Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
[Pd(NHC)(acac)Cl]: Well-Defined, Air-Stable, and Readily Available Precatalysts for Suzuki and Buchwald−Hartwig Cross-coupling (Transamidation) of Amides and Esters by N−C/O−C Activation Tongliang Zhou,‡ Guangchen Li,‡ Steven P. Nolan,§ and Michal Szostak*,†,‡
Org. Lett. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/16/19. For personal use only.
†
College of Chemistry and Chemical Engineering and Key Laboratory of Auxiliary Chemistry and Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science and Technology, Xi’an 710021, China ‡ Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States § Department of Chemistry and Center for Sustainable Chemistry, Ghent University, Krijgslaan 281, 9000 Ghent, Belgium S Supporting Information *
ABSTRACT: A general class of well-defined, air-stable, and readily available Pd(II)-NHC precatalysts (NHC = Nheterocyclic carbene) for Suzuki and Buchwald−Hartwig cross-coupling of amides (transamidation) and esters by selective N−C/O−C cleavage is reported. Since these precatalysts are highly active and the easiest to synthesize, the study clearly suggests that [Pd(NHC)(acac)Cl] should be routinely included during the development of new crosscoupling methods. An assay for in situ screening of NHC salts in this cross-coupling manifold is presented.
T
ligand facilitates insertion of the metal into challenging C(acyl)−X bonds.15 The flexible steric bulk of the NHC ligand promoting the reductive elimination step as well as mild reaction conditions in the absence of strong bases have played a central role in the production of high value structural motifs by N−C and O−C activations.16 To date, two general classes of Pd(II)− NHC precatalysts have been developed for the cross-coupling of C(acyl)−X bonds, namely, [Pd(NHC)(allyl)Cl] complexes, bearing an allyl-type throw-away ligand,17 and Pd−PEPPSI-type complexes,18 bearing N-heterocycles as the throw-away ligand. Herein, we report the third class of Pd(II)−NHC precatalysts for the cross-coupling of amides and esters (Figure 1B). Since these precatalysts are highly active, general, and among the easiest to synthesize from the corresponding NHC salts, the study clearly suggests that [Pd(NHC)(acac)Cl] should be routinely included during the development of new crosscoupling methods of bench-stable amide and ester electrophiles. We demonstrate that these well-defined, air-stable, and readily available [Pd(NHC)(acac)Cl] precatalysts promote Suzuki4,11 and Buchwald−Hartwig19,20 cross-coupling of amides (transamidation) and esters with exquisite selectivity at the C(acyl)− X (X = N, O) bond. A broad range of CO electrophiles as well as functional groups and sterically hindered amines are tolerated in this process. Crucially, we present an assay for in situ screening of NHC salts in this cross-coupling manifold,21 which should significantly facilitate the development of improved catalysts for N−C/O−C cross-coupling. Overall, the study should lead to
ransition-metal-catalyzed cross-coupling of amides by N− C activation has emerged as a powerful method for functionalization of the classically considered inert amide bonds of key significance in pharmaceuticals and natural products (Figure 1A).1−10 Studies have demonstrated close similarities of the amide and ester cross-coupling manifolds initiated by an oxidative addition of a low-valent metal into C(acyl)−X (X = N, O) bond, permitting to generalize the cross-coupling platform.11,12 The key advance in the area was identification of strongly σdonating Pd-NHC precatalysts,13,14 wherein the NHC ancillary
Figure 1. (a) Amides as new electrophiles in cross-coupling reactions. (b) This work: [Pd(NHC)(acac)Cl] general catalysts for N−C/O−C activation. © XXXX American Chemical Society
Received: March 25, 2019
A
DOI: 10.1021/acs.orglett.9b01053 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters further advances in important N−C/O−C cross-coupling reactions of unconventional amide and ester electrophiles. Notable features of our study include: (1) the present class of catalysts is the easiest to synthesize from the corresponding NHC salts, and (2) the use of acac throw-away ligand permits to in situ screen NHC salts in this cross-coupling manifold, which is not possible with any other throw-away ligands. Given that these catalysts are highly active, robust, and air-stable, these precatalysts should be routinely included in screening of the best catalysts for amide and ester cross-coupling. In the context of amide bond activation, our laboratory has introduced strategies that allow for the direct functional group interconversion of amides enabled by selective metal insertion into the N−C bond.2−4 Guided by the rational design of PdNHC catalysts, we hypothesized that the use of [Pd(NHC)(acac)Cl] precatalysts in conjunction with the facile synthesis of these complexes might allow for the general access to acyl-Pd intermediates by C(acyl)−X (X = N, O) bond activation. Since the synthesis of [Pd(NHC)(acac)Cl] proceeds in a single, highyielding, scalable, and operationally convenient step (Figure 2),22a,b,c these catalysts offer a key practical advantage, cf. other
Scheme 1. Scope of the [Pd(IPr)(acac)Cl]-catalyzed Suzuki− Miyaura Cross-coupling of Amidesa,b
Figure 2. Facile synthesis of well-defined, air-stable [Pd(NHC)(acac)Cl]. a
Conditions: amide (1.0 equiv), Ar−B(OH)2 (3.0 equiv), [Pd] (3 mol %), K2CO3 (3.0 equiv), THF (0.25 M), 110 °C, 16 h. bIsolated yields. See Supporting Information (SI) for details.
Pd(II)−NHCs. The very simple synthesis of [Pd(NHC)(acac)Cl] results from a facile displacement of a κ2-O,O-bound acac from the Pd(acac)2 precursor (routinely, >90−95% yield). After extensive optimization, we found that the Suzuki− Miyaura cross-coupling of a model N-Ph/Boc amide 1a activated by the N-carbamate group (RE = 7.2 kcal/mol, τ = 29.1°, χN = 8.4°; RE = resonance energy; τ = twist angle, χN = nitrogen pyramidalization angle)23 occurs in 89% yield using [Pd(IPr)(acac)Cl] (3 mol %) as a precatalyst (K2CO3, 3 equiv, THF, 110 °C) (eq 1).
(3a), electron-rich (3b−c), and electronically deactivated boronic acids (3d). Furthermore, steric-hindrance (3e), metasubstitution (3f), and boronic acids bearing electrophilic groups (3g) that would not be easily tolerated in the classic Weinreb ketone synthesis are tolerated. Moreover, fluorinated boronic acids (3h−i) and heterocyclic boronic acids (3j) serve as competent partners in this cross-coupling. The scope of the amide component is similarly broad and includes electronically diverse (3c′−d′), sterically hindered (3e′), heterocyclic amides (3k), and amides bearing electrophilic groups (3g′). At present, halides are not compatible with the Pd-NHC catalysis. Studies are in progress to develop new classes of catalysts that allow cross-coupling in the presence of halides. We were pleased to find that this readily available precatalyst is compatible with a broad range of amide precursors (Scheme 2, 1g−k).14 The generality of the coupling bodes well for its future applications in organic synthesis. To further expand the potential utility of [Pd(IPr)(acac)Cl] precatalyst, we examined its utility in the acyl-Buchwald− Hartwig coupling by N−C bond activation (transamidation) (Scheme 3).20 The rich variety of amides in medicinal chemistry and natural products has made the development of transamidation methods an intense research area.24 Acyl Buchwald− Hartwig cross-coupling provides a powerful and unconventional approach to amide transamidation,25 employing challenging less nucleophilic amines. We found that [Pd(IPr)(acac)Cl] is a highly effective precatalyst for this reaction (Scheme 3). As
Importantly, the cleavage of the N-activating group was not observed, consistent with the mild reaction conditions and the high reactivity of the [Pd(IPr)(acac)Cl] precatalyst. It should be noted that Pd-phosphane catalysts are not compatible with the direct activation of the amide bond in the synthetically useful NBoc amides,13 which effectively serve as 2° amide equivalents. Optimization revealed increase of the reaction yield with increasing the amount of base and boronic acid, consistent with the rate-limiting transmetalation. Note that the base and boronic acid are soluble under the reaction conditions. Other bases, including Na2CO3 and K3PO4, were less effective. With the optimum conditions in hand, the scope of the Suzuki−Miyaura cross-coupling was next evaluated (Scheme 1). As shown, the reaction is compatible with a broad range of electronically diverse boronic acids, including electron-neutral B
DOI: 10.1021/acs.orglett.9b01053 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
variety of amides (Scheme 4, 1g−k), attesting to the generality of the coupling.
Scheme 2. Amide Scope in the [Pd(IPr)(acac)Cl]-catalyzed Cross-couplinga
Scheme 4. Amide Scope in the [Pd(IPr)(acac)Cl]-catalyzed Buchwald−Hartwig Cross-coupling of Amidesa
a
See Scheme 1.
Scheme 3. Scope of the [Pd(IPr)(acac)Cl]-catalyzed Buchwald−Hartwig Cross-coupling of Amidesa,b
a
See Scheme 2.
To further explore the utility of [Pd(IPr)(acac)Cl], we investigated the Suzuki and Buchwald−Hartwig cross-coupling of a representative phenolic ester (Scheme 5).11,12,20 We were pleased to find that this catalyst can also be used for a catalytic ester to ketone and ester to amide interconversion. Scheme 5. Suzuki−Miyaura and Buchwald−Hartwig CrossCoupling of Esters using [Pd(IPr)(acac)Cl]
Kinetic profiling studies were performed to assess the performance of [Pd(IPr)(acac)Cl] in the Suzuki and Buchwald−Hartwig cross-coupling of amides (Scheme 6). Studies demonstrated facile activation under the developed conditions in both reactions (Suzuki: 10 min, 60% yield; 30 min, 75% yield; Buchwald-Hartwig: 30 min, 72% yield; 60 min, 77% yield). Turnover numbers (TON) of 570 and 410 were determined for the cross-coupling of amide 1a in the Suzuki
a
Conditions: amide (1.0 equiv), amine (2.0 equiv), [Pd] (3 mol %), K2CO3 (3.0 equiv), DME (0.25 M), 110 °C, 16 h. bIsolated yields.
shown, this precatalyst demonstrates broad compatibility. Electronically diverse (5a−c) and sterically hindered anilines (5d−f), including the extremely bulky 2,6-disubstituted aniline (5e) as well as N,N-disubstituted anilines (5f) and bulky primary amines (5g) readily participate in the coupling. Furthermore, the reaction is compatible with biologically relevant carbazoles (5h), sulfonamides (5i), and 2-aminobiphenyls (5j). As a distinguishing feature of the mild Pd-NHC catalysis platform, electrophilic functional groups such as esters and sulfonamides are compatible. It is worth noting that the coupling of both electronically deactivated (5k) and amides bearing electrophilic functional groups (5l) is achieved by this method. Pleasingly, the survey of amide precursors revealed that [Pd(IPr)(acac)Cl] effectively promotes the amidation of a
Scheme 6. Determination of TON in the Cross-Coupling using [Pd(IPr)(acac)Cl]
C
DOI: 10.1021/acs.orglett.9b01053 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters (4-MeO-C6H4-B(OH)2, [Pd(IPr)(acac)Cl], 0.10 mol %) and Buchwald−Hartwig (4-anisidine, [Pd(IPr)(acac)Cl], 0.10 mol %) reactions under the standard conditions, indicating high reactivity of the complex. This reactivity can be compared with allyl-type precatalysts [(Pd(IPr)(cin)Cl] (Suzuki: 10 min, 79%; 30 min, 92%; Buchwald-Hartwig: 30 min, 79%; 60 min, 87%). While the higher temperature is needed for precatalyst activation, these conditions employ mild carbonate base and are carried out in the absence of water, which does not lead to the formation of hydroxides in the reaction.17g A key advantage of [Pd(NHC)(acac)Cl] precatalysts hinges upon their facile synthesis (Figure 2). Accordingly, we hypothesized that the acac precatalyst platform might be deployed to rapidly evaluate the influence of the NHC by use of their salt precursors in the acyl cross-coupling without preforming of Pd(II)−NHC precatalysts in a separate step.22 As shown, rapid evaluation of a small subset of NHC salts revealed that IPr and IMes are the preferred NHC ligands for the Suzuki cross-coupling (Scheme 7), while saturated SIPr, aliphatic ItBu, ICy, and bulky IPr* are less preferred.
Scheme 8. In Situ Screening of NHC Salts in Buchwald− Hartwig Coupling of Amides Using [Pd(NHC)(acac)Cl]
Scheme 7. In Situ Screening of NHC Salts in Suzuki−Miyaura Coupling of Amides Using [Pd(NHC)(acac)Cl]
of NHC salts, which should significantly facilitate the development of optimized cross-coupling protocols of acyl electrophiles. The study vividly demonstrates that the class of [Pd(NHC)(acac)Cl] catalysts should be routinely considered in the development of C(acyl)−X cross-coupling reactions.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01053. Experimental procedures and characterization data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Steven P. Nolan: 0000-0001-9024-2035 Michal Szostak: 0000-0002-9650-9690 Notes
Likewise, IPr, IMes, and SIPr were identified as the most effective NHC ligands in the Buchwald−Hartwig cross-coupling (Scheme 8) with ItBu, ICy, and IPr* resulting in diminished yields. Note that control experiments against well-defined [Pd(IPr)(acac)Cl] (91% yield, Suzuki-Miyaura; 86% yield, amination) demonstrate that the difference in catalytic performance is very small. Allyl-based precatalysts cannot be used for in situ screening due to less efficient Pd-NHC formation.17 We believe this rapid screening assay should significantly facilitate the development of ligands and improved protocols for C−X acyl bond activation. In summary, we have reported a general class of well-defined, air-stable, and readily synthesized [Pd(NHC)(acac)Cl] precatalysts for Suzuki−Miyaura and Buchwald−Hartwig crosscoupling of amides (transamidation) and esters by selective N− C/O−C cleavage. These Pd(II)−NHC precatalysts are highly reactive and among the easiest to synthesize from the corresponding NHC salts. A broad range of amides, boronic acids, and anilines, including those with electrophilic groups, were coupled in high yields and with high cleavage selectivity. Crucially, we have developed an in situ assay for rapid screening
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Rutgers University and the NSF (CAREER CHE-1650766) are gratefully acknowledged for support. The Bruker 500 MHz spectrometer was supported by the NSF-MRI grant (CHE1229030). For work performed in Ghent, VLAIO (SBO project CO2PERATE) is gratefully acknowledged for partial support.
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REFERENCES
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DOI: 10.1021/acs.orglett.9b01053 Org. Lett. XXXX, XXX, XXX−XXX
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cyclic ligands, and that (2) the synthesis and purification of [Pd(NHC) (acac)Cl] is significantly easier than that of PdCl2-based precatalysts. (23) (a) Szostak, R.; Shi, S.; Meng, G.; Lalancette, R.; Szostak, M. Ground-State Distortion in N-Acyl-tert-butyl-carbamates (Boc) and NAcyl-tosylamides (Ts): Twisted Amides of Relevance to Amide N− C Cross-Coupling. J. Org. Chem. 2016, 81, 8091−8094. (b) Meng, G.; Shi, S.; Lalancette, R.; Szostak, R.; Szostak, M. Reversible Twisting of Primary Amides via Ground State N−C(O) Destabilization: Highly Twisted Rotationally Inverted Acyclic Amides. J. Am. Chem. Soc. 2018, 140, 727−734. (24) de Figueiredo, R. M.; Suppo, J. S.; Campagne, J. M. Nonclassical Routes for Amide Bond Formation. Chem. Rev. 2016, 116, 12029− 12122. (25) Ruiz-Castillo, P.; Buchwald, S. L. Applications of PalladiumCatalyzed C−N Cross-Coupling Reactions. Chem. Rev. 2016, 116, 12564−12649.
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DOI: 10.1021/acs.orglett.9b01053 Org. Lett. XXXX, XXX, XXX−XXX
