Catalytic Hydrolysis of Thiolates to Alcohols

Abstract

A new and efficient catalytic hydrolysis of aliphatic and aromatic thiolates under ambient conditions is presented. Previously, we have demonstrated (Ganguly et al., Inorg. Chem. 2018, 57, 11306–11309) the Co­(II) mediated stoichiometric hydrolysis of thiols to produce alcohols/phenols along with a binuclear dicobalt­(II)-hydrosulfide complex, [Co2(PhBIMP)­(μ2-SH)­(DMF)]2+ (1) (PhBIMP is the anion of 2,6 bis­[(bis­((N-1-methyl-4,5- diphenylimidazoylmethyl) amino)­methyl]- 4-methylphenol). In the present work, we have shown that the product of the stoichiometric reaction, 1, may act as an efficient catalyst for the catalytic hydrolysis of a broad range of aliphatic and aromatic thiolates in DMF at room temperature to produce alcohols/phenols. Complex 1 takes up a thiolate (RS–) and a water molecule to generate an active intermediate complex, [Co2(PhBIMP)­(μ2-SH)­(RS)­(H2O)]1+ (2), which, in turn, releases the alcohol/phenol (ROH), hydrosulfide (HS–), and regenerates 1.

Hydrolysis and Transfer Reactivity of the Coordinated Thiolate, Thiocarboxylate, and Selenolate in Binuclear Zinc(II) Complexes

“…Moreover, Zn­(II) could not mediate the C–S bond cleavage of thiolates in yet another binucleating ligand system, N -Et-HPTB 1– ( N -Et-HPTB 1– is the deprotonated version of N , N , N ′, N ′-tetrakis­[2-(1- ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane) , in one of our previous studies, although Fe­(II) and Co­(II) could mediate the hydrolysis reaction , using the same N -Et-HPTB 1– ligand. Therefore, it becomes quite apparent that Zn­(II) is indeed comparatively much less efficient in mediating the C–S bond cleavage of thiolates, and hence even with the use of the PhBIMP 1– ligand, which, by far, has been found to be the best binucleating ligand used for such hydrolysis reactions in our studies, – use of a base, such as Et 3 N, becomes absolutely necessary for the Zn­(II)-mediated C–S bond hydrolysis reaction to proceed in the present work. In line with this, stirring a mixture of 4a and water in DMF for 7 days (in the absence of Et 3 N) could not yield PhOH, and only the starting material ( 4a ) was recovered (Figure S53) instead of 2 .…”

“…Here, it may be noted that the role of Et 3 N was examined by us previously and it was found to be nonessential for the Co(II)-and Fe(II)-mediated C−S bond hydrolysis of thiolates using the same PhBIMP 1− ligand, 35,36 although the presence of Et 3 N was found to increase the yield marginally. We have explained in one of our previous reports that upon going from Fe(II) to Zn(II), the effective nuclear charge on the metal center increases, which in turn decreases the pK a of the metalbound water molecule and the nucleophilicity of the metalbound hydroxide ion.…”

“…Formation of hydrosulfide in the reaction solution was confirmed by the precipitation of insoluble CuS (yield = 35%) following literature procedures. 35,71 Furthermore, treatment of 5 with H 2 18 O in the presence of anhydrous Et 3 N in anhydrous DMF allowed the isolation of [Zn 2 (PhBIMP)(μ-18 OOCR)(MeCN)] 2+ (6 18O ) where the identity of 6 18O was confirmed by MS (m/z obtained for [Zn 2 (PhBIMP)(μ- 18 OOCPh)] 2+ = 703.2313, Figure S67; note that m/z obtained for [Zn 2 (PhBIMP)(μ-O 2 CPh)] 2+ = 702.2643, Figure S64) which again confirms the hydrolysis reaction of the coordinated thiobenzoate aided by the added water (here H 2 18 O). Compound, 6(BF 4 ) 2 , however, could also be prepared directly by addition of PhCOONa into a mixture of HPhBIMP and Zn(BF 4 ) 2 in EtOH, and identity of 6(BF 4 ) 2 was confirmed by unit cell determination of the single crystals, MS (Figure S68), and NMR spectroscopy (Figures S69 and S70).…”

“…While a large number of reports are available on the transition metal-mediated C–S bond cleavage of organosulfur substrates in general, – many of these reactions required either heating for a long period of time at high temperatures (≥100 °C) ,,,,, or the use of high-pressure Hg lamp irradiation for 12–25 h or photolysis (UV light) . In contrast to these harsh reaction conditions, the Co­(II)-mediated hydrolytic C–S bond cleavage of thiols, thioethers, and thiophenes at ambient conditions has recently been reported by us. , The initially proposed mechanism for the desulfurization of thiols was later supported by detailed characterization of the intermediates and the products using Co­(II), – Fe­(II), , and Ni­(II) in the presence of three different heptadentate N,O-donor ligands, N -Et-HPTB 1– , , BPMP 1– , – and PhBIMP 1– (Scheme , Chart ). The accepted order for the ease of C–S bond cleavage, namely, alkyl mercaptans < aromatic mercaptans < diphenyl sulfide < thiophene, dibenzothiophene, was also found to be obeyed by the Co­(II)-mediated C–S bond cleavage of organosulfur substrates in the presence of the HPhBIMP ligand .…”

Transition Metal Mediated Hydrolysis of C–S Bonds: An Overview of a New Reaction Strategy

“…Interestingly, the reaction systems involving HPhBIMP and either Co(II) or Fe(II) could also desulfurize elemental sulfur (S 8 ) to yield 6a(BF 4 ) 2 and 8a(BF 4 ) 2 in 53% and 22% yields, respectively, while the generation of S It was also found that the catalytic hydrolysis of thiolates was not favorable under acidic conditions, while the use of a base, such as Et 3 N, did not improve the results. Catalytic hydrolysis of a total of 11 substrates (Scheme 8) was explored at RT, 136 and the results (Table 2) showed that the catalysis was comparatively more efficient for aliphatic thiolates (yield of alcohols = 81−90%) than that for the aromatic thiolates (yield of phenols = 31−52%), which, however, was consistent with our previous reports on stoichiometric hydrolysis of thiolates 125 and the reported level of difficulty (alkyl mercaptans < aromatic mercaptans) for removing sulfur in crude oil by the HDS process. 7−9 The identity of the recovered catalyst (6a) was confirmed by unit-cell determination of the single crystals, mass spectrometry, and solution magnetic moment measurement by Evans' method (in comparison with an authentic sample of 6a), while the generation of hydrosulfide (HS − ) during the catalysis was confirmed by the precipitation of insoluble CuS.…”