Oxorhenium-catalyzed deoxydehydration of glycols and epoxides

Average reactions. Toxic solvents.



Deoxydehydration of glycols and epoxides catalysed by oxorhenium complexes is described in this paper. Methyl trioxorhenium (MTO) was the choice as the catalyst for the reactions. The use of other catalytic conditions for such reactions are reported in literature but the improvement claimed in this paper is described by the authors as “In the quest to develop new and cheaper DODH processes, at least in part, for the conversion of renewable cellulosic biomass into valuable chemicals we revisited the chemistry of MeReO3 and developed a modified process which requires a small amount of a primary or secondary alcohol as a reductant compared to earlier reports. 8,11

The Reaction of Interest:

Practical observations:

The use of MTO as catalyst was straightforward. The authors have mentioned that “The reaction between the glycol and 5-nonanol was first conducted in benzene with 10 mol % MTO in a thick-walled sealed glass tube at 90 ˚C, resulting in the quantitative conversion of styrene diol into styrene (97%)”. This process seems rather cumbersome and not high-yielding when the reaction was done in refluxing conditions using benzene. The yield of styrene (entry 1, Table 1) was only 78%.

The choice of benzene is not a good one. It is obsolete for practical purposes due to its carcinogenicity. The results obtained with toluene were even worse with very less yields.

The reaction times for all the reactions are extremely long and epoxides hardly produce the desired compounds.

The choice of alcohols for was fairly good, but the reactions do not work with smaller secondary alcohols.

The authors have wrongly mentioned rhenium as ruthenium in Results and Discussion section as “To understand the mechanism of ruthenium-catalyzed deoxy-dehydration, we examined the catalyst activity as a function of dependence on the solvent, temperature, and reductant”.

Modifications/ Comments:

I found the reports to be over-emphasizing the effectiveness of the procedure. In fact, the procedures from the reference no. 12 are far easier, less time consuming and high yielding.

The only really working reaction was the formation of styrene. Apart from that, the yields do not show it to be a promising methodology.

Use of benzene as solvent should be avoided altogether. Greener approaches should be explored for such processes.

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