摘要:
1975 645Kinetic Studies on Homogeneous Oxidation of Olefins with Palladium(ii)CatalystsTHE oxidationsolution underBy John S. Coe and John B. J. Unsworth, Chemistry Department, King's College, Strand, London WC2R 2LSThe oxidation of oct-1 -ene by p-benzoquinone, catalysed by bis(benzonitrile)dichloropalladium(ll). has beenstudied kinetically in mixtures containing 20% water and one of the following : 1,2-dimethoxyethane, diglyme.tetrahydrofuran, dioxan, or acetone. The results are similar for reactions in the aqueous-ether solvents and areconsistent with a mechanism basically similar to those currently under discussion for the catalytic oxidation oflower olefins in aqueous solution. The corresponding reaction of oct-1 -ene in aqueous acetone is complicated byinteractions between acetone and the palladiurn(ii) catalyst.of olefinic substances in homogeneousthe catalytic influence of palladium(r1)complexes has been studied extensively during the pastfew years.The recent account by Maitlisl containsreferences to most of the important work in this field.Effort has been concentrated mainly on determiningthe effects of various factors (the nature of the substrate,solvent, catalyst, and oxidising agent) on the productcomposition. The reaction generally results in carbon-oxygen bond formation but is frequently accompaniedby isomerisation and polymerisation. Even with thesimplest substrate, ethylene, one or more of at leastfifteen products may be obtained under various con-dit ions.Most of the kinetic studies previously reported referto reactions of olefins of low molecular weight(ethylene,2-10 propene,11J2 and butenes 12) in aqueoussolution and with the palladium complex initially in theform of the tetrachloropalladate(r1) ion.Kinetic studieshave also been reported for reactions in aqueous systemsof cyclohexene l3 and styrene and for ethylene inacetic acid,l7,l8 and the general kinetic problem has beendiscussed recently.l? l9In the present experiments our original object was todetermine conditions under which kinetic studies couldbe extended to a wide range of olefinic substances andpalladium(11) catalysts. Most olefinic compounds areof lower reactivity and have lower solubility in waterthan those previously studied.As more representativeof the class of substrate as a whole we have chosen tostudy several higher-molecular-weight terminal olefins,which have the advantage that, in the presence of water,they undergo reactions of simple stoicheiometry to formP. M. Maitlis, ' The Organic Chemistry of Palladium,'a P. M. Henry, J . Amer. Chem. SOC., 1964, 86, 3246.P. M. Henry, J . Amer. Chem. SOC., 1972, 94, 4437.J. Smidt, W. Hafner, R. Jira, R. Sieber, J. Sedlmeier, andA. Sabel, Angew. Chem. Internat. Edn., 1962, 1, 80.M. N. Vargaftik, I. I. Moiseev, and Ya. K. Syrkin, DokladyAkad. Nauk S.S.S.R., 1962, 147, 399.I. I. Moiseev, M. N. Vargaftik, and Ya. K. Syrkin, DokladyAkad. Nauk S.S.S.R., 1963,153, 140.M. N. Vargaftik, 0. G. Levanda, A. P. Belov, L. M. Zak-harova, and I. I.Moiseev, Kinetika i Kataliz, 1969,10, 1016.K. I. Matveev and N. B. Shitova, Kinetika i Kataliz, 1969,10, 717.P. Francois and Y. Trambouze, Bull. SOC. chim. France,1969, 51.lo R. Jira, J. Sedlmeier, and J. Smidt, Annalen, 1966, 693, 99.l1 I. I. Moiseev, M. N. Vargaftik, S. V. Pestrikov, 0. G.Levanda, T. N. Romanova, and Ya. A. Syrkin, Doklady Akad.Nnuk S.S.S.R., 1966,171, 1365.Academic Press, 1971, vol. 2, ch. 2.methyl ketonesz0 An attempt has been made tocombat the problem of low reactivity by using catalystsolutions of low chloride concentration. As shownbelow, this procedure led to mechanistic complicationswhich partly frustrated the initial objective. Thepresent experiments must therefore be regarded as ofan exploratory nature and represent the preliminaryresults of a continuing study of olefin oxidation inmixed solvents.Since some of the systems describedhere are not likely to be studied in more detail, it seemsappropriate to present the general results of the studyto date.EXPERIMENTALMaterials.-Palladium chloride (Johnson, Matthey) ,olefins (R. Emmanuel), tetrahydrofuran (thf), diglyme[bis(2-methoxyethyl) ether] , and 1,2-dimethoxyethane(B.D.H.) were all used as supplied. The hydroquinone(H2bqn) content of thf was determined by e.m.f. measure-ments as 0.098 g per 100 g, and this quantity was allowedfor in the kinetic experiments. Acetone (Fisons, SLR orAR) was heated under reflux for several hours overK[MnOJ, distilled, re-distilled over Na2C0,, and distilledagain before use.21 Dioxan (Fisons AR) was distilled oversodium immediately before use.Some samples of diglymeand 1,2-dimethoxyethane were distilled over sodiumimmediately before use, but these samples gave resultsidentical to those obtained with untreated materials.p-Benzoquinone (bqn) (Fisons, SLR) was crystallised fromlight petroleum (b.p. 80-100 "C) in the presence of activatedcharcoal and stored in a desiccator in the dark. Sampleswere recrystallised not more than 2 weeks before use. Thepalladium complexes listed in Table 1 were prepared bymethods given in the literature.22* 23 Other complexes usedwere samples prepared by earlier workers in this laboratory.l2 P. M. Henry, J . Amer. Chem. SOC., 1966, 88, 1596.l3 M.N. Vargaftik, I. I. Moiseev, and Ya. A. Syrkin, DokladyAkad. Nauk S.S.S.R., 1961, 139, 3196.l4 H. Okada and H. Hashimoto, Kog. Kag. Zosshi, 1966, 69,2137.l6 H. Okada, T. Noma, Y. Katsuyama, and H. Hashimoto,Bull. Chem. SOC. Japan, 1968,41, 1396.l6 L. M. Zakharova, M. N. Vargaftik, I. I. Moiseev, and L. A.Katsman, Kznetica i Kataliz, 1969, 10, 901; Izvest. Akad. NaukS.S.S.R., 1970, 700.l7 A. P. Belov, I. I. Moiseev, and N. G. Uvarova, Izvest Akad.Nauk S.S.S.R., 1966, 2224.P. M. Henry, J . Org. Chem., 1967,32,2575.I . I. Moiseev, Kinetica i Kataliz, 1970, 11, 342.2o E. W. Stern, Catalysis Rev., 1967,1, 73.21 E. Wedekind and T. Goost, Chem. Be?., 1916, 49, 946.22 M. S. Kharasch, R. C. Seyler, and F. Mayo, J . Amer. Chem.23 W.Kitching, C . J. Moore, and D. Doddrell, Inorg. Chem.,SOL, 1938, 60, 882.1970, 9, 541646 J.C.S. DaltonInstruments.-Varian 1200 and Pye 104 chromatographs(both with flame-ionisation detectors), Unicam SP 800 and500 spectrophotometers, a Perkin-Elmer 475 i.r. spectro-photometer, a Pye conductance bridge (No. 11700), and aPye precision vernier potentiometer (No. 7568) were used.TABLE 1Analyses of palladium complexesAnalyses/ %,P d C H N[PdCl,( NCPh) ,] Found: 27-2 45.1 2.8 7.4Calc. : 27.8 43.8 2-6 7.2[PdBr,(NCPh)J Found: 22.3 35.0 2.3 6.1Calc. : 22.5 35.6 2.1 6-9[PdCl,(NCMe),] Found: 40.9 19.3 2.6 11.2Calc. : 40.6 18.5 2.3 10.8iPdCl,(dmso) 2] Found: 31-7 15.0 5.7Calc. : 31.8 14.4 3.6[(PdCl,(dmso)f,] Found: 41.3 10.9 2.3Calc.: 41.6 9.4 2.4dmso = Dimethyl sulphoxide.E.M.F. Measurements.-Hydroquinone concentrationswere determined by measuring e.m.f. values of a cellconsisting of a cylindrical bright platinum electrode(2.5 x 0.75 cm radius), the sample solution, and a saturatedcalomel electrode (Radiometer K40 1) with a porous-plugjunction between the electrode and the sample. Theelectrodes were held by a Teflon bung in a flask (100 cm3)which was swept with nitrogen saturated with watervapour at the same vapour pressure as that of the sample.All measurements were made at 25.0 f 0.1 "C. Beforeeach measurement was made the assembly was washed withwater and then with a solution of l.OM-NaC1 containingO.OIM-HCI, and drained.* On average the cell took 10 mint o reach equilibrium.Reproducibility of the e.1n.f. valueswas k0.06 mV. Calibrations were carried out underconditions corresponding to those employed in the kineticexperiments and in all cases good straight lines wereobtained from plots of log ([bqn]/[H,bqn]) against e.m.f.Kinetic Expey.ivnents.-Reaction mixtures were preparedby weighing the catalyst and fi-benzoquinone into avolumetric flask (25 or 50 cm3), adding solvent, and bringingthe solution to 25 "C (&-O.l "C). A known quantity ofolefin was then added to start the reaction. Samples (2 or5 cm3) were taken at intervals and quenched by dilution to100 cm3 with 1-OM-NaC1 containing 0-OlM-HC1. Thissolution was then used for the e.m.f. measurement describedabove.Pyoducts.-For the reactions in aqueous dmso andaqueous dmf (NN-dimethylformamide) the amounts ofunreacted olefin and of octan-2-one were determined by ani.r. technique.Reaction mixtures were flooded with waterand extracted with n-hexane. The n-hexane extractswere washed with water, dried over Na,SO,, and made upto known volumes. Absorbances of these solutions weremeasured at 1 620 and 1 730 cm-l in 2 mm sodium chloridecells. Good straight lines were obtained for plots ofabsorbance against concentration in calibration experi-ments with oct-l-ene (absorbance at 1 620 cm-l) and withoctan-2-one (absorbance at 1 730 cm-l). Solutions contain-ing large proportions of ketone could not be analysedaccurately for olefin content. For reactions in aqueousacetone, products were examined by a chromatographictechnique.For experiments in which a high initial con-centration (0.25~) of olefin was used, reaction mixtureswere quenched with aqueous sodium chloride and extractedwith n-hexane. The n-hexane solutioiis were washed withwater, dried over Na,SO,, and mixed with n-nonane asinternal reference. Losses during the extraction procedurewere determined as not more than 7 and 9% for oct-l-eneand octan-2-one respectively. For reaction mixturescontaining a low concentration (less than 0 . 0 1 ~ ) of olefin,n-nonane was added directly to the reaction mixture as aninternal standard and a sample of the reaction mixture wasinjected into the gas chromatograph. Chromatographicanalyses were carried out using either a 5 f t x & in columnof dinonyl phthalate (15%) on 60/100 Embacol with anoven temperature of 130 "C, or a 20 f t x 4 in column ofApiezon L (iOyo) a t 150 "C.Octene isomers were notdistinguishable on these columns. In both cases theresponse of the flame-ionisation detectors for oct- l-ene andoctan-2-one relative to n-nonane was determined bycomparison of peak areas for solutions containing knownamounts of these three compounds. Some gas-chromato-graphic analyses were carried out using capillary columnsto determine the isomeric composition of the olefinic com-ponent of reaction mixtures.RESULTSSolvents and Palladium Complexes.-Conversion of oct- 1 -ene to octan-2-one, as measured by the i.r. techniquedescribed above, was only ca.70% complete after 5 h a t90 "C in aqueous dmso (20% water, v/v) containing p -benzoquinone (bqn). The quinone was unstable both inthese solutions and in similar solutions containing dmf inplace of dmso. Aqueous mixtures containing acetone,thf, diglyme, or 1,Qdimethoxyethane were found to bemore stable, and the oxidation reaction was faster in thesesolvents.The activity of several palladium(I1) complexes wasdetermined in aqueous acetone and in aqueous dioxan.The qualitative results were similar in these two solvents.Complexes were considered to be reactive if, on addition ofoct-l-ene (in the absence of quinone), a palladium pre-cipitate was formed within 1 h at room temperature. Theresults are: t (insoluble) [Pd(NH,),][SO,], K,[Pd(CN),],trans-[Pd(NO,),(NH,),], [Pd(NO,) (NH,),]Cl, [Pd (NO,)-(NHJJBr, and [Pd(en)Cl,] ; (soluble but unreactive)[Pd(NH,),]X, (X = C1, NO,, CN, ClO,, or CCl,CO,), cis-and trans-[PdCI,(NH,) J, trans-[PdCl,( py) J , [Pd(en) ,]C1,,[Pd (bipy) ,] [ClO,] ,, [Pd (apd) C11C1, Pd (edda), Na,[Pd (NO,) ,Iand cis- and trans-Pd(gly),; and (soluble and reactive)PdCl,, PdBr,, PdSO,, Pd(NO,),, Pd(C10,), (aq), cis-[Pd(NH3),(OH,),][C10,], (aq), M,[PdCl,] (M = I(, x a ,NH,, or NEt,), [PdC1,L2] (L = PhCN, MeCN, or dmso),K,[PdBr,], Na,[Pd(NO,),], and [{ PdCl,(dmso)},]. Fromthe last group, bis(benzonitrile)dichloropalladium(II) wasselected for further tests.In each case the solvent was amixture of one volume of water and four volumes of eitheracetone or one of the ethers listed above.Samples ofsolutions containing bqn and H,bqn were analysed for thelatter as in the kinetic experiments. Experiments werecarried out on similar solutions that contained in addition(a) oct-l-ene, (b) octan-2-one, (c) [PdCl,(NCPh),], and(d) octan-Zone and the palladium complex together. Inall cases the concentration of H,bqn remaining unchangedfor 3-5 h at 25 "C.* IM = 1 mol dm-3. t en = Ethylenediamine, py = pyridine, bipy = 2,2'-bipy-ridine, apd = 3-azapentane- l,Ei-diamine, edda = ethylenedi-imino-NN'-diacetato, and gly = glycinato1975 647Specbvn.-Spectra (u.v. and visible) of solutions of[PdCl,(NCPh)J in the 20% v/v aqueous solvents weresimilar (Amax. 4 1 G 4 2 2 nm, E,,, 210-290 1 mol-l cm-l).Solutions of PdCl,, [PdCl,(NCMe),], and [PdCl,(NCPh),]gave similar spectra in a given solvent.Only the solutionsin aqueous acetone showed significant changes with time.In this solvent, with 10-3iu-[PdC12(NCPh) J, the absorptionmaximum moved from 425 ( E 260) to 384 nm ( E 10601 mol-1 cm-l) in 7 11 at 25 "C. Addition of perchloric acidTABLE 2Initial gradients of plots of [H,bqn] against t for theoct-1-ene + bqn + H,O '-+c octan-2-one + H,bqnreaction (at 25 "C) :(a) [PdCl,(NCPh),] = 2.0 x 1 0 - 3 ~1,2-Dimethoxy-Dioxan 0 Diglyme ethane thf Acetone 6[01]0c vjd [01]0 z'i [01]0 vi [0130 ui [ol], vi1-83 1-7 2.92 4.4 3.86 2.8 3.99 4.5 0.96 0.421-99 1.6 3.91 4.8 8-31 6.8 7-81 6.8 1.33 0.652.61 2.2 7.73 7.1 12.6 8.3 11.4 7.8 1.47 1.073.78 3.5 14.5 9.7 17.6 9.3 14.0 8.0 2.25 1-453.78 3.5 21.3 10.3 22.6 9.0 16.4 8.5 3.81 2.033.86 2.9 25.3 10.5 23.3 9-7 4.87 2.87.90 4.8 7-12 2.711.1 5.5 12.8 4.014.7 6.020.9 6.5( b )Solvent [olj , 1 O3 [PdCI,- ViDioxan 8.0 1.01, 2*00,4*18 2*5,4*8, 9.1thf 14.0 1.03, 2*01,4.21 5.1,8.3, 18.0Dig1 yme 12.0 1*02,2*01,2*98, 3.5, 8.9, 14.5, 22.0Acetone b 3.8 0.49, 0.99, 2-00, 0.33, 1.0, 1.5, 3.3,(NCW 21 I M4.162.95, 3.52, 3.91 3*3,2.79-89, 11-7, 14.7,16.6, 19.9(G) [0110 = 3.8, [PdCl,(NCPh),] = [PdCl,(NCMe),] = 2.0 x247 1.79, 4*11,6*11, 12,40, 70, 130, 160,220, 310, 3701 0-3MVi8f -l Ageing period/h [PdCl,( NCPh) 2] [PdCl,( NCRSe)0.25 8.8 9.50.50 8-2 9.51.0 4.7 5.71.5 3.2 3.52.5 1.7 1.54.0 1.5 1.5(d) C01-j~ = 3.75, [PdCl,(NCPh),] =6 x 10-2~ in acetoneb1 03[NaC1] /hi00.150.501.001.502.502.0 x lo3, and LNaClO,] =Vi1.61.91-851-351.20.75a In all cases the solvent named is the organic component of'Aged ' solutions.[olJo = lo3 x Stoicheiometric concentration of oct-l-ene/M.a mixture containing 20% water (v/v).d 2'.- lo6 x Initial gradient/mol 1-1 s-1.to solutions of [PdCl,(ISCPh),] in the 20% v/v aqueoussolvents resulted in no changes in the initial spectra, exceptfor a small reduction in absorbance at all wavelengthsbelow 470 nm in the case of the 1,2-dimethoxyethanesolutions. In the aqueous-acetone solutions the intensitiesof the maxima in the spectra of the final solutions decreasedwith increasing perchloric acid concentration.Conductivity .-Conductivities of solutions of CPdC1,-(NCPh),) in 20% aqueous dioxan were not significantlydifferent from that of the solvent.A solution of thecomplex (0.001~) in aqueous diglyme had a conductivity4.9 x 10-5 S cm-1 at 25 "C compared with 2.9 xS cm-1 for the solvent. In both solvents the conductivityincreased only slightly with time, In aqueous acetonethe changes in conductivity with time were much moremarked and were dependent on the concentrations of[PdCl,(NCPh),] and quinone present. In the absence ofquinone, plots of conductivity against time for thesesolutions were sigmoid and the initial gradients increasedlinearly with the concentration of palladium complex.For a solution of given composition the time taken for theconductivity to reach a steady value was similar to thatrequired for the spectrum of the solution to assume itsfinal form.Kinetic i'l/l7easurements.-The primary results of thekinetic measurements gave the variation of H,bqn con-centration with time.From these data the initial gradients- 010 20t I min3xidation of oct-1-ene in aqueous acetone a t 25 "C and [catalyst]= 2.0 x 10%: (A) and (H), runs with [PdCl,(NCPh),] ascatalyst; (O), points obtained after adding a further quantityof oct-1-ene to the final reaction mixture from the run represen-ted by (A);] (e), [PdCl,(NCMe)d as catalystshown in Table 2 were determined (vi = (d[H,bqn]/dt)t-,o',.Similar results were obtained from experiments withhex- 1-ene, hept-1-ene, and dodec-1-ene, but, under com-parable conditions, showed a general decrease in reactivitywith increasing molecular weight.Solutions of [PdCl,(NCPh),] and of [PdCl,(NCMe),] inaqueous acetone showed an ageing ' effect.Initial ratesof reaction of oct-1-ene in these solutions varied with thetime that elapsed between preparing the solution of thecomplex and adding the olefin [Table 2(c)]. The ' ageing 'effect decreased and became negligible as the initial olefinconcentration was increased to ca. 0.25~. In cases wherethe ' ageing ' effect was significant [apart from the experi-ments listed in Table 2(c)], aqueous-acetone solutions of thepalladium complex and quinone were allowed to stand fora t least 3 h before adding olefin to start the reaction. Thereproducibility of individual runs is illustrated in theFigure, which also shows that the benzonitrile and aceto-nitrile complexes have the same catalytic activity [cf.also Table 2(c)].The results shown in the Figure alsodemonstrate that solutions of [PdCl,(NCPh),] retain th648 J.C.S. Daltonsame catalytic activity a t the end of a reaction as theypossessed a t the beginning.Some other factors were investigated briefly for thereactions of oct-1-ene in aqueous acetone. Benzonitrile(0-004-0.007~), sodium perchlorate and perchloric acid(each up to O - O ~ M ) , and the addition of up to 4% by volumeof n-nonane, octan-2-one, or benzene were all found tohave negligible effects on the course of the reactions.Addition of sodium chloride, up to ca.2-5 x lO-4~, in-creased the initial rate slightly. The initial rates decreasedas the chloride concentration was increased beyond thisvalue [Table 2 ( d ) ] . The complexes [PdCl,(dmso),] and[( PdCl,(dmso) ),I are somewhat less active catalysts thanthe benzonitrile complex. An attempt was made tocompare the catalytic activities of PdCl, and [PdCl,(NCPh),].However, it was found necessary to heat PdCl, underreflux with acetone for several hours in order to obtainsufficiently concentrated solutions. Initial rates of re-action of oct-1-ene were smaller when these solutions wereused in preparing reaction mixtures than for comparablesolutions of [PdCl,(NCPh) ,]. When the benzonitrilecomplex was heated under reflux in acetone prior to useits activity was reduced and the two palladium complexeswere then equally active as catalysts.Reaction Products.-Product analyses (using the chro-matographic techniques) were carried out on a number ofreaction mixtures from the oxidation of oct-1-ene inaqueous acetone with [PdCl,(NCPh),] as catalyst.In allcases octan-%one was the only oxidation product detected.With concentrations of olefin below 0.01111, reaction mixturesleft overnight were found to contain (within experimentalerror) quantitative yields of octan-%one. In experimentswith high olefin concentration (ca. 0 . 2 5 ~ ) analyses werecarried out with samples taken after intervals of up to2.5 h. In these cases the percentage completion of reactionwas calculated from the amount of octan-%one formed andfrom the amount of olefin (mixed isomers) remaining.Values agreed within experimental error.In a few casescomparisons were made between the extent of reaction asdetermined by the chromatographic technique and bye.m.f. measurements. Agreement was also found in thesecases. The unreacted olefin was examined in some mixturesafter ca. 90% conversion to octan-%one had occurred. Inthese cases 6-8% of the original olefin had isomerised.DISCUSSIONIt has frequently been observed 2o that oxidation of aterminal olefin under conditions similar to those usedhere results in formation of the corresponding methylketone as main product. Our results of productanalyses are, therefore, as might have been expected,and we assume that the stoicheiometry of the oxidationsreported in the present work niay be generally re-presented as in (1) where 01 = a terminal olefin.Thus,01 + bqn + H,O+ methyl ketone + H2bqn (1)during the course of a particular reaction, the concen-trations of chloride ion, hydrogen ion, and palladiumcomplex remain constant.Moiseev et ~ ~ 1 . 5 9 ~ 9 ~ ~ 9 ~ ~ and Henry 2,12 carried out kineticstudies on the oxidation of olefins of low molecularweight under conditions that in many cases gave first-order rate constants for individual runs. Our methodof following the reaction was essentially that of Vargaftiget aL5 but our reaction conditions differ from those usedin earlier kinetic studies in two mair, respects: the natureof the solvent, and the total chloride Concentration.These conditions, a high proportion of organic componentin the solvent and low chloride concentration, weredictated by the low solubilities and low reactivities ofthe olefins used here compared to those of ethyleneand others of low molecular weight.Under our con-ditions the results for individual runs did not fit a rateequation of simple order, and the present discussion istherefore limited to considering the values of the initialgradients given in Table 2. These values show that,within the experimental error of measuring initialgradients, (a) for a given solvent the values of 'iri approacha constant value as the stoicheionietric concentration ofolefin is increased {this limiting value is about the samefor diglyme, l,e-dimethoxyethane, and thf, but isslightly lower for dioxan and lower still for acetone[Table 2(a)]>, (b) initial rates show an approximatelylinear dependence on the palladium(I1) concentration,except in ' aged ' acetone solutions at high olefin con-centration [Table 2(b)] , and (c) the catalytic activity of afreshly prepared solution of [PdCl,(NCPh),] in acetoneis 6-7 times greater than that of an ' aged ' solution,i.e.one used several hours after preparation [Table 2(c)].Earlier kinetic studies 2-10 have shown that themechanism for oxidation of olefins with palladium( 11)catalysts probably involves rapid formation of a Z-complex of the olefin and its slow decomposition to givea ketone and Pdo. A PdII species may be regeneratedfrom the latter in the presence of a suitable oxidisingagent.With bqn as oxidant the mechanistic schememay be represented in outline as below:K k[PdII] + 01 __ [Pd(ol)] __+ [PdO] + ketonebqnFastIn this scheme [PdII] is a general representation of thecatalytic species, K is a composite constant involvingequilibrium constants for formation of the n-complexes,[Pd(ol)], and K is a composite rate constant. [PdO]Represents the reduced form of the catalyst. Its exactnature is not known, but it has only transient existenceand is rapidly oxidised by the quinone, bqn. The abovescheme leads to rate equation (2) where [ol] = concen-(2) d[H,bqnl - k~[oll[Pdlodt - (1 + J W I )tration of free olefin and [Pd], = stoicheiometricconcentration of catalyst.With low catalyst concen-trations, the value of [ol] at the beginning of a reactionis approximately equal to the stoicheiometric concen-tration of olefin. Thus, the variations in 'iri shown inTable 2(a) and (b) are largely consistent with the kindof behaviour predicted by equation (2). Similar resultsChem. SOC., 1974, 96, 1003.24 I. I. hloiseev, 0. G. Levanda, andM. N. Vargaftig, J . Amev1975(details not given here) with hex-l-ene and dodec-l-enein aqueous 1 ,Zdimethoxyethane show the expectedtrend of decreasing rates with increasing molecularweight of the olefin.To account for the detailed behaviour in individualruns it is necessary to obtain a suitable integrated formof equation (2). This is difficult because such formscontain the concentration of free olefin as a variable andthis quantity is not known from our measurements,except as a rather complicated function of the constantK .A more detailed analysis of the kinetic results istherefore a lengthy process 25 and, although it providesfurther support for equation (2), cannot convenientlybe reproduced here. A consideration of the detailedmechanism of these reactions would also require someknowledge of the equilibria among the palladiumspecies in solution. This is likely to be a complicatedsituation (probably involving several aquachloro-ionsand, possibly, dimeric forms) and has led to contro-versy3y24 even in the case of the reaction of ethylene inaqueous solution at high chloride concentration.Wetherefore defer discussion of these problems until ourcurrent work is more advanced. However, severalgeneral points emerge from considering the results ofthe present exploratory survey.Eflects of Solveizt Changes.-There are three ways inwhich the solvent might influence the present reactions.(i) By reaction with the substrate, thus determiningthe nature of the product and appearing in the stoicheio-metric equation for the overall reaction. In this respectthe high water content dominates and is expected tolead to the same product in all the solvents used. (ii) Byreaction with the catalyst, thus affecting the stoicheio-metric mechanism. (iG) By solvation and other' medium' effects, which are found in all reactions insolution and are generally less well defined than theeffects described under (i) and (ii).The reactions in aqueous-ether solvents were freefrom ' ageing ' effects and the general kinetic behaviourwas similar in all these solvents, suggesting that' medium ' effects may be responsible for any differences.For example, the slightly lower limiting rates in aqueousdioxan compared with the other ethers may be associatedwith its lower dielectric constant.(At 25 O C , 20%aqueous dioxan has E, = 10.7, whereas 20% aqueoussolutions of the other ethers have E , values in the range17-4-19.9.) On the other hand, the kinetic ' ageing 'effect in aqueous acetone [Table 2(c)] and the spectro-scopic and conductivity changes occurring in aqueous-acetone solutions of palladium (11) complexes overperiods similar to those required for ' ageing ' suggestthat influences of the type (ii), above, may be operativein this solvent.Complex formation between solventand catalyst probably accounts also for the markedreduction in rates of olefin oxidation observed in aqueousdmso and dmf.Efects of Various Catalysts.-The two complexes[ PdCl, (NCPh) ,] and [ PdC1, (NCMe) ,] have identicalcatalytic effects (see Figure) and may be considered asconvenient sources of readily soluble PdCl,, yieldingaquachloro- or chlorohydroxo-complexes on displace-ment of the nitrile ligands. From the qualitativeexperiments with several palladium(I1) catalysts (seeResults section), it appears that all the complexesclassified as ' reactive ' have in common that theywould all be expected to yield significant concentrationsof aqua-ions under the conditions used, whereas the'unreactive' group would be expected to be stable toaquation.These results are consistent with the viewthat a co-ordinated water molecule (or hydroxide ion)is required in an active catalytic complex.Eflect of Oxidant.-When all the bqn has been con-sumed in a particular run the catalyst is reduced by theolefin, and palladium metal precipitates. However, inour runs the oxidation reaction still proceeds withoutprecipitation of the catalyst even when the ratio[H,bqn] : [bqn] is quite high. The reduction potentialof the system bqn + 2Hi + 2e- __ H,bqn must bequite low under these conditions (eg. even with[H,bqn] : [bqn] = 1 : 1, the calculated value a t pH 7 isonly 0.29 V) and certainly considerably lower than thevalue required before oxidation of palladium metal toany aquachloro-complex should occur to any appreciableextent in, say, 1 0 - 3 ~ solution. This suggests thatelectron transfer occurs before breakdown of thepalladium-olefin complex to Pdo has gone very far.The results shown in the Figure demonstrate that thepalladium species retains its initial reactivity when usedagain for a second run. Thus, once the ' ageing ' periodis over, equilibria among the catalytic species appear toremain unchanged in the course of a run.We are currently studying some reactions similar tothose described above but under conditions such thatthe concentration of free olefin remains constant. Themore tractable kinetic equations obtained under theseconditions should, hopefully, lead to a better under-standing of the details of the mechanism.We thank Professor V. Gold for the initial impetus andfor his interest and advice in the continuation of this work,Mr. B. Fowler of the Esso Research Centre, Abingdon, forhis co-operation and guidance, Mr. R. D. Cole (EssoResearch Centre) for the chromatographic analyses withcapillary columns, the S.R.C. for a CAPS award (toJ. B. J. U.), and a referee for helpful comments that led tomodification of the Discussion section.[3/1500 Received, 17th July, 1973125 J. B. J. Unsworth, Ph.D. Thesis, London University, 1972
ISSN:1477-9226
DOI:10.1039/DT9750000645
出版商:RSC
年代:1975
数据来源: RSC