Aldrichimica Acta Vol. 45, No. 3 - Sigma-Aldrich

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Dalphos Ligands for Cross-Coupling A Dalphos ligand contains the bulky di(1-adamantyl)phosphino [P(1-Ad) 2 ] fragment. These chelating N,P ligands are useful for Pd-catalyzed C–N and C–C bond formation. The more reactive Mor-Dalphos improves the scope and utility of ammonia coupling at room temperature, and is also effective in the coupling of hydrazine and acetone. • Monoarylation of ammonia, hydrazine and acetone • Air-stable • Good functional group tolerance • Mild conditions To learn more about DalPhos ligands, visit Aldrich.com/dalphosacta Pd-Catalyzed Monoarylation of Ammonia, Hydrazine and Acetone 751618 Mor-DalPhos 751413 Me-DalPhos 759198 New References: (1) Lundgren, R. J.; Peters, B. D.; Alsabeh, P. G.; Stradiotto, M. Angew. Chem. Int. Ed. 2010, 49, 4071. (2) Lundgren, R. J.; Stradiotto, M. Angew. Chem. Int. Ed. 2010, 49, 8686. (3) Hesp, K. D.; Lundgren, R. J.; Stradiotto, M. J. Am. Chem. Soc. 2011, 133, 5194. (4) Lundgren, R. J.; Sappong-Kumankumah, A.; Stradiotto, M. Chem.—Eur. J. 2010, 16, 1983. (5) Hesp, K. D.; Stradiotto, M. J. Am. Chem. Soc. 2010, 132, 18026. 79653
VOL. 45, NO. 3 • 2012 Recent Advances in the Buchwald–Hartwig Amination Reaction Enabled by the Application of Sterically Demanding Phosphine Ancillary Ligands Dr. Rylan J. Lundgren Prof. Mark Stradiotto Keywords. amination; catalysis; cross-coupling; ligand design; palladium. Abstract. This review highlights a selection of important recent developments in the catalysis of the Buchwald–Hartwig amination reaction. These developments have been enabled through the use of sterically demanding phosphine ancillary ligands. Our aim is to inform the reader of recent methodology advances, and to encourage further development and understanding within the field of ancillary ligand design. Outline 1. Introduction 2. Selective Arylation of Challenging Substrates 2.1. Selective Monoarylation of Ammonia 2.2. Selective Monoarylation of Hydrazine 2.3. Regioselective Arylation of Imidazoles and 1,2,3-Triazoles 2.4. Amination of Aryl Mesylates 3. Chemoselective Arylation of Substrates Featuring Multiple N–H Functionalities 4. Towards a Universal Catalyst for the Buchwald–Hartwig Amination 5. Conclusions and Outlook 6. Acknowledgements 7. References 1. Introduction The palladium-catalyzed cross-coupling of (hetero)aryl (pseudo)halides and N–H containing substrates (i.e., Buchwald–Hartwig amination or BHA) has emerged as a highly effective C–N bond-forming protocol that is employed on both bench-top and industrial scales for the construction of (hetero)arylamines. 1 A prototypical cross-coupling reaction of this type involving 4-chlorotoluene and aniline is depicted in eq 1. In the time following the initial development of such catalytic cross-coupling reactions independently by Buchwald’s 2 and Hartwig’s 3 groups, significant research effort has been directed toward expanding the scope and utility of such methods, including by examining how the choice of solvent, base, palladium precursor, and, perhaps most notably, the ancillary co-ligand influences the course of the BHA reaction. 4 Early studies employed relatively simple triarylphosphine ancillary Rylan J. Lundgren and Mark Stradiotto* Department of Chemistry Dalhousie University 6274 Coburg Road P.O. Box 15000 Halifax, NS, Canada B3H 4R2 Email: mark.stradiotto@dal.ca ligands. However, the subsequent observation that the use of more structurally complex ancillary ligands can enable more challenging substrate pairings, often with excellent functional group tolerance and under milder conditions (e.g., low catalyst loadings, mild reaction temperatures), has given rise to the burgeoning field of ancillary ligand development within the domain of BHA catalysis. Indeed, it is now well-established that the use of sterically demanding and electronrich phosphine ligands, as well as N-heterocyclic carbenes, 5 serves to facilitate the formation of low-coordinate Pd(0) species that can readily engage in otherwise challenging C sp2–X bond oxidative addition reactions (e.g., X = Cl), as well as to encourage, through the alleviation of steric congestion, C–N bond reductive elimination from requisite L nPd(aryl)(NR 2) intermediates (R = aryl, alkyl, H, etc.) The aim of this review is to highlight a selection of important recent developments in the rapidly expanding field of BHA catalysis that have been enabled through the design and/or implementation of sterically demanding phosphine ancillary ligands. In particular, emphasis will be placed on examples from the recent literature in which the novel ligand architecture has played a central role in advancing the state of the art for reaction classes where achieving broad substrate scope, selectivity, and catalytic efficiency has otherwise proven to be particularly challenging. The ancillary ligands that will be discussed in this review include the bisphosphine JosiPhos ligand (L1) utilized by Hartwig’s group, biaryl monophosphine ligands (L2–L6) developed by Buchwald’s group, the P,N DalPhos ligands (L7 and L8) developed by our group, and the substituted indole ligand L9 disclosed by Kwong’s group (Figure 1). 2. Selective Arylation of Challenging Substrates Notwithstanding the tremendous progress that has been made with regard to expanding the scope of BHA in terms of both the (hetero)aryl (pseudo)halide and amine reaction partners, some transformations have until recently proven to be stubbornly problematic. Notable examples of such challenging transformations include the selective monoarylation of ammonia and hydrazine, the regioselective arylation of imidazoles and 1,2,3-triazoles, and the use of aryl mesylates as coupling partners. In the sections that follow, we discuss in detail the recent advances in ancillary ligand development that have enabled significant progress in addressing these reactivity challenges. 59
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