Friday, June 8, 2012

Hi Ho, Silver! (Benzoate)

Ever feel déjà vu? You know, that feeling of having been somewhere before, or crossing a black cat twice?
Carbon dioxide - Silver benzoate "puts a ring on it"
Source: Yamada, ACIEE, 2012
I've been stalked lately by a certain catalytic reagent: silver (I) benzoate. It's everywhere! Here, Charette's using it for C-H activation, with a little dash of Pd, to produce heterocyclic drug leads. Here, Toste's group turns up the selectivity on a gold hydroamination. Just this week, Yamada and coworkers showed lactone formation via carbon dioxide fixation (see above), using a bit of pressure, some base, guessed it, silver benzoate.

Makes me wonder: Did I somehow miss the signs? How long has this reagent been flying under my radar?

In 1957, the Lone Ranger
ruled the Silver Screen
Quite a while, it seems. Name reaction buffs - have you heard of the Prévost? First developed in the 1930s, it produces an anti vicinal diester, forming two new C-O bonds from opposite directions on neighboring carbons. Later investigations into the "wet Prévost" reaction favored cis diols; not too shabby when you consider that the related osmium or iodonium variants were still several decades away.

In its heyday, the Prévost helped synthesize steroids, alkaloids, and polymers, and found its way into the hands of a Who's Who of the O-Chem 'golden age': Woodward, Fieser, Carothers, Winstein, Streitweiser, Gilman, Djerassi, Fries, Benkeser, Sondheimer, van Tamelen, and Witkop. Perhaps you're more of a mechanisms person? Check out this sweet 18O labeling work, courtesy of a young University of Washington chemist named Ken Wiberg (1957).

What about silver benzoate makes it so darn useful for these oxidations? First, you have a d10 late metal species, which loves to coordinate to π-bonds (alkenes, alkynes, allenes). Second, the softer nature of the resonance-stabilized benzoate might switch on different catalytic pathways, say, changing from inner- to outer-sphere. Finally, to the benzoate itself: our buddy Ken showed that it adds twice, producing a transient 5-membered, cyclic species that ultimately permits the second benzoate addition.


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