Manganese acetate radical addition reactions

Despite the development of various intermolecular radical addition methods, those studies have rarely accommodated additional functionality, our discovery of the manganese-mediated photolysis conditions notwithstanding. Prior to that discovery, we began to elaborate an alternative strategy which employs temporary tethers ([115, 116] reviews of silicon-tethered reactions [117-120]) (silyl ether or acetal linkages) linking radical and acceptor. In this scenario the C-C bond is constructed via cyclization, in which internal conformational constraints can control diaster-eoselectivity. The tether itself would be converted to useful functionality upon cleavage, and once the tether is cleaved the net result may be considered as formal acyclic stereocontrol. ... 

Manganese(III) can oxidize carbonyl compounds and nitroalkanes to carboxy-methyl and nitromethyl radicals [186]. With Mn(III) as mediator, a tandem reaction consisting of an intermolecular radical addition followed by an intramolecular electrophilic aromatic substitution can be accomplished [186, 187). Further Mn(III)-mediated anodic additions of 1,3-dicarbonyl and l-keto-3-nitroalkyl compounds to alkenes and alkynes are reported in [110, 111, 188). Sorbic acid precursors have been obtained in larger scale and high current efficiency by a Mn(III)-mediated oxidation of acetic acid acetic anhydride in the presence of butadiene [189]. Also the nitromethylation of benzene can be performed in 78% yield with Mn(III) as electrocatalyst [190]. A N03 radical, generated by oxidation of a nitrate anion, can induce the 1,4-addition of aldehydes to activated olefins. NOj abstracts a hydrogen from the aldehyde to form an acyl radical, which undergoes addition to the olefin to afford a 1,4-diketone in 34-58% yield [191]. 

One of the most practical methods for the radical hydrophosphonylation of alkenes uses manganese (II) acetate to promote the addition reaction (Schane 4.75) [129], The chemistry was operationally trivial, and simply adding the reagents to a reaction vessel in air followed by stirring under air at 90 °C afforded fair to excellent yields of the alkylphos-phonates. For terminal alkenes, the reaction selectively formed the anti-Markovnikov... 

The low yield in this reaction might be caused by a number of reasons. First, the overall reaction is only rapid for readily enolizable compounds. 1,3-Dicarbonyl compounds will therefore be a better choice as compared to acetic acid. Second, to prevent oxidation of radical 54, it is advantageous to work with excess diene and therefore speed up trapping of 54 through diene addition. Finally, lactone 55 can, as an enolizable compound itself, also be oxidized by manganese(III) acetate and form various oxidation products. Shorter reaction time and the use of understoichiometric amounts of oxidant might therefore benefit the overall result. All these factors have been taken into account in the synthesis of bicyclic /-lactone 56, which has been obtained from cyanoacetic acid and 1,3-cyclohexadiene in 78% yield within 15 min reaction time (equation 25)60,88. 

Generation of the carbon based radical in these processes involves the prior formation of a complex between manganese(lll) and the enol of the carbonyl reactant. Intramolecular electron transfer occurs within this complex. Addition to the olefin then takes place within the co-ordination sphere of manganese. When manganese is present in catalytic amount, the relative values of the equlibrium constants between manganese and both the carbonyl compound and the alkene arc important. If the olefm is more strongly complexed then no radical can form and reaction ceases. Reactions are usually carried out at constant current and the current used must correspond to less than the maximum possible rate for the overall chemical steps involved. Excess current caused the anode potential to rise into a region where Kolbe reaction of acetate can occur and this leads to side reactions [28]. 

A group transfer tandem addition of bromotrichloromethane to diallyl amine 157 has been reported [95SC3529]. The radical reaction can be initiated using either azobisisobutyronitrile (AIBN) or manganese(III) acetate electrochemically. It should be noted that the cis diastereomer is formed as the major product. 

Manganese(m) acetate oxidation (cf. Vol. 3, p. 34) of camphene gives (186) as a 95 5 mixture by carboxymethyl radical insertion no rearranged products were obtained, in contrast to /3-pinene which gave Wagner-Meerwein products only, and no free-radical insertion.279 The E- and Z-isomers of (187) probably result from a non-concerted biradical intermediate formed by benzyne addition to camphene.280 Benzyl-lithium adds to the aminocamphor (188) exclusively from the exo-side whereas only the competing enolization reaction occurs with more sterically hindered organometallics.281... 

Manganese(III)-mediated radical reactions have become a valuable method for the formation of carbon-carbon bonds over the past thirty years since the oxidative addition of acetic acid (1) to alkenes to give y-butyrolactones 6 (Scheme 1) was first reported by Heiba and Dessau [1] and Bush and Finkbeiner [2] in 1968. This method differs from most radical reactions in that it is carried out under oxidative, rather than reductive, conditions leading to more highly functionalized products from simple precursors. Mn(III)-based oxidative free-radical cyclizations have been extensively developed since they were first reported in 1984-1985 [3-5] and extended to tandem, triple and quadruple cyclizations. Since these additions and cyclizations have been exhaustively reviewed recently [6-11], this chapter will present an overview with an emphasis on the recent literature. 

Radical reaction of [60]fullerene (459) with phosphites or phosphine oxide (460) mediated by manganese(III) acetate dihydrate in chlorobenzene under three dilferent reaction conditions afforded three different types of phosphorylated fullerenes hydrophosphorylated fullerenes (461), singly bonded fullerene dimers (462), and acetoxylated fullerene derivatives (463) (Scheme 114). In addition, interconversions among the three types of phosphorylated fullerene derivatives have also been investigated. ... 

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