J. Phys. Chem. B 1998, 102, 2841-2844
2841
Enantiomeric Differences in Ion Association in a Chiral Solvent Cheryl D. Stevenson,* Rosario M. Fico, Jr., and Eric C. Brown Department of Chemistry, Illinois State UniVersity, Normal, Illinois 61790-4160 ReceiVed: October 15, 1997; In Final Form: February 18, 1998
7Li NMR spectra of solutions of the lithium salts of the R and S chiral isomers of the sec-butoxycyclooctatetraene dianion (C8H7-OC4H92-) in S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane were found to be remarkably different. The lithium cations in both solutions exist as mixtures of solvent-separated ion pairs and contact ion pairs with the dianion of C8H7-OC4H9. However, the relative concentration of the contact ion pair is much greater for the R isomer of C8H7-OC4H92- than for the S isomer. The complexity of steric interactions involving the chiral solvent and the R and S sec-butoxy groups results in the solvent being much more capable of partially separating Li+ from the S isomer of C8H7-OC4H92- than from the R isomer.
Introduction
SCHEME 1
A wide variety of chiral host-chiral guest systems that exhibit consequent chiral recognition have been developed. The chiral host molecules can vary considerably and include the following: crown ethers,1 cyclodextrins,2 porphyrins,3 etc.4 In this work the chiral host is simply the solvent, and it is demonstrated that the chiral solvent {S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane} is much more capable of squeezing between the lithium cation and the dianion of S-sec-butoxycyclooctatetraene (1) than between lithium cation and the dianion of R-secbutoxycyclooctatetraene (2). This direct observation of different ion association states of the chiral isomers in an ion pair was realized by making use of the special features of the cyclooctatetraene system.
Cyclooctatetraene (C8H8) is the smallest stable nonaromatic annulene, and it and its derivatives represent one of the most intensively studied systems in terms of dynamical conformational processes.5-7 The 4n π-electron nature of the D2d groundstate symmetry and the antiaromatic nature of the D8h neutral molecule renders C8H8 particularly vulnerable toward twoelectron reduction (Scheme 1). The resulting dianion has D8h symmetry,8 is thermodynamically stable,9 and is resistant to thermal degradation.10 The planar geometry of C8H82- combined with its double negative charge renders it particularly vulnerable to ion association, which has been studied in a variety of ways.11 Most useful for this study is the fact that the NMR chemical shifts associated with the cyclooctatetraene dianion vary with the degree of ion association.12 In hexamethylphosphoramide (HMPA), the presence of substituents on the cyclooctatetraene dianion (as the sec-butoxy moiety) alters the nature of the ion association between the dianion and the cations.11b,c,13 In HMPA, one of the best cationsolvating solvents known,14 the ion association is best described
as an equilibrium mixture of the solvent-separated ions and the free unassociated ions (C8H82-//Li2+ ) C8H82-//Li+ + Li+ ) C8H82- + 2Li+).11 Hence, it was anticipated that the lithium counterions of the sec-butoxycyclooctatetraene (C4H9O-C8H7) dianion salt in S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane would exist as an equilibrium mixture of the contact ion pair and the solvent-separated ion pair (C4H9O-C8H72-, Li+//Li+ ) C4H9O-C8H72-//Li+2). Further, we were motivated to see if the intimate relationship that exists between the solvent, cation, and dianion might, in the presence of this chiral solvent, vary sufficiently with the chirality of the sec-butoxy substituent to allow NMR observation of solvent-ion pair chiral recognition. Results Cyclooctatetraene (C8H8) was exhaustively reduced in tetrahydrofuran (THF) with lithium metal, the solvent removed under reduced pressure, and S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane distilled onto the solid dianion salt. The resulting deep blue solution shows only one strong 7Li NMR resonance at -10.4 ppm (relative to lithium iodide in deuterated acetone), Figure 1. This is even further upfield than the chemical shift observed in THF (-8.55 ppm) or dimethoxyethane (-9.21 ppm) where both cations are bound to the C8H8 dianion as tight or contact ion pairs.12 The more downfield shift in the case of DME was explained in terms of more molecules of THF solvating the cations being able to disperse
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2842 J. Phys. Chem. B, Vol. 102, No. 16, 1998
Figure 1. (Upper) 7Li NMR spectrum of the dilithium salt of the cyclooctatetraene dianion in S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane. The large peak at 0.00 ppm is due to the LiI in acetoned6 standard. The Merck (Spartan Program) minimized structure of the double-tight dilithium ion pair solvated by two S,S-(+)-2,3-dimethoxy1,4-bis(dimethylamino)butane molecules is also shown. (Lower) 7Li NMR spectrum of the dilithium salt of the cyclooctatetraene dianion in hexamethylphosphoramide. The spectra were recorded at 298 K on a 300-MHz 1H spectrometer. The double wavy lines indicate solvent molecules.
the positive charge on the cation better than fewer molecules of DME. Hence, it is argued12 that there is a shorter interatomic distance in the contact ion pair in DME compared with that in THF. The 7Li chemical shift in S,S-(+)-2,3-dimethoxy-1,4-bis-
(dimethylamino)butane reflects a very intimate association between the Li+ and C8H82- (C8H82-, Li+2). The results of
Letters PM3 calculations (Figure 1) suggest that this solvent encapsulates the “backside” of the lithium ion via strong interactions with both oxygen and a nitrogen atom. This ion-solvent complex, which is sterically encumbered except on the surface approaching the dianion, can then approach the planar dianion from its concave face (containing the Li+), Figure 1. In hexamethylphosphoramide (HMPA), on the other hand, it is known that the C8H8 dianion-lithium ion pair exists in the form of solvent-separated ions pairs (C8H82-//2Li+ and C8H82-//Li+ + Li+).13b Our 7Li NMR in this solvent exhibits two lithium resonances: one for the solvent-separated Li+ at -0.72 ppm and a small peak for the free solvated Li+ at +0.81 ppm (Figure 1). 7Li NMR analysis of the R-sec-C H O-C H 2- solution 4 9 8 7 (generated via reaction 1) exhibits signals for the contact ion pairs (centered around -9.81 ppm) and solvent separated ion pairs (centered around -1.1 ppm), Figure 2. A PM3 calculation on the double-solvated dilithium salt of the R-sec-C4H9O-C8H7 dianion indicates that the dihedral angle between the plane of the ring and the oxygen-methine carbon bond is 75°, Figure 3. The substituent has the effect of breaking the symmetry across the plane of the eight-member ring resulting in different resonances for the endo- and exo-lithium ions. This results in the nonequivalency of the lithium cations. In fact, the PM3 calculation suggests that the endo-lithium cation has a larger positive charge density than does the exo-Li+, indicating that the former would appear slightly more downfield upon NMR analysis.
Chiral recognition of the ion-associated lithium salts was realized upon comparison of the 7Li NMR spectra for the R
Figure 2. 7Li NMR spectrum of the dilithium salt of the R (D and B) and S (C and A) chiral isomers of the sec-butoxycyclooctatetraene dianion in S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane. The large peak at 0.00 ppm is due to the LiI in acetone-d6 standard. The spectra were recorded at 298 K (C and D) and 223 K (A and B).
Letters
J. Phys. Chem. B, Vol. 102, No. 16, 1998 2843
and S forms of sec-butoxycyclooctatetraene dianion in the same chiral solvent (Figure 2). At ambient temperature, both solutions exhibited three major resonances, Figure 2C,D. The broad highfield NMR lines are clearly due to the lithium ions that are tightly ion-associated with the dianion (shown in 3, 4, and 5 where the doubly wavy line represents a molecule of solvent). They are shifted upfield owing to the close proximity of the two added electrons. The resonances for these tightly associated lithium ions are centered at -9.81 and -9.65 ppm (relative to LiI in acetone-d6) for the R and S dianions, respectively. These lines are relatively broad, as the tightly associated lithium ions in the double-tight ion pair (3) and in the monosolvent-separated ion pairs (4 and 5) all resonate in this area.
The resonances for the solvent-separated lithium ions in (loose exo-4 and loose endo-4) appear further downfield. Interestingly, the NMR spectra show that the dissociation of the tight ion pair to solvent-separated ion pair (sec-C4H9O-C8H72-, 2Li+ ) sec-C4H9O-C8H72-, Li+//Li+) is about an order of magnitude greater for the S isomer than it is for the R isomer of sec-C4H9O-C8H72- in S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane. Structures 3 and 4 represent chemically distinct species, and the narrower lines at the lower temperature leads to resolution of the resonances for the tightly ion associated Li+’s in the double-tight and monosolvent-separated systems (Figure 2B). In agreement with this interpretation is the fact that the smaller newly resolved resonance at -10.2 ppm (attributed to tight endo4) has about the same intensity as the peak at -1.17 ppm for the solvent-separated Li+ (loose exo-4). When one of the lithium ions becomes partially separated from the dianion owing to the formation of the solvent-separated ion pair, the other Li+ would become more intimately associated. The relative upfield position (-10.2 ppm) of the peak attributed to tight endo-4 is consistent with this. Figure 2A shows that very little of the S-sec-butoxycyclooctatetraene dianion remains in the form of the double-tight ion associated species (3) at 223 K, which gives rise to the peak at -9.64 ppm. The broad resonance under this sharper peak is attributed to a mixture of tight endo-4 and tight exo-5.
Figure 3. Structure, from two viewpoints, of the S,S-(+)-2,3dimethoxy-1,4-bis(dimethylamino)butane solvated dilithium salt of the R-sec-butoxycyclooctatetraene dianion generated from a PM3 calculation (protons not shown). The calculation was performed by constraining the cyclooctatetraenyl moiety to D4h symmetry with rC-C ) 1.43 and 1.36 Å. The ring C-H bond lengths were constrained to 1.08 Å. Note that the endo-lithium cation is predicted to be closer to the center of charge in the dianion than is the exo-lithium ion and that the exo-lithium ion is interacting with the oxygen atom of the sec-butoxy group of the cyclooctatetraenyl system.
systems are interpreted as being due to the fact that the equilibrium constant controlling the ion pair dissociation of the double-tight ion associated species to form the monosolventseparated species is shifted to the right in the case of the S dianion while it is shifted to the left in the case of the R isomer of the dianion (reactions 2 and 3, double wavy line indicates a molecule of solvent). The NMR signals for the endo- and exo-solvent-separated lithium ions (product side of reactions 2 and 3) are resolved. The resonances for the tightly associated lithiums are, however, incompletely resolved as there are four different tightly associated lithium ions (see structures 3, 4, and 5).
Conclusions The 7Li NMR signals for the R and S chiral isomers of the dilithium salts of the sec-butoxycyclooctatetraene dianion in a chiral solvent (S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane) are considerably different indicating solvent-ion pair chiral recognition. The differences in the NMR spectra for these
Most likely there are more complicated species present in these solutions, such as ion aggregates, that are not discussed
2844 J. Phys. Chem. B, Vol. 102, No. 16, 1998
Letters Acknowledgment. We thank the National Science Foundation (Grant CHE-9617066) and ISU Department of Chemistry for the Silicon Graphics computer system. References and Notes
above. In fact, careful inspection of the NMR spectra presented in Figure 2 reveals the presence of other species. This, however, does not alter our conclusions concerning reactions 2 and 3. Experimental Section The sec-butoxycyclooctatetraenes were prepared via the method of Krebs from bromocyclooctatetraene and the appropriate sec-butoxide, which were generated from the enantiomeric alcohols.11c,13a,15 Both the (R)- and (S)-sec-butanol were purchased from Aldrich Chemical Co. and converted to the alkoxides with potassium metal under an inert atmosphere. The sec-butoxycyclooctatetraenes were purified via distillation and collected from 40 to 42 °C at 2 µm. In separate glass apparatuses, 70 µL (0.365 mmol) of the (R)and (S) forms of sec-C4H9O-C8H7 were exhaustively reduced in THF with lithium metal under high vacuum. The THF was removed under high-vacuum conditions to reveal a dark purple solid. The evacuated glass apparatuses containing the dark purple solids were then connected to an evacuated distillation apparatus, and 1 mL of S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)butane was distilled from a bulb containing potassium metal onto the solid salt. After dissolution of the dianion, the apparatus was tilted to allow the solution to pour into an attached NMR tube, which was subsequently sealed from the apparatus. The 7Li spectra were recorded on a Varian (Gemini) 300 MHz NMR spectrometer. PM3 calculations were carried out using the Spartan 5.0 program from Wavefunction Inc.16 The structures were optimized using Merck force field.16 This leads to a planar D4h cyclooctatetraenyl moiety. The structure was subsequently optimized via the PM3 method, while constraining the ring to the planar configuration with C-C bond lengths of 1.43 and 1.36 Å. The atomic charges were obtained from a single-point PM3 calculation.
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