Natural abundance 17O NMR study of 2- and 4-substituted benzoyl chlorides

Spectrochimica Acta, Vol. 47A. No. 314. PP. 323-328, Printed in Great Britain

Natural abundance

1991 0

“0 NMR study of 2- and 4-substituted chlorides

05~8.539191 $3.00+0.00 1991 Pergamon Press plc

benzoyl

DAVID W. BOYKIN Department of Chemistry, Georgia State University, Atlanta, GA 30303-3083,U.S.A. (Received 25 July 1990; in final form and accepted 19 September

1590)

Abstract-Natural abundance “0 NMR chemical shift data for 17 ortho and para benzoyl chlorides recorded in acetonitrile at 75°C are reported. “0 NMR data for the para substituted benzoyl chlorides are correlated with “0 NMR data for similarly substituted acetophenones and methyl benzoates. The “0 NMR signals for ortho isomers are downfield (ca 30 ppm) from their para isomers; the downfield shifts are consistent with torsion angle change. The “0 NMR data for the para isomers gave good correlations with u+ constants and with dual substituent parameters (DSP).

I70 NMR spectroscopy is a sensitive method for assessment of the influence of electronic and geometric factors on the properties of carbonyl functional groups [ 11. The “0 NMR characteristics of carboxylic acids and a number of functional derivatives of carboxylic acids have been extensively studied [l, 21. The carbonyl 170 NMR signals for aromatic

carboxylic acids and esters have been shown to be sensitive to electronic effects [3,4] and quantitative relationships have been found between their 170 NMR chemical shift and torsion angle value [5]. Anhydrides [6,7] and amides [5,8,9] have been less intensively studied but reports have appeared which show their I70 NMR data to be sensitive to electronic and steric effects. In contrast, the application of 170 NMR methodology to acid chlorides, the most reactive of the carboxylic acid derivatives, has been limited. Representative I70 NMR data for several aliphatic acid chlorides [lo] and for benzoyl chloride [ll] have appeared, but no systematic investigation of aromatic acid chlorides has been reported. This study describes the effect of substituents of the carbonyl I70 NMR chemical shift for a series of orfho- and para-substituted benzoyl chlorides.

EXPERIMENTAL The acid chlorides (l-17) used in this study were commercially available from Aldrich. The “0 NMR spectra were recorded on a Varian VXR-400 spectrometer equipped with a 10 mm broadband probe. All spectra were acquired at natural abundance, at 75°C in acetonitrile (Aldrich, anhydrous gold label under nitrogen) containing 1% 2-butanone as an internal standard. The concentration of the compounds employed in these experiments was 0.5 M. The signals were referenced to external deionized water at 75°C. The 2-butanone resonance (558 f 1 ppm) was used as an internal check on the chemical shift measurements for these compounds. The instrumental settings were: spectral width 35 kHz, 2 K data points, 90” pulse angle (40~s pulse width), 200~s acquisition delay, 29 ms acquisition time. Typically 20 000 scans were required. The spectra were recorded with sample spinning and without lock. The signal-to-noise ratio was improved by applying a 25 Hz exponential broadening factor to the FID prior to Fourier transformation. The data point resolution was improved to rfrO.1ppm by zero filling to 8 K data points. The reproducibility of the chemical shift data is estimated to be better than f 1.0 ppm.

RESULTS

The “0 NMR chemical shift data for 11-substituted benzoyl chlorides and 6 o-substituted benzoyl chlorides recorded at natural abundance in acetonitrile at 75°C are listed in Table 1. The “0 NMR signal for benzoyl chloride (6) appears at 485.5 ppm, 323

DAVID W. BOYKIN

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Table 1. “0 NMR data for substituted benzoyl chlorides in acetonitrile* No.

Name

1

4-Nitrobenzoyl chloride Terphthaloyl, chloride 4-Cyanobenzoyl chloride 4-Trifluoromethylbenzoyl chloride 4-Chlorobenzoyl chloride Benzoyl chloride 4Fhrorobenzoyl chloride 4-Methylbenzoyl chloride 4-r-Butylbenzoyl chloride 4-Methoxybenzoyl chloride 4-Ethoxybenzoyl chloride 2-Trifluoromethylbenzoyl chloride 2Chlorobenzoyl chloride 2-Fluorobenzoyl chloride 2-Methylbenzoyl chloride 2-Methoxybenzoyl chloride Phthaloyl chloride

2 3 4 S 6 I 8 9 10 11 12 13 14 15 16 17

s(c=o)t 502.9 502.0 499.9 497.5 487.7 485.5 483.6 479.3 480.4 468.9 468.0 528.5 518.3 510.3 505.5 506.1 515.7

v1/2(C=O)$ 234 267 263 200 254 1.51 155 175 229 304 256 128 168 153 116 177 197

WP 583.6

68.6 98.0

59.5

v1/2(x)$ 442

340 213

192

* Data obtained at natural abundance from 0.5 M solutions at 75°C. t Chemical shift in ppm referenced to external water; 1% 2-butanone (558 + 1 ppm) internal reference. $ Linewidth (Hz) at half peak height; estimated error 10%) QChemical shift (ppm) of oxygen of substituent.

downfield of its reported value (452 ppm) for the neat liquid [ll]. The chemical shift value for 6 is upfield by lo-15 ppm of the values for aliphatic acid chlorides (also reported for neat liquids) [lOa]. The “0 NMR chemical shift values of p-substituted benzoyl chlorides range from 469 to 502 ppm for the p-methoxy to the p-nitro compound. The data shown in Table 1 for l-11 illustrate the effect of substituents on the carbonyl group’s “0 NMR chemical shift. The “0 NMR data for the o-substituted benzoyl chlorides are generally ca 30ppm downfield of the value for the corresponding p-substituted isomer; 17 is a notable exception. In Fig. 1, the data for o- and p-methylbenzoyl chloride illustrate the deshielding influence of or&o substitution. The range of “0 NMR chemical shift values for the ortho-substituted benzoyl chlorides is from 505.5 to 528.5 ppm. The band width for the “0 NMR signal of the ortho isomers is generally about 30% smaller than that of the paru isomers (cf. Fig. 1).

DISCUSSION

The high reactivity of acid chlorides with nucleophiles relative to amides and esters is often attributed to the fact that the chloride ion is a good leaving group [12]. The fact that the carbonyl”0 NMR signals for acid chlorides (485 ppm) are considerably downfield of their carboxylic analog esters [5] (345 ppm) and amides [5] (325 ppm) indicates greater double bond character than the mentioned analogs and suggests that factors other than leaving group capacity may play a role in the reactivity of this functional group. Electron attracting substituents cause downfield shifts in the “0 NMR signals of the para substituted benzoyl chlorides and electron donating substituents produce shielding shifts. Thus the effect of substituents on the “0 NMR signal of benzoyl chlorides is similar to that of substituents on various other aryl carbonyl systems [3,13-161. An important factor which influences the “0 NMR chemical shift of benzoyl chlorides appears to be the carbonyl n-electron density since an excellent correlation between the “0 NMR chemical shifts of benzoyl chlorides and acetophenones [13] is obtained (Fig. 2). Previous analysis of “0 NMR chemical shift data for acetophenones has shown a dependency on n-electron density [ 131. The acetophenone-benzoyl chloride correlation also suggests that the bond order electron density matrix of the Karplus-Pople

I’0 NMR study of 2- and 4-substituted benzoyl chlorides

325

expression [13, 171 is an important factor in determining the 170 NMR chemical shift of benzoyl chlorides. The slope of the line for the acetophenone-benzoyl chloride correlation is 0.9 which suggests that the benzoyl chlorides are about 10% more sensitive to substituent effects than the acetophenones. A good correlation between the carbonyl group I70 NMR chemical shifts of methyl benzoates [3] and benzoyl chlorides is expected and found (Fig. 2). For the latter case the slope of the line is cu 0.44 which indicates that the benzoyl chlorides are roughly twice as sensitive to substituent effects as methyl benzoates. The order of sensitivity to substituent effects, COCl - COCH3 > CO&H,, is consistent with the competing resonance interactions of functional group and aryl ring.

’ I

5&o

’ ‘17 ---ITT

540

c 5-o

-n

Fig. 1. Natural abundance “0 NMR spectrum of 4-methylbenzoyl chloride (8) (bottom) and 2-methyl benzoyl chloride (15) (top); signal at 558 ppm marked by asterisk denotes the internal standard 2-butanone.

DAVID W. BOYKIN

326

550.

525. 500. z = 475. I? z 0 450.

f z : ;

425. 400.

(b) methyl brnzoate9

350.

8;

c-. 3251 465

470

475

420

I70 NMR DRTA(PPM)

485

490

FOR BENZOYL

495

500

5

CHLORIDES

Fig. 2. Plot of “0 NMR chemical shifts of benzoyl chlorides vs (a) acetophenones “0 NMR chemical shifts and (b) methyl benzoates carbonyl group “0 NMR chemical shifts.

The benzoyl chloride “0 NMR chemical shift data gives an excellent correlation with (T+ constants [18] (Fig. 3). The expression for the line [6 = 21.7a+ + 485.7 (r = 0.99)] in Fig. 3 and the chemical shift data for 2 allows the estimation of a u+ value (0.75) for the COCl group. This result is in reasonable accord with the previously reported [19] value (0.79). Also, an excellent correlation is noted for the benzoyl chlorides’ “0 NMR chemical shift data using the dual substitutent parameter (DSP) [20] approach [6 = 21.30, + 22.1aR+, r =0.999]. The excellent correlation with o+ suggests that resonance interactions between substituents and the carbonyl group are important. A similar conclusion can be drawn based upon the DSP analysis since comparison of the coefficients for a, and oR+ contribution for the benzoyl chloride correlation is similar to that of the acetophenones [13] but is in contrast to correlations for 2-substituted thioxanthen-9-ones [21] and 3-substituted-9-fluorenones [16] which show a strong u, dependency. BROWN’S

b

0

-1

SIGMA

PLUS

1

CONSTANTS

Fig. 3. Plot of “0 NMR chemical shifts of benzoyl chlorides vs sigma plus constants.

“0 NMR study of 2- and 4-substituted benzoyl chlorides

327

The large downfield chemical shift for the o&o-isomers (12-17) is in the direction expected [2] for torsion angle rotation of the acid chloride functional group from the plane of the aryl ring. Quantitative relationships have been previously developed between “0 NMR chemical shifts and torsion angles for a number of functional groups [5]. However, no such relationship has been reported for aryl acid chlorides. Interpretation of MM2 results for the or&o-substituted benzoyl chlorides is problematic since it appears that repulsive van der Waals’ energies are not reduced to zero on torsion angle rotation for benzoyl chlorides with o&o substituents which contain highly electronegative atoms [2]. Consequently, no attempt to develop a quantitative relationship between the torsion angle values and “0 NMR data of the benzoyl chlorides for these substituents will be made. Nevertheless, it seems likely that the downfield shifts of the o&o-substituted benzoyl chlorides are largely a consequence of torsion angle rotation. MM2 calculations [22] estimate the torsion angle for 2-methylbenzoyl chloride (15) to be 29”. Thus, based upon one data point, it can be estimated that the change in “0 NMR chemical shift with torsion angle for benzoyl chlorides is 0.9ppm/deg. This value is larger than the ones noted for other related functional groups [5] and awaits confirmation using data from several other hindered aryl acid chlorides which do not include or&o substituents which contain electronegative atoms. The “0 chemical shift value for benzoyl chlorides (cu 485 ppm) is significantly downfield of that of related carboxylic acids (cu 250ppm), esters (cu 345 ppm) and amides (ca 325 ppm). Interestingly, the “0 NMR signal for acid chlorides appears in a region relatively close to that of ketones (ca 550ppm) and quite close to that of thiol esters [23] (cu 500 ppm). The “0 chemical shift for substituted benzoyl chlorides is sensitive to electronic [24] and geometric factors similar to substituent effects observed for other aryl carbonyl systems [2,13]. Acknowledgemenrs-Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research and to the NSF Instrumentation Program (CHEM-8409599).

REFERENCES [l] A. L. Baumstark and D. W. Boykin, in I70 NMR Spectroscopy in Organic Chemistry (edited D. W. Boykin), chaps 3, 4 and 8. CRC Press, Boca Raton, Florida (1990). [Z] D. W. Boykin and A. L. Baumstark, Tetrahedron 45,3613 (1989). [3] P. Balakrishnan, A. L. Baumstark and D. W. Boykin, Org. Magn. Reson. 22, 753 (1984). [4] D. Monti, F. Orsini and G. S. Ricca, Spectrosc. Len. 19, 91 (1986). [5] A. L. Baumstark, P. Balakrishnan, M. Dotrong, C. J. McCloskey, M. G. Oakley and D. W. Boykin, J. Am. Chem. Sot. 109, 1059 (1987). (61 P. Vasquez, D. W. Boykin and A. L. Baumstark, Magn. Reson. Chem. 24, 409 (1986). [7] D. W. Boykin, A. L. Baumstark, M. M. Kayser and C. M. Soucy, Can. /. Chem. 65, 1214 (1987). [8] D. W. Boykin, G. H. Deadwyler and A. L. Baumstark, Magn. Resort. Chem. 26, 19 (1988). [9] A. L. Baumstark, M. Dotrong, M. G. Oakley, R. R. Stark and D. W. Boykin, /. Org. Chem. 52, 3640 (1987). [lo] (a) C. Delseth, T. T.-T. Nguyen and J.-P. Kintzinger, Helv. Chim. Acta 63,498 (1980); (b) J. Reuben and S. Brownstein, J. Molec. Spectrosc. 23, 96 (1967); (c) H. A. Christ, P. Diehl, H. R. Schneider and H. Dahn, He/v. Chim. Acta 44, 865 (1961). [ll] C. P. Cheng, S. C. Lin and G.-S. Shaw, J. Magn. Resort. 69, 58 (1986). [12] A. Kivinen, Mechanisms of Substitution at the COX Group, in The Chemistry of Acyl Halides (edited by S. Patai). Wiley, New York (1972). [13] R. T. C. Brownlee, M. Sadek and D. J. Craik, Org. Magn. Reson. 21, 616 (1983). [14] T. E. St. Amour, M. I. Burgar, B. Valentine and D. Fiat, J. Am. Chem. Sot. 103, 1128 (1981). [I51 D. W. Boykin, A. L. Baumstark, P. Balakrishnan, A. Perjessy and P. Hrnciar, Spectrochim. Acta. 4OA, 887 (1984). [16] D. W. Boykin and B. Nowak-Wydra, Magn. Reson. Chem. in press. [17] M. Karplus and J. A. Pople, J. Chem. Phys. 38, 2803 (1963). [I81 H. C. Brown and Y. Ohamoto, J. Am. Chem. Sot. &IO,4978 (1958). [19] C. Hansch and A. Leo, Substituent Constants for Correlation Analysis in Chemistry and Biology,

p. 77.

Wiley, New York (1979). [20] S. Ehrenson, R. T. C. Brownlee and R. W. Taft, Prog. Phys. Org. Chem. (edited by R. W. Taft). Academic Press, New York, 10, 1 (1973).

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[21] J. S. Harwood and A. L. Ternay Jr, J. C/rem. Sot., Perkin Trans. II 1657 (1989). [22] U. Burket and N. L. Allinger, Molecular Mechanics. American Chemical Society, Washington, DC (1982); E. Esawa and H. Musso, Applications of Molecular Mechanics Calculations in Organic Chemistry, in Topics in Srereochemistry (edited by N. L. Allinger, E. L. Eliel and S. H. Wilen), p. 117. Wiley, New York (1982); MODEL Version KS2.94 available from Dr K. Steliou, University of Montreal. [23] D. W. Boykin, Specrrosc. Letr., in press. [24) Note added in proof. Related work on p- and m-substituted benzoyl chlorides appeared after this manuscript was submitted for publication: H. Dahn, P. PCchy and V. V. Toan, Angew. Chem. 102,681 (1990).