Nicholas reaction

Under the Nicholas reaction is defined as a chemical reaction that is used to implement stabilized propargyl cations with nucleophiles. The triple bond of propargyl alcohol used, or ether is hereby protected from the formation of the cation by Dicobalthexacarbonyl. By Nicholas reaction alkylated alkynes can be obtained.

Mechanism

Propargyl form of two resonance structures, so that a nucleophile may attack at two different positions and no single product is formed.

Forms a propargyl ether or a propargyl (R4 = H) from an acid-catalyzed propargylic. This is mesomeriestabillisiert (A and B ), allowing a nucleophilic attack at two different positions (R1 to R3 are alkyl or aryl group).

To increase the selectivity to the desired alkyne alklyierte wherein Nicholas reaction first the triple bond is protected by reaction with Dicobalthexacarbonyl (2). Dicobalthexacarbonyl can be obtained from dicobaltoctacarbonyl. The cation is then formed by reaction with a Lewis acid from the alcohol or from the ether ( 3a and 3b). Since the triple bond is protected, can not form the resonance structures shown in the upper diagram itself. The nucleophile remains only a position where it can attack (3a). Thus, only the desired alkylated alkyne is obtained, which is released by the subsequent oxidative cleavage of the cobalt complex (5).