Michael G. Organ

The Development of Bulky Palladium NHC Complexes for the Most-Challenging Cross-Coupling Reactions

Palladium-catalyzed cross-coupling reactions enable organic chemists to form C-C bonds in targeted positions and under mild conditions. Although phosphine ligands have been intensively researched, in the search for even better cross-coupling catalysts attention has recently turned to the use of N-heterocyclic carbene (NHC) ligands, which form a strong bond to the palladium center. PEPPSI (pyridine-enhanced precatalyst preparation, stabilization, and initiation) palladium precatalysts with bulky NHC ligands have established themselves as successful alternatives to palladium phosphine complexes. This Review shows the success of these species in Suzuki-Miyaura, Negishi, and Stille-Migita cross-couplings as well as in amination and sulfination reactions.

Structure–Activity Relationship Analysis of Pd–PEPPSI Complexes in Cross-Couplings: A Close Inspection of the Catalytic Cycle and the Precatalyst Activation Model

A series of Pd-N-heterocyclic carbene (Pd-NHC) complexes with various NHC, halide and pyridine ligands (PEPPSI (pyridine, enhanced, precatalyst, preparation, stabilisation and initiation) precatalysts) were prepared, and the effects of these ligands on catalyst activation and performance were studied in the Kumada-Tamao-Corriu (KTC), Negishi, and Suzuki-Miyaura cross-coupling reactions. The lowered reactivity of more hindered 2,6-dimethylpyridyl complex 4 in the Negishi and KTC reactions is consistent with slow reductive dimerisation of the organometallic reaction partner during precatalyst activation. Comparative rate studies of complexes 1, 4 and 5 in the KTC and Suzuki-Miyaura reactions verify that 4 activated more slowly than the others. A potential on/off mechanism of pyridine coordination to NHC-Pd(0) is also plausible, in which the more basic pyridine stays bound for longer.

Pd‐Catalyzed Aryl Amination Mediated by Well Defined, N‐Heterocyclic Carbene (NHC)–Pd Precatalysts, PEPPSI

Pd-N-heterocyclic carbene (NHC)-catalyzed Buchwald-Hartwig amination protocols mediated by Pd-PEPPSI precatalysts is described. These protocols provide access to a range of hindered and functionalized drug-like aryl amines in high yield with both electron-deficient and electron-rich aryl- and heteroaryl chlorides and bromides. Variations in solvent polarity, base and temperature are tolerated, enhancing the scope and utility of this protocol. A mechanistic rationalization for base strength (pKb) requirements is also provided.

Propargyl Amine Synthesis Catalysed by Gold and Copper Thin Films by Using Microwave-Assisted Continuous-Flow Organic Synthesis (MACOS)

An effective multi-component reaction (MCR) protocol has been developed for the construction of propargyl amines from aldehydes, amines and terminal alkynes by using microwave-assisted continuous-flow organic synthesis (MACOS). The process is catalysed by thin films of either copper or gold that achieve temperatures in excess of 900 degrees C when irradiated with low levels of microwave power. The process works equally well for premixed solutions of the three starting materials, or as three separate streams, which improves the combinatorial efficiency of the method. The process tolerates a wide variety of functional groups and heterocycles, and conversion over these diverse substrates ranges from 70-90 %.

Pd-PEPPSI-IPentCl: A Highly Effective Catalyst for the Selective Cross-Coupling of Secondary Organozinc Reagents†

No migration? No problem! A series of new N-heterocyclic carbene based Pd complexes has been created and evaluated in the Negishi cross-coupling of aryl and heteroaryl chlorides, bromides, and triflates with a variety of secondary alkylzinc reagents. The direct elimination product is nearly exclusively formed; in most examples there is no migratory insertion at all.

Amination with Pd–NHC Complexes: Rate and Computational Studies on the Effects of the Oxidative Addition Partner

Pd-PEPPSI-IPent, a recently-developed N-heterocyclic carbene (NHC) complex, has been evaluated in amination reactions with secondary amines and it has shown superb reactivity under the most mildly basic reaction conditions. Rate and computational studies were conducted to provide insight into the mechanism of the transformation. The IPent catalyst coordinates to the amine much more strongly than the IPr variant, thus favouring deprotonation with comparatively weak bases. Indeed the reaction is first order in base and only slightly more than zeroth order in amine.

Regioselective Cross-Coupling of Allylboronic Acid Pinacol Ester Derivatives with Aryl Halides via Pd-PEPPSI-IPent

The cross-coupling reactions of allylboronic acid pinacol ester derivatives with aryl and heteroaryl halides occurred with high selectivity (>97%) at the α-carbon of the allylboron reagent in the presence of Pd-PEPPSI-IPent precatalyst and 5 M KOH in refluxing THF. In the case of trisubstituted allylboronates with different substituents on the olefin, minor olefin geometry isomerization was observed (E/Z ≈ 80/20).

An Efficient Low‐Temperature Stille–Migita Cross‐Coupling Reaction for Heteroaromatic Compounds by Pd–PEPPSI–IPent

The reactivity of Pd-PEPPSI (Pyridine, Enhanced, Precatalyst, Preparation, Stabilization, and Initiation) precatalysts in the Stille-Migita cross-coupling reaction between heteroaryl stannanes and aryl or heteroaryl halides was evaluated. In general, Pd-PEPPSI-IPent (IPent=diisopentylphenylimidazolium derivative) demonstrated high efficiency over a variety of challenging aryl or heteroaryl halides with thiophene-, furan-, pyrrole-, and thiazole-based organostannanes when compared with Pd-PEPPSI-IPr (IPr=diisopropylphenylimidazolium derivative). The transformations proceeded at appreciably lower temperatures (30-80 degrees C) than triarylphosphine-based Pd catalysts, improving the scope of this useful carbon-carbon bond-forming process.

Amination with PdNHC Complexes: Rate and Computational Studies Involving Substituted Aniline Substrates

The amination of aryl chlorides with various aniline derivatives using the N-heterocyclic carbene-based Pd complexes Pd-PEPPSI-IPr and Pd-PEPPSI-IPent (PEPPSI=pyridine, enhanced precatalyst, preparation, stabilization, and initiation; IPr=diisopropylphenylimidazolium derivative; IPent= diisopentylphenylimidazolium derivative) has been studied. Rate studies have shown a reliance on the aryl chloride to be electron poor, although oxidative addition is not rate limiting. Anilines couple best when they are electron rich, which would seem to discount deprotonation of the intermediate metal ammonium complex as being rate limiting in favour of reductive elimination. In previous studies with secondary amines using PEPPSI complexes, deprotonation was proposed to be the slow step in the cycle. These experimental findings relating to mechanism were corroborated by computation. Pd-PEPPSI-IPr and the more hindered Pd-PEPPSI-IPent catalysts were used to couple deactivated aryl chlorides with electron poor anilines; while the IPr catalysis was sluggish, the IPent catalyst performed extremely well, again showing the high reactivity of this broadly useful catalyst.

High yielding alkylations of unactivated sp3 and sp2 centres with alkyl-9-BBN reagents using an NHC-based catalyst: Pd-PEPPSI-IPr

High yielding, room temperature cross couplings of unactivated alkyl bromides and aryl bromides/chlorides with alkyl-9-BBN reagents has been achieved using an NHC-based catalyst (Pd-PEPPSI-IPr) via a general, functional-group tolerant and easily implemented protocol.