Recent advances in the applications of [1.1.1]propellane in organic synthesis

Palladium-Catalyzed Ligand-Controlled Switchable Hetero-(5 + 3)/Enantioselective [2σ+2σ] Cycloadditions of Bicyclobutanes with Vinyl Oxiranes

For nearly 60 years, significant research efforts have been focused on developing strategies for the cycloaddition of bicyclobutanes (BCBs). However, higher-order cycloaddition and catalytic asymmetric cycloaddition of BCBs have been long-standing formidable challenges. Here, we report Pd-catalyzed ligand-controlled, tunable cycloadditions for the divergent synthesis of bridged bicyclic frameworks. The dppb ligand facilitates the formal (5+3) cycloaddition of BCBs and vinyl oxiranes, yielding valuable eight-membered ethers with bridged bicyclic scaffolds in 100% regioselectivity. The Cy-DPEphos ligand promotes selective hetero-[2σ+2σ] cycloadditions to access pharmacologically important 2-oxabicyclo[3.1.1]heptane (O-BCHeps). Furthermore, the corresponding catalytic asymmetric synthesis of O-BCHeps with 94-99% ee has been achieved using chiral (S)-DTBM-Segphos, representing the first catalytic asymmetric cross-dimerization of two strained rings. The obtained O-BCHeps are promising bioisosteres for ortho-substituted benzenes.

DFT Insight into a Strain-Release Mechanism in Bicyclo[1.1.0]butanes via Concerted Activation of Central and Lateral C-C Bonds with Rh(III) Catalysis

Transition-metal-catalyzed, strain-release-driven transformations of "spring-loaded" bicyclo[1.1.0]butanes (BCBs) are considered potent tools in synthetic organic chemistry. Previously proposed strain-release mechanisms involve either the insertion of the central C-C bond of BCBs into a metal-carbon bond, followed by β-C elimination, or the oxidative addition of the central or lateral C-C bond on the transition metal center, followed by reductive elimination. This study, employing DFT calculations on a Rh(III)-catalyzed model system in a three-component protocol involving oxime ether, BCB ester, and ethyl glyoxylate for constructing diastereoselective quaternary carbon centers, introduces an unusual strain-release mechanism for BCBs. In this mechanism, the catalytic reaction is initiated by the simultaneous cleavage of two C-C bonds (the central and lateral C-C bonds), resulting in the formation of a Rh-carbene intermediate. The new mechanism exhibits a barrier of 21.0 kcal/mol, making it energetically more favorable by 11.1 kcal/mol compared to the previously suggested most favorable pathway. This unusual reaction mode rationalizes experimental observation of the construction of quaternary carbon centers, including the excellent E-selectivity and diastereoselectivity. The newly proposed strain-release mechanism holds promise in advancing our understanding of transition-metal-catalyzed C-C bond activation mechanisms and facilitating the synthesis of transition metal carbene complexes.

Photochemistry in Medicinal Chemistry and Chemical Biology

Photochemistry has emerged as a transformative force in organic chemistry, significantly expanding the chemical space accessible for medicinal chemistry. Light-induced reactions enable the efficient synthesis of intricate organic structures and have found applications throughout the different stages of the drug discovery and development processes. Moreover, photochemical techniques provide innovative solutions in chemical biology, allowing precise spatiotemporal drug activation and targeted delivery. In this Perspective, we highlight the already numerous remarkable applications and the even more promising future of photochemistry in medicinal chemistry and chemical biology.

Photo-induced radical transformations of tosyl cyanide

Tosyl cyanide is a commonly used reagent for cyanation and sulfonylation in organic synthesis and pharmaceutical chemistry. The photocatalytic transformations of tosyl cyanide are generally conducted under mild conditions. This minireview summarizes the recent progress of radical-involved transformations of tosyl cyanide via photo-induced cyanation or sulfonylcyanation.

Silver-Catalyzed Dearomative [2π+2σ] Cycloadditions of Indoles with Bicyclobutanes: Access to Indoline Fused Bicyclo[2.1.1]hexanes

Bicyclo[2.1.1]hexanes (BCHs) are becoming ever more important in drug design and development as bridged scaffolds that provide underexplored chemical space, but are difficult to access. Here a silver-catalyzed dearomative [2π+2σ] cycloaddition strategy for the synthesis of indoline fused BCHs from N-unprotected indoles and bicyclobutane precursors is described. The strain-release dearomative cycloaddition operates under mild conditions, tolerating a wide range of functional groups. It is capable of forming BCHs with up to four contiguous quaternary carbon centers, achieving yields of up to 99 %. In addition, a scale-up experiment and the synthetic transformations of the cycloadducts further highlighted the synthetic utility.

Conquering the Synthesis and Functionalization of Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs) have become established as attractive bioisosteres for para-substituted benzene rings in drug design. Conferring various beneficial properties compared with their aromatic "parents," BCPs featuring a wide array of bridgehead substituents can now be accessed by an equivalent variety of methods. In this perspective, we discuss the evolution of this field and focus on the most enabling and general methods for BCPs synthesis, considering both scope and limitation. Recent breakthroughs on the synthesis of bridge-substituted BCPs are described, as well as methodologies for postsynthesis functionalization. We further explore new challenges and directions for the field, such as the emergence of other rigid small ring hydrocarbons and heterocycles possessing unique substituent exit vectors.

Designing a Propellane-based Nonlinear Optically Active System Absorbing in Three Different Wavelength Regions

The aim of this work is to demonstrate the possibility of using propellane in designing a molecule that can absorb in three different wavelength regions and their nonlinear optical (NLO) activity can be fine-tuned by varying the three wings. We considered 22 tailor-made propellane derivatives consisting of phenyl, naphthyl, and biphenyl wings for this purpose. Using the state-of-the-art linear and quadratic response methods within TD-DFT and RI-CC2 theories and a suitable generalized few-state model that quantifies the effect of orientation of different transition moments on NLO properties, we discussed how and why the linear and nonlinear optical activity of propellane vary when the three wings are assembled successively to construct a full-propellane.

Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight

We leveraged the recent increase in synthetic accessibility of SF5 Cl and Ar-SF4 Cl compounds to combine chemistry of the SF5 and SF4 Ar groups with strain-release functionalization. By effectively adding SF5 and SF4 Ar radicals across [1.1.1]propellane, we accessed structurally unique bicyclopentanes, bearing two distinct elements of bioisosterism. Upon evaluating these "hybrid isostere" motifs in the solid state, we measured exceptionally short transannular distances; in one case, the distance rivals the shortest nonbonding C⋅⋅⋅C contact reported to date. This prompted SC-XRD and DFT analyses that support the notion that a donor-acceptor interaction involving the "wing" C-C bonds is playing an important role in stabilization. Thus, these heretofore unknown structures expand the palette for highly coveted three-dimensional fluorinated building blocks and provide insight to a more general effect observed in bicyclopentanes.

The emerging role of radical chemistry in the amination transformation of highly strained [1.1.1]propellane: Bicyclo[1.1.1]pentylamine as bioisosteres of anilines

Bicyclo[1.1.1]pentylamines (BPCAs), emerging as sp³-rich surrogates for aniline and its derivatives, demonstrate unique structural features and physicochemical profiles in medicinal and synthetic chemistry. In recent years, compared with conventional synthetic approaches, the rapid development of radical chemistry enables the assembly of valuable bicyclo[1.1.1]pentylamines scaffold directly through the amination transformation of highly strained [1.1.1]propellane. In this review, we concisely summarize the emerging role of radical chemistry in the construction of BCPAs motif, highlighting two different and powerful radical-involved strategies including C-centered and N-centered radical pathways under appropriate conditions. The future direction concerning BCPAs is also discussed at the end of this review, which aims to provide some inspiration for the research of this promising project.

One step synthesis of unsymmetrical 1,3-disubstituted BCP ketones via nickel/photoredox-catalyzed [1.1.1]propellane multicomponent dicarbofunctionalization

Bicyclo[1.1.1]pentanes (BCPs), utilized as sp3-rich bioisosteres for tert-butyl- and aryl groups as well as internal alkynes, have gained considerable momentum in drug development programs. Although many elegant methods have been developed to access BCP amines and BCP aryls efficiently, the methods used to construct BCP ketones directly are relatively underdeveloped. In particular, the preparation of unsymmetrical 1,3-disubstituted-BCP ketones remains challenging and still requires multiple chemical steps. Herein, a single-step, multi-component approach to versatile disubstituted BCP ketones via nickel/photoredox catalysis is reported. Importantly, installing a boron group at the carbon position adjacent to the BCP structure bypasses the limitation to tertiary BF3K coupling partners, thus expanding the scope of this paradigm. Further transformation of disubstituted-BCP ketones into a variety of other BCP derivatives demonstrates the synthetic value of this developed method.

Synthesis of Sulfur-Substituted Bicyclo[1.1.1]pentanes by Iodo-Sulfenylation of [1.1.1]Propellane

Thiols easily react with [1.1.1]propellane to give sulfur-substituted bicyclo[1.1.1]pentanes in radical reactions, but this reactivity is not replicated in the case of heterocyclic thiols. Herein, we address this issue by electrophilically activating [1.1.1]propellane to promote its iodo-sulfenylation with 10 classes of heterocyclic thiols in two protocols that can be conducted on a multigram scale without exclusion of air or moisture.

A Practical and Scalable Approach to Fluoro-Substituted Bicyclo[1.1.1]pentanes

After more than 20 years of trials, a practical scalable approach to fluoro‐substituted bicyclo[1.1.1]pentanes (F‐BCPs) has been developed. The physicochemical properties of the F‐BCPs have been studied, and the core was incorporated into the structure of the anti‐inflammatory drug Flurbiprofen in place of the fluorophenyl ring.

First-order hyperpolarizabilities of propellanes: elucidating structure-property relationships

Following recent experimental work demonstrating strong nonlinear optical properties, namely second harmonic generation of light, in crystals composed of 16,20-dinitro-(3,4,8,9)-dibenzo-2,7-dioxa-5,10-diaza[4.4.4]propellane molecules [A. Miniewicz, S. Bartkiewicz, E. Wojaczyńska, T. Galica, R. Zaleśny and R. Jakubas, J. Mater. Chem. C, 2019, 7, 1255-1262] in this paper we aim to investigate "structure-property" relationships for a series of 16 propellanes presenting a wide palette of substituents with varying electron-accepting/donating capabilities. To that end, we use electronic- and vibrational-structure theories and a recently developed generalized few-state model combined with a range-separated CAM-B3LYP functional to analyze electronic and vibrational contributions to the first hyperpolarizability for the whole series of molecules. The variations in computed properties are large among the studied set of substituents and can reach an order of magnitude. It has been demonstrated that the maximum values of frequency-independent first hyperpolarizability are expected for strong electron-accepting NO₂ substituents, but only at the preferred position with respect to the electronegative oxygen atom in the 1,4-oxazine moiety. This holds for electronic as well as vibrational counterparts.

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Synthesis of Tetrasubstituted Alkenyl Nitriles via Cyanocarbene Addition of [1.1.1]Propellane

Herein, we report the metal-free synthesis of methylenecyclobutane containing tetrasubstituted alkenyl nitriles via a strain-release driven addition reaction of [1.1.1]propellane under mild conditions. Using this strategy, TMSN3 was shown to...

Photoredox-catalyzed hydroxymethylation of β-ketoesters: application to the synthesis of [3.3.3] propellane lactones

Photoredox-catalysed hydroxymethylation of β-ketoesters substituted by an allyl subunit on the α-position afforded directly the corresponding bicyclic lactones possessing both a hydroxy group and an unsaturation. A subsequent regioselective iodoetherification led to the formation of original [3.3.3] propellane structures. Substitution of the iodine atom by various nucleophiles afforded highly functionnalized structures including triazolomethyl derivatives.

Electrophilic Activation of [1.1.1]Propellane for the Synthesis of Nitrogen-Substituted Bicyclo[1.1.1]pentanes

Strategies commonly used for the synthesis of functionalised bicyclo[1.1.1]pentanes (BCP) rely on the reaction of [1.1.1]propellane with anionic or radical intermediates. In contrast, electrophilic activation has remained a considerable challenge due to the facile decomposition of BCP cations, which has severely limited the applications of this strategy. Herein, we report the electrophilic activation of [1.1.1]propellane in a halogen bond complex, which enables its reaction with electron‐neutral nucleophiles such as anilines and azoles to give nitrogen‐substituted BCPs that are prominent motifs in drug discovery. A detailed computational analysis indicates that the key halogen bonding interaction promotes nucleophilic attack without sacrificing cage stabilisation. Overall, our work rehabilitates electrophilic activation of [1.1.1]propellane as a valuable strategy for accessing functionalised BCPs.

Large-Scale Synthesis and Modifications of Bicyclo[1.1.1]pentane-1,3-dicarboxylic Acid (BCP)

In flow photochemical addition of propellane to diacetyl allowed construction of the bicyclo[1.1.1]pentane (BCP) core in a 1 kg scale within 1 day. Haloform reaction of the formed diketone in batch afforded bicyclo[1.1.1]pentane-1,3-dicarboxylic acid in a multigram amount. Representative gram scale transformations of the diacid were also performed to obtain various BCP-containing building blocks—alcohols, acids, amines, trifluoroborates, amino acids, etc.—for medicinal chemistry.

Bioisosteres of the Phenyl Ring: Recent Strategic Applications in Lead Optimization and Drug Design

The benzene moiety is the most prevalent ring system in marketed drugs, underscoring its historic popularity in drug design either as a pharmacophore or as a scaffold that projects pharmacophoric elements. However, introspective analyses of medicinal chemistry practices at the beginning of the 21st century highlighted the indiscriminate deployment of phenyl rings as an important contributor to the poor physicochemical properties of advanced molecules, which limited their prospects of being developed into effective drugs. This Perspective deliberates on the design and applications of bioisosteric replacements for a phenyl ring that have provided practical solutions to a range of developability problems frequently encountered in lead optimization campaigns. While the effect of phenyl ring replacements on compound properties is contextual in nature, bioisosteric substitution can lead to enhanced potency, solubility, and metabolic stability while reducing lipophilicity, plasma protein binding, phospholipidosis potential, and inhibition of cytochrome P450 enzymes and the hERG channel.

1-Aza-2,4-disilabicyclo[1.1.0]butanes with Superelongated C-N σ-Bonds

Two silylene molecules oxidatively react under formal [1 + 2 + 1]-cycloaddition to the C≡N bond of nitriles to yield 1-aza-2,4-disilabicyclo[1.1.0]butanes (L)(Cl)Si[μ-η^1,2-NC(p-RC₆H₄)]Si(Cl)(L) (L = PhC(NtBu)₂, R = CF₃ (2), F (3), Cl (4), Br (5)). The strongly folded bicyclic SiCNSi cores in 2-5 feature inverted bridgehead carbon atoms and superelongated C-N bonds [1.745(12) to 1.801(2) Å], exceeding the lengths of C-N single bonds in known silaaziridines by up to 23%. Detailed bonding analysis discloses C-N bonding interactions, sharing far-reaching similarities with the central C-C bond in [1.1.1]propellane.

Strain-release 2-azaallyl anion addition/borylation of [1.1.1]propellane: synthesis and functionalization of benzylamine bicyclo[1.1.1]pentyl boronates

We report a 3-component reaction between N-benzyl ketimines, [1.1.1]propellane, and pinacol boronates to generate benzylamine bicyclo[1.1.1]pentane (BCP) pinacol boronates. These structures are analogous to highly sought diarylmethanamine cores, which are common motifs in bioactive molecules. We demonstrate the versatility of the boronate ester handle via downstream functionalization through a variety of reactions, including a challenging Pd-catalyzed (hetero)arylation that exhibits a broad substrate scope. Together, these methods enable the synthesis of high-value BCP benzylamines which are inaccessible by existing methods. Furthermore, we demonstrate the successful application of these newly developed (hetero)arylation conditions to a variety of challenging tertiary pinacol boronates, including nitrogen-containing heterocycles, 1,1-disubstituted cyclopropanes, and other BCP cores.

Iminyl radical initiated sulfonylation of alkenes with rongalite under photoredox conditions

A photoredox-catalyzed reaction of oximes, rongalite and electrophiles is accomplished, affording pyrrole-substituted aliphatic sulfones or sulfonamides in moderate to good yields.