Reductive Coupling Reactions of Aryl Aldehydes

To date, no report has appeared using manganese metal. We wish to report a mild method for reductive coupling reactions of aryl aldehydes, aryl ketones, andaldimines [100]. 

Various aromatic aldehydes were reacted with the active manganese prepared from using manganese chloride. Table 8.24 reports that simple treatment of aryl aldehydes with active manganese gives the corresponding 1,2-diols in moderate to good isolated yields under very mild conditions, rt in THF. The results are shown in Table 8.24. 

It is proposed that the reaction proceeds via a single-electron transfer (SET) [72b]. With the active manganese supplying the electrons, the ratio of manganese to aldehyde was found to be very important with respect to overall yield. Entry 1 in Table 8.24 shows that a higher yield is obtained using 2 equiv of Mn and 1 equiv of aldehyde. This appears to be general for all aldehydes and ketones. All of the reactions with different mole ratios were carried out at rt in 

30 min. Reaction temperature had little effect on the yield. Reactions at it and 0°C in 30 min gave 64 and 62% isolated yields, respectively (Table 8.24, entry 1). 

In addition to the coupling of aryl aldehydes, it should be mentioned that a multicompound mixture was obtained from the reactions using aliphatic aldehydes with Mn. According to Yanada s work, the radical intermediates from the aliphatic aldehydes are unstable and do not have enough time to react with each other to give diols. 

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