Electrophilic transformations induced by heteropoly acids: applications and structural studies

C. R. Acad. Sci. Paris, t. 1, Skrie II c, p. 381-396, 1998 Chimie des surfaces et catalyse/Surface chemistry and catalysis

Electrophilic transformations induced by heteropoly acids: applications and structural studies+ BBa ToRoK”*,

h.rpzid MOLNhRb

’ Organic Catalysis Research Group of the Hungarian Academy of Sciences, J&.sefAttila University, H-6720 t6r 8, Hungary E-mail: [email protected] I’ Department of Organic Chemistry. J6zscfAtrila University, H-6720 Szeged, IXm t@r8, Hungary

Szeged,

IXn

(Received 14 February 1997, accepred 27 March 1938)

Abstract - The transformations ofoxygen-containing compounds and hydrocarbons were studied in the presence of heteropoly acids (H,[SiM o,~O~~~], H,]PMo,~O~~~], H#iW,2040] and H3[PW,,0,~0]), and a salt (CS,,,H,,~]PV(‘,,O~,]). First, the reactions of oxygen-containing compounds were studied including the transformations of monohydroxy compounds (dehydration, derivatization), aliphatic and diary1 dials (dehydration, rearrangements), and epoxides (ring opening, isomerization). Reactions of hydrocarbons were also investigated, with emphasis on alkylation reactions. As a main conclusion, it was found that seleccivities strongly depend on the acid strength of the catalyst and the structure of the organic substrates. In some casf~ the products were isolated in excellent yield and selectivity to open new applications for organic syntheses and fine chemical production. The catalysts were also studied by spectroscopic (UV-VIS, FT-IR, ESR) and thermoanalytical (DSC) methods. 0 AcadCmie des Sciences/Elsevicr, Paris heteropoly dehydration

acid

I electrophilic

/ rearrangement

reaction

I oxygen-containing

compound

I hydrocarbon

I protection

I

/ akylation

RCsumC - Transformations Clectrophiles induites par des hktkropolyacides : applications et Ctudes structurales. Nous avons &udiP les transformations de composts contenant de l’oxyg&ne et d’hydrocarbures en presence des h&tropolyacides : (Hd[SiMo,2040], H,[PM oIrO~J, H4~SiW,,0401 et H,3[I’W,20,,J et d’un se1 CS,,~H” j[PW,,O,,,]. Ces transformations ont et& Ctudiees sur les composts oxyg&Gs suivants : des compost% monohydroxylis (dishydratation, dCrivation), des diols aliphatiques et diaryI& (dkshydratation et r&arrangements) et des ipoxydes (ouverture du cycle, isom&isation). Des rtactions sur des hydrocarbures ont aussi it6 ten&es, en mettant I’accent sur les rCactions d’alkylation. En conclusion, nous avons trouv6 que les sPlectivit& dtpendent essentiellement de I’acidite du catalyseur et de la structure des substrats organiques. 1)ans certains cas, l’excellent rendement et la sClectiviti des produits isol& ouvrent de nouvelles perspectives g la synth&c organique et g la chimie fine. Les catalyseurs ont Ctt anal+ par des mkthodes spectroscopiques (UV-VIS, FT-IR, ESR) et thermoanalytiques (DSC). (R’.esumtl r6digt: par la redaction) 0 Academic des Sciences/Elsevier, Paris hCtCropolyacide rtkwrangement

Communicated

/ rt!action / alkylation

by Fran+

Clectrophile

/ composk

oxygCnC

I protection

/ ddshydratation

/

MATHEY.

* Corrcspondencc and reprints. j Presenred at thr symposium on Rmwt Progreu on /‘ot$mwxctaht~ 1251~8069/98/00010381

/ hydrocarbure

0 AcadCmie

des Sciences/Elsevier,

Paris

Chemistry

Paris, I 396

381

B. Tiiriik,

A. Mohir

Version franqaise

abrkgke

un grand inttret s’est port6 sur la synInduits par les problPmes de pollution de I’environnement, th&se de nouveaux catalyseurs acides utilisables en catalyse homogtne. En effet se posent par exemple les remplacements de HF et H,SO, dans un certain nombre de processus industriels. Les h&tropolyacides, de par leur stabilitC et leur comportement catalytique, ont deja Ptt introduits sur le plan industriel en pttrochimie au Japon. Les hCt&opolyoxoanions acides catalysent I’hydratation des al& nes, la polym&isation du tttrahydrofuranne, l’oxydation de la methacroltine en acide m&hacrylique. Nous avons utilise Hq[SiW,,O,,,], H:,[PMo,~O~~)], H,[PW lIO’tO] et CS~,~H~~,~[PW,~O~~] dans les transformations de composts oxygCn& essentiellement dans les r&actions d’Climination de groupes alcooliques. La dishydratation de compos& renfermant un seul goupe hydroxo (C,H, ,OH) conduit B l’oltfine correspondante ou B un milange d’alcknes avec un tres bon rendement. La rtaction de protection des groupes OH par la reaction de tCtrahydropyrannylation fonctionne en catalyse acide par les htdropolyanions en phase homogkne et conditions deuces. Notons que la rCaction inverse de dPprotection peut, elle aussi, ?tre catalysCe par les m@mes catalyseurs. En ce qui conClectrophile en prtsence cerne les alcools aliphatiques ou les diols 1,3-, leur deshydratation d’h&ropolyacides conduit i des transformations compliquees avec formation de di&nes, d’alcools satur& ou insaturCs et de carbonyles. En revanche, la dtshydratation des diols 1,2- est plus stlective. Le 2,3-butanediol conduit h la 2-butanone avec une excellentc sPlectivitP en catalyse supporde. Les homologues supCrieurs (1,4-, I ,5-, I ,6-diols) peuvent ctre d&hydratCs aussi bien en milieu homogtne qu’htttrogtne pour former les &hers cycliques correspondants avec un bon rendement et une excellente s?lectivit&. L’alkylation de d&iv& aromatiques par catalyse acide selon la rkaction de Friedel-Crafts est l’une des mCthodes les plus utilisbes. En particulier l’adamantylation de d&iv& benzkniques sur des solides superacides, PW,,/SiO, a t+tGr&emment d&rite. Dans ce type de r&action, la position para est bloqute et la position or& devient attaquable par un reactif electrophile tel que NO,BF,. La sPlectivit6 de cesr&actions dipend fortement dex propriitCs acidesdu catalyseur : SiW,L, le plus faible des acides est le plus selectif et la quantita dc Liz&-isombre obtenue dansle casdu tolu&ne augmente avecla force de l’acide. k cBtC de ces&t&s de la rtactivitC, des caractirisations structurales du catalyseur apr&sla reaction ont Ptt d&eloppt-es. (Version franc;aiseabrtgCe r&digie par la rkdaction).

1. Introduction Due to environmental considerations and safety concerns there is a strong driving force in the industrial practice to find alternatives to very hazardous and corrosive materials such as HF or sulfuric acid. As is well known, these chemicals are used in a large number of commercial processes[I]. One possibility is to use solid acid catalysts as replacements for homogeneous ones in industrial and laboratory processes.On the other hand, the use ofenvironmentally friendly, easily recyclable catalysts in the homogeneous phase is also preferred. A class of inorganic materials called heteropoly acids (HPAs) are good candidates for both purposes. Although, in most cases,they work in the homogeneous phase, their easy recovery and environmentally safer handling make them an attractive alternative. Furthermore, the

382

recent discovery of insoluble acidic cesium salts of HPAs with outstanding properties as solid acid catalyst has the potential of an even greater practical significance. Due to their unique structural properties heteropoly acids exhibit bifunctional catalytic behavior [2]. They can be applied in reactions requiring electrophilic catalysis (alkylation, acylation, isomerization, hydration and dehydration), and they also promote oxidations [S12]. Heteropoly acids have recently been introduced, mainly in Japan, as a new classof catalytic materials into several petrochemical processes[I, 3-91. They are used as acid catalysts in the hydration of alkenes and the polymerization of tetrahydrofuran, and asoxidation catalysts in the oxidation of methacrolein to methacrylic acid. As a consequence of some unique properties (multifunctional character,

Electrophilic

heterogeneous, pseudo-liquid, homogeneous and phase-transfercatalysis) and favorable technological features, there is still a growing interest in the exploration of new applications. In the present paper we summarize our recent results on the use of heteropoly acids and derivatives in organic transformations and give a short treatment of our findings with respect to the changesof the catalytic materials occurring during the reactions. The aim of this project has been to find new applications of HPAs in the transformations of organic compounds. A careful analysis of the leading recent review papers of the field 12,3, 7-121 revealed that the application possibilities of HPAs in numerous classesof organic compounds are still unexplored. Since we have a long-lasting interest in the chemistry of various oxygen-containing compounds induced by electrophilic catalysts [13-l 51, our primary efforts have been concentrated on studying the transformations of alcohols, diols and oxiranes. Although FriedelCrafts alkylation catalyzed by HPAs is a wellstudied field, adamantylation, a unique synthetic reaction was also included since it has never been studied. New, interesting findings could be expected from these studies, thereby extending the scope of applications of HPAs. To underline the significance of these studies referencesshould be made to some interesting and important recent findings. These include a novel method to prepare supported HPAs [ 161, the synthesis of acidic cesium salts either supported [ 171 or immobilized into a silica matrix [ 18, 191, and the preparation of Y-type zeolite encaging 12-phosphomolybdic acid [2O]. In addition, traditional supported heteropoly acid catalysts are still intensively studied [2 l-251. All these recent developments indicate that the searchfor new catalytic applications of HPAs is continuing to be in the forefront of catalysis research.

2. Structure and acidity of heteropoly acids In our studies four heteropoly acids, namely H,,[SiMo,2040] (denoted as SiMo,,),

transformations

induced

by heteropoly

acids

organic synthesis. In some cases,the recently described CS~.~H,,,[PW,~O~~] (CSJ’W,~) prepared according to a literature method (see experimental section) was also applied. Due to their commercial availability the structure of these acids, the so-called Kegginstructure, is well known and well characterized. This structure was first suggestedby Keggin in 1933 [26] and confirmed and refined in numerous structure determinations ever since [2]. I.ater, Baker and Figgis [271 proposed several isomers of the basic Keggin structure. The determination of the acid strength of these heteropoly acids was the subject of extended research.It was pointed out that since the acid strength of PW, Z in the solid state (Ho c -13. ~6) exceedsthat of concentrated sulfuric acid (1’-I,,= -12) it is a superacid [28]. Izumi and coworkers determined the relative acid strength of a seriesof heteropoly acids basedon the hydrogen bonding with chloral hydrate and the order was found to IX l’W,z > PMo,, > SiW,, > SiMo,, [29]. However, on the basisof N l-i, temperature programmed desorption experiments the sequence I’W,, > SiW,, > PMo,, > SiMo,, [ 30) was observed. The same order was proposed by a more recent investigation [IO] and this wasproven at leastwith the tungsten-containing acids in homogeneous solutions as well [31]. New results are available for the acidity of PW, J showing that in acetic acid the three protons dissociate independently [32]. PW,L, therefore, behaves like the solid acids having acid sites of the same strength, and is significantly stronger than perchloric acid or sulfuric acid. ln contrast, Drag0 et al. showed that PW,, furnishes only one strong proton for catalytic reactions [33]. Unfortunately, similar studiesfor the comparison of the acidity of different HPAs are lacking. Moreover, we are not aware of any exact acidity value for CsZ,J’Wi2. However, the product distribution in FriedelCrafts nlkylation suggeststhat it is a stronger acid than the parent PW, (see discussion below). As this short discussionshowsthe question of the acidity of HPAs is still a controversial subject.

H,~LPMq2C)~01 (I’Mq,), H+[SiW120f,,,1 (SiW,,), and HI[PW120401 (PW12), wcx

3. Electrophilic

mainly usedascatalyst. Although a wide variety of heteropoly acids is described, these four compounds are commercially available having a great potential to be applied asacid catalysts for

In our studies the strongest (PW,J and the weakest (SiMo,,) acid were always tested in all electrophilic transformations studied. How-

reactions

383

B. TSriik,

A. Mohir

resistant to oxidizing and reducing agents and stable under neutral and basic conditions which give tetrahydropyranylation a significant interest in organic synthesis [34]. Numerous solid and liquid acids were found to be effective in forming tetrahydropyranyl ethers by the reaction of alcohols and phenols with 3,4-dihydro2H-pyran and new reports are still being published disclosing the use of new reagents [35371. The mild, homogeneous, heteropoly acidcatalyzed tetrahydropyranylation ensures an effective synthetic step and provides an attractive and viable alternative to the known protection of various hydroxyl compounds. Moreover, deprotection can also be carried out with the same catalyst with similar ease and effectiveness [ 381.

ever, when it seemed necessary all four acids and the salt were applied to establish more accurate correlations between acid strength and activity/selectivity. In some cases, results obtained with other well known solid acids (Amberlyst, zeolites, Nafion-H, etc.) are also listed for comparison.

3.1. Reactions

of oxygen-containing

compounds Of the electrophilic transformations, the elimination of alcoholic hydroxyl group is an interesting and important reaction since many different products (alkenes, dienes, ethers and carbonyl compounds) may be formed, depending on the reaction conditions and the structure of the starting materials. Using monobydroxy compounds the dehydration produces either an olefin or the mixture of possible alkenes in good yield (Jigures I and 2). The yield is excellent even with the weakest acid tested and no significant differences are observed with the various acids. It is important to point out that all four HPAs used in our studies are soluble in alcohols. All transformations discussed here are, consequently, considered to be homogeneous reactions.

All four acids (PMo,*, SiW,,, PW,,, and SiMo,,) were tested in the tetrahydropyranylation of benzyl alcohoi and all were found to be effective. As a result, systematic studies were carried out with I’W,, (the strongest acid) and SiMolz (the weakest acid). All types of alcohols (primary, secondary, tertiary, benzylic, and allylic) undergo facile tetrahydropyranylation to form tetrahydropyranyl ethers in high yields (tuble 4. Of the various phenols studied only phenol gave satisfactory yield, whereas substituted derivatives, independently on the type of the second substituent, reacted sluggishly giving low yields.

OH SiMolp distillation

*

I

()

The removal of the protective tetrahydropyranyl group can be carried out under similar mild and convenient reaction conditions. The results (table II) indicate that the efficiency of the heteropoly acid-catalyzed deprotection is also highly satisfactory.

95% Figure 1. Silicomolybdic cyclohexanol. Figure l’acide

1. IXhydratation silicomolybdique.

acid catalyzed

dehydraration

du cyclohexanol

catalysPe

of par

Similarly to alcohols aliphatic and diary/ dials readily undergo electrophilic dehydration reactions [ 15,39,40]. However, only a single paper was found in the literature concerning the heteropoly acid-catalyzed dehydration of these compounds [41].

Another transformation of monohydroxy compounds studied is tetrahydropyranylation. This reaction is one of the most useful methods for the protection of alcoholic and phenolic hydroxyl groups due to the highly stable 2-tetrahydropyranyl ether products. They are

CHaCH3 HO

z;$zion

*

As a result, a systematic study was undertaken of this subject with a series of 1,2-, 1,3-, 1,4-,

+ CH3yCH3

CHqCH2

CH3 CH3

31 Figure

2. Formation

of isomeric

Figure dique.

2. Formation

d’isomkres

384

penrenes du pent&ne

in the dehydratation dans la dtshydratation

%

of’2- methyl-butan-2-01 du 2-mkhylbutan-2-01

CH3

65% catalyzed catalyske

by silicomolybdic par I’acide

acid.

silicomolyb-

Electrophilic Table

I. Tetrahydropyranylation

Tableau

of alcohols

I. T&rahydropyrannylation

catalyzed

des alcools

by PW,,

catalyke

and SiMoIL

par PW,2 heteropoly

Alcohol

induced

by heteropoly

[38].

et SiMo,,

[38].

R.T.

Time

Yield

(o/o)’

PW,,

SiMo,L

1

I-Octanol

30 min

88

88

2

I -Decanol

30 min

72

83

3

Isobutyl

30 min

96

94

4 5

Tctrahydrofurfuryl Benzyl alcohol

2h 30 min

84(63)” 90(73)”

72 76

6

2-Octanol

30 min

80

90

7

Cyclopentanol

30 min

92(80jh

92

8

Cyclohexanol

30 min

84

83

9

tut-Amy1

4h

93

93

10 11

Ally1 alcohol Phenol

30 min 30 min

96 64

96 80

d GC yields.

’ Isolated

Table II. PW,,

catalFed

yields.

alcohol alcohol

alcohol

All values

deprotection

Tableau II. Deprotection

des &hers

in the tables are given of tetrahydropyranyl

in mol%.

ethers

tPtrahydropyrannylCs

(381.

catalysPe heteropoly methand,

par PW,,

acid ~ R.T.

[38].

R-OH

Entry

R

Time

Yield 90

1

l-Octyl

4h

2

1-Dccyl

4

4

TetrahydrofUrfLryl

4h

5

Benzyl

4

6

2-Ocryl

2h

85

7

Cyclopentyl

4h

84”

8 9

Cyclohexyl Phenyl

30 min 30 min

93 71

’ GC

yields.

h Isolated

acids

acid

CH&,

Entry

transformations

11

(%)a

94 90”

11

73”

yields.

1,5- and 1,6-diols [42,43]. Reactions were carried out in the homogeneous or the heterogeneous phase. All four heteropoly acids were used either as neat acids or supported on silica (for details, seeexperimental section). Preliminary studies with 1,3-diols showed that these compounds underwent very complex transformations (formation of dienes, saturated and unsaturated alcohols and carbonyl compounds). This is in accordance with earlier results observed in the presenceof highly active electrophilic catalysts [44, 451. Since our attempts to find reaction conditions resulting

in selective reactions failed, no further studies were conducted with these compounds. 1,2-Dials, in contrast, underwent more selective transformations. All four heteropoly acids studied exhibited high activity in the transformation of 1,2-dials, with the exception of 1,2ethanediol which was unreactive. The main products were carbonyl compounds formed in the pinacol rearrangement and their dioxolane derivatives the products of a secondary reaction (schemeI). The best selectivities were achieved with PW,,. The pinacol rearrangement, i.e. the formation of carbonyl compounds through the

385

6. TSik,

A. Molndr

*

*

Me-CfH-tfH-Me OH

Tableau

Selectivities III.

in the I’W,?

S&ctivids

catalyzed

Selectivities

are given in mol%

Table

IV. Effect

Tableau

de I’acidite

of pure

isomers

of 2.3-bucancdiol

pures du 2,3-hutanediol

(harch

[4,?].

catalys6e

reactor)

par I’&‘;,

HeterogeneoLls

[42].

(few

system)

.MC30

r~xrmic

VWS”

I t 39

0 + 3 32 + 63

21 +o 79 + 0

5.3 + .3 (mols of products

of acid strength

IV. Effet

Me

The effect of increasing acid strength was tested in the dehydration of 2,3-dimethyl-2,3butanediol (pinacol). Side-reactions include fragmentation to acetone and 1,2-elimination to produce dienes. As it is seen in table IL’ the

d’isom&res

Homogeneous ~

+ dioxolane + dioxolane

Me

rable transition state without Me-Me gauche interaction for hydride migration and the high migratory aptitude of the hydride anion. The relatively high amount of aldehyde formed can be explained with the high acidity of PW,, stabilizing the open carbocation involved in the process.

dehydrarion

dam la dkhydratation

2-Me-propanal 2-Rutanone

Me x Me

1.

elimination of water and concomitant 1,2 rearrangement occurs with high selectivity both in the homogeneous phase and with heterogeneous catalysts. The data unambiguously show that the formation of dioxolanes is negligible in the heterogeneousflow system due to the short contact time (table III). The data in table ZII show some interesting changes.2JButanediol yields 2-butanone with high selectivity under heterogeneous conditions. In the reaction with the racemic compound almost exclusive formation of the ketone was observed. This can be attributed to a favoIII.

Me

OH

Scheme

Table

Me

formed

on the selectivity

sur la s6lectivit6

divided

by mol of starting

of the HPA catalyzed

de la dPshydratation

compounds

dehydrarion

du pinacol

rlrremic 4+0 96 + 0 reacted

of pinacol

catalys6e

multiplied

by 100).

1421.

par let hPt&opolyacides

[42].

fyle fyle CHz=C-C=CH2

bfe lyle Me-F-F-Me

Me Me-&---C-Me

OH OH

bile 8 Homogeneous SiMo 12 2,3-Dimethyl,.Lbutadiene 3,3-Dimethyl-2-butanone In certain

386

reactions

2,3-dimethyl-

I’M0

(batch ,?

I5 .44

20 43

I -butrne

and acetone

reactor)

SiW,L 29 71

Heterogeneous I’W I?

I’W,,/Si02(HT)

22 78

wcrc also detected.

(flow

I’W,,ISiOL(WE)

45 55 For HT

system)

and W’E see experimental

2s 75 section.

Electrophilic

pinacol rearrangement (the formation of 3,3dimethyl-2-butanone) took place with increasing selectivity as a function of acid strength. This is in agreement with earlier observations that active sites of low acidity catalyze I,2-elimination over pillared clays [46] while stronger acid sites generate the rearranged product. Heterogenized heteropoly acids were also studied in this reaction. PW,,/Si02 heat treated at 300 “C (PW,,/SiO,(HT)) exhibited the same activity as the homogeneous catalyst, whereas the selectivity decreased considerably. The activity of the catalyst after water etching (PW,,/SiO,(WE)) was slightly lower, but the selectivity was again similar to that in the homogeneous reaction. According to a recent paper [47], heat treatment results in the formation of an amorphous heteropoly acid with mixed acid sites. Subsequent water etching was found to remove the unchanged crystalline heteropoly acid to ensure a more uniform active site distribution and lower activity. Higher aliphatic diol homologues (1,4-, 1,5and 1,6-diols) can be dehydrated under either

~H2CH2CH2CH2CH2~H2 OH

-

induced

by heteropoly

acids

homogeneous or heterogeneous conditions to produce the corresponding cyclic ethers in high yield with excellent selectivity (the yield of cyclic ethers are usually higher than 96 %) [42]. The only exception is 1,6-hexanediol, which, in addition to oxacycloheptane, yields five- and six-membered cyclic ethers. In this case, the weaker acid SiMo, z gives higher selectivity due to its limited ability to induce ring contraction through carbocationic isomerization processes (scheme 2). Cyclic ether formation with significant synthetic interest is completely stereospecific, giving &s-2,5-dimethyltetrahydro-furan from racem.-2,5-hexanediol and tram-2,5dimethyltetrahydrofuran from meso-2,5-hexanediol (scheme 3). 1,2-Eliminations cannot occur in the dehydration of diary/ dials. Therefore, in the transformation of isomeric hydrobenzoins the pinacol rearrangement to yield carbonyl compounds is the main reaction direction [43]. Of the two rearranged products the selectivity of aldehyde formation is characteristically higher (table V). Moreover, diphenylacetaldehyde is

SiMolz PW12

- Hz0

OH

Scheme

transformations

66% 64%

2.

meso-2,5hexanediol

trarw2,5dimethyLtetrahydrofuran

Scheme

3.

387

6. Tiiriik, Table

br. Molt&

V. Selectivities

Tableau

in the transformation

V. SClectivitCs

of isomeric

dam la transformation

hydrobenzoins

des hydrobenzdines

[43]. isomkres

[43].

OH Ph

Ph +

- pwor

PhxcHo+phLPh

thermal

1

OH

Microwave

+phaPh 2

activation,

+ph&Ph 3

4

Thermal

mew

0

activation

(155

rucmic

mtT0

SiMo,, i

i

14

2

23

37

3+4 Conversion

32 45 19

44 24

,’ + 2 % unidentified

SiW 12

PMo,,

PW,,

Tableau

VI.

in the cransformation

SClectivit&

dans la transformation

l-Phenyl-2-ptopanone Conversion

388

temperature;

SiMo,,

pw,,

84

78

81

75

85

33 24 29

15

11

11

13

9.1 100

6” 100

IO“ 100

7 6” 100

1 100

of isomeric

reaction

exclusive formation of 1 -phenyl-2-propanone took place, regardlessof the stereochemistry of the oxiranes. Earlier studies with deuterium-labeled compounds indicated selective benzyl C-O bond fission [48]. The formation of methyl-benzyl ketone can be attributed first to the significant stability difference between the benzylic and the secondary carbocations, and to exclusive hydride migration. The fast rotation in the open benzylic carbocation formed from both isomersensuresthe necessaryconformation for hydride migration. Steric interaction between the methyl and phenyl groups in the transition state formed from the cis isomer can account for the higher reactivity of the tran~ compound (scheme4).

3.2. Reactions of hydrocarbons Various transformations of hydrocarbons are frequently studied reactions due to their industrial and synthetic importance [ 11. Alkykztion of aromatic compoundsvia acid-catalyzed Friedel-

2-methyl-3-phenyloxiranes

[43].

des 2-mtthyl-3-phknyloxiranes

SiMo IL

Room

PWU

compound.

Selectivities VI.

SiMo 12

43

formed with very high selectivity in the presence of PW,,, or under thermal conditions. These changes are considered as further proof of the involvement of the highly stable benzylic carbocation in the rearrangement. Noteworthy, however, is the formation of benzil and benzophenone, indicating the involvement of oxidative processes.As it is shown, the contribution of these processes increases in the presence of weaker, molybdenum-containing acids during microwave irradiation. Microwave heating requires the presenceof polar moleculesin the reaction mixture. In our case,both the reacting diol and the water content of crystalline HPAs satisfy this requirement. It appearsthat the extremely fast heating processin the microwave reaction prefers the formation of oxidized products. One of the synthetically useful reactions of cyclic ethersis their isomerization through ring opening. 2-Methyl-3-phenyloxiranes undergo fast and selective ring opening in benzene (table VI). As expected, I’W,, (the strongest acid) exhibits the highest activity. In most cases,

Table

“C)

isomkres

PMo,:

SiW

[43].

1:

PW,,

cis

tram

cis

tram

cis

tram

cis

tram

100 54

05 57

100 10

88 18

100 20

100 100

100

100 100

SiW,J

or 10 min

time:

30 min (SiMoIL,

PMo,~,

(PW,,).

100

Electrophilic

transformations

induced

by heteropoly

acids

Me 0

Me

Me

OH PhCHp-C-Me b Scheme

4.

P

m Scheme

Crafts reaction is one of the most often used synthetic reactions. Among these reactions the adamantylation of benzene derivatives over solid superacids including PW,,/SiO, was recently described to be an effective and selective method for positional protection of these compounds [49]. Introducing the bulky adamantyl group, the para-position can be blocked

5.

and the or&o-positions become attackable by electrophilic reagents such as NO,BF,. After the second substitution the adamantyl group can be cleaved simply with the same solid superacidsor the adamantyl cation can be recycled by transadamantylation. As a consequence, this method permits the selective synthesis of or&o-substituted aromatics.

389

B. TiirSk,

A. Moldr

into the phenomena observed the time dependence of the reaction was studied over each catalyst. The activity and selectivity data are included in table VU, while the changes in selectivity asa function of reaction time are displayed in j&re 3. It is seen,that the para isomer is formed with high selectivity in the early period of the reaction with each catalyst. However, when the starting material is completely consumed a sudden increase in the meta to para ratio can be observed in the presence of tungsten-containing acids. These data indicate that the formation of thepara isomer takes place under kinetic control, and the isomeric distribution reaches the thermodynamic equilibrium after 30 min of reaction time. The acid strength of the catalysts, however, unambiguously plays a crucial role in determining the selectivity. As known the determination of acid strength of heteropoly acids was the subject of extended research [IO, 301 (vide supra). In the light of literature data, the strong influence ofthe catalysts on the regioselectivity can be explained as acidity dependence. The stronger W-containing acids are able to induce the para to meta isomeriza-

It was also pointed out, however, that selectivity strongly depends on the acid strength of the catalyst. In the presence of strong acids (e.g. Nafion-H or PW,,) high amount of the metaproduct is formed via the isomerization in the intermediate (probably, o-complex) phase (scheme .5). This observation prompted us to carry out a study with a series of heteropoly acids providing a wide range of acidity [50]. However, it is essential to note that the acids either in neat or in supported or salt form are not soluble in the aromatic reactants used. As a consequence, this alkylation reactions can be considered as heterogeneouscatalytic transformations in sharp contrast with the reactions of oxygen containing compounds (vide supra). The results obtained using toluene as starting material are tabulated in table VU together with earlier results with several solid acids 1491for comparison. As it is shown, the tendency with heteropoly acids as catalysts are the same as observed carlier: SiMo, ,, the weakestacid exhibits the highestselectivity, and the amount of the isomerized product (the meta-isomer) increaseswith the acid strength. However, to get more insight

Table

VII.

Tableau

Heteropoly VII.

acid catalyzed

Adamantylacion

Catalyst

adamantylation

du toI&ne

catalys&

of toluene

par les h&topolyacides

Conversion

t

(h)

(%I

[49, 501.

Reaction (mm01

[49, 501.

rate

g-’ min-‘)

Adamantane !%)

VI

P

(%)

(%I

Nafion-H

1.5

100

30

70

,4mherlyst

1.5

1 00

-

9

91

HY

1.5

100

-

28

71

-

10

90

-

7

93

13 60

87

-

67

-

29 66

33 71 34

Sulfated

ZrOZ

SiMo 12 PMo,, SiW,2 I’W,, SiMo,jSiL), CS,,,PW12

(H’l‘)

Reactions

were carried

390

9

2

54

2 1.5

100

0.55 0.09

2

100 100

2

100

1 I

1 00 100

0.57 0.77 0.77

out at reflus

temperature

0.18

(= 111 “C).

40

Electrophilic

transformations

0.8 ,

induced

by heteropoly

acids

1

0.7 0.6 --a--SiWl2 +PMol2 *Sit&l2 +SIMol2/Si02(HTJ

0

Figure 3. Effect toluene.

of reacrion

5

IO

15 20 time (min)

time on rhe isomeric

Figure 3. Effct du temps de rkacrion catalysCe par les hCtCropolyacides

distribution

sur la dixrihution

of the catalysts

One of the most important questions in catalytic reactions is what happens to the catalyst during a reaction since this can affect the lifetime, reproducibility or the regeneration of the catalytic material. A test of the catalyst before and after reaction, consequently, provides valuable information about the actual rransformations of the catalyst. In some cases, therefore, we studied the heteropoly acids to get more insight into the changes taking place during the catalytic cycle.

4.1. Thermal

stability

The thermal stability of the acids was studied by differential scanning calorimetry (DSC) [5 11. It is a very useful and effective method requiring only a very small amount of material. Figure 4 shows the results of these measurements of all four heteropoly acids. Since rhe catalysts were used in microwave assistedreactions aswell, it is important to know if microwaves induce any transformation of the heteropoly acids themselves. Observations

30

of heteropoly

dea isomkres

tion, providing the thermodynamic equilibrium, while the weaker MO-containing acids cannot. As an additional factor the accessibility of surface acid sites determines the isomerization rates. The relatively high number of acid sites on the surface of the cesium salt requires the shortest reaction time to reach the equilibrium.

4. Transformation during reactions

25

35

acid catalyzed

dam l’adamantylation

Friedel-Crafts

adamanrylation

of

du toluke

de type Fridel-Crafts

obtained with the acids irradiated by microwaves, therefore, are also included in figure 4. As it is seen the molybdenum-containing acids gave one broad endothermic peak (SiMo,2) or separate peaks in the same region @‘MO,,), and one exothermic peak at higher temperature. In contrast, the spectra of tungsten-containing acids have Two separate endothermic peaksat lower temperatures and one at very high temperature (above 800 K). The endothermic peaks at low temperatures can be attributed to the water content of the heteropoly acids. The appearancr of these separate peakscan be interpreted by the existence of different types ofwater in the crystals [2] asit was recently observed by IR and Raman measurements 1521.The water content can be removed and build up again without significant changes in the structure of the heteropoly acid 1511.As an important additional observation it is noted that microwave treatment doesnot result in any significant changesin the DSC patterns.

4.2. Catalyst

stability

Working with molybdenum-containing compounds like SiMo12 we always encountered the formation of the well-known heteropoly blue, which is still extensively studied [53] despite the researchstarted almost 200 years ago. The phenomenon is due to the susceptibility of molybdenum to reduction and results in slow deactivation. This phenomenon is most pro-. nounced in the presenceof reducing agents or in redox reactions. To our surprise, however, ir was observed in the electrophilic reactions we studied, e.g. in the transformation of diols. In our studies various spectroscopies (UV-VIS,

391

B. T&Sk, A. Moln6r

Temperature

(K)

r

PW

PW

T-----------AVW

884.5 K 461.5 K

--r-\f”----------I-----280 410

.----A-1------540

670

Temperature

Figure4.

DSC

curves

Figure 4. Courbes

DSC

of neat and microwave des hkdropolyacides

irradiated utilisPs

392

-- 7930

(I<)

heteropoly

acids used in reactions.

dans les Actions

FT-IR, ESR) were used [54]. Unfortunately, IR spectra were hardly informative, but in the seriesof UV-VIS spectra the continuous formation of heteropoly blue was observed cfisure 5). The parallel investigations with ESR showed the formation of MO(V)-containing species (&WY G). The MO(V) concentration goes through a maximum and, eventually, decreases to zero. The IJV-VIS and ESR results indicate that MO(W) is partly reduced to MO(V) species. This reduction step becomes possible by the formation of products readily undergoing oxidation (alkenes, dienes, aldehydes etc.). The reasonfor the diminishing intensity of the ESR signal is the formation of diamagnetic bridged dimer complexes through organic bridges [551. Another explanation could be the further reduction of MO(V) to MO(W). This interpretation, however, can be ruled out on the basisof the blue shift (860 nm -+ 720 nm) observed in the UV-VIS spectra [561. The transformation of SiMo,, caused slight or significant decrease in acidity and activity

T-800

ZI Yetat pur et sow irradiation

par micro-ondes.

depending on the amount of the organic compound transformed. In the reactions of 1,2and 1,3-dials, the deactivation is relatively fast due to extensive fragmentation and the formation of carbonyl compounds. In cyclodehydrations, i.e. during the transformation of 1,4diols and higher homologues, deactivation is much less pronounced. These observations indicate that, in agreement with literature findings [29], the activity of heteropoly blue in acid-catalyzed reactions is smaller than that of the parent heteropoly acid. This is due to the increased basicity of the reduced heteropoly anion resulting in a decreasedacidity.

5. Conclusions Heteropoly acids were shown to exhibit excellent catalytic properties in the electrophilic transformation of oxygen-containing compounds. A comparison of the activity of four heteropoly acids studied (SiMo,2, PMo12, indicated increasing activity SiW, z, and PW,,)

Electrophilic

300

540

420

transformations

669

induced

by heteropoly

acids

780

1

0.0 b. 190 Figure 5. UV-VIS of 1,4-bucanediol

MO& -(P&l

332

474

spectra of the reaction mixture catalyzed by H41SiMo,2C),,,]

Figure 5. Spectres UV-visibles du m&nge apt& divers temps de rkaction.

616

withdrawn

rCactionnel

after appropriate

de la dkhydraracion

with increasing acid strength. The CS~,~PW~~ salt wasfound to display an activity in aromatic aikylation that is even higher than that of the parent acid. The useofheteropoly acidsascatalyst provides selective methods for the synthesis of various organic compounds (carbonyl compounds, cyclic ethers, protected alcohols, and substituted aromatics). Further studieswith heteropoly acidswill certainly result in useful, new applications of rhese versatile catalytic materials. A recent trend in catalysisis to replaceliquid catalystswith solids, and neat materials with supported ones. In this wap expensive chemicals can be usedmore economically, even in continuous flow reactor systems. Heterogenization of heteropoly acids, Le. to prepare supported heteropoly acids which are stable under reaction conditions. is an area

758 reaction

900

rimesfrom the

du 1.4-butanediol

Am1 dehydration

reaction

catalysPc par H4[SiMo,2C),0]

of great perspectives. Depending on the support (usually SiO, 1571 and activated carbon [58]) the acid strength of the resulting catalyst can be manipulated and even shape selective catalysts (using montmorillonites [22] or zeolites [59]> can be tailored. Since rhe known solid superacids (Nafion-H or SbCI, intercalated graphite, etc. [GOJ)are not stableat higher temperatures (above 250 “C), the application of supported heteropoly acids exhibiting high thermal stability is of high practical significance. The possibilities of the preparation and application of acidic salts such as Cs&W12 have not been fully explored yet. Isoburane-&fin alkylarion [I], an area of great practical importance reviewed by Corma [61], is still the subject of recent research [62, 631. Unfortunately,

393

B. TiirGk,

I& Molnir

identified on the basis of their ‘H-, ‘“C-NMR (Bruker AM400) and mass spectra (HP5890 GC coupled with an HP-5970 mass spectrometer). The detailed spectral characterization of the new compounds were published earlier 1491. The quantitative product distributions were determined with a Hewlett-Packard GC (50 m HP-1 capillary column, FID). In some cases the individual compounds were identified by comparison with authentic samples or by using an MS library search method.

'1

6.1. Catalysts 0

5 10

20

30

40

50

io

80

time (min)

the results observed with solid acids such as zcolites, superacidic sulfated zirconia, and with the commercial hcteropoly acids arc not very promising: although product distributions are satisfactory, the lifetime of catalysts is quite short not meeting industrial requirements. Cs2,,PW,, tested bv Misonos group was found, however, to be extremely effective in the alkylation process giving much higher quality alkylate than other solid acids 1641. ‘l‘he superacidic sulfated zirconia used as a reference cata-lyst produced satisfactory alkylate quality with low amount of light ends (i:5-C7 hvdrocarbons, 0.9 O/b), moderate perccnrage 6f heavy ends (higher than C9, 26.6 o/i)), and good TMP/DMH (trimethylpentanes to dimethylhexanes) ratio. Results with PW,, was very similar. However, Csl,,PW,, gave an increased alkylate quality (almost 90 ‘!& C8, including 63 (31ofTMP), and the yield was three times as high as with PW,, (79.4 o/o vs. 25 o/i)), Systematic studies with acidic salts will certainly result in further improvement\.

H~JPW,,O,,,] . I-l,JI’~~q~(~& . -2GH,O, - 19HL0 and Hq[SiMolrO& . - 14H,O were purchased from Aldrich, while H,~[SiW,ZO,~o] -26Hz0 was a Riedel-de Haen product. Supported and heat treated cata!y,sts were prepared by impregnating Davisil sthca gel (Aldrich, ‘IYpr 62, 60-200 mesh, S,
6. Experimental

6.2. General procedure for tetrahydropyranylation in the presence of HPA catalysts 1381

All chemicals used in our studies except the catalysts listed below were purchased from Aldrich, Fluka and Rcanal. The products were

3,4-dihydro-2H-pyran (30 mmol, 2.52 g) dissolved in CH,Cl, (5 mL) was added to a stirred solution of benzyl alcohol (20 mmol,

394

Electrophilic

2.16 g) and PW12 (0.056 g) in CH,CI, (25 mL) at room temperature for 5 min. Stirring was continued for an additional 30 min. The solution was first filtered through a short column of basic alumina then silica gel, washed with a small amount of CH,CIZ and evaporated to give the pure tetrahydropyranyl ether (2.8 g, 73 o/o). 6.3. Typical reaction procedure for &protection of tetrabydropyranyl ethers by HPA catalysts 1381 ‘l‘he solution of the 2-tetrahydropyranyl ether of benzylic alcohol (10 mmol, 1.32 g) and PW,? CO.056 g) in MeOH (IO mL) was stirred at room temperature for 30 min. After a workup identical to that used in tetrahydropyranylation benzylic alcohol was isolated (0.79 g, 7.3

%j.

6.4. Transformation of dioL in the presence of HPAs 1421 Reactions in the homogeneous phase were carried out by reacting a mixture of 0.02 mol of diol and heteropoly acid corresponding to a catalyst quantity of 0.1 mol% at 150-I 80 “C and collecting the products that distilled off (denoted as Homogeneous). Supported catalysts were also used in a similar way. Reactions were usually complete in 30-60 min depending on the reactivity of the dial. A customary vertical fixed-bed reactor (length: I60 mm, inner diameter: 20 mm) electrically heated with a split-tube furnace was used for heterogeneous reactions (denoted as Heterogeneous). 1 g of the supported heteropoly acid was kept at 200 “C for 1 h before the diol was introduced with a syringe pump at a feeding rate of 1.5 g h-’ . Except when noted, conversions were 100 %.

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6.7. Kinetic

studies for adamantykion

rru

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