Predominance of the second cycle in homogeneous Os-catalyzed dihydroxylation: the nature of Os(vi) → Os(viii) reoxidation and unprecedented roles of amine-N-oxides

Abstract

The homogenous Os-catalyzed dihydroxylation of alkenes has been widely applied in organic synthesis. Mechanistic studies on the diol formation have been performed but concentrated on the osmylation step to form...

Elucidating the Mechanism of Tetrahydrofuran-Diol Formation through Os(VI)-Catalyzed Oxidative Cyclization of 5,6-Dihydroxyalkenes Ligated by Citric Acid

“…This discloses that in the absence of Bro̷ nsted acids, reoxidation to Os(VIII) is highly preferred despite its slowness and, consequently, entering the second cycle pathway through a fast osmylating of an alkene. 15 Broadly, this is in agreement with the Sharpless Os(VI)-catalyzed dihydroxylation of olefins ligated with citric acid in the presence of a stronger co-oxidant such as NMO (Figure 1c). 14a In this regard, to understand the effect of co-oxidants and validate our results further computations on the reoxidation Os(VI) → Os(VIII) citrate, different co-oxidants are considered (see Figure SI1).…”

“…This value closely aligns with the energy calculated for the formation of 1,2-vicinal diol through Os-catalyzed dihydroxylation of alkenes (ΔG r cycle = −43.6.0 kcal/mol). 15 According to the calculated energy profile appeared in Figure 7 and based on the energetic span model, 18 it is evident that the reoxidation step on TS19 is not the rate-determining state or step. Instead, it is the insertion step through TS12 that serves as the ratedetermining step and thus acts as a turnover-limiting step (TLS).…”

“…15,17 Complexation of PNO with Os(VI) is thermodynamically exergonic of ΔG = −12.1 kcal/mol with a sigma character bonding formation between O of PNO and Os atoms at 2.10 Å. 15 Now, the PNO-complexed Os(VI) glycolate undergoes either reoxidation to Os(VIII) or gaining a proton from the solution. In the absence of Bro̷ nsted acid, the reoxidation Os(VI) → Os(VIII) trioxoglycolate (Int7 → Int8) is kinetically slow with a barrier of 24.8 kcal/mol via TS3 as a moderately endergonic step (ΔG = 7.9 kcal/mol).…”

Impact of counteranions on N-heterocyclic carbene gold(i)-catalyzed cyclization of propargylic amide

“… 53,54 We recently used this model for determining the turnover-limiting state in the homogenously Os-catalyzed dihydroxylation of alkene and successfully predicted it to be the reoxidation step rather than hydrolysis. 55 …”

“…53,54 We recently used this model for determining the turnover-limiting state in the homogenously Os-catalyzed dihydroxylation of alkene and successfully predicted it to be the reoxidation step rather than hydrolysis. 55 This model requires two fundamental terms: the TOFdetermining transition state (TDTS) and the TOF-determining intermediate (TDI). The calculated catalytic cycle presented in Fig.…”