Kinetic and spectroscopic study of the hydrogen-transfer reaction from 2-propanol to cyclohexanone catalyzed by [IrH2(pz)(Hpz)(PPh3)2] (Hpzpyrazole)

Journal of Molecular Catalysis, 87 (1994)151-160 Elsevier Science B.V., Amsterdam

151

M292

Kinetic and spectroscopic study of the hydrogen-transfer reaction from 2-propanol to cyclohexanone catalyzed by [IrHs(~x)

WPZ)

(PPh&l

W~z=PyrazoW

Miguel A. Esteruelas*, Maria P. Garcia, Marta Martin and Luis A. 01-o Departamento de Quimica Inorgcinica, Znstituto de Cienciu de Materiales de Aragh, Universidad de Zaragoza, CSIC, 50009 Zarogoza (Spain) (Received April 16,1993; accepted August 6,1993)

Abstract The complex [IrHz(pz) (Hpz) (PPhs),] (1) (Hpz=pyrazole) has been shown to be an efficient catalyst for the hydrogen transfer reaction from 2-propanol to cyclohexanone. The rate of formation of cyclohexanol was found to be inhibited by the addition of pyrazole. The data of the reaction can be accommodated by a rate expression of the form r= a [ Ir ] &b [ Hpz ] ’ + 1, where [ Ir]r,,tiand [Hpz] are catalyst precursor and added pyrazole concentrations, respectively, and a and b constants. The mechanism deduced on the basis of this rate law and spectroscopic observations proceeds in accordance with the following set of reactions: [IrH,(pz) (Hpz(PPh,),] (~)+HPz, 2+ (CHs)zCHOH= [IrHz{OCH(CH&}(PPhs),] (l)=[IrH,(pz)(PPhS),l (S)+HPZ, 3+[IrH~(PF%)~l (4)+(CH&CO, ~+c-C~H~~O~W-I~(~-C~H~~O)(PP~~~I (6), 5-r [IrH2(c-CsH110)(PPh,),] (6), 6+ (CH&$HOH+3+CBH110H, where the /?-elimination process on the OCH ( CH3)z group linked to the metallic center of 3 is the slow step of the reaction. Key words: cyclohexanone; hydrogen transfer, iridium; kinetics; P-propanol; pyrazole; NMR spectroscopy

Introduction

Reduction reactions of unsaturated organic substrates with molecular hydrogen are very effective, and they have played a key role in the fundamental understanding of catalytic reactions [ 11. However for the reduction of some organic compounds, e.g. ketones, the use of hydrogen transfer catalysis can be an alternative method, more effective than the reduction with molecular hydrogen, if the hydrogen donor and the catalyst are appropriately selected [ 21. The reduction of ketones by hydrogen transfer catalysis generally requires the use of alcohols as the donor species, mainly 2-propanol. Catalysts are generally cobalt [ 31, rhodium [ 41, iridium [ 51, iron [ 61, ruthenium [ 71 and osmium [8] complexes with tertiary phosphine ligands or nitrogen containing *Corresponding author. Fax. ( +34-76)567920

0304-5102/94/$07.00 0 1994- Elsevier Science B.V. All rights reserved. SSDZ 0304-5102(93)E0226-7

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chelating ligands such as 2,2’-bipyridine, l,lO-phenanthroline and their methyl or methoxy derivatives [2a]. Although numerous hydrogen transfer catalysts have been reported, a detailed discussion on the probable mechanisms of these processes is not feasible at the moment because the data are scarce and incomplete. The proposal of a sensible catalytic cycle requires careful kinetic and spectroscopic studies of the reactions [ 91. In this line, we had previously reported kinetic and spectroscopic investigations of the hydrogen-transfer reactions from 2-propanol to cyclohexanone [lo] and benzylideneacetone [ 111 catalyzed by the complexes [OsHX(CO) (PiPr,),] (X=H, Cl) and [H(CO) (PPh,),Ru(pbim)Ir(COD)] (b im = 2,2’ -biimidazolate ) , respectively. As a continuation of our work on the mechanisms of the hydrogen-transfer reactions from 2-propanol to ketones, we now report a kinetic and spectroscopic investigation of the hydrogen-transfer reaction from 2-propanol to cyclohexanone (eqn. 1) catalyzed by the complex [ IrH2 (pz) (Hpz) ( PPh3)2] (Hpz=pyrazole). C-C6H1,,0+ (CH3)&HOH+~-C6H110H+

(CH3)&0

(1)

Results The reactions were carried out in 2-propanol/dichloromethane (3:l) at 60’ C. In order to determine the rate dependence on the various reaction components, hydrogen transfer runs were performed at different catalyst and substrate concentrations. Furthermore, because the catalyst is coo&natively saturated and the creation of a coordination vacancy must be necessary for the catalysis, the effect of the addition of pyrazole to the catalytic solution was also investigated. The initial rates for these experiments are collected in Table 1. Figure 1 shows that the concentration of the catalyst has a linear relationship with the initial rate of the reduction. The initial rate may be regarded as first order dependent on the concentration of the catalyst, which is expressed in the form:

r=41&bt

(2)

where [ Ir ] rot is the initial concentration of catalyst precursor and a is a constant. From Fig. 1, a value of 2.7~ 10S3 s-l was obtained for a. As seen from Fig. 2, the initial rate of the hydrogen transfer is independent on the cyclohexanone concentration. This zero-order dependence can be understood as a consequence either that the ketone coordinates to the metal of the catalyst after the rate limiting step or that the cyclohexanone coordinates to the catalyst species before the rate limiting step to yield quantitatively Ir-c-C,H,,O species ( [ Ir ] Tot= [Ir-c-CsH1lO] ). We believe that the first way seems to be more probable, because in the iridium complexes with phosphines

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et al. /J. Mol. Catal. 87 (1994) 151-160

153

TABLE 1 Kinetic data for the reduction of cyclohexanone by hydrogen transfer from 2-propanol catalysed by [IrHz(pz) WPZ) W%Ll (1O’M)

[ Cyclohexanone] (M)

0.80

0.250

2.13

1.00 1.39 1.60 1.80 2.50 1.39 1.39 1.39 1.39 1.39 1.39 1.39 1.39 1.39 1.39

0.250 0.250 0.250 0.250 0.250 0.099 0.175 0.250 0.310 0.372 0.500 0.250 0.250 0.250 0.250

3.40 4.04 4.82 5.07 6.61 4.56 4.50 4.10 4.08 3.97 4.10 1.48 1.11 0.89 0.58

[IrlTot

P, 106

WPZI

(M s-l)

(10’M)

1.39 2.79 4.19 5.58

1

IIrh4

2

xl@

3

(M)

Fig. 1. Plot of the rats of the hydrogen-transfer reaction from 2-propanol to cyclohexanone versus [Ir]lMcat.alyzedby [IrH,(pz)(Hpz)(PPhs),] in2propanol/dichloromethane (31) at6O”C (0.25 M cyclohexanone ) .

or pyrazole as ancillary ligands the coordination of ketones is rather weak. In order to confirm this point, a spectroscopic study of the catalytic reaction was carried out, and in fact, the ‘lP{lH} NMR spectra of different runs showed only a signal at 19.8 ppm, assigned to the complex [ IrH2 (pz) (Hpz) (PPh,), ] by comparison of these spectra with that of a pure sample.

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*/ h

6-

r

?

4___._o_ncl~-_o-_o-

Ll-

x L

2-

0-l 0,4

0.2

a0

08

(M)

[Cyclohexanonel

Fig. 2. Plot of the rate of the hydrogen-transfer reaction from 2-propanol to cyclohexanone verzuz [ cyclohexanone ] catalyzed by [ IrH2 (pz) (Hpz ) (PPh&] in 2-propanol/dichloromethane (3:l) at6O”C (1.39X10-9M [IrH,(pz)(Hpz)(PPh,)2]).

01 0

10 IHpz)2

Fig. 3. Plot of [Ir]&r dichloromethane (3:l) cyclohexanone ) .

20 xlw

30

(M*)

verzuz [Hpz]’ catalyzed by [IrHZ(pz) (Hpz) (PPh,),] at 60°C (1.39x1O-3 M [IrHZ(pz)(Hpz)(PPh3)J,

in 2-propanol/ and 0.25 M

The effect of the addition of pyrazole to the catalytic solution is shown in Fig. 3. The initial reduction rate decreases by addition of pyrazole to the reaction system. The plot of [ Ir]rJr versus [Hpz12 is linear with a positive interception on the y axis. The relation is expressed in the form: (3) where [ Hpz] is the concentration of added pyrazole and a and b are constant. From Fig. 3, values of 1.6 x 10B3 s-l and 9.2 x 10’ Mm2 were obtained for a and

MA. Esteruelas et al. /J. Mol. Catal. 87 (1994) 151-160

155

b, respectively. In the absence of pyrazole eqn. 3 is converted into eqn. 2. In accordance with this, the value calculated for a from Fig. 3 agrees well with that calculated from Fig. 1.

Discussion In light of eqn. 2, the following set of reactions can be proposed as catalytic cycle for the hydrogen-transfer reaction from 2-propanol to cyclohexanone catalyzed by [IrHz(pz) (Hpz) (PPh,),]. [IrHApz) (ypz)

W-W21 g NM.$WPh,M

[IrHdpz~W%M

+ (CW&HOH s [IrH,{OCH(CsH,),}(PPh,),]

[IrH,{OCH(CSH3)2}(PPh,),lf W-M~Ph,M [IrH&-C,H?O)

(4)

+Hpz

M-M~Ph,),l+

+Hpz

(5)

(CHd&O

(6)

[IrHs(c-C6HfOI (PPh,),]

+c-GH&+

(7) (3)

(PPW21+ W-b(c-C6HgO) WE%,),]

[IrH2(c-C6Hf;O) W&M

+ WM&HOH -, [IrH,{OCH(CsH,),}(PPh,),l +c-C,H,,OH

(9)

where eqn. 6 is the slow step of this catalytic reaction. Thus, the rate of formation of cyclohexanol follows the kinetic law:

r= dW&,OW

=

dt

431

(10)

-x=M31

The concentration of the key intermediate 3 can be determined as follows: [IdTot

=

w+w+[31

since [l] = [2] [Hpz]/K, [ Hpz] 2/K&, and finally

(11) and [2] = [3] [Hpz]/&,

we have [l] = [3]

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M.A. Esteruelas et al. /J. Mol. Catal. 87(1994) 151-160

&&

[IrlTot

[31 = [H~z]~+K,[H~z]

+K&

(12)

1 is the only spectroscopically detected species in the course of the catalysis, suggesting that ( [ Hpz] 2+&KS) >> K4 [ Hpz] . In this way, [ 31 can be written as: [IdTot

[31=([Hpz]2/K&5)+1

(13)

Combining eqns. 10 and 13, we obtain eqn. 14, k3[1rlTot

r= ( [H~z]~/K,K,)

+l

(14)

which agrees well with the experimental data (see eqn. 3). Furthermore, combining eqns. 3 and 14 we can get the values of b and K,K,. The value of & is between1.6~10-3and2.7x10-3s-1,whilethevalueofK4K,isabout1.1x10~7 M2. From our previous work [ 10,111 on the mechanism of the hydrogen-transfer reactions from 2-propanol to ketones catalyzed by hydride complexes of platinum group metals can be inferred that, in general, these reactions involve four steps: (i) coordination of the ketone to a coo&natively unsaturated hydride-metal intermediate, (ii) formation of alkoxy-metal species by insertion of the hydride ligand into the ketonic double bond, (iii) exchange of the alkoxy group by reaction with the hydrogen donor, which also acts as solvent of the reaction, and (iv) a/3-elimination process. The mechanism described by eqns. 4-9 is in agreement with this proposal. However, it has special features related to the presence of the pyrazole and pyrazolate ligands in the catalytic precursor. The complex [ IrH, (pz) (Hpz) ( PPh3) 2] is coordinatively saturated and its catalytic activation seems to involve the dissociation of the pyrazole ligand (eqn. 4). The dissociation of monodentate nitrogen donor ligands from the metallic center of hydrogen transfer catalysts has been previously observed, even for coordinatively unsaturated species. In contrast to these systems, related catalysts containing phosphine groups as ancillary ligands remain coordinated the phosphine ligands during the catalysis [4a]. The reaction shown in eqn. 4 leads to the coordinatively unsaturated intermediate 2, which could subsequently coordinate the ketone or alternatively react with 2-propanol to give pyrazole and 3 (eqn. 5). Although the formation of 3 according to eqn. 5 is not a thermodynamically favoured process due to the lower pK, of pyrazole in comparison to the pK, of 2-propanol, in light of eqn. 3, this process seems to be the only one available under catalytic conditions. The highest concentration of P-propanol under the reaction conditions must favour the formation of [IrH2(pz){ (CH&CHOH}W%)21, in comparison with [ IrH2 (pz) (cC,H,oC) W’h,),l, and therefore the reaction shown in eqn. 5. The metal-

M.A. Esteruekas et al. /J. Mol. Catal. 87 (1994) 151-160

157

isopropoxide intermediate 3 gives the coordinatively unsaturated trihydride 4 with formation of acetone via a b-elimination reaction (eqn. 6). The related [ IrH, ( PiPrs )2] has been recently proposed as an active catalytic intermediate in the reduction with molecular hydrogen of benzylideneacetone to 4-phenylbutan-Z-one [ 121. The insertion of cyclohexanone into one of the three hydride ligands of 4 must involve the initial coordination of the substrate (eqn. 7). The insertion reactions are generally viewed as concerted processes involving four-center intermediates, consequently, the formation of 6 from 4, most probably involves a Ir ($-O=Cb) intermediate [lo]. Equation 9 illustrates step (iii) of the general mechanistic scheme mentioned above. Exchange between the hydrogen-bonded alcohol and the coordinated alkoxide has been previously demonstrated [ 131. This exchange most probably proceeds via a hydrogen-bonded adduct, some examples of which have been characterized [ 141. In an excess of pyrazole, the formation of cyclohexanol could also be promoted by this ligand, according to eqn. 15. [IrH2(c-CGH110)

W%hl

+HP-

W%(pz)

+c-C,H,,OH

U’Phhl (15)

Schrock and Osborn [ 151 have previously suggested that during the hydrogenation of ketones catalyzed by solvento complexes of the type [RhH,S,G-‘R,M+ @= solvent), the second proton-transfer step could be promoted by small quantities of water. In the same line, Sanchez-Delgado [ 161 et al. have observed that the hydrogenation rates of ketones and aldehydes catalyzed by [RuHCl(CO) (PPh,),] are increased by the addition of small amounts of acetic acid or water, which is explained in terms of a hydrolytic cleavage of an alkoxy-metal intermediate. Although these processes are very similar to that described in eqn. 15, there are significant differences between them. For the systems of Schrock and Osborn and Sanchez-Delgado et al., the hydrolytic cleavages of the alkoxy-metal intermediates are proposed as steps of the catalytic cycles, and consequently increases in the initial rates of the reactions are observed by addition of water or acetic acid. However, in our case, the formation of cyclohexanol according to eqn. 15 should lead to the side catalytic intermediate 2, which could return to the cycle before the rate-limiting step (eqn. 5)) decreasing the initial rate of the reaction.

Concluding remarks This study has revealed that the complex [IrH,(pz) (Hpz) (PPh,),] catalyzes the hydrogen-transfer reaction from 2-propanol to cyclohexanone. In accordance with the kinetic and spectroscopic studies carried out, the mechanism of the reaction can be summarized by the Scheme 1, where the b-elimi-

158

MA. Esteruelaa et al. /J. Mol. Catal. 87 (1994) 151-160

(

/

c-G~HIoO

IlrH3(r12-c-C,H,oO)(PPhd21 Scheme1. nation process on the OCH ( CH3)2 group linked to the metallic center of the intermediate 3 is the slow step of the catalytic reaction.

Experimental General comments All manipulations were conducted with rigorous exclusion of air. Solvents were dried by known procedures and distilled under nitrogen prior .to use. to use. Complex (Probus) was distilled prior Cyclohexanone was prepared by reaction of the pentahydride [IrHAps) WPZ) W%M [ IrHs ( PPh3)2] with pyrazole in toluene as solvent [ 171. Physical measurements 31P(1H} NMR spectra were recorded on a Varian XL 200 spectrophotometer at 80.984 MHz, chemical shifts are reported relative to phosphoric acid at 85% as external reference. Samples for recording these spectra were prepared in 5-mm-diameter tubes under the same conditions employed for the catalytic reactions. These samples were then introduced into l-cm-diameter tubes containing CDCL. The analysis of the products of the catalytic reactions was carried out on

MA. Ester&as

et al. /J. Mol. Catal. 87 (1994) 151-160

159

a Perkin Elmer 8500 gas chromatograghwith a flame ionization detector and using an FFAP on Chromosorb GHP SO/l00 mesh (3.68 m x l/8 in.) column, at 120°C. The reduction products were identified by comparison of their retention times with those observedfor pure samples.Initialrate datawerefitted by conventional linear regressionprogramsby plotting ns& (moles of hydrogenatedsubstrate) versustime. Kinetics of the hydrogen-transfer reaction from 2-propanol to cyclohexarwne

The reactionswerecarriedout undernitrogenin a mixtureof 2-propanol/ dichloromethane (3:l) with magnetic stirring.The equipment consisted of a 50 ml two-necked flash fitted with a Suba-sealto allow samplesto be removed without opening the system. In a typical procedure,a solution of the catalyst ( NL(pz) (Hpz)(P%)21 1 in 3 ml of 2-propanol and 1 ml of dichloromethane was refluxedand a solution of the substrate (cyclohexanone) in 3 ml of 2-propanol and 1 ml of dichloromethanewas then injected. For the reactionsin the presenceof pyrazolethe procedurewas as follows: to a solution of the catalyst (0.01 mmol) in 3 ml of 2-propanol and 1 ml of dichloromethanewas added a Hpz solution 22.3x 10m4M in 2-propanol and 2 mmol of the substratein the appropriateamount of 2-propanol to complete a total volume of 8 ml.

Acknowledgment

We thank the DGICYT (Project PB 92-0092, Programa de Promotion Generalde1Conocimiento) and EEC (Project Small Molecules:SelectiveProcesses and Catalysis) for financial support. M.M. thanks the DGA (Diputaci6n Generalde Aragon) for a grant.

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