Alternative CO Sources

After extensive screening of various aldehydes to optimize the reaction conditions, it was found that aromatic aldehydes were able to serve as a carbon monoxide source, in which the electronic nature of the aldehydes is responsible for their ability to transfer CO efficiently [24]. Consequently, aldehydes bearing electron-withdrawing substituents are more effective than those bearing electron-donating substituents, with pentafluoro-benzaldehyde providing optimal reactivity. Interestingly, for all substrates tested the reaction is void of any complications from hydroacylation of either the alkene or alkyne of the enyne. Iridium and ruthenium complexes, which are known to decarboxylate aldehydes and catalyze the PK reaction, demonstrated inferior efficiency as compared to 

The origination of CO from the aldehyde was confirmed by mechanistic studies, in which labeling experiments demonstrated that the majority of the CO was derived from the aldehyde. For example, when the reaction was carried out with 1.2 equiv of aldehyde under a partial pressure of CO instead of argon, only about 10% of the iso-topically labeled product was formed. This indicated that the source of the CO incorporated into the PK product was derived primarily from the aldehyde (Eq. 10) [24c]. An additional experiment with 2-naphthylaldehyde 49 (2 equiv.) demonstrated that only 1 equiv. of aldehyde was consumed (Eq. 11) [24aj. 

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Regarding the CO source, alternative CO sources should be discovered and applied in organic synthesis. Besides M(CO)x, formic acid and aldehyde, can we use CO2 in carbonylation by in situ reduction ... 

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