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Arylation of carbons with active hydrogen...
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Author Topic: Arylation of carbons with active hydrogen...
Teonanacatl
Member posted 12-14-1999 03:30 PM
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Unactivated aryl halides in the presence of a strong base will arylate at carbons with an active hydrogen...ie. CH3-CO-HCH-CO-CH3...March describes this mechanism as occuring through the benzyne mechanism...I understand what benzyne is and how it comes about, but I can't understand how this applies in this situation...I'll show the reaction he used as an example:
the way that I thought this occured was movement of electons from the carbonyl bond to the CO-CH2 bond, following attack of the aryl group at the halide bond after activation of the CH2 group by strong base...but how does this involve a benzyne mechanism? Any thoughts/answers?
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K.C. Nicolaou
Member posted 12-15-1999 02:14 AM
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Strong base removes HX from the halobenzene to give benzyne. Strong base deprotonates the carbonyl conpound to form an enolate. The nucleophillic carbon of the enolate attacks the very unstable triple bond of benzyne to give a new C-C bond and an phenyl anion(not delocalized) at the ortho position. The anion then abstracts a proton from something to give the arylated carbonyl compound.
rev drone
Member posted 12-15-1999 07:29 AM
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KCN, Teo,
...However, a distinction needs to be made, between the Sn1 benzyne mechanism and the Srn1 mechanism: two entirely different animals. In the case of Srn1 enolate additions to arylhalides, no benzyne is formed; it follows a free-radical mechanism. In this case, a ketone such as (let's just pick one at random) acetone, is deprotonated and stabilized by a strong base. Meanwhile, the aryl halide, by either transition metal catalysis, electrolylsis or photoexcitation, dehalogenates by releasing a halogen free-radical. This dehalogenation process releases a halogen free radical only-- no proton. The nucleophilic enolate attacks the elctron-deficient benzene radical, an electron transfer takes place, and the reaction is complete. In the end, the result is phenylacetone and alkali halide salt.
Ref's available upon request (hey, didn't I write an FAQ on this? Oh yeah, now I remember: http://rhodium.lycaeum.org/chemistry/dro
-the good reverend drone
Teonanacatl
Member posted 12-15-1999 08:19 AM
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Hmmm...it appears that my image is not coming through...thanks K.C. and Rev....the specific information I'm looking at is in March's Adv. Org. Chem. reaction 3-14 (p.662 of 4th ed.)...basically formation of an indole using this method with the halide ortho to the reactive chain containing the active hydrogen...this is what struck me as strange for mention of an benyl mechanism, since based on where the bond is formed, it doesn't look like a benyne intermediate could be formed...there's no place for loss of H from the aromatic ring here...so it sounds like Drone's example of an Srn1 type reaction...I looked back at your FAQ (I have read it before...just never occured to me that it applied to this situation)...anyways, what do you think, would this be an Srn1 type mechanism, or an Sn1 benzyne type? (By the way, the reason this isn't being posted on DMT world, is that there's almost no one left there
...the place is being overrun with nonsense posts...
K.C. Nicolaou
Member posted 12-15-1999 11:13 PM
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Shit, they never taught me that one in school. All they told us is "there is no such thing as aromatic nucleophillic substition, there is no such thing as aromatic nucleophillic substitution..." ad nauseum. Just thought I'd take a stab at it. Time to ask those fuckers for a refund.
Teonanacatl
Member posted 12-15-1999 11:41 PM
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Hee hee...no worries...exactly why I couldn't figure this one out...once Drone mentioned it, I looked it up...but basically I got the same thing..."there is no such thing as aromatic nucleophilic substitution, there is no such thing...
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Danger...ethnic digestion tabernacles on now; privacy robotic doubles, rolling rehab threat machines, reformulators, heightened insensitivity vehicles, erotic invisible empires on now...
rev drone
Member posted 12-16-1999 07:58 AM
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No such thing as aromatic nucleophilic substitution!? Good grief! I would ask for my money back! March devotes 27 pages to this (650-676)! Admittedly, the Sn2 reaction mechanism doesn't work so swimmingly in the case of explaining aromatic nucleophilic substitutions, but other mechanisms acount for this quite elegantly.
As for the the indole synthesis on p. 663 in March, if you look again, you'll see there is a proton adjacent to the chloride that allows for benzyne formation.
-the good reverend drone
