J. Org. Chem. 1989,54, 3847-3852
3847
Regioselectivity in the Semiconductor-Mediated Photooxidation of 1,I-Pentanediol Marye Anne Fox,*vt Haruo Ogawa,t and Pierre Pichat' Department of Chemistry, University of Texas, Austin, Texas 78712, and Equipe CNRS Photocatalyse, Ecole Centrale de Lyon, BP 163, 69131 Ecully Ceder, France Received December 28, 1988 Optimum conditions have been established for the selective semiconductor-photocatalyzed oxidation by long-wavelength ultraviolet light of the primary alcohol functionality in 1,4-pentanediol. On platinized (2%) T i 0 2 powder suspended in oxygenated aqueous (4 vol % ) acetonitrile, the initial rate ratio for oxidation of the primary/secondary alcohol site was >7. Analysis of further oxidation products allowed for mechanistic delineation of the course of the semicondudor-mediated reaction. The selectivity is attributed to the essential role of adsorption, with the critical photoinduced electron transfer occurring a t the surface of the irradiated particle. Zr02 and S n 0 2samples were much less active than T i 0 2 as photocatalysts. The effects of oxygen pressure, metal cocatalyst loading, and water content of acetonitrile are discussed.
Although selective redox activation of organic substrates has been the goal of many recent photoelectrochemical ~tudies,l-~ most investigations have been concerned with functional-group activation or with accelerated photooxidation of one substrate from a mixture. Little information is available concerning the ability of an excited semiconductorto selectively initiate redox reactivity at one site in a multifunctional molecule. The present study was undertaken to establish whether irradiated metal oxide semiconductor suspensions could induce preferential oxidation at one site of a diol and to delineate those conditions under which optimal regioselectivity could be attained. 1,4-Pentanediolwas chosen as the model compound for mechanistic investigation. We find indeed that a regiochemical preference for oxidation of the primary functionality is observed, with aldehyde being produced about 7 times more efficiently than ketone in the initial stages of the reaction.
Experimental Section Materials. T i 0 2 [anatase; Matheson, Coleman, & Bell (- 10 m2 g-l) or Degussa P-25 (50 m2 g-l)] was heated overnight at 125 "C in a vacuum oven before use. Nonporous Zr02 (49 m2 g-') was prepared in a flame reactor and S n 0 2 (25 m2g-l) was obtained by precipitation of SnCl, by NH,OH at pH 7. Hexachloroplatinic acid (H2PtCl6.6Hz0; MFG Corp.) and acetonitrile (Fisher Scientific, HPLC grade) were used without further purification. Methylene chloride was spectroscopic grade and was dried over molecular sieves before use. All aqueous solutions or suspensions were prepared with triply distilled water. 4-Hydroxypentanal(2) and tetrahydro-5-methyl-2-furanol (4) were synthesized by reduction of y-valerolactone! All other material^^,^ used to identify the photooxidation products were commercially available: 4oxopentanol 3 (Fluka), 4-oxopentanoic acid 8 (Aldrich), 5methyltetrahydrofuran-2-one 9 (Aldrich), and 4,5-dihydro-2methylfuran 10 (Aldrich). The alcohols (1,4-pentanediol, 1pentanol, and 2-pentanol (Aldrich)) were redistilled before use, and a center cut of the distillate, shown to be pure (>99%) by gaslliquid chromatography (GLPC), was used for the adsorption isotherm experiments. Catalyst Preparation. Platinized T i 0 2 catalysts were prepared by the method of Kraeutler and Bard.' In a typical example, anatase powder (3 g) and an appropriate amount of H2PtClg6H20were suspended in a buffered aqueous acetic acid solution (20 mL) maintained a t p H 5 by the addition of Na2C03. After the solution was homogenized for 5 min in an ultrasonic bath, the resulting slurry was transferred to a quartz reaction vessel. The suspension was used either directly or after bubbling University of Texas.
* Ecole Centrale de Lyon. 0022-3263/89/1954-3847$01.50/0
Table I. TiOz Adsorption Equilibria"for 1-Pentanol and 2-Pentanol Kabc
solvent on TiOzn dry CH3CN 2% aq CH3CN 2% aqCH3CN CH& on 0.5% Pt/TiOz dry CH3CN CHzCl2
temp,b (1-pentanol) Kabc (1-pentanol)/ OC X lo3 Kab (2-pentanol) 23 23 0 0
4.8 35 45 12
5.6 2.2 1.3 0.93
23 0
2.5 7.5
5.0 1.1
Langmuir adsorption isotherms on dried, deaerated Ti02 (anatase), surface area -10 m2/g. b*l "C. CAverageof three determinations. Experimental precision was better for the ratio of adsorption equilibrium constants (f12%) than for the values of the constants themselves (*31%). (I
with nitrogen for 15 min to eliminate dissolved air. Irradiation (1000-W Hg-Xe lamp) was carried out under a slow constant stream of nitrogen with constant stirring until the chloroplatinic acid had completely decomposed (5 h). After irradiation, the suspension was filtered. T h e filtrate was washed with water several times until the washings were neutral. The solid was then dried at 130 "C overnight, before being stored under nitrogen at room temperature. Other P t / T i 0 2 samples were prepared by impregnating T i 0 2 (Degussa P-25) with an aqueous solution containing the required amount of H2PtC&. T h e resulting sample was reduced under flowing hydrogen at 480 "C as previously described? The resulting Pt particle sizes were nearly independent of the loading level attained. Photocatalytic Oxidations. A Pyrex test tube (20 mm i.d.) was connected to a condenser and sealed with a rubber septum. Two Teflon tubes (2 mm i.d.) pierced the septum, respectively for delivering or removing gases over the solution. (1)Fox, M. A. Acc. Chem. Res. 1983, 16, 314. (2) Fox, M. A. Top. Curr. Chem. 1987, 142, 72 and references cited
therein. (3) Fox, M. A. In Photocatalysis and Environment. Trends and Applications; Schiavello, M., Ed.; Kluwer Academic Publishers: Dordrecht, 1988; 445.
(4)Corey, E. J.; Nicolaou, K. C.; Toru, T. J.Am. Chem. SOC.1975,97, 2287. (5) The structure of the hemiketal 5 was inferred from its NMR spectrum and by its thermal dehydration to 4,5-dihydro-2-methylfwan (10).
(6) Although firm structural assignments for 6 and 7 are not required for the arguments to be presented below, the structures offered in Scheme I are further supported by the observation that both are cleanly converted to keto acid 8 during an oxidative workup. (7) Kfaeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 4317. (8) Pichat, P.; Disdier, J.; Courbon, H.; Herrmann, J. M.; Mozzanega, M. N.; Disdier, J. Nouv.J. Chim. 1981, 5, 627.
0 1989 American Chemical Society
3848 J. Org. Chem., Vol. 54, No. 16, 1989
Fox et al.
Scheme I. Products Formed in the Photocatalyzed Oxidation of
-ao
QOH 4
9
OH
/ L
O
1
OH
L CHO
7
2
H
3
ii 0
(- m-
5
Results and Discussion Reaction Profile. Irradiation of a suspension of semiconductor metal oxide photocatalyst in the presence of 1,4-pentanediol(l) led to the formation of aldehyde 2 and ketone 3, (eq 1 and Scheme I). The initial rates of photooxidation to produce 2 or 3 depended on the metal
H
2
0
10
The photocatalyst (20 mg unless otherwise indicated) was suspended via ultrasonication in a solution of 1,4-pentanediol(25 mL, 0.02 M) in water, acetonitrileor mixtures of these solvents. The envelopinggas (nitrogen,air,or oxygen) was bubbled through the suspension (ca. 6 mL/min) for 15min, and a positive pressure of the gas was maintained during the irradiation. The samples were irradiated, with vigorous stirring, at ca. 60 "C in a Ftayonet Photochemical Reactor equipped with "black lights" blazed at 350 nm (384 W)for varying periods. The reaction mixtures were analyzed on a Hewlett-Packard Model 5890 gas chromatograph equipped with a Model 3393A integrator and a DB-5 capillary column (0.53 mm X 15 m). The progress of the reaction was followed by GLPC analysis of aliquot samples. Dodecane and/or 1,4-dioxanewere used as internal standards. The reaction products were identified by comparison of retention volumes, spectroscopicproperties,and/or gas chromat.ography/massspectroscopicfragmentationpatterns (Finnegan 4023 automated GC/MS equipped with an INCOS data system and a 25-m DB-1 capillary column) with those of authentic samples. Adsorption Isotherms. Langmuir adsorption isotherms were determined by shaking a dried deaerated sample of TiOz in a specified solvent mixture made 2.2 X M in alcohol in an automatic shaker for 8 h (Table I). The initial and final concentrations of alcohol were determined by GLPC as above. The solution temperature was maintained at 23 "C or 0 "C ( i l "C) during the adsorption by immersing the tightly capped flasks during the shaking in a constant temperature bath.
1
mixture of dimeric diastereomers
3
1ooq
T i me/ h Figure 1. Reaction profile for the TiOzphotocatalyzedoxidation of 1,4-pentanediol:20 mg of TiOz suspended in aerated acetonitrile (25 mL) containing 1,4-pentanediol(0.02 M), h > 350 nm.
loading of the TiOz photocatalyst. A t low conversions (60%), these compounds, which together with their cyclic isomers 4 and 5 were the only isolable products, were obtained in high chemical yield (nearly q ~ a n t i t a t i v e ) .A5 ~ the reaction proceeded to higher conversion, the initial products were themselves photooxidized, ultimately producing a complex mixture of products, as shown in Figure 1. These products can be rationalized by the routes shown in Scheme I, in which compounds 2, 3, 8, and 9 were identified by comparison with authentic samples, and compounds 6 and 7 are tentative assignments based on their mass spectral fragmentation patterns? Although no absolute determination of the rate constants for these transformations was undertaken, our results allow us to (9) Yields of 2 reported represent the combined amounts of 2 and its cyclic hemiacetal 4, and those of 3 represent the sum of contributions from 3 and its hemiketal isomer 5.
J. Org. Chem., Vol. 54,No.16,1989
Semiconductor-MediatedPhotooxidation of Polyols
3849
Scheme 11. Proposed Mechanism for the SemiconductorPhotocatalyzed Oxidation of Alcohols
iz
d
hcB +
15-
R2CHOH
-
R,CHbH
%
R,C-OH
(b)
9 \ .-E 10v) L
Pc
E 5-
IP "
6
1
2
3
Time
/
4
5
6
h
Figure 2. Effect of light on the course of the TiOpphotocatalyzed oxidation of l&pentanediol. Conditions as in Figure 1.
order the relative rates of photooxidation in the following order:lOJ1 Further oxidation of 10, the dehydration
P product derived from hemiketal5, gave rise to a complex mixture of dimeric dihydrofurans analogous to those described by Kisch.12 That the reaction was photocatalytic was established by determining a rate profile for the disappearance of starting material. As shown in Figure 2, oxidative consumption of the reactant ceased immediately (within experimental error) when the light source was extinguished. The activity of the fresh photocatalyst was highest in the initial stages of the reaction because of a decrease in the concentration of the reactant and competitive adsorption of products as the reaction proceeded. Thus, although the reaction rate decreased somewhat after repeated light on-off cycles, the photocatalyst retained most of its activity even after prolonged irradiation for many hours. Mechanism. We infer that the mechanism involved in these transformations is parallel to that thought to occur in the photocatalytic oxidation of monoalcohols (Scheme II).2J3 An electron-hole pair, eq a, is created as a result of optical excitation of the semic~nductor.l-~ Since the valence band edge of the excited semiconductor defines the hole's oxidizing power, a photogenerated hole in n-TiO, (+2.4 V vs SCE in a~etonitrile'~) is highly active and capable of initiating a thermodynamically favorable single (10) Pichat, P. In Organic Phototransformationsin Nonhomogeneous Media; Amer. Chem. SOC.Sympos. Ser. 1985, 278, 21. (11) Pichat, P.; Disdier, J.; Herrmann, J. M.; Vaudano, P. Nouu. J . Chim. 1986,10,545. (12) Zeug, N.; Buecheler, J.; Kisch, H. J . Am. Chem. SOC.1985, 107, 1459. (13) The entire suggested mechanism is not specifically cited in any
of the following studies, but can be reasonably inferred from the individual steps addressed therein: (a) Yamagata, S.; Nakabayashi, S.; Sancier, K.; Fujishima, A. Bull. Chem. SOC.Jpn. 1988,61,3429. (b) Cundall, R. B.; Rudham, R.; Salim, M. J . Chem. SOC. Faraday Trans. 1 1976,72, 1642. (c) Ait-Ichou, I.; Formenti, M.; Teichner, S. J. Stud. Surf. Catal. 1984,19, 297. (d) Walker, A.; Formenti, M.; Meriaudeau, P.; Teichner, S. J. Catal. 1977,50,237. (e) Cunningham,J.; Hodnett, B. K. J. Chem. SOC.,Faraday Trans. 1 1981,77,2777. (0Blake, N. R.; Griffin, G. L. J . Phys. Chem. 1988, 92, 5697. (14) Fox, M. A.; Chen, C. C. J . Am. Chem. SOC.1981, 103, 6757.
0
/ air
1
2 3 4 5 Time / h Figure 3. The effect of oxygen on the TiOz photocatalyzed oxidation of 1,4-pentanediol. Conditions as in Figure 1except that the suspension was maintained under oxygen (o),air (o), or nitrogen (A). electron oxidation of an adsorbed alcohol, eq b. Deprotonation of this species will be rapid, producing an a-hydroxy radical of reasonable stability. Alternatively, this radical can be formed from the alcohol dissociatively adsorbed on the basic sites of TiOFl6 This latter intermediate is extremely easily oxidized,15and a modestly doped TiO, particle, whose Fermi level is poised near the conduction band edge, will easily accept an additional electron, generating a protonated carbonyl compound, eq c. Interaction of this intermediate with the basic sites on Ti02, with solvent, or with a photogenerated base completes the sequence for formation of the observed product. An alternative route to carbonyl compound from the a-hydroxy radical would involve chemical trapping by dioxygen to form a peroxy radical, decomposition of which would be expected to occur within milliseconds to produce the carbonyl product, eq d.17 In aerated solutions, the photogenerated electron can be trapped by adsorbed oxygen to form superoxide, or other negatively charged adsorbed oxygen species,l0eq e. Hydrogen peroxide thus formed by sequential electron and (15) Lilie, J.; Beck, G.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1971, 75, 458. (16) Pichat, P.; Mozzanega, M. N.; Courbon, H. J . Chem. SOC., Faraday Trans. 1 1987, 83, 697:. (17) Cunningham, J.; Srijaranai, S., private communication.
3850 J. Org. Chem., Vol. 54, No. 16, 1989
Fox et al. respectively). Both primary and secondary alcohols should be exothermically oxidized when adsorbed onto photoexcited Ti02 Since rates of electron-hole recombinaiion are extremely fast on irradiated semiconductor particles,22the rate of interfacial carrier trapping must also be rapid to be competitive. While relatively little is known on rates of electron transfer for adsorbed species, mathematical models have suggested very high rates for both electron and hole trapping.23 If we assume that the rate of electron transfer governs photooxidizability, two modes are available to predict relative reactivity. In a model derived from semiconductor band theory,%hole trapping by adsorbates whose oxidation potentials lie within the energy gap of the semiconductor would be governed by the exothermicity of the requisite energy transfer, the more exothermic reaction occurring more readily. On the other hand, if both electron transfers are highly exothermic, and if the adsorbed species behave as do solvated participants in homogeneous electron transfer reactions, then reaction rates may be governed by the reorganization required during the critical electron transfer.25 For very highly exothermic reactions (i.e., in the Marcus inverted region) in solution, rate decreases have been observed with increased exothermicity in both intermolecular26and intramolecular2' electron transfers of high exothermicity. If pure thermodynamic restraints were operative, the band model should prevail and the preferential oxidation of the secondary site would be predicted. If the transfers are so exothermic as to reach within the Marcus inverted region, however, the reverse prediction might be realized. If the electron transfer is not the rate-determining step, the relative rates of deprotonation of the adsorbed radical ion might also be important. Unfortunately, little is known concerning the rates of radical ion deprotonation on metal oxide surfaces, and a resolution of this question awaits a kinetic characterization of the relevant dark secondary chemical reactions occurring on the surface. Adsorption Effects. An additional method for selective activation may lie in preferential adsorption effects. Although two-site adsorption has been suggested as important in the oxidation of diols in conventional electrolytic cells,z8the suggested biradical intermediate seems not to be important under the low-current density conditions attained in photoelectrochemical cells, for intermediate biradical formation should lead directly to the further oxidized keto aldehyde 6 rather than to the observed initial oxidation products 2 and 3. Formation of these monooxidation products is more consistent, then, with single-site adsorption as has been observed in the electrochemical oxidation of 1,2-propanedi01.~With a single adsorption site, that region of the molecule more closely associated with the surface would be predicted to suffer the more rapid oxidation.
l o vl
Po2 Figure 4. The dependence of regioselectivity S (A)and initial on oxygen concentration in the TiOzphotoreaction rate R (0) catalyzed oxidation of l,4-pentanediol. Conditions as in Figure 3.
proton transfer rapidly decomposes on illuminated Ti02 to water and oxygen. In the absence of oxygen, the electrons can accumulate a t the platinum cocatalyst, eq f.l0 These electron trapping modes are consistent with the observation that while native Ti02is at least 100 times less active in the absence of air (Figure 3), platinized Ti02 remains active under nitrogen. The requirement for oxygen in photocatalyzed oxidations occurring on nonplatinized Ti02 presumably derives from the necessity of trapping the conduction band electron. In the absence of an appropriate trap or charge accumulation center, electron-hole recombination (reverse of eq a) ensues. Since this energy dissipative recombination is inhibited by adsorbed oxygen, it is reasonable to expect that the rate of photooxidation should be increased with increasing steady-state concentrations of dissolved oxygen (Figure 3). Selectivity for the primary alcohol site was slightly greater in pure O2than in air (Figure 4).18 The data obtained under a N2 atmosphere are not directly comparable since the dehydrogenation was not catalytic in the absence of deposited Pt. These data do, however, agree with the observed trend toward decreased selectivity with reduced oxygen pressures. Thermicity of Electron Transfer. A relationship between thermodynamics and kinetics might provide a reasonable vehicle to explain the observed acceleration of the photooxidation a t the primary rather than the secondary site if the rate-determining step of the photocatalysis were known. It is not. Although charge density requirements for the photooxygenation of 1,l-diphenylethylene have been determined,lg the observed p value (-0.8) can be interpreted as consistent with either formation of the cation radical (i.e., interfacial electron transfer) or with charge localization in the cation radical (i.e., by chemical trapping as a reactive free radical). The ionization potentials of primary alcohols are slightly more positive than branched isomers (viz., 10.37 eV for 1-butyl alcoholz0 and 10.25 eV for tert-butyl alcohol,21 (18) Selectivity is reported in this f i i r e as the ratio of the initial rates of primary vs secondary alcohol oxidation (Le., (2 4)/(3 + 5)) extrapolated to zero conversion. The curves are smooth line fits to the three data points. (19) Fox, M. A.; Chen, C. C. Tetrahedron Lett. 1983, 24, 547. (20) Katsumata, S.; Iwai, T.; Kimura, K. Bull. Chem. SOC.Jpn. 1973, 46, 3391. (21) Robin, M. B.; Kuebler, N. A. J. Elec. Spectr. Rel. Phenom. 1972, 1, 13.
+
(22) For example, see: Serpone, N.; Sharma, D. K.; Jamieson, M. A.; Graetzel, M.; Ramsden, J. Chem. Phys. Lett. 1985, 115, 473. (23) Nosaka, Y.; Fox, M. A. J. Phys. Chem. 1988,92, 1893. (24) Gerischer, H. In Physical Chemistry: An Advanced Treatise; Eyring, H., Henderson, D., Jost, W., Eds.; Academic Press: New York, 1970; Vol. IX A, p 463. (25) Marcus, R. A. J . Chem. Phys. 1956, 24, 966; Ann. Reu. Phys. Chem. 1964,15, 155. (26) Gould, I. R.; Ege, D.; Mattes, S. L.: Farid, S. J. Am. Chem. SOC. 1987, 109, 3794. (27) Miller, J. R.: Calcaterra, L. T.: Closs, G. L. J. Am. Chem. SOC. 1984, 106, 3047. (28) (a) Horanyi, G.; Torkos, K. J. ElectroanaL Chem. 1981,125, 105. (b) Sokolova. E. I.: Electrochim. Acta 1979.24.147. (c) Ocon. P.: Beden, B.; Huser, H.; Lamy, C. Electrochim. Acta 1987, 32, 387. (29) Huser, H.; Leger, J. M.; Lamy, C. Electrochim. Acta 1988, 33, 1359.
J. Org. Chem., Vol. 54, No. 16, 1989
Semiconductor-MediatedPhotooxidation of Polyols
-10
T
3851
50 I
I
-8 -6
v, lfl
-4
"I 2
O
0
1
2
3
4
5
Pt content/"+% Figure 5. Dependence of regimelectivity S (A)and initial reaction on platinum loading in the TiOz photocatalyzed oxirate R (0) dation of 1,4-pentanediol. Conditions as in Figure 1.
1O
8
L
f
O
I
0! O 0
5
10
H20 content
50
20
15
Hx)
/ vOlYo
Figure 6. Dependence of regioselectivity S (A)and initial reaction rate R (0) 1 on solvent water content in the TiOz photocatalyzed oxidation of 1,4-pentanediol. Conditions as in Figure 1.
In the photocatalytic experiment, the extremely short lifetime of the electron-hole pair makes it mandatory for the oxidizable molecule to be adsorbed onto the photocatalyst surface before excitation. The Langmuir adsorption isotherm model%has been shown to be effective in describing such binding?J3b-30Table I summarizes the relative adsorption constants Kadsfor appropriate monofunctional alcohol models for 1 on TiOz and platinized TiOz, assuming that the greater part of the adsorbed alcohol is undissociated. Clearly, primary alcohols are more effectively adsorbed than are secondary substrates, i.e., they interact more easily with the surface. Accordingly, adsorption effects can be significant in controlling the preferential oxidizability of the primary alcohol group. Although the absolute adsorption constant of 1-pentanol increases in 2 % aqueous CHBCNas compared with dry CH3CN,the ratio of the adsorption constants of 1-pentanol and 2-pentanol decreases so that the increase in selectivity, Figure 6, cannot be explained on this basis only. Reduced adsorption is observed on the platinized particles. As part of the alcohol is likely adsorbed dissociatively on the basic ~
~
~~
~~~
~
(30) Pichat, P.; Herrmann, J. M. In Photocatalysis-Fundamentakr of Applications; Serpone, N., Pelizzetti, E., E%.; Wiley Interscience: New York, in press, and references cited therein.
(31) Jaffrezic-Renault, N.; Pichat, P.; Foissy, A.; Mercier, R. J . Phys. Chem. 1986, 90,2733. (32) Since a more complex mixtures of products (at least some of which are chlorinated) is formed from photocatalytic alcohol oxidation in methylene chloride, this reaction has not been fully characterized. T h e results reported in the text represent the ratio of products 2 + 4 and 3 + 6 formed after 10% disappearance of starting material. (33) (a) Hope, G.A.; Bard, A. J. J . Phys. Chem. 1983, 87, 1979. (b) Aspnes, D.E.; Heller, A. Ibid. 1983,87, 4919.
3852 J . Org. Chem., Vol. 54, No. 16, 1989
Fox et al.
Table 11. Rate and Selectivity Effects on the Photocatalyzed Oxidation of 1 on Various Metal Oxide Semiconductor Powdersa photocatalyst weight [HZ019 mg [O,], wt % rate (initial), mmol/h (g-cat)-' selectivity (1/2) air TiOp 20 4.5 f 0.4 4.3 f 0.3 air Zr02 20 0.40 0.05 2.6 f 0.6 air 20 SnOz 2.0 f 0.3 0.15 f 0.04 5 air TiOp 9.0 f 1.0 4.3 f 0.1 18 f 1 5 TiOz 3.5 f 0.3 02 23 2 5 3.4 f 0.1 TiOz-0.1% Pt 0 2 25 f 2 5 3.6 f 0.2 Tio2-0.5% Pt 0 2 29 f 1 5 4.7 f 0.5 TiOz-1% Pt 0 2 Ti02-2% Pt 29 f 1 7.2 0.2 5 0 2 Tio2-5% Pt 30 1 5 3.3 0.1 0 2
*
* *
* *
"Each experiment was done by using 25 mL of 1,4-pentanediol (0.01 M) in aerated CH3CN under UV (350 nm) irradiation.
selectivity observed for the two highest Pt contents cannot be easily rationalized. The water content of the solvent also influenced the reaction appreciably (Figure 6). Water competes with alcohol for adsorption on the catalyst.8 This should decrease the conversion of l. However, water increases the polarity at the surface and thus assists charge transfer and stabilizes charged intermediates. Unlike the effect of surface-attached platinum, water exhibited a parallel influence on reaction rate and site selectivity. As a result, the oxidation of 1 is faciliated at low H 2 0 contents. The diminished selectivity observed as the water content increases probably reflects a shift from the direct oxidation suggested in Scheme I1 to one mediated by intermediate formation of the more indiscriminant hydroxy radical.34 Other Photocatalysts. The rationale for the observation that titanium dioxide was a much more active photocatalyst than two other metal oxide samples of comparable band gaps, Table 11, must be quite complex. Changing the identity of the photocatalyst modifies not only the band positions (and hence interfacial electron transfer energetics) but also the adsorption sites (nature and number), adsorption equilibria, surface charge, density of surface states and traps, and rates of electron-hole recombination. As discussed above, this should affect the selectivity as well (Table 11). Optimized Regioselectivity. Upon attempts to optimize simultaneously all the experimental variables, the reactivity and selectivity profiles shown in Figure 7 were obtained. Thus, a 7:l ratio is observed for the kinetic preference for oxidation of the primary site in 1,Cpentanediol on 2 w t % platinized TiO, in 4% aqueous acetonitrile under a slow stream of oxygen. This ratio is com(34) Ward, M. D.; Razdil, J. F.; Graselli, R. K. J. Org. Chem. 1984,88, 4210.
parable to that observed in the ruthenium complex catalyzed dehydrogenation of unsymmetrically substituted diolss and is parallel to that reported earlier by Pattenden and co-workers who found much higher yield and cleaner product mixtures from primary alcohols photooxidized on TiO, suspended in benzene.36
Conclusions We attribute the observed regioselective oxidation or dehydrogenation of the primary alcohol in 1,Cpentanediol to site selective adsorption on the active photocatalyst by the less sterically encumbered primary alcohol. Preferential adsorption of the primary alcohol is clearly observed in the competitiveadsorption on TiO, of 1-and 2-pentanol dissolved in acetonitrile. When the absolute adsorption increases (with increasing water content or decreasing temperature), the observed selectivity decreases. This selective adsorption is also solvent dependent, with little selective adsorption observed in methylene chloride. Finally, although the presence of platinum particles on TiOz is not required for efficient photocatalytic oxidation of 1 in the presence of oxygen, partial platinization does influence the activity and selectivity of this reaction, presumably by affecting the separation of photoproduced charged0and the surface properties of the semic~nductor.~~ Acknowledgment. This work was supported by the U.S. Army Research Office. We gratefully acknowledge travel support by the NSF-CNRS, which made possible the collaboration between the groups in Austin and Lyon. We thank Mme. M. N. Mozzanega (CNRS) for the preparation of some of the Pt/Ti02 samples. (35) Ishii, Y.; Osakada, K.; Ikariya, T.; Saburi, M.; Yoshikawa, S. J. Org. Chem. 1986,51, 2034. (36) Hussein, F. H.; Pattenden, G.; Rudham, R.; Russell, J. J. Tetrahedron Lett. 1984,3363.
