5134
I$'. BRUCEBLACK AND ROBERT E. LCTZ [CONTRIBUTIOS FROM THE COBB CHEMICAL
LABORATORY O F THE
Vol. 77
v S I V E R S I T Y OF VIRGIXIA]
Ultraviolet Absorption Spectra of Chalcones. Identification of Chromophores' BY iv, BRUCEBLACK'AND ROBERTE.LUTZ RECEIVED AUGTST 16, 1954 This paper deals with cis- and trans-chalcones, cis-trans photoequilibria, ultraviolet absorption spectra and effects of substituents on the ethylene linkage or in either or both phenyl nuclei, including the effects of o-methyls. The middle wave lcngth ultraviolet absorption bands a t 250-270 r n M of chalcones having the cis configuration, of chalcones with a- or 4substituents, of mesitalacetophenone, and of phenylbenzoylacety-lene, are unequivocally identified as benzoyl or acrylophenone bands by the disappearance of these bands when two o-methyls are substituted into the benzoyl group t o destroy its normal planarit>-. The cinnamoyl system appears primarily t o be responsible for the intensity of the long wave length maximum above 285 mM. Para substitution of nitro into the heuzoyl group of cis-chalcone and cis-or-bromochalcone almost doubles absorptivity a t the long wave length ultraviolet band; it is suggested that the styrene system in this and other related chalcones functions to some extent as a partially independent chromophore.
Of the two or three important regions of ultraviolet absorption which often appear simultaneously in the chalcone series (I and 11) i t has been sug-
cis isomerza whereas t h a t of the cu-bromochalcones lies almost entirely on the side of the cis (also in isooctane) . j b The photoequilibrium spectra of chalcone derivaAr CO;\r' Xr R I tives which have been isolated in only one form C-C Ar = ,ir>l C -C (trans) have proved to be of value in assigning configurations and may be useful to some extent in esK R timating roughly the cis-trans ratio involved (see 1 Li( Table I,!. gested t h a t the longest wave length band (usually The non-formation of other than the cis and trans above 2% mp) is due to the molecule as a whole3 compounds during irradiation was shown by the and not just to the cinnamoyl portion4and t h a t the constancy of the photoequilibrium maxima under shortest wave length bands (below ca. 240 mp) are prolonged irradiation and by inspection of the slope due to secondary absorption of the main chronia- and nature of the absorption curves. At 0.00005 -11 p h ~ r e . The ~ bands in the intermediate area (250- dilutions under exposure to sunlight or sunlamp E O mp) which often appear and which may at times the photoequilibria always were attained within a overlap the others, appear to be due to a partially few seconds or minutes, whereas dimerization which independent chromophore (the benzoy13s4a,6or is a likely competing reaction normally requires a acrylophenone group), and will be referred to as long exposure time' and would be detectable by a middle wave length bands whenever they clearly continued rise in benzoyl group absorptivity and appear. The present investigation which includes drop in chalcone absorptivity. further studies on cis-trans isomerism, photoequilibThe cis isomers of chalcones bearing neither ethylria and the effect of para substitution, has as its enic nor 0- or 0'-substituents, which are the "labile" main purpose the unequivocal identification of the isomers, show lower absorptivities a t the long wave middle wave length bands. length maxima than do the trans isomers; but only cis Isomers and cis-trans Photoequi1ibria.-Five in the cis isomers do the spectra regularly show a cis-chalcone derivatives (three new) have been pre- directly observable middle wave length band in the pared by sunlight inversions of the trans com- 260 m p region attributable to the benzoyl or acrylopounds. In 0.00005 JI ethanol solutions the photo- phenone group. Chalcones of this type when preequilibrium concentrations of cis isomer varied pared by acid- or base-catalyzed condensation of from 57-92yG and were determined by proportions aldehydes and acetophenones always have been from the absorptivities, in four cases from both the trans. The absorption phenomena have been cormiddle and long wave length maxima and in the related with the configurational differences and fifth from the long wave length maxima only (see with relative planarities of the cis and tmns sysTable 1 6 ) . The photoequilibrium of the parent tems as evidenced from examination of scale inodon the side of the els.' IVhere only the tvans forms have been isochalcones in isooctane lies lated the configurations are established unequivoi I ) This project was supported in part h y a contract with the Office of Ordnance Research. T h e work is described in a doctorate dissertacally as trans by comparison of their spectra with tion B E . ) , University of Virginia, X a y , 1954, a n d was reported those of photoequilibrium mixtures arrived a t by a t the Septemher Meeting of the American Chemical Society, S e w irradiation of dilute solutions of these compounds ; York, N. 17.,1954 (.Ibstracts, B, 0 0 - 0 ) . ( 2 ) Du Pont C u Postgraduate FeIlo\$-, 1933-1934. in going from the one chalcone (trans! to the photo(3) H. H. Szmant and A . J Basso. THISJOI:RSAL, 7 4 , 4397 (19.52~1. equilibrium mixture the long wave length band f\\'
(4) (a) E. K . Katzenelleniiugen and (>. R . I;. Branch, ibid., 69, 1615 ( 1 9 i i ) ; I b ) L . 1;. b'ergusun and K . P. Barnes, i b i d . , 70, 3907
(1948). ( 5 ) (a) I lr n 10.000 a t c i i . 300 m;i; for p-nitrostyrene. .k,,,:~\ 303 mu ( e 11.300) in methanol [hl. Pe?temer, T 1.anger and F. \Ian chen, Iloiznish., 68. 326 (1931iIl. i 2 3 ) Cf. l ; . i i ; i i . a - ~ ~ h p n y l - 4 - n i t r o c h 3 l c o nirei ~ ~ .iri mhere tiotll 1 1 1 ( ~ rir-p-nitrostilbene and the tiails-p-nitrochalcone systems seem to he functioning simultaneously as partially independent chromophore( c j . communication which appeared since this paper was submitted, Y Urushibara, F. Iimura and IC. Ikeda, J . C h e m . P h y s . , 22, 1913 (lQt5-!) 1. (24) Microanalyses by Miss Patricia L. Paynter; alkoxy1 determinations b y Clark Rficroanal~ticalLaboratoties. ( 2 5 ) E. P. Kohler and C . R . Addinall, THISJ O U R N A L , 52, 3728 (1930). i 2 6 ) H \Vieland, tis;. , 37, 1148 11904)
Oct. 3, 1933
IDENTIFICATION OF CHROMOPHORES OF CHALCONES
3139
and sunlamp for 5 days. Crystallization from ethanol and ethoxy compound remained unchanged on vacuum sublimafrom isooctane gave unchanged material. Concentration tion. Attempts t o dehydrobrominate mesitalacetophenone of the isooctane and cooling in the "deep freeze" gave lemon- dibromide by the action of ethanolic potassium acetate were unsuccessful; using a glacial acetic acid-potassium acetate yellow clusters; recrystallized, m.p. 33-33.5". mixture, only trans-mesitalacetophenone (the debrominated Annl. Calcd. for C16H1402: C , 80.65; H , 5.92. Found: product) could be isolated. Mesitalacetophenone was the C , 80.76; H , 6.00. Stobbe, et U Z . , ' ~were , ~ unsuccessful in obtaining the cis first product isolated by earlier authorslIa who were successful isomer of 4-methoxychalcone by the action of sunlight on in obtaining a small yield of a-bromomesitalacetophenone. In attempting dehydrobromination with an ethanolic soluiodine-containing solutions of the trans isomer, by heating tion of triethylamine only the debroniination product, the trans isomer above its melting point, or by irradiation trans-mesitalacetophenone, was isolated in 63 % yield. of its solution in acetic acid, ethanol or chloroform, both cis-2,4,6 -Trimethylchalcone (cis-mesitalacetophenone) in the presence of air or in vacuo. Illumination of the solid was obtained by exposure of a benzene solution of the trans trans isomer (1000 hr.) gave a dimer of m.p. 164'. compound of m.p. 96-101' in sunlight for one day; light t~an,s-~-Bromo-4-methoxychalcone~~ was prepared from yellow crystals from ethanol, m.p. 65-66' (mixture melting 25 g. of 4-methoxychalcone dibromide of m . p . 132-134' by treatment with 40 ml. of triethylamine in 200 ml. of acetone point with trans isomer, 52-70"j. Anal. Calcd. for CI8Hl80: C, 86.36; H , 7.25. Found: a t room temperature (reagent grade acetone is used because C, 86.24; H, 7.02. the 0-bromine is so reactive that it is replaced readily by an alkoxy group even during crystallization from alcohol2'). Mesitalacetomesitylene Dibromide .-Bromine was added After 4 hr. the amine hydrobromide (73%) was filtered off. dropwise t o a solution of trans-rne~italacetornesitylene~0 in Evaporation under reduced pressure left a residual oil which 100 ml. of hot glacial acetic acid; recrystallization from ethwas crystallized from ethanol; 7.5 g. (38%), light yellow, anol gave a yield of 70%, m.p. 93-97'; recrystallized, m.p. m.p. 80-90'; recrystallized, m.p. 94.5-96' (P. and S.27 99-101 O . 94.50). Anal. Calcd. for Cn,Hn4Br20: C, 55.77; H, 5 . 3 5 . cis-~-Bromo-4-methoxychalcone .-A solution of 8.0 g. of Found: C, 55.65; H, 5.14. the trans isomer of m.p. 94.5-96" in 2.5 1. of 95% ethanol An attempt to prepare a-bromomesitalacetomesityleneby was exposed for one week to sunlight and sunlamp. Evapdehydrobromination of mesitalacetomesitylene dibromide oration left starting material and oil. The oil was taken up (acetone-triethylamine) gave instead 36% of the debromin isooctane and on cooling a t -20" more trans isomer came inated product, truns-mesitalacetomesitylene (identified by down sloivly during one week; the solution was decanted mixture m.p.). and again placed in the "deep freeze." Bright yellow crystrans-4-Methoxy-2',4',6'-trimethylchalcone (trans-4tal clusters of cis isomer appeared and grew slowly, m.p. 52methoxybenzalacetomesitylene)31was obtained in two poly55'; recrystallized, m.p. 54-54.5'. morphic modifications. In the first condensation experiment -4nnl. Calcd. for C1&3Br0~: C, 60.58; H , 4.13. a 50% yield of yellow lower melting polymorph (new) was Found: C, 60.68; H, 4.13. obtained; recrystallized twice from ethanol and thrice from ~rans-4-Bromochalcone~8 was obtained in 97y0 yield by isooctane, m.p. 78-79'. condensation of 4-bromobenzaldehyde and acetophenone in Anal. Calcd. for C I ~ H ~ ~C,O81.39; ~: H , 7.19. Found: 370 potassium hydroxide-85yo ethanol (5 min. a t room temC, 81.52; H, 7.09. perature and 5 hr. a t 0 " ) ; m.p. 127-128' (W.z8 123.5'). After standing for one week the m.p. of this analytical Anal. Calcd. for C15HIIBrO: C, 62.74; H , 3.86. Found: sample had changed to 104-106", and no more of the lower C, 62.47; H, 3.75. melting polymorph could be produced. All subsequent trens-3-Methyl~halcone~~ was prepared by addition of a condensations of anisaldehyde and acetomesitylene yielded solution of 2
