Bulky N-Heterocyclic Carbene IPr* in Selected Organo- and Transition

Article pubs.acs.org/Organometallics

Bulky N-Heterocyclic Carbene IPr* in Selected Organo- and Transition Metal-Mediated Catalytic Applications János Balogh, Alexandra M. Z. Slawin, and Steven P. Nolan* EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, U.K. S Supporting Information *

ABSTRACT: A series of studies were conducted to probe the stability and reactivity of a very sterically encumbered Nheterocyclic carbene. The X-ray structure of the NHC IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2ylidene) was determined. IPr* was used as an organocatalyst in transesterification reactions. Steric and electronic parameters characterizing IPr* were determined via the synthesis of a nickel-carbonyl complex, [Ni(CO)3(IPr*)]. A related complex, [(Cp*)Ru(IPr*)Cl] (Cp* = η5-C5Me5), was prepared and characterized by X-ray crystallography, and its catalytic performance in the racemization of chiral alcohols is reported. The catalytic performance of the NHC and of its transition metal derivatives permit establishing the standing of this uniquely bulky member among the NHC family.

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INTRODUCTION Since Arduengo1 reported the first isolation of a free Nheterocyclic carbene (NHC), the synthesis of new NHCs has become a burgeoning area of research mostly due to their numerous applications.2a,b NHCs have served (and continue to serve) as ancillary ligands on many transition metal centers.3a−c Free NHCs are also able to act as organocatalysts, effectively promoting a variety of transformations, most notably the transesterification of various esters.4 NHC ligands are wellknown to be very strong σ-donors.5a−d The electronic parameters of NHC ligands just like those of tertiary phosphine ligands can be quantified by IR measurements of the carbonyl stretching frequencies (ν) in the related [Ni(CO)3(NHC)] complexes.6,7a−e In order to compare the steric features of various NHC ligands, Nolan and Cavallo have proposed the percent buried volume (%Vbur) model,8a a numerical value that represents the percent volume of a sphere occupied by the atoms of the NHC.8a−d With a combination of σ-donation and steric bulk, NHCs often stabilize low-coordinate, catalytically active species. In this context, there is a fundamental need to synthesize and investigate new sterically hindered NHC ligands.3b,d,9 In order to achieve such novel architectures, one of the pivotal structural parameters that can be modified to tune the electronic features in the imidazole ring is the Nsubstituents.3b In 2010, Marko and co-workers described the synthesis of the extremely sterically demanding 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene (IPr*) (1) (Figure 1).10 The preparations of its silver and rhodium © 2012 American Chemical Society

Figure 1. IPr* ligand reported by Markó.

complexes were also carried out in this initial report. Our group reported recently gold complexes of 1.11 We now report studies focusing on the use of the free NHC as an organocatalyst in transesterification reactions; as part of this aspect of the study, we confirmed the structural features of the free NHC by X-ray crystallography. Steric and electronic parameters characterizing IPr* were studied. The complex [Ru(Cp*)(IPr*)Cl] (Cp* = η5-C5Me5) was prepared and characterized by X-ray crystallography, and its catalytic performance in the racemization of alcohols was tested.

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RESULTS AND DISCUSSION Structural Characterization of IPr*. As part of our ongoing research aimed at synthesizing new [M(IPr*)Ln] complexes (M = metal), we initially undertook the challenge of structurally characterizing the free IPr* (1) by X-ray Received: February 9, 2012 Published: March 26, 2012 3259

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Table 1. Influence of Temperature on Catalytic Activitya

crystallography. The HCl salt of IPr* was prepared as described by Markó et al.10 Subsequently, a two-step methodology11 led to the desired free carbene. Single crystals of IPr* were grown by slow vapor diffusion of pentane into a saturated benzene solution.12 The X-ray crystal structure of 1 is presented in Figure 2.

convd entry 1 2 3 4

reagent b

2 2b 3c 3c

T (°C)

3h

24 h

yielde

24 50 24 50

17% 27% 46% 61%

45% 69% 67% 78%

n.if. 67% n.if. 74%

a

Reaction conditions: 4 (1 mmol). b2 (10.8 mmol), 1 (2.5 mol %). c3 (1.2 mmol), 1 (0.5 mol %). dConversion determined by GC as an average of two runs. eIsolated yield after chromatography on silica gel. f Not isolated.

NHC (e.g., ICy) one hour was required to obtain 5 quantitatively (100%) at room temperature under the conditions a, while only five minutes was needed to reach almost full conversion (97%) using IMes under conditions b. We attempted to obtain an increased catalytic activity by raising the temperature. At elevated temperature (50 °C) reasonable conversions (69% and 78%) could be achieved (Table 1, entries 2 and 4), highlighting the thermal stability of 1. These poorer catalytic results can be attributed to the bulkiness of 1, where steric demands are presumably responsible for the difficulties in generating the acylazolium intermediate.13 Electronic and Steric Properties of 1. Synthesis of [Ni(CO)3IPr*]. NHC ligands are very strong σ-donors.14 Tolman’s pioneering work quantifying the electronic properties of tertiary phosphine ligands6 has been expanded to include NHCs.5d,7a−e It has been shown that the NHCs’ electrondonating character surpasses that of the most basic tertiary phosphines.7a,b Investigating where 1 fit on the ligand sterics and aiming to define its electronic features quantitatively, a substitution reaction involving IPr* and Ni(CO)4 (6) (eq 1) was carried out.

Figure 2. ORTEP representation of IPr* (1) showing 50% thermal ellipsoid probability. H atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): N5−C39, 1.441(3); N2−C6, 1.439(3); C1−N5−C39, 123.7(2); C1−N2−C6, 122.2(2).

The NHC N−C−N angle in 1 was measured to be 101.2(2)°, which is slightly reduced compared to the same metrical parameter for Arduengo’s carbene IAd (N,N′-di(adamantly)imidazole-2-ylidene) (102.2°).1 The most significant difference found between these two NHCs was the distance between the N2(5)−C1‑Ad′ in IAd and the corresponding N2(5)−C6(39) in IPr*. This value for IPr* is 1.440 Å, which is shorter than the corresponding value of IAd (1.484 Å), hinting at the inferior steric bulk of the N-substituents in 1. Organocatalysis. N-Heterocyclic carbenes have the ability to promote a wide variety of organocatalytic transformations.4a−d Among several applications, NHCs are efficient catalysts in transesterification reactions.4e−h Near-quantitative conversions are achieved when NHCs are employed as organocatalysts in the conversion of methyl or vinyl acetate into benzyl acetate using benzyl alcohol.4e Motivated by these results, a reactivity screening was performed in order to assess the activity of IPr* in this transesterification reaction. Benzyl alcohol was selected as the benchmark substrate.4e Two different nucleophilic reagents were tested under various reaction conditions. In the first case (condition a) 2.5 mol % of 1, methyl acetate (2), and solvent-free conditions were used. Molecular sieves (4 Å) were required to absorb the liberated methanol. In the second case (condition b), vinyl acetate (3) in THF was utilized as the nucleophile reagent, employing a reduced quantity of NHC catalyst (0.5 mol %). The reactions were carried out at room temperature in order to be able to compare the results with literature data.4e Moderate conversions were observed for both transformations, even after 24 h (Table 1, entries 1 and 3). These results show that 1 possesses lower catalytic activity than IPr and falls far behind ICy (N,N′-dicyclohexylimidazol-2-ylidene) and IMes (N,N′dimesitylimidazol-2-ylidene), which were found to have the best catalytic activity in this transformation. Using alkyl-bearing

The analogous transformation for a number of other NHCs has previously been reported.7b,c In most cases, saturated offwhite [Ni(CO)3(NHC)] complexes were generated. The reaction of the bulkiest NHCs (N,N′-di(adamantly)imidazole2-ylidene (IAd) and N,N′-di(tert-butyl)imidazole-2-ylidene (ItBu)) with Ni(CO)4 led to the unprecedented, coordinatively unsaturated, orange-red colored complexes [Ni(CO)2(IAd)] and [Ni(CO)2(ItBu)]. To obtain the nickel carbonyl product, a solution of IPr* in THF was treated with a slight excess of Ni(CO)4 in a reaction 3260

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that was carried out overnight (time not optimized).15 Following the evolution of CO, an off-white product (7) was isolated. The color of compound 7 hinted at its saturated character. This was also suggested by its IR spectrum, where three carbonyl bands can be observed (2052.7, 1997.5, and 1973.7 cm−1). These values permit concluding that the sterically demanding IPr* possesses similar electronic properties to IPr. Single crystals of complex 7 suitable for X-ray analysis were grown by slow vapor diffusion of pentane into a saturated solution of benzene.12 The ORTEP representation of this complex is shown in Figure 3.

formation of coordinatively unsaturated [Ni(CO)2(IPr*)] might have been expected, but this is not observed. These observations suggest that although IPr* seems to be highly sterically demanding, the bulkiness of this ligand is conformationally flexible. Our observations are in accord with those of Marko,10 where, in the present case, IPr* was found to be able to adjust its orientation about the metal to afford [Ni(IPr*)(CO)3]. We calculated a “pseudo-%Vbur” for the free carbene 1 in order to assess its original sterical parameter (Table 2, entry 8). Ligand 1 appears to be considerably flexible (46.3 in free IPr* to 34.6 in the [Ni(CO)3(L)] complex according to these straightforward calculations). The conformational flexibility can be beneficially exploited, as proven by Glorius using the Pd complex of the sterically demanding bioxazoline-derived carbene in Suzuki cross-coupling reactions.9b Synthesis of [Cp*Ru(IPr*)Cl] and Its Catalytic Activity in Alcohol Racemization. Among the large number of ruthenium catalysts described in the literature, NHC-ligated ruthenium complexes, analogues of the Shvo catalyst (Figure 4, complexes A and B),16 were found to be useful promoters of alcohol racemization.17a−k This transformation is an essential step in the production of optically pure compounds by dynamic kinetic resolution.18k Recently, several very efficient ruthenium complexes have been disclosed17g−j (Figure 4, complexes C and D). Among them [Cp*Ru(NHC)Cl] (Cp* = pentamethylcyclopentadienyl)-type 16-electron Ru complexes have shown excellent efficiency in the racemization, and they could be easily obtained by reacting [Cp*RuCl]418 with a NHC (eq 2).17g−i,19a−c Inspired by these precedents, [Cp*Ru(IPr*)Cl] (9) was prepared, and its catalytic activity in the racemization of alcohols investigated.

Figure 3. ORTEP representation for [Ni(CO)3(IPr*)] (7) showing 50% thermal ellipsoid probability. H atoms were omitted for clarity. Selected bond distances (Å) and angles (deg): C1−Ni1, 1.971(3); N1−C73, 1.787(3); N1−C74, 1.794(4); N1−C72, 1.809(3); O72− C72, 1.150(4); O73−C73, 1.134(4); O74−C74, 1.147(4); C73− Ni1−C1, 112.98(13); C74−Ni1−C1, 119.24(13); C72−Ni1−C1, 107.79(13).

Complex 7 displays the expected tetrahedral geometry about the metal center. Surprisingly, despite the steric significance of the IPr* ligand, the coordination of three carbonyl ligands is possible. In order to compare the bulkiness of 1 to the NHCs mentioned above and to quantify the steric property of the ligand, the %Vbur was calculated as 34.6%. By comparing the % Vbur of IPr* with the values characterizing other NHCs, it is clear that the steric demand of IPr* is similar to the most bulky NHCs, IAd and ItBu (Table 2, entries 4 and 5). In this context, Table 2. Steric and Electronic Properties of the Ligands in the [Ni(CO)n(NHC)] Complexesa entry 1 2 3 4 5 6 7 8

complex t

[Ni(CO)3(P Bu3)] [Ni(CO)3(IMes)] [Ni(CO)3(SIMes)] [Ni(CO)2(IAd)] [Ni(CO)2(ItBu)] [Ni(CO)3(IPr)] [Ni(CO)3(IPr*)] free IPr*

A1b (cm−1)

%Vburc

2056.1 2050.7 2051.5 2007.2 2009.7 2051.5 2052.7

30.0 26.0 27.0 37.0 37.0 29.0 34.6 46.3

The new complex 9 is prone to decomposition under the synthetic conditions, and only very low yields of the desired complex are obtained. Single crystals of complex 9 were grown by slow vapor diffusion of pentane into a saturated solution of benzene, and its structure was determined by X-ray analysis.12 An ORTEP representation of this complex is shown in Figure 5. The %Vbur of ligand 1 in complex 9 was determined to be 31.1% (Table 3, entry 5).8c,d,20 On the basis of this value (Table 3, entry 5), the bulkiness of 1 in the ruthenium complex is approximately the same as its aromatic congeners IMes and IPr

a

For literature reference, see refs 7b, c, and 20. bA1 stretching frequency determined in CH2Cl2 at room temperature. cCalculation parameters: sphere radius, 3.00 Å; distances for the metal−ligand bond, 2.00 Å; hydrogen atoms were omitted; scaled Bondi radii were used as recommended by Cavallo.8c,d

the ligand-exchange behavior of IPr* in the Ni(CO)4 reaction is unexpected, as the %Vbur value for IPr* indicated that the 3261

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Figure 4. The Shvo catalyst (S) and analogous ruthenium complexes (A−D) involved in alcohol racemization.

The relationship between the bulkiness of the ligand and the catalyst efficacy was investigated. Unsurprisingly, based on the steric assessment described above, the catalytic activity of 9 is similar to that of the IMes-ligated Ru complex. The transformation was carried out with extended reaction time (24 h), but no further racemization was observed. Although the catalytic efficacy of 9 could be increased by using elevated temperature (50 °C), complete racemization could not be achieved (eemax(120 h) = 72%).

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CONCLUSION As part of our ongoing research on NHC ligands and the potential benefits they can bring to catalytic species, the structure of IPr* was determined by X-ray crystallography to assess its steric and electronic properties. The formation of the saturated [Ni(CO)3(IPr*)] was observed in the reaction of IPr* and Ni(CO)4, which supports the concept of the conformational flexibility of this NHC. IR analysis and calculation of %Vbur were carried out in order to obtain quantifiable values for the electronic and steric properties of the ligand. These measurements revealed the flexibility of the substituents on the N-aryl ring. IPr* was tested also as organocatalyst in a transesterification reaction. During this reaction elevated temperatures were required to achieve reasonable yields. The novel [Cp*Ru(IPr*)Cl] complex was prepared, and the catalyst was employed to mediate the racemization of chiral alcohol. From these structural and catalytic studies, the trend is clear in that the IPr* ligand is very bulky, but this steric encumbrance is not static and the ligand can adapt as a function of ancillary environment. This behavior could be beneficial in a number of catalytic transformations, and these are presently being explored in our laboratories.

Figure 5. ORTEP representation of [Cp*Ru(IPr*)Cl] (9) showing 50% thermal ellipsoid probability. H atoms were omitted for clarity. Selected bond distances (Å) and angle (deg): C1−Ru1 2.110(5), Ru1−Cl1 2.304(3), C1−Ru−Cl1 93.13(16).

Table 3. Comparison of %Vbur of [Cp*Ru(IPr*)Cl] and Its Efficiency toward Alcohol Racemization with Analogues in the Literaturea

entry

complexb

%Vburc

eed (%)

1 2 3 4 5

[Cp*Ru(IMes)Cl] [Cp*Ru(IAd)Cl] [Cp*Ru(IPr)Cl] [Cp*Ru(ICy)Cl] [Cp*Ru(IPr*)Cl]

31.4 34.0 31.2 25.6 31.1

87 71 66 >1 88d

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a

See ref 17g. bReaction conditions: catalyst (2 mol %), NaOtBu (5 mol %), and toluene (2 mL); reaction time: 30 min. cThe parameters used for SambVca calculations were identical to those of ref 17g. d Determined by chiral stationary-phase HPLC, average of two runs.

ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures leading to 7 and 9, CIF files and crystallographic data for 1, 7, and 9, and procedures for transesterification and alcohol racemization. NMR spectra for

(Table 3, entries 1 and 3), again demonstrating the flexibility of IPr*. 3262

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(10) Berthon-Gelloz, G.; Siegler, M. A.; Spek, A. L.; Tinant, B.; Reek, J. N. H.; Markó, I. E. Dalton Trans. 2010, 39, 1444−1446. (11) Gómez-Suárez, A.; Ramón, R. S.; Songis, O.; Slawin, A. M. Z.; Cazin, C. S. J.; Nolan, S. P. Organometallics 2011, 30, 5463−5470. (12) CCDC-864265 (1), CCDC-864266 (7), and CCDC-864267 (9) contain the supplementary crystallographic data for this contribution. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. (13) He, L.; Jian, T.-Y.; Ye, S. J. Org. Chem. 2007, 72, 7466−7468. (14) Jacobsen, H.; Correa, A.; Costabile, C.; Cavallo, L. J. Organomet. Chem. 2006, 691, 4350−4358. (15) Special care has been taken while manipulating the EXTREMELY TOXIC Ni(CO)4. All manipulations were carried out in the glovebox, using additional protective gloves. Ni(CO)4 was constantly maintained at −50 °C. The solutions containing the NHC ligands were cooled to −50 °C, and Ni(CO)4 was added via syringe at the same temperature. The slight excess of Ni(CO)4 at the end of each reaction (carried out in Schlenk glassware inside the glovebox) was evaporated and trapped in a toluene solution containing phosphines. (16) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodosh, D. F. J. Am. Chem. Soc. 1986, 108, 7400−7402. (17) (a) Choi, J. H.; Kim, Y. H.; Nam, S. H.; Shin, S. T.; Kim, M.-J.; Park, J. Angew. Chem., Int. Ed. 2002, 41, 2373−2376. (b) Choi, J. H.; Choi, Y. K.; Kim, Y. H.; Park, E. S.; Kim, E. J.; Kim, M.-J.; Park, J. J. Org. Chem. 2004, 69, 1972−1977. (c) Csjernyik, G.; Bogar, K.; Bäckvall, J.-E. Tetrahedron Lett. 2004, 45, 6799−6802. (d) MartinMatute, B.; Edin, M.; Bogar, K.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2004, 43, 6535−6539. (e) Karvembu, R.; Prabhakaran, R.; Natarajan, K. Coord. Chem. Rev. 2005, 249, 911−918. (f) Conley, B. L.; Pennington-Boggio, M. K.; Boz, E.; Williams, T. J. Chem. Rev. 2010, 110, 2294−2312. (g) Bosson, J.; Nolan, S. P. J. Org. Chem. 2010, 75, 2039−2043. (h) Bosson, J.; Poater, A.; Cavallo, L.; Nolan, S. P. J. Am. Chem. Soc. 2010, 132, 13146−13149. (i) Nun, P.; Fortman, G. C.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2011, 30, 6347−6350. (j) Manzini, S.; Urbina-Blanco, C. A.; Poater, A.; Slawin, A. M. Z.; Cavallo, L.; Nolan, S. P. Angew. Chem., Int. Ed. 2012, 51, 1042−1045. (k) Ebbers, E. J.; Ariaans, G. J. A.; Houbiers, J. P. M.; Bruggink, A.; Zwanenburg, B. Tetrahedron 1997, 53, 9417−9456. (18) Fagan, P. J.; Ward, M. J.; Calabrese, J. C. J. Am. Chem. Soc. 1989, 111, 1698−1719. (19) (a) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 2370−2375. (b) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674−2678. (c) Jafarpour, R.; Stevens, E. D.; Nolan, S. P. J. Organomet. Chem. 2000, 606, 49−54. (20) For a review on %Vbur, see: Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841−861.

all complexes (7 and 9) and for the product of the transesterification, 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The ERC (Advanced Investigator Award FUNCAT) is gratefully acknowledged for support. S.P.N. is a Royal Society-Wolfson Research Merit Award holder.

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REFERENCES

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