Enough maudlin overtures. Now, on to the fun!
Strem has, as any synthetic guru would attest, the highest-quality metal precursors in the biz.* Now, you could spend a weekend cracking ampoules to find out, or just open to the Supporting Information of one of Jeff Bode's recent publications in Org. Lett. Perhaps you remember this reaction - SnAP synthesis of saturated heterocycles - best from a cheeky Derek Lowe tweet:
That's in reference to the stoichiometric incorporation of tin** in the reagent, which serves as a linchpin for the eventual transmetalation to a copper species and ring closure, neatly without disturbance of the ipso heteroatomic group.
Well, much to my surprise, Prof. Bode has climbed on the recent trend of showing one's work through tactful inclusion of smartphone pics to buoy up procedure adoption. Especially with fussy transition metals, valency, contaminants, poor environment, and a whole host of other factors lead to catalyst poisoning and color changes. In the SnAP case, the litmus test seems to be formation of a correctly ligated Cu(II) ion in lutidine relative to the (probable) hexaaquo cuprate species formed as a blue heterogeneous train wreck.
The kicker? The fairly indiscreet preference for the Strem copper(II) precursor over all other suppliers. Look at the change! Night and day, and key to making these reactions work.
You couldn't buy better advertising than this....right, Strem?
Bravo, Bode group! I look forward to seeing your colorful coupling chemistry in future reads.
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*Dear Strem: please send non-sequential $50 bills to See Arr Oh at Big City Company, USA
**SnAP. Get it? [drum kit]
Cu(OTf)2 is best handled in glovebox for consistent results - I spent one entire humid Boston summer working with super-Lewis acidic Cu(II) complexes (with SbF6- counterion they even react with MTBE and some chlorinated solvents like DCE). The color change from pale green to deep blue was a sign things got humid, and much less reactive - in my case the attenuated hydrated Cu catalyst was actually an advantage.
ReplyDeleteCompanies like Alfa or Aldrich packing hygroscopic materials into wide mouth bottles under moist air is unfortunately all too common problem. They have an all-purpose filling station and not enough qualified/motivated people doing the job the right way.
Close to twenty years ago, upon learning that I was colorblind, a Chinese coworkers said "you'd never be allowed to become a chemist in China". Maybe she had a point.
ReplyDeleteA lot of catalytic and organometallic chemistry is still a rather mysterious black box. I recall a particularly finicky Grignard reaction back in grad school that kept giving me mostly dimers and other intractable by-products, despite my best efforts to keep things anhydrous, oxygen free, or trying various halogen and electrophilic partners, even resorting to Rieke magnesium (the purest available), all to no avail. The great Professor Mel Newman was still roaming the labs at OSU at the time and suggested that it is sometimes a trace metal impurity that can make all the difference between having a successful outcome or the production of a tar ball of by-products, besides no reaction at all.
ReplyDeleteIn my case, he suggested a trace of iron as the adulterant, citing several anecdotal examples he was aware of related to my issue. And guess what? Right, the reaction still failed! Sometimes you eat the bear, sometimes...