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I am trying to understand this quote in a section on "Biotransformations of Organic Molecules" from a handbook I am reading:

[...] Euglena gracilis could convert acetophenone to 2-phenylethanol; however, its enantioexcess is very poor (10%) (Takahashi, 1994).

Ref.: Takahashi, H., 1994. Biotransformation of terpenoids and aromatic compounds by some microorganisms. PhD thesis. Tokushima Bunri University, pp. 1–115.

What does "enantioexcess" mean in this context? Neither of those molecules are chiral, right? I cannot spot any asymmetric C atoms. Nor any higher order symmetries.

I looked up Wikipedia entries on both molecules and neither mention any d / l isomers.

What am I missing?

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    $\begingroup$ Scifinder search for "Euglena gracilis acetophenone" doesn't bring up anything except for one Chinese patent, CN 103757061, which suggests that the product is 1-phenylethanol. $\endgroup$
    – orthocresol
    Commented Dec 29, 2016 at 9:03
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    $\begingroup$ It should have been 1-phenylethanol, I guess. Look at the molecules again. Suppose you reduce the carbonyl group; where would be your hydroxyl? $\endgroup$
    – Ivan Neretin
    Commented Dec 29, 2016 at 9:10
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    $\begingroup$ I didn't read the patent in detail because technical documents in different languages are really difficult to read. However I did spot that the reduction product is (S)-1-phenylethanol. (These authors also got >99% e.e., so...) Anyway, I would assume it is a typo and the authors meant 1-phenylethanol, because that is the product of direct reduction of acetophenone and it is chiral. $\endgroup$
    – orthocresol
    Commented Dec 29, 2016 at 9:11
  • $\begingroup$ @orthocresol That makes a lot of sense. I think you are right and the Handbook or the original author got his ones & twos mixed up. I was just wondering if talking about an enantioexcess in the context of 2-phenylethylalcohol made any sense. I think not. $\endgroup$
    – curious_cat
    Commented Dec 29, 2016 at 9:16
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    $\begingroup$ @curious_cat You think right; ee in non-chiral molecules wouldn't make any sense. $\endgroup$
    – Ivan Neretin
    Commented Dec 29, 2016 at 9:30

1 Answer 1

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The Ph.D. thesis that is cited is probably near-impossible to obtain and may very well be in Japanese, which certainly doesn't help.

2-Phenylethanol is not a chiral compound, so as Ivan pointed out, the concept of enantiomeric excess (ee) makes no sense. The fact that the direct reduction product of acetophenone (e.g. via hydride transfer) is 1-phenylethanol strongly suggests that this is nothing more than a typo. However, I thought that it might be fun to go to great lengths to verify the applicability of Occam's razor to this situation.

A SciFinder search for "acetophenone Euglena gracilis" did throw up one Chinese patent, CN 103757061. There is a translation on Google Patents. The patent describes the reduction of various prochiral ketones with various microphytes (also known as microalgae) to form chiral alcohols, with near-perfect enantioselectivity.

I managed to figure out the technical terms in the patent. E. gracilis in Chinese appears to be 纤细裸藻 (xian1 xi4 luo3 zao3). Acetophenone is 苯乙酮 (ben3 yi3 tong2). The patent does not, in fact, describe the reduction of acetophenone by E. gracilis, and the text in Table 2 is so small that I cannot quite make out what compounds they tested (they seem to be aliphatic). Perhaps they simply did not include this combination in the patent if the enantioselectivity they obtained was very low.

However, there are experiments with different species that lead to the reduction of acetophenone to "S-苯乙醇" (ben3 yi3 chun2). This translates to (S)-phenylethanol, and leaves open the possibility of it being either 2-phenylethanol or 1-phenylethanol. Since only the 1-phenyl isomer is chiral, I assume the reduction product is (S)-1-phenylethanol.

This is supported by Figure 1 in the patent. I have scribbled some loose translations. In hindsight I think "Photosystem" should be "Light-harvesting complex". The text translates to "Light-harvesting system" which is a bit of a mix and match of the two, so I wasn't sure what to go for.

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Having $\ce{R_1 = Me}$ and $\ce{R_2 = Ph}$ would mean acetophenone is reduced to 1-phenylethanol.

In conclusion, I would therefore assume with confidence that the handbook contains a typo and the authors meant 1-phenylethanol.

(On a side note, the Chinese also got >95% ee for all their acetophenone reductions, go figure...)

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    $\begingroup$ It’s always good to have a Chinese around who can answer these types of questions <3 $\endgroup$
    – Jan
    Commented Dec 29, 2016 at 23:33

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