Epoxidation species

As discussed above, the activation barrier of solvated complexes (type b) is larger, since the starting complex is more stabilized than the TS. Notwithstanding this barrier increase, solvated TSs reside at much lower (absolute) energies than their unsolvated counterparts. However, hydrated TSs are entropically disfavored (by 9 kcal/mol in the gas phase) with respect to the corresponding water-free TSs. These observations prevent a definitive decision whether 5 or 5b is the active epoxidant species in solution. 

Stabilized ketene 6S. For l, 2 -disubstituted epoxide, species 6S undergoes 6-endo-dig electrocyclization (path b) [24] to form the six-membered ketone 66, ultimately giving naphthol products. l, 2, 2 -Trisubstituted epoxide species 6S undergoes 5-endo-dig cyclization (path a) to give the ketone species 67, finally producing l-alkylidene-2-indanones. The dialkyl substituent of the epoxide enhances the 5-endo-dig cyclization of species 65 via formation of a stable tertiary carbocation 67. We observed similar behavior for the cyclization of (o-styryl)ethynylbenzenes [15, 16]. Formation of 2,4-cyclohexadien-l-one is explicable according to 6-endo-dig cyclization of a ruthenium-stabilized ketene, vhich ultimately afforded the observed products [25]. 

Epoxides. Epoxy compounds react with the carboxyl groups of CTPB to form polyesters. The reaction rates and extent of reaction of a number of epoxides have been determined with the model compound hexanoic acid (6). It was found that most epoxides undergo side reactions (as evidenced by the more rapid consumption of epoxide species) but that at least one difunctional epoxide, DER-332 (Dow Chemical Co.) (Table IV), exhibits a clean reaction with carboxylic acids, even in the presence of ammonium perchlorate. 

The results of model compound studies with three different types of epoxides, obtained in the presence and absence of ammonium perchlorate are shown in Figures 4, 5, and 6. The epoxide DER-332 shows a uniform rate of disappearance for the acid and epoxide species in this reaction. In the presence of ammonium perchlorate, the rate is increased, and a minimum of side reactions occur. Similar data but faster reaction rates are obtained with Epon X-801, but the consumption of epoxide species by side reactions is increased, particularly in the presence of ammonium perchlorate. On the other hand, the epoxide ERLA-0510 (Table IV), which contains a basic nitrogen, shows a reaction rate which is an order of magnitude greater than that for DER-332, accompanied by a substantial increase in side reactions. In the presence of ammonium perchlorate, the side reactions of ERLA-0510 predominate. In all probability, the side reactions of the multifunctional epoxides studied are homopolymerization. 

In the seoond, the halogen is displaced nucleophilically by nxygen and a transient epoxide species is formed which, in the presence of MgXg, rearranges to a carbonyl compound. The latter in turn reacts with Grignard reagent to give the observed products. A similar mechanism has been advanced by House 13-814 in a related problem (Eq. 167. ... 

In each of the above reactions, more than one species is present in solution. Enantioselection is determined by the propensity of each ligand to form an active catalyst related to 4, the enantioselectivity of the active catalyst and the relative rate of epoxidation by the catalyst compared to other epoxidizing species in solution. At least for ligands containing 1,2- and 1,3-diols in which the alcohols are a. to esters, NMR evidence suggests that dimeric structures related to 4 are the predominant species in solution. It is with these ligands that the highest enantioselectivities are achieved. 

Here, an a-bromocarboxylate, which has been treated with a base, reacts with a carbonyl containing compound to form an epoxide species. 

Note that even critical phenomena for the catalyst-inhibitor conversion were observed in alkene epoxidation by O2/IBA system in the presence of PWnTi and PWnV catalysts [19]. All these data provide evidence in favor of the radical chain mechanism of the epoxidation in most 02/IBA/Co(II) systems studied where acylperoxyradicals are most probably the main epoxidizing species. 

Aerobic epoxidation of different alkenes, including a number of natural terpenes, efficiently occurs under mild reaction conditions in the presence of isobutyraldehyde as a reductant and MNaY and MNaZSM-5 type zeolites (M=Co(II), Cu(II), Ni(II) and Fe(III)) as catalysts. Yields of the epoxidation products vary from 80 up to 99% depending on the olefin and catalyst. The reaction proceeds via chain radical mechanism, acylperoxy radicals being the main epoxidizing species. 

The story of mctai-salen catalyzed olefin epoxidation began in 80s. when Kochi cl al. found that (salen)chromium(111) [2-3] and (salcn)mangancsc(lll) 4] complexes catalyze the epoxidation of unfunctionalized olefins using iodosylbcnzenc (PhlO) as terminal oxidant. Kochi was also the first one to describe the effect of the ligand substituent and donor additives (c.g. pyridinc-N-oxido) on the epoxidation, and to study the reaction kinetics and mechanism of the proposed systems (by UV-Vis). The epoxidizing species in both systems were postulated to be oxo(salen)mctal(V) species. 

Scheme 5. An alternative mechanism. Radical addition (instead of base-catalyzed abstraction) leading to species X is followed by formation of a ferryl heme species (Compound II) and a proposed epoxide species (55). Formation of an epoxide has also been suggested from computational work (51) (it was also considered in an earlier study, but initially considered energetically unfavorable in the gas phase (50)). Electrophilic addition (Scheme 4) could also involve epoxide formation through a similar (two-electron) mechanism. Possible ring opening of the epoxide is also indicated (third step).

These subsequent epoxidizing species can reform P04[W(0)(02)2]4 " upon reaction with as little as one equivalent of H202-... 

Bruice, T.C. and G. He (1991). Nature of the epoxidizing species generated by reaction of aUcyl hydroperoxides with iron(III) porphyrins. Oxidations of cis-stilbene and (Z)-l,2-bis(trans-2, trans-3-diphenylcyclopropyl)ethene by tert-BuOOH in the presence of [meso-tetrakis(2,4,6-trimethylphenyl)porphinato]-, [meso-tetrakis(2,6-dichlorophenyl)-porphinato]-, and [meso-tetrakis(2,6-dibromophenyl)porphinato]rron(III) chloride. J. Am. Chem. Soc. 113, 2747-2753. 

Yet, in related studies, mechanistic studies and theoretical calculations on (salen)AfX/nucleophile-mediated formation of cyclic carbonates from CO2 and PO, the authors proposed a bi-metaUic mechanism, involving a nucleophilic attack at an Al-coordinated epoxide species by an Al-bound nucleophile adduct [56]. Subsequent CO2 insertion into the newly formed Al-alkoxide bond, likely to be the rate-determining step of the all process, would then afford the corresponding Al-carboxylate derivative. Overall, further studies are certainly required for a complete understanding of the factors controlling and affecting Al-mediated CO2/ epoxide copolymerization reactions so that to allow the development of more active Al catalytic systems. 

Here should be also mentioned the importance of frilerene (C-60) ozonolysis [70] resulting in the obtaining of mixture containing various oxidation products such as ketone, ester and epoxides species. The intermediate in this reaction can act as oxygen atom carrier thus yielding phenols [88]. 

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