Novel Base-Free Catalytic Wittig Reaction for the Synthesis of Highly Functionalized Alkenes

A highly efficient catalyst system for base-free catalytic Wittig reactions has been developed and optimized. Initially, several potential (pre)catalysts as well as different silanes as reducing agents were screened. A system based on a readily available phosphine oxide as precatalyst and trimethoxy silane as reducing agent proved to be optimal. The effect of various Brønsted acidic additives was studied. Subsequently, the reaction conditions were optimized and standard reaction conditions were determined. Under these conditions the scope of this new protocol was evaluated. Nine activated olefins and 33 aldehydes were converted into 42 highly functionalized alkenes. Notably, aromatic, aliphatic as well as heteroaromatic aldehydes could be converted, giving the desired products in isolated yields up to 99 % and with good to excellent E/Z selectivities. These results underline the remarkable efficiency of this protocol considering the complexity of the reaction mixture and the four reaction steps that proceed in parallel in one pot.

Mechanistic Insights into the Mode of Action of Bifunctional Pyrrolidine-Squaramide-Derived Organocatalysts

The catalytic modes of action of three squaramide-derived bifunctional organocatalysts have been investigated using DFT methods. The [5+2] cycloaddition between oxidopyrylium ylides and enals was used as the model reaction. Two primary modes were possible for the different catalysts studied. The preference for one mode over the other was due to the possibility of additional favorable π-π interactions between the hydrogen-bond activated pyrylium ylide and an electron-deficient aromatic ring bonded to the squaramide NH group. The model can be extended to other reactions catalyzed by the same catalysts, such as formal [2+2] cycloadditions between nitroalkenes and α,β-unsaturated aldehydes. The computational results were in excellent concurrence with the available experimental reports on the observed total enantioselectivity and differences in diastereoselectivity depending on the substrate and the reaction.

Methyltriphenylphosphonium Methylcarbonate, an All-In-One Wittig Vinylation Reagent

The methyltriphenylphosphonium methylcarbonate salt [Ph3 PCH3 ][CH3 OCO2 ], obtained directly by quaternarization of triphenylphosphine with dimethylcarbonate, is a latent ylide that promotes Wittig vinylation of aldehydes and ketones. Alkenes are obtained simply by mixing [Ph3 PCH3 ][CH3 OCO2 ] and the carbonyl and heating in a solvent (no base, no halides, and no inorganic byproducts). Deuterium exchange experiments and the particularly short anion-cation distance measured by XRD in [Ph3 PCH3 ][CH3 OCO2 ] allowed to explain the nature and reactivity of this species. Green chemistry metrics (atom economy, mass index, environmental factor) indicate that this vinylation procedure is more efficient than comparable ones. Deuterated [Ph3 PCD3 ][CH3 OCO2 ] promoted the synthesis of deuterated olefins.

Moving toward Ylide-Stabilized Carbenes

The effect of ylide substitution at the α position to the carbene carbon (Cc ) atom on the stability and σ-donating ability of a number of cyclic carbenes has been studied theoretically. The stabilities of all of the carbenes were investigated from an evaluation of their singlet-triplet energy gaps and stabilization energies. All carbenes were found to have a stable singlet state. The energy of the σ-symmetric lone-pair orbital at the Cc atom increases as a result of the introduction of ylide centers near to the Cc atom. This indicates an enhanced σ-donating ability of the ylide-containing carbenes. The calculated carbonyl-stretching frequencies of the corresponding rhodium complexes, proton affinities, and nucleophilicity index values correlate well with the σ basicity of the carbenes.

Preparation of Furo[3,2-c]coumarins from 3-Cinnamoyl-4-hydroxy-2H-chromen-2-ones and Acyl Chlorides: A Bu3P-Mediated C-Acylation/Cyclization Sequence

A Bu3P-mediated cyclization reaction of 3-cinnamoyl-4-hydroxy-2H-chromen-2-ones though electrophilic addition of acyl chlorides towards the synthesis of highly functionalized furo[3,2-c]coumarins bearing a phosphorus ylide moiety is described. These unprecedented cyclization reaction proceeds under mild reaction conditions within short reaction times (1 min to 1 h), and can be further applied in the synthesis of alkenyl-substituted furo[3,2-c]coumarins by the treatment with carbonyl electrophiles under basic conditions.

Catalytic Asymmetric Synthesis of Epoxides from Aldehydes Using Sulfur Ylides with In Situ Generation of Diazocompounds

A practical, general, and convergent route to epoxides with control of the relative and absolute stereochemistry has been achieved by generating the reactive intermediate (the diazo compound) in situ from tosylhydrazone salts (see scheme, PTC=phase-transfer catalyst, Ts=toluene-4-sulfonyl). High yields (58-82 %), high d.r. (88:12-98:2), and high ee values (87-94 %) have been obtained using a new class of stable chiral sulfides at low catalyst loading (5 mol %) and [Rh₂ (OAc)₄ ] (0.5 mol %).

Thioformaldehyde S-sulfide (Thiosulfine)

Matrix isolation spectroscopy allows the direct identification of ylide 1 and its cyclic isomer 2. They were obtained by pyrolysis of 1,2,4-trithiolane under high vacuum; the cyclic compound forms from 1 by thermal ring closure in a kinetically controlled reaction.