Homogeneous catalytic hydrogenation of olefins using rhodium(I) compounds with 2-aminopyridine derivatives as catalysts

Journal of Molecular Catalyszs, 38 (1986)

309 - 314

309

HOMOGENEOUS CATALYTIC HYDROGENATION OF OLEFINS USING RHODIUM(I) COMPOUNDS WITH Z-AMINOPYRIDINE DERIVATIVES AS CATALYSTS M. ZUBER, W. A. SZUBERLA Institute of Chemistry, (Poland)

and F P. PRUCHNIK*

University of Wrodhw,

(Received February 25,1986,accepted

50-383

Wro&w,

14 Jolzot-Cune

Street

July 16,1986)

Summary The catalytic (C4H7)+

Q-

activity

NH R

of the Rh,Cl,(C&H&

sYste ms (R = n-propyl,

+ Q-NHR

n-butyl,

and Rh&-

n-amyl,

cyclohexyl;

C4H, = 2-methylallyl) in the homogeneous hydrogenaC8H14 = cyclooctene; tion of olefins was examined. The influence of the R substituents and of the Rh:ligand ratio on the hydrogenation rate of various olefins was defined.

Introduction Rhodium complexes belong to the group of the most active catalysts for olefin hydrogenation reactions [l]. The majority of papers concerning olefin hydrogenation describe rhodium complexes with phosphines, phosphites or arsines. Our studies [2 - 41 have revealed that rhodium or iridium complexes with nitrogen-containing ligands form extremely active catalytic systems for olefin hydrogenation. Among several catalytic systems, the most active is in ethanol [3,4]. RhZC12(C8H14)4 + 2-aminopyridine (C sH 14= cydooctene) The catalyst formed in situ in this systeni is about 10 times more active than the Wilkinson catalyst RhCl(PPhs), and also more active than RuHCl(PPhs)s [3,41. Herein we report the results of our studies on homogeneous olefin hydrogenation

in catalytic

(C4H7)+

NHR in ethanol

R = n-propyl,

n-butyl,

-NHR

systems RhZC12(CsH14)4 +

n-amyl,

(C8H14 = cyclooctene, cyclohexyl).

*Author to whom correspondence

Catalysts

and RhCl*-

C4H7 = 2-methylallyl, formed

in situ in these

should be addressed. @ Elsevler SeauolalPrmted

in The Netherlands

310

systems are very active and their activity could be compared with that of a system containing 2aminopyridine as the ligand.

Experimental Rh,CI,( C8H1J4 and RhC12(CdH,) complexes were prepared by described methods

[ 5,6].

2-Aminopyridine

derivatives (

NHR, where R = nQ-propyl, n-butyl, n-amyl, cyclohexyl) were obtained by the Chichibabin reaction [7]. Equimolar quantities of 2aminopyridine and sodium amide in xylene were refluxed for about 2 h (till ammonia evolution ceased). The mixture was treated with the appropriate alkyl bromide (1 mol RBr per 1 mol 2aminopyridine) and the solution obtained was further refluxed for about 2 h. Sodium bromide was filtered from the hot solution and xylene distilled from the filtrate. Then the dark brown residue was distilled fractionally under vacuum and the appropriate fraction recrystallized from n-hexane. Hydrogenation rate measurements were made in a constant volume apparatus [ 81. Olefins were purified by the Wilkinson method [ 91. Results and discussion The results of the olefin hydrogenation reactions are presented in Tables 1 - 3. The catalytic activity of the systems under investigation is very high and could be compared with that of the catalytic systems where TABLE 1 Maximum hydrogenation CF-

rates of cyclohexene

in EtOH using RhzClz(CsH14)4 +

NHR as catalyst

R

Rh:L

Turnover number (mol Hz h-l (mol Rh)-l)

n-propyl n-butyl n-amyl cyclohexyl

1:2

5000 4200 3000 1600

n-butyl n-amyl cyclohexyl

1:4

2500 2450 700

n-propyl

1:lO

400

Reaction conditions: [Rh&&(CsH&] = 0.89 X 10e4 M, [cyclohexene] 30 OC; V&utic.n = 22 cm3, PH, = 0 1 MPa -Pm,,,.

= 0.46 M; T =

311

2-aminopyridine was used as a ligand [4]. The catalytic activity depends upon the alkyl substituent R in the ligand and on the Rh:ligand molar ratio. Varying R has a significant influence on the hydrogenation rate. In the cyclohexene hydrogenation reactions, the replacement of 2-n-propylaminopyridine by the n-butyl derivative, or the replacement of 2-n-butylaminopyridine by 2-n-amylaminopyridine suppressed the hydrogenation rate by about 20 - 30%. The hydrogenation rate of the catalytic systems with 2-cyclohexylaminopyridine is lowered even more. Such a decrease m the catalytic activity was not observed upon replacement of 2-n-butylaminopyridine by the n-amyl derivative with a Rh:ligand ratio of 1:4 (Table 1). However, the lowest hydrogenation rates were always observed for catalytic systems with 2-cyclohexylsminopyridine as a ligand (Tables 1, 2). At the Rh:ligand = 1:4 molar ratio, (Table 2) variations of the R substituent (R = n-propyl, n-butyl or n-amyl) have only a small influence on the catalytic activity; a considerable decrease in the catalytic activity is always observed for catalytic systems with 2-cyclohexylaminopyridine as a ligand. As follows from the literature data [lo, 111, the basicity changes of the primary or secondary aliphatic amines with changes of the R substituent are very small (for primary amines Kb X lo4 = 4.7,4.1 and 4.4 for R = n-Pr, n-Bu and cyclohexyl respectively). The same can be expected for 2-aminopyridine TABLE 2 Maximum hydrogenation catalyst

rates of olefins in EtOH using Rh2Cl~(CsH14)4 +

Q-

NHR as

Olefm

R

Turnover number (mol Hz h-r (mol Rh)-‘)

1-pentene

n-butyl cyclohexyl

4800 2500

2,4,4trimethyl1-pentene

n-butyl cyclohexyl

550 200

2-pentene

n-butyl

1800

2,4,4-trimethyl2-pentene

n-butyl cyclohexyl

200 >lO

2-octene

n-propyl n-butyl n-amyl

1100 1050 1150

3-heptene

n-butyl

cyclohexene

n-butyl n-amyl cyclohexyl

Reaction

conditions:

V *,,htbn = 22 cm3, Pq

850 2600 2450 700

[RhaCla(CsHr&] = 0.89 x 10e4 M, [olefin] = 0.45 M; T = 30 “C; = 0 1 MPa -Pvapour; Rh:L = 1:4.

312

derivatives. Therefore the changes in the electron density at the nitrogen atom should be small, and steric effects in 2aminopyridine derivatives could be responsible for the hydrogenation rates in the catalytic systems studied. When R = cyclohexyl, steric hindrance would be large enough to impede the amine nitrogen coordination with the rhodium atom, thus reducing the catalytic activity. In line with this hypothesis, the data of Table 2 indicate a dependence of the catalytic activity also upon the steric situation at the hydrogenated double bond. The highest hydrogenation rates were observed for the terminal olefins. Hydrogenation rates are lower for the internal olefins and lowest of all for branched-chain olefins, where significant steric hindrance occurs. TABLE 3 Maximum hydrogenation rates of cyclohexene m EtOH using RhClz(C4H,) as catalyst, Rh:L molar ratio 1:4, reaction conditions as m Table 1 R

Turnover number (mol Hz h-l (mol Rh)-‘)

n-butyl n-amyl cyclohexyl

1500 300 500

+eNHR

The data in Table 3 revealed that the catalytic activity of the systems containing Rh2C12(CsH,&, as a catalytic precursor exceeds that of the systems containing RhCl,(C,H,). Differences in activity of catalysts prepared from RhzClz(CsH,& and RhClJ&H,) are marked, and thus it could be concluded that both rhodium compounds produce catalysts containing complexes of different composition and structure. In the presence of Rh&( CsHi,& + 2-cyclohexylaminopyridine catalytic system (Rh:L = 1:4) the bands at 11600 cm-’ and 16000 cm-’ are observed during the hydrogenation of 2,4,4-trimethyl-2-pentene and at 11200,14 700 and 19 000 cm-’ in the case of 2-heptene hydrogenation. The spectra of the catalysts prepared from Rh2C1,(CsH,,& and 2-n-butylaminopyridine (Rh:L = 1:4) show bands at 11100 and 13 800 cm-’ (sh) for the hydrogenation of 2-heptene and a band at 18 700 cm-’ for 2,4,4trimethyl2-pentene. This is indicative of the formation of polymeric Rh(1) complexes with Rh-Rh interaction [12]. The charge transfer bands Rh + aminopyridine and Rh + olefin in the mononuclear complexes appear at energies similar to the MLCT bands (Metal-to-Ligand Charge Transfer) in complexes RhCI(CO)(PR&, i e. cu. 25 000 - 30 000 cm-‘. For binuclear L4Rh-RhL4 complexes the energy of this transition declines by cu. 6000 - 8000 cm-’ because of the formation of the Rh-Rh bond. The lower the MLCT energy, the higher will be the n value in the polynuclear complexes of the structure:

313

L

L

L

L

L - --Rh

--Rh

Rh L

:_ L

L

L

0 n

L

For n > 5 they usually appear in the range 15 000 - 8000 cm-’

[ 13,141.

NHR should change only Qslightly, depending on the kind of olefin [15 - 171. E.g. for [PtCls(C,H,)]the d(Pt) + n*(olefin) bands appear at 34 400, 37 700 and 41600 cm-’ and for [PtC13(CH&HCH20H)]- at 33 800, 37 400 and 41200 cm-’ [15]. For Positions of CT bands Rh -+ olefin and Rh +

the PtC12(N?R)(C&)

complex, the bands at 38 750,36 350 and 32 500

and d(Pt) + n*(L) transicm-’ are due to the n(L) + n*(L), d(Pt) +x*(&H,) tions [18]. Thus, the changes in the spectra of catalytic systems are caused not only by coordination of different olefins to rhodium but also by formation of complexes of different structure, containmg different amounts of Rh atoms. The composition of the complexes changes with a change in the rhodium compound concentrations and with the kind of olefin. For this reason, the isolation of the catalytically active complexes and their physicochemical investigations were impossible. The kind of complexes formed depends upon the electronic and steric properties of the olefin; because the concentration of olefin is the same in each case, thus the dielectric constants of mixtures of solvent + olefin are also essentially the same. Catalytic systems with 2-aminopyridine derivatives containing asymmetric R substituents are still under investigation.

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314 10 J D. Roberta and M C Caserio, Basw Princrples of Organic Chemistry, W A. RenIamin, New York, 1964. 11 T W Graham Solomons, Organzc Chemistry, Wiley, New York, 1976. 12 G. L Geoffrey, M. S Wrighton, G. S. Wammond and H. B. Gray, J Am. Chem Sot , 96 (1974) 3105, K R. Mann, J. G. Gordon and H. B. Gray, ibid, 97 (1975) 3555. 13 K R. Mann, N. S. Lewis, R. M Williams, H. B. Gray and J. G. Gordon II, Znorg Chem., 17 (1978) 828. 14 A. L. Baich, ACS Symp Ser , 155 (1981) 167 and references cited therein. 15 R. G. Denning, F. R. Hartley and L. M. Venanzi, J. Chem. Sot. A, (1967) 1322. 16 H. Huber, G. A. Ozin and W. J. Power, J. Am. Chem. Sot., 98 (1976) 6508. 17 G. A. Ozin and W. J. Power, Znorg Chem , 17 (1978) 2836. 18 M A. Meester, D. J. Stufkens and K. Vrieze, Znorg Chim Acta, 14 (1975) 33