The regioselective formation of the 6-lithio derivative of 1,5-dichloro-2,4-dimethoxybenzene (i.e., 12) by directed ortho metalation (DoM) with nBuLi in THF is described. Although literature reports suggest direct deprotonation at C6, a series of time-course and labelling studies has revealed that deprotonation rather occurs exclusively at C3 followed by isomerization of the anion to C6. By contrast, when DoM was performed in Et₂O, deprotonation again occurred selectively at C3, but now no isomerization occurs, and electrophilic capture produces the regioisomer of that produced in THF. In these labeling studies, it has been found that deuterium has an enormous kinetic isotope effect (KIE) that suppresses not only the original DoM reaction at C3 when deuterium is present there, but also suppresses isomerization to C6 when the label is at that site. Remarkably, this “protecting-group” role of the deuterium is unique to THF; in ether, full deprotonation of the deuterium at C3 was observed.

A single set of reaction conditions for the palladium-catalyzed amination of a wide variety of (hetero)aryl halides using primary alkyl amines has been developed. By combining the exceptionally high reactivity of the Pd-PEPPSI-IPent^Cl catalyst (PEPPSI=pyridine enhanced precatalyst preparation, stabilization, and initiation) with the soluble and nonaggressive sodium salt of BHT (BHT=2,6-di-tert-butyl-hydroxytoluene), both six- and five-membered (hetero)aryl halides undergo efficient and selective amination.

The ability to cross-couple secondary alkyl centers is fraught with a number of problems, including difficult reductive elimination, which often leads to β-hydride elimination. Whereas catalysts have been reported that provide decent selectivity for the expected (non-rearranged) cross-coupled product with aryl or heteroaryl oxidative-addition partners, none have shown reliable selectivity with five-membered-ring heterocycles. In this report, a new, rationally designed catalyst, Pd-PEPPSI-IHept^Cl, is demonstrated to be effective in selective cross-coupling reactions with secondary alkyl reagents across an impressive variety of furans, thiophenes, and benzo-fused derivatives (e.g., indoles, benzofurans), in most instances producing clean products with minimal, if any, migratory insertion for the first time.

A single set of reaction conditions for the palladium-catalyzed amination of a wide variety of (hetero)aryl halides using primary alkyl amines has been developed. By combining the exceptionally high reactivity of the Pd-PEPPSI-IPent^Cl catalyst (PEPPSI=pyridine enhanced precatalyst preparation, stabilization, and initiation) with the soluble and nonaggressive sodium salt of BHT (BHT=2,6-di-tert-butyl-hydroxytoluene), both six- and five-membered (hetero)aryl halides undergo efficient and selective amination.

The ability to cross-couple secondary alkyl centers is fraught with a number of problems, including difficult reductive elimination, which often leads to β-hydride elimination. Whereas catalysts have been reported that provide decent selectivity for the expected (non-rearranged) cross-coupled product with aryl or heteroaryl oxidative-addition partners, none have shown reliable selectivity with five-membered-ring heterocycles. In this report, a new, rationally designed catalyst, Pd-PEPPSI-IHept^Cl, is demonstrated to be effective in selective cross-coupling reactions with secondary alkyl reagents across an impressive variety of furans, thiophenes, and benzo-fused derivatives (e.g., indoles, benzofurans), in most instances producing clean products with minimal, if any, migratory insertion for the first time.

A single pass flow diazotization/Mizoroki–Heck protocol has been developed for the production of cinnimoyl and styryl products. The factors that govern aryl diazonium salt stability have been examined in detail leading to the development of a MeOH/DMF co-solvent system in which the diazonium salts can be generated in the presence of all other reaction components and then coupled selectively to give the desired products. Finally the key role of the reaction quench for flow reactions has been demonstrated.

Herein, we describe a highly regio- and stereoselective radical-mediated and molecular-oxygen (O₂)-dependent hydrostannylation of phenyl propargylic alcohols and their derivatives. There is a significant steric effect on the stereoselectivity of the tin-radical addition. Further, the uncatalyzed regio- and stereoselective hydrostannylation of aryl propargylic alcohols with nBu₃SnH and Ph₃SnH is also described and occurs with near titration kinetics. Although the uncatalyzed addition with nBu₃SnH gave a remarkable γ-regioselectivity irrespective of the electronic nature of the aryl moiety, addition with Ph₃SnH appears to be driven by the electronic nature of the aryl alkynes.

Current state-of-the-art protocols for the coupling of unreactive amines (e.g., electron-poor anilines) with deactivated oxidative-addition partners (e.g., electron-rich and/or hindered aryl chlorides) involve strong heating (usually >100 °C) and/or tert-butoxide base, and even then not all couplings are successful. The aggressive base tert-butoxide reacts with and in many instances destroys the typical functional groups that are necessary for the function of most organic molecules, such as carbonyl groups, esters, nitriles, amides, alcohols, and amines. The new catalyst described herein, Pd-PEPPSI-IPent^Cl-o-picoline, is able to aminate profoundly deactivated coupling partners when using only carbonate base at room temperature.

The role of halide salt additives has been investigated in the Negishi reaction involving aryl zinc reagents. Diarylzincs readily transmetallate to Pd in relatively non-polar media (e.g., THF) with zero salt present and coupling proceeds. Arylzinc halides (ArZnX) fail to couple in THF without salt, but do couple with it. However, unlike alkylzincs that form higher-order zincates in order to facilitate transmetallation, all that is required with arylzincs in an increase in solvent dielectric as even ZnX₂ works as an additive, which completely terminates alkylzinc coupling.

Current state-of-the-art protocols for the coupling of unreactive amines (e.g., electron-poor anilines) with deactivated oxidative-addition partners (e.g., electron-rich and/or hindered aryl chlorides) involve strong heating (usually >100 °C) and/or tert-butoxide base, and even then not all couplings are successful. The aggressive base tert-butoxide reacts with and in many instances destroys the typical functional groups that are necessary for the function of most organic molecules, such as carbonyl groups, esters, nitriles, amides, alcohols, and amines. The new catalyst described herein, Pd-PEPPSI-IPent^Cl-o-picoline, is able to aminate profoundly deactivated coupling partners when using only carbonate base at room temperature.

The role of halide salt additives has been investigated in the Negishi reaction involving aryl zinc reagents. Diarylzincs readily transmetallate to Pd in relatively non-polar media (e.g., THF) with zero salt present and coupling proceeds. Arylzinc halides (ArZnX) fail to couple in THF without salt, but do couple with it. However, unlike alkylzincs that form higher-order zincates in order to facilitate transmetallation, all that is required with arylzincs in an increase in solvent dielectric as even ZnX₂ works as an additive, which completely terminates alkylzinc coupling.

A series of new, easily activated NHC–Pd^II precatalysts featuring a trans-oriented morpholine ligand were prepared and evaluated for activity in carbon-sulfur cross-coupling chemistry. [(IPent)PdCl₂(morpholine)] (IPent=1,3-bis(2,6-di(3-pentyl)phenyl)imidazol-2-ylidene) was identified as the most active precatalyst and was shown to effectively couple a wide variety of deactivated aryl halides with both aryl and alkyl thiols at or near ambient temperature, without the need for additives, external activators, or pre-activation steps. Mechanistic studies revealed that, in contrast to other common NHC–Pd^II precatalysts, these complexes are rapidly reduced to the active NHC–Pd⁰ species at ambient temperature in the presence of KOtBu, thus avoiding the formation of deleterious off-cycle Pd^II–thiolate resting states.

The activation of PEPPSI precatalysts has been systematically studied in Pd-catalysed sulfination. Under the reactions conditions of the sulfide and KOtBu in toluene, the first thing that happens is exchange of the two chlorides on the PEPPSI precatalyst with the corresponding sulfides, creating the first resting state; it is via this complex that all Pd enters the catalytic cycle. However, it is also from this same complex that a tri-Pd complex forms, which is a more persistent resting state. Under standard reaction conditions, this complex is catalytically inactive. However, if additional pyridine or a smaller base (i.e., KOEt) is added, this complex is broken down, presumably initially back to the first resting state and it is again capable of entering the catalytic cycle and completing the sulfination. Of note, once the tri-Pd complex forms, one equivalent of Pd is lost to the transformation. Related to this, the nature of the cation of the sulfide salt and solvent dielectric is very important to the success of this transformation. That is, the less soluble the salt the better the performance, which can be attributed to lowering sulfide concentration to avoid the movement of the Pd-NHC complex into the above described off-cycle sulfinated resting states.

Generalized iodolactonization conditions have been developed for the formation of both cis- and trans-γ-lactones from 3-pentenoic acid derivatives containing various protected alcohol moieties. Kinetic control of the reaction using iodine and base at 0 °C for 6 h provided the cis-lactone with good selectivity (75:25), and thermodynamic control at room temperature for 24 h provided the trans-lactone with excellent selectivity (98:2).

Palladium-catalyzed cross-coupling reactions enable organic chemists to form CC 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.

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.

Palladiumkatalysierte Kreuzkupplungen versetzten Organiker in die Lage, C-C-Bindungen gezielt und unter milden Reaktionsbedingungen zu einzuführen. Während Phosphanliganden in diesen Umsetzungen am intensivsten erforscht wurden, sind seit kurzem auf der Suche nach leistungsfähigeren Katalysatoren für Kreuzkupplungen auch N-heterocyclische Carbenliganden (NHC), die eine starke Bindung zum Palladiumzentrum bilden, beachtet worden. PEPPSI-Pd-Präkatalysatoren (“Pyridin-verstärkte Präkatalysatorherstellung, -stabilisierung und -initiierung”) mit sperrigen NHC-Liganden haben sich mittlerweile als Alternative neben Palladium-Phosphan-Komplexen etabliert. Über ihre Leistungsfähigkeit in Suzuki-Miyaura-, Negishi- und Stille-Migita-Kreuzkupplungen sowie in Aminierungen und Sulfinierungen berichtet dieser Aufsatz.

A collection of highly functionalized, trisubstituted olefin building blocks (templates) have been developed for their Pd-catalyzed elaboration into more complex structures, including natural products and compounds of medicinal chemistry interest. The Pd-mediated transformations involve a combination of allylic substitution and cross-coupling reactions. The templates have been designed such that these processes proceed not only with a high degree of regio- and stereoselectivity, but the transformations can all be conducted sequentially in the same flask using the same catalyst.

Negishi cross-coupling reactions were analyzed in solution by mass spectrometry and NMR spectroscopy to identify both the effect of LiBr as an additive as well as the purpose of 3-dimethyl-2-imidazolidinone (DMI) as a co-solvent. The results suggest that the main role of DMI is to facilitate a higher order bromozincate formation during the addition of LiBr.

An overview of Pd-PEPPSI complexes in the Negishi cross-coupling reaction is presented. Preliminary evaluations of differentially substituted imidazolium salts that generate highly active cross-coupling catalysts in situ provide the foundation for the development of an air- and moisture-stable NHC-based precatalyst: Pd-PEPPSI-IPr. The application of Pd-PEPPSI-IPr in sp³–sp³, sp³–sp² (and vice versa), and sp²–sp² Negishi cross-couplings is reviewed. This allowed the systematic development of Pd-PEPPSI-IPent, a more sterically demanding, second-generation catalyst that outperforms the IPr analogue in a variety of sp²–sp² Negishi cross-couplings. General routes for the preparation of organozinc reagents are also examined, and an additives study reveals the probable transmetallating species that is operative in these cross-couplings. Mechanistic considerations of the Negishi cross-coupling reaction based on experimental and computational studies are summarized and evaluated.

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–Pd⁰ is also plausible, in which the more basic pyridine stays bound for longer.

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 °C) than triarylphosphine-based Pd catalysts, improving the scope of this useful carbon–carbon bond-forming process.

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 °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 %.

A method has been devised for the microwave-assisted, continuous-flow preparation of indole alkaloids by a two-step aryl amination/cross-coupling sequence of bromoalkenes and 2-bromoanilines. This process requires both the presence of a metal-lined flow tube (a 1180 micron capillary) and the Pd PEPPSI-IPr catalyst; without either, the catalyst or the film, there is zero turnover of this catalytic process. A silver film has been shown to provide some conversion (48–62 %), but optimal results (quantitative) across a variety of bromoalkenes and bromoanilines were achieved by using a highly porous palladium film. Possible roles for the Pd film are considered, as is the interplay of the catalyst and the film.

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 (pK_b) requirements is also provided.

Thin gold films on the surface of glass capillaries have proven to be highly active catalysts for the rapid hydrosilylation of alkynes that are flowed through the reactor while being heated by microwave irradiation. The films are able to be reused at least five times with no loss of activity and with no detectable levels of gold showing up in the hydrosilylated products.

Two strategies, namely a cross-metathesis/ring-closing metathesis and Pd-catalyzed Stille allylation/Nozaki–Hiyama–Kishi coupling, are examined for the preparation of neodolabellane-type diterpenoids 1 and 2. Whereas the first approach possessed synthetic limitations, the latter was successfully employed to provide compounds 1 and 2 in 8.8 % (14 steps) and 8 % (15 steps) overall yields, respectively.