Radical Scission of Symmetrical 1,4-Dicarbonyl Compounds:  C−C Bond Cleavage with Titanium(IV) Enolate Formation and Related Reactions

Abstract

Reaction of Ti(NRAr^1)_3 (1, R = C(CH_3)_3, Ar^1 = 3,5-C_6H_3Me_2) with 0.5 equiv of symmetrical 1,4-diketones (ArCOCH_2)_2 (Ar = p-Tol or p-MeOC_6H_4) in hydrocarbon solvents at ≤25 °C resulted in carbon−carbon bond cleavage with clean formation of titanium-bound enolates, 1-OC(CH_2)Ar. Treatment of Ti(NRAr^1)_3 with esters or amides of succinic acid, under the same mild conditions, smoothly produced titanium(IV) compounds containing the corresponding amide or ester enolate moiety. The amide enolate condenses with benzaldehyde in an aldolic fashion. Differences in the observed reactivity of amido-enolate vs ketone-derived enolate toward aldol condensation were interpreted with the help of computational methods. Upon reaction with Ti(NRAr^1)_3, para-substituted acetophenones yielded equal amounts of enolate and alkoxide products. Under similar experimental conditions, acetophenone itself produced quantitatively a species whose proposed structure incorporates characteristics reminiscent of a Gomberg dimer. This intermediate decomposes cleanly to the expected enolate and alkoxide mixture upon heating. Ti(NRAr^1)_3 reductively complexes substrates such as N-methyl phthalimide. Treatment of Ti(NRAr^1)_3 with 0.5 equiv of o-bromophenyl allyl ether resulted in bromine atom abstraction followed by cyclization of the intermediate aryl radical to generate a titanium-bound 3-methylenedihydrobenzofuran product.

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