Peculiarities of β‐Pinene Autoxidation

Abstract

The thermal oxidation of the renewable olefin β-pinene with molecular oxygen was experimentally and computationally investigated. Peroxyl radicals abstract weakly bonded allylic hydrogen atoms from the substrate, yielding allylic hydroperoxides (i.e., myrtenyl and pinocarvyl hydroperoxide). In addition, peroxyl radicals add to the C=C bond of the substrate to form an epoxide. It was found that a relatively high peroxyl radical concentration, together with the high rate of peroxyl cross-reactions, make radical-radical reactions surprisingly important for this particular substrate. Approximately 60 % of these peroxyl cross-reactions lead to termination (radical destruction), keeping a radical chain length of approximately 4 at 10 % conversion. Numerical simulation of the reaction-based on the proposed reaction mechanism and known or predicted rate constants-demonstrate the importance of peroxyl cross-reactions for the formation of alkoxyl radicals, which are the precursor of alcohol and ketone products.

Catalytic Epoxidations with Peroxides: Molybdenum Trioxide Species as the Origin of Allylic Byproducts

“…Insights from aerobic β‐pinene oxidation : To further corroborate these results, we investigated the influence of MoO 2 (acac) 2 on the autoxidation of β‐pinene (343 K, 1 atm O 2 ). Indeed, in agreement with the proposed homolytic decomposition of the trioxide intermediate, a much higher reaction rate could be observed, due to a catalytic chain initiation, complementing the thermal initiation17 (Figure 4). Moreover, as a result of concurrent Mo VI ‐catalyzed Sharpless‐type epoxidation, the selectivity towards the β‐pinene oxide product was three times higher than under uncatalyzed conditions (i.e., 27 % instead of 9 %, at 10 % conversion).…”

“…The k term refers to the peroxyl‐radical termination rate constant and k init,noncat to the rate constant of the noncatalyzed initiation rate reaction between ROOH and RH 17. Substituting Equations (14) and (15) in Equation (13) leads to the ratio of squared rates [Eq.…”

“…n ‐Nonane (Sigma Aldrich,>99 %) was added to the freshly distilled (−)‐β‐pinene (1 mol %, Sigma Aldrich, 99 %) substrate and used as an inert internal standard. Product characterization was performed as described elsewhere 17. Quantum chemical calculations were performed with the Gaussian09 software22 at the UB3LYP/6–311++G(df,pd)//UB3LYP/6–31G(d,p) level of theory,23 by using a LANL2DZ basis set24 with an additional f‐polarization function25 for Mo.…”

Oxidation

“…Peroxy radicals also add readily to C¼C double bonds, forming either epoxides and alkoxy radicals (Twigg mechanism) [379][380][381], or polyperoxides and their decomposition products (epoxides, aldehydes, alcohols, and ketones) (Mayo mechanism). Reactions of Hydroperoxides.…”

Synthesis of Nopinone from β‐Pinene – A Journey Revisiting Methods for Oxidative Cleavage of C=C Bonds in Terpenoid Chemistry

“…Use of a binuclear Fe complex as a model for methane monooxygenase and aqueous H 2 O 2 afforded – besides 1 (27 % yield, 66 % selectivity) – pinocarvone ( 27 , Scheme ), trans ‐pinocarveol ( 28 ), myrtenal ( 29 ), and myrtenol ( 30 ) after oxidation at 25 °C (12 h) 91. Autoxidation of 2 also led to 27 – 30 together with a series of other products, but 1 was formed in a side reaction with <1 % selectivity only 92. Oxidation of limonene or α‐pinene under similar conditions afforded a series of oxidation products attributable to allylic oxidation rather than oxidative cleavage.…”