Electronic Effects in Migratory Groups. [1,4]- versus [1,2]-Rearrangement in Rhodium Carbenoid Generated Bicyclic Oxonium Ylides

Abstract

The variety of alpha-diazo beta-keto esters (3a-f, 8a-f) with varying substituents (ED/EW) on the phenyl ring of the O-benzyl group were prepared. The rhodium(II) acetate catalyzed decomposition of diazo compounds in benzene reflux conditions. The ratio of 1,4 versus 1,2 migration product was determined. It was found that an increase in electron density on the benzylic carbon of the migrating group prefers 1,4 migration products (4, 9) while a decrease in electron density leads to a preponderance of 1,2 migration products (5, 10). The results obtained were correlated to the mechanistic aspect of the product selectivity. The intermediacy of the intramolecular oxonium ylide formation was demonstrated by crossover experiments. The preference for the formation of 2,3 sigmatropic rearrangement product over 1,2 and 1,4 was demonstrated by performing the reaction with alpha-diazo beta-keto esters (13a, 13b) with O-allyl and O-propargyl at C3. The effect of solvent, temperature, and mole percentage of rhodium(II) acetate was also studied.

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“…Pleasingly, it was found that the use of o ‐ethynylanisole substrate 3 a completely suppressed the formation of side product 2 a and furnished the desired 3‐coumaranone 4 a in 65 % NMR yield. Interestingly, no oxonium ylide [1,2]‐shift product was observed in this case 6b. g …”

Advances in [1,2]-Sigmatropic Rearrangements of Onium Ylides via Carbene Transfer Reactions

“…The orientation of the metal carbenoid from the same side as the large ester substituent seems to be energetically favored as the pyramidal form of nitrogen outmaneuvers the effect of steric repulsion (30 vs. 31). The stereoselective formation of spirocyclic ylide (32) and following [1,2]-sigmatropic rearrangement, lead to the formation of quinolizidine (29) in high yield and excellent stereoselectivity (Scheme 6a), which is an analogue of the lupinine alkaloid. 16 Moreover, West et al investigated 2-silylpyrrolodine diazoalkane (34) under similar reaction conditions, in which the silyl group acted as a hydroxy surrogate.…”

“…They can undergo three major reaction pathways: (a) [1,2]-sigmatropic rearrangement that leads to formal homologation and ring expansion (e.g., 94), (b) in the presence of an -vinyl substituent, [2,3]-sigmatropic rearrangement can occur to give a ring-expanded product (e.g., 95), or (c) -hydride elimination leading to ring opening of the oxygen heterocycle to give allyl ethers (e.g., 96) (Scheme 12). 29 Scheme 12 Reaction pathways of oxonium ylides: [1,2]-and [2,3]-sigmatropic rearrangements and -hydride elimination One of the first examples of such a ring-expansion reaction of oxonium ylides was reported by Hillman and Gutsche in the reaction of 2-phenyl-1,3-dioxolane (99) with ethyl diazoacetate (97) under heating at 150 °C to give 1,4dioxane 102. Hillman proposed the thermal decomposition of the diazoalkane to the corresponding free carbene 98.…”

C–H Functionalization on Carbohydrates

“…The electronic effects governing the reaction pathways for the [1,2]‐ and [1,4]‐migrations were studied in more detail and it was found that the selectivities correlated with the electron density on the 3‐ O ‐benzylic carbon, with an increase in this favoring the [1,4]‐migration and vice versa …”