Synthesis of Homoallylic Sulfones through a Decarboxylative Claisen Rearrangement Reaction

Carboxylate-Catalyzed C-Silylation of Terminal Alkynes

A carboxylate-catalyzed, metal-free C-silylation protocol for terminal alkynes is reported using a quaternary ammonium pivalate as the catalyst and commercially available N,O-bis(silyl)acetamides as silylating agents. The reaction proceeds under mild conditions, tolerates a range of functionalities, and enables concomitant O- or N-silylation of acidic OH or NH groups. A Hammett ρ value of +1.4 ± 0.1 obtained for para-substituted 2-arylalkynes is consistent with the proposed catalytic cycle involving a turnover-determining deprotonation step.

Carboxylate Catalysis: A Catalytic O-Silylative Aldol Reaction of Aldehydes and Ethyl Diazoacetate

A mild catalytic variant of the aldol reaction between ethyl diazoacetate and aldehydes is described using a combination of N,O-bis(trimethylsilyl)acetamide and catalytic tetramethylammonium pivalate as catalyst. The reaction proceeds rapidly at ambient temperature to afford the O-silylated aldol products in good to excellent yield, and the acetamide byproducts can be removed by simple filtration.

Structural and Mechanistic Insights into the C-C Bond-Forming Rearrangement Reaction Catalyzed by Heterodimeric Hinokiresinol Synthase

Hinokiresinol synthase (HRS) from Asparagus officinalis consists of two subunits, α and β, and catalyzes an unusual decarboxylative rearrangement reaction of 4-coumaryl 4-coumarate to generate (Z)-hinokiresinol with complete stereoselectivity. Herein, we describe the mechanism of rearrangement catalysis and the role played by the heterodimeric HRS, through structural and computational analyses. Our results suggest that the HRS reaction is unlikely to proceed via the previously hypothesized Claisen rearrangement mechanism. Instead, we propose that the 4-coumaryl 4-coumarate substrate is first cleaved into coumarate and an extended p-quinone methide, which then recombine to generate a new C-C bond. These processes are facilitated by proton transfers mediated by the basic residues (α-Lys164, α-Arg169, β-Lys168, and β-Arg173) in the cavity at the heterodimer interface. The active site residues, α-Asp165, β-Asp169, β-Trp17, β-Met136, and β-Ala171, play crucial roles in controlling the regioselectivity of the coupling between the fragmented intermediates as well as the stereoselectivity of the decarboxylation step, leading to the formation of the (Z)-hinokiresinol product.

Palladium-catalyzed asymmetric allylic alkylation (AAA) with alkyl sulfones as nucleophiles

An efficient palladium-catalyzed AAA reaction with a simple α-sulfonyl carbon anion as nucleophiles is presented for the first time. Allyl fluorides are used as superior precursors for the generation of π-allyl complexes that upon ionization liberate fluoride anions for activation of silylated nucleophiles. With the unique bidentate diamidophosphite ligand ligated palladium as catalyst, the in situ generated α-sulfonyl carbon anion was quickly captured by the allylic intermediates, affording a series of chiral homo-allylic sulfones with high efficiency and selectivity. This work provides a mild in situ desilylation strategy to reveal nucleophilic carbon centers that could be used to overcome the pK_a limitation of “hard” nucleophiles in enantioselective transformations.

A variety of “hard” α-sulfonyl carbanions of aryl, heteroaryl and alkyl sulfones were successfully employed as nucleophiles in palladium-catalyzed asymmetric allylic alkylation with excellent enantioselectivities.

Design of bifunctional chiral phenanthroline ligand with Lewis basic site for palladium-catalyzed asymmetric allylic substitution

Conceptually new bifunctional chiral ligands with Lewis basic site were developed for Pd-catalyzed asymmetric allylic substitution.

Claisen rearrangements of equilibrating allylic azides

Equilibrating mixtures of allylic azide-containing allylic alcohols or allylic 2-tolylsulfonylacetic esters undergo Johnson-Claisen or Ireland-Claisen rearrangement reactions to give unsaturated γ-azidoesters and -acids, respectively. Decarboxylation of the acids under basic conditions gives azidosulfones, with moderate to high diastereoselectivity.

Aldehyde- and ketone-induced tandem decarboxylation-coupling (Csp(3)-Csp) of natural alpha-amino acids and alkynes

An interesting aldehyde- and ketone-induced intermolecular tandem decarboxylation-coupling (Csp(3)-Csp) catalyzed by copper with use of natural alpha-amino acids as starting materials is developed under neutral conditions with the production of CO(2) and H(2)O as the only byproducts. Various functionalized nitrogen-containing compounds were obtained by this method. In these processes, interesting regioselectivites of the alkylation were observed, which has been rationalized by the relative stability of proposed resonance structures based on computation methods.

Palladium-catalyzed decarboxylative rearrangements of allyl 2,2,2-trifluoroethyl malonates: direct access to homoallylic esters

Homoallylic esters are obtained in a single transformation from allyl 2,2,2-trifluoroethyl malonates by using a Pd(0) catalyst. Facile decarboxylation of allyl 2,2,2-trifluoroethyl malonates is attributed to a decrease in pK(a) compared to allyl methyl malonates. Subsequent reduction of the homoallylic 2,2,2-trifluoroethyl ester provides a (hydroxyethyl)cyclopentenyl derivative that represents a key intermediate in the synthesis of carbocyclic nucleosides. A select allyl 2,2,2-trifluoroethyl malonate undergoes a decarboxylative Claisen rearrangement to provide a regioisomeric homoallylic ester.

Decarboxylative allylation using sulfones as surrogates of alkanes

Alpha-sulfonyl functional groups are traceless activating groups that facilitate catalytic decarboxylative allylations in high yield yet can be cleaved to allow the synthesis of simple allylated alkanes. Substrate studies suggest that decarboxylation to form an alpha-sulfonyl anion is rate-limiting. Furthermore, the anion is formed regiospecifically under formally neutral conditions.

Asymmetric Decarboxylative Claisen Rearrangement Reactions of Sulfoximine-Substituted Allylic Tosylacetic Esters

Allylic acetate esters containing a variety of N-arylsulfonyl sulfoximines on the acetyl residue have been prepared and submitted to the decarboxylative Claisen rearrangement reaction. Rearranged products were isolated in generally good yields, and diastereoselectivities up to 82:18 have been obtained. The N-(2,4,6-triisopropylphenylsulfonyl)-S-phenyl sulfoximine moiety gave the best selectivity. The stereochemistry of the major isomer was established by X-ray crystallography. A model to explain the stereochemical course of the rearrangement is proposed.

Decarboxylative Claisen Rearrangement Reactions of Allylic Tosylmalonate Esters

[reaction: see text] Two different combinations of silylating agent and base are used for one-pot [3,3]-sigmatropic rearrangement-decarboxylation reactions of tosylmalonic mono(allylic) esters under mild conditions, providing the products of formal regiospecific allylation of methyl tosylacetate at the more substituted allylic terminus.