Effect of ortho substituents on carbonyl carbon ¹³C NMR chemical shifts in substituted phenyl benzoates

Abstract

Effect of ortho substituents on carbonyl carbon 13 C NMR chemical shifts in substituted phenyl benzoates Vilve Nummert a , Mare Piirsalu a , Vahur Mä emets a , Signe Vahur a and Ilmar A. Koppel a * 13 C NMR spectra of 37 ortho-, meta-, and para-substituted phenyl benzoates, containing substituents in benzoyl and phenyl moiety, 4 ortho-substituted methyl and 5 ethyl benzoates as well as 9 R-substituted alkyl benzoates have been recorded. The influence of the ortho substituents on the carbonyl carbon 13 C NMR chemical shift, d CO , was found to be described by a linear multiple regression equation containing the inductive, s I , resonance, s-R , and steric, E B s , or y substituent constants. For all the ortho-substituted esters containing substituents in the acyl part as well as the phenyl part, the substituent-induced reverse inductive effect (r I < 0), the normal resonance effect (r R > 0), and the negative steric effect (d ortho < 0) with the E B s were observed. In the case of ortho substituents in the phenyl part, the resonance effect was negligible. Due to inductive effect, the ortho electron-withdrawing substituents showed an upfield shift or shielding of the carbonyl carbon, while the electron-donating substituents had an opposite effect. Because of the sterical consequences, ortho substituents revealed a deshielding effect on the 13 C NMR chemical shift of the carbonyl carbon. For all the meta-and para-substituted esters, the reverse substituent-induced inductive and resonance effects (r I < 0, r R < 0) were found to be significant. In alkyl benzoates, the alkyl substituents showed the reverse inductive and steric effects. The log k values for the alkaline hydrolysis in water, aqueous 0.5 M Bu 4 NBr and 2.25 M Bu 4 NBr, and the IR frequencies, n CO , for the ortho-, meta-, and para-substituted phenyl benzoates and alkyl benzoates were correlated nicely with the corresponding 13 C NMR substituent chemical shifts, Dd CO .ORTHO EFFECT ON CARBONYL CARBON CHEMICAL SHIFTS 1.5) þ (58.4 AE 3.4)s I þ (32.6 AE 4.1)E B s , R ¼ 0.991, s 0 ¼ 0.138, n ¼ 8.

Influence of ortho substituents on 17O NMR chemical shifts in phenyl esters of substituted benzoic acids

“…Recently we found [1,8] good correlations between the log k values of the alkaline hydrolysis of ortho-substituted phenyl benzoates and the infrared stretching frequencies of carbonyl group, ν CO , as well as the 13 C NMR chemical shifts, δ CO , in case the additional resonance and steric scales were included. The aim of the present paper was to check the applicability of the Charton equation to describe the influence of ortho substituents on the δ( 17 O) values in phenyl esters of orthosubstituted benzoic acids, (X-C 6 H 4 CO 2 C 6 H 5 ), and to compare the substituent effects in the δ( 17 O) values with those in the alkaline hydrolysis, [6] the infrared stretching frequencies, ν CO , [9] and the 13 C NMR chemical shifts, δ CO .…”

“…In the rigid systems the steric influence of ortho substituents was explained by the van der Waals interactions between the ester group and the adjacent bulky substituent. [41,42] In the previous papers we found that in substituted phenyl benzoates, (X-C 6 H 4 CO 2 C 6 H 5 , C 6 H 5 CO 2 C 6 H 4 -X) the influence of ortho substituents on the log k values in the alkaline hydrolysis, [1][2][3][4][5][6][7] the carbonyl carbon 13 C NMR chemical shifts, δ CO , [8] and the carbonyl carbon infrared stretching frequencies, ν CO , [9] was precisely described with the Charton equation [45] using the inductive, σ I , [8,46] resonance, σ°R, [8,47] and steric, E S B , [6,48] substituent constants. Nearly the same results were obtained when for ortho substituents the Charton steric scale, υ, [49] based on van der Waals radii, was used.…”

“…Nearly the same results were obtained when for ortho substituents the Charton steric scale, υ, [49] based on van der Waals radii, was used. In the phenyl esters of ortho-substituted benzoic acids (X-C 6 H 4 CO 2 C 6 H 5 ) the contribution of ortho inductive and ortho steric components were found to be the dominant factors in the alkaline hydrolysis in F o r P e e r R e v i e w 3 water, [6] the carbonyl carbon infrared stretching frequencies, ν CO , [1,9] for cis conformers as well as in the 13 C NMR chemical shifts, δ CO : [8] log k ortho (H 2 In reaction series considered (Eqns (1)- (2)) the resonance term was negligible. In the 13 C NMR chemical shifts, δ CO , [8] the value of (ρ R ) ortho has the opposite sign ((ρ R ) ortho = +1.6) as compared to that for para derivatives ((ρ R ) para = − 1.00).…”

“…The aim of the present paper was to check the applicability of the Charton equation to describe the influence of ortho substituents on the δ( 17 O) values in phenyl esters of orthosubstituted benzoic acids, (X-C 6 H 4 CO 2 C 6 H 5 ), and to compare the substituent effects in the δ( 17 O) values with those in the alkaline hydrolysis, [6] the infrared stretching frequencies, ν CO , [9] and the 13 C NMR chemical shifts, δ CO . [8] For the comparison, the δ( 17 O) values for substituted nitrobenzenes, [34,35] acetophenones, [36,38] and benzoyl chlorides, [39][40] published in literature were correlated with the Charton equation as well.…”

“…In our previous papers the influence of substituent effects, especially ortho effect, on the rates of the alkaline hydrolysis in various media (as shown in References [1][2][3][4][5][6][7] ), the carbonyl carbon 13 C NMR chemical shifts, δ CO , [8] and the carbonyl carbon infrared stretching frequencies, ν CO , [9] in substituted phenyl benzoates, (X-C 6 H 4 CO 2 C 6 H 5 , C 6 H 5 CO 2 C 6 H 4 -X) were studied. In the present paper we extend our study of the ortho substituent effect to the To the best of our knowledge, no study of the ortho, meta, and para substituent effects on the δ( 17 O) values for phenyl esters of substituted benzoic acids, X-C 6 H 4 CO 2 C 6 H 5 , could be found in the literature.…”