Direct epoxy hydroxylation of hydroperoxy homoallylic alcohols: multidentate oxygen donor and oxygen acceptor substrates in titanium(IV)-catalyzed epoxidations

Abstract

The hydroperoxy homoallylic alcohols (2?*,5*)-2a and (S*,S*)-2&-d, readily available through the photooxygenation of chiral allylic alcohols 1, were converted into epoxy diols 4 under the catalytic action of Ti(Oi-Pr)4. In these epoxy hydroxylations, the hydroperoxides 2 play the double role as oxygen atom donor and, in form of the in situ generated corresponding unsaturated diols 3, as substrate for oxygen transfer. Compared to Ti(IV)-catalyzed epoxidations of unsaturated diols by -BuOOH, the advantage of this approach is that a large rate enhancement is obtained. Moreover, with the exception of (S*JS*)-2A, all reactions proceeded in unusually high diastereoselectivity. These results are rationalized in terms of the ability of the hydroxy-functionalized hydroperoxides (oxygen atom donors) as well as the corresponding unsaturated diols (oxygen atom acceptors) to chelate to the titanium metal in the catalytically operating template. For (S*,S*)-3a-c, bidentate binding is feasible, while for (7?*,£*)-3a and (S*,S*)-3d, this is difficult due to unfavorable steric interactions. Important for synthetic applications is the fact that allylic alcohols 1 can directly be converted into epoxy diols 4 in a one-pot, two-step procedure simply by adding catalytic amounts of Ti(Oi-Pr)4 to a photooxygenated solution of 1.Epoxy alcohols are versatile building blocks of high synthetic utility which are abundantly used in organic chemistry.1 Of the numerous methods available, metal-catalyzed epoxidations of allylic alcohols with hydroperoxides2 represent a practical route, of which the Sharpless epoxidation2d~s is undoubtedly the most valuable prototype for this purpose.An alternative, related oxyfunctionalization constitutes the direct conversion of olefins into epoxy alcohols by photooxygenation, followed by subsequent transformation of the intermediary allylic hydroperoxides under the catalytic action of transitionmetal catalysts.3 In this epoxy hydroxylation, the allylic alcohol, which is the actual epoxidized substrate, is generated in situ from the allylic hydroperoxide by oxygen transfer during the epoxidation. Hence, the allylic hydroperoxide plays the double role as oxygen donor and, after oxygen transfer, as oxygen acceptor. Therefore, no separate hydroperoxide is necessary as the oxygen atom donor reagent (Scheme I).Recently we reported that the epoxy hydroxylation methodology

Technical Note: Synthesis of isoprene atmospheric oxidation products: isomeric epoxydiols and the rearrangement products <i>cis</i>- and <i>trans</i>-3-methyl-3,4-dihydroxytetrahydrofuran

“…Acid hydrolysis of 2 followed by epoxidation with mCPBA gave the diastereomeric mixture IEPOX-2 in 62 % yield. The 1 H NMR spectrum of IEPOX-1 is identical to published spectra (Chiappe et al, 2000;Adam and Nestler, 1993) in which assignment of NMR signals was based on the spectral characteristics of close structural analogs (Adam and Nestler, 1993). In the 13 C NMR spectrum of IEPOX-1, two sets of signals in a 2 : 1 ratio are assigned to the erythro and threo diastereomers, respectively, based on the 13 C NMR shifts which are in accord with the published report.…”

“…The 1 H NMR spectrum ( Fig. S3) is identical to published spectra (Adam and Nestler, 1993;Adam et al, 1997).…”

Technical Note: Synthesis of isoprene atmospheric oxidation products: isomeric epoxydiols and the rearrangement products <i>cis</i>- and <i>trans</i>-3-methyl-3,4-dihydroxytetrahydrofuran

“…Acid hydrolysis of 2 followed by epoxidation with mCPBA gave the diastereomeric mixture IEPOX-2 in 62 % yield. The 1 H NMR spectrum of IEPOX-1 is identical to published spectra (Chiappe et al, 2000;Adam and Nestler, 1993) and Nestler, 1993). In the 13 C NMR spectrum of IEPOX-1, two sets of signals in a 2 : 1 ratio are assigned to the erythro and threo diastereomers, respectively, based on the 13 C NMR shifts which are in accord with the published report.…”

Catalytic Partial Oxidation of Cyclohexane by Bimetallic Ag/Pd Nanoparticles on Magnesium Oxide

“…To further extend our mechanistic model beyond the study of CHHP decomposition, we also considered the role and decomposition of tert ‐butyl hydroperoxide (TBHP). TBHP is a common radical initiator, as well as an oxygen donor that can undergo homolytic cleavage of the O−O bond in a similar manner to that experienced by CHHP [Eqs. ]. …”

“…To furthere xtend our mechanisticm odel beyond the study of CHHP decomposition, we also considered the role andd ecompositiono ftert-butyl hydroperoxide( TBHP). TBHP is ac ommon radicali nitiator, [40] as well as an oxygen donor [41] that can undergo homolytic cleavage of the OÀOb ond in as imilar manner to that experienced by CHHP [Eqs. (9)(10)(11)].…”