The Influence Of Aryl-Aryl Interactions In The Photochemistry Of Some 1,3-Diarylpropanes
Abedawn I. Khalaf,1 Clive E. Badman,2 Marcus P. Ennis,2 William M. Horspool,2 and Qaisar Sultana2
1
Department of Pure & Applied Chemistry, WestCHEM, Strathclyde University, Thomas Graham
Building, 295 Cathedral Street, GLASGOW G1 1XL
2
Department of Chemistry, Dundee University, Dundee DD1 4HN, UK
Correspondence should be addressed to Abedawn I. Khalaf, abedawn.khalaf@strath.ac.uk
1. Abstract
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.
2. Keywords
1,3-diarylpropanols, photosolvolysis, photodechlorination
3. Introduction
For 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. Experimental
A 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 7677oC).
(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-3phenylpropyl)benzene 2c as a colourless oil [22] (0.85g, 80%), B.P. 158-160oC/0.88 mmHg. The nonirradiated solution gave the starting material back. IR (NaCl): 700, 750, 1510, 1610, 1100, 2800-3000 cm.-1
1
H 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), 6.737.33(9H, m). Microanalysis: Found: C, 79.80; H, 7.93 Calculated for C 17H20O2 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. 132134oC/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. 146148/0.3 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), 2.492.73(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 Discussion
We 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
C6H5
4-Me2NC6H4
1a
C5H5FeC5H4 4-MeOC6H4
1b
C5H5FeC5H4 4-MeOC6H4
1c
C6H5
C6H5
1d
C6H5
3,4-methylenedioxy C6H3
1e
C6H5
4-MeOC6H4
1f
C6H5
3-MeOC6H4
1g
R
OH
MeO
H
OH
OH
OH
OH
Consequently 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
Ar2
C6H5
C6H5
2a
C6H5
3,4-methylenedioxy C6H3
2b
C6H5
4-MeOC6H4
2c
C6H5
3-MeOC6H4
2d
Quantitative studies indicate that the nature of the substitution in Ar 2 (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 Ar 2
(compound 5). As the electron donating ability of the substituents in Ar 2 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 (Ar 2 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. References
1. 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. Chem. Soc., 1963, 85, 992-997.
7. Jaarinen, S., Niiranen, J., Koskikallio, J., International. J. Chem. Kinetics, 1985, 17(9), 925-930.
8. Cristol, S. J., Greenwald, B. E., Tetrahedron Lett., 1976, 25, 2105-2108.
9. Kochi, J. K., Ratcliff. M. A., J. Org. Chem., 1971, 36(21), 3112-3120.
10. Maycock, A. L., Berchtold, G. A., J. Org. Chem., 1970, 35(8), 2532-2538.
11. Hirayama, F., J. Chem. Physics, 1965, 42(9), 3163-3171.
12. Grimshaw, J., De Silva, A., J. Chem. Soc., Perkin Transactions 2, 1981, 7, 1010-1014.
13. Bunce, N. J., Ravanal, L., J. Amer. Chem. Soc., 1977, 99(12), 4150-4152.
14. Kohler, E. P. and Chadwell, H. M., Organic Synthesis, Collective Volume 1, 1944, 71.
15. Silver, N. L., Boykin, D. W., J. Org. Chem., 1970, 35(3), 759-764.
16. Kochetkov, N. K., Belyaev, V. F., Zhurnal Obshchei Khimii, 1960, 30, 1495-1497.
17. Bhat, B. A., Dhar, K. L., Puri, S. C., Saxena, A. K., Shanmugavel, M., Qazi, G. N., Bioorg. & Med.
Chem. Lett., 2005, 15(12), 3177-3180.
18. Bushby, R. J., Ferber, G. J., J. Chem. Soc., Perkin Transactions 2: 1976, 14, 1683-1688.
19. Fukuzawa, S., Fujinami, T., Yamauchi, S. and Sakai, S. 1986, J. Chem. Soc. Perkin Transactions 1, 11,
1929-1932.
20. Rondestvedt, C. S., J. Amer. Chem. Soc., 1951, 73(10), 4509-4511.
21. Andrade, C. K. Z., Silva, W. A., Letters in Org. Chem., 2006, 3(1), 39-41.
22. Hixson, S. S., Garrett, D. W., J. Amer. Chem. Soc., 1974, 96(15), 4872-4879.
23. Hixson, S. S., Tetrahedron Lett., 1971, 44, 4211-4214.