Journal of Molecular Catalysis, 43 (1987) 21 - 29 27

Letter

Correlation of Rates of Alkene Hydrogenation Catalyzed by Rhodium Complexes

M. KRAUS

Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague 6Suchdol (Czechoslovakia)

(Received March 17, 1987; accepted July 13,1987)

The application of linear free energy relationships (LFER) to rate and equilibrium data of organic reactions yields important information on the mechanism and allows prediction of the reactivity of related compounds. It has been shown that the LFER are useful also in the field of heterogeneous catalysis [ 1 - 31, where it is possible e.g. to analyze the steric and electronic contributions to the reactivity of alkenes in their hydrogenation on metals and sometimes also on non-metals [ 3,4]. A difficulty in these analyses is the fact that the relative changes in the extents of electronic and steric influences are approximately the same with straight chain alkyl groups; however, with highly branched and substituted alkyl groups the contri- butions to reactivity can be distinguished.

Recently Zuber et al, [5] measured the rates of alkene hydrogenation catalyzed by rhodium complexes containing 2-(N-alkylamino)pyridines, and from their data a consistent and sufficiently broad series of C5 to Cs alkenes can be extracted to justify an attempt to correlate the rates against structural parameters of the substituents on the C=C group. As substituent parameters for the LFER, the u* constants for polar effects and various constants for steric effects have been tested; their numerical values were taken from a critical compilation [6]. The most successful correlation, based on standard steric E, constants, is shown in Fig. 1. It confirms quantitatively and specifies the qu~itative conclusion of the authors that the predomin~t structural influence upon reactivity is steric hindrance. Other authors [ 7 1, starting from another series of alkenes hydrogenated with a phosphine- rhodium complex, have assumed both electronic and steric effects to play a role. However, a consistent series taken from their broader set of alkenes (l- hexene, l-dodecene, cis-2-pentene, 2-methyl-l-pentene and cis-4-methyl-2- pentene) also give better correlation against the sum of steric E, constants than against the sum of polar u* constants.

These correlations are of more general interest because the same conclusion on the predominance of steric hindrance by the substituents upon the reactivity of the C=C double bond has been drawn from the corre-

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Fig. 1. Correlation of rates of alkene hydrogenation catalyzed by Rh&Xa(CsHr& + C,H$&2--NH-C&I9 [ 5 ] in the coordinates of the Taft-Pavelich equation log I = a + s c Es. 1 l-Pentene (substituents on the double bond R’R2C=CR3R4 H, H, H, Pr); 2 2- pentene (H, Me, H, Et); 3 2-octene (H, Me, H, Am); 4 3-heptene (H, Et, H, Pr); 5 2,4,4- trimethyl-l-pentene (H, H, Me, neoAm); 6 2,4,4-trimethyl-2-pentene (H, tBu, Me, Me). Correlation coefficient 0.97, slope s = 0.35 * 0.04.

lation of the data on heterogeneously catalyzed hydrogenations [ 3, 41. A more detailed analysis indicated that on metals, the steric influence is exerted in the adsorption step, i.e. in the formation of the surface complex. Jardine and McQuiliin IS, 91 obtained a direct correlation between the rates of he~rogeneously and homogeneously catalyzed hydrogenations of cyclo- alkenes, and concluded on the -basis of correlations of the rates with equi- librium of complexation that the structure of the alkene influences mainly the first step of the hydrogenation i.e. the formation of the intermediate catalytic complex, in both cases.

All the correlations mentioned here establish a link between homo- geneous and heterogeneous catalysis; the common features of alkene hydro- genation are thus manifested.

Other possibility, which is unique for metal complex catalysis, is to look for quantitative relations between structural parameters of the ligands and observed rates. The data of Homer et al. [lo], concerning hydroge- nation of 1-hexene catalyzed by rhodium triarylphosphine complexes, can be treated in this way. From their set, six catalysts form a consistent series of puru-substituted phenyls in the phosphine ligands. The result of the LFER analysis is presented in Fig. 2. A part of the scatter of the points may be caused by slightly differing Rh:P ratios. Nevertheless, the correlation confirms the qualitative conclusion of the authors [lo] that electron- donating substituents increase and electron-accepting ones decrease the hydrogenation rate.

Zuber et al. [ 51 also studied changes in the ligand by varying the alkyl R in the 2-(N-alkylamino)pyridine component of the catalyst. The Taft or Taft-Pavelich equations should describe this case. Best correlation is shown in Fig. 3, which suggests the electronic nature of the influence of the alkyl substituen~. The correlation of the rates against steric constants of the

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Fig. 2. Correlation of rate of l-hexene hydrogenation catalyzed by Rh&lz(CeHr& + @-XC&&P [lo] in the coordinates of the Hammett equation log r = a + p U. 1 Xr = NMes, Xz=Xs=H; 2 Xr=Xz=Xs=OMe; 3 X1=Xz=Xs=Me; 4 Xr=Xz=x~=H;5 X1 = COMe, X2 = Xs = H; 6 Xr = Xz = X3 = Cl. Correlation coefficient 0.95, slope p = -0.80 ?; 0.12.

Fig. 3. Correlation of rates of cyclohexene hydrogenation catalyzed by RhzC12(CsHr& + C5H$I-2-NH-R [ 51 in the coordinates of the Taft equation log r = a + p* IT*. 1 R = Pr, 2 R,= Bu, 3 R = Am, 4 R = cycloHe. Correlation coefficient 0.99, slope p* = 3.34 f 0.27.

alkyls was poor. This result is in contrast to the views of the authors [ 51, who also assume steric hindrance. The sign of the slope indicates further that the activity of the catalyst is decreased by electron donation to the NH group; this means that the substituen~ act in opposite direction than in the preceding series of phosphine ligands. Therefore, the question arises as to whether the assumption of the authors [ 51 that the NH group participates as a base (electron donor) is correct.

These examples demonstrate the usefulness of the LFER for exami- nation of mechanisms of metal complex catalysis. However, full utilization of the possibilities of correlation analysis would require some advance planning of the composition of reactants or catalysts, which should be sufficiently populated in series and should cover a wider range of structural parameters.

References

1 M. Kraus, Adu. Catal., 17 (1967) 75. 2 Y. Yoneda, Proc. 4th Int. Congr. Catai., Moscow 1968, Vol. 2, Akademirli Kiado,

Budapest, 1971, p. 449. 3 M. Kraus, Adu. Catal, 29 (1980) 151. 4 M. Kraus, Izu. Khim., 17 (1984) 117. 5 M. Zuber, W. A. Szuberla and F. P. Pruchnik, J. Mol. Cataf.. 38 (1986) 315. 6 C. Hansch and A. Leo, Substituent Constants for Correlation Analysis in Chemistry

and Biology, Wiley, New York, 1979. 7 F. H. Jardine, J. A. Osborn and G. Wilkinson, J. Chem. Sot., A (1967) 1574. 8 I. Jardine and F. J. McQuillin, J. Chem. Sot., Chem. Commun., (1969) 502. 9 I. Jardine and F. J. McQuillin, Tetrahedron Lett., (1968) 5189.

10 3. Horner, H. Biithe and H. Siegel, Tetrahedron Lett., (1968) 4023.