Visible-Light-Induced Regioselective Radical-Polar Crossover 1,4-Hydrophosphinylation of 1,3-Enynes: Access to Trisubstituted Allenes Bearing a Phosphine Oxide Group

The radical 1,4-functionalizations of 1,3-enynes have emerged as a powerful strategy for the synthesis of multisubstituted allenes. However, the phosphorus-centered radical-initiated transformations remain largely elusive. Herein, visible-light photoredox catalytic regioselective radical hydrophosphinylation of 1,3-enynes with diaryl phosphine oxides as phosphinoyl radical precursors has been realized. This protocol features mild conditions, a wide substrate scope, and good functional group tolerance, producing a diverse range of phosphinoyl-substituted allenes in moderate to good yields with high atom economy. Detailed mechanistic experiments revealed a radical-polar crossover process in the reaction.

Nickel-catalyzed switchable 1,3-dienylation and enantioselective allenylation of phosphine oxides

The development of general catalytic methods for the regio- and stereoselective construction of phosphoryl derivatives from identical substrates remains a formidable challenge in organic synthesis. Enabled by the newly developed BDPP-type ligands, we disclosed a nickel-catalyzed allenylation of phosphine oxides rationally and predictably, allowing the construction of versatile chiral allenylphosphoryl derivatives with high enantiopurity (up to 94% e.e.). Alternatively, using an achiral phosphine ligand dcypbz under acidic conditions, we achieved a regiochemical switch of the 1,3-dienylation to afford functionalized phosphinoyl 1,3-butadienes (up to 93% yield). The salient features of this method include switchable reactivity, broad substrate scope, readily available feedstock, single-step preparation, and high asymmetric induction.

HFIP-Catalyzed Difluoroalkylation of Propargylic Alcohols to Access Tetrasubstituted Difluoroalkyl Allenes

A hexafluoroisopropanol (HFIP)-catalyzed difluoroalkylation of propargylic alcohols with difluoroenoxysilanes to access structurally diverse tetrasubstituted difluoroalkyl allenes has been developed. This convenient procedure enables the rapid construction of highly functionalized multisubstituted fluorinated allenes in a mild and straightforward way. Furthermore, the synthetic potential of this methodology has been demonstrated by the facile synthesis of various structurally interesting fluorine-containing molecules such as gem-difluorosubstituted dihydropyran, tetrasubstituted CF₂H-allene, and multisubstituted fluorinated cyclopentanone derivatives.

Catalysis and regioselectivity in hydrofunctionalization reactions of unsaturated carbon bonds. Part III

The review addresses the possibility of obtaining Markovnikov and anti-Markovnikov isomers in the reactions of unsaturated hydrocarbons with organophosphorus and organosulfur compounds having P–H and S–H bonds using metal salts or complexes as catalysts.

The bibliography includes 247 references.

Three-Component Reactions of α-Amino Acids, p-Quinone Monoacetals, and Diarylphosphine Oxides to Selectively Afford 3-(Diarylphosphinyl)anilides and N-Aryl-2-diarylphosphinylpyrrolidines

The three-component reactions of α-amino acids, p-quinone monoacetals (or p-quinol ethers), and diarylphosphine oxides have been developed for the synthesis of 3-(diarylphosphinyl) anilides and N-aryl-2-diarylphosphinylpyrrolidines. The transformations may involve the in situ generation of conjugated azomethine ylides or 2-azaallyl anion species from the reaction of α-amino acids and p-quinone monoacetals, which are further trapped by diarylphosphine oxides.

Zinc-catalyzed regioselective C–P coupling of p-quinol ethers with secondary phosphine oxides to afford 2-phosphinylphenols

A highly regioselective C–P coupling reaction of p-quinol ethers with secondary phosphine oxides is developed as a new synthesis method for 2-phosphinylphenols by using Zn(OTf)₂ as the catalyst.

Palladium-catalyzed microwave-assisted Hirao reaction utilizing the excess of the diarylphosphine oxide reagent as the P-ligand; a study on the activity and formation of the “PdP2” catalyst

The microwave-assisted Hirao reaction of bromobenzene and diarylphosphine oxides was performed at 120 °C using triethylamine as the base, and 5% of palladium acetate as the catalyst in ethanol. 5% Excess of the >P(O)H reagent served as the reducing agent, while another 10% as the preligand (in the >POH tautomeric form). It was found that the P–C coupling reaction was significantly faster with (2-MeC6H4)2P(O)H (A) and (3,5-diMeC6H3)2P(O)H (B), than with Ph2P(O)H (C) and (4-MeC6H4)2P(O)H (D). Moreover, species A and B could be applied as selective P-ligands in the reaction of bromobenzene with C or D. Dependence of the effectiveness of “PdP2” catalysts with diarylphosphine oxide preligands on the methyl substituents followed a reversed order as the reactivity of the diarylphosphine oxide species in the P–C coupling itself. Formation of the “PdP2” catalyst from palladium acetate and diarylphosphine oxide has never been studied, but now it was evaluated by us at the B3LYP level of theory applying 6-31G(d,p) for C,H,P,O and SDD/MW28 for Pd including the explicit-implicit solvent model. The novel mechanism requiring three equivalents of the >P(O)H species for each of the palladium acetate molecule was in agreement with the preparative experiments. The ligation of palladium(0) with different P(III) species comprising the >POH form of the >P(O)H reagent was also studied, and the critical role of the steric hindrance on the ligation, and hence on the activity of the “PdP2” catalyst was substantiated. Last but not least, the influence of the Me substituents in the aromatic ring of the P-reagents on the energetics of the elemental steps of the Hirao reaction itself was also evaluated.

Copper-catalyzed decarboxylative cyclization via tandem C–P and C–N bond formation: access to 2-phosphorylmethyl indoles

A copper-catalyzed decarboxylative cyclization of ethynyl benzoxazinanones with P(O)H compounds has been developed for the synthesis of 2-phosphorylmethyl indoles.

Scope and advances in the catalytic propargylic substitution reaction

Nucleophilic displacement of the propargylic alcohol is one of the sought-after methods in the current scenario. The highly nucleophilic alkyne functional moiety along with its considerably acidic terminal hydrogen atom allows the propargylic unit to play a crucial role in organic synthesis by offering a handle for further synthetic transformations. Until 2000, the most fundamental propargylic substitution reaction was the Nicolas reaction, a multi-step transformation, developed in 1972, which involved cobalt as a stoichiometric promoter. Therefore, the direct catalytic substitution of propargylic alcohols was a highly desirable method for development. The pioneering work on the Ru-catalyzed propargylic substitution reaction in 2000 encouraged many researchers to develop several novel catalytic propargylic substitution reactions, which have made rapid progress since then. The purpose of this review is to emphasise the involvement of diverse types of Lewis acid, transition metal and Brønsted acid catalysts in the propargylic substitution reaction and provide an updated summary of the recent developments in this field. The selected examples presented here are the most significant and relevant ones and we believe that this will help the readers to comprehend the scope of the propargylic substitution reaction with diverse types of catalysts and will envisage the scientific community for the future developments in this field.

Direct nucleophilic displacement of the alpha-hydroxy of the propargylic alcohol is one of the sought-after methods in the current scenario. An updated summary of the recent developments in this field is presented here.

Synthesis of benzothiophene and indole derivatives through metal-free propargyl-allene rearrangement and allyl migration

An efficient base-catalyzed protocol for the synthesis of benzothiophene is described. The reaction proceeds via base-promoted propargyl-allenyl rearrangement followed by cyclization and allyl migration. Phosphine-substituted indoles can be synthesized by a similar strategy.

Exploring allene chemistry using phosphorus-based allenes as scaffolds

In this paper, we review some of our results on cycloaddition and cyclization reactions of allenylphosphonates/and allenyl phosphine oxides. Thus nitro-substituted propargylic alcohols react with P(III)–Cl substrates to lead to unprecedented phosphono-benzazepines or -hydroxyindolinones. A similar reaction using a higher stoichiometry of P(III)–Cl precursor has led to the first observation of spontaneous resolution by crystallization in allene chemistry. In the reaction of these phosphorus based allenes with diphenyl isobenzofuran (DPBF), depending on the substituents, both [α, β] and [β, γ]-cycloaddition products are formed. Extension of this work to sulfur based allenes and leading to a new [4+2] cycloadduct with DPBF is reported. A novel cyclization reaction of a functionalized allenylphosphine oxide with diethylamine leading to 3-diethylamino-4-diphenylphosphinoyl-1-naphthol is discovered. Base-catalyzed reactions of allenylphosphonates with 2-substituted benzaldehdes lead to novel thiochromans, chromenes, and quinolines. Allenylphosphonates undergo phosphonylation in the presence of tetrabutyl ammonium fluoride (TBAF). Vinyl azides derived from allenylphosphonates undergo thermolysis to form bis-phosphonopyrazines. [Pd]-catalyzed reactions of allenes with 2-iodophenols, 2-iodobenzyl alcohol and 3-iodo-indole-2-carboxylic acids lead to a variety of benzofurans, benzopyrans and benzopyranones, respectively.

Silver-catalyzed phosphonylation of unprotected propargylic alcohols for the synthesis of allenylphosphoryl compounds

A facile and efficient method for the synthesis of a variety of allenylphosphine oxides by silver-catalyzed phosphonylation of unprotected propargylic alcohols with P(O)H compounds has been established.