Phosphonations, manganese acetate

Dimethyl H-phosphonate reacts with calcium dichloride at 70 °C, with manganese acetate at 112 °C, and with calcium nitrate at 115 °C. Kinetic investigations [304] have shown that the reaction takes place as a bimolecular substitution of the second order. There is no difference in the reaction rate of the substitution of both alkyl groups. In general, it decreases with increase in the length of the carbon chain in the alkoxy group. 

P-Ketoesters are oxidized at the a-position with oxygen in the presence of either manganese(II) acetate " or cobalt(II) chloride." Cyclic allyl phosphonates give y-acetoxy a,p-unsaturated phosphonates when they are exposed to oxygen in HOAc containing PdCl2 and isopentyl nitrite. " Tertiary amine oxides are formed under cooxidation conditions, that is, O2, Fe203, and isovaleraldehyde. "... 

The functionalization of C—H bonds remains one of the most difficult challenges in synthetic chemistry, and while a number of catalysts have been developed to promote specific reactions, general approaches are quite rare. Many of these reactions were proposed to occur through radical processes however, the precise mechanism for many reactions remains unknown. Several versions of this chemistry have been used to generate arylphosphonates, and the nature and reactivity of phosphorus-based radicals have been reviewed [396]. Some of the earliest reports on this chemistry were based upon using anionic oxidation [397], peroxides [398], cerium nitrate [399], and manganese(lll) acetate [400]. The following sections will outUne specific examples of successful C—H phosphonations. 

SCHEME 4.247 Manganese(III) acetate-promoted phosphonation of heteroarenes [403]. 

An effective C—H phosphonation of pyrimidylphosphonates has been reported (Scheme 4.250) [405]. Manganese (HI) acetate served as the promoter for this chemistry, and simply heating to 50 or 80 C in acetic acid successfully phosphonated the C-5 position on the pyrimidine. Curiously, the cerium nitrate [399] was an ineffective catalyst for this transformation. An attractive aspect of this chemistry was that the primary and secondary alcohols on the sugar did not need to be protected prior to the phosphonation reaction. One drawback to this approach was that three equivalents of the manganese reagent and four equivalents of the secondary phosphite were needed for high conversions. However, these issues are overshadowed by the ability to functionalize a C—H bond under such mild conditions. Overall, this is a very attractive process for the synthesis of pyrimidylphosphonates. 

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