Fe(III)-Catalyzed Hydroallylation of Unactivated Alkenes with Morita–Baylis–Hillman Adducts

Directed Hydrogen Atom Transfer for Selective Reactions of Polyenols

Directed hydrogen atom transfer to alkenes is described. The process is catalyzed by iron complexes and allows for the site-selective hydrofunctionalization of polyenols. Experimental data suggest that coordination of the hydroxy group to the iron hydride intermediate plays an important role in preferential engagement of the allylic alcohol motif and provides a new basis for selectivity in radical hydrofunctionalization events. As a proof of concept, β- and γ-amino alcohols are prepared from the corresponding polyenols in a selective manner.

Synthesis of functionalized alkenes via Cu(I)-catalysed allylation of acetanilides using Morita-Baylis-Hillman bromides

An easy access to functionalized alkenes has been developed by the C-H functionalization of anilides with Morita-Baylis-Hillman (MBH) bromides in the presence of copper chloride, TBHP and acetic acid. Unsubstituted as well as ortho/meta-substituted anilides exclusively give rise to the para-allylated products, whereas para-substitution brings about the formation of ortho-allylated anilides.

Formal Glycosylation of Quinones with exo-Glycals Enabled by Iron-Mediated Oxidative Radical-Polar Crossover

The intermolecular C-O coupling reaction of 1,4-quinones with exo-glycals under iron hydride hydrogen atom transfer (HAT) conditions is described. This method provides a direct and regioselective access to a wide range of phenolic O-ketosides related to biologically relevant natural products in diastereomeric ratios up to >98:2 in the furanose and pyranose series. No trace of the corresponding C-glycosylated products that might have resulted from the radical alkylation of 1,4-quinones was observed. The results of mechanistic experiments suggest that the key C-O bond-forming event proceeds through an oxidative radical-polar crossover process involving a single-electron transfer between the HAT-generated glycosyl radical and the electron-acceptor quinone.

Chemoselective tandem SN2'/SN2''/inter- or intramolecular Diels-Alder reaction of γ-vinyl MBH carbonates with phenols and o-hydroxychalcones

An unprecedented consecutive S_N2'/S_N2'' addition of phenol to γ-vinyl MBH carbonate forms a reactive diene intermediate, followed by a dimerization/elimination process to give functionalized exocyclohexenes with excellent chemoselectivity. When using o-hydroxychalcones as pronucleophiles, an S_N2'/S_N2''/intramolecular Diels-Alder reaction sequence occurs, selectively producing a series of pharmaceutically intriguing tricyclic chromane derivatives with good diastereoselectivity. The good nucleophilicity and fair nucleofugicity exhibited by phenoxy anion play a pivotal role in the S_N2'' addition step.

Regio- and Stereoselective Electrochemical Alkylation of Morita-Baylis-Hillman Adducts

Electrosynthesis is effectively employed in a general regio- and stereoselective alkylation of Morita-Baylis-Hillman compounds. The exposition of N-acyloxyphthalimides (redox-active esters) to galvanostatic electroreductive conditions, following the sacrificial-anode strategy, is proved an efficient and practical method to access densely functionalized cinnamate and oxindole derivatives. High yields (up to 80%) and wide functional group tolerance characterized the methodology. A tentative mechanistic sketch is proposed based on dedicated control experiments.

Catalytic Enantioselective Reductive Cross Coupling of Electron-Deficient Olefins

We report an enantioselective reductive cross coupling of electron-deficient olefins. Using a visible-light-driven cooperative photoredox and chiral Brønsted acid-catalyzed reaction with a Hantzsch ester as the terminal reductant, various cyclic and acyclic enones with 2-vinylpyridines were converted in high yields (up to 93%) to a wide range of enantioenriched pyridine derivatives featuring diverse γ-tertiary carbon stereocenters with good to excellent enantioselectivities (up to >99% ee).

Catalyst-Controlled Regiodivergent Synthesis of α/β-Dipeptide Derivatives via N-Allylic Alkylation of O-Alkyl Hydroxamates with MBH Carbonates

A controllable and regiodivergent N-allylation reaction involving readily available O-alkyl hydroxamates derived from natural α-amino acids has been developed, allowing regiospecific access to α/β-dipeptides containing α-unsaturated β-amino acids moieties in moderate to good yields. The regioselectivity could be conveniently switched by alternation of the catalysts and solvents.

A combined experimental and computational study of NHC-catalyzed allylation of allenoate with MBH esters: new regiospecific and stereoselective access to 1,5-enyne

An NHC-catalyzed regiospecific allylation of α-substituted allenoates with MBH carbonates derived from aryl aldehyde furnished highly functionalized 1,5-enynes.

Enantioselective Radical SN2-Type Alkylation of Morita-Baylis-Hillman Adducts Using Dual Photoredox/Palladium Catalysis

An enantioselective radical alkylation of 4-alkyl-1,4-dihydropyridines with Morita-Baylis-Hillman (MBH) adducts has been reported. The S_N2-type products are predominant. This reaction is enabled by dual photoredox/palladium catalysis. The alkylation products are provided in good yields with good regio- and enantioselectivity. The use of Ding's spiroketal-based bis(phosphine) (SKP) ligand is crucial to achieving satisfactory regio- and enantioselectivity. The resultant α,β-unsaturated ester can be easily reduced to a synthetically useful chiral allyl alcohol.

Methodologies for the synthesis of quaternary carbon centers via hydroalkylation of unactivated olefins: twenty years of advances

Olefin double-bond functionalization has been established as an excellent strategy for the construction of elaborate molecules. In particular, the hydroalkylation of olefins represents a straightforward strategy for the synthesis of new C(sp³)–C(sp³) bonds, with concomitant formation of challenging quaternary carbon centers. In the last 20 years, numerous hydroalkylation methodologies have emerged that have explored the diverse reactivity patterns of the olefin double bond. This review presents examples of olefins acting as electrophilic partners when coordinated with electrophilic transition-metal complexes or, in more recent approaches, when used as precursors of nucleophilic radical species in metal hydride hydrogen atom transfer reactions. This unique reactivity, combined with the wide availability of olefins as starting materials and the success reported in the construction of all-carbon C(sp³) quaternary centers, makes hydroalkylation reactions an ideal platform for the synthesis of molecules with increased molecular complexity.

Iron-catalyzed hydrogen atom transfer induced cyclization of 1,6-enynes for the synthesis of ketoximes: a combined experimental and computational study

An iron-catalyzed reductive radical cyclization/hydro-oximation of 1,6-enynes with tBuONO was developed, leading to functionalized benzofuran, benzothiophene, and cyclopentenyl-based ketoximes.

Synthesis of naphthodihydrofurans via an iron(iii)-catalyzed reduction radical cascade reaction

A convenient method for the synthesis of naphthodihydrofurans has been developed by iron(iii)-catalyzed cascade reaction of reducing radicals.

A Construction of α-Alkenyl Lactones via Reduction Radical Cascade Reaction of Allyl Alcohols and Acetylenic Acids

An iron-catalyzed cascade reaction of radical reduction of allyl alcohols and acetylenic acids to construct polysubstituted α-alkenyl lactones has been developed. In this paper, various allyl alcohols can form allyl ester intermediates and are further transformed into alkyl radicals, which form products through intramolecular reflex-Michael addition. In addition, this method can be used to prepare spirocycloalkenyl lactones. Interestingly, this protocol can be used to synthesize the skeleton structure of natural products. Moreover, the product can be further transformed into a β-methylene tetrahydrofuran and tetrahydrofuran diene.

Lewis Acid Catalyzed Transfer Hydromethallylation for the Construction of Quaternary Carbon Centers

The design and gram-scale synthesis of a cyclohexa-1,4-diene-based surrogate of isobutene gas is reported. Using the highly electron-deficient Lewis acid B(C6 F5 )3 , application of this surrogate in the hydromethallylation of electron-rich styrene derivatives provided sterically congested quaternary carbon centers. The reaction proceeds by C(sp3 )-C(sp3 ) bond formation at a tertiary carbenium ion that is generated by alkene protonation. The possibility of two concurrent mechanisms is proposed on the basis of mechanistic experiments using a deuterated surrogate.

A Radical-Polar Crossover Annulation To Access Terpenoid Motifs

A new catalytic radical-polar crossover annulation between two unsaturated carbonyl compounds is described. The annulation proceeds under exceptionally mild conditions and provides direct and expedient access to complex terpenoid motifs. Application of this chemistry allows for synthesis of forskolin, a densely functionalized terpenoid, in 14 steps from commercially available material.

Roles of Iron Complexes in Catalytic Radical Alkene Cross-Coupling: A Computational and Mechanistic Study

A growing and useful class of alkene coupling reactions involve hydrogen atom transfer (HAT) from a metal-hydride species to an alkene to form a free radical, which is responsible for subsequent bond formation. Here, we use a combination of experimental and computational investigations to map out the mechanistic details of iron-catalyzed reductive alkene cross-coupling, an important representative of the HAT alkene reactions. We are able to explain several observations that were previously mysterious. First, the rate-limiting step in the catalytic cycle is the formation of the reactive Fe-H intermediate, elucidating the importance of the choice of reductant. Second, the success of the catalytic system is attributable to the exceptionally weak (17 kcal/mol) Fe-H bond, which performs irreversible HAT to alkenes in contrast to previous studies on isolable hydride complexes where this addition was reversible. Third, the organic radical intermediates can reversibly form organometallic species, which helps to protect the free radicals from side reactions. Fourth, the previously accepted quenching of the postcoupling radical through stepwise electron transfer/proton transfer is not as favorable as alternative mechanisms. We find that there are two feasible pathways. One uses concerted proton-coupled electron transfer (PCET) from an iron(II) ethanol complex, which is facilitated because the O-H bond dissociation free energy is lowered by 30 kcal/mol upon metal binding. In an alternative pathway, an O-bound enolate-iron(III) complex undergoes proton shuttling from an iron-bound alcohol. These kinetic, spectroscopic, and computational studies identify key organometallic species and PCET steps that control selectivity and reactivity in metal-catalyzed HAT alkene coupling, and create a firm basis for elucidation of mechanisms in the growing class of HAT alkene cross-coupling reactions.

Ligand-Promoted Iron(III)-Catalyzed Hydrofluorination of Alkenes

An iron-catalyzed hydrofluorination of unactivated alkenes has been developed. The use of a multidentate ligand and the fluorination reagent N-fluorobenzenesulfonimide (NFSI) proved to be critical for this reaction, which afforded various fluorinated compounds in up to 94 % yield.

A hydrate salt-promoted reductive coupling reaction of nitrodienes with unactivated alkenes

Transition metal-catalyzed reductive coupling has emerged as a powerful method for the construction of C-C bonds. Herein, a crystalline hydrate, Na2HPO4·7H2O, has been disclosed as an effective promoter for the reductive coupling of nitrodienes with unactivated alkenes to afford diverse dienes with various functionalities in an open-flask manner. The mechanism study has revealed that Na2HPO4·7H2O accelerates the in situ generation of active silane PhSi(OEt)H2 and prevents the deactivation of catalyst. The approach can increase the efficiency of previous reductive coupling reactions as well.

Reductive coupling of alkenes with unsaturated imines via a radical pathway

A reductive coupling of alkenes with unsaturated imines via a radical pathway is described.

Visible light enabled γ-trifluoromethylation of Baylis–Hillman acetates: stereoselective synthesis of trisubstituted alkenes

Metal-free γ-trifluoromethylation of Baylis–Hillman acetates is reported using Langlois’ reagent under visible light photoredox catalysis.

Base-promoted regiodivergent allylation of N-acylhydrazones with Morita–Baylis–Hillman carbonates by tuning the catalyst

A catalyst-controlled regiodivergent allylation reaction of N-acylhydrazones (NAHs) with Morita–Baylis–Hillman (MBH) carbonates has been developed, paving a new avenue for the diversification of NAH.

Highly selective AlCl3 initiated intramolecular α-alkylation of α,β-unsaturated lactams and lactones

An unprecedented example of AlCl3 initiated intramolecular α-alkylation of α,β-unsaturated lactams and lactones is reported. A variety of substrates containing an intramolecular diene yield exclusively regioselective six-membered ring products. This reaction protocol generates a new stereo centre which may be of high interest for the functionalization of bioactive coumarin and quinolinone derivatives.

A cascade synthesis of S-allyl benzoylcarbamothioates via Mumm-type rearrangement

A catalyst and solvent free synthesis of S-allyl benzoylcarbamothioates has been achieved from the in situ generated benzoylcarbonimidothioates obtained by reacting MBH alcohols with aroyl isothiocyanates. An intramolecular thia-Michael addition of the in situ generated adduct triggers a Mumm-type rearrangement leading to a stereoselective synthesis of highly functionalised S-allyl benzoylcarbamothioates.