Nickel-Mediated Alkyl-, Acyl-, and Sulfonylcyanation of [1.1.1]Propellane

Photocatalytic Difunctionalization of [1.1.1]Propellane

The hunt for new molecular structures to improve the efficacy of biologically active molecules is at the forefront of pharmaceutical chemistry. So synthetic chemists have always been busy in the last few decades in synthesizing and testing new molecular frameworks which would work as more efficient bioisosteres of present bioactive functional groups. In this area, bicyclo[1.1.1]pentane (BCP) framework has been identified as a promising candidate. It is being utilized as a bioisostere of aryl, tert-butyl, alkynes, etc. in pharmaceutical chemistry. Now the major precursor of various BCP derivatives is [1.1.1]propellane and functionalization of [1.1.1]propellane has drawn widespread attention of the organic chemist community. Over the past two decades, the use of visible light in organic synthesis has rapidly gained popularity, as it represents one of the most efficient approaches aligned with the principles of green and sustainable chemistry, and several interesting papers covering the photocatalytic difunctionalization of [1.1.1]propellane have also been published in the last decade. This particular field has really attracted the attention of organic chemist community. That is why we decided to compile a review article covering the articles related to difunctionalization of [1.1.1]propellane under photocatalytic conditions. Here in this review, we have categorized and discussed the articles under three categories, namely i) without using any catalyst, ii) using organocatalysts, and iii) using metal catalysts for a deeper understanding of various key aspects of these transformations.

Photoredox/Copper Cooperatively Catalyzed Arylalkynylation of [1.1.1]Propellane

We present a sp2-sp3/sp3-sp bond formation reaction through a three-component coupling strategy involving terminal alkynes, [1.1.1]propellane, and aryl thianthrenium salts that are prepared from arenes. The reaction employs a dual photo/copper catalysis system and provides a streamlined approach for assembling 1-alkynyl-3-aryl bicyclo[1.1.1]pentane derivatives with a broad spectrum of functional group compatibility. Mechanistic studies suggest that the generation of aryl radicals and copper alkynide intermediates was involved.

Recent Advances in Radical Coupling Reactions Directly Involving Bicyclo[1.1.1]pentane (BCP)

BCP (bicyclo[1.1.1]pentane) is an ideal saturated carbon bioisostere, instead of the traditional benzene group, which has been extensively developed. As a novel building block, BCP could be directly involved in a variety of synthetic methods and widely used in the last-stage modification of drugs, attracting much attention from organic chemists and pharmacists. Radical-type cross-coupling reactions involving BCP enable the simultaneous formation of multiple chemical bonds (e.g., C-C, C-N, C-B, C-S, and C-Si) through metal catalysis, photocatalysis, metal-photo synergistic catalysis, and other catalytic systems. Various radical precursors have been explored, facilitating cross-coupling reactions that directly incorporate BCP. This review highlights these state-of-the-art radical couplings of BCP since 2017, organized by reaction components with emphasis on the scope of substrates, reaction mechanisms, and synthetic applications.

At the Speed of Light: The Systematic Implementation of Photoredox Cross-Coupling Reactions for Medicinal Chemistry Research

The adoption of new and emerging techniques in organic synthesis is essential to promote innovation in drug discovery. In this Perspective, we detail the strategy we used for the systematic deployment of photoredox-mediated, metal-catalyzed cross-coupling reactions in AbbVie's medicinal chemistry organization, focusing on topics such as assessment, evaluation, implementation, and accessibility. The comprehensive evaluation of photoredox reaction setups and published methods will be discussed, along with internal efforts to build expertise and photoredox high-throughput experimentation capabilities. We also highlight AbbVie's academic-industry collaborations in this field that have been leveraged to develop new synthetic strategies, along with discussing the internal adoption of photoredox cross-coupling reactions. The work described herein has culminated in robust photocatalysis and cross-coupling capabilities which are viewed as key platforms for medicinal chemistry research at AbbVie.

Benzyl Alcohol Functionalization of [1.1.1]Propellane with Alkanes and Aldehydes

Bicyclo[1.1.1]pentanes (BCPs) play a crucial role in drug discovery research as C(sp3)-rich bioisosteres of benzene rings. However, the preparation of BCPs with strong alkane C(sp3)-H bonds has not been reported to date. In this study, we reported a method for light-induced benzyl alcohol functionalization of [1.1.1]propellane with aliphatic hydrocarbons (which have not previously been explored for this purpose) and aldehydes under metal- and photocatalyst-free conditions. The BCP products could be transformed into various useful derivatives, demonstrating the utility of the method. Notably, we achieved the synthesis of functionalized BCPs with simple alkanes.

Light-enabled scalable synthesis of bicyclo[1.1.1]pentane halides and their functionalizations

In 2012, bicyclo[1.1.1]pentanes were demonstrated to be bioisosteres of the benzene ring. Here, we report a general scalable reaction between alkyl iodides and propellane that provides bicyclo[1.1.1]pentane iodides in milligram, gram and even kilogram quantities. The reaction is performed in flow and requires just light; no catalysts, initiators or additives are needed. The reaction is clean enough that, in many cases, evaporation of the reaction mixture provides products in around 90% purity that can be directly used in further transformations without any purification. Combined with the subsequent functionalization, >300 bicyclo[1.1.1]pentanes for medicinal chemistry have been prepared. So far, this is the most general and scalable approach towards functionalized bicyclo[1.1.1]pentanes.

Photochemical tandem reaction of nitrogen containing heterocycles, bicyclo[1.1.1]pentane, and difluoroiodane(III) reagents

A visible light-induced difluoroalkylation/heteroarylation of [1.1.1]propellane with nitrogen containing heterocycles and difluoroiodane(III) reagents was achieved. Various heteroarenes and difluoroiodane(III) reagents exhibited good compatibility, yielding the desired products in moderate to good yields. The accessibility of the reagents and the mild reaction conditions establish this method as an alternative and practical strategy for accessing diverse 1-difluoroalkyl-3-heteroaryl bicyclo[1.1.1]pentanes (BCPs).

Photocatalyzed 1,3-Bromodifluoroallylation of [1.1.1]Propellane with α-Trifluoromethylalkenes and KBr Salts

Herein we unveil a visible-light-driven transition-metal-free 1,3-bromodifluoroallylation of [1.1.1]propellane. This reactivity is harnessed through organophotocatalysis, providing practical synthetic pathways to 1-brominated-3-gem-difluoroallylic bicyclo[1.1.1]pentane derivatives, particularly derived from readily available α-trifluoromethylalkenes and inexpensive KBr salts utilized as precursors for bromine radicals. Mechanistic investigations reveal that bromide anions quench the excited state of the photocatalyst, leading to the formation of bromine radicals, which react in a strain-release radical addition process rather than hydrogen atom abstraction with [1.1.1]propellane.

Air-Stable Arene Manganese Complexes as Catalysts for the Syngas-Assisted Direct Reductive Amination, Cyanation of Aldehyde, and CO2 Fixation by Epoxide with High Functional Groups Tolerance

Manganese complexes [(arene)Mn(CO)3]+ were prepared in one step from arenes and Mn(CO)5Br. They were found to be efficient catalysts in the carbonyl cyanation with TMSCN, CO2 fixation by epoxides, and direct reductive amination in the presence of syngas. The amination reaction tolerated various reducible functional groups. The synergy of carbon monoxide and hydrogen in syngas provides high efficiency of the catalytic system. The developed protocols do not require an inert atmosphere, and the catalysts can be handled in air.

1,3-Difunctionalization of [1.1.1]propellane through iron-hydride catalyzed hydropyridylation

Current methodologies for the functionalization of [1.1.1]propellane primarily focus on achieving 1, 3-difunctionalized bicyclo[1.1.1]pentane or ring-opened cyclobutane moiety. Herein, we report an innovative approach for the 1, 3-difunctionalization of [1.1.1]propellane, enabling access to a diverse range of highly functionalized cyclobutanes via nucleophilic attack followed by ring opening and iron-hydride hydrogen atom transfer. To enable this method, we developed an efficient iron-catalyzed hydropyridylation of various alkenes for C - H alkylation of pyridines at the C4 position, eliminating the need for stoichiometric quantities of oxidants or reductants. Mechanistic investigations reveal that the resulting N-centered radical serves as an effective oxidizing agent, facilitating single-electron transfer oxidation of the reduced iron catalyst. This process efficiently sustains the catalytic cycle, offering significant advantages for substrates with oxidatively sensitive functionalities that are generally incompatible with alternative approaches. The strategy presented herein is not only mechanistically compelling but also demonstrates broad versatility, highlighting its potential for late-stage functionalization.

Bicyclopentylation of Alcohols with Thianthrenium Reagents

Herein we present the first method for the synthesis of bicyclo[1.1.1]pentyl (BCP) alkyl ethers from alcohols. The reaction uses BCP-thianthrenium reagents and is catalyzed by a dual copper/photoredox catalyst system. Unlike known alkylations of tertiary alcohols via carbocation intermediates, our Cu-mediated radical process circumvents the labile BCP carbocations. The approach demonstrates a broad tolerance for functional groups when applied to primary, secondary, and even tertiary alcohols. In addition, we highlight the utility of this method in late-stage functionalizations of both natural products and pharmaceuticals as well as in the rapid construction of BCP analogs of known pharmaceuticals that would otherwise be difficult to access.

Strain-Release Photocatalysis

The concept of strain in organic compounds is as old as modern organic chemistry and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate─with significant driving force─rigid aliphatic systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an additional dimension to strain-release processes by leveraging the energy of photons to create chemical complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis─with emphases on mechanisms, catalytic cycles, and current limitations─the unique chemical architectures that can be produced, and possible future directions.