Convergent Deboronative and Decarboxylative Phosphonylation Enabled by the Phosphite Radical Trap "BecaP"

Visible Light-Mediated Late-Stage Thioetherification of Mercaptopurine Derivatives

We disclose herein a novel and general radical approach to alkylthiopurines, encompassing 4 types of thiopurines, as well as their corresponding ribosides. This strategy is achieved through visible light-mediated late-stage functionalization of the sulfur atoms of mercaptopurines. The in situ-generated disulfide was proposed as the pivotal neutral intermediate for this transformation. We present herein a novel photo-mediated homolytic C-S bond formation for the preparation of alkylthiopurines and alkylthiopurine nucleosides. Despite the presence of reactive sites for the Minisci reaction, chemoselective S-alkylation remained the predominant pathway. This method allows for the late-stage introduction of a broad spectrum of alkyl groups onto the sulfur atom of unprotective mercaptopurine derivatives, encompassing 2-, 6-, and 8-mercaptopurine rings. Organoborons serve as efficient and eco-friendly alkylating reagents, providing advantages in terms of readily availability, stability, and reduced toxicity. Further derivatization of the thioetherified nucleosides, together with anti-tumor assays, led to the discovery of potent anti-tumor agents with an IC50 value reaching 6.1 μM (Comp. 31 for Jurkat).

Photoinduced Remote C(sp3)-H Phosphonylation of Amides

The present study reports an unprecedented protocol for the phosphonylation of unactivated C(sp3)-H bonds. By utilizing 1 mol % 4DPAIPN (1,2,3,5-tetrakis(diphenylamino)-4,6-dicyanobenzene) as the catalyst, satisfactory yields of γ-phosphonylated amides are obtained through a visible-light-induced reaction between N-((4-cyanobenzoyl)oxy)alkanamides and 9-fluorenyl o-phenylene phosphite at room temperature. This protocol demonstrates broad substrate scope and wide functional group compatibility.

Deoxygenative Transformation of Alcohols via Phosphoranyl Radical from Exogenous Radical Addition

A general approach to the direct deoxygenative transformation of primary, secondary, and tertiary alcohols has been developed. It undergoes through phosphoranyl radical intermediates generated by the addition of exogenous iodine radical to trivalent alkoxylphosphanes. Since these alkoxylphosphanes are readily in situ obtained from alcohols and commercially available, inexpensive chlorodiphenylphosphine, a diverse range of alcohols with various functional groups can be utilized to proceed deoxygenative cross-couplings with alkenes or aryl iodides. The selective transformation of polyhydroxy substrates and the rapid synthesis of complex organic molecules are also demonstrated with this method.

Construction of C-P Bonds from Free Cyclobutanone Oximes and Chlorophosphines via Radical-Radical Coupling

Herein, we report a catalyst-free reaction of cyclobutanone oximes with chlorophosphines (R2PCl), which forms a fragile C═N-O-PR2 species that undergoes N-O homolysis, fragmentation, and radical-radical coupling, leading to the formation of cyano-containing phosphine oxides in good yields. The reaction features an in situ activation of cyclobutanone oximes for radical generation, in which R2PCl plays a dual role as both an activator and a reactant.

Development of a General Organophosphorus Radical Trap: Deoxyphosphonylation of Alcohols

Here we report the design of a general, redox-switchable organophosphorus alkyl radical trap that enables the synthesis of a broad range of C(sp3)-P(V) modalities. This "plug-and-play" approach relies upon in situ activation of alcohols and O═P(R2)H motifs, two broadly available and inexpensive sources of molecular complexity. The mild, photocatalytic deoxygenative strategy described herein allows for the direct conversion of sugars, nucleosides, and complex pharmaceutical architectures to their organophosphorus analogs. This includes the facile incorporation of medicinally relevant phosphonate ester prodrugs.

Synthesis of γ-Oxo-phosphonates via N-Heterocyclic Carbene-Catalyzed Acylphosphorylation of Alkenes

In this study, we present an N-heterocyclic carbene-catalyzed method for the radical acylphosphorylation of alkenes. Electrochemical investigations were employed to identify an appropriate class of oxime phosphonates capable of undergoing a single-electron transfer (SET) with Breslow enolates. The resulting phosphoryl radicals were effectively coupled with diverse styrenes and aldehydes to yield a variety of γ-oxo-phosphonates. Both radical clock experiments and electrochemical studies support our reaction design, and a plausible mechanism for the organocatalytic transformation is proposed.

Photoinduced Decarboxylative Radical Phosphinylation

Reported herein is an unprecedented protocol for C(sp3 )-phosphinylation. With 1 mol % 4CzIPN (1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) as the catalyst, the visible light induced reaction of redox-active esters of aliphatic carboxylic acids with dimethyl arylphosphonites or diethyl alkylphosphonites at room temperature provides the corresponding decarboxylative phosphinylation products in satisfactory yields. The protocol exhibits broad substrate scope and wide functional-group compatibility, enabling the late-stage modification of complex molecules and rapid synthesis of bioactive phosphinic acids such as glutamine synthetase phosphinothricin and a kynureninase inhibitor. A radical-polar crossover mechanism involving the formation and subsequent oxidation of phosphoranyl radicals followed by nucleophilic demethylation (or deethylation) is proposed.

Photoredox-Catalyzed Selective α-Scission of PR3-OH Radicals to Access Hydroalkylation of Alkenes

Photoinduced generation of phosphoranyl radicals offers a versatile strategy to access a variety of synthetically valuable radicals. A long-standing challenge remains in the regulation of phosphoranyl radical to undergo α-scission pathway, although the β-scission mode has been intensively studied. We herein developed an unprecedented protocol for selective α-scission of the P(OH)R3 radical intermediate under photocatalytic conditions. This efficient P-C bond cleavage via α-scission of the P(OH)R3 radicals has been successfully utilized in the alkylation/fluoroalkylation of alkenes.

Synthesis of tertiary alkylphosphonate oligonucleotides through light-driven radical-polar crossover reactions

Chemical modification of nucleotides can improve the metabolic stability and target specificity of oligonucleotide therapeutics, and alkylphosphonates have been employed as charge-neutral replacements for naturally-occurring phosphodiester backbones in these compounds. However, at present, the alkyl moieties that can be attached to phosphorus atoms in these compounds are limited to methyl groups or primary/secondary alkyls, and such alkylphosphonate moieties can degrade during oligonucleotide synthesis. The present work demonstrates the tertiary alkylation of the phosphorus atoms of phosphites bearing two 2'-deoxynuclosides. This process utilizes a carbocation generated via a light-driven radical-polar crossover mechanism. This protocol provides tertiary alkylphosphonate structures that are difficult to synthesize using existing methods. The conversion of these species to oligonucleotides having charge-neutral alkylphosphonate linkages through a phosphoramidite-based approach was also confirmed in this study.