Cationic iron complexes carbanions

Although alkynes are highly reactive toward a wide range of transition metals, few instances of metal-catalyzed reactions of carbanions with alkynes are known. The most extensively developed system involves cationic iron complexes of internal alkynes. These complexes underwent alkylation by a range of carbanions to produce stable [Pg.582]

Cationic iron-alkene complexes also participate in an unusual cycloaddition process, wherein electron-deficient alkenes are attacked by nucleophilic o -allylic Fp complexes, generating stabilized carb-anions and cationic alkene-iron complexes. Attack of the carbanion on the alkene forming a five-membered ring completes this process (Scheme 13). Oxidative removal of the iron produces useful organic compounds.19-21... 

By reaction of cationic carbonyl complexes with lithium carbanions, neutral acyl complexes are prepared. Whereas treatment of [> -CpFe(CO)3]BF4 with (a) PhLi gives the expected > -CpFe(CO)2 [C(0)Ph] in 80% yield, with (b) MeLi only traces of > -CpFe(CO)2 [C(0)Me] can be detected . This complex and other phosphane-substituted acyl compounds of the type f -CpM(CO)L[C(0)Me] [M = Fe, Ru L = CO, PPh3, P(hex)j], as well as >/ -CpMo(CO)2P(hex)3[C(0)Me] (prepared by different routes), are protonated with and alkylated with [R3 0]BF4 reversibly, yielding cationic hydroxy- and alkoxy(methyl)carbene complexes, respectively . The formation of the ( + )- and ( —)-acetyl complex / -CpFe(C0)(PPh3)[C(0)Me] from the ( + )-and ( —)-conformers of optically active > -CpFe(C0XPPh3)[C(0)0-menthyl] and MeLi occurs with inversion of configuration at the asymmetric iron atom . 

Nucleophilic attack on neutral complexes of ii -diene ligands is less common than attack on cationic complexes of these ligands but is known. In one case, Semmelhack reported the reactions of iron complexes of acyclic ii -dienes with reactive carbanions such as LiC-MCjCN and LiCHPhj (Equation 11.47). At -78 °C, the reaction is rapid, and kinetically controlled. Attack occurs at an unsubstituted, internal position to give an unstable o-alkyl Ti -olefin complex. This addition is reversible below 0 °C, and the more stable product is then generated from nucleophilic addition at a terminal position of the diene to give the thermodynamically more stable T -allyl complex. Cyclohexadiene complexes are similarly alkylated by a range of carbanions. ... 

Methodology of this type has also been applied to the synthesis of the related compounds trichodiene and trichodermol, and also to the alkaloid limaspermine. The following protocol is representative of carbanion addition to cationic tricarbonyl(Ti -cyclohexadienyl)iron complexes. 

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