Stereoconvergent [1,2]- and [1,4]-Wittig rearrangements of 2-silyl-6-aryl-5,6-dihydropyrans: a tale of steric vs electronic regiocontrol of divergent pathways

Mechanism of Iron-Catalyzed Oxidative α-Amination of Ketones with Sulfonamides

We report the mechanism of the iron-catalyzed oxidative α-amination of ketones with sulfonamides. Using linear free energy relationships, competition experiments, and identification of reaction intermediates, we have found that the mechanism of this reaction proceeds through rate-limiting electron transfer to 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) from an iron enolate in the process of forming an α-DDQ adduct. The adduct then serves as the electrophile for substitution with sulfonamide nucleophiles, accelerated by iron and additional DDQ. This mechanistic study rules out formation of an α-carbocation intermediate and purely radical mechanistic hypotheses.

Cobalt-catalysed [1,2]-Wittig rearrangement of ethers to secondary alcohols

Stable ethers are successfully transformed into secondary alcohols via C-O bond activation using a simple cobalt pincer catalyst. Mechanistic studies indicate the involvement of radical pairs, and their sequential recombination and the subsequent hydrolysis results in the formation of secondary alcohols.

Access to Diarylmethanols by Wittig Rearrangement of ortho-, meta-, and para-Benzyloxy-N-Butylbenzamides

The N-butyl amide group, CONHBu, has been found to be an effective promoter of the [1,2]-Wittig rearrangement of aryl benzyl ethers and thus allow the two-step synthesis of isomerically pure substituted diarylmethanols starting from simple hydroxybenzoic acid derivatives. The method is compatible with a wide range of functional groups including methyl, methoxy, and fluoro, although not with nitro and, unexpectedly, is applicable to meta as well as ortho and para isomeric series.

Silylcyclopropanes by Selective [1,4]-Wittig Rearrangement of 4-Silyl-5,6-dihydropyrans

4-Silyl-5,6-dihydropyrans undergo remarkably selective [1,4]-Wittig rearrangements to give silylcyclopropanes in good yields. The selectivity is independent of the silyl group, but it is influenced by the electronic character of the migrating center. Electron-rich and electron-neutral (hetero)aryl groups and aliphatic substituents at the migrating center lead to exclusive [1,4]-migration, whereas electron-deficient aryl groups predominantly afford [1,2]-Wittig products.

Phosphite mediated asymmetric N to C migration for the synthesis of chiral heterocycles from primary amines

A phosphite mediated stereoretentive C-H alkylation of N-alkylpyridinium salts derived from chiral primary amines was achieved. The reaction proceeds through the activation of the N-alkylpyridinium salt substrate with a nucleophilic phosphite catalyst, followed by a base mediated [1,2] aza-Wittig rearrangement and subsequent catalyst dissociation for an overall N to C-2 alkyl migration. The scope and degree of stereoretention were studied, and both experimental and theoretical investigations were performed to support an unprecedented aza-Wittig rearrangement-rearomatization sequence. A catalytic enantioselective version starting with racemic starting material and chiral phosphite catalyst was also established following our understanding of the stereoretentive process. This method provides efficient access to tertiary and quaternary stereogenic centers in pyridine systems, which are prevalent in drugs, bioactive natural products, chiral ligands, and catalysts.

Dicarbonylative benzannulation of 3-acetoxy-1,4-enynes with CO and silylboranes by Pd and Cu cooperative catalysis: one-step access to 3-hydroxyarylacylsilanes

A new, general Pd/Cu-cocatalysed dicarbonylative benzannulation of 3-acetoxy-1,4-enynes with CO and silylboranes is described. The method utilizes CO as both a one-carbon (C1) unit and an external addition functional reagent to achieve an unprecedented dicarbonylative benzannulation process, and represents a facile, efficient route to 3-hydroxyarylacylsilanes. Mechanistically, the silyl-Cu intermediate formed from CuF2 and silylboranes, and silyl-Pd intermediate generated by transmetallation are two key factors for successfully targeting the reaction and selectivity.

A removable functional group strategy for regiodivergent Wittig rearrangement products

[1,2] and [2,3] Wittig rearrangements are competing reaction pathways, often leading to uncontrollable product distribution. We employ a single removable functional group to fulfill the dual role of attaining a reversible [2,3] and stabilizing radical intermediate for the [1,2] path to obtain both the Wittig products selectively for a broad range of substrates.

Reagent-controlled regiodivergence in the [3,3]-sigmatropic rearrangement of N-(acyloxy)enamides

A regiodivergent [3,3]-sigmatropic rearrangement of N-(acyloxy)enamides based on the N–O bond cleavage is achieved by changing reaction conditions.

Mechanistic investigation in the [1,4] and [1,2] Wittig rearrangement reactions: a DFT study

The mechanistic pathways for the [1,4] and [1,2] Wittig rearrangements of 2-silyl-6-aryl-5,6-dihydro-(2H)-pyrans have been studied at the M06-2X/6-31+G(d,p), 6-311++G(d,p) and cc-pVTZ level of theory. The crucial C-O bond cleavage step in the mechanism has been analysed initially, using two model reactions covering aliphatic as well as cyclic allylic ethers. The barriers for the one-step as well as two-step pathways have been calculated and the mechanisms for both the [1,4] and [1,2] Wittig rearrangement reactions are predicted to occur through a two-step mode. An energetic analysis of the reaction pathways reveals that the [1,4]-rearrangement has a lower barrier than the [1,2]-Wittig rearrangement. The C-O cleavage transition state was found to have the highest barrier and is thus the rate determining transition state for all of the studied molecules. This is in agreement with the previously published experimental studies. The role of the allylic trimethylsilane group in the stabilization of the intermediate anions of the Wittig reactants has also been investigated while comparing it with the phenyl and allylic t-butyl groups through Natural Bond Orbital (NBO) calculations.

Diaza [1,4] Wittig-type rearrangement of N-allylic-N-Boc-hydrazines into γ-amino-N-Boc-enamines

Diaza [1,4] Wittig-type rearrangement of N-allylic-N-Boc-hydrazines into γ-amino-N-Boc-enamines was demonstrated.

Synthesis of 4-trifluoromethanesulfonate substituted 3,6-dihydropyrans and their application in various C–C coupling reactions

Triflic acid is used for Prins cyclization of homopropargyl alcohols with aldehydes to give 3,6-dihydro-2H-pyran-4-yl trifluoromethanesulfonates. The dihydropyran is converted into 4-alkyl and aryl substituted products using cross-coupling reactions.

Stereospecific Intramolecular Reductive Cross-Electrophile Coupling Reactions for Cyclopropane Synthesis

The stereospecific reductive cross-electrophile coupling reaction of 2-aryl-4-chlorotetrahydropyrans to afford disubstituted cyclopropanes is reported. This ring contraction presents surprises with respect to the stereochemical outcome of reaction of the alkyl halide moiety. While cross-coupling and reductive cross-electrophile coupling reactions of alkyl halides are typically stereoablative, using a chiral catalyst to set the stereocenter, this transformation proceeds with high stereochemical fidelity at the alkyl halide and ether bearing stereogenic centers. This approach provides straightforward access to highly substituted cyclopropanes in two steps from commercially available aldehydes.

Base-promoted [1,4]-Wittig rearrangement of chalcone-derived allylic ethers leading to aromatic β-benzyl ketones

Strong base promoted [1,4]-Wittig rearrangement of allylic ethers was developed in this work, in which the reaction provided a facile approach to the synthesis of aromatic β-benzyl ketones under controllable radical reaction conditions.

Construction of an all-substituted pyrrolidine derivative with multiple stereogenic centers and Betti-base-derived γ-amino alcohols by [1,2]-Wittig rearrangement

The construction of Betti base-derived γ-amino alcohols and all-substituted pyrrolidine derivative with multiple stereogenic centers has been developed successfully through neighboring lithium-assisted [1,2]-Wittig rearrangement (NLAWR).