A Mild Photocatalytic Synthesis of Guanidine from Thiourea under Visible Light

Abstract

In this work, we developed the catalytic guanylation of thiourea using Ru(bpy) 3 Cl 2 as a photocatalyst under irradiation by visible light. The conversion of various thioureas to the corresponding guanidines was achieved using 1−5 mol % of photocatalyst in a mixture of water and ethanol at room temperature. Key benefits of this reaction include the use of photoredox catalyst, low-toxicity solvents/base, ambient temperature, and an open-flask environment.The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.orglett.0c02770.General procedure, materials and methods, copies of 1 H and 13 C NMR and HRMS spectra of compounds 2a−3j, and EDX-SEM spectrum of PbSO 4 (PDF)

De novo design and synthesis of dipyridopurinone derivatives as visible-light photocatalysts in productive guanylation reactions

“… 24 These results indicated that the reactive oxygen species might be ground-state oxygen ( 3 O 2 ), however, a minor contribution from other active oxygen species could not be completely excluded. Based on the mechanistic studies and previous reports, 19,24 a although the mechanism of this visible-light-catalyzed guanylation reaction was not unequivocally demonstrated, a reasonable possibility is outlined in Scheme 2e . The tautomer I of diphenylthiourea is transformed into a radical intermediate II through a proton-coupled electron transfer (PCET) to the excited state catalyst DP4*.…”

“…Very recently, Wacharasindhu's group reported the first photocatalytic process for guanylation of thioureas under visible light. 19 They screened the common visible-light organic PCs including eosin Y, rose bengal, and safranin O, as well as a transition-metal photoredox catalyst Ru(bpy) 3 Cl 2 . Among them, only Ru(bpy) 3 Cl 2 showed good catalytic activity and enabled the guanylation of diaryl thioureas with 1° and 2°-alkyl amines in high yields ( Fig.…”

“…2c ). 19 To date, the photocatalytic transformation of alkyl thioureas with aryl amines, biologically important amino acids and aminonitrile to the corresponding guanidines, especially valuable cyclic guanidines remains hugely challenging and is still not realized. Herein, we wish to present a visible-light photocatalytic and productive synthesis of diverse guanidines and cyclic guanidines by employing DP4 as an oxidative PC ( Fig.…”

“…Notably, the phenolic OH and carboxyl groups were well tolerated in the guanidine (10) formation, which seldom survived in previous methods. Gratifyingly, in contrast to Ru(bpy) 3 Cl 2 , 19 DP4 enabled the transformation of alkyl and benzyl thioureas in good to excellent yields (11–15). The dialkyl thiourea T16 was inactive for this transformation, the desired product 16 was not observed and T16 was recovered in 91%.…”

Photoredox-Promoted Selective Synthesis of C-5 Thiolated 2-Aminothiazoles from Terminal Alkynes

“…The thiyl radical, upon addition to the alkyne, gives vinyl radical 1c , which undergoes hydroperoxide insertion to afford peroxy intermediate 1d . In another parallel photocatalytic cycle (PC-2), the thiol tautomer of thiourea engages in single electron transfer to Ru 2+* (SET), generating thiyl radical cation 1e , which after deprotonation affords intermediate 1f and Ru 1+ . The thiol radical 1f attacks intermediate 1d , which undergoes facile peroxide cleavage, generating intermediate 1g and a hydroxyl radical .…”