H NMR Study of The Mechanism of Ethyleneglycol Monoacetate Formation in Oxidative Acetoxylation of Ethylene Catalyzed by Pd(II) Complexes

G. Centi and F. Trifiro' (Editors), New Developments in Sekctive Oxidation 0 1990 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

213

'H NMR STUDY OF THE MECHANISM OF P T H Y L E ~ G L Y C O LMONOACETATE FORMATION IN OXIDATIVE ACETOXYLATION OF ETHYLENE CATALYZED BY Pd(I1) COMPLEXES E.V. K.I.

GUSEVSKAYA, 1.E. ZAMARAEV

BECK, A.G.

STEPANOVj V.A.

LIKHOLOBOV a d

Institute of Catalysie, Novosibirsk 630090, USSR SUMMARY A detailed mechanism of ethylene oxidation by Pd(N0 )ClL complexes (nt2,3; LPCD CN) in a chloroform-acetic acid %ixt&e is studied by 1H N d R spectroscopy. The end reaction products are ethyleneglycol monoacetate (EGMA), aueteldehyde, nitroethylene and com ounds with the general formula CH3-CHXY (X,Y u OH, OAc, C1, NO whoee ratio depends upon the solvent composition. Kinet?c and speotral data obtained indicate the formation of a number of intermediates. The structure and route8 of decomposition of the intermediates to EGMA and other reaction products are suggested.

p,

INTRODUCTION Oxidation of olefins catalyzed by Pd(I1) complexes is a rapidly developing trend in selective synthesis of oxygen-containing organic compounds. The main product of d -olefins oxidation in acetic acid solutions containing ealts of nitric acid and palladium(I1) is glycol monoacetate, while in the absence of nitrate ions carbonyl compounds and vinyl ethers are formed (ref. 1). The mechanism of formation of glycol monoacetates from &-olefins has been studied for Pd(OAc)@iN03/HOAc (ref. 2) and Pd(N02)C1(CH3CP3)2/HOAc (ref. 3) systems using lithium nitrate or nitro ligands in Pd(I1) complexes labelled by heavy isotopes of oxygen. It has been established that the resulting glycol monoacetate contains labelled oxygen in the carbonyl position of the acetate group. Based on data obtained on distribution of labelled oxygen in reaction products, the authors have suggested the mechanism of glycol monoacetate formation; however, the structure of intermediates has not been confirmed by spectroscopy. The objective of this work was to etudy the mechanism of ethylene oxidation by Pd(I?On)C1L2 complexes in chloroform-acetic acid solution by 1 H IrJIy[R spectroscopy.

214

METHODS Pd(NOn)C1L2 complexes were prepared as in (ref. 4 ) . ‘H lyMR spectra were recorded using a Bruker CXP-300 spectrometer with a magnetic field induction of 7 T. Chemical shifts of signals were measured with respect to the internal reference hexamethyldisiloxane. The temperature of samples was continuously monitored with a precision of l o by a W-1000 thermocouple. In all experiments, the concentration of palladium in solution was 2 ~ 1 Ml”; 0 ~ ~5tlO mol o f ethylene per palladium i o n being introduced into the solution of complex. CDC13 and CD3COOD(DOAc) were used as solvents. RESULTS AND DISCUSSION

Addition of ethylene (I) to solution8 of Pd(HOn)C1L2 complexes in chloroform-acetic acid medium (content of DOAc is O-lo%) gives rise to the appearance of several lines in the NMR spectra. Analysis of the change8 In the line intensities with reaction time permitted us to isolate groups of lines, whose inteneities varied in the same manner and that could,for this reason,be attributed to the same compounds. For this purpose the parameters of J(H-H) of multiplet lines were also used. Reaction products Seven groups of lines that do not disappear for a long period of time can be assigned to end reaction products whose ratio depends upon concentration of DOAc in solution. Acetaldehyde (11) ( 8 2.17 pprn (d), = 9.73 ppm (qd)) is the main product (95-97% per reacted olefin) of the reaction in chloroform; its yield tends t o decrease with increasing concentration of DOAc in solution. During ethylene oxidation in chloroform nitroethylene (111) 8 7.14 pprn (dd)) is ( 8 = 5.91 ppm (dd), s = 6.65 ppm (dd), accumulated (up to 5%) with a long induction period; in the presence of DOAc nitroethylene is formed in trace amounts, In solutions containing DOAc one of the products of ethylene oxidation ie EGMA (IV) ( 6 P 3.77 ppm (m), 8 = 4.14 ppm (m)), in glacial DOAc the yield of E G U ie 95-97%. In the range of DOAc concentrations 2-20 ~01.4% ethylene oxidation gives rise to the formation of compounds V - V I I I (total yield up to 45$), whose NMR spectra are similar in line structures and positions ( 6 m 1.35-1.71 ppm (d) and a 6.46-6.97 ppm (qd) with intensity ratio 3 : l ) . Analysis of M6R spectra of comP

-

215

pounds V-VIII and peculiarities of their accumulation in solution permitted us to suggest the following compoeition for theee products: CH CH(OAc12 (V), CH3CH(OAc)(OH) (VI), CH3CH(OAc)(Cl) (VII) 3 and CH3CH(OA~)(N0,) (VIII). Intermediates Based on the initial increase and subsequent decrease of their intensities with time the groups of linee IX-XVI (see table 1) seem to belong to intermediatea formed during the reaction. As a result of 'H NMR studies of the kinetics of ethylene oxidation by Pd(NOn)C1L2 complexes at various concentrations of DOAc in chloroform, we have registered intermediatea that may be responsible f o r the formation of observed reaction producte. The maximum observed line intensities of the intermediatee formed during ethylene oxidation in CDC13 solutions with different concentrations of DOAc are shown in Fig. 1 f o r Pd(N03)C1L2 and in Fig. 2 for Pd(N02)C1L2. An analysis of 1H IWdR spectra of intermediates and kinetic curves of accumulation-decomposition o f the intermediates and end products at various concentrations of DOAc allowed us t o suggest the structures of compounds IX-XVI (table 1) as well ae the possible routes of their formation and decomposition. Mechanism o f ethylene oxidation Palladium complexes with NO2 ligande in chloroform solutions exist as two isomers: Pd(ONO)C1L2 (complex A) and Pd(N02)C1L2 (complex B) (ref. 5); in the presence of DOAc Pd(OAc)C1L2 oomplex C) may be aleo formed. Then it is reasonable to suggeet that in the first step of ethylene oxidation displacement of the neutral liganda from complexee A,B,C and Pd(N03)C1L2 (complex D) and formation o f the corresponding SE-olefin complexes of palladium A, 2, C and 2 take place. Due to insertion of coordinated ethylene into Pd-0 bonds in complexes A, 2 and 2 and into the Pd-N bond in complex l3, organopalladium intermediates XII, XI, IX and XIII, reepectively, are formed (table 1). A wide variety of ethylene oxidation products is determined by the step of decompoaition of organometallic compounds IX, XI-XI11 Pd-CH2 CH2Z. The transformation of these key intermediatea depend on the nature of subetituent 2, ligands in the palladium complex and solvent composition. Based on the results of IR and NMR spectroscopy studies on the mechanism of ethylene oxidation by Pd(I1) complexes ( f o r chloroform solution8 the reeults have been

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216

25

0

*

Fig. 1. D’isxlmum observed intensities of lines of intermediates registered during ethylene interaction with Pd(N03)C1L2 VS. solvent compoeition at 295 K O

I, %

I,%

25

75 I

&

50 I

0

25 I

% CDCL3

125

XVI

xv

7

*

Fig, 2. hbximum observed intensities of NMR lines of intermediates registered during ethylene interaction with Pd(N02)C1L2 vs. solvent composition at 295 K. the toLine intensities in spectra are given per one ial quantity of reacted (during observation timeproton; ethylene is taken as 100%. The yield of products is also given per reacted ethylene

.

217

TABLE 1

Characteristics o f 'H NMR spectra lines attributed to reaction intermediates and their propoeed structuree Corn- Line struc- 8(ppm) J(H-H) (He) pound ture

IX X

a triplet b triplet a triplet

b triplet

XI

a triplet b triplet

IntenProposed structure sity rati0 8 b I

1.61

7.3

4.30

7.3

I

1.56 3.72

6.2 6.2

I I

1.67 3.92

a broad line 2.373.25 b triplet 4.054.3 XI11 a triplet 1.08 b triplet 4.22 XI1

6.8 6.8

a ' d P '

/

I I

-

I

6.3

I

/

/

Pd

I

7.2

I

/

,pd\ \

/

,pd\

XIV

a doublet

b triplet XV*

2.34 9.56

3.5 3.5

2

I

a multiplet 4.29

I

4.89 b broad multiplet

I

OAc

a

b

a

b

CH2-YH2 0n0

9-FH2 N02 a

H

,Pd\ \

b

CHrF%

'

\

7.2

OH

-\

a \

b

CH2-FH2

CHz-C,

/

&0

H b

8

-H3c XVI

a triplet

b triplet

* The spectrum typical for a four-spin system with JAA, = Jm JAB, = JAtB JAtB, 4.55 H z ~

1.4 Hz; JBB, = 3.1 HI;

0

3

31

218

published in (refs, 5 , 6 ) ) we propose the following possible mechanism of the formation of 1 , l - (i.e. containing an ethylidene fragment) and 1,2-additlon products. 1.2-Addition Droducts. D u r i n g ethylene oxidation by Pd(N02)C1L2 EGMA seems to form at least by three parallel routes via key intermediate E:

5%'

CH -CHz

a 0 A c

L

2 0 \O

IV Organometallic intermediate E may be formed: from a-nitritoethylpalladium complex XI1 via reesterification-byacetic acid (route 1); by heterolysis of the Pd-C bond in J-nitroethylpalladium complex XI11 under the influence of DOAc resulting in 1nitro-2-acetoxyethane XY followed by oxidative addition of the Pd(0) complex to the C-M bond in intermediate XV (route 2) and by direct acetoxypalladation of ethylene in palladium complex with nitro ligand (route 3 ) . Then intramolecular rearrangement of intermediate E leads to the Pd(I1) complex with a hydroxyalkyl ligand and acetylnitrite XVI. Decomposition of complex XVI to form EGMA and nitrosyl complex of Pd(I1) by heterolyeis of the Pd-C bond under the action of the coordinated molecule of acetylnitrite. It should be noted, that the mechanism proposed here is consistent with stereochemical data, labeling studies and the regiochemistry observed in (refs, 2,3,7). It has been established that during ethylene oxidation by Pd(N03)ClL2 EGW is formed directly from the ethylene nitrate

219

complex of Pd(I1). The mechaniem of interaction of the ethylene nitratopalladation product IX with DOAc 8eme to be sfmllar to r . 2 for the nitrite eyetern. Although in the nitrate eyetern the EGMA formation intermediate analogoue to XV nae not found probably due to it6 high reactivity, intermediate X (analogous to XVI), which might contain aoetylnitrate a8 8 possible ligand, was registered. Heterolgeie of the Pd-C bond in the complex X under the action of the coordinated acetylnitrate molecule yields EGIYUL. Unlike acetylnitrite, acetylnitrate can easily be dieplaced from the palladium complex followed by decompoeition of the ethylene oxypalladation product into 1,l-addition produats (mainly acetaldehyde) by 4 -hydrlde elimination. 1.1-addition products. me products of 1,l-addition (acetaldehyde and CH3CHXY) seem to form during the decomposition of or-nopalladium intermediatee IX-XI11 via the following eeheme: (a) reversible 6 - 3 -rearrangement of complexee IX-XIII; (b) 8Z 6-transformation of hydridepalladiumolefin oomplexee via the attack of coolrliaated vinyl ether by the nualeophile 1 leading to the formation of either regietered intermediate XIV (aoetaldehyde preouraor) or complex 0 (preoureor of 0 5 C H X Y producte); (c) decomposition of hydride complexes XIV and 0 via reductive elimination producing acetaldehyde snd compounds Y-VIII:

-

I/

t

R =NO?(complex H (complex

NO [ c o m p l e x CI

X

= OAC.

X

CH, -CH \

L/

Pd /

-

2-

?L

R= OH ( c o m p l e x OAc(camp1ex NO2 [ c o m p l e x

X = OAC, C I

L/

b

L'

h

L /

b

IX), XI, XII)

-L

-4

L -

XI, XI).

m)

CH3CHRX

V-Vm

+ PdoL2

XIV

220

During the formation of acetaldehyde the Pd(0) complexes are oxidized by nitroxgl or nltroayl chloride (or by the corresponding acetates); in the other cases the palladium black is formed, along with the products of 1,l-addition. In chloroform, during the decomposition of intermediate XI11 nitroethylene is formed, as ha8 been deacribed by us in (ref. 5).

REFERENCES P.M. Henry, Palladium-catalyzed oxidation of hydrocarbons, Reidel, Dordrecht, 1980, p. 99. V.A. Likholobov, N . I . Kuznetsova, M.A. Fedotov, Yu.A. Lokhov and Yu.1. Y e m k o v , Interaction between oxidants and olefine in solutions containing palladium complexes, in: 6th Nat. Symp. Recent Advances in Catalyeie and Catalytic Reaction Engineering, Pune, India, 1983, pp. 217-228. F. Maree, S.E, Diamond, F.J. Regina and J.P. Solar, Bomnation of glycol monoacetates in the oxidation of olefine catalyzed by metal nitro complexes: mono- VS. bimetallic system, J. Am. Chem. SOC., 107 (1985) 3545-3552. I,E. Beck, E.V. Gusevskaya, V.A. Likholobov and Yu.1. Yermakov, Synthesis of Pd(I1) nitro and nitrate complexes and studies of their reactivity towards oxidation of olefins in organic solvents, React. Xinet. Catal. Lett., 33 (1987) 209-214. E.V. Gusevskaya, I.B. Beck, A.C. Stepanov, V.A. Likholobov, V.M. Nekipelov, Yu.1. Yenaakov and K.I. Zamaraev, Study on the meohanism of ethylene oxidation by a nitrite com lex of palladium in chloroform medium, J. Molec. Catal., 37 f1986) 177-188. I.E. Beck, E.V. Gusevskaya, A.G. Stepanov, V.A. Likholobov, V.M. Nekipelov, Yu.1. Yermakov and K . I . Zamaraev, Study of the mechanlem of ethylene oxidation by palladium(I1) complexes containing nitro and/or nitrato ligande in chloroform, J. Molec. Catsl., 50 (1989) 167-179. Jan-L. Backvsll and A. Henmnnn, A cromment on the recently proposed mechaniem f o r the oxidation of olefins with J. Am. Chem. Soc., 108 (1986) 7107-7108. PdCl(HO,)(CH-,CN),,