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).

Visible-Light-Induced Highly Site-Selective Direct C-H Phosphorylation of Pyrrolo[2,3-d]pyrimidine Derivatives with H-Phosphine Oxides

An efficient and highly regioselective C6-phosphorylation protocol for pyrrolo[2,3-d]pyrimidine (7-DAP) derivatives with various H-phosphine oxides induced by visible light at room temperature is described for the first time. This protocol has been successfully achieved by the combination of Na2-eosin Y as a photocatalyst and LPO as an oxidant under transition metal- and additive-free conditions. The broad substrate scope, good functional group tolerance, excellent regioselectivity, and air tolerant conditions make this process favorable for the functional modification of pyrrolo[2,3-d]pyrimidine scaffold and enrich the phosphorylated 7-DAP compounds for further biological evaluation.

An Efficient Direct Arylation Polycondensation via C-S Bond Cleavage

The direct arylation polycondensation (DArP) has become one of the most important methods to construct conjugated polymers (CPs). However, the homocoupling side-reactions of aryl halides and the low regioseletive reactivities of unfunctionalized aryls hinder the development of DArP. Here, an efficient Pd and Cu co-catalyzed DArP was developed via inert C-S bond cleavage of aryl thioethers, of which robustness was exemplified by over twenty conjugated polymers (CPs), including copolymers, homopolymers, and random polymers. The capture of oxidative addition intermediate together with experimental and theoretic results suggested the important role of palladium (Pd) and copper (Cu) co-catalysis with a bicyclic mechanism. The studies of NMR, molecular weights, trap densities, two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS), and the charge transport mobilities revealed that the homocoupling reactions were significantly suppressed with high regioselectivity of unfunctionalized aryls, suggesting this method is an excellent choice for synthesizing high performance CPs.

Room Temperature Anhydrous Suzuki-Miyaura Polymerization Enabled by C-S Bond Activation

The Suzuki-Miyaura cross-coupling is one of the most important and powerful methods for constructing C-C bonds. However, the protodeboronation of arylboronic acids hinder the development of Suzuki-Miyaura coupling in the precise synthesis of conjugated polymers (CPs). Here, an anhydrous room temperature Suzuki-Miyaura cross-coupling reaction between (hetero)aryl boronic esters and aryl sulfides was explored, of which universality was exemplified by thirty small molecules and twelve CPs. Meanwhile, the mechanistic studies involving with capturing four coordinated borate intermediate revealed the direct transmetalation of boronic esters in the absence of H2 O suppressing the protodeboronation. Additionally, the room temperature reaction significantly reduced the homocoupling defects and enhanced the optoelectronic properties of the CPs. In all, this work provides a green protocol to synthesize alternating CPs.

Efficient room temperature catalytic synthesis of alternating conjugated copolymers via C-S bond activation

Structural defects in conjugated copolymers are severely detrimental to the optoelectronic properties and the performance of the resulting electronic devices fabricated from them. Therefore, the much-desired precision synthesis of conjugated copolymers with highly regular repeat units is important, but presents a significant challenge to synthetic materials chemists. To this end, aryl sulfides are naturally abundant substances and offer unrealized potential in cross-coupling reactions. Here we report an efficient room temperature polycondensation protocol which implements aryl disulfide C-S activation to produce defect-minimized semiconducting conjugated copolymers with broad scope and applicability. Thus, a broad series of arylstannanes and thioethers are employed via the present protocol to afford copolymers with number-average molecular weights (M_ns) of 10.0–45.0 kDa. MALDI and NMR analysis of selected copolymers reveals minimal structural defects. Moreover, the polymer trap density here is smaller and the field effect mobility higher than that in the analogous polymer synthesized through thermal-activation Stille coupling.

Structural defects in conjugated copolymers are severely detrimental to the optoelectronic properties. Here the authors show an efficient room temperature polycondensation protocol which implements aryl disulfide C-S activation to produce defect-minimized semiconducting conjugated copolymers.

Microwave-Assisted Palladium-Catalyzed Cross-Coupling Reactions: Generation of Carbon–Carbon Bond

Cross-coupling reactions furnishing carbon–carbon (C–C) bond is one of the most challenging tasks in organic syntheses. The early developed reaction protocols by Negishi, Heck, Kumada, Sonogashira, Stille, Suzuki, and Hiyama, utilizing palladium or its salts as catalysis have, for decades, attracted and inspired researchers affiliated with academia and industry. Tremendous efforts have been paid to develop and achieve more sustainable reaction conditions, such as the reduction in energy consumption by applying the microwave irradiation technique. Chemical reactions under controlled microwave conditions dramatically reduce the reaction time and therefore resulting in increase in the yield of the desired product by minimizing the formation of side products. In this review, we mainly focus on the recent advances and applications of palladium catalyzed cross-coupling carbon–carbon bond formation under microwave technology.

Tandem Double-Cross-Coupling/Hydrothiolation Reaction of 2-Sulfenyl Benzimidazoles with Boronic Acids

A new tandem palladium-catalyzed reaction involving a Suzuki-Miyaura coupling, a desulfenylative coupling, and a hydrothiolation of a triple bond is reported. Notably, the desulfenylative coupling occurs without copper(I) assistance and the hydrothiolation is totally regioselective and stereoselective. The overall process results in the double incorporation of the boronic acid and the reincorporation of the sulfenyl moiety into the product structure. Starting from 2-(bromobenzylsulfenyl)-1-propargyl benzimidazoles, the transformation led to variously substituted benzimidazoles bearing a stereodefined alkenyl sulfide.

C-H Imidation of 7-Deazapurines

We developed and presented here a ferrocene-catalyzed C-H imidation of 7-deazapurines (pyrrolo[2,3-d]pyrimidines) with N-imidyl peroxyesters. The reactions occur regioselectively at position 8 in 7-deazapurines, leading to a series of 8-succinimido-, phtalimido-, or naphthalimido-7-deazapurine derivatives. Attempted hydrazinolysis of resulting 8-imidyl-7-deazapurines led to corresponding 8-amino-7-deazapurine, which was very unstable and quickly decomposed.

Regioselective C–H and N–H functionalization of purine derivatives and analogues: a synthetic and mechanistic perspective

This perspective provides a systematic and concise overview of the recent development in C–H/N–H bond functionalization in purine derivatives and analogues.