AvBaeyer
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Cobalt (II) Chloride catalysis of ester formation
Introduction
Recently Loptr referenced a paper which claimed that p-toluensulfonate ester formation is catalyzed by cobalt (II) chloride hexahydrate [1]. A
follow-on paper [2] from this research group claimed that acetylation of alcohols was also catalyzed by cobalt (II) chloride hexahydrate. Two things
bothered me about these papers. First, the claims seemed just too good to be true. Second, I could not I could not come up with a believable mechanism
for the catalyzed reactions. The authors postulate a mechanism for the acetylation reaction [2] but it does not appear to be reasonable.
Given my doubts about the reported results, I decided to repeat an experiment described in the acetylation paper. One of the deficiencies of the paper
is that the authors report no data regarding the progress of the reaction, i.e. , no hplc, no tlc or any other analytical method for following the
reaction. I chose to react benzyl alcohol with acetic acid following procedure B as described in the experimental section. Benzyl alcohol was chosen
since I had authentic benzyl acetate to use as a product standard. Table 1 in the paper shows a one hour reaction time and 98% yield (entry 1). I
scaled up the reaction somewhat to help with eventual work-up and product isolation. I also decided to follow the reaction by tlc.
Experimental
Analytical method. The reaction mixture supernatant (5 drops) was added to ca. 0.5 ml 5% potassium carbonate solution. The mixture was gently shaken
until CO2 stopped. The mixture was extracted with ca. 0.2 ml ether and a 5 micro-L aliquot was spotted on a silica gel tlc plate. A control sample of
benzyl acetate was also spotted in a second lane. The plate was eluted with 75% hexane – 25% ethyl acetate. In this system the starting benzyl
alcohol showed an Rf = 0.6 and benzyl acetate showed Rf = 0.85. Plates were visualized with uv and iodine.
Benzyl alcohol (0.54 g, 5 mmole) and cobalt (II) chloride hexahydrate (0.06g, 0.025 mmol, 5 mole%) were taken up in acetic acid (5ml). The reaction
mixture was immersed in a preheated bath at 60C and stirred magnetically. Aliquots were analyzed after about 20 minutes and 1 hour. The 20 minute
point showed no evidence of benzyl acetate. After 1 hour there was maybe a hint of benzyl acetate formation. The reaction was discontinued at this
point.
Results and Discussion
The results from my experiment show no formation of benzyl acetate during the course of the reaction time reported by the authors [2]. It is
reasonable to expect that one would see the appearance of the ester during the course of the reaction if the catalyst was acting as posited. I submit
that there is no catalytic activity associated with cobalt (II) chloride hexahydrate. If the authors really isolated benzyl acetate, where did it come
from? The most reasonable explanation at this point is that the ester forms during the concentration step in the work up. At that point, the benzyl
alcohol is present in a large excess of acetic acid which is being concentrated under vacuum. This procedure would remove not only acetic acid but
also water resulting from the in situ auto-esterification. This explanation is obviously speculative. I do not have access to a rotary evaporator. It
would be informative if someone could evaporate a mixture of benzyl alcohol and acetic acid in the proportions described in the paper and determine if
the ester is formed.
Overall, I submit that the acetylation paper is incorrect in claiming any catalysis by cobalt (II) chloride hexahydrate. I also submit that the
tosylation paper has serious questions regarding cobalt (II) chloride hexahydrate catalysis but this remains to be shown by further experiments.
[1] https://www.sciencemadness.org/whisper/viewthread.php?tid=91...
[2] S. Velusamy, et a., Tetrahedron 61, 2011-2015 (2005)
AvB
Attachment: Cobalt(II)-catalyzed direct acetylation of alcohols with acetic acid.pdf (170kB)
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DJF90
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Nice work AvB. Its not unusual to see methodology such as this in the literature where the claims are a little farfetched or beyond reasonable
mechanistic explaination. Its good that you've taken the time to give it a go, and have reported it as not providing the stated product.
Perhaps the authors confused anhydrous acetic acid with acetic anhydride...
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