Dialkyl Ether Formation by Nickel‐Catalyzed Cross‐Coupling of Acetals and Aryl Iodides

A new substrate class for nickel‐catalyzed C(sp³) cross‐coupling reactions is reported. α‐Oxy radicals generated from benzylic acetals, TMSCl, and a mild reductant can participate in chemoselective cross‐coupling with aryl iodides using a 2,6‐bis(N‐pyrazolyl)pyridine (bpp)/Ni catalyst. The mild, base‐free conditions are tolerant of a variety of functional groups on both partners, thus representing an attractive C? C bond‐forming approach to dialkyl ether synthesis. Characterization of a [(bpp)NiCl] complex relevant to the proposed catalytic cycle is also described.     

Nickel‐Mediated Inter‐ and Intramolecular Reductive Cross‐Coupling of Unactivated Alkyl Bromides and Aryl Iodides at Room Temperature

A nickel‐mediated intermolecular reductive cross‐coupling reaction of unactivated alkyl bromides and aryl iodides at room temperature has been developed and successfully extended to less explored intramolecular versions and tandem cyclization‐intermolecular cross‐coupling. Highly stereoselective (or stereospecific) synthesis of linear‐fused perhydrofuro[2,3‐b]furan (pyran) and spiroketal skeletons allows rapid access to these useful building blocks, which would be potentially valuable in the synthesis of relevant natural products. A rational explanation for the formation of contiguous stereogenic centers is given.     

Secondary Alkyl Halides in Transition‐Metal‐Catalyzed Cross‐Coupling Reactions

Secondary, but second to none : The use of secondary alkyl halides in transition‐metal‐catalyzed cross‐coupling reactions (see scheme) has advanced significantly over the last five years. Selected examples of these transformations are examined, including mechanistic and stereochemical aspects.

     

Computational Insight into Nickel‐Catalyzed Carbon–Carbon versus Carbon–Boron Coupling Reactions of Primary,Secondary, and Tertiary Alkyl Bromides

The nickel‐catalyzed alkyl–alkyl cross‐coupling (C?C bond formation) and borylation (C?B bond formation) of unactivated alkyl halides reported in the literature show completely opposite reactivity orders in the reactions of primary, secondary, and tertiary alkyl bromides. The proposed Ni^I/Ni^III catalytic cycles for these two types of bond‐formation reactions were studied computationally by means of DFT calculations at the B3LYP level. These calculations indicate that the rate‐determining step for alkyl–alkyl cross‐coupling is the reductive elimination step, whereas for borylation the rate is determined mainly by the atom‐transfer step. In borylation reactions, the boryl ligand involved has an empty p orbital, which strongly facilitates the reductive elimination step. The inability of unactivated tertiary alkyl halides to undergo alkyl–alkyl cross‐coupling is mainly due to the moderately high reductive elimination barrier.     

Cobalt‐Catalyzed Negishi Cross‐Coupling Reactions of (Hetero)Arylzinc Reagents with Primary and Secondary Alkyl Bromides and Iodides

We report a cobalt‐catalyzed cross‐coupling of di(hetero)arylzinc reagents with primary and secondary alkyl iodides or bromides using THF‐soluble CoCl₂?2 LiCl and TMEDA as a ligand, which leads to the corresponding alkylated products in up to 88 % yield. A range of functional groups (e.g. COOR, CN, CF₃, F) are tolerated in these substitution reactions. Remarkably, we do not observe rearrangement of secondary alkyl iodides to unbranched products. Additionally, the use of cyclic TBS‐protected iodohydrins leads to trans‐2‐arylcyclohexanol derivatives in excellent diastereoselectivities (up to d.r.=99:1).     

Nickel‐Catalyzed Cross‐Coupling of Non‐activated and Functionalized Alkyl Halides with Alkyl Grignard Reagents

Reacting in the 'Ni'ck of time : The title reaction is realized by using an isolated Ni^II complex ( 1 ). The catalysis tolerates a wide range of important functional groups that are often incompatible with Grignard reagents in cross‐coupling reactions.

     

Palladium‐Catalyzed Heck‐Type Cross‐Couplings of Unactivated Alkyl Iodides

A palladium‐catalyzed, intermolecular Heck‐type coupling of alkyl iodides and alkenes is described. This process is successful with a variety of primary and secondary unactivated alkyl iodides as reaction partners, including those with hydrogen atoms in the β position. The mild catalytic conditions enable intermolecular C? C bond formations with a diverse set of alkyl iodides and alkenes, including substrates containing base‐ or nucleophile‐sensitive functionality.     

Microwave‐Assisted Heterogeneous Cross‐Coupling Reactions Catalyzed by Nickel‐in‐Charcoal (Ni/C)

A study involving the relatively rare combination of heterogeneous catalysis conducted under microwave conditions is presented. Carbon–carbon bond formation, including Negishi and Suzuki couplings, can be quickly effected with aryl chloride partners by using a base metal (nickel) adsorbed in the pores of activated charcoal. Aminations were also studied, along with cross‐couplings of vinyl alanes with benzylic chlorides as a means to stereodefined allylated aromatics. Reaction times for all these processes are typically reduced from several hours to minutes in a microwave reactor.     

Nickel‐Catalyzed Cross‐Coupling of Functionalized Difluoromethyl Bromides and Chlorides with Aryl Boronic Acids: A General Method for Difluoroalkylated Arenes

Transition‐metal‐catalyzed difluoroalkylation of aromatics remains challenging despite the importance of difluoroalkylated arenes in medicinal chemistry. Herein, the first successful example of nickel‐catalyzed difluoroalkylation of aryl boronic acids is described. The reaction allows access to a variety of functionalized difluoromethyl bromides and chlorides, and paves the way to highly cost‐efficient synthesis of a wide range of difluoroalkylated arenes. The notable features of this protocol are its high generality, excellent functional‐group compatibility, low‐cost nickel‐catalyst, and practicality for gram‐scale production, thus providing a facile method for applications in drug discovery and development.     

Nickel‐Catalyzed Stereoselective Dicarbofunctionalization of Alkynes

A nickel‐catalyzed three‐component reaction involving terminal alkynes, boronic acids, and alkyl halides is presented herein. Trisubstituted alkenes can be obtained in a highly regio‐ and stereocontrolled manner by the simultaneous addition of both aryl and alkyl groups across the triple bond in a radical‐mediated process. The reaction, devoid of air‐ and moisture‐sensitive organometallic reagents and catalysts, is operationally simple and offers a broad scope and functional‐group tolerance.     

Nickel‐Catalyzed Csp2Csp3 Bond Formation by CarbonFluorine Activation

We report herein a general catalytic method for Csp²?Csp³ bond formation through C?F activation. The process uses an inexpensive nickel complex with either diorganozinc or alkylzinc halide reagents, including those with β‐hydrogen atoms. A variety of fluorine substitution patterns and functional groups can be readily incorporated. Sequential reactions involving different precatalysts and coupling partners permit the synthesis of densely functionalized fluorinated building blocks.     

Nickel‐Catalyzed Monoarylation of Ammonia

Structurally diverse (hetero)aryl chloride, bromide, and tosylate electrophiles were employed in the Ni‐catalyzed monoarylation of ammonia, including chemoselective transformations. The employed JosiPhos/[Ni(cod)₂] catalyst system enables the use of commercially available stock solutions of ammonia, or the use of ammonia gas in these reactions, thereby demonstrating the versatility and potential scalability of the reported protocol. Proof‐of‐principle experiments established that air‐stable [(JosiPhos)NiCl₂] precatalysts can be employed successfully in such transformations.     

Iron‐Catalyzed 1,2‐Addition of Perfluoroalkyl Iodides to Alkynes and Alkenes

Iron catalysis has been developed for the intermolecular 1,2‐addition of perfluoroalkyl iodides to alkynes and alkenes. The catalysis has a wide substrate scope and high functional‐group tolerance. A variety of perfluoroalkyl iodides including CF₃I can be employed. The resulting perfluoroalkylated alkyl and alkenyl iodides can be further functionalized by cross‐coupling reactions. This methodology provides a straightforward and streamlined access to perfluoroalkylated organic molecules.     

Nickel‐Catalyzed Alkyl–Alkyl Cross‐Couplings of Fluorinated Secondary Electrophiles: A General Approach to the Synthesis of Compounds having a Perfluoroalkyl Substituent

Fluorinated organic molecules are of interest in fields ranging from medicinal chemistry to polymer science. Described herein is a mild, convenient, and versatile method for the synthesis of compounds bearing a perfluoroalkyl group attached to a tertiary carbon atom by using an alkyl–alkyl cross‐coupling. A nickel catalyst derived from NiCl₂?glyme and a pybox ligand achieves the coupling of a wide range of fluorinated alkyl halides with alkylzinc reagents at room temperature. A broad array of functional groups is compatible with the reaction conditions, and highly selective couplings can be achieved on the basis of differing levels of fluorination. A mechanistic investigation has established that the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) inhibits cross‐coupling under these conditions and that a TEMPO–electrophile adduct can be isolated.