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Chiral Reactions With Chiral Electrons

Here’s a weird one for the Strange Things to Do With Chirality file. A multi-center team (Hebrew Univ., Weizmann Inst., Univ. Modena, and Pitt) report that electron spin, of all things, can be a chiral reagent. Some readers will sit up at that phrase, and others will (understandably) wonder what I’m talking about. Perhaps that happens most mornings around here, but I’m pretty sure about this instance.

I Am Not a Physicist, but here’s the brief background. Even us knuckle-dragging synthetic organic types are aware that electrons come in “spin-up” and “spin-down” forms (we had to fill out those electron configuration diagrams for test questions back in first-year chemistry, for one thing). But we certainly don’t spend much time thinking about them. What that means, though, is that electrons have a type of intrinsic angular momentum, a built-in property that’s quantized (they can’t spin a bit faster or a bit slower) and cannot be shed or cancelled out. And since electrons have linear momentum as well, they end up with helicity. If the spin and the linear momentum are in the same direction, that’s right-handed/positive helicity, and if they’re opposite each other (no other angles are possible, quantization and all), then it’s left-handed/negative. You can sort of imagine them corkscrewing through space, although the standard warning applies: if you think of elementary particles as little round physical objects, spinning on their axes and caroming along like ping-pong balls, you will come to grief because the reality is a lot stranger and very hard (perhaps even impossible, for humans) to have a really accurate mental picture of.

There’s been a lot of work put into exploiting an electron’s spin to go along with its charge (which we exploit constantly, every time we turn on an electric switch) which defines the field of “spintronics”. As for chemistry, it’s been shown that chiral molecules can act as “spin filters” in electrochemistry, which is the background of this latest paper. In it, it’s further demonstrated that such spin-polarized electrons can do chiral chemistry. There are key background experiments with association of chirally biased molecules on electrode-surface monolayers, but I wanted to highlight this part: an enantioselective reduction which indeed seems to be determined purely by the spin of the electrons coming from the anode.

The setup is an electrode that has nanometer-scale layers of nickel and gold, and has a strong permanent magnet next to it which can be flipped up (parallel) or down (antiparallel) normal to the surface. Charged particles with spin have a magnetic moment, so this will affect them. One of the great triumphs of quantum electrodynamics, QED, is the prediction of the electron’s magnetic moment (well, its g-factor constant) to an insane degree of accuracy – quantum mechanics as a whole is unnervingly robust, but this part is especially covered with a solid layer of armor.

Shown at rate are data from the reduction of camphorsulfonic acid to the corresponding borneol.  You can see that the cyclic voltammetry curves are different when you start with either the R and S enantiomers and run the magnet up/magnet down experiments. But since I realize that most people’s eyes glaze over at the sight of CV plots, check out the next two. Panel D is a circular dichroism experiment – the straight-across black line is the CD spectrum of racemic CSA, and by gosh, it’s flat, as it should be. The blue and red lines are measured after the electrochemical reduction, though, magnet-up and magnet-down, and it clearly seems to have produced an enantiomeric excess by decomposing one component of the racemic mixture more than the other. And indeed, panel E shows this as a function of time. The longer you run the current, the more chiral the resulting solution becomes, and the sign depends perfectly on how you have the magnet oriented to the electrode surface.

The enantiomeric excesses themselves are not huge – they get up to around 11% e.e. – but that’s way more than you need to prove an effect. As the authors note, if you used a more magnetic layer on the electrode, you’d presumably see an even more pronounced effect. You don’t get chiral resolution out of nothing, and the only thing chiral in this whole setup is the helicity of the electron spins. We’re going to have to add another one to the list of unusual ways to influence chirality, and it will be quite interesting to see if something like this can be turned into preparative chemistry!

19 comments on “Chiral Reactions With Chiral Electrons”

  1. Jake O says:

    Camphorsulfonic acid is already chiral, so they’re really making diastereomers. Unless I totally misunderstand the experiment (wouldn’t be the first time!) I’d like to see this repeated on a prochiral substrate.

    1. DrT says:

      I’d also like to see that experiment, but my understanding is that they’re not making diastereomers, but starting with a racemic mixture and selectively reducing just one enantiomer to borneol, thus enriching the amount of the other enantiomer of CSA in solution. I wish the reviewers on the paper had insisted on using the term “enantiomer-selective” rather than “enantioselective”, since the later brings my mind to a different place? (Just my two cents).

  2. Uncle Al says:

    The 1970s (PMID: 896189) knew beta-decay isotopes selectively decomposed thin layers of solid racemic amino acids and such, William Bonner. However, helicity (in which direction does the Earth spin?) is not chirality. Helicity reverses sense if you look from the other side, Helicity at relativistic speeds increasingly becomes chirality.

    Helicity becomes chirality if you enforce orientation viewpoint. As with magnetic field choosing for C-13 molecules in bond scission and recombination photochemistry…it’s a trippy effect but not a production technique. Tesla+ permanent magnet Halbach arrays are not expensive.

    Saturate the effect. See whether you earn citations or a patents.

  3. Dr Acoustix says:

    Not that I want to be picky, but don’t electrons come from the cathode? (Not the anode as written in the blog.)

    1. Pedwards says:

      Electrons always flow from the negatively charged anode to the positively charged cathode.

      1. Karl says:

        Except in vacuum tubes, where they flow from the negatively charged cathode to the positively charged anode (plate).

      2. colintd says:

        I think the confusion is with the “inside” and “outside” of a cell. Outside a cell, electrons flow in the wires from the cathode (the negative terminal) to the anode (the positive terminal). Inside the cell, the electrons go from the anode to the cathode. It has to be this way otherwise all the electrons would end up in a big pile (sorry for the pun).

      3. Nick K says:

        I always thought that the anode was positively charged. After all, oxidations occur there, not at the cathode.

      4. Another Guy says:

        The historical definition of a cathode is the electrode to which cations migrate. Cations got their name based on this observation. Now we know that cations are positively charged and therefore are attracted to a region where cations are being depleted due to electrochemical reduction. I believe this definition goes way back to the Michael Faraday era. In a simple galvanic cell based on zinc and copper electrodes, the copper metal is the cathode since zinc will donate electrons (which travel through the external wire) towards the copper cathode.

    2. JM says:

      Well, the anodic reaction is the source of electrons, which are then pumped through the external circuit (e.g. potentiostat) to the cathode.

      Having said that, In this context, I think you’re right that it would be better to say the “cathode”, since from the point of view of the reduction reaction happening there, the local source (particularly of the spin-filtered electrons) is the cathode.

  4. Industry Guy says:

    Could this be the basis of chirality in the building blocks of life?

    1. rtw says:

      I was thinking the same thing.

  5. Eugene says:

    I do not understand enough about QM but it appears that the circularly polarized laser light, which has to do with the spin of the photon, and this effect which takes advantage of the spin of the electron seem to be related.

  6. Cb says:

    Origin of chirality is close to the big bang; today real enantiomers do not exist since the real mirror image of a chiral molecule would have positrons, anti-protons and anti-neutrons. A racemic mixture would be quite explosive;-) So what we call enantiomers are a sort of diastereomers

  7. David E. Young, MD says:

    Here is something on the side. If there are right and left that can be determined easily, might their be a way to utilize this concept to make a fast computer? I suppose you can but it probably would not be fast.

    1. Biochemist says:

      Well, yes it’s being investigated. There are some links to reviews on the wiki pages for spintronics.

  8. forwardslashs says:

    So if I want a better ee in my electrochemical reduction, I could always just spin my hotplate in the opposite direction?

  9. Mates' rates says:

    Is “shown at rate” a phrase that I should know? “Shown at right” might be the intent?

  10. JG4 says:

    I’m not the sharpest knife in the drawer, irrespective of what I may have been in the past, but that should be “shown at right…”

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