ULTRAVIOLET ABSORPTION OF SUBSTITUTED PHENYL AND POLYCYCLIC ARYL CHALCONES

Departinent of Clzenlistry arzd Pzrerto Rico Nzrclear Center, University of Puerto Rico, niIayagues, Plrerto Rico Departnzent of Chemistry, Brzlnel College, London

Received February 14, 1964

ABSTRACT

The ultraviolet and near-visible light absorption of a nuniber of substituted trar~s-chalco~les are reported.

Chalcones ( I ) possess a conlplex conjugate system since the carbony1 group is inter- posed between phenyl and styryl groups. trans-Chalcones generally show two main regions of selective ultraviolet absorption, one near 230 n ~ p (band 11), the other beyond 300 inp (band I) (see Table I). These two nlaxima were formerly attributed (1) to the partial benzoyl (PhCO-) and cinna~noyl (PhCI-I=CI-ICO-) chroinophores, respectively, but the long wavelength band (band I) is now believed to involve the whole conjugate system (2). A third band, a t 205 mp, has been observed (3) and may be attributed to a modified E-band of the phenyl groups (cf. benzene A,,, 205 mp, E 6 300 (4)).

A so-called "middle" band, near 250 imp, is shown by cis-chalconcs (5) and by certain trans-chalcones (cf. compounds 5, 7, and 21, Table I) and this probably results from the partial (5) benzoyl (cf. PhCHO-, A,,, 248 mp, E 12 700 (6)) or benzoyl-vinyl (PhCOCH=CH-) chroinophores (cf. PhCH=CHCOCH3, A,,,, 286 mp, E 23 000 (7)).

Szinant and Basso (2) have discussed the effect of para- and ineta-substituents on the long wavelength band and have noted that a substituent produces a larger bathochronlic displacement when i t is located on ring A rather than on ring B (cf. Table 11). In the case of band 11, examination of the present data (Table I) and those reported in the literature (2, 3, 7-9) proved unsuccessful in arriving a t general conclusions. The situatioil is complex since the singlet ?r + ?r* transitions (K-bands) of most siinple substituted aryl groups (contributing here as partial chrornophores) fall within the region of absorption of band I1 (cf. styrene, A,,, 249 mp, E 10 000 (10); phenol, X,,, 270 mp, E 1 450 (11)).

ortho-Substituents are known to cause large hypsochromic effects in certain chalcones (2). Thus, negative values were shown for 2-chlorochalcones (8, 12) and for certain ortho-nitro (7) and mesityl derivatives (lb, 7). Similarly, 2-chloro-4'-methoxychalcone (compound 18) exhibits a AX = -22 mp, compared to the calculated absorption (2). While one ortho-substituent in ring B may give a bathochroinic displaceillent (e.g. 2'-hydroxychalcone, AX = 3-17 mp), the introduction of a further ortho-substituent may give an overall hypsochroinic effect (e.g. 2'-hydroxy-6'-metl~oxychalcone, AX = - 16 mp (9b)). A splitting of the K-band, with reduction of the intensity of absorption, has been observed in 2-methoxychalcone which exhibited A,,, 297 and 348 mp, E 15 400 and 14 800,

Canadian Journal of Chemistry. Volume 42 (1964)

2580

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WHEELER ET AL.: CFIALCONES 2581

TABLE I

Absorption of substituted phenyl chalcones

Substituents Band I1 Band I°

No. Ring A Ring B M.p. LUX (6.d L u x (4-1 AA

1 - 56" 226(10 5 0 0 ) ~ 310(27 000) 4'-Br 104" 229(10 550) 320(28 200)

$0 2 3-Me 3 4-Me 4'-OH 143" 230(7 700) 320(16 300)

+ 10

386(6 400) + 10

4 4-Me 4'-OMe 95" 235(10 500) 298*(28 300) 316(31300)

+ 6

5 4-OH 2'-OH 110" 222(15 200) 285(19 600) $25 243(8 700) 335(35 500)

6 4-OMe 4'-Me 90" 236(11 100) 350(28 700) $40 7 4-OMe 4'-Br 141°d 240(11400)d 291 (10 200)d $34

276(11400) 344(28 400) 8 4-OMe 4'-OMe 99" 236(13 OOO)a 350(32 000) $40 9 4-OMe 3'-NO2 167"l 244(23 400)l 352(23 000)l

4'-Me 133" 255(6 750) 318(31 700)g $42

10 4-No? 4'-Me 101" 266(16 100) 418(27 9OO)h

$8 11 4-NMe? $108 12 4-NMez 4'-Br 153" 275(20 100) 326(3 100) $110

420(36 300) 13 4-XMez 3'-OH 164" 268(26 900) 337(4 600) $110

420(31400) 14 3-OH - 138" 246(11300) 312(15 900)' $ 2 15 3-OH 4'-Me 133" 260(8 000) 308(25 100) -2 16 3-OH 4'-OMe 135" 282(11800) 327(27 00O)i

3-OH 3'-OH 185" 252(11700) 308(20 200) $87

17 - 18 2-CI 4'-OMe 72" 227(13 900) 299(15 300) -11 19 2-OMe 4'-Br 62" 280(12 600) 293 (13 950)

350(12 050) $40

20 3,4-CH?Oz 3'-OH 181' 258(13 800) 310(9 800) 357 (23 900)

$47

2 1 3,4-CHz02 2'-OH 134" 248(10 000) 322(4 750)k $63 284(5 700) 373(9 100)

22 - 3'-OH 127" 227(10 950) 308(25 500)' -2 -

"All data from the present work were obtained using 95% ethanol. unless otherwise stated. Amax in mp. asterisk denotes inflection; m lar extinctions in parenthesis.

%Ax refers to K-band (band I). CReference 2 quotes 230(8 900). 312(26 700); ref. 6 quotes 205(15 800). 226(10 400), 308(25 400); ref. 9 quotes 228(9 800).

W I Q I ~ / L snn) .,- *-.,--,. %eference 8 quotes m.p. 142.5"' and 265(16 500). 348.5(19 500). eReference 8 gives 231(14 400). 343(23 500). l ~ e f e r e n c e 8 quotes m.p. 166-168". and 242(2(3 600). 353(14 500). 04-Nitrochalcone: 315.5(18 000) (ref. 8) or 316(28 000) (ref. 11). h - ..;.- - - - - ~ -~

~ . ~ ~ m ~ t h v l a m i n o c l ~ a l c b n e : -428(39 800) (ref. 12). {ief&ie 2 rep&tsm~xima a t 2i6 ;6;d 307 mp. '

j3 4'-Dihydroxychalcone: 242 and 321 mp (ref. 8). li2~-~ydroxy-3.4-dimethoxycl1alcone (ref. 9n) Amax at 247, 264, and 371 mp and inflection a t 316 mr. lReported Amnx a t 315 mp (ref. 96).

TABLE I1

Effects of para-substituents on the I<-band of chalcone

Substituent A A Mean

Ring A -NMeZ 116 (la), 112,* 105 (1) $111 -0Me 36 (7), 35,* 33 (8, 12) $34 -NO? 8 (81, 6 (12), 5.5 (7), 8* $6.5 -Me 4,* 8* $ 6

Ring B -NMe2 84 (2), 74 (1) $79 -0Me 17 (9a), 15,* 14 (9a)

11 (9b, 12) $ 14 -NO? 7 (7), 5 (2) + 6 -Me -4,* 9,* 14 (9a) $6

*Data from Table I.

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2582 CANADIAN JOURNAL OF CHEMISTRY. VOL. 42, 1964

respectively (8). A similar effect is noticed in 2-methoxy-4'-bromochalcone (compound 19) where three long wavelength bands are observed. Data recently obtained by Noyce and Jorgenson (8) on corresponding 2- and 4-substituted chalcones allow the calculation of the hypothetical angle of deformation, which may be responsible for the lowered in- tensity of the I<-band of the 2-isomer. Assuming that the inolar extinction of the I<-band for the 2-isomer would be that of the 4-isomer (€0) in the absence of an angular deforination of one phenyl chromophore relative to the other, then the deformation angle (8) in the 2-isomer may be calculated froin the equation (13) cos" = cEa/cO, where c, is the value for the 2-isomer. The values so obtained are nearly equal for methyl-(3j0), chloro-(3G0), or inethoxy-(39") substituents, and are similar to those observed for 2;2'-disubstituted azoxybenzenes (14).

Polycyclic Chalcones The K-bands of a series of chalcone derivatives with one of its aryl groups a naphthyl

or phenanthryl residue are given in Table 111. Substitution for phenyl in ring A results

Absorption of polycyclic aryl chalcones

Arl Arz Solvent K-band Ax

Phenyl Phenyl EtOH 310(27 000) - 1-Naphthyl Phenyl EtOH 352(14 700) +42 2-Naphthyl Phenyl EtOH 328(24 300) 1-Phenanthryl Phenyl EtOH 3178(7 100) +7

+IS

9-Phenanthryl Phenyl EtOH 352(13 200) + 42 Phenyl 2.Naphthyl Dioxan 313(19 000) +3 Phenyl 2-Phenanthryl CHClP 316(20 600) +6 Phenvl 3-Phenanthrvl CHC13 328(20 000) +18

"Inflection.

in bathochro~nic shifts of the K-band inaxima in increasing order of conjugating powers, phenyl < 1-phenanthryl < 2-naphthyl < 1-naphthyl < 9-phenanthryl, and in ring B, phenyl < 2-naphthyl < 2-phenanthryl < 3-phenanthryl. Significantly the effect is more pronounced in ring A than in ring B. Certain anthryl chalcones were also examined, but their K-bands correspond in wavelength with, and are submerged in, the absorption of the anthracene moiety.

EXPERIbIENTAL Cl~alcones

These were prepared by condensation of the appropriate acetophenones and benzaldehydes (15) and were recrystallized before use. Compounds 3, 13, 17, and 21 (Table I ) were also purified by chromatography on alumina from benzene and hexane. Their melting points are recorded in Table I. The preparations of the polycyclic chalcones (Table 111) will be published in connection with other work.

Speclra The ultraviolet and near visible spectra were determined, unless otherwise stated, using solutions in

95% ethanol, which had been purified by refluxing with zinc dust and sodium hydroxide. The dioxan was purified by refluxing with, and distilling from, sodium, using only the center fraction. Chloroform was analytical grade. Measurements were made with a Bausch and Lomb spectronic 505 spectrophotometer with 1.0 cm cells. Concentratioils were such as to give optical densities of 0.5-0.8. Several spectra were rechecked on a Beckman DU spectrophotometer.

ACKNOWLEDGMENTS

The authors are grateful t o Dr. J . R. Gwilt for a gift of chalcones and the National Science Foundation for an Undergraduate Research Participant grant. The Puerto Rico

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WHEELER ET AL.: CHALCONES 2583

Nuclear Center is operated by the University of Puerto Rico for the U.S. Atomic Energy Commission.

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flavanoid compounds. Edited by T. A. Geissman. The Pergamon Press, Ltd., London. 1962. p. 141. 10. 0. H. WHEELER and C. B. COVARRUBIAS. Can. J. Chem. 40, 1224 (1962). 11. L. DOUB and J. M. VANDENBELT. J. Am. Chenl. Soc. 69, 2714 (1947); 71, 2414 (1949). 12. V. ALEXA. Buletinul Societatea de Chimie din RoinBnia, A, 18, 93 (1936). 13. H. B. KLEVENS and J . R. PLATT. J. Am. Chem. Soc. 71, 1714 (1949). 14. P. H. GORE and 0. H. WHEELER. J . Am. Chem. Soc. 78, 2160 (1956). 15. W. DAVEY and J. R. GWILT. J. Chern. Soc. 1008 (1957).

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