α-Ferrocenylalkyl carbonates: Reagents for ferrocenylalkylation reactions under mild neutral conditions

Abstract α -Ferrocenylalkyl carbonates were demonstrated to be the species for ferrocenylalkylation reactions. The α -ferrocenylalkyl carbonates FcCH(R)OCOOEt ( 1a-c , Fc = ferrocenyl) were easily generated in situ by a reaction of the α -ferrocenyl substituted alcohols FcCH(R)OH ( 5a-c ) with equimolar amounts of n -BuLi followed by EtOCOCl in THF or Et 2 O at 20 °C. Due to the low thermal stability, carbonates 1a-c readily undergo a heterolytical decay furnishing the α -ferrocenyl carbocations FcCH(R) + ( 2a-c ) and carbonate anion R’OC(O)O − ( 3 ). The last one reversibly loses a molecule of CO 2 giving rise anion EtO − that is capable of deprotonating a pre-nucleophile NuH (MeCOCH 2 COMe, dimethyl malonate, etc.) giving rise the corresponding nucleophile Nu − . The interaction between 2a-c and Nu − produces the α -ferrocenylalkylation products FcCH(R)Nu ( 4a-c) in moderate to high yields. The sequence above is a novel α -ferrocenylalkylation procedure easily proceeding under mild and neutral conditions. Due to these features, it may be useful for α -ferrocenylalkylations of either thermally labile compounds or the substrates susceptible to acidic conditions. The stability of carbonates 1a-c was found to decrease as the stability of the corresponding carbocations 2a-c increases. With 2-mercapto-1-methylimidazole, the α -ferrocenylalkylations proceed as the N -alkylation processes.