Manganese reagents

Acylation reactions can also be greatly improved in this way, with t-alkyl- or sec-alkyl-manganese reagents reacting with acid chlorides in excellent yields [123]. The related addition-elimination to 3-ethoxy-2-cyclohexenone is also improved, resulting after acidic aqueous workup in 3-methyl-2-cyclohexenone [125]. The perilla-ketone 126 was prepared in an improved yield using copper(I) catalysis (Scheme 2.58) [129]. 

Scheme 2.59. Copper-catalyzed alkylation of alkyl manganese reagents.

Scheme 2.60. Copper-catalyzed acylation of benzylic manganese reagents.

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

Figure 4.11. Examples of redox-initiated radical reactions. Samarium diiodide reduction of the bromide gives a radical that cyclizes faster than the second reduction reaction. Manganese triacetate oxidation of the P-keto ester gives an enol radical that is not further oxidized by the manganese reagent. The IBX oxidizes anilides to the corresponding radicals. Hexamethylphosphoramide = HMPA and Tetrahydrofuran = THE.

This reaction is clearly related to forming esters and lactones by reaction of carboxylic acids with alkenes (15-6), but the manganese reagent leads to... 

Manganese. Li and Chan [34] and Rieke [35] have independently reported that manganese reagents can accomplish the pinacol coupling of aromatic carbonyl compounds. In the Li/Chan study, reaction of an array of aromatic aldehydes proceeded in good to excellent yield in the presence of Mn/Ac0H/H20, albeit with poor diastereoselectivity (Eq. 3.15). Under these conditions, aliphatic aldehydes are reduced to the corresponding alcohol, and ketones (aromatic or aliphatic) do not react. 

A manganese reagent prepared from RLi or RMgCI and MnCb reacts with cyclohexenone in a 1,4-addition (Scheme The structure of the reagent is not known and the formula e.g. RaMn ) is at-... 

Chlorination of Alkenes. A novel stereospecific dichlorination of electron rich alkenes has been reported using a manganese reagent generated from Benzyltriethylammonium Chloride and oxalyl chloride (eqs 13-17). No oxygenation b)q)roducts are observed. 

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