Epoxides oxidative addition

It is now well estabUshed that the mechanism of the transition metal-catalyzed coupling reactions of CO, and epoxides includes three main steps, epoxides oxidative addition, CO2 insertion, and reductive elimination of carbonates. The important issue of the mechanism is whether first to activate the epoxides or C02- In the following paragraphs, we will discuss a couple of examples to illustrate the mechanistic details. 

Allylic acetates are widely used. The oxidative addition of allylic acetates to Pd(0) is reversible, and their reaction must be carried out in the presence of bases. An important improvement in 7r-allylpalladium chemistry has been achieved by the introduction of allylic carbonates. Carbonates are highly reactive. More importantly, their reactions can be carried out under neutral con-ditions[13,14]. Also reactions of allylic carbamates[14], allyl aryl ethers[6,15], and vinyl epoxides[16,17] proceed under neutral conditions without addition of bases. 

A very common combination in Pd-catalyzed domino reactions is the insertion of CO as the last step (this was discussed previously). However, there is also the possibility that CO is inserted as the first step after oxidative addition. This process, as well as the amidocarbonylation, the animation, the arylation of ketones, the isomerization of epoxides, and the reaction with isonitriles will be discussed in this section. 

The oxidation of allenylsulfonamides 75 is also possible by using DMDO [23], Unlike the corresponding reaction of allenyl acids, oxidation of allenyl sulfonamides usually cannot be stopped after the formation of the allene oxide 76 but proceeds further to the spirodiepoxide intermediate 77, finally giving hydroxypyrrolidinone 78 and hydroxypiperidone 79, respectively (Scheme 17.23). Similarly to y-allenyl alcohols, aldehydes and acids, five-membered heterocycles, e.g. 80, are also formed from y-allenylsulfonamides. In the latter case the reaction can be terminated after the first epoxidation by addition of p-toluenesulfonic acid. 

The oxidative addition of hydroxide anion to nitroalkenes, e.g. ( )-2-nitro-2-butene, which leads to epoxides, proceeds by way of radical anions (equation 135)448. 

Uracils and related pyrimidines undergo oxidative addition to the 5,6-double bond, and the reaction with a number of oxidants to form 5,6-epoxides and 5,6-diols was discussed in CHEC-II(1996) <1996CHEC-II(6)93>. Oxidative halogenation can also occur <1996SC3583, 1998NN1125>, as shown by the formation of 5-bromo-5,6-dihydro-6-methoxyuracil 100 from uracil 99 by treatment with a mixture of potassium bromate and potassium bromide in the presence of Dowex ion-exchange resin in methanol <1996SC3583>. 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

Selective polymerization, enantiomers, 185 Semico rrin-copper complexes, 199 Sharpless epoxidation, racemic alcohols, 45 Side-chain units, prostaglandins, 310 Sigmatropic reactions, 222 Silanes, oxidative addition, 126 Silica gel, 285, 352... 

The two basic mechanistic possibilities for C-P450 epoxidations are summarized in Scheme 54. Path a represents an entirely covalent pathway involving oxidative addition... 

Cationic t 3-allyltetracarbonyliron complexes are generated by oxidative addition of allyl iodide to pentacarbonyliron followed by removal of the iodide ligand with AgBF4 under a carbon monoxide atmosphere [35]. Similarly, photolysis of vinyl epoxides or cyclic vinyl sulfites with pentacarbonyliron or nonacarbonyldiiron provides Jt-allyltricarbonyliron lactone complexes. Oxidation with CAN provides by demetallation with concomitant coupling of the iron acyl carbon to one of the termini of the coordinated allyl moiety either [3- or 8-lactones (Scheme 1.12) [36, 37]. In a related procedure, the corresponding Jt-allyltricarbonyliron lactam complexes lead to P- and 8-lactams [37]. 

These iron porphyrin derivatives selectively catalyzed cyclohexane hydroxidation, where (FeTNMCPP)Cl displayed the higher stability. Cyclooctane epoxidation on the same mimic proceeded with multiple oxidant addition. 

Alkyl halides (particularly bromides) undergo oxidative addition with activated copper powder, prepared from Cu(I) salts with lithium naphthalenide, to give alkylcopper species10. The alkyl halides may be functionalized with ester, nitrile and chloro functions ketone and epoxide functions may also be tolerated in some cases11. The resulting alkylcopper species have been shown to react efficiently with acid chlorides, enones (conjugate addition) and (less efficiently) with primary alkyl iodides and allylic and benzylic bromides (equations 5 and 6). If a suitable ring size can be made, intramolecular reactions with epoxides and ketones are realized. 

In contrast, trans olefins are often poor substrates for CPO. When the double bond is far from the chain terminus (i.e., from r/.v-3-alkenes upward), allylic hydro-xylation accompanies the epoxidation. In addition, with terminal monosubstituted olefins, heme alkylation occurs, thus producing inactivation of CPO. 1-Alkenes can be profitably oxidized to epoxides by CPO only when they are not monosubstituted. A detailed description of the yields and e.e. for CPO-catalyzed epoxidation of olefins has been reported by Adams and coworkers [23]. 

Benzodithioles 285 (R1 = H) and 7,8-dimethyl-l,5-dihydro-2,4-benzodithiepins286 (R = H) have been used as precursors of formyl and acyl anion derivatives. The lithiation of compounds 285 takes place at —30 °C with n-BuLi and these anions are stable for long periods of time at this temperature454. They react with alkyl iodides, carbonyl compounds and epoxides, the addition to cyclohex-2-enone taking place at the carbonyl group. The deprotection has also been carried out with mercury(II) oxide and BF3 OEt2-... 

Early investigations on the oxidative addition of halogens to alkenes have mainly been focused on the preparation of epoxides from lower alkenes such as ediylene and propene. ... 

The acetate unit is particnlarly convenient, since it is obtained from the corresponding alcohol and is relatively stable toward solvolysis, Sn2 reactions, and so on. Many other electronegative units are also effective, including halide and tosylate. Less obvious is the high reactivity of allylic sulfones and allylic nifro compounds toward the oxidative addition. Three special cases are outlined here. First, an allylic epoxide can undergo rapid oxidative addition of the allylic C-O bond under mild conditions, leading to the zwitterion or equivalent (Scheme 13). Proton abstraction can lead... 

Oxidative addition to vinyl epoxides (obtained from epoxidation of dienes) affords a ferralactone XL where E = O. This procedure tolerates a varied substitution... 

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