An Examination of the Scope and Stereochemistry of the Ireland-Claisen Rearrangement of Boron Ketene Acetals

Copper-Catalyzed Reductive Ireland-Claisen Rearrangements of Propargylic Acrylates and Allylic Allenoates

The copper-catalyzed reductive Ireland-Claisen rearrangement of propargylic acrylates led to 3,4-allenoic acids. The use of silanes or pinacolborane as stoichiometric reducing agents and triethylphosphite as a ligand facilitated the divergent and complementary selectivity for the synthesis of diastereomeric anti- and syn-rearranged products, respectively. Copper-catalyzed reductive Ireland-Claisen rearrangement of allylic 2,3-allenoates proceeded effectively only when pinacolborane was used as a reductant to generate various 1,5-dienes in excellent yields and with good diastereoselectivities in some cases. Mechanistic studies showed that the silyl and boron enolates, rather than the copper enolate, underwent a stereospecific rearrangement via a chairlike transition state to afford the corresponding Claisen rearrangement products.

Mechanistic Insight into the 1,3,2-Diazaphospholene-Catalyzed Reductant (HBpin/NH3BH3)-Controlled Reaction of Allyl 2-Phenylacrylate: Claisen Rearrangement or Hydrogenation?

Mechanistic study on the 1,3,2-diazaphospholene (1)-catalyzed reduction reaction of allyl 2-phenylacrylate 4 with HBpin or ammonia borane (AB) was systematically performed by the density functional theory (DFT) method. When HBpin is employed as the reductant, the reductive Ireland-Claisen (IC) rearrangement reaction occurs. First, the active species P-hydrido-1,3,2-diazaphospholene 3 is generated through the metathesis reaction of 1 with HBpin. Next, the terminal C═C double bond of 4 is inserted into the P-H bond of 3 to produce 6a through the 1,2-addition (Markovnikov) step, which is followed by the pinB-H bond activation to afford key boron enolate 8. Then, 8 undergoes the [3,3] rearrangement that is followed by the alcoholysis reaction with methanol leading to the final product γ,δ-unsaturated carboxylic acid. The [3,3] rearrangement step is the rate-determining step with the Gibbs energy barrier (ΔG^≠) and Gibbs reaction energy (ΔG) of 23.9 and -27.5 kcal/mol, respectively. When AB is employed as the reductant, the transfer hydrogenation reaction occurs through two comparable pathways, 1,2- and 1,4-transfer hydrogenation pathways. The former pathway directly leads to the hydrogenation product with the ΔG^≠ and ΔG values of 22.4 and -27.7 kcal/mol, respectively. The latter pathway produces an enolate intermediate (rate-determining step, ΔG^≠/ΔG = 24.1/-0.3 kcal/mol) first, which then prefers to undergo the enol-keto tautomerism instead of the [3,3] rearrangement to afford the hydrogenation product. Obviously, the generation of the boron enolate plays a crucial role in the reductive IC rearrangement reaction because it prevents the enol-keto tautomerism.

A borane-mediated palladium-catalyzed reductive allylic alkylation of α,β-unsaturated carbonyl compounds

The development of the palladium-catalyzed allylic alkylation of in situ generated boron enolates via tandem 1,4-hydroboration is reported. Investigation of the reaction revealed insights into specific catalyst electronic features as well as a profound leaving group effect that proved crucial for achieving efficient allylic alkylation of ester enolates at room temperature and ultimately a highly preparatively useful synthesis of notoriously challenging acyclic all-carbon quaternary stereocenters. The method demonstrates boron enolates as viable pro-nucleophiles in transition-metal catalyzed allylic alkylation, potentially opening up further transformations outside their traditional use.

Why do thioureas and squaramides slow down the Ireland-Claisen rearrangement?

A range of chiral hydrogen-bond-donating organocatalysts was tested in the Ireland–Claisen rearrangement of silyl ketene acetals. None of these organocatalysts was able to impart any enantioselectivity on the rearrangements. Furthermore, these organocatalysts slowed down the Ireland–Claisen rearrangement in comparison to an uncatalyzed reaction. The catalyst-free reaction proceeded well in green solvents or without any solvent. DFT calculations showed that the activation barriers are higher for reactions involving hydrogen-donating organocatalysts and kinetic experiments suggest that the catalysts bind stronger to the starting silyl ketene acetals than to transition structures thus leading to inefficient rearrangement reactions.

A 1,3,2-Diazaphospholene-Catalyzed Reductive Claisen Rearrangement

1,3,2-Diazaphospholenes (DAPs) are an emerging class of organic hydrides. In this work, we exploited them as efficient catalysts for very mild reductive Claisen rearrangements. The method is tolerant towards a wide variety of functional groups and operates at ambient temperature. Besides being enantiospecific for substrates with existing stereogenic centers, the diastereoselectivity can be switched by varying solvents and DAP catalysts. The reaction kinetics show direct rearrangements of O-bound phospholene enolates and provide a proof-of-principle for catalytic enantioselective reactions.