Synthesis and Catalytic Activity of Heterobimetallic Rare Earth–Zinc

Article pubs.acs.org/Organometallics

Synthesis and Catalytic Activity of Heterobimetallic Rare Earth−Zinc Ethyl BINOLate Analogues of Shibasaki’s Catalysts Ismael Nieto, Alfred J. Wooten,§ Jerome R. Robinson, Patrick J. Carroll, Eric J. Schelter,* and Patrick J. Walsh* P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States S Supporting Information *

ABSTRACT: Shibasaki’s heterobimetallic complexes [M3(THF)n][(BINOLate)3RE] (BINOLate = 1,1′-bi-2-naphtholate; RE = LnIII, YIII; M = Li+, Na+, K+) are among the most general and highly enantioselective catalysts known. Their structures, however, have been limited to group I metals in the peripheral sites. We envisioned that the utility of this class of catalysts could be broadened by the synthesis of new members. Herein, we report the first synthesis of Shibasaki-type catalysts that incorporate divalent Zn2+ ions in the peripheral positions. The compounds (EtZn)3(THF)2(BINOLate)3RE(THF) (RE = LaIII, PrIII, EuIII) are easily prepared from the corresponding tris(silylamide) precursors RE[N(SiMe3)2]3, 3 equiv of (S)-BINOL, and 3 equiv of ZnEt2 in 68−86% crystalline yields. The compounds are isostructural with known [Li3(THF)4][(BINOLate)3RE(THF)] catalysts. We have demonstrated that the (EtZn)3(THF)2(BINOLate)3RE(THF) complexes are catalytically active in the enantioselective addition of diethylzinc to benzaldehyde with moderate enantioselectivities.

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INTRODUCTION The heterobimetallic complexes [M3(THF)n][(BINOLate)3RE] (REMB, Figure 1; RE = La−Lu, Y; M =

selectivity in comparison with monometallic Lewis acid catalysts.1 The REMB framework is easily tuned by variation of the central and peripheral metal ions’ ionic radii, offering an array of accessible catalysts from a single enantioenriched ligand (BINOL). Despite their utility in asymmetric catalysis, there have been few reports incorporating other metal ions into the heterobimetallic system. One approach toward this goal is to substitute metals with different oxidation states for the central 3+ metal ion. Recently, we demonstrated that the REMB framework can support tetravalent central metal ions (Ce4+, U4+), which perform differently from their trivalent counterparts in catalytic asymmetric reactions.2 Another approach is to replace the peripheral main-group metals (Li+, Na+, and K+) with alternate metal cations. Such a change would be expected to have a pronounced impact on the catalytic behavior and lead to new modes of reactivity. Herein, we report the first examples of substitution of the peripheral metal cations in Shibasaki’s catalysts with the introduction of Zn2+, resulting in the synthesis and characterization of rare earth−zinc ethyl heterobimetallic complexes, (EtZn)3(THF)2(BINOLate)3REIII(THF) (RE = La, Pr, Eu). We have determined that (EtZn)3(THF)2(BINOLate)3RE(THF) complexes are isostructural with their lithium analogues

Figure 1. Shibasaki’s REMB catalysts: RE = LnIII, YIII; M = Li+, Na+, K+; n = 1, 2.

Li, Na, K; BINOLate = 1,1′-bi-2-naphtholate), developed by Shibasaki and co-workers, have proven to be highly enantioselective in a broad range of mechanistically diverse reactions.1 These multifunctional catalysts contain two unique Lewis acid components (REIII and M+) and Brønsted-basic BINOLate oxygens within close proximity of each other. This characteristic facilitates intramolecular cooperativity in reactivity and is proposed to be responsible for the high levels of © 2013 American Chemical Society

Received: September 20, 2013 Published: December 2, 2013 7431

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Scheme 1. Synthesis of (EtZn)3(THF)2(BINOLate)3RE(THF)a

a Legend: (a) THF, room temperature, 3 days, RE = La ((S)-1-La), Eu ((S)-1-Eu); (b) THF, room temperature, 1 day, RE = La ((S)-1-La), Pr ((S)1-Pr), Eu ((S)-1-Eu).

Figure 2. Thermal ellipsoid plot of (S)-1-Eu with ellipsoids set at the 50% probability level.

metals with cations of size similar to that of lithium. The Li+ cation has an ionic radius of 0.59 Å with CN = 4,6 and the Zn2+ cation has an ionic radius of 0.6 Å with CN = 4.6 Further, zinc(II) is known to form asymmetric catalysts with BINOLatebased ligands.7 Synthesis and Characterization of (EtZn)3(THF)2(BINOLate)3RE(THF) Complexes. Two routes to synthesize zinc derivatives of REMB complexes were envisioned. The first was a salt-metathesis reaction with REX3 (X = halide, pseudohalide), and the second was protonolysis using RE[N(SiMe3)2]3.1−3,8 Our first approach to incorporate zinc into the REMB framework involved combining a THF solution of anhydrous RE(OTf)3 and (S)-BINOL with excess Et2Zn. We expected that diethylzinc would react rapidly with the acidic BINOL to generate BINOLate(ZnEt)2 in situ. The BINOLate(ZnEt)2 was anticipated to undergo metathesis with RE(OTf)3 to form the targeted products. Indeed, reactions performed with La(OTf) 3 or Eu(OTf) 3 generated the desired (EtZn)3(THF)2(BINOLate)3RE(THF) (RE = La ((S)-1-La), Eu ((S)-1-Eu)) (Scheme 1, route (a)). Although we were able

and have demonstrated their catalytic activity in enantioselective ethyl addition to benzaldehyde.

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RESULTS AND DISCUSSION We have recently shown that the primary determinants in the ability of Shibasaki’s complexes [M3(THF)n][(BINOLate)3RE] to bind substrate analogues and/or Lewis basic ligands at the lanthanide cation are dependent on the size of the main group metal cation and its ability to form π-arene type noncovalent interactions with the BINOLate ligands.3 When the main-group metal is lithium, binding of substrate analogues to the rare-earth cation occurs readily and can be observed both in the solid state by X-ray crystallography and in solution by NMR spectroscopy. Larger alkali metals significantly reduce the ability of the RE to obtain a seven-coordinate geometry; when M = Na, only the early lanthanides (La−Gd) coordinate water at the RE center in the solid state,1,4 and M = K derivatives will not bind water in the solid state or solution.5 The lithium derivatives have been applied to the widest variety of asymmetric transformations; therefore, we initiated our efforts to replace the peripheral 7432

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Table 1. Selected Bond Distances (Å) for (EtZn)3(THF)2(BINOLate)3RE(THF) and [Li3(THF)4][(BINOLate)3RE(THF)] ((S)-1-RE and (S)-2-RE) RE−OBINOLate (avg) RE−OTHF M−OBINOLate (avg) Zn−C (av) M−OTHF (avg) M3−E displacementa a

(S)-1-La

(S)-1-Pr

(S)-1-Eu

(S)-2-La

(S)-2-Pr

(S)-2-Eu

2.445(9) 2.520(4) 1.989(9) 1.952(11) 2.172(6) 0.6747(3)

2.412(32) 2.4844(15) 1.990(34) 1.950(3) 2.172(21) 0.65608(9)

2.352(11) 2.424(6) 2.004(6) 1.947(16) 2.156(8) 0.6373(3)

2.447(10) 2.567(5) 1.882(33) N/A 1.930(29) 0.767(4)

2.402(9) 2.542(4) 1.882(24) N/A 1.929(23 0.7490(3)

2.355(9) 2.482(4) 1.878(26) N/A 1.931(26) 0.737(3)

Illustration detailing M3−RE displacement shown in Figure 3.

to obtain crystalline product from route (a)), removal of the major byproduct, EtZn(OTf), was difficult due to the similarity of its solubility properties to those of (S)-1-RE. To avoid the difficult separation and to raise the yields of (S)-1-RE, an improved synthesis was devised employing RE[N(SiMe3)2]3 precursors.9 Combination of 1 equiv of RE[N(SiMe3)2]3 and 3 equiv of (S)-BINOL followed by addition of 3 equiv of Et2Zn (Scheme 1, route (b)) led to the formation of (S)-1-RE (RE = La ((S)-1-La), Pr ((S)-1-Pr), Eu ((S)-1-Eu)), which were obtained in 76, 68, and 86% crystalline yields, respectively. Attempts to use alternate zinc reagents, such as Zn[N(SiMe)3]2 and EtZnOTf, in place of Et2Zn led to intractable mixtures.10,11 After obtaining crystalline products, we then proceeded with characterization of the (S)-1-RE complexes for comparison with the known Shibasaki REMB complexes. The 1H and 13C{1H} NMR spectra of (S)-1-La, (S)-1-Pr, and (S)-1-Eu were recorded in THF-d8 and were similar to reported spectra for the [Li3(THF)n][(BINOLate)3RE(THF)] (RE = La ((S)-2-La), Pr ((S)-2-Pr), Eu ((S)-2-Eu)) complexes.1−5,8,12 As expected, (S)-1-La is diamagnetic, with resonances ranging from 7.78 to −1.01 ppm, while (S)-1-Eu and (S)-1-Pr exhibit paramagnetically shifted and broadened resonances ranging from 18.78 to −7.98 ppm and 13.86 to −6.33 ppm, respectively (spectra are provided as Supporting Information). The resonances for the zinc-bound ethyl groups of (S)-1-RE are diastereotopic due to the chiral environment of the BINOLate ligands. Characterization of the solid-state structures of (S)-1-La, (S)-1-Pr, and (S)-1-Eu was performed by single-crystal X-ray diffraction. The complexes (S)-1-RE are isostructural with their [Li3(THF)4][(BINOLate)3RE(THF)] ((S)-2-RE) analogues. The crystal structure of the europium derivative (S)-1-Eu is shown in Figure 2. Like the [Li3(THF)4][(BINOLate)3RE(THF)] complexes, the zinc analogues (EtZn)3(THF)2(BINOLate)3RE(THF) exhibit seven-coordinate RE cations comprising six BINOLate oxygen atoms and one bound THF molecule at the rare-earth cation. The REO7 cores of these complexes are trigonally compressed capped octahedra, with THF acting as the capping group.3 Table 1 gives selected bond lengths (Å) of (S)-1-RE and the lithium analogues (S)-2-RE for comparison.3 The RE−O distances of the (S)-1-RE complexes decrease in the order La > Pr > Eu, as anticipated on the basis of the decreasing ionic radii of these cations.3 Likewise, the degree of displacement of the central RE ion from the plane formed by the three peripheral metal ions also decreases in the order La > Pr > Eu, as observed for the (S)-2-RE complexes (Figure 3).13 These displacements, however, are about 0.1 Å less than those for the analogous (S)2-RE complexes.3 The RE−O distances for (S)-1-RE and (S)2-RE are similar. In contrast, the Zn−O distances of (S)-1-RE are longer than the Li−O distances in (S)-2-RE by about 0.1 Å,

Figure 3. M3−RE displacement measurement (Å) of the distance between the plane as defined by the three outer-sphere metal ions to the central RE. Measurements were calculated with CrystMol.13

and longer than the Zn−C distances of (S)-1-RE by 0.2 Å. This difference in shorter Zn−C bonds in comparison with Zn−O bonds has been reported for complexes such as the [Zn4(Et)2(OEt)2(MaIO)4] (MaIO = 3-hydroxy-2-methyl-4Hpyran-4-one-based O,O′-bidentate ligand) tetrameric cluster and the {Li(THF)(EtZn)3(OiPr)4} cubane complex.14 The coordination environments of the zinc ions in (S)-1-RE consist of two four-coordinate zinc centers and one threecoordinate zinc, resulting in a total of three ethyl-bound and two THF-bound zinc centers. Although less common, threecoordinate zinc complexes are known in the literature.15 After determining the solid-state structures of the (S)-1-RE complexes and characterizing their solution structures by NMR spectroscopy, we shifted our attention to their reactivity. Reactivity Studies of (EtZn)3(THF)2(BINOLate)3RE(THF) Complexes with Benzaldehyde. With the identities of (S)1-RE (RE = La, Pr, Eu) confirmed, we set out to probe the reactivity of these new complexes. We were especially interested in the potential reactivity of the ZnEt+ groups, as positioning of the EtZn+ moiety within the BINOLate cleft could greatly affect the (S)-1-RE complex’s ability to act as a heterobimetallic catalyst. To begin our reactivity studies, we investigated a classic benchmark reaction for organozinc compounds: the catalytic asymmetric addition of zinc alkyl groups to aldehydes. Organozinc reagents have not been used for asymmetric additions catalyzed by the REMB framework, although the stoichiometric addition of alkyl reagents to aldehydes promoted by (S)-2-La has been investigated by Aspinall and co-workers.8 In the reported work, methyl- and nbutyllithium were used for the additions to various aldehydes. Under their optimized conditions, benzaldehyde underwent selective methylation to afford (S)-1-phenyl-1-ethanol in 84% ee and 46% yield. Other substrates exhibited addition reactions in moderate to excellent yields (31−93%) with low to moderate enantioselectivities (33−74%).8 We first investigated the reactivity of (S)-1-Eu with benzaldehyde at room temperature. In both coordinating (THF) and noncoordinating (toluene) solvents, no transfer 7433

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Further reaction optimization involved use of an additive, triphenylphosphine oxide (TPPO), on the basis of reports of improved enantioselectivity with [Li3(THF)n][(BINOLate)3Y] and TPPO in cyano-ethoxycarbonylation of benzaldehyde with (S)-2-La.17 The results in Table 3 showed an increase in

of the ethyl group from (S)-1-Eu to benzaldehyde was observed over the course of 24 h by 1H NMR, suggesting that the ethyl groups were only weakly nucleophilic. Noyori and co-workers observed similar behavior with their amino alcohol based (2S)-DAIB (=(2S)-(−)-3-exo-(dimethylamino)isoborneol), which reacted with dialkylzinc reagents to form the alkyl zinc complexes ((2S)-DAIB)ZnR (R = Me, Et). The three-coordinate ((2S)-DIAB)ZnR, which is in equilibrium with its dimer, required excess dialkylzinc reagent to form ((2S)-DAIB)ZnR·ZnR2. This bimetallic intermediate exhibited high reactivity in the alkylation of aldehydes.16 Likewise, in our system, addition of an extra 1 equiv of diethylzinc to (S)-1-Eu and benzaldehyde resulted in the formation of 1-phenyl-1propanol in 61% yield and 46% ee (determined by gas chromatography). After optimization of the conditions, the stoichiometric ethylation in toluene using (S)-1-Eu and diethylzinc produced 1-phenyl-1-propanol in 97% GC yield and 47% ee. Control experiments using (S)-BINOL and diethylzinc in the absence of (S)-1-Eu produced 47% GC yield and 44% ee in toluene over 24 h. Encouraged by our initial results from the stoichiometric reactions, we pursued the application of (S)-1-RE in the ethylation under catalytic conditions. The catalytic ethyl additions were performed by premixing 10 mol % of (S)-1-RE or (S)-2-RE in toluene with 3 equiv of diethylzinc followed by addition of benzaldehyde and stirring for 48 h at room temperature (Table 2). Overall, the (S)-1-RE

Table 3. Ethyl Addition on Benzaldehyde Catalyzed by (S)1-RE/TPPOa

catalyst

amt of X (mol %)

GC yield (%)

ee (%)

1 2 3 4 5 6 7

(S)-1-La (S)-1-Pr (S)-1-Eu (S)-2-La (S)-2-Pr (S)-2-Eu (S)-BINOL

10 10 10 10 10 10 30

93 96 96 90 98 98 49

34 44 51 30 37 38 44

catalyst

amt of X (mol %)

amt of Y (mol %)

GC yield (%)

ee (%)

1 2 3 4 5 6 7 8

(S)-1-La (S)-1-Pr (S)-1-Eu (S)-1-Eu (S)-1-Eu (S)-BINOL (S)-BINOL (S)-BINOL

10 10 10 10 10 30 30 30

15 15 15 30 45 15 30 45

95 89 99 99 99 61 99 99

40 53 70 69 67 27 13 10

a

Reaction conditions: 0.1−0.3 mmo of l (S)-1-RE ((S)-BINOL) in 2 mL of toluene, 0.15−0.45 mmol of TPPO, 3 mmol of Et2Zn in toluene (1.5 M), and 1 mmol of benzaldehyde. The ee and yield (in percent) were determined by gas chromatography integration against tetradecane (C14H30) internal standard.

Table 2. Ethyl Addition on Benzaldehyde Catalyzed by (S)1-RE and (S)-2-REa

entry

entry

enantioselectivity for (S)-1-La, (S)-1-Pr, and (S)-1-Eu to 40, 53, and 70% ee, respectively, with 15 mol % of the TPPO additive (entries 1−3). Increasing the amount of TPPO to 30 and 45 mol % (entries 4 and 5) showed minimal perturbation of the enantioselectivity and suggests that a (S)-1-Eu:TPPO ratio of ∼1:1 is optimal. Control experiments similar to those in Table 2 were performed (Table 3, entries 6−8) to ensure that the improved performance of (S)-1-Eu in the presence of TPPO was not due to the formation of active Zn(BINOLate) species. The results from these (S)-BINOL/TPPO combinations showed a dramatic decrease in enantioselectivity as the amount of TPPO was increased from 0 to 45 mol %. These results provide further support that the catalytically active species formed from (S)-1-Eu are not Zn(BINOLate) species. On the basis of our experiments involving TPPO, we propose the following: (1) that the improved enantioselectivity for the ethylation of benzaldehyde involves the formation of (S)-1-RE· TPPO adduct as an active species and (2) that the (S)-1-RE complexes do not decompose into active Zn(BINOLate) species under catalytic conditions. Given the improved selectivity upon addition of TPPO to (S)-1-Eu, and our interest in the interactions between Et2Zn and (S)-1-Eu, we pursued NMR studies to help elucidate the solution behavior. NMR Studies on (S)-1-Eu. Given that no ethylation was observed on combining (S)-1-Eu and benzaldehyde in the absence of diethylzinc, we were interested in determining whether (S)-1-Eu and diethylzinc interacted in solution. Therefore, we employed 2D 1H NMR exchange spectroscopy (EXSY)18 to study equimolar amounts of (S)-1-Eu and diethylzinc in THF-d8. The compiled 2D EXSY spectra (see the Supporting Information) displayed exchange cross peaks between the methylene and methyl protons associated with

a Reaction conditions: 0.1 mmol of (S)-1-RE ((S)-2-RE) in 2 mL of toluene, 3 mmol of Et2Zn in toluene (1.5 M), and 1 mmol of benzaldehyde. The ee and yield (in percent) were determined by gas chromatography on the basis of an average of two trials, with 0.5 mmol of tetradecane (C14H30) as internal standard. A 0.3 mmol portion of (S)-BINOL was used instead of catalyst for entry 7.

complexes (entries 1−3) were more enantioselective than their (S)-2-RE (entries 4−6) counterparts by about 4−12% ee, with the smaller RE cations providing higher levels of enantioselectivity. In order to address the concern that Zn(BINOLate) species are the active catalysts in the presence of (S)-1-RE, we used 30 mol % (S)-BINOL instead of (S)-1-RE or (S)-2-RE under our catalytic conditions (entry 7). While the level of enantioselectivity upon use of 30 mol % (S)-BINOL was 44%, it is noteworthy that the conversions were significantly lower than those with (S)-1-RE, suggesting that the catalysts formed from these precursors are indeed different. 7434

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Figure 4. Partial 1H EXSY NMR spectrum (δ 1.5 to −8.5 ppm) of equimolar (S)-1-Eu:Et2Zn at 298 K in THF-d8 (τM = 100 ms). The exchange cross-peaks (gray circled regions) between the zinc methylene resonances (noted in black for (S)-1-Eu and green for Et2Zn) are shown.

Figure 5. Selected 31P{1H} NMR (THF-d8) spectra of (S)-1-Eu/TPPO combinations.

NMR spectra from these experiments are shown in Figure 5, with full experimental details provided as Supporting Information. Free TPPO appears as a singlet at 24 ppm in the 31P{1H} NMR spectrum (Figure 5A). The combination of (S)-1-Eu with 1 equiv of TPPO results in a dramatic upfield shift in the 31P{1H} NMR to −80 ppm (Figure 5B), supporting the notion that the TPPO is binding to the paramagnetic Eu3+ cation of (S)-1-Eu. Interestingly, our observation is in contrast to those of Shibasaki and co-workers, where addition of TPPO to (S)-2-Y results in binding at the Li+ centers.17 Addition of

both zinc species for mixing times (τM) of 20−100 ms, indicating ready exchange between (S)-1-Eu and diethylzinc at room temperature (Figure 4). This result indicates that the ethyl groups in the (S)-1-RE complexes are capable of undergoing ligand exchange reactions. The exchange of diethylzinc with the zinc-based heterobimetallic complex is reminiscent of the exchange of diethylzinc with ((2S)DAIB)ZnR.16 Further insight was gained by observation of 31P{1H} NMR of (S)-1-Eu with varying amounts of TPPO. Selected 31P{1H} 7435

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ethyl group to the carbonyl carbon from the coordinated ZnEt2, as illustrated in the proposed transition state (Figure 7).

another 1 equiv of TPPO to (S)-1-Eu gives rise to two signals in the 31P{1H} NMR spectrum at 28 and −79 ppm. The 31 1 P{ H} NMR chemical shifts in Figure 5B−D indicate that the TPPO binds to (S)-1-Eu and remains relatively constant on the NMR time scale at −79 ppm in comparison with the second signal at 28 ppm. The 31P{1H} NMR signal at 28 ppm is assigned to TPPO bound at the Zn2+ cation. Examination of a reaction with (S)-1-Eu (1 equiv), TPPO (1.5 equiv), ZnEt2 (6 equiv), and benzaldehyde (3 equiv) after 30 min at room temperature by 31P{1H} NMR similarly exhibited the two resonances (Figure 5E). On the basis of the results of the NMR studies and catalytic reactions outlined above, we propose the following key ethyl zinc exchange dynamics. Exchange of the ethyl groups between (S)-1-Eu and diethylzinc, as observed in the 2D EXSY spectra, can be envisioned to occur by multiple pathways. Given that diethylzinc is unsaturated and known to bind to Lewis bases,19 we favor a pathway that involves binding of the diethylzinc to a BINOLate oxygen of (S)-1-Eu (Figure 6). Cleavage of a Zn−O

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CONCLUSIONS Shibasaki’s [M3(THF)n][(BINOLate)3RE] catalysts exhibit excellent enantioselectivities in a broad range of catalytic asymmetric transformations.1−5 Among this family of catalysts, the lithium derivatives [Li3(THF)n][(BINOLate)3RE], have been applied to the widest variety of asymmetric transformations, making them an attractive framework to target for new analogues. In this report, we have expanded Shibasaki’s family of [M3(THF)n][(BINOLate)3RE] catalysts by preparing the first members to contain metal ions outside group I in the peripheral sites. Our choice of main-group metal was based on the hypothesis that zinc(II) ions could be substituted for lithium(I) ions because of their similar ionic radii. The complexes synthesized herein, (EtZn)3(THF)2(BINOLate)3RE(THF) (RE = La ((S)-1-La), Pr ((S)-1-Pr), Eu ((S)-1Eu)), have been prepared by the reaction of the readily available rare-earth tris(silylamides) RE[(N(SiMe3)2]3 with 3 equiv of (S)-BINOL and 3 equiv of diethylzinc in 68−86% yield. The compounds (S)-1-RE are isostructural with their lithium counterparts [Li3(THF)4][(BINOLate)3RE(THF)] ((S)-2-RE). We have also explored the application of (EtZn)3(THF)2(BINOLate)3RE(THF) in the enantioselective addition of diethylzinc reagents to benzaldehyde. Enantioselectivities as high as 70% were achieved in the presence of TPPO with (S)-1-RE under catalytic conditions. More importantly, control experiments with (S)-2-RE and (S)BINOL provide circumstantial evidence that the active catalyst in these reactions is not formed from decomposition to generate Zn(BINOLate)-based species. NMR studies on (S)-1Eu and diethylzinc indicate exchange of the ZnEt groups by 1H NMR EXSY. NMR studies on (S)-1-Eu and TPPO suggest TPPO binds tightly to (S)-1-Eu at the Eu3+ ion. On the basis of these experiments, we hypothesize that the (EtZn)3(THF)2(BINOLate)3RE(THF) complexes are the active species and are indeed viable catalysts for this reaction. Further studies will focus on the synthesis of related [(LnM)3(THF)n][(BINOLate)3RE(THF)] complexes incorporating different metal−ligand adducts in the peripheral sites and expanding the reactions that are catalyzed by (EtZn)3(THF)2(BINOLate)3RE(THF) complexes.

Figure 6. Possible reaction pathway (clockwise) for exchange of ethyl groups between (S)-1-Eu and diethylzinc.

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bond of the heterobimetallic followed by ethyl transfer exchanges the ethyl groups and the zinc cations. With a grasp of the spectroscopy in support of ethyl zinc exchange dynamics, a broader mechanism for the alkylation can be proposed. A reasonable reaction pathway for the catalytic asymmetric ethyl addition to benzaldehyde is presented in Figure 7. On the basis of the binding of TPPO to (S)-1-Eu and the increased enantioselectivity in the presence of TPPO, we hypothesize that TPPO forms an (S)-1-Eu·TPPO adduct, which is present in the catalytic cycle. Coordination of diethylzinc to the BINOLate oxygen atom of (S)-1-Eu·TPPO increases the Lewis acidity of the adjacent ZnEt moiety, which is then better able to activate the benzaldehyde substrate. At the same time, the exogenous ZnEt2 ethyl groups become more electron rich on coordination to the BINOLate oxygen of (S)-1-Eu·TPPO. This synergistic interaction enables the binding of benzaldehyde at the Lewis acidic zinc center and facilitates the delivery of the

EXPERIMENTAL SECTION

General Methods. All reactions and manipulations were carried out under an inert atmosphere in a Vacuum Atmospheres drybox with attached MO-40 Dritrain or by using standard Schlenk or vacuum line techniques. Glassware was oven-dried overnight at 150 °C prior to use. 1 H and 13C{1H} NMR spectra were obtained on a Bruker DRX 500 spectrometer. 31P{1H} NMR spectra were obtained on a Bruker DMX300 spectrometer. Chemical shifts were recorded in units of parts per million downfield from residual proteo solvent peaks (1H) or characteristic solvent peaks (13C{1H}). The 31P{1H} spectra were referenced to external solution H3PO4 at 0 ppm. Elemental analyses were performed at the University of California, Berkeley Microanalytical Facility, and at Robertson Microlit Laboratories using a PerkinElmer Series II 2400 CHNS analyzer. Analyses of enantiomeric excess and experimental yields pertaining to the formation of (S)-1phenylpropanol were performed by gas chromatography using a Hewlett-Packard 6990 or Agilent Technologies 7890A GC instrument with a Beta-Dex column. 7436

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Figure 7. Proposed reaction pathway for the catalytic addition of ethyl groups to benzaldehyde promoted by (S)-1-Eu and diethylzinc. Materials. Tetrahydrofuran (THF), hexanes, dichloromethane (CH2Cl2), and toluene were purchased from Fisher Scientific. The solvents were sparged for 20 min with dry N2 and dried using a commercial two-column solvent purification system comprising columns packed with Q5 reactant and neutral alumina, respectively (for hexanes and toluene), or two columns of neutral alumina (for THF and CH2Cl2). Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., and stored over potassium mirror overnight prior to use. RE(OTf)3 (RE = La, Pr, Eu; OTf = triflate; purchased from Strem Chemicals Inc.) were heated at 150 °C for 12 h at ∼100 mTorr prior to use. Mesitylene was stirred with silica gel for several hours, distilled under N2, and stored in a glovebox prior to use. Tetradecane and benzaldehyde were stirred with MgSO4 for 1 day under N2, distilled under reduced pressure, and stored in a glovebox prior to use. RE[N(SiMe3)2]3·(toluene)0.5 (RE = La, Pr, Eu) were prepared according to literature procedures.9 The complexes [Li3(THF)4][(BINOLate)3RE](THF) (RE = La ((S)-2-La), Pr ((S)2-Pr), Eu ((S)-2-Eu)) were prepared according to literature procedures.3 Unless otherwise specified, all other reagents were purchased from Sigma-Aldrich, Fisher Scientific, or Strem Chemicals and used as received. X-ray Crystallography. X-ray intensity data were collected on a Brüker APEXII CCD area detector employing graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at a temperature of 143(1) K. In all cases, rotation frames were integrated using SAINT,20 producing a listing of unaveraged F2 and σ(F2) values, which were then passed to the SHELXTL21 program package for further processing and structure solution on a Dell Pentium 4 computer. The intensity data were corrected for Lorentz and polarization effects and for absorption using SADABS.22 The structures were solved by direct methods (SHELXS97).23 Refinement was by full-matrix least squares based on F2 using SHELXL-97.23 All reflections were used during refinements. Nonhydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. General Synthetic Procedure and Characterization Data for (EtZn)3(THF)2(BINOLate)3RE(THF) ((S)-1-RE) Complexes. In a 50 mL Schlenk flask, RE[N(SiMe3)2]3·0.5(toluene) (1 equiv) and (S)BINOL (3 equiv) were combined with 10 mL of THF and the reaction solution was stirred for 2 h at room temperature. A solution of Et2Zn (3 equiv) in THF (5 mL) was then added dropwise to the reaction mixture. After 1 day of stirring at room temperature, the volatiles were removed under reduced pressure. The residue was washed with hexanes and then dried under reduced pressure to

provide the crude product in >90% yield. Crystalline product was obtained by dissolving the crude product in minimal THF followed by layering the product solution with hexanes (30/70 volume/volume ratio). The product was collected by filtration after 2 days. The characterization details of the (S)-1-RE complexes, including 1H and 13 C{1H} NMR spectral assignments (noted in Figure 8), yields, and elemental analyses are as follows.

Figure 8. Diagram illustrating the numbering system used in the 1H NMR spectral assignments of the (EtZn)3(THF)2(BINOLate)3RE(THF) complexes. (EtZn)3(THF)2(BINOLate)3La(THF) ((S)-1-La): using 500 mg (0.75 mmol) of La[N(SiMe3)2]3·(toluene)0.5, 650 mg (2.25 mmol) of (S)-BINOL, and 0.23 mL (2.25 mmol) of Et2Zn yielded 858 mg (0.58 mmol) of crystalline product (76% yield). 1H NMR (500 MHz, THFd8, 25 °C): δ 7.78 (H3, d, J = 6.0 Hz, 6H), 7.71 (H4, d, J = 8.1 Hz, 6H), 7.08 (H6 and H7, m, 12H), 6.92 (H8, m, 6H), 6.80 (H9, d, J = 8.6 Hz, 6H), 0.16 (ZnCH2CH3, t, J = 8.0 Hz, 9H), and −1.00 (ZnCH2CH3, br q, J = 7.9 Hz, 6H) ppm. 13C{1H} NMR (500 MHz, THF-d8, 25 °C): δ 159.0, 136.0, 129.6, 129.4, 128.2, 127.0, 125.8, 124.3, 122.6, 119.5, 12.0, and −2.2 ppm. Anal. Calcd for C78H75O9Zn3La: C. 62.81; H, 5.07. Found: C, 62.48; H, 5.14. (EtZn)3(THF)2(BINOLate)3Pr(THF) ((S)-1-Pr): using 500 mg (0.74 mmol) of Pr[N(SiMe3)2]3·0.5(toluene), 635 mg (2.22 mmol) of (S)-BINOL, and 0.23 mL (2.22 mmol) of Et2Zn yielded 750 mg (0.50 mmol) of crystalline product (68% yield). 1H NMR (500 MHz, THFd8, 25 °C): δ 13.86 (H9, br s, 6H), 9.75 (H8, br s, 6H), 8.18 (H7, br s, 6H), 7.52 (H6, br s, 6H), 7.23 (ZnCH2CH3, br s, 3H), 6.85 (ZnCH2CH3, br s, 3H), 5.22 (ZnCH2CH3, br s, 9H), 4.14 (H4, br s, 6H), and −6.33 (H3, br s, 6H) ppm. 13C {1H} NMR (500 MHz, THF-d8, 25 °C): δ 170.2, 145.5, 142.7, 135.0, 134.1, 129.0, 128.8, 7437

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Et2Zn (neat, 20 μmol) was added to the NMR tube. This solution was set aside for 10 min before the 1H NMR was acquired. Following initial 1H NMR of the (S)-1-Eu/Et2Zn solution, 1H NMR EXSY experiments were performed at variable mixing times (τM) from 0 to 100 ms. 1H, 1H COSY, and 1H EXSY NMR spectra are provided as Supporting Information. Procedures for 1H and 31P{1H} NMR studies on (S)-1-Eu/ TPPO Combinations. 1H and 31P{1H} NMR spectra are provided in the Supporting Information. (S)-1-Eu with Sequential TPPO Additions. In a J. Young NMR tube were placed 50 mg (33 μmol) of (S)-1-Eu, 5 μL of mesitylene (33 μmol), and 0.5 mL of THF-d8, and the tube was sealed with a threaded Teflon cap. The 1H NMR spectrum of the resulting solution was obtained as a reference spectrum (Figure S54A, Supporting Information). In a separate NMR tube were placed TPPO and 0.5 mL of proteo-THF, the tube was sealed with a Teflon cap, and the 31 1 P{ H} NMR spectrum was obtained (Figure S55A, Supporting Information). After spectrum A was obtained, 10 mg of TPPO (33 μmol) was added to the NMR tube, and this mixture was allowed to sit for 30 min. The 1H and 31P{1H} NMR spectra were then acquired (Figures S54B and S55B, Supporting Information). After spectrum B was obtained, 10 mg of TPPO (33 μmol) was added to the NMR tube, the solution was allowed to stand for 30 min, and then 1H and 31 1 P{ H} NMR spectra were again acquired (Figures S54C and S55C, Supporting Information). Finally, 20 mg of TPPO (66 μmol) was added to the NMR tube and, after standing for 30 min, the 1H and 31 1 P{ H} NMR spectra were obtained (Figures S54D and S55D, Supporting Information). (S)-1-Eu with Sequential Additions of Benzaldehyde, TPPO, and Et2Zn. In a J. Young NMR tube were placed 50 mg (33 μmol) of (S)1-Eu, 5 μL of mesitylene (33 μmol), and 0.5 mL of THF-d8 and the tube was sealed with a threaded Teflon cap. The 1H NMR spectrum was then obtained (Figure S56A, Supporting Information). Next, 10 μL of benzaldehyde (99 μmol) was added to the NMR tube and the 1 H NMR was again acquired after 30 min (Figure S56B, Supporting Information). An additional 10 mg of TPPO (33 μmol) was added and the procedure repeated (Figures S56C and S57A, Supporting Information). Finally, 20 μL of Et2Zn (198 μmol) was added to the NMR tube and the 1H and 31P{1H} NMR spectra (Figures S56D and S57B, Supporting Information) were again obtained after 30 min.

126.3, 126.2, 126.0, 18.9, and 7.1 ppm. Anal. Calcd for C78H75O9Zn3Pr: C, 62.72; H, 5.06. Found: C, 62.35; H, 4.99. (EtZn)3(THF)2(BINOLate)3Eu(THF) ((S)-1-Eu): using 680 mg (1.0 mmol) of Eu[N(SiMe3)2]3·0.5(toluene), 888 mg (3.0 mmol) of (S)-BINOL, and 0.318 mL (3 mmol) of Et2Zn yielded 1.33 g (0.88 mmol) of crystalline product (86% yield). 1H NMR (500 MHz, THFd8, 25 °C): δ 18.78 (H3, br s, 6H), 9.77 (H4, br s, 6H), 7.48 (H6, br s, 6H), 6.46 (H7, br s, 6H), 4.44 (H8, br s, 6H), 1.32 (H9, br s, 6H), −3.74 (ZnCH2CH3, br s, 9H), and −7.54 (ZnCH2CH3, br s, 3H), and −7.98 (ZnCH2CH3, br s, 3H) ppm. 13C {1H} NMR (500 MHz, THFd8, 25 °C): δ 165.6, 133.8, 132.4, 129.5, 124.8, 123.8, 119.9, 119.1, 117.1, 98.2, 6.9, and −4.9 ppm. Elemental analyses for (S)-1-Eu were consistently low in carbon on three attempts, presumably due to incomplete combustion of the sample. The best results for elemental analysis are as follows. Anal. Calcd for C70H59O7Zn3Eu: C, 61.81; H, 4.37. Found: C, 61.29; H, 4.96. General Method for Catalytic Ethyl Addition of Benzaldehyde by (S)-1-RE and (S)-2-RE. In a 20 mL microwave vial were placed 0.1 mmol of (S)-1-RE (or (S)-2-RE or 0.3 mmol (S)-BINOL), 0.5 mmol of tetradecane (130 μL), and 2 mL of toluene, and the vial was sealed with a rubber septum. In a separate 20 mL microwave vial, 3 mmol of neat Et2Zn (300 μL) was diluted with 2 mL of toluene and the vial was sealed and chilled to −35 °C. Once the Et2Zn solution had cooled, it was added dropwise into the (S)-1-RE/tetradecane solution. This (S)-1-RE/tetradecane/Et2Zn solution was stirred for 30 min at room temperature. After the allotted time, 1 mmol of benzaldehyde (100 μL) was added to (S)-1-RE/tetradecane/Et2Zn solution by syringe. After 2 days of stirring, the sealed reaction vial was removed from the drybox, diluted with 20 mL of CH2Cl2, and quenched with 1 mL of saturated NH4Cl solution. The quenched reaction mixture was stirred for 30 min, and then MgSO4 (∼250 mg) was added and the mixture was stirred for another 30 min. The reaction solution was then filtered through a silica plug to remove the insoluble material. The filtrate was collected, and CH2Cl2 was evaporated under reduced pressure, leaving a product residue. An aliquot of the product residue (10−15 mg) was diluted with CH2Cl2 (10 mL) and analyzed by gas chromatography. Conditions used for the gas chromatography were as follows: 1 μL autoinjection, oven inlet 250 °C, FID detector 270 °C, rate 1.3 mL/min, oven 115 °C (30 min run). From the experimental trials: tR(benzaldehyde) = 6.4 min, tR(tetradecane) = 19.5 min, tR((R)1-phenyl-1-propanol) = 26.2 min, tR((S)-1-phenyl-1-propanol) = 27.4 min. The observed retention times of R and S products are consistent with those in the literature.24 General Method for Catalytic Ethylation of Benzaldehyde by (S)-1-RE/TPPO. In a 20 mL microwave vial, 0.1 mmol of (S)-1-RE (or 0.3 mmol of (S)-BINOL), 0.5 mmol of tetradecane (130 μL), 0.15−0.45 mmol of TPPO, and 2 mL of toluene were added and the vial was sealed with a rubber septum. In a separate 20 mL microwave vial, 3 mmol of neat Et2Zn (300 μL) was diluted with 2 mL of toluene and sealed and the Et2Zn solution was chilled to −35 °C. Once the Et2Zn solution had cooled, it was added dropwise into the (S)-1-RE/ TPPO/tetradecane solution. This (S)-1-RE/TPPO/tetradecane/ Et2Zn solution was stirred for 30 min at room temperature. After the allotted time, 1 mmol of benzaldehyde (100 μL) was added to the reaction mixture by syringe. After 2 days of stirring, the sealed reaction vial was removed from the drybox, diluted with 20 mL of CH2Cl2, and quenched with 1 mL of saturated NH4Cl solution. The quenched reaction solution was stirred for 30 min, and then MgSO4 (∼250 mg) was added to solution and the mixture was stirred for another 30 min. The mixture was filtered through a silica plug to remove the insoluble material. The filtrate was collected, and CH2Cl2 was evaporated under reduced pressure, leaving a product residue. An aliquot of the product residue (10−15 mg) was diluted with CH2Cl2 (10 mL) and analyzed by gas chromatography. Procedure for 2D 1H EXSY NMR of Equimolar (S)-1-Eu/Et2Zn Solution. (S)-1-Eu (30 mg, 20 μmol) and 0.4 mL of THF-d8 were added to a J. Young NMR tube and sealed with a threaded Teflon cap. The sealed NMR tube was interrogated by 1H NMR and COSY 1H NMR. The COSY spectrum was used as a control for the following EXSY experiments. After the COSY NMR data were obtained, 2 μL of

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, figures, and CIF files giving experimental details, characterization data, and CIF files for (S)-1-La, (S)-1-Pr, and (S)-1-Eu and text giving experimental details used for ethylation of benzaldehyde catalyzed by (S)-1-RE and (S)-2RE. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for E.J.S.: [email protected]. *E-mail for P.J.W.: [email protected]. Present Address §

(A.J.W.) Battelle Memorial Institute, 2900 Fire Rd., Suite 201, Egg Harbor Township, New Jersey 08234, United States. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest. Notes

The authors declare no competing financial interest. 7438

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ACKNOWLEDGMENTS I.N. acknowledges the “Postdoctoral Fellowship for Academic Diversity” at the University of Pennsylvania for partial support. E.J.S. and P.J.W. acknowledge the University of Pennsylvania and the NSF (CHE-1026553 and CHE-0840438 for an X-ray diffractometer).

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