2.1.1 Introduction

Manganese (Mn) is the 12^th most abundant element (0.1%) on earth’s crust with atomic number 25. Though manganese exists with the oxidation states from –3 to +7, the common oxidation states are +2, +3, +4, +6 and +7. The +2 oxidation state, which has a pale pink color due to spin forbidden d-d transition is found in living organisms for essential functions. The manganese in the oxidation state +7 is deep purple in colour and a strong oxidizing agent (Mn⁺⁷ + 5e- → Mn⁺²).

2.1.2 Manganese(III)  Reagents and Catalysts

2.1.2.1 Selective Oxidation of Benzylic and Allylic Alcohols

A combination of Mn(OAc)₃ and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) has been used for the selective oxidation of benzylic and allylic alcohols. The reaction works under mild conditions at ambient temperature (Scheme 1).

Scheme 1

 

2.1.2.3 Oxidation of Sulfides to Sulfoxides

Oxidation of sulfides to sulfoxides is one of the important transformations in organic synthesis. Sulfides could be selectively oxidized to sulfoxides in good yields with hydrogen peroxide in the presence of a manganese(III) Schiff-base complex 1 under ambient conditions (Scheme 2).

Scheme 2

2.1.2.4 Asymmetric Epoxidation of Alkenes

Jacobsen and Katsuki groups have explored asymmetric expoxidation of unfunctionalized alkenes using chiral Mn(III)-salen complexes in the presence of terminal oxidants such as PhIO and NaOCl (Scheme 3-4).  The most interesting feature of the reaction is that simple alkenes are oxidized with high asymmetric induction. This process has now been extensively used in pharmaceutical industries.