• Direct 'uphill' isomerization of numerou

    From ScienceDaily@1:317/3 to All on Thu Feb 24 21:30:42 2022
    Direct 'uphill' isomerization of numerous olefin classes

    Date:
    February 24, 2022
    Source:
    Princeton University
    Summary:
    Researchers have used excited-state electron transfer events to
    'pump' olefin molecules up and then have them fall back down
    through a series of favorable steps to ultimately transform into
    a less stable isomeric form.

    This essentially allows internal olefins, or alkenes, to migrate
    along the carbon chain to the terminal position -- a kind of
    musical chairs that shunts the double bond to a position of less
    overall stability.



    FULL STORY ========================================================================== Chemists have long sought methods to convert more stable internal olefins
    into less stable terminal olefins. Isomerization reactions that proceed
    against a thermodynamic bias, as this one would, are generally challenging
    for conventional thermal catalysis.


    ==========================================================================
    With an innovative use of photocatalysis and chromium co-catalysis to
    provide an energy offset, however, the Princeton University Department
    of Chemistry reports a general method to generate less-stable olefins
    from internal olefins catalytically.

    Researchers in the Knowles Lab used excited-state electron transfer
    events to "pump" olefin molecules up and then have them fall back down
    through a series of favorable steps to ultimately transform into a less
    stable isomeric form.

    This essentially allows internal olefins, or alkenes, to migrate along
    the carbon chain to the terminal position -- a kind of musical chairs
    that shunts the double bond to a position of less overall stability.

    The research marks an exciting development because terminal olefins
    are useful starting points in a wide range of chemical processes and as building blocks in industrial products. This research gives scientists
    a new tool to generate more of them.

    The lab's paper on the research, Contra-Thermodynamic Positional
    Isomerization of Olefins, was recently published in the Journal of the
    American Chemical Society (JACS).

    The realization of contra-thermodynamic alkene isomerization has been
    a long- standing challenge for catalysis. By combing through and then
    combining some of the precedents he found in literature, Kuo Zhao,
    a fifth-year graduate student in the lab, was able to come up with a light-driven method for the direct 'uphill' isomerization of numerous
    olefin classes.

    In the paper, Zhao describes how stepwise proton coupled electron transfer (PCET) activation of a more thermodynamically stable olefin substrate
    is mediated by an excited-state oxidant and a Bro/nsted base to give
    an allylic radical. That radical, in turn, is captured by a chromium
    (II) co-catalyst to generate an allylchromium(III) intermediate, which eventually undergoes regioselective protodemetalation to deliver a
    less-stable terminal alkene.

    "This work is another example from our lab of using photochemistry
    to accomplish an uphill process. Traditionally, this is impossible,
    by definition, using conventional ground state catalysis. But with photochemistry, this can be achieved catalytically," said Zhao. "When I
    looked back into it, I found from the literature that there's a way to
    solve this problem by introducing chromium catalysis.

    "Basically, you generate an allyl chromium intermediate and do an in
    situ protodemetalation to get the terminal olefin, which is the olefin
    you want in this transformation.

    "As to how to use it, we'll leave that to future chemists," Zhao
    added. "But we're showing in this research that this previously impossible transformation is now possible." Zhao said he worked on the problem
    for about a year, dipping into literature and then trying a series of transition metal catalysts to drive the transformation, which resulted
    in a low yield of the desired olefin and sent him back to the drawing
    board. Other graduate students within the Knowles lab had been working
    on this problem over the years utilizing only photoredox catalysis.

    "The Knowles lab had been looking to solve this problem for
    a number of years, but with little success," said Professor
    of Chemistry Robert Knowles. "By merging chromium catalysis and
    photoredox catalysis, Zhao was able to solve this challenging problem." ========================================================================== Story Source: Materials provided by Princeton_University. Original
    written by Wendy Plump.

    Note: Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Kuo Zhao, Robert R. Knowles. Contra-Thermodynamic Positional
    Isomerization of Olefins. Journal of the American Chemical Society,
    2021; 144 (1): 137 DOI: 10.1021/jacs.1c11681 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/02/220224140647.htm

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