Axial Tri-tert-butylphosphane Coordination to Rh2(OAc)4: Synthesis

Sep 13, 2016 - The introduction of strong σ-donor axial ligands to the Rh–Rh metal bond has been utilized as an effective way to provide new chemic...
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Axial Tri-tert-butylphosphane Coordination to Rh2(OAc)4: Synthesis, Structure, and Catalytic Studies Jiantao Tan,†,‡ Yi Kuang,†,‡ Yi Wang,§ Qingfei Huang,*,† Jin Zhu,† and Yuanhua Wang*,§ †

Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610046, People’s Republic of China University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § College of Chemistry, Sichuan University, Chengdu 610046, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: The introduction of strong σ-donor axial ligands to the Rh−Rh metal bond has been utilized as an effective way to provide new chemical reactivities to bimetallic dirhodium(II) complexes. In this report, Rh2(OAc)4 complexes with axial bulky alkylphosphane ligands (PR3), in particular P(t-Bu)3, were prepared and characterized. The net σ-donation from the PR3 to the Rh−Rh bond is the result of the competition between the electron-donating ability of the R group and the steric profile of the PR3 at the Rh2 core. Analysis of the crystal structure data showed that the strong σ-donor P(t-Bu)3 coordinates to the rhodium with an amount of σ-donation to the rhodium similar to that of the aryl phosphane ligand PPh3, but has an unusually long Rh−P bond distance (2.663 Å). During catalytic trials to synthesize 3-aryl-3-hydroxy-2-oxindole by the addition of arylboronic acids to isatin derivatives, this longer Rh−P bond distance in Rh2(OAc)4(P(t-Bu)3)2 (Cat-1) facilitates substitution of one of the axial phosphane ligands by the arylboronic acid. This σ-donating effect greatly accelerated the arylation reaction in comparison to alternative catalysts. Additionally, Rh2(OAc)4 was easily recovered after completion of the reaction.

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provide Rh2 complexes with new chemical reactivities.6 This simple and efficient strategy was developed on the basis of the observation that during a reaction only one of the two rhodium atoms acts as a catalytic site at a time.1,7 There is a high degree of potential cooperativity and electronic communication between the two rhodium metal sites, leading to the hypothesis that a strongly σ-donating ligand axially coordinated to one of the Rh atoms would influence the electrophilicity of the other Rh atom across the Rh−Rh bond.6 Though the effect is rather subtle, axial ligands such as N-heterocyclic carbene (NHC) ligands,8a,b phosphanes,8c phosphites,8d pyridines,8e−h and so forth have previously been known to play an important role in improving the selectivity for classic metal carbene reactions catalyzed by Rh2 complexes. Due to the unique structure of Rh2 complexes the axial sites are extraordinarily unrestricted, making it possible for the various σ-donors to approach each other.1,2 The use of strong σ-donating axial ligands has led to many new and useful catalytic reactions. One example is the use of Rh2/NHC complexes to catalyze arylation reactions between aromatic aldehydes and arylboronic acids in order to synthesize secondary alcohols, as reported by Gois and co-workers (Figure 2).9 In this case the NHC ligands were strongly σ-donating toward the Rh−Rh bond and led to a significant effect on the Rh2 complex reactivity. It was also determined that NHC ligands with higher stereodemand, such as N,N-(2,6diisopropylphenyl)imidazolidene (IPr), are required in order to support the stability of the Rh2/NHC complexes as well as

INTRODUCTION Dirhodium(II) complexes have a well-defined bimetallic dimer structure and are well-known for their effectiveness as catalysts for carbene and nitrene species generation in organic synthesis.1 These complexes are air stable and typically have a dimeric “paddle wheel” structure with D4h symmetry surrounding a Rh−Rh bond with four bridging ligands and two axial ligands (Figure 1).1,2 In order to efficiently tune the electronic

Figure 1. Structure of dirhodium(II) carboxylate catalysts.

properties of the Rh2 complexes as well as their catalytic selectivity, bridging ligands have been modified to generate a variety of nitrene- and carbenoid-based dirhodium complexes.3 The axial sites of the complexes are electrophilic and are usually occupied by solvent molecules which are labile and are easily displaced by alternative reactants.1−4 Historically it was understood that the axial ligands played a less important role in catalysis in comparison to the bridging ligands.3,5 However, recently the introduction of strong σ-donor axial ligands to the Rh−Rh metal bond has been used as an effective strategy to © XXXX American Chemical Society

Received: June 13, 2016

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DOI: 10.1021/acs.organomet.6b00477 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 2. Arylation of aldehydes catalyzed by Rh2/axial ligand complexes.

prevent the decoordination of the axial NHC ligands. The various coordination geometries of the selected σ-donors to the Rh2 core can result in a change in stereoelectronic effects and lead to different reactivities for the Rh2 complexes. This was the case in our research, when an unexpected reactivity was observed for dirhodium(II) acetate with axial n-butylphosphane ligands used to catalyze the reaction between aromatic aldehydes and arylboronic acids. Instead of the production of secondary alcohols as predicted, the final products were ketones in neat water which were formed via secondary alcohol intermediates and then slow oxidation to ketone (Figure 2).10 This difference may be explained by the geometry of the σdonors: phosphane ligands have a conical shape, as opposed to NHC ligands, which have a planar shape. This result also suggests that the alkylphosphane ligands donate higher electron density in comparison to arylphosphane ligands and therefore should be selected as the preferred axial ligands to provide strong σ-donation to the Rh−Rh bond. The electron-donating ability of alkyl groups increases according to the following trend: methyl < ethyl < isopropyl < cyclohexyl (Cy) < tert-butyl (t-Bu).11 Therefore, the strong σdonating ability of the phosphane ligands is due to the large alkyl substituents, which may result in steric repulsion of the metal. However, the unrestricted nature of the axial sites on the Rh2 complex would accommodate axial phosphane ligands with cone angles as great as 180° without steric difficulty. This allows for preparation of Rh2 complexes with large axial alkylphosphane ligands, including tri-tert-butylphosphane (P(tBu)3) and tricyclohexylphosphane (PCy3). The axial coordination of PCy3 to Rh2(OAc)4 has been previously described by Yu and co-workers and was seen to facilitate ligand exchange of the bridging ligands as well as affect the oxidation potential of the complex during oxidative alkenylation of arenes.12 Chang and co-workers further proposed that PCy3 and the NHC ligands would axially coordinate to prepared dirhodium(II) complexes simultaneously as a precatalyst during C−H activation reactions.13 It was determined that PCy3 acted to stabilize the active monocoordinated catalytic species Rh2(OAc)4(NHC). Although these previous studies indicated that axial alkylphosphane ligands have strong effects on Rh2(OAc)4, detailed structures of the adduct complexes have yet to be reported. In this work, we continue to expand the understanding of the fine-tuning effects of axial phosphane ligands on dirhodium(II) complex chemistry. Here we report the efficient preparation of Rh2(OAc)4 complexes with large, axially ligated alkylphosphane ligands, specifically the complexes Rh2(OAc)4(P(t-Bu)3)2 (Cat1) and Rh2(OAc)4(PCy3)2 (Cat-2). Detailed crystal structures are provided, and the catalytic performance of these complexes toward arylation reactions using arylboronic acids and isatin derivatives are investigated.

from their acid forms in situ by deprotonating the species using a base.14 To prepare the Rh2(OAc)4(PR3)2 complexes, the airstable ligands tri-tert-butylphosphane tetrafluoroborate ([(tBu)3PH]BF4) and tricyclohexylphosphane tetrafluoroborate ([PCy3H]BF4) were reacted with Rh2(OAc)4 in DME (dimethoxyethane)/H2O under a nitrogen atmosphere and using K2CO3 as a base. Happily, the red Cat-1 and brown Cat-2 complexes were seen to precipitate directly from the reaction vessel and were able to be recovered using simple filtration (eq 1). These obtained complexes were tolerant to degradation by air and could be stored on the bench. Rh 2(OAc)4 + [(t ‐Bu)3 PH]BF4 K 2CO3

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Rh 2(OAc)4 (P(t ‐Bu)3 )2 DME/H 2O, room temp, N2 96%

Cat ‐ 1

Rh 2(OAc)4 + [PCy3H]BF4 K 2CO3

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Rh 2(OAc)4 (PCy3)2 DME/H 2O, room temp, N2 95%

Cat ‐ 2

(1)

Crystals suitable for X-ray diffraction were obtained according to the following procedure: Rh2(OAc)4 (5.8 mg, 1 equiv), [(t-Bu)3PH]BF4 (44 mg, 10 equiv), and K2CO3 (69 mg, 40 equiv) were placed in a Schlenk tube under a nitrogen atmosphere followed by the addition of Et2O (1 mL) and H2O (1 mL). The mixture was observed to immediately turn red and was stirred at room temperature for 30 min. The mixture was then allowed to stand and separate, and the upper red ether solution was transferred to a nitrogen-filled Schlenk tube. Cat-1 single crystals were grown by slow evaporation of the ether (Figure 3).

Figure 3. ORTEP crystal structure of complex Cat-1. The molecular structure is depicted in an ellipsoid style at the 50% probability level.

Cat-2 single crystals were prepared following a similar procedure (Figure 4). In order to better understand the structural differences induced by the addition of axial bulky alkylphosphane ligands, the obtained crystal structures were compared with structures of previously reported axially ligated dirhodium(II) complexes.9,15 Of particular interest to our investigation was a comparison of the Rh−Rh bond and Rh−P bond lengths. It had been reported that the strongly σ-donating axial ligands coordinated to the rhodium led to a shorter Rh−ligand bond, while also acting to weaken and lengthen the Rh−Rh bond by

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RESULTS AND DISCUSSION A challenge of the proposed synthesis is that P(t-Bu)3 and PCy3 are sensitive to oxidant and are difficult to handle. On the basis of previous reports, it is possible to release P(t-Bu)3 and PCy3 B

DOI: 10.1021/acs.organomet.6b00477 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

of PPh3, the longer Rh−P bond distance acts to weaken the σdonating abilities of the axial phosphane ligands toward the Rh−Rh bond and therefore results in the net σ-donating ability of P(t-Bu)3 or PCy3 being similar to that of PPh3. Furthermore, in addition to the electron-donating ability, the steric profile of the σ-donors also has a great influence on the σ-donation abilities of the ligands to the metal. The Rh2(OAc)4(PR3)2 complexes may have longer Rh−P bond distances due to the bulkiness of the phosphane ligands. Steric crowding of bulky phosphane ligands such as P(t-Bu)3 and PCy3 cause either a distortion of the coordination geometry at the rhodium or an adjustment of the internal geometry of the phosphane ligands themselves.17 Because the axial site of the dirhodium core has square-planar geometry, the bulky P(t-Bu)3 is not hindered from approaching the rhodium, resulting in a minimal steric interaction. In order to determine the amount of distortion to the coordination geometry at the rhodium, Rh−Rh−P angles were measured. In the case of Cat-1, the Rh−Rh−P linkage deviated only slightly (3.5°) from linearity (Table 1, entry 5). This result indicated that internal geometric distortion in the phosphane ligand was responsible for the relatively longer Rh− P bond observed for Rh2(OAc)4(PR3)2 complexes. Angular symmetric deformation coordinates (S4′ values) were also used to evaluate the geometric distortion at the phosphorus.18 These values are directly related to the flattening or pyramidality of the phosphane ligands and correlate to hybridization of the lone pair at the phosphorus. On the basis of the data collected from the crystal structures, the calculated S4′ values clearly showed that a degree of distortion had occurred at the phosphorus. The least distorted complex, Rh2(OAc)4(P(OPh)3)2, had a large S4′ value (56.3°) (Table 1, entry 3), while Cat-1 had a much smaller S4′ value (13.9°) (Table 1, entry 5). This small S4′ value is due to the large t-Bu group in the P(t-Bu)3 ligand forcing the geometry of P(t-Bu)3 to become more planar. This considerably flattened phosphane ligand resulted in poor σ-orbital overlap between the phosphorus and the rhodium and therefore decreased the σ-donation of the P(tBu)3 toward the Rh−Rh core and elongated the Rh−P bond. This extraordinarily long Rh−P bond distance also suggests that Rh−P π back-bonding is also very weak and the P(t-Bu)3 is purely σ-donating. With the structural information in hand, we then set out to examine the potential of Rh2(OAc)4(PR3)2 complexes as catalysts for organic reactions. The arylation reaction of N-Bn isatin (1a) with phenylboronic acid (2a) was used to investigate how various axially ligated phosphane ligands affected catalytic performance.19 First, 1 mol % of Rh2(OAc)4 was combined with 2.5 mol % of PPh3 in DME/water at room temperature in order to easily generate the Rh2(OAc)4(PPh3)2 complexes as red precipitates in situ. This step was followed by the addition of 1 equiv of N-Bn isatin (1a), 1.1 equiv of phenylboronic acid (2a), and 5 mol % of K2CO3. The reaction was kept at 90 °C and allowed to proceed for 4 h, after which the desired product 3aa was obtained in 80% isolated yield (Table 2, entry 1). Using these same reaction conditions, Rh2(OAc)4 ligated with the poor σ-donor P(OPh)3 was not able to catalyze this reaction (Table 2, entry 2). Subsequent control investigations confirmed that Rh2(OAc)4 and ligands are both necessary for the reaction to occur (Table 2, entries 3 and 4). Results from these tests indicate that axial phosphane ligands do act to increase the reactivity of Rh2(OAc)4. Notably, high yields of 3aa were obtained using Cat-1 and Cat-2 formed in situ (Table 2, entries 5 and 6). Surprisingly, the P(t-Bu)3 ligand accelerated

Figure 4. ORTEP crystal structure of complex Cat-2. The molecular structure is depicted in an ellipsoid style at the 50% probability level.

adding electrons into the σ*-orbital of the Rh−Rh MO.16 These effects were measured by examining changes to the Rh− Rh bond length. For instance, in comparison to the dirhodium(II) parent structure, the strong σ-donor IPr NHC ligated dirhodium(II) complex Rh2(OAc)4(IPr)29 had a longer Rh−Rh bond length (2.473 Å) as well as a shorter Rh−ligand bond distance (Rh−L, 2.228 Å). As shown in Table 1, among the Rh 2 (OAc) 4 (PR 3 ) 2 complexes examined, the length of the Rh−Rh bond measured Table 1. Selected Bond Distances, Bond Angles, and Torsion Angles from Crystal Structure Data of Selected Axially Ligated Dirhodium(II) Complexes

entry

axial ligand (L)

Rh−Rh (Å)

Rh−O (Å)

Rh−L (Å)

Rh−Rh−L (deg)

S4′ (deg)

1 2 3 4 5

H2O15a P(OPh)315b PPh315b PCy3 P(t-Bu)3

2.385 2.443 2.451 2.457 2.454

2.039 2.039 2.045 2.055 2.046

2.310 2.412 2.477 2.509 2.663

176.47 179.90 175.52 176.39 176.52

56.3 35.7 27.3 13.9

for the selected axially ligated complexes did indeed increase in comparison to that of the dirhodium(II) parent structure and varied overall from 2.443 to 2.457 Å. These data support the notion that axial ligands with strong σ-donation do indeed increase the Rh−Rh bond length. However, the data for Rh−P bond distances measured were not consistent with the previous report.16 Due to the poor σ-donating ability of P(OPh)3 (Table 1, entry 2), the Rh2(OAc)4(P(OPh)3)215b complex had the shortest Rh−Rh bond distance measured (2.443 Å) and a longer Rh−P bond distance (2.412 Å). However, the other Rh2(OAc)4(PR3)2 complexes with strong σ-donors all had longer Rh−P bonds (2.477−2.663 Å) (Table 1, entries 3−5) in comparison to Rh2(OAc)4(P(OPh)3)2. The Rh−P bond distance of Cat-2 (2.509 Å) (Table 1, entry 4) was slightly longer than that measured for Rh2(OAc)4(PPh3)215 (2.477 Å) (Table 1, entry 2). It was also noted that the Rh−P bond in Cat-1 had an unusually long length (2.663 Å) (Table 1, entry 5). As can be seen in Table 1, the Rh−Rh bond distances for Cat-1 and Cat-2 were 2.454 and 2.457 Å, respectively, and showed little difference in comparison to the value (2.451 Å) measured for the analogous PPh3 complex. Although the σdonating abilities of P(t-Bu)3 and PCy3 are stronger than that C

DOI: 10.1021/acs.organomet.6b00477 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 2. Optimization of Reaction Conditionsa

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

[Rh2] Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Cat-1 Rh2(OAc)4 Cat-1 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4(IPr)2

ligand

base

PPh3 P(OPh)3

K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

PPh3 (Cy3PH)BF4 [(t-Bu)3PH]BF4 P(t-Bu)3 [(t-Bu)3PH]BF4 [(t-Bu)3PH]BF4 [(t-Bu)3PH]BF4 [(t-Bu)3PH]BF4 [(t-Bu)3PH]BF4

KOH NaOH Na2CO3 NaHCO3 K2CO3

time

yield (%)b

4h 8h 4h 4h 4h 20 min 20 min 20 min 4h 4h 20 min 20 min 20 min 20 min 8h

80 trace n.r. n.r. 92 92 93 96 n.r. n.r. 65 80 90 91 trace

a

Unless otherwise noted, all reactions were carried out with 1a (71.1 mg, 0.30 mmol), 2a (40.0 mg, 0.33 mmol), Rh2(OAc)4 (1.0 mol %), ligand (2.5 mol %), and K2CO3 (5.0 mol %) in 1 mL of 1/1 (v/v) DME/H2O at 90 °C under N2. bIsolated yield. n.r. = no reaction.

Scheme 1. Arylation of Derivatives of Isatins with 2aa

a Unless otherwise noted, all reactions were carried out with 1 (0.30 mmol), 2a (40.0 mg, 0.33 mmol), Rh2(OAc)4 (1.0 mol %), [(t-Bu)3PH]BF4 (2.5 mol %), and K2CO3 (5.0 mol %) in 1 mL of 1/1 (v/v) DME/H2O at 90 °C under N2. Yields of isolated products are reported.

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DOI: 10.1021/acs.organomet.6b00477 Organometallics XXXX, XXX, XXX−XXX

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Organometallics this arylation reaction and resulted in a reaction time of only 20 min (Table 2, entry 6).20 The end point of the reaction was determined according to a color change of the reaction mixture (see the Supporting Information for details). The direct application of P(t-Bu)3 in the reaction also gave good results (Table 2, entry 7). The preprepared Cat-1 worked very well as a catalyst, with an excellent 96% yield of product 3aa obtained after only 20 min (Table 2, entry 8), demonstrating the efficiency of this bimetallic Rh(II) catalytic system. In addition, it was determined that the presence of base was required for the reaction to progress (Table 2, entries 9 and 10). As a next step, we briefly screened the base used in the reaction (Table 2, entries 11−14). The addition of Na2CO3 or NaHCO3 gave results nearly identical with those seen for K2CO3 (Table 2, entries 13 and 14), whereas the use of strong bases (KOH and NaOH) gave lower yields of product (Table 2, entries 11 and 12). Use of Rh2(OAc)4(IPr)2 as a catalyst was also tested and was shown to be less efficient, a result confirming the hypothesis that axial phosphane ligands are significantly different from NHC ligands (Table 2, entry 15). Once the optimized reaction conditions had been identified, the next step was to investigate the reaction of isatin substrates bearing substituents on the nitrogen atom or the aromatic rings (Scheme 1). The arylation of N-substituted isatins (1b−f) proceeded efficiently with 2a in DME/H2O and gave the products in good yields (83−98%) in less than 1 h. For the non N-substituted isatin 1g, no product 3ga was detected in the DME/H2O mixture. This is mostly likely due to the inhibiting effects of the nitrogen atom, which can block the active site of the Rh2 complex. Next, N-Bn isatin derivatives 1h−n with various substituents on the aromatic rings were tested under similar reaction conditions. Derivatives 1h−l reacted smoothly with 2a to afford the corresponding products 3ha−3la with yields ranging from 75 to 98%. These results showed that steric hindrance had obvious effects on the reaction. In the case of NBn-4-bromoisatin (1n) the reaction was unsuccessful, particularly in comparison to N-Bn-7-fluoro-isatin (1l), which had a product (3la) yield of 85% in DME/H2O. Notably, the product of 3ka with the bromo group untouched showed high selectivity and is therefore attractive for further synthetic elaboration. It was also determined that the N-Bn isatin derivatives containing a nitro group (1m) were not tolerated in this procedure, though with less steric hindrance. Reactions between several arylboronic acids 2b−i and 1a were also investigated under the previously determined optimal conditions (Scheme 2). In the cases of highly electron withdrawing 4-chlorophenylboronic acid 2e and 4-trifluoromethylphenylboronic acid 2g, products 3ae,ag were obtained in excellent yields. Interestingly, electron-rich 4-methoxyphenylboronic acid 2d also gave good yields (77%) of the arylation product 3ad. However, 4-cyanophenylboronic acid 2f was an exception and resulted in only 40% yield of the addition product 3af. Incubation of 2-hydroxyphenylboronic acid 2b with 1a afforded the product 3ab in low yields (