OH by triplet ketone is nearly thermoneutraP4 and styrene's triplet energy is at least 10 kcal/mol lower than that of acetophenone.25 This explanation is preferred since it explains why benzophenone, with no u-hydrogens, can be photoreduced by benzhydrol with a quantum efficiency approaching unity.26
Disproportionation/Coupling Competition. Tables I1 and IVIX all allow some assessment to be made of the extent of disproportionationthat occurs between two different hemipinacol radicals. Lewis14and we15 have both found some 2% from selfreaction of two AH radicals, and Lewis has reported 15% from two PH radicals and 41% from two iBH radicals. Table I1 shows that indeed 16%PH2 is formed relative to (PH)? from the mixture of A and P. However, the AH2 represents over 4% of the (AH)2. The extra AH2 must arise from the cross-disproportionationand representssome 4% of the AH-PH coupling product. Apparently, the cross-reaction resembles the AH-AH reaction in that AH abstracts H from PH, but not vice versa. The data in Tables IV-VI corroborate that AH2 formation depends on the PH concentration,equalingsome 8% of the AH-PH. The percentage of PH2 formation relative to (PH)z varies only slightly as the AH/PH ratio drops, as would be the case if PH2 is formed only from two PH radicals.
The same type of comparisons were not made for A-iB. However, Table VI1 shows that the percentage of iBH2 relative to (BH)2 equals 22 f 2% over a 7-fold range of AH/BH ratios, only half the reported percentage.14 Given the negligible amount of PH2 that comes from AH-PH reaction, it is doubtful that much iBH2 would arise from iBH-AH reaction. Instead, we suspect that coupling is not as disfavored in benzene solvent as in the more alcoholicmixtures used in other studies, in which the hemipinacol radicals are heavily hydrogen-bonded and thus bulkier.9
Summary Rate constants for hydrogen atom exchange between the hemipinacolradical of acetophenoneand four other ketones were determined from measurement of acetophenone yields when varying concentrations of another ketone are irradiated with 1-phenylethanol. As the starting ketone concentration increases, so does the amount of exchange relative to pinacol, while the
Wagner et al.
13374 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993
pinacol contentreflects decreasingamountsof the original alcohol. Rate constants k,, of (3.7-8.6) X 103 M-1 s-l were deduced for hydrogen transfer to propiophenone, isobutyrophenone,p-methylacetophenone, and p-chloroacetophenone. Equilibrium constants for the hydrogen transfer were determinedfrom the product ratios obtained by irradiating a mixture of two ketones with 2-propanol;from these k- values of 11.5,57, 13, and 1.8 X lo3 M-1 s-1, respectively, were deduced for hydrogen transfer to acetophenone from the four other ketone hemipinacol radicals. These exchange rate constants depend more on the structure of the radical than on that of the ketone. Quantum yields for pinacol formation from 1-phenylethanoldo not exceed 50%;this maximum quantum efficiency rises to 71% for 1-phenylethanol-O-d. This inverse isotope effect indicates that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionationof the initial radical pair back to reactants.
Acknowledgment. This work has been supportedby continuing grants from NSF over the past 15 years. We thank Prof. Tito Scaiano for measuring the bimolecular decay rates of hemipinacol radicals.
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(9) Naguib, Y . M.A.; Steel, C.; Cohen, S.G. J. Am. Chem. Soc. 1985, 108, 128. (10) Naguib, Y.M.A.; Steel, C.; Cohen, S . G. J. Phys. Chem. 1988,92, 6574. (1 1) Demeter, A.; Usz16, B.;BCrces, T. Ber. Bunsen-Ges. Phys. Chem. 1988,92, 1478. (12) Puchalski, A. E. Ph.D. Thesis, Michigan State University, 1980. (13) Zhang, Y. M.S. Thesis, Michigan State University, 1989. (14) Lewis, F. D.; Magyar, J. G. J. Org. Chem. 1972, 37, 2102. (15) Wagner, P. J.; Truman, R. J.; Puchalski, A. E.;Wake, R. J . Am. Chem. Soc. 1986, 108,7727. (16) Scaiaino, J. C.; Wagner, P. J. Unpublished results. (1 7) (a) Wagner, P. J.; Kochevar, I. E.; Kemppainen, A. E.J. Am. Chem. Soc. 1972,947489. (b) Bays, J.P.;Encinas,M.V.;Small,Jr.,R.D.;Scaiano, J . C. J . Am. Chem. Soc. 1980,102,727. (18) Hammond, G. S.;Baker, W. P.; Moore, W. M. J. Am. Chem. Soc. 1%1,83,2795. Wagner,P. J.SteadyState Kinetics. In HandbookofOrganic Chemistry;Scaiano, J . C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 2, p 251. (19) Wagner, P. J.; Puchalski, A. E.J. Am. Chem.Soc. 1980,102,7138. (20) Schonberg, A.; Mustafa, A. J. Chem. Soc. 1944,67. (21) Sec ref 3 in: Fischer, H. J. Am. Chem. Soc. 1986,108, 3925. (22) Weiner, S. A. J. Am. Chem. Soc. 1971, 93,425. (23) Paul, H.; Small, R. D., Jr.; Scaiano, J. C. J. Am. Chem. Soc. 1978, 100,4520. (24) Walling, C.; Gibian, M.J. J. Am. Chem. Soc. 1965,87, 3361. (25) Lamola, A. A.; Hammond, G. S.J. Chem. Phys. 1965,13, 2129. (26) Wagner, P. J. Mol. Photochem. 1%9, I , 71.
