Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Cheese_Burgers on August 05, 2014, 12:29:58 PM
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In reactions such as the ullmann, sonogashira, and heck I am finding literature that shows the substitution of Halides I, Br, and Cl substituted compounds with triflates (trifluorosulfonate ester groups).
I assume that other tosylates did not work and thus are not reported in literature. Does anyone have any idea why they did not work? For example, why wouldn't a regular paratolulene tosylate work?
How is palladium coordinating with the triflate species for any of these reactions?
Thanks
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I assume it's something to do with the stability of the triflate as a leaving group (pKa of TfOH ~ -13!) being more similar to that of the halogens than the tosylate (pKa of TsOH ~ -3). I guess by the Hammond postulate, this also means that oxidative insertion is fast because the C-X bond is weak? There is no special coordination between the triflate and palladium, it just acts as a "pseudohalogen" - and undergoes the same oxidative insertion mechanism you would draw for a normal cross coupling.
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I assume that other tosylates did not work and thus are not reported in literature.
Tosylates do work and have been widely reported - this kind of tosylate reactivity has been known for around a decade.
See for starters: http://scholar.google.co.uk/scholar?q=tosylate+coupling&btnG=&hl=en&as_sdt=0%2C5&as_vis=1
Triflates are more reacive though, probably for the reasons clarkstill outlined.
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Thank you so much!