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2,3-Dimethyl-2,3-diphenylbutane (Dicumyl) (63). A solution of 2-bromo-2-phenylpropane (10.0 g, 50 mmol) in anhydrous diethyl ether (25 mL) was stirred with magnesium turnings (0.60 g, 0.025 g atom) overnight. The reaction mixture was poured into aqueous $\ce{NH4Cl}$ solution (100 mL, 5%) and extracted with dichloromethane (100 mL). The solvent was evaporated, and the solid residue was recrystallized from 95% ethanol to give 2,3-dimethyl-2,3-diphenylbutane (2.4 g, 47%).

Does anyone have any intuition as to how this reaction is working mechanistically? Is it a Grignard formation and then homocoupling event? Can anyone see a reason for why an ortho methyl substituent on the phenyl ring might interfere with this reaction?

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  • $\begingroup$ Clarification: am I correct that they don't care about the atmosphere, i.e. no argon/nitrogen atmosphere is implied in the reaction? $\endgroup$ – permeakra Feb 15 '18 at 10:14
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Well, that's interesting as nothing is added here. Today Palladium-catalyzed cross-couplings are quite common, but on a historical side, they took their idea from the very first reactions of sodium and organo-chlorides (Wurtz-coupling) and the magnesium mediated formation of biaryls. Back then transition metal salts were added which lead to the idea to use $\ce{Pd}$ which is way more carbophilic and can easily enter and leave the catalytic cycle. I have heard that the idea for the cross-coupling derived from the Schlenk-equilibrium, so your Grignard-reagent dimerizes and then equilibrates to from a $\ce{MgBr2}$ and a $\ce{MgR2}$. I'd love to say now that this simply reductively eliminates to form $\ce{Mg}$ and your product, but I have my doubts (although I have heard people talk about it before).

So in your case, we probably have to go the Wurtz path and say that the negatively polarized carbon on your Grignard-reagent nucleophilically attacks the $\ce{C-Br}$ bond in an $\ce{S_{N}2}$ reaction. If you used sodium the bond would be much more ionic and you'd probably also end up in simply deprotonating the second equivalent of starting material forming a 1-propene derivate.

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  • $\begingroup$ > negatively polarized carbon on your Grignard-reagent nucleophilically attacks the C-Br bond || Not going to happen with tertiary carbon. $\endgroup$ – permeakra Feb 15 '18 at 10:11
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The ammount of magnesium was 25 mmol, so only half of the 2-bromo-2-phenylpropane was turned into magnesium bromide.

After a molecule of magnesium bromide formed, I would guess it undergo a elimination of $\ce{MgBr^+}$ moity since it is on a tertiary (and also benzyl) carbon and form a relatively stable carboanion. The carboanion then attacks one of the 2-bromo-2-phenylpropane molecules forming the product. But I also wouldn't rule out $\ce{S_{N}2}$ type substitution.

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    $\begingroup$ I don't know what the exact mechanism is, but I would be very surprised to see a naked carbanion without any sort of metal. I've never seen or heard of such a thing before, except in highly simplified mechanisms (which incidentally give a horribly wrong impression that organolithiums react almost instantaneously). Aromatic rings don't even stabilise negative charges all that much; the pKa of toluene is still very high, ~40. With two electron-donating methyl groups, one would expect 2-phenylpropane to be even less acidic than toluene. $\endgroup$ – orthocresol Feb 14 '18 at 23:50

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