Inhibition of Allylic Substitution and Isomerization by - American

Chapter 26

The Importance of Hydrogen Bonding to Catalysis in Ionic Liquids: Inhibition of Allylic Substitution and Isomerization by [bmim][BF ] Downloaded by UNIV OF GUELPH LIBRARY on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch026

4

James Ross and Jianliang Xiao* Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom

Neutral allylic substitution reactions, in which a base is generated in situ and which hence require no external bases, can be significantly retarded when carried out in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF ]). Evidence suggests that this is due to the base or base precursor entering into hydrogen bonding with the imidazolium cation, and interestingly, this intermolecular interaction can be exploited to suppress unwanted allylic isomerization. 4

Hydrogen bonding has an ubiquitous influence in chemistry and biochemistry (/). The effect of hydrogen bonding on reaction chemistry in common molecular solvents is well documented and understood (2). Hydrogen bonding in solvents based on room temperature ionic liquids is a relatively new subject. In fact, the perception of hydrogen bonding in imidazolium ionic liquids, the most extensively investigated ionic liquids to date, was still controversial in the mid 1980s (J, 4). Thanks to the pioneering studies of several

314

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

315 research groups, it is now well established that imidazolium cations and their associated anions form hydrogen bonds both in the solid states and in solution (5-6). The three ring protons of the 1,3-dialkylimidazolium cation can act as hydrogen bond donors and interact with counter anions such as CI", OTf and BF " acting as hydrogen bond acceptors, which can also enter into hydrogen bonding with external hydrogen bond donors such as H2O. Of the three imidazolium ring protons, the H proton appears to form the strongest hydrogen bond. However, whilst the concept of hydrogen bonding in ionic liquids has generally been accepted and explosive growth in research on reaction chemistry in these solvents has been witnessed in the past few years (7-72), little attention has been paid to the potential effects of hydrogen bonding on catalyzed reactions in ionic liquids (13, 14). We report herein that the capability of imidazolium ionic liquids for hydrogen bonding can impose significant effects on neutral allylic alkylation reactions and, interestingly, the effect could be harnessed to suppress allylic isomerization (15), a reaction that may deteriorate the stereochemistry of asymmetric allylic substitution (16). 4

Downloaded by UNIV OF GUELPH LIBRARY on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch026

2

Results and Discussion Following on from our earlier studies on Pd(0)-catalyzed Tsuji-Trost allylic substitution reactions under basic conditions in imidazolium ionic liquids (17, 18), we turned our attention to the neutral variants in an attempt to determine the scope of such reactions in ionic liquids. The allylic alkylation of phenylallyl carbonate 1 with dimethyl malonate 2 was first chosen as a model neutral TsujiTrost reaction to study in the ionic liquid [bmim][BF ] (eq 1). The 4

Pd(OAc) ,2 mol% PPh ,8 mol% 2

*OC0Me + MeOaCs^COaMe 2

3

solvent, 22 °C

C0 Me

(1)

2

reaction requires no external bases, as decarboxylation of MeOC0 ", which results from the oxidative addition of 1 to Pd(0), generates C 0 and OMe" (15, 16). Although methanol (79) has a higher ρΚ than a dialkylimidazolium ion (29 vs ca. 24 in DMSO) (20, 27), the methoxide is expected to preferentially deprotonate the malonate 2, which is about 7 orders of magnitude more acidic than the imidazolium cation (22). Furthermore, even if deprotonation of the solvent cations took place, the so generated dialkylimidazol-2-ylidene would readily deprotonate the malonate to give the required nucleophile to attack the 2

2

Λ

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

316 palladium-allyl intermediate and complete the catalytic cycle (20, 21). After first confirming the reaction to be rapid in THF, reaching complete conversion within 20 min (23), we carried out the same reaction in [bmim][BF ]. Surprisingly, the reaction in [bmim][BF ] afforded only 38% conversion of 1 after 5 h, and complete conversion giving 3 in 90% isolated yield only after an extended reaction time of 30 h. Even more strikingly, the reaction in THF was significantly inhibited by the addition of a small quantity of [bmim][BF ] and the rate was progressively decreased by introducing more [bmim][BF ]. Thus, the reaction in THF reached a conversion of only 63% after 45 min when 4 equiv (relative to the catalyst) of [bmim][BF ] was added, and the conversion was further decreased to 46% in the presence of 10 equiv [bmim][BF ] in the same reaction time, indicating that some key intermediate is involved in a preequilibrium with the imidazolium additive. A very revealing experiment is the comparison of the reaction performed in [bmim][BF ] with that in 1-buty1-2,3dimethylimidazolium tetrafluoroborate ([bdmim][BF ]), in which the H ring proton is replaced with a methyl group. In [bdmim][BF ] at 50 °C, the conversion of 1 was 89% in 30 min. By way of contrast, in [bmim][BF ] at the same temperature a conversion of 48% was reached only after 5 h reaction time, suggesting that the retarding effect of [bmim][BF ] relates to its H proton. The oxidative addition of 1 to Pd(0)-PPh leads to two ionic species, MeOC0 * and a Pd-allyl cation, and as such should not be slowed down on going from THF to an ionic medium. Therefore it is probably the nucleophilic attack step that is affected in the ionic liquid. The allylic alkylation of phenylallyl acetate 4 with methyl nitroacetate 5 was similarly retarded when conducted in the ionic liquid (eq 2). This is again a neutral reaction, because the Ο Ac ion generated in the oxidative addition of 4 to Pd(0) is basic enough to deprotonate 5 (ρΚ = 8.0 in DMSO) (24), which is much more acidic than 2. Indeed, the reaction in THF was complete within 0.5 h when catalyzed by Pd(0)-PPh at 75 °C. Repeating this reaction in [bmim][BF ], only 26% conversion was reached after 0.5 h. 4

4

4

4


4

4

4

2

4

4

4

2

4

3

2

Λ

3

4

Pd(OAc) ,2 mol% PPh ,8 mol% 2

3

OAc

OaN^COaMe

(2)

solvent, 75 °C

C0 Me 2

5

The observed inhibition of the neutral Tsuji-Trost reaction by [bmim][BF ] could be accounted for by assuming that the OAc" or MeOC0 " ion generated in the oxidative addition is strongly solvated or "trapped" by hydrogen bonding with the imidazolium cations and is thus made unavailable to deprotonate the 4

2


317

Bu

Me MeOC0 ' 2

MeOC0 '

0 COMe

2

C0

MeOC0 " Downloaded by UNIV OF GUELPH LIBRARY on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch026

2

HNu

+

MeO"

MeOH

MeO

+

2

2

+

Nu"

Ph I Pd

+

L'

Nu"

*• Ph

Nu +

Pd(0)L

2

\

Scheme 1. Solvation of the carbonate by hydrogen bonding and effect on the nucleophilic attack at the Pd-allyl intermediate. hydrocarbon acids to give the required nucleophiles for subsequent nucleophilic attack (Scheme 1). OAc' is known to form a strong hydrogen bond to the H proton of imidazolium ring in both solution and solid states (25, 26). A *H NMR experiment, in which the concentration of [bmim][BF ] was kept constant while that of [NBu ][OAc] was varied in CDCI3, revealed that the H proton chemical shift moved to lower field in almost a linear fashion with increasing concentration of OAc' until the molar ratio of acetate/bmim reached a value of ca. 2; thereafter the H chemical shift was little affected with additional acetate. Only insignificant changes were observed for the H and H ring protons. These observations suggest that the H proton hydrogen bonds to OAc" and each H proton is approximately involved with two acetate ions (27). Similar observations have been made concerning imidazolium and halide ions (28). Amatore, Jutand and co-workers have recently shown that the oxidative addition of allylic carbonates to Pd(0) is reversible and the resulting carbonate anion does not decarboxylate as fast as previously thought (16). Hydrogen bonding should certainly enhance its stability. Indeed, it is known that decarboxylation can dramatically be decelerated by using dipolar protic solvents capable of hydrogen bonding (29). The notion that OAc" or MeOC0 " could not function efficiently as a base or base precursor due to hydrogen bonding explains the effects of the added [bmim][BF ] on the reaction of 1 and 2 in THF. An increasing concentration of imidazolium cation augments the extent of hydrogen bonding and lowers the concentration of free base and nucleophile. The result is slower nucleophilic addition (Scheme 1). 2

4

2

4

2

4

5

2

2

2

4


318 There exists a possibility that the sluggish reaction between 1 and 2 in [bmim][BF ] could stem from the formation of some inactive dialkylimidazol-2ylidene complexes of palladium via deprotonation of the imidazolium cation by OMe" (30-32). This appears to be unlikely. No Pd-carbene complexes were ever detected by NMR in stoichiometric reactions of Pd(0)-PPh with 1 in the presence of [bmim][BF ] at room temperature. Furthermore, the aforementioned effect of [bmim][BF ] on the allylic alkylation in THF casts doubts on this possibility, since the activity of the hypothesised Pd-carbene species would not be affected by additional imidazolium cations. In addition, the reaction of 7 and 2 shown below in eq 3, which involves an in situ generated alkoxide and reaches completion in both THF and [bmim][BF ] within 1 h, suggests that either Nheterocyclic carbenes are not formed by alkoxide deprotonation of the imidazolium cation or, if formed, have no effect on the palladium catalysis. The success of this reaction also indicates that strong bases such as the alkoxides do not form stable hydrogen bonds with the acidic imidazolium H ring proton in the presence of much stronger acids. 4

3

4


4

4

2

Pd(OAc) ,2 mol% P(4-C H OMe) ,8 mol% 2

+ MeOgC^COgMe

e

4

3

H O ^ ^ ^ ^ v ^ C O g M e (3) Χ C0 Me

solvent, 22 °C. 1 h

Λ

â J

2

Applying the hydrogen bonding proposition between OAc" or MeOC0 " and [bmim][BF ], it was possible to suppress the Pd(0)-catalyzed isomerization of allylic acetates, which may lead to the loss of regio- and stereo-chemistry in stereospecific allylic substitution (16). The isomerization is thought to be due to the key oxidative addition step being reversible, and unequivocal evidence for this has recently been laid out (16). As OAc' can be trapped by hydrogen bonding with [bmim][BF ], allylic isomerization resulting from reversible attack by the acetate would be expected to be retarded when allyic substitution or isomerization is carried out in an imidazolium ionic liquid. This is indeed the case. Treating 9 with 5 mol% Pd(0)-PPh in CH C1 afforded 35% of product 10 after 1 h, comparable with the equilibrium value reported in the literature (Scheme 2) (33). However, the same reaction appears to be completely suppressed in [bmim][BF ]. Thus, 10 was not detected in the crude reaction mixture even after 20 h. The isomerization of 9 in CH C1 was even inhibited by a sub-stoichiometric quantity of [bmim][BF ]. Thus, in the presence of only 0.15 2

4

4

3

2

2

4

2

2

4


319

I

Pd

Pd(0)L

2

L

+ OAc" \

R1(II)X

OAc

2

10 Downloaded by UNIV OF GUELPH LIBRARY on July 19, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch026

PdX Scheme 2. Isomerization of allylic acetate 9 by palladium catalysis.

equiv [bmim][BF ] relative to 9, only 12% of 10 was formed after 3 h. Similar results were obtained starting from 10. Thus, while 58% of 9 was formed after 1 h in CH C1 ,10 remained intact in [bmim][BF ] even after 20 h. Remarkably, the isomerization can be brought about when a Pd(II) catalyst is employed. Thus, Treatment of 9 with [PdCl (MeCN) ] in [bmim][BF ] afforded 20% of 10 in 1 h reaction time; the same reaction in CH C1 gave 33% of 10. As indicated, this reaction proceeds intra-molecularly via a cyclic intermediate and involves no ionized acetate ions (34); therefore it is not expected to be suppressed by hydrogen bonding. Differences between CH C1 and [bmim][BF ] in other solvent properties possibly account for the different extent of isomerization in these solvents. These results show that if the isomerization of an allylic acetate is to be carried out in imidazolium ionic liquids, Pd(II), rather than Pd(0), should be the catalyst of choice. In summary, neutral Tsuji-Trost reactions can be considerably retarded in dialkylimidazolium ionic liquids and our results suggest that this is due to hydrogen bonding between the H ring proton of fbmim][BF ] and OAc* or MeOC0 " ions. Being strongly solvated by the ionic liquid via hydrogen bonds, the anions could not function as an effective base to deprotonate a HNu nucleophile, thus rendering slow the nucleophilic attack at the Pd(II)-allyl intermediate. We further demonstrated that this hydrogen bonding could be exploited to suppress unwanted, Pd(0) catalyzed isomerization of allylic acetates. Taken together, these results highlight the potential effect of dialkylimidazolium cations as hydrogen bond donor on any reactions that are performed in ionic liquids containing such cations or in their presence and involve transition and/or ground state with hydrogen bond acceptor characteristics. 4

2

2

4

2

2

4

2

2

2

4

2

4

2


2

320

Experimental Section All reactions were carried out in oven-dried glassware under argon, using standard Schlenk and vacuum line techniques. [bmim][BF ] and [bdmim][BF ] were prepared according to published procedures and vacuum-dried and stored under argon (55). THF was freshly distilled from sodium benzophenone under nitrogen immediately prior to use. CH C1 was freshly distilled from calcium hydride under nitrogen immediately prior to use. Phenylallyl carbonate 1 was synthesized according to a literature method (56). The synthesis of compound 10 was adapted from a literature method (57). Compounds 2, 4, 5, 7 and 9, [NBu ][OAc], Pd(OAc) , Pd(dba) and PPh were purchased from commercial suppliers and used as received without further purification. Typical Tsuji-Trost reaction in [bmim][BF ] as exemplified for the allylic alkylation of 1 by 2: Pd(OAc) (4.5 mg, 2 mol%) and PPh (21.0 mg, 8 mol%) were stirred in [bmim][BF ] (2 ml) at 80 °C for 20 min under an atmosphere of argon and then allowed to cool to room temperature. 1 (192.2 mg, 1 mmol) and 2 (198.2 mg, 1.5 mmol) were added and the mixture stirred under argon. The progress of reaction was monitored by *H NMR. Upon completion, 3 was isolated in analytically pure form in 90% yield (38). Typical allylic isomerization in [bmim][BF ] as exemplified for 9: Pd(dba) (28.7 mg, 5 mol%) and PPh (26.2 mg, 10 mol%) were stirred in [bmim][BF ] (2 ml) at 80 °C for 20 min and then allowed to cool to room temperature. 9(114.2 mg, 1.0 mmol) was added and the mixture stirred under an atmosphere of argon. Upon completion, 9 and 10 were isolated quantitatively relative to the quantity of starting 9; the ratio of the two was determined by *H NMR (55). 4


2

4

2

2

4

2

3

4

2

3

4

4

3

2

4

Acknowledgement: We are grateful to the EPSRC for a studentship (JR) and the Industrial Partners of the Leverhulme Center for Innovative Catalysis for support. We also thank Johnson Matthey for the loan of palladium.

References 1.

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33. D. C. Braddock, A. J. Wildsmith, Tetrahedron Lett 2001, 42, 3239. 34. Crilley, M . M . L.; Golding, B. T.; Pierpoint, C. J. Chem. Soc. Perkin Trans. 1998, 2061. 35. Holbrey, J. D.; Seddon, K. R. J. Chem.Soc.,Dalton Trans. 1999, 2133. 36. Lehmann, J.; Lloyd-Jones, G. C. Tetrahedron 1995, 51, 8863. 37. Hayashi, T.; Yamamoto, Α.; Ito, Y.; Nishioka, E.; Miura, H.; Yanagi, K. J. Am. Chem. Soc. 1989, 111, 6301. 38. Trost, B. M . ; Hachiya, I. J. Am. Chem. Soc. 1998, 120, 1104.


Inhibition of Allylic Substitution and Isomerization by - American

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