Allenylphosphonates/Allenylphosphine Oxides as Intermediates/Precursors for Intramolecular Cyclization Leading to Phosphorus-Based Indenes, Indenones, Benzofurans, and Isochromenes

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Dearomative 1,6-addition of P(O)–H to in situ formed p-QM-like ion pairs from 2-benzofuryl-ols to C3-phosphinoyl hydrobenzofurans

We report a dearomative C3-phosphorylation and a tandem C3-phosphorylation/aromatization of 2-benzofuryl-ols with P(O)–H species to afford C3-phosphinoyl hydrobenzofurans and benzofurans, respectively.

Copper-catalyzed tandem phosphorylative allenylation/cyclization of 1-(o-aminophenyl)prop-2-ynols with the P(O)–H species: access to C2-phosphorylmethylindoles

A novel method to synthesize C2-phosphorylmethylindoles via the carbocation formation initiated tandem phosphorylative allenylation/cyclization of 1-(o-aminophenyl)prop-2-ynols with the P(O)–H species has been developed.

Reactivity of allenylphosphonates/allenylphosphine oxides – some new addition/cycloaddition and cyclization pathways

In this paper, we highlight some addition/cycloaddition reactions of allenylphosphonates and allenylphosphine oxides, mostly based on the work done in our laboratory. Thus the electrophilic addition of iodine monochloride (ICl) with allenylphosphine oxides affords cyclic phosphonium salts rather than γ-chloro-β-iodovinylphosphine oxides (NMR, HRMS, X-ray) that exhibit rather unusual downfield shifts in the 31P NMR spectra. These compounds undergo hydrolysis to afford γ-hydroxy-β-iodovinylphosphine oxides; the hydroxymethyl group in these compounds can be oxidized by Dess-Martin periodinane to afford the corresponding aldehyde-substituted vinylphosphine oxides. A [2+2] cycloaddition product of an allenylphosphonate has also been structurally characterized. Other reactions that are highlighted include those leading to (Z)/(E)-β-aminovinylphosphonates, β-ketophosphonates (and their utility in Horner-Wadsworth-Emmons reaction), indolyl/furanyl/isocoumaranyl/naphthyl phosphine oxides, thiophosphorylated phosphonates and azo-substituted coumarin phosphonates.

Metal-free cascade rearrangement/radical addition/oxidative C–H annulation of propargyl alcohols with sodium sulfinates: access to 2-sulfenylindenones

A facile protocol for the construction of 2-sulfenylindenones via one-pot cascade rearrangement/radical addition/oxidative C–H cyclization of propargyl alcohols with sodium sulfinates has been developed.

Highly functionalised (γ-azido/γ-fluoro-β-iodo/)vinyl derivatives from phosphorus based allenes or allenoates: I⋯O halogen bonding interactions

γ-Azido/γ-fluoro-β-iodo-vinyl phosphine oxides/phosphonates/esters/sulphone and a γ-diodoallene were synthesised from phosphorus-based allenes or allenoates or a sulphur based allene; in many cases, I⋯O halogen bonding is observed in the solid state.

Organobase catalyzed straightforward synthesis of phosphinyl functionalized 2H-pyran cores from allenylphosphine oxides and 1,3-diones

A cost-effective system is revealed here to construct polyfunctionalized 2H-pyran cores containing phosphinyl groups with an organobase as a catalyst.

Construction of Polysubstituted 1,2-Dihydrocyclobuta[b]naphthalenes and 1,2-Dihydrocyclobuta[b]anthracenes with Photoluminescence

A facile protocol for the synthesis of polysubstituted 1,2-dihydrocyclobuta[b]naphthalenes and 1,2-dihydrocyclobuta[b]anthracenes was developed via a sequence of base-promoted 1,5-H shift, intramolecular [2 + 2] cycloaddition, and aromatization. The synthesized 1,2-dihydrocyclobuta[b]anthracenes exhibited bright blue emissions in solution and strong yellow emissions in solid, which made them possible candidates for optoelectronic conjugated materials.

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.

1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles

1,3-Dien-5-ynes have been extensively used as starting materials for the synthesis of a wide number of different carbo- and heterocycles. The aim of this review is to give an overview of their utility in organic synthesis, highlighting the variety of compounds that can be directly accessed from single reactions over these systems. Thus, cycloaromatization processes are initially commented, followed by reactions directed toward the syntheses of five-membered rings, other carbocycles and, finally, heterocycles. The diverse methodologies that have been developed for the synthesis of each of these types of compounds from 1,3-dien-5-ynes are presented, emphasizing the influence of the reaction conditions and the use of additional reagents in the outcome of the transformations.

Base catalysed intermolecular cyclisation of N-protected o-amino benzaldehyde/acetophenone with phosphorus/sulphur based allenes: facile synthesis of substituted quinolines

Base catalyzed reaction of N-Bz protected o-aminobenzaldehyde with allenylphosphonates affords O-phosphorylated quinolines via a route involving a thermal rearrangement.

Convenient synthesis of allenylphosphoryl compounds via Cu-catalysed couplings of P(O)H compounds with propargyl acetates

A novel Cu-catalysed substitution reaction of propargyl acetates with P(O)H compounds is developed to afford allenylphosphoryl compounds in high yields. A plausible mechanism involving the nucleophilic interception of Cu-allenylidene intermediates is proposed.

Construction of highly functionalized naphthalenes using an in situ ene–allene strategy

Construction of highly functionalized naphthalene derivatives remains a challenging task for organic chemists because of the effect of the substituent.

Synthesis of 1,4-dihydrophosphinoline 1-oxides by acid-promoted cyclization of 1-(diphenylphosphoryl)allenes

Phosphonoallenes in Brønsted acids (TfOH, FSO₃H, H₂SO₄) gave the corresponding 1,2-oxaphosphol-3-enium ions, that were studied by means of NMR and DFT calculations.

Synthesis of γ-hydroxypropyl P-chirogenic (±)-phosphorus oxide derivatives by regioselective ring-opening of oxaphospholane 2-oxide precursors

The synthesis of P-chirogenic (±)-phosphine oxides and phosphinates via selective nucleophilic ring opening of the corresponding oxaphospholanes is described. Two representative substrates: the phosphonate 2-ethoxy-1,2-oxaphospholane 2-oxide and the phosphinate 2-phenyl-1,2-oxaphospholane 2-oxide were reacted with various Grignard reagents to produce a single alkyl/aryl product. These products may possess further functionalities in addition to the phosphorus center such as the γ-hydroxypropyl group which results from the ring opening and π-donor moieties such as aryl, allyl, propargyl and allene which originates from the Grignard reagent.

Acid-promoted transformations of 1-(diphenylphosphoryl)allenes: synthesis of novel 1,4-dihydrophosphinoline 1-oxides

1-(Diphenylphosphoryl)alka-1,2-dienes in acids (TfOH, FSO₃H, H₂SO₄) give (3-hydroxyalk-1-en-1-yl)diphenylphosphine oxides, that are further converted into 1-phenyl-1,4-dihydrophosphinoline 1-oxides. The latter compounds are directly formed from these 1,2-dienes under the action of AlCl₃.

Spontaneous resolution upon crystallization of allenyl-bis-phosphine oxides

The first example of ‘spontaneous resolution by crystallization’ in allene chemistry, by means of crystal structures and solid state CD spectra of the R and S enantiomers, is presented.

α-Allenyl Ethers as Starting Materials for Palladium Catalyzed Suzuki-Miyaura Couplings of Allenylphosphine Oxides with Arylboronic Acids

We disclose here the first palladium-catalyzed Suzuki-Miyaura couplings of aryl ethers functionalized allenylphosphine oxides with arylboronic acids. This new methodology with α-allenyl ethers as starting materials provides a novel approach to generate phosphinoyl 1,3-butadienes and derivatives with medium to excellent yields. The reaction tolerates a variety of functional groups to afford ranges of structurally diverse substituted phosphionyl 1,3-butadienes. For unsymmetrical allene substrates, high stereospecific additions to give E-isomers are usually observed. On the basis of the known palladium and allene chemistry, a mechanism is proposed.

Central-axial-central chirality transfer: asymmetric synthesis of highly substituted indenes bearing a stereogenic quaternary carbon center from optically active propargyl alcohols

An asymmetric synthesis of highly substituted indenes 3, bearing a quaternary stereogenic carbon center, has been developed via the central-axial-central chirality transfer from optically active propargyl alcohols 1. This transformation involves the addition/rearrangement of 1 and ynamides 2 to give tetra-substituted allenes 4 and further cyclization of 4.

Assembly of indenamine derivatives through in situ formed N-sulfonyliminium ion initiated cyclization

An expedient route to structurally diverse indenamine derivatives through condensation of the readily accessible substituted cinnamylaldehydes and sulfonylamines under the catalysis of FeCl3 has been developed, featuring high efficiency in the generation of two bonds and one ring in a single-step and water as the only by-product.

Allenylphosphine oxides as simple scaffolds for phosphinoylindoles and phosphinoylisocoumarins

A range of phosphinoylindoles was prepared in one-pot from functionalized propargyl alcohols and a suitable P(III) precursor via a base-mediated reaction. The reaction proceeds via the intermediacy of allenylphosphine oxides. Similarly, phosphinoylisocoumarins were prepared from allenylphosphine oxides in a trifluoroacetic acid-mediated reaction; the latter also acts as a solvent. Interestingly, in the presence of wet trifluoroacetic acid, in addition to phosphinoylisocoumarins, phosphorus-free isocoumarins were also obtained. Key products were characterized by single crystal X-ray crystallography.

NHC–gold(i) catalysed [4 + 2] cycloaddition–acyclic addition of dialkyl substituted propargylic esters with 1,3-diphenylisobenzofuran: synthesis of novel benzo[c]fluorenols and substituted dienes

A new class of benzofluorenols is generated via a novel gold carbene complex [IPrAuCl/AgSbF₆] catalysed cycloaddition of propargylic esters with 1,3-diphenylisobenzofuran.