Lewis acid adducts of a,p-unsaturated carbonyl and nitrile componnds. A calorimetric study

RONALD F. CHILDS,' D. LINDSAY MULHOLLAND, AND ALAN NIXON Department of Chemistry, McMaster University, Hamilton, Ont., Canada L8S 4M1

Received July 20, 1981

RONALD F. CHILDS, D. LINDSAY MULHOLLAND, and ALAN NIXON. Can. J. Chem. 60,809 (1982). Enthalpies of complexation of the a,P-unsaturated Lewis bases crotonaldehyde, 1; methyl crotonate, 2; mesityl oxide, 3; and

crotononitrile, 4; with each of the Lewis acids SnCl,, TiCl,, SbCl,, and BC1, were measured in solution in CW,Cl,. The results indicate that while a qualitative agreement exists in the ranking of Lewis acids for each of the Lewis bases, there is only a modest quantitative correlation. Similarly, when compared with an nmr-derived Lewis acid scale, the enthalpies were found to agree with the qualitative ranking of Lewis acids, but exhibited only a poor correlation.

RONALD F. CHILDS, D. LINDSAY MULHOLLAND et ALAN NIXON. Can. J. Chem. 60,809 (1982). On a mesure, en solution dans le CH,Cl,, les enthalpies de complexation des bases de Lewis a,P-non saturees, crotonaldehyde

(I), crotonate de mtthyle (2), oxyde de mesityle (3) et crotononitrile (4) avec chacun des acides Lewis suivants: SnCl,, TiCl,, SbC1, et BC1,. Les resultats indiquent que des accords qualitatifs permettent d'ordonner chaque acide de Lewis pour chacune des bases de Lewis; toutefois la correlation quantitative n'est que modeste. De la mCme maniere, si on compare les enthalpies avec une tchelle d'acide de Lewis derivee de la rmn, on peut qualitativement ordonner les acides de Lewis mais la correlation est faible.

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Lewis acids are used to catalyse a variety of synthetically useful reactions of a,P-unsaturated carbonyl and nitrile compounds. In a previous paper we have reported a systematic examination of the nmr spectra of the complexes formed be- tween some of these unsaturated bases and Lewis acids and have proposed an acid scale which, it was suggested, reflected the amount of charge induced at C3 of the base on complexation (1).

There has been much discussion about and many attempts at correlating Lewis acid1Lewi.s base interactions (2). These include, among others, the hardlsoft concepts introduced by Pearson (3), the donorlacceptor numbers of Cutmann (4), Drago's E/C equation (3, and theoretical treatments of Mulliken and Pearson (6) and Klopman (7). The equilibrium constant or the heat of reaction is generally used to measure the strength of a Lewis acidlbase reaction.

In our attempts to quantify the catalytic effects of Lewis acids on the reactions of unsaturated car- bonyls and nitriles, it seemed of interest to compare the nmr-based scale mentioned above with the strength of the acidlbase interaction. As complex formation was essentially complete for nearly all the systems we had studied it seemed appropriate to base such a comparison on the heats of reaction. There have been some ,previous reports of the measurement of the BIi for reaction of unsaturated bases with Lewis acids (8) but insufficient for a meaningful comparison with our nmr-based scale.

'Author to whom correspondence may be addressed.

We report here a range of calorimetric measure- ments of these systems and examine the correla- tions with the nmr scale.

Results and discussion Calorimetric meas~rrements

Ideally the enthalpy measured should be that of the interaction of the acid and base in the gas phase. However, since solvation energies are likely to be small in inert solvents, the gas phase conditions can be approximated by taking the heat of molecular complexation to be the difference in molar heats of solution of the base in a solution of the Lewis acid and that in the same solvent with no acid present (4, 9-11).

The solution calorimeter used was a modified version of a design of Arnett and Petro (10). The precision of the calorimeter was checked by mea- suring the enthalpy of reaction of water with FS0,H (measured -12.23 T 0.08 kcallmol, literature - 12.2 T 0.1 (12) and - 11.95 T 0.04 kcal/mol(13)). Further checks were made by measuring the heat of solution of various bases in a SbC1,/C2H4C12 solu- tion and comparing with literature values.

Methylene chloride was used as the solvent with the unsaturated carbonyl and nitrile compounds of interest in this study. This solvent is inert to the Lewis acids used. The bases used were crotonal- dehyde, 1; methyl crotonate, 2; mesityl oxide, 3; and crotononitrile, 4. The heats of reaction of each of these bases with BCl,, SbCl,, TiCl,, and SnC1, were measured and the results shown in Table 1.

0008-4042/82/060809-04$01 .OO/O 01982 National Research Council of CanadaIConseil national de recherches du Canada

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CAN. J. CHEM. VOL. 60, 1982

TABLE 1. Enthalpies of complexation (AH) of a,P-unsaturated bases with various Lewis acidsu

Lewis base BCI, SbC1, TiCI, SnC1, -AH,'

Crotonaldehyde 24.4k0.3 21.2k 0.4 14.0k0.6 12.2k0.4 0.78k0.3 Methyl crotonate 19.9k0.4 18.0k0.4 13.6k0.7 6.1k0.3 0.64k 0.05 Mesityl oxide 25.4k0.3 24.3k0.5 17.9k0.3 12.1k0.2 0.60k0.02 Crotononitrile 19.2k0.2 15.3k0.3 11.5k0.3 (5.6k0.2)b 0.60k0.04

"As defined by equatlon given in Experimental. In dichloromethane solution at 295 K. OValue corrected for incomplete complexation. Measured value = 3.8 f 0.1 kcalimol. cHeat of solution of base in CH,CI,.

Nature of complexes formed Before discussing these heats of reaction it is

important to establish the nature of the complexes formed. In all measurements the Lewis acid was held in excess and 1: 1 complexes were formed (1). Checks were made for complete complexation by running 'H nmr spectra at 25°C on solutions of comparable concentrations to those used in the calorimetry. In all but one case, variation of the acid to base ratio did not alter the chemical shifts of the resonances of the base indicating that the base was essentially completely complexed.

The one exception was crotononitrile and SnCl,, where considerable variation was detected, Table 2. From a plot of 1lA6 of H3 against the reciprocal of the Lewis acid concentration (Fig. I), the equilibrium constant was found to be 5.31 L mol-I.z The value of the equilibrium constant found here for 4 and SnCl, corresponds well with that reported by Brown and Kubota (14) for ben- zonitrile/SnCl, ( K = 4.17 L mol-I).

Under the calorimetric conditions used the mag- nitude of the equilibrium constant between 4 and SnC1, is such that only some 58% of 4 would be complexed. This contrasts markedly with the 'H nmr measurements at -20°C reported earlier (1) where it appeared that essentially all of 4 was complexed. The entropy term for this equilibrium was found to be -16 eu. Using the linear en- thalpylentropy relationship of Laane and Brown (15), a value of -22 eu might be expected.

Enthalpy correlations Plots of the enthalpies of reaction of the Lewis

2A6 is the chemical shift of the fast exchange average of a resonance due to the complex and the free base measured relative to the chemical shift of the free base.

TABLE 2. Lewis acid concentrations and chemical shift differences for the com- plexation of crotononitrile 4 with SnC14a

[SnC1,1 141 (M) (MI 63 ( P P ~ ) 86, (ppm)

"3-H shift of free base is 6 6.77.

acids with the Lewis bases against the nmr-derived scale (1) are shown in Fig. 2. Each of the bases shows a qualitative correlation between the enthal- pies and nmr rating of the Lewis acids. In each case the ranking of the Lewis acids is the same. How- ever, with the exception of mesityl oxide, the correlations are not very good quantitatively.

If the nmr scale we advanced previously is indeed indicative of the amount of positive charge

FIG. 1. Plot of 1/86 of the 'H chemical shift of H3 of crotononitrile versus l/[SnC14].

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CHILDS ET AL.: 2 811

LA A 6 - scale

FIG. 2. Plots of the enthalpy of reaction of Lewis acids with crotonaldehyde (@); mesityl oxide (+); methyl crotonate (*); and crotononitrile (0) against nmr Lewis acid scale.

induced on the carbon framework of these unsatur- ated bases on complexation, then qualitatively this is related to the heat of formation of the complexes. It should be stressed, however, that the Lewis acids selected for this study are all simple inasmuch as they are monomeric in solution. No correlation would be anticipated with dimeric systems such as Al,Br, or the alkylaluminum halides unless allow- ance was made for the heats of dissociation of the Lewis acids.

In the nmr work it was found that each base had a different sensitivity (slope parameter) to change of the Lewis acid. Thus crotonaldehyde was much more sensitive to change of the Lewis acid than pent-3-en-2-one. Crotononitrile was found to be the least sensitive. There appears to be no correlation between the calorimetric data presented in this paper and these base slope parameters. As can be seen from Fig. 2, the slopes of the AH vs. nmr scale plots for all the bases used are qualitatively the same.

In conclusion, it remains to be seen whether the parameters measured for these Lewis acid com- plexes of the unsaturated bases can be used to predict reactivity in their catalysed reactions.

Experimental General

All materials were purified as described previously (1). The 'H nmr spectra were obtained on a Bruker WP 80 Fourier

Transform spectrometer operating at 80 MHz equipped with a Bruker B-VT 1000 variable temperature controller. Samples were run at 25°C in CD,CI, as solvent.

Equilibrium measurements for crotononitrile/SnCl, A series of samples of 4 (0.04 M) in CD,Cl, (0.3 mL) were

prepared and SnC1, added to them such that the acid concentra- tion varied between 0.2 to 0.8 M . The 'H nmr chemical shifts of 4 were measured at 25'C and are shown in Table I . The equilibrium constant was estimated from a slightly modified Benesi-Hildebrand plot (16). The slope is given by liAiioK (where A60 is the chemical shift of the complex relative to the free base) and the intercept is given by l/A60. The slope was 0.48 T 0.02 mol/L and intercept 2.55 T 0.05 giving a value of K = 5.31 T 0.19 L mol-' (16). All other combinations of Lewis acids - bases used in this study gave invariant chemical shift differences on comparable changes of the Lewis acid concentra- tion.

The calorimeter employed was based on the design by Arnett and Petro (lo) with the following modifications. The calorimeter vessel was reduced in size to approximately 125 mL. As a result, the volume of solvent used could be reduced to 60 mL. The bottom of the vessel was made flat except for a V-shaped indentation which acted as a bearing for the Teflon tip of the stirrer. This allowed better immersion of the glass probes containing the thermistor and heaters. The reduction in volume of solvent caused an increase in sensitivity of the calorimeter, decreasing the heat input for balance and calibration runs. For the balance and calibration heaters, Philips metal film precision resistors type MR 25 F (50 ohm) were used. These were in series and mounted in 5 mm od Pyrex glass tubes partially filled with liquid paraffin as a heat transfer fluid. The resistance of these heaters was carefully monitored during use, but heat transfer was found to be so effective that no significant change in resistance took place even with prolonged use. Two 12-V lead-acid accumulators acted as power supplies. These were independently connected to the balance and calibration heaters and the applied voltage was measured using a digital multimeter (Keithley 169). The duration of the calibration heating cycle was measured to 0.01 s using an interconnected digital timer.

Calorimetric proced~tre The dried calorimeter vessel was charged with dried CH,Cl,

(60 mL) and blanketed with nitrogen. The Lewis acid was added to the calorimeter with a syringe so as to give a 0.2 to 0.3 M solution. (At least two completely independent runs were carried out for each acidlbase combination with different acid concentrations being used in each.) The calorimeter was balanced by adjusting the balance heater at ca. 25'C and a series of at least eight electrical calibrations was conducted.

The Lewis base (5-20 mg) was added using a weighed 25 p L syringe. The enthalpies quoted in Table 2 are the arithmetic

TABLE 3. Enthalpy of complexation of crotonaldehyde with SbC1,"

-

Run Electrical calibrationb Wt. sample (mg) AH, (kcal/mol)

1 0.1083$.0.0016 7.27 21.20 8.13 21.57 7.50 22.06 7.57 22.16

2 0.1068f 0.0028 7.31 21.81 4.33 22.47 6.29 22.37 6.49 22.15

"At 298 K with SbCI, d~ssolved In 60 mL CH,CI, 'In units of calones per centimeter pen deflect~on Each callbratton 1s a mean of 8

measurements

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812 CAN. J. CHEM. VOL. 60, 1982

mean of at least six injections carried out over two separate runs. A typical set of results is shown in Table 3. To obtain the heat of complexation a separate experiment was conducted in which no Lewis acid was added to the calorimeter and the heat of solution subtracted from the previous value. (AH,,,,,. = AH, - AH, where AH,,,,,, = heat of complexation; AH, = heat of solution of base in CH2C1, containing the Lewis acid; AH2 = heat of solution of base in CH,C12 solution.) A further set of electrical calibrations was carried out at the end of the run.

I. R. F . CHILDS, D. L. MULHOLLAND, and A. NIXON. Can. J. Chem. 60, 801 (1982).

2. W. B. JENSEN. Chem. Rev. 78, l(1978). 3. R. G. PEARSON. J. Am. Chem. Soc. 85, 3533 (1963). 4. (a ) V . GUTMANN and R. SCHMID. Coord. Chem. Rev. 12,

263 (1974); (b) V . GUTMANN. Coord. Chem. Rev. 15, 207 (1975).

5. R. S. DRAGO and B. B. WAYLAND. J. Am. Chem. Soc. 87, 3571 (1965).

6. R. S. MULLIKEN and W. B. PEARSON. Molecular com- plexes: a lecture and reprint volume. Wiley-Interscience, New York, NY. 1969.

7. (a ) G. KLOPMAN. J. Am. Chem. Soc. W, 223 (1968); (b) I n Sigma molecular orbital theory. Edited by 0 . Sinanoglu and K. B. Wiberg. Yale University Press, New Haven, Conn. 1970. pp. 115-120; (c) In Chemical reactivity and reaction paths. Edited by G. Klopman. Wiley-Interscience, New York, NY. 1974. Chapt. 4.

8. J. BRUNN and C. BECK. Z. Chem. 17, 296 (1977). 9. T . F. BoLLEsand R. S. DRAGO. J. Am. Chem. Soc. 87,5015

(1965). 10. E. M. ARNETT and C. PETRO. J. Am. Chem. Soc. 100,5402

(1978). 11. G. OLOFSSON, I. LINDQVIST, and S. SUNNER. Acta Chem.

Scand. 17, 259 (1963). 12. E. M. ARNETT, R. P. QUIRK, and J. J. BURKE. J. Am.

Chem. Soc. 92, 1260 (1970). 13. G. W . RICHARDS and A. A. WOOLF. J. Chem. Soc. A, 470

(1968). 14. T . L. BROWN and M. KUBOTA. J. Am. Chem. Soc. 83,331

(1961). 15. J. LAANE and T. L. BROWN. Inorg. Chem. 3 , 148 (1964). 16. H. A. BENESI and J. H. HILDEBRAND. J. Am. Chem. Soc.

71, 2703 (1944).

ADDITIONS AND CORRECTIONSIAJOUTS ET CORRECTIONS &

J. A. Howard and S. B. Tong. Metal complexes as antioxidants. 7. A kinetic and product study of the reaction of tertiary alkylperoxy radicals with cobalt(1I) acetylacetonate.

Page 1962. This paper should have been Part 8 in the series on metal complexes as antioxidants. In addition, the acetophenone and a-cumyl alcohol yields for the fourth entry in Table 3 are back-to-front. Thus the alcohol yield should have been 0.033 M and the acetophenone yield 0.002 M.

Page 2253. In the discussion section of the paper the corrected ratio of [PhCMe2360,']/[PhCMe202'] should have been 0.015 and not 0.05 and Nangia and Benson's estimated of M , , O was -20.2 kcal mol-I and not - 10.2 kcal mol-I.

Vincenzo Malatesta and Angelo Mugnoli. The crystal and molecular structure of N,N1-ethylenebis(benzoy1acetonimin- ato)-nickel(I1).

Page 2766. The value for P in both the English and French abstracts should be P = 96.68(2)"; the value given in the text requires no correction.

J. A. Howard, J. B. Bennett, and 6. Bmnton. Absolute rate constants for hydrocarbon autoxidation. 30. On the self- reaction of the a-cumylperoxy radical in solution.

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