Chromium, pentacarbonyl reaction with amines

The nitrogen atom in a-ferrocenylalkylamines generally shows the same reaction pattern as that in other amines alkylation and acylation do not provide synthetic problems. Due to the high stability of the a-ferrocenylalkyl carbocations, ammonium salts readily lose amine and are, therefore, important synthetic intermediates. Acylation of primary amines with esters of formic acid gives the formamides, which can be dehydrated to isocyanides by the standard POClj/diisopropylamine technique (Fig. 4-16) [92]. Chiral isocyanides are obtained from chiral amines without any racemization during the reaction sequence. The isocyanides undergo normal a-addition at the isocyanide carbon, but could not be deprotonated at the a-carbon by even strong bases. This deviation from the normal reactivity of isocyanides prompted us to study the electrochemistry of these compounds, but no abnormal redox behaviour, compared with that of other ferrocene derivatives, was detected [93]. The isocyanides form chromium pentacarbonyl complexes on treatment with Cr(CO)s(THF) (Fig. 4-16) and electrochemistry demonstrated that there is no electronic interaction between the two metal centres. 

Substituted bicyclo[ . 1.0]alkanes may also be obtained by condensation of secondary amines with 2-haloketones. A variety of nucleophilic reactions can be carried out on the intermediate cyclopropaniminium salt 116251 (Scheme 108). Competing alkene scission and cyclopropanation occurs on reaction of enamines with pentacarbonyl-chromium carbene complexes252 (Scheme 109). N-Silylated allylamines and their derived N-silylated enamines undergo rhodium or copper catalysed cyclopropanation by methyl diazoacetate253 (Scheme 110). 

Similarly, the pentacarbonyl[(2-cyclopropyl-2-dimethylaminovinyl)ethoxycarbene]chromium complex 27, prepared from pentacarbonyl[cyclopropylethynyl(ethoxy)carbene]chromium 19 and dimethylamine (or other secondary amines), reacted with pent-l-yne or phenylethyne to give 1-propyl- or 1-phenyl-substituted 5-cyclopropyl-5-dimethylamino-3-ethoxycyclopenta-1,3-diene in 62-77% yield.With substituents other than cyclopropyl yields were drastically lower unless the reaction of the carbene complex was performed in pyridine or acetonitrile. 

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