Solvent and Temperature Effects on Diastereodifferentiating Paternó

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Solvent and Temperature Effects on Diastereodifferentiating Patern
o-B€ uchi Reaction of Chiral Alkyl Cyanobenzoates with Diphenylethene upon Direct versus Charge-Transfer Excitation Kazuyuki Matsumura, Tadashi Mori,* and Yoshihisa Inoue* Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan [email protected] Received July 6, 2010

In the Patern
o-B€ uchi reaction of chiral p-cyanobenzoates (1) with 1,1-diphenylethene (2), we revealed that the excited charge-transfer (CT) complex formed upon selective excitation at the CT band is distinctly different in structure and reactivity from the conventional exciplex generated through the direct excitation of acceptor 1 which subsequently associates with donor 2. Thus, the favored diastereoface upon photocycloaddition, as well as the temperature- and solvent-dependent behavior of the product’s diastereoselectivity, were highly contrasting, often opposite, to each other upon direct versus CT excitation. From the activation parameters obtained by the Eyring analyses of the diastereoselectivity, we are able to infer that the conventional exciplex is relatively flexible and susceptible to the environmental variants, whereas the CT complex is better π-π stacked and more rigid in the ground state and also in the excited state, leading to the significantly smaller differential activation enthalpies and entropies. More interestingly, the signs of the differential activation parameters determined for direct and CT excitation are consistently opposite to each other and the isokinetic temperatures calculated therefrom differ significantly, unambiguously revealing the distinctly different nature in structure and reactivity of these two excited-state complex species. Thus, the combined use of irradiation wavelength, temperature, and solvent provides us with a convenient, powerful tool not only for elucidating the mechanistic details of photoreaction but also for critically controlling the stereochemical outcomes of photochirogenic reaction.

Introduction The Patern
o-B€ uchi reaction,1 i.e., [2 þ 2] photocycloaddition of a ketone to an alkene leading to oxetane formation, has attracted much attention from the mechanistic and synthetic (1) (a) Patern o, E.; Chieffi, G. Gazz. Chim. Ital. 1909, 39, 341–361. (b) B€ uchi, G.; Inman, C. G.; Lipinsky, E. S. J. Am. Chem. Soc. 1954, 76, 4327– 4331. (2) (a) D’Auria, M.; Racioppi, R. Eur. J. Org. Chem. 2010, 3831–3826. (b) Abe, M. J. Chin. Chem. Soc. 2008, 55, 479–486. (c) D’Auria, M.; Emanuele, L.; Racioppi, R. Adv. Photochem. 2005, 28, 81–127. (d) Griesbeck, A. G. Synlett 2003, 451–472. (e) Vargas, F.; Rivas, C. Int. J. Photoenergy 2000, 2, 97–101. (f) Bach, T. Synlett 2000, 1699–1707. (g) Bach, T. Synthesis 1998, 683–703. (h) Jones, G. Org. Photochem. 1981, 5, 1–122. (i) Horspool, W. M. Photolysis of Carbonyl Compounds, in Specialist Periodical Reports, Photochemistry, vol. 11, 283-300. (Ed. Bryce-Smith, D.) RSC Publishing, UK (1981). (j) Arnold, D. R. Adv. Photochem. 1968, 6, 301–423.

DOI: 10.1021/jo101332x r 2010 American Chemical Society

Published on Web 07/27/2010

viewpoints,2 as the photochemical route to oxetane is more advantageous in several aspects than conventional thermal o-B€ uchi reaction were ones.3 Mechanistic studies on Patern
carried out in considerable detail to reveal the vital role of temperature in determining the product selectivity.4 The knowledge accumulated through these studies provides the crucial clues to effectively control the distribution and stereochemistry of oxetane products. More recently, asymmetric Patern
oB€ uchi reactions of chirally modified substrates were also studied (3) (a) Hoffmann, N. Chem. Rev. 2008, 108, 1052–1103. (b) Jimenez, M. C.; Miranda, M. A. Oxygen-containing functions, in Specialist Periodical Reports, Photochemistry, vol. 37, 149-174. (Ed. Albini, A.) RSC Publishing, UK (2009). (c) See also: Roth, H. D. Angew. Chem., Int. Ed. Engl. 1989, 28, 1193–1207. (4) Buschmann, H.; Scharf, H.-D.; Hoffmann, N.; Esser, P. Angew. Chem., Int. Ed. Engl. 1991, 30, 477–515.

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JOC Featured Article extensively to find that a wide variety of chiral oxetanes were produced in generally high diastereoselectivities.5 A considerable amount of effort has been devoted to the manipulation of regio- and diastereoselectivities in asymmetric Patern
o-B€ uchi reactions,6 and in particular, the use of allylic strain and hydrogen-bonding interaction were demonstrated to be highly effective in controlling the stereochemical consequence of the diastereodiffeentiating [2 þ 2] photoreaction.7 However, the precise control of the face selectivity of Patern
o-B€ uchi reaction is still a challenging task. The excited-state dynamics of a donor-acceptor system can be manipulated by changing the excitation wavelength, but the focus is rather on the photophysical aspects in the literature,8 and the importance of excitation wavelength on Patern
o-B€ uchi reaction has not been fully recognized or examined in detail.9 We have recently investigated the temperature dependence behavior of the diastereodifferentiating [2 þ 2] photocycloaddition of chiral alkyl benzoates (1) to 1,1-diphenylethene (2) (Scheme 1).10 The stereochemical outcome was critically affected by the mode of excitation, and the direct excitation of acceptor 1 and the selective excitation at the charge-transfer (CT) band gave the same oxetane but in entirely different diastereoselectivities, indicating the existence of two distinct excitedstate complex species that are not equilibrated each other. The CT-band excitation of he ground-state complex has been a target of intensive studies,11 but the observed difference in photochemical outcome upon direct versus CT excitation was rather (5) (a) D’Auria, M.; Racioppi, R. Cur. Org. Chem. 2009, 13, 939–954. (b) Demuth, M.; Mikhail, G. Synthesis 1989, 145–162. (b) See also: Gotthardt, H.; Lenz, W. Angew. Chem., Int. Ed. Engl. 1979, 18, 868. (6) (a) Adam, W.; Stegmann, W. J. Am. Chem. Soc. 2002, 124, 3600–3607. (b) Adam, W.; Stegmann, W.; Weik€ otz, S. J. Am. Chem. Soc. 2001, 123, 2452–2453. (c) Hoffmann, N.; Buschmann, H.; Raabe, G.; Scharf, H. D. Tetrahedron 1994, 50, 11167–11186. (d) Buschmann, H.; Hoffmann, N.; Scharf, H. D. Tetrahedron: Asymmetry 1991, 2, 1429–1444. (e) Buschmann, H.; Scharf, H.-D.; Hoffmann, N.; Plath, M. W.; Runsink, J. J. Am. Chem. Soc. 1989, 111, 5367–5373. (f) Pelzer, R.; Juetten, P.; Scharf, H. D. Chem. Ber. 1989, 122, 487–491. (g) Hoffmann, N.; Scharf, H.-D.; Runsinkt, J. Tetrahedron Lett. 1989, 30, 2637–2638. (h) Runsink, J.; Koch, H.; Nehrings, A.; Scharf, H. D.; Nowack, E.; Hahn, T. J. Chem. Soc., Perkin Trans. 2 1988, 49–55. (i) Herzog, H.; Koch, H.; Scharf, H.-D.; Runsinkt, J. Tetrahedron 1986, 42, 3547–3558. (j) Koch, H.; Runsink, J.; Scharf, H. D. Tetrahedron Lett. 1983, 24, 3217–3220. (k) Jarosz, S.; Zamojski, A. Tetrahedron 1982, 38, 1453–1456. (7) (a) Bach, T.; J€ odicke, K.; Kather, K.; Fr€ ohlich, R. J. Am. Chem. Soc. 1997, 119, 2437–2445. (b) Bach, T.; J€ odicke, K.; Kather, K.; Hecht, J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2271–2273. (c) Griesbeck, A. G.; Bondock, S. J. Am. Chem. Soc. 2001, 123, 6191–6192. (d) Adam, W.; Stegmann, V. R. Synthesis 2001, 1203–1214. (e) Adam, W.; Peters, K.; Peters, E. M.; Stegmann, V. R. J. Am. Chem. Soc. 2000, 122, 2958–2959. (f) Bach, T.; Bergmann, H.; Harms, K. J. Am. Chem. Soc. 1999, 121, 10650–10651. (g) Abe, M.; Kawakami, T.; Ohata, S.; Nozaki, K.; Nojima, M. J. Am. Chem. Soc. 2004, 126, 2838–2846. (h) Abe, M.; Fujimoto, K.; Nojima, M. J. Am. Chem. Soc. 2000, 122, 4005–4010. (8) (a) Nicolet, O.; Banerji, N.; Pages, S.; Vauthey, E. J. Phys. Chem. A 2005, 109, 8236–8245. (b) Levy, D.; Arnold, B. R. J. Phys. Chem. A 2005, 109, 2113–2119. (c) See also: Mohammed, O. F.; Vauthey, E. J. Phys. Chem. A 2008, 112, 3823–3830. (d) Wang, Y.; Haze, O.; Dinnocenzo, J. P.; Farid, S.; Farid, R. S.; Gould, I. R. J. Org. Chem. 2007, 72, 6970–6981. (9) Gotthardt, H.; Lenz, W. Tetrahedron Lett. 1979, 20, 2879–2880. (10) Matsumura, K.; Mori, T.; Inoue, Y. J. Am. Chem. Soc. 2009, 131, 17076–17077. (11) (a) Gonzalez-Bejar, M.; Stiriba, S.-E.; Miranda, M. A.; Perez-Prieto, J. Org. Lett. 2007, 9, 453–456. (b) Haga, N.; Takayanagi, H.; Tokumaru, K. J. Chem. Soc., Perkin Trans. 2 2002, 734–745. (c) Sun, D.; Hubig, S. M.; Kochi, J. K. J. Org. Chem. 1999, 64, 2250–2258. (d) Hubig, S. M.; Sun, D.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 2 1999, 781–788. (e) Bosch, E.; Hubig, S. M.; Lindeman, S. V.; Kochi, J. K. J. Org. Chem. 1998, 63, 592–601. (f) Haga, N.; Nakajima, H.; Takayanagi, H.; Tokumaru, K. J. Org. Chem. 1998, 63, 5372–5384. (g) Takahashi, Y.; Ohaku, H.; Nishioka, N.; Ikeda, H.; Miyashi, T.; Gormin, D. A.; Hilinski, E. F. J. Chem. Soc., Perkin Trans. 2 1997, 303–308. (h) Haga, N.; Nakajima, H.; Takayanagi, H.; Tokumaru, K. Chem. Commun. 1997, 1171–1172.

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Matsumura et al. SCHEME 1. Diastereodifferentiating [2 þ 2] Photocycloaddition of Chiral Alkyl Benzoates 1 to 1,1-Diphenylethene 2 upon Direct and Charge-Transfer Band Excitation

small or controversial, except for the clear excitation-wavelength dependence observed in the diastereodifferentiating [2 þ 2] photocycloaddition of stilbene to chiral fumarate.12 In this study, we closely investigated the solvent and temperature effects on the diastereodifferentiating photocycloaddition of chiral alkyl cyanobenzoates 1 to diphenylethene 2 to elucidate the nature and the difference of two excited-state complex species generated upon direct and CT excitation. Interestingly, the CT, rather than direct, excitation led to lower diastereoselectivities in general, which is in keen contrast to the photocycloaddition of stilbene to chiral fumarate, where the CT excitation consistently affords better diastereoselectivities. Although the photoreaction of stilbene with fumarate is not very clean (accompanying the E-Z isomerization and dimerizaiton of stilbene), the reaction of 1 with 2 exclusively affords oxetane 3, allowing more detailed examinations of the product selectivities. The differential activation parameters obtained by the Eyring analysis of the diastereoselectivities of 3 obtained under a variety of conditions provide us with rich insights into the photocycloaddition mechanism and the nature of excited-state intermediates, which will be discussed below. Results and Discussion Ground-State Interaction of Cyanobenzoate 1a with Diphenylethene 2. It was reported that despite no clear indication of the ground-state interaction between electron-deficient aromatic esters, such as dimethyl tere- and isophthalate, and several olefinic donors in the UV spectra, these donor-acceptor pairs smoothly underwent the Patern
o-B€ uchi reaction to give the corresponding oxetanes upon photoexcitation.13 Because such interactions are elusive in general, it is likely that the failure to detect the ground-state interaction was due to the insufficient formation of the CT complex under the experimental conditions employed. Indeed, we were able to observe weak but appreciable UV spectral changes only by mixing donor 2 and acceptor 1a at relatively high concentrations (∼0.2 M). The new absorption band, visible at the red edge of the original spectrum, can be ascribed to the formation of donor-acceptor or charge-transfer complex. Association constants (KCT) were determined in a variety of solvents by using the modified Benesi-Hildebrand method; the original Benesi-Hildebrand treatment14 was avoided in the present case, as the method is not suitable for accurately determining a (12) (a) Saito, H.; Mori, T.; Wada, T.; Inoue, Y. J. Am. Chem. Soc. 2004, 126, 1900–1906. (b) Saito, H.; Mori, T.; Wada, T.; Inoue, Y. Org. Lett. 2006, 8, 1909–1912. (13) (a) Shigemitsu, Y.; Nakai, H.; Odaira, Y. Tetrahedron 1969, 25, 3039–3043. (b) Shigemitsu, Y.; Katsuhara, Y.; Odaira, Y. Tetrahedron Lett. 1971, 12, 2887–2890. (c) Kateuhara, Y.; Shigemitsu, Y.; Odaira, Y. Bull. Chem. Soc. Jpn. 1971, 44, 1169. (14) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703– 2707.

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Matsumura et al. TABLE 1. Association Constants (KCT) of Charge-Transfer Complex of 1a with 2 in Some Solventsa dipole moment dielectric solvent (μ)/debye constant/ε λedge/nmb KCT/M-1 methylcyclohexane toluene tetrahydrofuran acetonitrile

0 0.37 1.68 3.94

2.02 2.38 7.39 37.5

337 343 328 338

0.04 (0.03)c 0.8 0.02 0.2

a Determined by the modified Benesi-Hildebrand method for solutions of [1a] = [2] = 0.2-0.07 M at 25 °C. For data in methylcyclohexane, see ref. 10. bEdge of the absorption spectrum of CT complex (defined as the wavelength at which the absorbance becomes 0.005), which was obtained by subtracting the spectral sum of 1a and 2 from the observed spectrum. cAt 50 °C.

low K value. Thus, we recorded the UV spectral changes upon simultaneous reduction of the donor and acceptor concentrations from 0.2 M to 0.07 or 0.08 M, keeping the 1:1 stoichiometry. The deviation of the observed spectrum from the spectral sum of each component at a given concentration indicates the net absorption of the CT complex, which appeared at 300-340 nm (Figure S1, Supporting Information). The absorbance change in the CT band region was plotted against the product of the donor and acceptor concentrations,15 followed by the curve fitting procedure, to afford the association constants listed in Table 1. The detailed discussion on the evaluation of such low KCT values for weak CT complexes may be found in the literature.16 The association constant for the 1a/2 pair is comparable to or slightly lower than the typical values reported for the conventional CT complexes formed between aromatic donors and tetracyanoethene or chloranil.17 The relatively low KCT value found for 1a/2 pair may be attributed at least in part to the steric hindrance of the acceptor’s ester moiety. The KCT values are known to be significantly affected by the steric inhibition between donor and acceptor, in addition to the electrostatic and stacking interactions.18 The KCT value became considerably larger in polar acetonitrile than in less polar methylcyclohexane or tetrahydrofuran, indicating that the charge delocalization within the complex plays some role to stabilize the complex. The much larger KCT value in toluene may be attributed to the effective stabilization of the complex through the π-stacking interaction with toluene molecules. The complexation thermodynamic parameters could not be determined, as the difference in KCT determined at 25 and 50 °C (in methylcyclohexane) was so small to give a reliable van’t Hoff plot. Due to the low KCT values in all the solvents, relatively high donor/acceptor concentrations were used to keep a satisfactory amount of the complex in the solution. However, by irradiating the CT band at 330 nm through a band-path filter (with a xenon light source), the selective excitation of the CT complex was possible (vide infra). The ground-state interaction was further examined by circular dichroism (CD) spectroscopy,19 as cyanobenzoate (15) Arnold, B. R.; Euler, A.; Fields, K.; Zaini, R. Y. J. Phys. Org. Chem. 2000, 13, 729–734. (16) (a) Zaini, R.; Orcutt, A. C.; Arnold, B. R. Photochem. Photobiol. 1999, 69, 443–447. (b) Exner, O. Chemom. Intell. Lab. Syst. 1997, 39, 85–93. (17) (a) Frey, J. E.; Andrews, A. M.; Ankoviac, D. G.; Beaman, D. N.; Du Pont, L. E.; Elsner, T. E.; Lang, S. R.; Zwart, M. O.; Seagle, R. E.; Torreano, L. A. J. Org. Chem. 1990, 55, 606–624. (b) Frey, J. E.; Aiello, T.; Beaman, D. N.; Combs, S. D.; Fu, S.-L.; Puckett, J. J. J. Org. Chem. 1994, 59, 1817–1830. (c) Frey, J. E.; Aiello, T.; Fu, S.-L.; Hutson, H. J. Org. Chem. 1996, 61, 295–300. (18) Rathore, R.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 9393–9404. (19) (a) Hembury, G. A.; Borovkov, V. V.; Inoue, Y. Chem. Rev. 2008, 108, 1–73. (b) Berova, N., Nakanishi, K., Woody, R. W. Circular Dichroism: Principles and Applications, 2nd ed.; Wiley: New York, 2000.

1a has a chiral handle. The electronic CD is frequently used as a tool for analyzing the conformational behavior of chiral molecules in solution.20 Thus, the CD spectral changes of a methylcyclohexane solution of 1a upon addition of 2 were examined. Although no appreciable induced CD associated with the complexation developed at 25 °C under the same conditions employed for the photoreaction, a fairly weak but appreciable negative Cotton effect was observed for the CT band at around 320-330 nm decreasing the temperature down to -25 °C (Figure S2, Supporting Information). Note that cyanobenzoate 1a and the related (S,S)-bis(2-methylpropyl) terephthalate were not fluorescent at 25 °C in the presence and absence of donor 2; nevertheless, achiral methyl terephthalate was subjected to the fluorescence study.13 Diastereodifferentiating [2 þ 2] Photocycloaddition of 1a to 2 upon Direct versus CT Excitation. Photoirradiation of a concentrated solution of 1a and 2 (0.2 and 1.0 M, respectively) afforded diastereomeric oxetane 3a in moderate to good combined yields in all of the solvents examined (Table 2). The photoreaction was essentially very clean, and only small amounts of minor byproduct were detected by HPLC analysis; therefore, the low material balance and modest chemical yield would be ascribed at least in part to the less accurate evaluation of consumed 1a in particular at low conversions. The diastereomeric excess (de) of photoadduct 3a was determined by HPLC with satisfactory accuracy (error (3%), and the effects of irradiation wavelength, temperature, and solvent on the de of 3a obtained upon photoirradiation of a mixture of 1a and 2 were systematically examined. (a) Effect of Irradiation Wavelength. The effect of irradiation wavelength on the product’s de was examined with a toluene solution of 1a and 2 at low conversions.10 The de of 3a was kept almost constant upon irradiation at 254-290 nm but gradually decreased by shifting the irradiation wavelength to 310-320 nm, and eventually the epimeric product was obtained upon excitation at 330 nm. When irradiated at 340 nm, the starting material was recovered even after prolonged irradiations. We initially investigated the wavelength effect on the diastereoselectivity of 3a at 254 nm (from a lowpressure mercury lamp) and at 313 nm (from a high-pressure mercury lamp fitted with a Pyrex filter), and the temperaturedependent de values obtained at each wavelength were subjected to the Eyring analysis by plotting the relative rate constant forming diastereomeric 3a as a function of reciprocal temperature. The de values obtained upon excitation at 254 nm gave a good linear Eyring plot over the entire temperature region employed. However, the de data obtained at 313 nm led to a curved plot (Figure S3, Supporting Information), revealing operation of two distinct diastereodifferentiation mechanisms that are switched at the middle of the temperature range employed (ca. 15 °C). The Eyring plot for the de’s obtained at 330 nm became normal again, giving a single straight line of oppositely signed slope without showing any curvature over the entire temperature range. Hence, the spectral overlap between 1a and the CT complex is likely to be responsible for the curved Eyring plot and the mixed diastereodifferentiation mechanism observed at 313 nm. We therefore chose the excitation wavelengths of 290 and 330 nm (20) (a) Mori, T.; Inoue, Y.; Grimme, S. J. Org. Chem. 2006, 71, 9797– 9806. (b) Mori, T.; Inoue, Y.; Grimme, S. J. Phys. Chem. A 2007, 111, 7995– 8006. (c) Mori, T.; Inoue, Y.; Grimme, S. J. Phys. Chem. A 2007, 111, 4222– 4234.

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TABLE 2. Diastereodifferentiating [2 þ 2] Photocycloaddition of (S)-1Methylpropyl p-Cyanobenzoate 1a to 1,1-Diphenylethene 2 upon Direct and Charge-Transfer Band Excitation in a Variety of Solventsa excitation irrad temp/ % % % solvent modeb time/h °C convc yieldd dee methylcyclohexane

direct

CT

toluene

tetrahydrofuran

acetonitrile

2% acetonitrile in methylcyclohexane

3

10

direct

3

CT

6

direct

3

CT

5

direct

3

CT

7

direct

3

CT

8

50 25 0 -25 -50 50 25 0 -25 -50 50 25 0 -25 -50 50 25 0 -25 -50 50 25 -50 50 25 -50 50 25 -20 -40 50 25 -20 -40 50 25 -20 -40 50 25 -20 -40

31 31 33 32 32 20 23 31 26 26 18 25 18 26 24

12 18 8 13 17 24 20 25 22 23 23 27 26 26 14

12 12 11 11 9 5 4 4 4 6 12 13 9 10 10 4 4 5 4 5 9 7 4 4 5 4 4 4 5 6 11 15 15 14 3

77 64 63 43 34 3 4 3 24 26 74 65 68 54 37 -11 -4 4 6 21 57 54 32 -12 -8 12 -13 -12 -11 -11 -3 -8 -10 -11 1

22 22 22 10 6 21 20

3 4 4 4 3 3 4

-2 -4 -2 16 13 0 3

f f f f f

a

[1a] = 0.2 M, [2] = 1.0 M. For data in methylcyclohexane, see ref. 10. Irradiation was performed at 290 nm (direct excitation) or at 330 nm (CT excitation) using an appropriate band-path filter fitted to the light source LAX-101 (100 W xenon lamp). cConsumption of 1a determined by HPLC. dHPLC yield of 3a. eThe absolute configurations of diastereomeric 3a were established by X-ray crystallography (ref 10), and the positive and negative de values indicate the predominant formation of (10 S,2R)- and (10 S,2S)-3a, respectively. fNot determined. b

for direct and selective CT excitation, respectively, throughout the study. Although the possible contribution of hidden CT bands at shorter wavelengths is difficult to strictly exclude, the same de values obtained at 254 and 290 nm indicate the insignificant contribution of the excited CT complex in the direct excitation. (b) Effect of Triplet Quencher on the Diastereoselectivity. While typical Patern
o-B€ uchi reactions of ketones with olefins undergo via triplet biradical intermediates,21 the photoaddition of cyanobenzoate 1 with diphenylethene 2, in both (21) (a) Freilich, S. C.; Peters, K. S. J. Am. Chem. Soc. 1981, 103, 6257– 6259. (b) Freilich, S. C.; Peters, K. S. J. Am. Chem. Soc. 1985, 107, 3819– 3822. (c) Yang, N. C.; Loeschen, R. L.; Mitchell, D. J. Am. Chem. Soc. 1967, 89, 5465–5466.

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direct and CT excitation, is thought to proceed in the singlet manifold, as judged from the triplet quenching experiments shown below. This view also agrees with the results of previous studies on the photocycloaddition reactions of electron-deficient o-B€ uchi aromatic esters with olefinic donors.13 Thus, the Patern
reaction of 1a and 2 upon direct and CT excitation was examined in the presence of (E)-1,3-pentadiene (0.2-0.4 M), whose triplet energy, ET=248 kJ mol-1, is reasonably lower than that of methyl p-cyanobenzoate, ET=301 kJ mol-1.22 The addition of diene lowered the conversion but enhanced the de in both excitation modes (Table S1, Supporting Information). On the contrary, the yield and de of 3a obtained upon direct and CT excitation were almost unaffected (within the experimental error) in the presence of molecular oxygen added as a triplet quencher,23 suggesting that the triplet species are not the major species in the present photoreaction.10 As the Eyring treatment of the de data always afford good linear plots (vide infra), a single diastereodifferentiation mechanism should be operative under the reaction conditions employed. We may conclude, therefore, that 1,3-pentadiene behaves not only as a triplet quencher but also acts as a chemical quencher. It seems that the singlet manifold of the cyanobenzoate is also quenched by this diene presumably via energy or electron transfer process. Accordingly, the intervention of triplet species in the overall mechanism and its effect on de in the reaction of 1 with 2 are insignificant under our experimental conditions. (c) Temperature and Solvent Effects on the Photoreaction. For better comparison of the direct versus CT excitation and also the diastereodifferentiation mechanisms operative in both excitation modes, we performed the photoreaction of 1a and 2 at various temperatures ranging from þ50 to -50 °C to obtain the results shown in Table 2. Attempts to further lower the reaction temperature down to -70 °C in toluene resulted in extremely low conversion and yield (3a in