MAGNETIC RESONANCE IN CHEMISTRYMagn. Reson. Chem. 2001; 39: 463–465

17O NMR studies on (E)-3-arylidenechromanone and-flavanones derivatives†

Gabor Toth,1∗ Andras Simon,1 Albert Levai,2 Hanspeter Kahlig3 and Hermann Kalchhauser3

1 Technical Analytical Research Group of the Hungarian Academy of Sciences, Institute for General and Analytical Chemistry of the Budapest Universityof Technology and Economics, Szent Gellert ter 4, H-1111 Budapest, Hungary2 Department of Organic Chemistry, Debrecen University, P.O.Box 20, H-4010 Debrecen, Hungary3 Universitat Wien, Institut fur Organische Chemie, Wahringer Str. 38, A-1090 Vienna, Austria

Received 11 January 2001; Revised 10 April 2001; Accepted 17 April 2001

17O NMR data for some 3-arylidenechromanones and -flavanones are discussed in terms of mesomeric andsteric substituent interactions. The transmission of long-range substituent effects was studied. 17O NMRchemical shifts were correlated with Hammett sp

+ values for 4′-substituted derivatives. Copyright 2001John Wiley & Sons, Ltd.

KEYWORDS: NMR; 13C NMR; 17O NMR; (E)-3-arylidenechromanones; (E)-3-arylideneflavanones; substituent effects;Hammett �p

C

INTRODUCTION

17O NMR spectroscopy continues to develop as an extremelyuseful technique for studying structural and electronic effectsin organic and inorganic compounds. During the last decadea huge set of 17O NMR data have been measured andreported. Collections of these data provide the chemist withvaluable databases.1 – 3

There are different ways to overcome the baselineproblems caused by acoustic ringing, e.g. zeroing thefirst points of the FID and reconstructing them usinga backward linear prediction algorithm,4,5 application ofmultipulse methods by appropriate phase cycling (RIDEversions)6 – 8 and extended spin–echo experiments.6 Mostrecently, a further improved RIDE sequence has beenpresented utilizing adiabatic inversion pulses, providingeffective excitation in a broad frequency domain.9

The (E)-3-arylidenechromanones (1) and -flavanones (2)are useful intermediates for, e.g., spiroepoxides, spiropyra-zolines and different condensed nitrogen heterocycles.10,11

For 3-arylideneflavanones we have found that the six-membered heterocycle can adopt two energetically differentand rapidly interconverting envelope conformers where thephenyl group in position 2 is in either axial or equatorial

ŁCorrespondence to: G. Toth, Technical Analytical Research Groupof the Hungarian Academy of Sciences, Institute for General andAnalytical Chemistry of the Budapest University of Technologyand Economics, Szent Gellert ter 4, H-1111, Budapest, Hungary.E-mail: g-toth@tki.aak.bme.hu†This paper is dedicated to Professor Dr Istvan Bitter on theoccasion of his 60th birthday.Contract/grant sponsor: Hungarian Scientific Research Fund;Contract/grant number: OTKA T 026264; OTKA T 029171; OTKA D032830.Contract/grant sponsor: Austrian–Hungarian ResearchAgreement; Contract/grant number: A-4/98.

orientation (Scheme 1, A and B, respectively), and C-2 islocated out of the plane formed by the other atoms of thering.12 Owing to the C-3 exo-double bond, the axial positionof the phenyl ring is more favourable owing to 1,3-allylicstrain13 and steric interactions with the peri-positioned ˇ-aryl substituent of the exo-double bond. The vicinal couplingbetween H-2 and C-8a [3J(H-2,C-8a)] is diagnostic for thesteric position of the 2-phenyl group and allows a nearlyquantitative description of the conformational equilibrium(A : B D 9 : 1).12

O

O

Ph

Ar

O

OPhAr

2

4

8a

A B

Scheme 1. Conformational equilibrium.

RESULTS AND DISCUSSION

Investigating the transmission of the substituent effects on13C chemical shifts of 3-arylidenechromanones (1) and -flavanones (2), we found that υ(C-4) is virtually unaffectedby the electronic character of R2.11 As it is known that the 17ONMR chemical shifts are sensitive to long-range substituenteffects, we set out to investigate the 17O NMR spectra of 1 and2 (Scheme 2). The 17O and some characteristic 13C chemicalshifts are summarized in Table 1.

In the parent chromanone, the chemical shifts of O-1 andO-4 were found in CDCl3 at 70 and 515 ppm, respectively.14

The presence of a phenyl group in position 2 in flavanoneleads to a deshielding of the endocyclic O-1 by about 23 ppm

DOI: 10.1002/mrc.877 Copyright 2001 John Wiley & Sons, Ltd.

464 G. Toth et al.

9

4'3'2'

1'

8a8

76

54a

43

2

O

O

R1

R2

1: R1 = H; 2: R1 = Ph

a b c d e f g h

R2 p-NMe2 p-MeO m-MeO o-MeO p-EtO p-Me o-Me p

i j k l m n o

R2 H p-F p-Cl m-Cl o-Cl p-Br p-NO2

-iPr

Scheme 2. Structure of compounds 1 and 2.

(ˇ-effect), whereas the carbonyl oxygen is hardly affected(C3 ppm) by the equatorial phenyl group in the υ-position.14

Owing to the conjugation with the endocyclic double bondin chromone and flavone, O-4 experienced low-frequencyshifts of 69 and 92 ppm, respectively. In contrast, the signalsof O-1 were shifted to higher frequency by 92 and 67 ppm.14

p-Methoxy substitution of the 2-phenyl group of flavoneresulted in a shielding of 10 ppm at the remote O-4 nucleieight bonds away.15

Introduction of a benzylidene substituent in position 3of a chromanone system affects O-4 strongly by shifting itssignal by 45 ppm to lower frequency.14 Here, both mesomericand steric effects have to be considered. No 17O data onsubstituted 3-arylidene chromanones and the analogousflavanones are available in the literature.

Interesting remote substituents effects were observedwhen the 17O chemical shifts of the parent compounds (1i and2i) were compared with those of their 40-substituted deriva-tives. Electron-donating groups resulted in shielding ofthe nucleus of O-4, whereas electron-attracting substituentscaused deshielding. The υ(O-4) data for both series gave afair correlation16 with Hammett �p

C values for 40-substitutedderivatives (Table 1, Fig. 1).

Analogous substituents effects and correlations withHammett �p

C constants were observed for (E)-benzal-indanones (� D 9.2)17 and chalcones (� D 10.0).18 Themeasured � values for 1 and 2 (5.2 and 7.4, respectively)are lower in the analogues mentioned above, indicating

Table 1. 17O and (selected) 13C chemical shifts (ppm) and Hammett correlations with �pC and �p values for 1 and 2

[toluene-d8 –toluene mixture (2 : 3) at 343 K]

Compound υ(O-4)a,b υ(O-1) υ(C-4) υ(C-9)c,d υ(C-3)a,b υ(C-2)a,b υ(C-7)a,d υ(C-100)d υ(C-200)d

1a 476 67 180.9 137.6 127.3 68.6 134.8 — —1b 480 67 181.0 136.6 129.7 68.1 135.2 — —1f 484 67 181.1 136.8 130.9 68.0 135.3 — —1h 484 68 181.4 137.1 130.9 68.0 135.5 — —1i 486 68 181.3 136.9 131.6 67.8 135.5 — —1k 486 68 180.8 135.2 132.1 67.6 135.6 — —1o 488 — 180.6 134.3 133.8 67.4 135.9 — —� 5.186 — — �2.175 2.607 �0.487 0.443 — —r2e 0.962 — — 0.835 0.999 0.965 0.975 — —2a 475 91 181.5 140.0 128.9 79.0 135.1 139.6 128.32b 482 101 181.7 138.9 131.3 78.6 135.5 139.2 128.12e 484 88 181.7 139.0 131.2 78.6 135.5 139.2 128.12f 484 93 181.7 139.1 132.6 78.5 135.6 139.1 128.02h 489 92 181.7 139.1 132.6 78.5 135.6 139.1 128.02i 488 95 181.7 138.9 133.4 78.3 135.7 139.0 128.02j 489.5 91 181.6 137.8 133.1 78.2 135.8 138.8 128.02k 490 97 181.5 137.5 133.7 78.2 135.9 138.8 127.92n 491 94 181.5 137.5 133.8 78.2 135.9 138.7 127.92o 92 89 181.2 136.1 135.9 78.0 136.2 138.4 127.8� 7.377 — — �2.588 2.788 �0.419 0.706 �0.781 —r2e 0.901 — — 0.905 0.994 0.953 0.987 0.972 —2c 489 942d 490 962g 491 952l 490.5 942m 494 93

a Correlation for 1 with �pC.

b Correlation for 2 with �pC.

c Correlation for 1 with �p.d Correlation for 2 with �p.e r2, Coefficient of determination (square of product–moment correlation coefficient).

Copyright 2001 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2001; 39: 463–465

17O NMR studies on (E)-3-arylidenechromanone and -flavanones 465

y = 7,377x + 488,53 r2 = 0,9012

y = 5,1864x + 485,04 r2 = 0,9617

470

475

480

485

490

495

500

−2,00 −1,50 −1,00 −0,50 0,00 0,50 1,00σp+

17O

ch

emic

al s

hif

ts

Figure 1. Correlation of the 17O chemical shifts of O-4 and theHammett parameter �p

C of 1 (diamonds) and 2 (squares).

that these derivatives are less sensitive to substituenteffects and, therefore, pointing to a lower degree ofconjugation. Coplanarity of the 9-aryl, C-3 C-9, and C-4 O groups exhibits the highest conjugation, whereastwisting of these moieties in 1 and 2 due to the stericrepulsion between 9-aryl and H2C-2 or 9-aryl and PhCH-2, respectively, diminishes conjugation. Comparing the O-4chemical shifts of the 3-arylidene chromanones with thoseof the corresponding flavanones, only small differenceswere observed. A deshielding of up to C5 ppm was foundupon introduction of an aryl moiety on C-2 (υ-position),thus increasing the electron-withdrawing effect of R2. It isinteresting to observe that the dependence of υ(O-4) valueson the electronic effect of the substituent in position 40 issignificantly stronger in the case of 3-arylideneflavanonesthan in the corresponding chromanones. As a result, thedeshielding of C2 ppm in the parent flavanone (2i) turnsinto a shielding of �1 ppm if the proton at position 40 isreplaced by the strongly electron-donating NMe2 group (2a).

In the preferred conformation of 3-arylideneflavanones,the 2-phenyl group occupies the axial position, but alsoin this arrangement it shows no υ-steric interaction withO-4 which should result in a strong (ca C25 ppm)deshielding, as has been observed for peri-substituted 2,2-dimethylchromanone.14

The introduction of ortho-substituents in 2d, 2g and 2mresulted in a deshielding of O-4 of the order of 5–8 ppm thatcan be explained by the twisting of the 9-aryl group due tothe steric requirements of the ortho-substituents. In the case ofthe meta-substituted compounds 2c and 2l there are neithersteric nor mesomeric effects. Only small inductive effectsshould be considered, and the υ(O-4) values are, therefore,similar to the value observed for the parent compound 2i.

EXPERIMENTAL

The syntheses of the compounds investigated have been pub-lished earlier.19 – 21 13C and 17O NMR spectra were recordedat concentrations above 0.5 M in a toluene-d8–toluene mix-ture (2 : 3) at 343 K in 10 mm sample tubes using Bruker

Avance DRX-500 and DRX-400 WB spectrometers. The chem-ical shifts are given on the υ-scale and are referenced toexternal water (17O; υ D 0 ppm) and to the methyl signalof the solvent (13C; υCD3 D 20.4 ppm). Data for 17O mea-surements: spectral width, 600 ppm; 90° pulse length, 12 µs;number of scans, 6 ð 105 –24 ð 105 depending on concentra-tion and line width (ca 600–1200 Hz); 2 K data points wereacquired and zero-filled to 8K, corresponding to 0.07 ppm perpoint digital resolution. The pulse programs required weretaken from the Bruker software library (zg, aring2). Beforedata acquisition, pre-scan delays of 100 µs (zg, all compoundswith exception of 1f and 2j, processed with backward linearprediction) or 20 µs (aring2, 1f, 1h, 2f, 2j; aring2 with adiabaticpulse 2f) were applied. The different methods resulted in thesame 17O chemical shifts and linewidths. The reproducibilityof 17O chemical shifts on the same sample with differentspectrometers and with different pulse programs proved tobe within š0.5 ppm.

AcknowledgementsThis project was supported by the Hungarian Scientific ResearchFund (OTKA T 026264, T 029171 and D 032830) and Aus-trian–Hungarian Research Agreement: A-4/98. A.S. thanks theJ. Bolyai foundation for a grant.

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Copyright 2001 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2001; 39: 463–465