The irradiation of 1,3-diarylpropanols in acidic methanol results in their conversion to the corresponding methyl ethers. This reaction and that of the photodechlorination of some 1,3-diarylpropanes is influenced by the presence of electron donating substituents in the aryl group remote from the reactive site

Badman, Clive E.

Ennis, Marcus P.

Horspool, William M.

Khalaf, A.I.

Name not available

The influence of aryl-aryl interactions in the photochemistry of some 1,3-Diarylpropanes

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The Influence Of Aryl-Aryl Interactions In The Photochemistry Of Some 1,3-DiarylpropanesAbedawn I. Khalaf,1 Clive E. Badman,2 Marcus P. Ennis,2 William M. Horspool,2 and Qaisar Sultana21Department of Pure & Applied Chemistry, WestCHEM, Strathclyde University, Thomas Graham Building, 295 Cathedral Street, GLASGOW G1 1XL2Department of Chemistry, Dundee University, Dundee DD1 4HN, UKCorrespondence should be addressed to Abedawn I. Khalaf, abedawn.khalaf@strath.ac.uk (​mailto:abedawn.khalaf@strath.ac.uk​)1. AbstractThe irradiation of 1,3-diarylpropanols in acidic methanol results in their conversion to the corresponding methyl ethers. This reaction and that of the photodechlorination of some 1,3-diarylpropanes is influenced by the presence of electron donating substituents in the aryl group remote from the reactive site.2. Keywords1,3-diarylpropanols, photosolvolysis, photodechlorination3. IntroductionFor some years we have been studying the photochemical reactions of 1,3-diaryl systems to evaluate the influence of substituents in the aromatic ring distant from the reaction centre. We were especially intrigued by the report by Ullman and his co-workers [1] who demonstrated that the 1,3-diarylpropane derivatives 1a would undergo photosolvolysis resulting in the exchange of the hydroxyl group for a methoxyl substituents. Our earlier studies [2-4] had identified another example of such a system where the irradiation of the ferrocenyl ether 1b resulted in the reductive loss of the methoxy substituents on the propane chain yielding 1c. Often the irradiation of benzyl derivatives [3] leads to both homo- and heterolytic reactions [2a, b] and in the cases above it appears that 1a reacts solely in the heterolytic mode while 1b yields product exclusively from a homolytic process (see table 1).4. ExperimentalA general procedure was used for the preparation of the following chalcones [14]:(2E)-1,3-Diphenyl-2-propen-1-one, mp 55-57oC (lit.[14] mp 55-57oC).(2E)-3-(4-Methoxyphenyl)-1-phenyl-2-propen-1-one, mp 75-77oC (lit.[15] mp 73-74oC; lit.[16] mp 76-77oC).(2E)-3-(1,3-Benzodioxol-5-yl)-1-phenyl-2-propen-1-one, mp 124-126oC.(2E)-3-(3-Methoxyphenyl)-1-phenyl-2-propen-1-one, mp 58-59oC (lit. [15] mp 59-61oC).The corresponding alcohols were prepared by the method of Bushby and Ferber [17, 18].1d: 1,3-Diphenyl-1-propanol as a colourless oil  [18, 19].1e: 3-(1,3-Benzodioxol-5-yl)-1-phenyl-1-propanol as white crystals  mp 93-94oC (lit.[21] mp 95-96oC].1f: 3-(4-Methoxyphenyl)-1-phenyl-1-propanol as a white crystals, mp 65-67oC (lit.[20] mp 65-65.8oC).1g: 3-(3-Methoxyphenyl)-1-phenyl-1-propanol as yellow oil.Photolysis of 3-(4-methoxyphenyl)-1-phenyl-1-propanol 1f:The alcohol (1.00g, 4.13 mmol) was dissolved in methanol (250 mL, dry) to which HCl (0.5 mL, 1M) was added. A sample (10 mL) was stored in a sample tube in the dark for the duration of the experiment. The bulk of the solution was put in an immersion apparatus and irradiated for 48h. Both solutions were neutralized with NaHCO3 and the solvent was evaporated from each under reduced pressure, and then the organic material was extracted with chloroform and water. Evaporation of the organic layer after drying (MgSO4) under reduced pressure gave the required product: 1-methoxy-4-(3-methoxy-3-phenylpropyl)benzene 2c as a colourless oil [22] (0.85g, 80%), B.P. 158-160oC/0.88 mmHg. The non-irradiated solution gave the starting material back. IR (NaCl): 700, 750, 1510, 1610, 1100, 2800-3000 cm.-1 1H NMR (CDCl3): 1.67-2.33(2H, m), 2.51-2.73(2H, t), 3.20(3H, s), 3.76(3H, s), 3.98-4.16(1H, t), 6.73-7.33(9H, m). Microanalysis: Found: C, 79.80; H, 7.93 Calculated for C17H20O2 Required C, 79.69; H, 7.81%.Similarly the following were prepared:2d: 1-Methoxy-3-(3-methoxy-3-phenylpropyl)benzene: as a colourless oil [22] (91% yield), B.P. 132-134oC/0.1 mmHg. IR (KBr): 700, 760, 1100, 1600, 1480, 1350, 1450, 3000-2800 cm.-1 1H NMR (CDCl3): 1.96-2.13(2H, m), 2.56-2.78(2H, t), 3.22(3H, s), 3.78(3H, s), 3.98-4.18(1H, t), 6.73(4H, m), 7.24(5H, m). Microanalysis: Found: C, 79.65; H, 8.04 Calculated for C17H20O2 Required C, 79.69; H, 7.81%.2b: 3-(1,3-Benzodioxol-5-yl)-1-phenylpropyl methyl ether: as a colourless oil  (86% yield), B.P. 146-148/0.3 mmHg). IR (KBr): 700, 760, 800, 880, 1600, 1100 cm.-1 1H NMR (CDCl3): 1.78-2.18(2H, m), 2.44-2.67(2H, t), 3.16(3H, s), 3.93-4.11(1H, t), 2.82(2H, s), 6.62(3H, d), 7.22(5H, m). Microanalysis: Found: C, 75.64; H, 6.87 Calculated for C17H18O3 Required: C, 75.56; H, 6.67%.2a: (3-Methoxy-3-phenylpropyl)benzene: as a colourless oil [23] (81% yield), B.P. 114-116oC/0.5 mmHg. IR (KBr): 700, 750, 1100, 1450, 1490, 1600, 2800-3040 cm.-1 1H NMR (CDCl3): 1.80-2.22(2H, m), 2.49-2.73(2H, t), 3.16(1H, s), 3.91-4.09(1H, t), 6.67-7.74(10H, m). Microanalysis: Found: C, 84.99; H, 7.92 C16H18O Required: C, 84.96; H, 7.96%.5. Results and DiscussionWe were interested in the behaviour of diarylpropanes where the aryl substitution was not as heavily weighted in favour of a charge transfer as the system described by Ullman [1].Table 1: Substituted diarylpropanes 1.Compound	Ar1	Ar2	R1a1b1c1d1e1f1g	C6H5C5H5FeC5H4C5H5FeC5H4C6H5C6H5C6H5C6H5	4-Me2NC6H44-MeOC6H44-MeOC6H4C6H53,4-methylenedioxy C6H34-MeOC6H43-MeOC6H4	OHMeOHOHOHOHOHConsequently the synthesis of the derivatives 1d-g was carried out and the photochemical activity studied. The synthesis of these compounds 1d-g is readily achieved using base catalysed condensation of the appropriate aldehyde with acetophenone to yield the substituted chalcone. Lithium aluminium hydride reduction of the enones affords the 1,3-diarylpropanol derivatives. The irradiation of these derivatives at 254nm in quartz tubes in dry methanol solution and under an atmosphere of deoxygenated nitrogen failed to yield detectable amounts of product. The reaction also failed when repeated under identical conditions to the above but with added acid. Reaction did take place, however, when the propanols 1d-g were irradiated under nitrogen in methanol to which hydrochloric acid (1x10-4M) had been added. Dark control reactions failed to yield products implying that the reaction observed on irradiation was truly photochemical. The products in each case were readily identified, by NMR spectroscopy and independent synthesis, as the corresponding methyl ethers 2a-d.Table 2: Methyl ethers formed from the irradiation of 1d, 1e, 1f, and 1g.Compounds	Ar1	Ar22a2b2c2d	C6H5C6H5C6H5C6H5	C6H53,4-methylenedioxy C6H34-MeOC6H43-MeOC6H4Quantitative studies indicate that the nature of the substitution in Ar2 (compounds 1d-g) is important in that those with oxygen substituents 1e-g react to form product faster than does the parent 1d. Furthermore, there are rate differences dependent upon the substitution pattern as can be seen from the ratios of reactivity i.e. 1d: 1e: 1f: 1g = 1.0: 4.5: 4.5: 9.5. Thus the m-methoxy substituent in 1g has the greatest effect on the photochemical process [3]. In view of the involvement of acid, and since the overall process is exchange of a hydroxyl function for a methoxy group, it seems feasible that a carbocationic process is operative involving an intermediate ion (e.g. 3). Analysis of the reaction mixture does not show the presence of products other than the ethers, thus the ion 3 does not fragment in the manner described by Ullman [1] for the propanol 1a.Hirayama [2-6] demonstrated from a study of exciplex emission that the aryl-aryl interaction in hydrocarbon systems was at an optimum when the aryl groups were separated by three carbons. If this arrangement is applicable to the situation described above, then it is possible to explain the stabilisation of the intermediate 3 and also to account for the enhancement of the hydroxyl-methoxyl exchange process in terms of an interaction such as that shown in 4 where electron donation takes place from the electron rich aryl ring (Ar2) to the electron deficient benzyl carbocation [7-12].A similar aryl-aryl interaction is also possible in the 1,3-diarylpropanes 5 and, to evaluate the effect of this on the photochemical reactions these compounds were synthesised from the appropriate 1,3-diarylpropanol by dehydration over potassium hydrogen sulphate and catalytic reduction of the resultant olefins. The photochemistry of the propanes 5a-c was simple, and each one undergoes photodechlorination [5] yielding 6a-c on irradiation of deoxygenated solution through quartz with a low pressure mercury lamp.A more detailed study showed that the efficiency of the photodechlorination of the propane 5a was dependant on the nature of the solvent in which the reaction was carried out (the relative quantum yields for the dechlorination of 5a were: EtOH: C6H12: MeOH: MeCN = 1.00: 0.82: 0.75: 0.63). Furthermore, the efficiency of the dechlorination is also dependent upon the nature of the substituents in the aryl ring Ar2 (compound 5). As the electron donating ability of the substituents in Ar2 is increased (measured by the decrease in the ionization potential) so the efficiency of the dechlorination decreases (relative quantum yields for dechlorination in methanol were 5a: 5b: 5c = 1.00: 0.24: 0.02, and in cyclohexane 5a: 5b = 1.00: 0.32). These results do not appear to be too dependant on solvent polarity. It is clear, however, that in this set of examples the electron donating ability of the aryl ring (Ar2 in 5) plays a major part in determining how efficient the photodechlorination is. Again the Hirayama model is an adequate description of how the interaction between the aryl groups takes place, but in this instance donation of electrons to the reacting site, i.e. to the ring from which dechlorination takes place, makes the dechlorination less efficient. These results are broadly in line with the proposal of Bunce and Ravanal [13] who studied the photodechlorination of 5a in cyclohexane.Interestingly the ferrocenyl derivative 5d is also photoreactive and undergoes photodechlorination in methanol to yield (6d, quantum yield = 0.025x10-2) with modest efficiency. The behaviour of this molecule 5d is somewhat different from the previous examples in that it is recovered unchanged when the irradiation is carried out in cyclohexane solution.6. References1. Lin, C-I., Singh, P., Ullman, E. F., 1976, J. Amer. Chem. Soc., 98(21), 6711-6713.2a. Lin, C-I., Singh, P., Ullman, E. F., 1976, J. Amer. Chem. Soc., 98(24), 7848-7850.2b. Rowe, G. T., Rybak-Akimova, E. V., Caradonna, J. P., Chem.-A Eur. J., 2008, 14(27), 8303-8311.2c. Yoon, U. C., Mariano, P. S., J. Photoscience, 2003, 10(1), 89-96.2d. Zhang, G., Li, W., Chu, B., Su, Z., Yang, D., Org. Electronics, 2009, 10(2), 352-356. 2e. Amada, S., Kishita, S., Nakai, S., Takada, M., Hosomi, M., Chemosphere, 2008, 73(6), 1005-1010.3. Baker, C., Horspool, W. M., J. Chem. Soc., Chem. Commun., 1972, 22, 1236-1237.4. Baker, C., Horspool, W. M., J. Chem. Soc., Perkin Transactions 1: 1979, 9, 2298-302.5. Kosmrlj, B., Sket, B., J. Org.  Chem., 2000, 65(21), 6890-6896.6. Zimmerman, H. E., Somasekhara, S., J. Amer. 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The influence of aryl-aryl interactions in the photochemistry of some 1,3-Diarylpropanes

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