Article pubs.acs.org/Organometallics
Uranium(IV) BINOLate Heterobimetallics: Synthesis and Reactivity in an Asymmetric Diels−Alder Reaction Jerome R. Robinson, Patrick J. Carroll, Patrick J. Walsh,* and Eric J. Schelter* P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: The first heterobimetallic BINOLate complexes incorporating uranium were prepared, and their reactivity in an asymmetric Diels−Alder reaction was investigated. The contributions of both the Li+ and UIV cations to the reaction selectivity were addressed through control of the two different Lewis acidic centers. The presence of an anionic ligand in the seventh coordination site of the central uranium cation resulted in enhanced selectivity compared to the RE(III) catalyst with the same alkali metal cation and represents the highest enantioselectivities obtained with a uranium-based catalyst to date. Additionally, we describe a simple workup procedure to obtain organic products free of the trace radioactivity present in the reaction mixtures.
■
INTRODUCTION Asymmetric catalysis is a powerful method for the preparation of optically active compounds. The enantioenriched products prepared in this way find applications in pharmaceuticals, materials, and synthesis.1 One successful class of asymmetric catalysts is based on multifunctional bimetallic compounds, which often exhibit enhanced reactivity and selectivity arising from cooperative interactions between neighboring sites on the catalyst.2 Among the most successful multifunctional bimetallic asymmetric catalysts are the rare-earth/alkali-metal/1,1′-binaphtholate (REMB; 1:3:3) complexes whose applications were pioneered by Shibasaki and co-workers (Figure 1).1a,b,2a,3 These complexes feature two unique Lewis acids: a central RE(III) cation and three alkali metal (M = Li, Na, K) cations. The compounds also contain six Brønsted-basic oxygen atoms of the BINOLate ligands that can serve to deprotonate the substrate and to shuttle protons between the catalyst and intermediates.
The REMB system is highly tunable, and simple changes in the identity of RE and M retain the catalyst scaffolding, but result in dramatic differences in reactivity and enantioselectivity. Despite the success and tunability of the REMB system in catalysis,2a,3 there has been no investigation of the oxidation state of the central metal ion in catalysis for these frameworks (Figure 1). We hypothesized that oxidized REMB analogues would be attractive for catalysis, because an increase in oxidation state at the central cation is expected to increase the Lewis acidity at both unique metal sites. Among the lanthanides, cerium is the only member that has an accessible tetravalent state.4 In contrast, early actinides readily support high oxidation states; uranium cations display oxidation states ranging from +3 to +6.5 Depleted uranium, primarily 238U, is a weak alpha emitter allowing for safe experimental work with simple monitoring procedures.6 In this work we disclose the synthesis of the first tetravalent uranium BINOLate complexes and benchmark their reactivity and enantioselectivity against established lanthanide catalysts using an asymmetric Diels−Alder test reaction. The results of this benchmarking show the highest enantioselectivities obtained with a uranium-based catalyst to date and provide evidence that the uranium−lithium heterobimetallic complexes catalyze the Diels−Alder reaction by activation of the dienophile at the lithium cations. Initial investigations provide evidence that installation of an anionic ligand in the seventh coordination site of the REMB framework enhances selectivity Special Issue: Recent Advances in Organo-f-Element Chemistry Received: December 7, 2012 Published: February 18, 2013
Figure 1. REMB complex framework. © 2013 American Chemical Society
1493
dx.doi.org/10.1021/om3011849 | Organometallics 2013, 32, 1493−1499
Organometallics
Article
Scheme 1. Synthesis of [Li3(sol)n][(BINOLate)3U-Cl] (sol = THF (1-U), DMEDA (2-U)) and [Li3(THF)5][(S)-(6,6′-Br2BINOLate)3U−(S)-(6,6′-Br2-H1BINOL)] (3-U)
U(IV)−Cl distance at 2.6718(7) Å is consistent with those reported for other uranium(IV) aryloxide chloride complexes.9
and suggests that selectivity may be further controlled by choice of the anionic ligand. Finally, we describe a simple workup procedure to obtain organic products free of the trace radioactivity present in the reaction mixtures.
■
RESULTS AND DISCUSSION Synthesis of U(IV)-BINOLate Complexes. Our initial approach to the synthesis of uranium-BINOLate complexes focused on isolating an isostructural uranium(III) REMB framework starting from either UI3 or U[N(SiMe3)2]3 in THF. In our hands, however, only uranium(IV) μ-hydroxoBINOLate clusters were isolated using those starting materials. In contrast, using the tetravalent UCl4 allowed for the synthesis of [Li3(THF)5][(BINOLate)3U-Cl] (1-U) as a green crystalline solid in 74% yield (Scheme 1). Additionally, formation of 1-U was found to be insensitive to stoichiometry; reaction of excess (7 equiv) Li2[(S)-BINOLate] at temperatures up to 80 °C in THF did not result in elimination of the fourth equivalent of chloride anion as LiCl. Surprisingly, the reaction of monodeprotonated Li[(S)-H 1 -BINOLate] with [((SiMe3)2N)3U-Cl] did not result in 1-U as a major product, as judged by 1H NMR, indicating UCl4 is superior for obtaining 1U. 1-U is isostructural to the recently reported [Li3(THF)5][(BINOLate)3Ce-Cl] (1-Ce),7 where the geometry of the central Ce(IV) or An(IV) ions is best described as a distorted trigonal prism. The use of (S)-BINOL to prepare 1-U leads to the expected Λ configuration of the complex, which was observed for 1-Ce and is similar to the reported trivalent lanthanides (Figure 1).3i,7,8 The U−OBINOLate distances range from 2.2939(17) to 2.3676(19) Å with Li−OBINOLate distances ranging from 1.888(5) to 1.925(5) Å, which are similar to the trivalent and tetravalent REMB (M = Li) frameworks. Also, the
Figure 2. Thermal ellipsoid plot of [Li3(THF)5][(BINOLate)3U-Cl] (1-U).
The NMR data for 1-U are unusual compared to those of the reported REMB complexes. The REMB complexes typically exhibit D3 symmetric structures in solution due to rapid exchange of THF coordinated to the RE ion with THF solvent.3i,8 In contrast, due to the chloride ligand bound at the uranium(IV) center, 1-U displays lower symmetry (C3) in THF-d8 at room temperature, as verified by 1H and 13C{1H} NMR (Figure 3). Twelve unique 1H resonances that represent 1494
dx.doi.org/10.1021/om3011849 | Organometallics 2013, 32, 1493−1499
Organometallics
Article
Figure 3. (a) 1H (top), (b) 7Li{1H} (top inset), and (c) 13C{1H} NMR (bottom) spectra of 1-U. Resonances belonging to (S)-BINOLate ligands are marked with *’s.
the inequivalent top and bottom faces of the BINOLate ligand framework are observed for the C3 symmetric 1-U at room temperature. Furthermore, 13C{1H} NMR data for 1-U showed 17 of the 20 expected resonances (Figure 3c); the two C− OBINOLate resonances, which typically fall between 160 and 180 ppm, were not observed. Their close proximity to the paramagnetic uranium(IV) center likely results in substantial line broadening of the C−OBINOLate resonances. The absence of the other BINOLate 13C resonance is suggested to be due to accidental equivalence. Finally, we observe one paramagnetically shifted 7Li signal (Figure 3b, 11.8 ppm), which is consistent with three equivalent Li+ cations on the NMR time scale. After successful isolation of 1-U, we pursued the synthesis of related U(IV) tris(BINOLate) complexes to study structure− function relationships.8a,b In the REMB framework, the DMEDA analogues have proven to be useful mechanistic probes due to DMEDA’s strong chelating preferences for the Li centers. Simple ligand substitution of 1-U with 3 equivalents of DMEDA resulted in crystallization of the diamine-bound complex [Li3(DMEDA)3][(BINOLate)3U-Cl] (2-U) in quantitative yield (Scheme 1 and Figure 4). 2-U is structurally similar to 1-U, with comparable metrical parameters about their uranium cores. In THF-d8 solutions the 7Li{1H} NMR of 2-U displays a single resonance (10.6 ppm) shifted upfield by 1.2 ppm relative to 1-U, indicating the different Li+ coordination environments for 1-U and 2-U in solution. Substitution of the BINOL ligand framework in the 6,6′positions alters the steric and electronic properties of the complex and has been shown to improve catalyst selectivity.3b,10 Deprotonation of (S)-6,6′-Br2-BINOL with 2 equiv-
Figure 4. Thermal ellipsoid plot of [Li3(DMEDA)3][(BINOLate)3UCl] (2-U).
alents of LiOtBu or LiN(SiMe3)2 in THF results in the corresponding Li2[(S)-6,6′-Br2-BINOLate] salt. Surprisingly, treatment of UCl4 in THF with 3 or 4 equivalents of Li2[(S)6,6′-Br2-BINOLate] resulted in the isolation of the tetrasubstituted compound [Li3(THF)5][(6,6′-Br2-BINOLate)3U-(6,6′Br2-H1BINOL)] (3-U, Scheme 1). It appears that the more electron-deficient [Li3(THF)5][(6,6′-Br2-BINOLate)3U-Cl] undergoes further substitution with 6,6′-Br2 -BINOLate 1495
dx.doi.org/10.1021/om3011849 | Organometallics 2013, 32, 1493−1499
Organometallics
Article
Reactivity of U(IV)-BINOLate Complexes. With this series of compounds in hand, we investigated the reactivity of 1-U, 2-U, and 3-U to determine the impact of the tetravalent oxidation state on the catalytic activity and mode of action of fblock tris(BINOLate) frameworks. Sasai, Shibasaki, and coworkers reported an asymmetric Diels−Alder reaction of an oxazolidinone substrate using the REMB framework, which undergoes cycloaddition by a Lewis acid catalyzed mechanism.3b The Diels−Alder reaction was chosen as a test reaction for the uranium-BINOLate complexes for two reasons: (1) optimized conditions for the reaction are anhydrous, allowing for facile correlation of solid state and solution catalytic species, and (2) the selectivity for this Diels−Alder reaction is reportedly dependent upon the size of the central lanthanide(III) ion.3b One concern with the use of uranium as a catalyst is the radioactivity of the element, even in its depleted form, which is primarily 238U. For our investigations, uranium was easily removed by passing the reaction mixtures through a short silica plug followed by an aqueous workup (Figure 6 and Supporting
(Scheme 1), which is in contrast to 1-U, which will not undergo further substitution with excess Li2[(S)-BINOLate]. While the source of the proton present at the dangling aryloxide moiety is not known, optimized and reproducible yields of ∼50% were obtained using 4 equivalents of the Li2[(6,6′-Br2-BINOLate], with separation of impurities afforded by three subsequent recrystallizations from THF/pentane. The X-ray structure of 3U (Figure 5) shows a seven-coordinate uranium center that
Figure 5. Thermal ellipsoid plot of [Li3(THF)5][(S)-(6,6′-Br2BINOLate)3U−(S)-(6,6′-Br2-H1BINOL)] (3-U). Solvent was removed for clarity.
Figure 6. Radioactivity of Diels−Alder reaction using different workup protocols. (1) 10 mol % 1-U (no workup); (2 and 3) short silica plug filtration followed by aqueous extraction. 2 = aqueous layer, 3 = organic layer, 4 = laboratory background.
adopts a slightly distorted trigonal prismatic geometry. The terminal U−OBINOLate bond distance of 2.221(5) Å is ∼0.1 Å shorter than the Li-bridged U−OBINOLate bond distances, which is expected with the increased electron density at the BINOLate alkoxide in the absence of bridging lithium interactions. Motivated by the complete aryloxide substitution observed for 3-U, we were compelled to probe the reactivity of the U−Cl bond of 1-U to generate coordinatively unsaturated, cationic uranium(IV) BINOLate complexes. Salt metathesis reactions of 1-U with Na[BArF4], BArF4 = B[3,5-C6H3-(CF3)2]4, in THF or toluene resulted in a color changes from green to brown solutions and complete consumption of 1-U as judged by 1H NMR. The salt metathesis reactions yielded mixtures of products, as judged by 1H NMR spectroscopy. The only crystalline products obtained from these reactions were low yields of poorly defined uranium(IV) μ-hydroxo-BINOLate trimeric clusters, akin to those obtained from reaction of uranium(III) starting materials with BINOLate salts. With the hope of isolating well-defined, cationic uranium-BINOLate products, the Lewis basic donor triphenylphosphine oxide or pyridine was added to the reaction mixtures of 1-U with Na[BArF4]. The presence of such additives, however, did not affect the outcomes of the reactions. On the basis of these experiments it seems likely that cationic, coordinatively unsaturated uranium-BINOLate compounds were not stable under the conditions specified.
Information). Quantitative assessment of these procedures was performed using a Beckman-Coulter LS 6500 liquid scintillation counter by following the β-decay of solution aliquots with the addition of scintillation cocktail. In Figure 6, entry 1 is the radioactivity (counts per minute, CPM) for a solution of ∼10 mol % of 1-U, whereas entry 4 is an average background reading from our lab (
