New methodologies for oxidation using peroxides mediated by Oxo-Chromium(VI) and Oxo-Boron(III) compounds

Abstract

Part A: Oxidative Transformations with PDC/t-BuOOH Part A of this thesis focuses on oxidative transformations using the pyridinium dichromate (PDC)/tert-butyl hydroperoxide (t-BuOOH) reagent system. Chapter 1 describes a general methodology for the direct conversion of acetals to esters. A variety of acetals (13 examples) were smoothly converted to their corresponding esters using PDC/t-BuOOH in good yields. This reagent system exhibited remarkable selectivity, unattainable by other known reagents for this transformation. To the best of our knowledge, this constitutes the first satisfactory methodology for the selective oxidation of acetals of ?,?-unsaturated aldehydes and acetals bearing remote carbon–carbon double bonds or hydroxyl groups. Chapter 2 explains an interesting reaction of PDC/t-BuOOH with 1,3-dienes. Both cyclic and acyclic 1,3-dienes (11 examples) reacted with PDC/t-BuOOH to afford 7-tert-butyldioxy enones, which were subsequently converted to 7-hydroxy enones. Even a catalytic amount of PDC (10 mol%) achieved this oxidation with good conversion. This provides an efficient route to key intermediates in the total synthesis of prostaglandins and compactin, such as 7-hydroxy enones, from readily available and inexpensive starting materials like cyclopentadiene and 1,3-cyclohexadiene. Conjugated dienes with benzylic centers at both termini gave enediones upon treatment with PDC/t-BuOOH. However, when substantial steric hindrance was present around the diene moiety, only allylic oxidation was observed, as in the case of ?-ionyl acetate. Chapter 3 investigates the nature of the reactive species in the PDC/t-BuOOH system (1:1.5). The reactivity of this system with conjugated dienes showed remarkable similarity to singlet oxygen reactions, but experimental studies convincingly ruled out singlet oxygen involvement. Reactions with stilbene and styrene suggested that the PDC/t-BuOOH system behaves similarly to the chromium trioxide/t-BuOOH system but is milder. This was further supported by its reaction with tetrahydrofuran (THF), which produced a mixture of lactone and lactol. Based on these observations, a plausible mechanism for the oxidation of conjugated dienes with PDC/t-BuOOH has been proposed.

Part B: Oxo-Boron(III) Mediated Oxidations Part B of the thesis includes studies on oxo-boron(III) mediated oxidations. Chapter 1 presents a facile oxidative cleavage of cyclic acetals (9 examples) to their respective esters using an inexpensive reagent system: sodium perborate/acetic anhydride. This reagent system is readily available, non-toxic, and scalable, making it a promising protocol for synthesizing commercially important products such as monoesters of ethane-1,2-diol. The oxidation of 2-(9-decenyl)-1,3-dioxolane with perborate/acetic anhydride clearly demonstrates the selectivity of this system. Interestingly, 2-aryl-1,3-dioxolanes without other substituents on the acetal ring were converted to 2-acetoxyethyl esters. Chapter 2 describes studies on sugar–borate mediated oxygenation using t-BuOOH as the terminal oxidant. A novel species, ?-cyclodextrin–borate, was prepared by mixing boric acid (7 equivalents) with ?-cyclodextrin (1 equivalent) and removing water via azeotropic distillation. Although its precise structure remains unclear, this species proved useful as a catalyst for the oxygenation of alkenes (5 examples) with t-BuOOH. Aromatic alkenes were converted directly to ?-tert-butyldioxy alcohols, unlike earlier methods that required epoxides as starting materials. Mechanistic studies supported the intermediacy of epoxides. The ?-dioxy alcohols were further converted to the corresponding diols. The proposed structure of ?-cyclodextrin–borate suggests an extended cylindrical cavity, indicating potential applications in supramolecular chemistry.