Epoxidation radical-based

The radical mechanism has also been proposed as a general mechanism for oxidation of alkenes and aromatics, but several objections have been raised because of the absence of products typically associated with radical reactions. In classical radical reactions, alkenes should react also at the allylic position and give rise to allyl-substituted products, not exclusively epoxides methyl-substituted aromatics should react at the benzylic position. The products expected from such reactions are absent. Another argument was made against the radical mechanism based on the stereoselectivity of epoxidation. Radical intermediates are free to rotate around the C C bond, with the consequence that both cis- and /rani-epoxides are formed from a single alkene isomer, contrary to the evidence obtained with titanium silicates (Clerici et al., 1993). 

A radical-based Wagner-Meerwein-type rearrangement has been reported where the radical was generated from the epoxide using CP2T1CI as the radical source.163... 

Other radical-based transformations are ruthenium-catalyzed oxidative dimerizations of phenols [263] and reductive dimerizations [264], The isomerization of chiral c/s-epoxides to tram-epoxides catalyzed by 2-10 mol% TpRu(py)2Cl proceeds at 100 °C in 95-98% yields with inversion of configuration [265], A radical or SN2 mechanism was discussed for this process. 

Epoxidation. Hoft and Ganschow report that this reagent converts olefins into epoxides, Schiff bases into oxaziridines, and tertiary amines into N-oxides. Epoxidation can be performed more simply by addition of benzoyl isocyanate to a solution of the alkene in THF containing excess anhydrous H2O2 and a trace of a radical inhibitor. Under these conditions phenanthrene is converted at 25° into biphenyl-2,2 -dicarboxylic acid. 

Apparently, pre-irradiating polyethylene containing a HALS with a low pressure Hg lamp imparts some surface stabilization toward sunlight exposure. Anthracene-HAS stabilizer molecules are found to be more effective stabilizers than the separate molecular moieties, while the nature of the polymer composition affects the distribution of nitroxyl radicals in stabilized materials. " Polyethylene with terminal HAS groups has been found to be very light stable, as were reactive acrylic coatings with acrylated HAS systems. A new epoxide amine based HAS has been found to be quite effective in... 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

This is interesting when one considers the effect of synergists on the synthetics. All of the synthetics mentioned above are based on chrysanthemum monocarboxylic acid and in the case of allethrin, cyclethrin, and furethrin on the alcohol moiety there is only one double bond. When checked against the standard synergists, these synthetics do not show the degree of synergism shown by pyrethrins and this may be because of the fact that there is only one double bond for epoxidation, compared with two in the pyrethrolone radical, and therefore the synergist would not block this epoxidation step as effectively. 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

The calculations also suggested that 5 was favored over 6 by 2.4 kcal mol 1 as found experimentally. The explanation was based on the higher stability of a tertiary alkoxide compared to a primary alkoxide [15], which outweighed the opposite trend for radical stabilization. Epoxide opening was irreversible [16]. 

The radical versus electrophilic character of triplet and singlet carbenes also shows up in relative reactivity patterns shown in Table 10.1. The relative reactivity of singlet dibromocarbene toward alkenes is more similar to that of electrophiles (bromination, epoxidation) than to that of radicals ( CC13). Carbene reactivity is strongly affected by substituents.61 Various singlet carbenes have been characterized as nucleophilic, ambi-philic, and electrophilic as shown in Table 10.2. This classification is based on relative reactivity toward a series of different alkenes containing both nucleophilic alkenes, such as tetramethylethylene, and electrophilic ones, such as acrylonitrile. The principal structural feature that determines the reactivity of the carbene is the ability of the substituents to act as electron donors. For example, dimethoxycarbene is devoid of electrophilicity toward... 

Co and Cu exchanged X and Y zeolites catalyze the decomposition of t-butyl hydroperoxide with generation of t-butoxy and t-butylperoxy radicals. When this decomposition is performed in the presence of olefins, such as cyclohexene or 1-octene, the corresponding epoxides are formed with selectivities ranging from 10 to 50% based on decomposed t-butyl hydroperoxide... 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

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