TY - JOUR
T1 - Kinetics of sulfur-transfer from titanocene (poly)sulfides to sulfenyl chlorides
T2 - rapid metal-assisted concerted substitution
AU - Helou de Oliveira, Pedro H.
AU - Boaler, Patrick J.
AU - Hua, Guoxiong
AU - West, Nathan M.
AU - Hembre, Robert T.
AU - Penney, Jonathan M.
AU - Al-Afyouni, Malik H.
AU - Woollins, J. Derek
AU - García-Domínguez, Andrés
AU - Lloyd-Jones, Guy C.
N1 - Publisher Copyright:
© 2024 The Royal Society of Chemistry.
PY - 2024/6/19
Y1 - 2024/6/19
N2 - The kinetics of sulfur transfer from titanocene (poly)sulfides (RCp2TiS5, Cp2TiS4CMe2, Cp2Ti(SAr)2, Cp2TiCl(SAr)) to sulfenyl chlorides (S2Cl2, RSCl) have been investigated by a combination of stopped-flow UV-Vis/NMR reaction monitoring, titration assays, numerical kinetic modelling and KS-DFT calculations. The reactions are rapid, proceeding to completion over timescales of milliseconds to minutes, via a sequence of two S-S bond-forming steps (k1, k2). The archetypical polysulfides Cp2TiS5 (1a) and Cp2TiS4C(Me2) (2a) react with disulfur dichloride (S2Cl2) through rate-limiting intermolecular S-S bond formation (k1) followed by a rapid intramolecular cyclization (k2, with k2 ≫ k1 [RSCl]). The monofunctional sulfenyl chlorides (RSCl) studied herein react in two intermolecular S-S bond forming steps proceeding at similar rates (k1 ≈ k2). Reactions of titanocene bisthiophenolates, Cp2Ti(SAr)2 (5), with both mono- and di-functional sulfenyl chlorides result in rapid accumulation of the monothiophenolate, Cp2TiCl(SAr) (6) (k1 > k2). Across the range of reactants studied, the rates are relatively insensitive to changes in temperature and in the electronics of the sulfenyl chloride, moderately sensitive to the electronics of the titanocene (poly)sulfide (ρ(Ti-(SAr)) ≈ −2.0), and highly sensitive to the solvent polarity, with non-polar solvents (CS2, CCl4) leading to the slowest rates. The combined sensitivities are the result of a concerted, polarized and late transition state for the rate-limiting S-S bond forming step, accompanied by a large entropic penalty. Each substitution step {[Ti]-SR′ + Cl-SR → [Ti]-Cl + RS-SR′} proceeds via titanium-assisted Cl-S cleavage to generate a transient pentacoordinate complex, Cl-[Cp2TiX]-S(R′)-SR, which then undergoes rapid Ti-S dissociation.
AB - The kinetics of sulfur transfer from titanocene (poly)sulfides (RCp2TiS5, Cp2TiS4CMe2, Cp2Ti(SAr)2, Cp2TiCl(SAr)) to sulfenyl chlorides (S2Cl2, RSCl) have been investigated by a combination of stopped-flow UV-Vis/NMR reaction monitoring, titration assays, numerical kinetic modelling and KS-DFT calculations. The reactions are rapid, proceeding to completion over timescales of milliseconds to minutes, via a sequence of two S-S bond-forming steps (k1, k2). The archetypical polysulfides Cp2TiS5 (1a) and Cp2TiS4C(Me2) (2a) react with disulfur dichloride (S2Cl2) through rate-limiting intermolecular S-S bond formation (k1) followed by a rapid intramolecular cyclization (k2, with k2 ≫ k1 [RSCl]). The monofunctional sulfenyl chlorides (RSCl) studied herein react in two intermolecular S-S bond forming steps proceeding at similar rates (k1 ≈ k2). Reactions of titanocene bisthiophenolates, Cp2Ti(SAr)2 (5), with both mono- and di-functional sulfenyl chlorides result in rapid accumulation of the monothiophenolate, Cp2TiCl(SAr) (6) (k1 > k2). Across the range of reactants studied, the rates are relatively insensitive to changes in temperature and in the electronics of the sulfenyl chloride, moderately sensitive to the electronics of the titanocene (poly)sulfide (ρ(Ti-(SAr)) ≈ −2.0), and highly sensitive to the solvent polarity, with non-polar solvents (CS2, CCl4) leading to the slowest rates. The combined sensitivities are the result of a concerted, polarized and late transition state for the rate-limiting S-S bond forming step, accompanied by a large entropic penalty. Each substitution step {[Ti]-SR′ + Cl-SR → [Ti]-Cl + RS-SR′} proceeds via titanium-assisted Cl-S cleavage to generate a transient pentacoordinate complex, Cl-[Cp2TiX]-S(R′)-SR, which then undergoes rapid Ti-S dissociation.
U2 - 10.1039/d4sc02737j
DO - 10.1039/d4sc02737j
M3 - Article
AN - SCOPUS:85197880819
SN - 2041-6520
VL - 15
SP - 11875
EP - 11883
JO - Chemical Science
JF - Chemical Science
IS - 30
ER -