Tricarbonyl iron formylation

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared. Table 1 shows that the rates of substitution for (a) formylation by phosgene and V,iV-dimethylformamide, (b) acetylation by acetic anhydride and tin(IV) chloride, and (c) trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235) are all in the sequence furan > tellurophene > selenophene > thiophene. Pyrrole is still more reactive as shown by the rate for trifluoroacetylation, by the relative rates of bromination of the 2-methoxycarbonyl derivatives (pyrrole > furan > selenophene > thiophene), and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3]+ (Scheme 5) (2-methylindole ss V-methylindole > indole > pyrrole > furan > thiophene (73CC540)). 

Reaction of Complex III with acetyl chloride and aluminum chloride under typical Friedel-Crafts conditions affords acetylcyclobutadiene-iron tricarbonyl (VII) in high yields. The corresponding benzoyl derivative is similarly prepared with benzoyl chloride. Formylation with AT-methyl-formanilide and POCI3 produces cyclobutadienecarboxaldehyde-iron tricarbonyl (VIII), while chloromethylation with formaldehyde and HCl affords the chloromethyl derivative (IX). 

Chiral electrophilic cyclopropanes (63) are prepared in high enantiomeric excess starting from butadiene-iron tricarbonyl complexes (60) containing a non-complexed double bond. Reaction with diazomethane and decomposition of the resulting pyrazolines (61) in the presence of Ce" gave the corresponding chiral cyclopropanes (62). Breakdown of the dienic substituent of electrophilic cyclopropane (62) by means of ozonization resulted in the formation of formyl-substituted electrophilic cyclopropane (63) still carrying the asymmetric centre (equation 10) " . ... 

Enantiopure 2-formyl-3,4-dimethylphosphaferrocene has been prepared by column chromatography of the aminals from reaction with (i ),(i )-l,2-di(iV-methylamino)cyclohexane. Chiral pinene-fused cyclopentadienyl-containing phosphaferrocenes have also been prepared directly from the iron tricarbonyl complex PCp Q CO)2 z /-butyl-phosphole, where PCp = pinene-fused cyclopentadiene. A crystal structure of the product phosphaferrocene is shown as... 

Tricarbonyliron complexes of conjugated trienes react with diazoalkanes at the free (uncom-plexed) double bond. In the synthesis of dimethyl 2-formylcyclopropane-l, 1-dicarboxylate (48), the ceric ion served the double function of catalyzing the deazetization and removing the tricarbonyl iron protecting group. When the optically active iron carbonyl complex was used, the addition of diazomethane gave selectively one diastereomer and this was used to make optically active dimethyl 2-formylcyclopropane-l,1-dicarboxylate (>90% ee). A similar route was employed to make the optically active formyl cyclopropanes 49, precursors to optically active cis- and tran.v-chrysanthemic acids. 

Chiral tricarbonyl(butadienyl)iron complexes are easily accessible by resolution and show excellent diastereofacial selectivities with a variety of reagents55. The tricarbonyl(trienyl)iron complex 2 and methyl diazoacetate (copper bronze catalysis) give only two cis 7/m -isomeric cyclopropanes 5< These can be separated by column chromatography, and each diastereomer transformed into methyl traits- or 7.r-2-formyl-3.3-dimcthylcyclopropanecarboxylate (hemica-ronic aldehyde) by destructive cleavage of the diene complex auxiliary. The enantiomeric excess in these compounds is above 90%. 

The reactivity of olefins toward electrophilic reagents is modified considerably on coordination to a transition metal. Cycloctatetraene, for example, is usually polymerized by electrophilic reagents. However, tricarbonyl(cyclo-octatetraene)iron undergoes a facile formylation in reasonable yield. The coordinated formyl compound XIX can be used in further synthetic transformations, as shown below, and the tricarbonyliron group readily removed by oxidation with ceric ion [Eq. (55) (Johnson el al., 1969, 1971)J. 

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