Hydroxycarbene grouping

Older IR-imwstigalions by Kolbel et at. led to the assumption of surface formyl or hydroxycarbene groups [113]. More recent measurements on supported ruthenium under synthesis conditions showed absorptions for mr ecularly adsorbed CO and for formate and CH species. As a result of deuieration experiments, however, the latter were concluded not to be reaction intermediates (113. 106], It was also shown that the production of methane and ethane continued for a significant period after CO had been removed from tlic reaction mixture and after the disappearance of all IR-observable COads species [106J. It was concluded that product formation occurs via carbidic intermediates. /n-situ IR studies at higher pressures (3 bar) revealed formation of CHjf species with absorptions in the 3000 cm region (114). 

The formation of hydroxycarbenes would be promoted by 0-protonation of a coordinated carbonyl group, followed by a hydride transfer 7]. 

The reaction of hexacarbonyls of group VIB gives the anionic [MCOR(CO)5] complexes. Their protonation with mineral acids in water gives acetaldehyde (R = Me) and benzaldehyde (R = Ph), respectively. The formation of benzoin and benzhydrol derivatives in the Fe(CO),-LiR reaction is suggested to occur via C—C coupling of the unstable hydroxycarbene with benzaldehyde, whereas benzhydrol originates from further attack of lithiated Fe(CO)5 [Eq. (0] by LiR, followed by hydrolysis. 

An alternative route to surface-methylene groups involves stepwise reduction of carbon monoxide rather than dissociative chemisorption. Scheme 4 describes a number of plausible intermediates, all of which have been seen as isolable, well-characterized molecular compounds [11] carbonyl (a), formyl (b), hydroxycarbene (c), hydroxymethyl (d), and methylene (e). They are summarized in Ref. [8-12]. 

M—C TTT 0 and are susceptible to nucleophilic attack (p.332). Alkyl and aryl-lithium reagents add to a carbonyl group of the hexacarbonyls M(CO) (M = Cr. Mo, W) to give an anionic species which can be formulated either as an acyl or a carbene complex. Treatment with a strongly electrophilic trialkylox-onium salt (a source of R" ) or protonation followed by methylation of the resulting hydroxycarbene derivative, affords the product. 

A third and very practical alternative is the resolution of racemic acyliron complexes by crystallization with (5)-(+)- or (/ )-(-)-camphersulfonic acid via the diastereomeric hydroxycarbene salts and subsequent neutralization. Conversion of the enantiomerically pure acetyliron complexes to a-alkoxy complexes and subsequent trimethylsilyl triflate-mediated removal of the methoxy group provides enantiomerically pure (ethylidene)iron complexes.Methylation of alkynoyliron complexes with Meerwein s salt has been reported to give cationic methoxy-substituted alkynylcarbenes, which can be transformed into the corresponding (aminocarbene)iron complexes upon reaction with primary amines. " Meerwein s salt can also be used to form cationic... 

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