enone system. The tertiary alcohol of 14 was readily converted to the corresponding TMS ether 17. The potassium dienolate of 17 was generated with potassium bis(trimethvlsilv1)amide and subseauentlv treated with the prove to be particularly challenging.We are currently attempting to extend the findings, described herein, in programs directed toward the total svntheses of tax01 and Dotentially useful analogs thereof. Dahs oxazkd&el6 to give diol 18 &er aqueous workup." Formally, C-4 of 18 can be viewed as corresponding in stereochemistry to C-2 of 1. However, the preservation of this stereogenicity throughout the steps leading to 1 could -Acknowledgment. We thank V. Parmakovich and B.SPorer from the Department of Chemistry of Columbia University for mass spectral analyses. Supplementary Material Available: Procedures and spectral data (1H a d 13C NMR, IR, HRMS) for compounds 4-18 (47 is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the journal, a d can be ordered from the ACS; see current masthead page for ordering information. (16) (a) Davis, F. A.; Vishwakarma, L. c.; Billmers, J. M.; Finn, J. J. Org. Chem. 1984,49,3241. (b) Vihwakarma, L. C.; Stringer, 0. D.; Davis, F. A. Org. Synth. 1987,66, 203.(17) Aqueous workup readily the TMS ether. Flash chromatography using a 20-402 EtOAc in CHzClz gradient elution was needed to remove the N-phenylsulfonamide byproduct.
2-Propanol is oxidized by tetrapropylammonium perruthenate (TPAP) in a reaction that is second order in TPAP and first order in 2-propanol. One of the products, believed to be ruthenium dioxide, is an effective catalyst for the reaction, making it an autocatalytic process. The rate of oxidation is relatively insensitive to the presence of substituents. Primary kinetic deuterium isotope effects are observed when either the hydroxyl or the α hydrogen is replaced by deuterium. The only product obtained from the oxidation of cyclobutanol is cyclobutanone, indicating that the reaction is a two-electron process. Tetrahydrofuran is oxidized at a rate that is several orders of magnitude slower than that observed for 2-propanol, suggesting that the reaction of TPAP with alcohols may be initiated by formation of perruthenate esters. A tentative mechanism consistent with these observations is proposed.Key words: oxidation, alcohols, tetrapropylammonium perruthenate, reaction mechanism, autocatalysis.
The kinetics of the oxidation of 2-propanol by ruthenium tetroxide in aqueous perchloric acid solutions have been investigated. The results indicate that the two mechanisms pertain; at moderate acidities (1-6.5 M HCIO,) the rate determining step involves hydride abstraction while at very high acid concentrations the rate determining step becomes carbonium ion formation.Les cinktiques de I'oxydation du propanol-2 par le tktroxyde de ruthenium dans des solutions aqueuses d'acide perchlorique ont etC Btudites. Les resultats indiquent que deux mkanismes sont concernts; a une acidit6 modkrke (1-6.5 M HCIO,), ]'&tape qui determine la vitesse implique un arrachement d'hydrure alors qu'i une concentration &levee en acide I'btape qui determine la vitesse correspond a la formation d'un ion carbonium.
Possible mechanisms for the oxidation of alcohols by dioxoruthenium(VI) complexes are critically evaluated. Rate constants for the reduction of trans-[(TMC)RuVI(O)2]++ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) by substituted benzhydrols are correlated more satisfactorily with Hammett σ substituent constants ( rho = -1.44 ± 0.08, r2 = 0.98) than with σ + substituent constants ( rho = -0.72 ± 0.11, r2 = 0.83). Similar observations for the oxidation of substituted benzyl alcohols have recently been reported, confirming that the transition state for these reactions is not carbocation-like. Primary deuterium isotope effects indicate that cleavage of the α -C-H bond is rate-limiting. The lack of an observable O-D isotope effect and the ease of oxidation of ethers indicates that the presence of a hydroxyl is not essential. The previously reported observation that cyclobutanol is quantitatively converted into cyclobutanone by dioxoruthenium(VI) complexes eliminates free-radical intermediates from consideration as part of the mechanism, and negative entroπes of activation (-Δ Sdouble dagger = 96-137 J mol-1 K-1) suggest a structured transition state. Only two of eight possible reaction mechanisms considered were found to be consistent with the available data. A critical analysis of the available data indicates that a 2 + 2 (C-H + Ru font 35137 roman T O) addition and a reaction initiated by ligand formation through the interaction of the reductant's HOMO with the oxidant's LUMO are the most likely reaction mechanisms.Key words: oxidation, alcohols, ruthenium(VI), mechanism, substituent effects.
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