Synthesis of E-Alkyl Alkenes from Terminal Alkynes via Ni-Catalyzed Cross-Coupling of Alkyl Halides with B-Alkenyl-9-borabicyclo[3.3.1]nonanes

A Process Chemistry Benchmark for sp2-sp3 Cross Couplings

As sp²-sp³ disconnections gain acceptance in the medicinal chemist's toolbox, an increasing number of potential drug candidates containing this motif are moving into the pharmaceutical development pipeline. This raises a new set of questions and challenges around the novel, direct methodologies available for forging these bonds. These questions gain further importance in the context of process chemistry, where the focus is the development of scalable processes that enable the large-scale delivery of clinical supplies. In this paper, we describe our efforts to apply a wide variety of standard, photo-, and electrochemical sp²-sp³ cross-coupling methods to a pharmaceutically relevant intermediate and optimize each through a combination of high throughput and mechanistically guided experimentation. With data regarding the performance, benefits, and limitations of these novel methods, we evaluate them against a more traditional two-step palladium-catalyzed process. This work reveals trends and similarities between these sp²-sp³ bond-forming methods and suggests a path forward for further refinements.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Antiparasitic Constituents of Beilschmiedia louisii and Beilschmiedia obscura and Some Semisynthetic Derivatives (Lauraceae)

The MeOH/CH₂Cl₂ (1:1) extracts of the roots and leaves of Beilschmiedia louisii and B. obscura showed potent antitrypanosomal activity during preliminary screening on Trypanosoma brucei brucei. Phytochemical investigation of these extracts led to the isolation of a mixture of two new endiandric acid derivatives beilschmiedol B (1) and beilschmiedol C (2), and one new phenylalkene obscurene A (3) together with twelve known compounds (4–15). In addition, four new derivatives (11a–11d) were synthesized from compound 11. Their structures were elucidated based on their NMR and MS data. Compounds 5, 6, and 7 were isolated for the first time from the Beilschmiedia genus. Additionally, the NMR data of compound 4 are given here for the first time. The isolates were evaluated for their antitrypanosomal and antimalarial activities against Tb brucei and the Plasmodium falciparum chloroquine-resistant strain Pf3D7 in vitro, respectively. From the tested compounds, the mixture of new compounds 1 and 2 exhibited the most potent antitrypanosomal activity in vitro with IC₅₀ value of 4.91 μM.

Nickamine and Analogous Nickel Pincer Catalysts for Cross-Coupling of Alkyl Halides and Hydrosilylation of Alkenes

Ligand development plays an essential role in the advance of homogeneous catalysis. Tridentate, meridionally coordinating ligands, commonly termed pincer ligands, have been established as a privileged class of ligands in catalysis because they confer high stability while maintaining electronic tenability to the resulting metal complexes. Pincer ligands containing "soft" donors such as phosphines are typically used for late transition-metal ions, which are considered "soft" acids. Driven by our interest to develop base-metal catalysis and in view of the "hard" character of base-metal ions, our group explored a pincer ligand containing only "hard" nitrogen donors. A prototypical nickel complex of this ligand, "Nickamine", turned out to be an efficient catalyst in a wide range of organic reactions. Because of its propensity to mediate single-electron redox chemistry, Nickamine is particularly suited to catalyze cross-coupling of nonactivated alkyl halides through radical pathways. These coupling partners have been challenging substrates for traditional, palladium-based catalysts because of difficult oxidative addition and nonproductive β-H elimination. The high activity of Nickamine for cross-coupling leads to high chemoselectivity and functional group tolerance, even when reactive Grignard reagents are employed as nucleophiles. The scope of the catalysis is broad and encompasses sp³-sp³, sp³-sp², and sp³-sp cross-coupling. The defined nature of Nickamine facilitated the mechanistic study of cross-coupling reactions. Experiments involving radical-probe substrates, presumed intermediates and dormant species, kinetics, and density functional theory computations revealed a bimetallic oxidative addition pathway. In this pathway, two Ni centers each provide one electron to support the two-electron activation of an alkyl halide substrate. The success of Nickamine motivated our systematic structure-activity studies aiming at improved activity in certain reactions through ligand modification. Indeed, better catalysts have been developed for cross-coupling of secondary alkyl halides as well as direct alkynylation of alkyl halides. The improvement is attributed to a more accessible Ni center in the new catalysts than in Nickamine. Surprisingly, the improvement could be obtained simply by replacing a dimethyl amino group in Nickamine with a pyrrolidino group. During the study of the catalytic cycle of Nickamine in cross-coupling reactions, we synthesized the corresponding Ni-H species. Consequently, we explored the catalytic application of Nickamine in Ni-H mediated reactions, such as hydrosilylation. To our delight, Nickamine is a chemoselective catalyst for hydrosilylation of alkenes while tolerating a reactive C=O group. An analogous Ni pincer complex was found to catalyze unusual hydrosilylation reactions using alkoxy hydrosilanes as surrogates of gaseous silanes.

Alkenylation of unactivated alkyl bromides through visible light photocatalysis

Two visible-light driven alkenylation reactions of unactivated alkyl bromides, which were enabled by the use of Ir(dF(CF₃)ppy)₂(dtbbpy)PF₆ as the photocatalyst and (TMS)₃SiH as the atom transfer reagent to activate the alkyl bromides, were described for the first time.

Palladium-Catalyzed Reductive Coupling Reaction of Terminal Alkynes with Aryl Iodides Utilizing Hafnocene Difluoride as a Hafnium Hydride Precursor Leading to trans-Alkenes

Herein, we describe a reductive cross-coupling of alkynes and aryl iodides by using a novel catalytic system composed of a catalytic amount of palladium dichloride and a promoter precursor, hafnocene difluoride (Cp₂ HfF₂ , Cp=cyclopentadienyl anion), in the presence of a mild reducing reagent, a hydrosilane, leading to a one-pot preparation of trans-alkenes. In this process, a series of coupling reactions efficiently proceeds through the following three steps: (i) an initial formation of hafnocene hydride from hafnocene difluoride and the hydrosilane, (ii) a subsequent hydrohafnation toward alkynes, and (iii) a final transmetalation of the alkenyl hafnium species to a palladium complex. This reductive coupling could be chemoselectively applied to the preparation of trans-alkenes with various functional groups, such as an alkyl group, a halogen, an ester, a nitro group, a heterocycle, a boronic ester, and an internal alkyne.

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

1,1-Disubstituted olefin synthesis via Ni-catalyzed Markovnikov hydroalkylation of alkynes with alkyl halides

A Ni-catalyzed Markovnikov hydroalkylation of alkynes with alkyl halides is described. The reaction proceeds smoothly without the use of sensitive organometallic reagents and shows good functional-group compatibility, enabling the efficient synthesis of a variety of 1,1-disubstituted olefins. It also provides a straightforward approach for the modification of complex organic molecules.