Dynamic Supramolecular Chemistry: The Role of Hydrogen Bonding

Ionic Liquids Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 04/05/16. For personal use only.

Chapter 19

Dynamic Supramolecular Chemistry: The Role of Hydrogen Bonding in Controlling the Selectivity of Diels-Alder Reactions in Room-Temperature Ionic Liquids Alick R. Sethi and Tom Welton Department of Chemistry, Imperial College, London SW7 2AY, United Kingdom

The Diels-Alder cycloaddition reaction between methyl acrylate and cyclopentadiene has been investigated in a number of air and moisture stable ionic liquids. The endo/exo ratio of the reaction has been used as a probe of the nature of the solvents. We have demonstrated that hydrogen bonding between the cation of the ionic liquid and the methyl acrylate is the principal interacting controlling the product selectivity.

Introduction Room-temperature ionic liquids are liquids that are constituted entirely of ions. Hence, they provide a solvent environment that is quite unlike any other available at room temperature. They have recently excited much interest in synthetic and catalytic chemistry. However little is known about how the use of an ionic liquid solvent can effect the reactions o f solute species. With their unique character, the ionic liquids may induce solvent effects on a wide range of processes. Hence, the investigation of the effect of the solute microenvironment of reactivity in these solvents forms a central theme in our research. In this paper we report, for the first time, how a specific solvent-solute interaction in room-temperature ionic liquids can lead to changes in both the rate and product selectivity of a reaction.

© 2002 American Chemical Society

241


242 The Diels-Alder reaction remains one of the most useful carbon-carbon bond-forming reactions in organic chemistry. It is highly "atom efficient but suffers from being an addition process with a negative entropy of reaction. As such, the use of high temperatures to give useful reaction rates has a detrimental effect on the position of the reaction equilibrium. Hence, there is a great deal of interesst in accelerating Diels-Alder reactions at low temperatures. Solvents offer one of the ways in which these reactions may be manipulated. Here we are using the reaction of cyclopentadiene with methyl acrylate, which leads to a mixture of exo and endo products as a probe of the solvent behaviour. We have previously reported some initial results of this investigation and others have reported similar reactions in ionic liquids. ' ' ' The reaction of cyclopentadiene with methyl acrylate has been widely investigated in a range of molecular solvents and solvent influences on the endo/exo selectivity of the reaction are well known. They may be viewed as being due to the "polarity" of the solvent leading to the stabilisation of the more polar {endo) activated complex. The effect has also been attributed to solvophobic interactions that generate an "internal pressure" and promote the association of the reagents in a "solvent cavity" during the activation process. A s highly ordered hydrogen-bonded solvents, ionic liquids have the potential to be high internal pressure solvents and to have dramatic effects on Diels-Alder reactions. Although not only a solvent effect, the addition of a Lewis acid is also known to have a dramatic effect on these reactions. 2

3 4 5 6

7

8

9

Results In this paper we seek to determine how ionic liquids affect the progress of the addition of methyl acrylate to cyclopentadiene (Scheme 1). In a typical reaction, methyl acrylate and cyclopentadiene, both freshly distilled, were added to the freshly dried ionic liquid directly and the mixture was stirred at 25 °C for 72 hours. The product was extracted from the ionic liquid with either diethyl ether leaving the pure ionic liquid, which can be reused.

Scheme 1. The reaction of cyclopentadiene and methyl acrylate Table I shows the endo/exo selectivity for the reaction in three ionic liquids, [ E t N H ] [ N 0 ] (6.7:1), [bmim][BF ] (4.6:1) and [bmmim][BF ] (2.9:1). These compare to 6.7:1 for methanol, 5.2:1 for ethanol, 4.2:1 for acetone and 2.9:1 for diethyl ether, under similar conditions. Clearly the ionic 3

3

4

4

7

243 liquids show a spread of behaviours that is just as great as that seen for molecular solvents. It has previously been shown that all three of the ring protons in JV,7V-disubstituted imidazolium cations can hydrogen bond to anions and that the strongest of these interactions occurs at the 2-position of the ring. With its N - H protons, ethyl ammonium nitrate is expected to have even stronger interactions. Hence, the selectivity of the reaction appears to follow degree of cation-anion hydrogen bonding. This would seem to support an "internal pressure" explanation for the differences in the observed selectivities.


10

Table I. The Diels-Alder addition of methyl acrylate and cyclopentadiene in three ionic liquids.

Ionic Liquid

endo/exo ratio

[bmmim][BF l [bmim][BF r [EtNH ][N0 ]

2.9:1 4.6:1 6.7:1

a

4

4

3

3

a

3

+

b

+

[bmim] is the l-butyl-3-methylimidazolium cation; [bmmim] is the 1butyl-2,3-dimethylimidazolium cation To investigate this phenomenon further, we compared the endo/exo ratio for the reaction in 5 ionic liquids with a common cation but with different anions (Table II). Since the cation remains the same in all of these liquids, its ability to hydrogen bond donate remains constant. However, the ability of the anion to hydrogen bond accept changes with the different ionic liquids. For the [bmim] cation, the *H nmr chemical shift of the proton of the 2-position of the imidazolium ring (H ) in a neat ionic liquid can be used as a measure of the degree of hydrogen bonding between the cation and anion. The greater the chemical shift then the greater the hydrogen bonding. +

2

10

10

Table II. The Diels-Alder addition of methyl acrylate and cyclopentadiene endo/exo ratio as a function of hydrogen bonding of [bmim] to the anions. +

Ionic Liquid [bmim][CH S0 ] [bmim][TfO] [bmim][BF ] [bmim][C10 ] [bmim][PF ] 3

3

4

4

6

2

endo.exo ratio

ô(H )/ppm

3.8:1 4.5:1 4.6:1 4.7:1 4.8:1

8.54 7.86 7.63 7.84 7.34

E

N T

0.62 0.64 0.67 0.65 0.67

If the explanation of the difference in the endo/exo ratios in different ionic liquids was that hydrogen-bonding interactions between the cation and

244 anion of the ionic liquids lead to increased solvophobic interactions and hence internal pressure, it would be expected that as the chemical shift of H increased the endo/exo ration would increase. This is clearly not the case, indeed the opposite is true (Table II). Hence another explanation is required. It is well known that Lewis acid catalysts can have a dramatic effect on both the rates and selectivities of Diels-Alder reactions. This occurs by the Lewis acid coordinating to the carbonyl oxygen of the methyl acrylate. This leads to an enhancement of the effect of the electron withdrawing group, further polarisation of the double bond of the dienophile and better H O M O L U M O overlap in the activated complex of the reaction. It has been shown that in a Lewis basic [emim]Cl-AlCl3 (48 mol % AICI3) ionic liquid the endo/exo ratio of the addition of methyl acrylate to cyclopentadiene is 5.25:1 under conditions similar to ours. B y changing to an acidic regime the (51 mol % AICI3) ratio leaps to 19:1. We have ourselves used scandium(III) triflate as a Lewis acid catalyst in a [bmim][TfO] ionic liquid and achieve an endo/exo ratio of 16.5:1. This led us to investigate the possibility that a similar interaction was occurring in the ionic liquids themselves (Figure 1). Indeed, it has been suggested that, in Diels-Alder reactions in dichloromethane with added imidazolium salts, their role is to act as Lewis acid catalysts. 2


9

5

5

11

Figure 1. The hydrogen bond (Lewis acid) interaction of an imidazolium cation with the carbonyl oxygen of methyl acrylate in the activated complex of the Diels-Alder reaction. Ν

Table II also shows the normalized Reichardt's Ε τ polarity scale values for the ionic liquids. In ionic liquids this scale is dominated by the ability of the solvent to stabilize the ground state of the dye through hydrogen bonding to the phenoxide site of the dye, giving a measurement of the liquid's ability to hydrogen bond to a solute. It can be seen that the endo/exo ratio correlates well with the E value of the ionic liquid, and hence the ability of the liquid to hydrogen bond to a solute. If the cation has been unchanged, its ability to act as a hydrogen-bond donor has been unchanged, so why is an effect seen? This can be explained by the observation that the selectivity of the reaction decreases as the cation-anion hydrogen bonding increases. The interaction with the methyl acrylate is via a hydrogen bond between the cation and the carbonyl oxygen of the electron withdrawing group. Clearly there is a competition between the anion and the solute for 12

N

T


245 the proton. Although the individual hydrogen bonds to the cations would be expected to be much weaker than that to the methyl acrylate, the anions are available in much higher concentrations. Hence, the endo/exo ratio and associated acceleration of the Diels-Alder addition of cyclopentadiene and methyl acrylate in ionic liquids is controlled by the ability of the liquid to act as a hydrogen-bond donor (cation effect) moderated by its hydrogenbond acceptor ability (anion effect). This may be described in terms of two competing equilibria. The cation can hydrogen bond to the anion: [bmimf + A" ^

^

[bmim]...A [[bmim]...A] ^ eqm ~

[[bmim]1[A-] The cation can hydrogen bond to the methyl acrylate: +

[bmim] + M A _

[bmim]...MA [[bmim]...MA] Κ

eqm

=

~

~

—

—

_

+

[[bmim] ][MA] It can be clearly seen that the concentration of the hydrogen bonded cationmethylacrylate adduct is inversely proportional to the equilibrium constant for the formation of the cation-anion hydrogen bonded adduct ( K ' ) . The room-temperature ionic liquids give substantial endo selectivity enhancements, and associated rate enhancements, in the reaction o f cyclopentadiene with methyl acrylate when compared to non-polar solvents. Hence, they offer the potential to be useful solvents for Diels-Alder cycloadditions, and related reactions, particularly for moisture and oxygen sensitive reagents. The greatest selectivities will be observed in ionic liquids with the strongest hydrogen-bond donor capacity coupled with the weakest hydrogen-bond acceptor ability. Further to this we can predict that this behaviour will be general and in any reaction where a reactive centre is activated by a neighbouring electron withdrawing group such an ionic liquid will further enhance its reactivity. We are continuing to investigate these propositions. eqm

References

1

(a) T. Welton, Chem. Rev., 1999,99, 2071; (b) K.R. Seddon and J.D. Holbrey, Clean Products and Processes, 1999, 1, 223; (c) P. Wasserschied and W . Keim, Angew. Chem. Int. Ed. Engl., 2000, 39, 3772. A . Sethi, T. Welton and J. Wolff, Tetrahedron Lett., 1999, 40, 793. D. A. Jaeger and C. E. Tucker, Tetrahedron. Lett, 1989, 30, 1785. M. J. Earle, P. B. McCormac and K. R. Seddon, Green. Chem., 1999, 23. 2

3

4

246 5

C. W. Lee, Tetrahedron Lett., 1999, 40, 2461. P.Ludley and Ν. Karodia, Tetrahedron Lett., 2001, 42, 2011. J. A. Berson, Z. Hamlet and W. A. Mueller, J. Am. Chem. Soc., 1962, 84, 297. R. Breslow, Acc. Chem. Res., 1991, 24, 159. U. Pindur, G. Lutz and C. Otto, Chem. Rev., 1993, 93, 741. A. G. Avent, P. A. Chaloner, M. P. Day, K. R. Seddon and T. Welton, J. Chem. Soc., Dalton Trans., 1994, 3405. J. Howarth, K. Hanlon, D. Fayne and P.McCormac, Tetrahedron Lett., 1997, 38, 3097. M. J. Muldoon, C. M. Gordon and I. R. Dunkin, J. Chem. Soc, Perkin 2, in press.

6

7

8

9


10

11

12

Dynamic Supramolecular Chemistry: The Role of Hydrogen Bonding

Recommend Documents