BF3-Promoted Ring Expansion of Iminylphosphiranes and Acylphosphiranes for Divergent Access to 1,2-Azaphospholidines and 1,2-Dihydrophosphetes.

Ring expansion of strained small rings provides an efficient method for the synthesis of various high-value carbocycles and heterocycles. Here we report BF3·Et2O as both an activating reagent and fluorine source, enabling ring expansion of phosphirane and P-F bond formation. Treatment of 1-iminylphosphirane complexes with BF3·Et2O resulted in 1,2-azaphospholidines, while the reaction of 1-acylphosphirane complexes with BF3·Et2O afforded 1,2-dihydrophosphetes. The reaction path was tuned by the nucleophilicity of the N and O atoms toward the intermediate phosphenium cation.

Towards tetrasubstituted furans through rearrangement and cyclodimerization of acetylenic ketones.

Cyclodimerization of readily accessible acetylenic ketones facilitated by a phosphane-borane complex under basic conditions is achieved. This methodology allows one-pot synthesis of phosphorus-involved tetrasubstituted furans via the construction of a C-P bond and a furan ring within a single procedure. A plausible reaction mechanism is proposed.

2H-Phosphindole-Enabled Dearomatization and [4+2] Cycloaddition of (Hetero)Arenes.

The heavier main group multiple bonds offer an effective tool for small molecule activation. Transient 2H-phosphinidole working as a reactive phosphadiene system undergoes phospha-Diels-Alder reaction with a wide range of non-activated aromatic carbocycles and heterocycles, including naphthalene, anthracene, phenanthrene, furan, thiophene, pyrrole, pyridine, and benzo-fused heterocycles, affording concise access to a range of polycyclic fused rings feature with phosphorus at the bridgehead. These results demonstrate that non-activated (hetero)arenes are capable of acting as 2π systems in [4+2] cycloaddition with highly reactive 2H-phosphindole complex.

Phosphorus-Involved Wagner-Meerwein Rearrangement of Phosphiranes: An Entry to Four-Membered Phosphacycles.

Phosphenium ions [R2P]+ are important and highly reactive dicoordinate phosphorus species. Herein, we report a rearrangement of the carbocation into the phosphenium cation driven by ring strain. This phosphorus-involved Wagner-Meerwein rearrangement pathway converted the 1-acylphosphirane complex into phosphetane and 1,2-dihydrophosphete derivatives depending on the reaction temperature. The generation of the intermediate phosphenium cation was identified by the intramolecular reaction with ether, which also disclosed its strong Lewis acidity. This work expands the boundary of the phosphorus-carbon analogy.

Activation of CS2 with the 2H-Phosphindole Complex to Construct P,S-Polycycles.

The activation of CS2 by the 2H-phosphindole complex with a low-coordinate phosphadiene moiety is reported. The successive hetero-Diels-Alder reaction between 2H-phosphindoles and CS2 constructs two bridged rings and one spirocycle simultaneously, affording structurally complex P,S-polycyclic products. The two 2H-phosphindoles approach the C═S bond in a head-to-head disposition to minimize steric hindrance. This work reveals the unique reactivity of low-coordinate organophosphorus species and their potential applications in small molecule activation.

Dearomatization [4+2] Cycloaddition of Nonactivated Benzene Derivatives.

Dearomatization reactions have recently emerged as a powerful tool for the rapid buildup of molecular complexity. Here, an unparalleled thermal dearomatization [4+2] cycloaddition reaction between benzene derivatives and a 2H-phosphindole tungsten complex was reported. The unique reactivity of the in situ-generated 2H-phosphindole complex toward benzene was revealed by density functional theory calculations. We thus provide new insights into the dearomatization of nonactivated arenes and pave the way for the manipulation of the dearomatization for further applications.

Insight into fragmentation of a phosphirane to form phosphinidene complexes: an illustration with the 1-phenylselenylphosphirane W(CO)5 complex.

Density functional theory (DFT) calculations with 1-phenylselenylphosphirane complex 1 provide an insight into phosphirane fragmentation to phosphinidene complexes. FMO and ELF analyses show that the cleavage of two P-C σ bonds of phosphirane proceeds via an asynchronous concerted pathway. Transient [PhSeP-W(CO)5] was generated by dissociation of 1 at 90 °C and trapped with different reagents. The 1-phenoxylphosphirane complex undergoes [1 + 2] retroaddition at a comparatively higher temperature which implies that the lone pair of the adjacent atom center of phosphorus plays a major role in phosphirane fragmentation.

Intramolecular Activation of Enones by Electrophilic Phosphinidene Complexes to Construct 2-Phosphafurans.

Herein, we report a facile and highly atom-economic approach to 2-phosphafurans by using simple 2-chloroethylphosphine and acetylenic ketones. The key step of this protocol utilizes the Lewis acidity of electrophilic phosphinidenes to induce an intramolecular cyclization with enones. Dearomative hetero-Diels-Alder reactions of 2-phosphafurans provide two series of bicyclic phosphacycles. This rare synthetic application of Lewis acidity of electrophilic phosphinidene complexes represents a new frontier of phosphinidene chemistry.

FeCl2 Catalyzed Three-Component Reactions of Phospholes, Pyrrolidine, and Ketones (Aldehydes): Chemoselective Synthesis of 1-Phosphafulvenes.

We have developed an unprecedented approach for the synthesis of transient 1-phosphafulvenes through three component reactions of phospholes. The generation of 1-phosphafulvenes was demonstrated by in situ [6 + 4] cycloaddition with 2H-phospholes and [6 + 6] self-dimerization. The [6 + 4] and [6 + 6] reaction pathway could be modulated by the starting ketones and aldehydes. The construction of 1-phosphafulvenes is illustrated by a proposed mechanism combining nucleophilic addition of phospholide to the iminium or isomerized azomethine ylide and a [1,5]-shift of phosphole.

Nonbenzenoid aromaticity of 1-phosphafulvenes: synthesis of phosphacymantrenes.

The coordination chemistry of 1-phosphafulvenes was investigated by employing their [6 + 4] adducts or α-C2-bridged biphospholes as a precursor. Unbridged phosphacymantrenes arise from 1-phosphafulvenes via proton abstraction. α-C2-bridged biphosphacymantrenes are probably yielded by the reductive coupling of 1-phosphafulvene with Mn2(CO)10. The coordination behavior of 1-phosphafulvenes is comparable to that of pentafulvenes, which again demonstrates the phosphorus-carbon analogy in low-coordinate organophosphorus chemistry.

1,1-Addition of α-C2-Bridged Biphospholes with Alkynes.

An unusual chemoselective 1,1-addition of α-C2-bridged biphospholes to terminal alkynes is reported. The developed protocol provides simple access to the unknown 1,3-diphosphepines, which has potential applications in the coordination and catalyst chemistry. Their Pd and Mo complexes were studied by single-crystal X-ray diffraction analysis. This method features excellent chemoselectivity, high step and atom economy, mild reaction conditions, and wide substrate scope.

The chemistry of phosphirane-substituted phosphinidene complexes.

Thermolysis of the 1,1'-biphosphirane pentacarbonylmetal complex offers access to 3,4-dihydro-1,2-diphosphete and diphosphorus (P2) intermediates. A control reaction proves that this step-wise reaction is initiated by the dissociation of a W(CO)5 group. DFT calculations predict that further transformations proceed via a transient phosphiranylphosphinidene complex. The formation of the 3,4-dihydro-1,2-diphosphete derivative is thermodynamically favored, while a kinetic process provides the diphosphorus (P2) complex.

An Approach to Peri-Fused Heterocycles: A Metal-Mediated Cascade Carbonylative Cyclization/Dearomatic Diels-Alder Reaction.

A short and efficient route to peri-fused heterocycles has been developed. The transition metal in cooperation with the imino directing group cleaves the C-P bond of the phosphirane, followed by 1,1-insertion of CO and reductive elimination to give a reactive azaphosphacyclohexone derivative. The further dearomatic Diels-Alder reaction of the in situ-generated azaphosphacyclohexone with alkenes provides an array of annulated heterodecalins with high molecular complexity and atomic utilization.

Transition-Metal-Like Reversible Cycloadditions of [tBuSP-W(CO)5 ] with Alkenes and Alkynes.

tert-Butylthiophosphinidene complex [tBuSP-W(CO)5 ] was generated by dissociation of 1-(tert-butylthio)phosphirane-W(CO)5 complex under mild conditions. The formation of transient [tBuSP-W(CO)5 ] was indicated by trapping reactions with 2,3-dimethyl-1,3-butadiene, alkynes, phenanthrene-9,10-dione, and methanol. The LUMO of [MeSP-W(CO)5 ] is significantly lower in energy than those of [Me2 NP-W(CO)5 ], [MeOP-W(CO)5 ], and [Me2 PP-W(CO)5 ]. The HOMO of [MeSP-W(CO)5 ] contains a significant contribution from the in-plane lone pair of P and the LUMO shows a typical π* characteristic. Since stabilized by sulfur lone pair and coordinated by W(CO)5 , [tBuSP-W(CO)5 ] undergoes facile and reversible cycloadditions with alkenes and alkynes.

An approach to 7-aza-1-phosphanorbornane complexes: strain promoted rearrangement of 1-iminylphosphirane complexes and cycloaddition with olefins.

1-Iminylphosphirane W(CO)5 complexes react with alkenes under an atmosphere of CO at 130 °C to form the original 7-aza-1-phosphanorbornane complexes. The reaction works well with both electron-rich and electron-poor alkenes. On the basis of DFT calculations, we propose a mechanism including the rearrangement of phosphirane into a dipolar five-membered ring and the following cycloaddition of this dipolar intermediate with alkenes.

The coordination chemistry of phosphinidene sulfides. Synthesis and catalytic properties of Pd4 and Pt4 clusters.

7-Phosphanorbornene sulfides were used as [RP = S] precursors. The reaction of these precursors with [M(PPh3)4] (M = Pd, Pt) yields star-like M4 clusters in which the central core is coated by three RP = S units acting as 4-electron µ2-P, η2-P = S ligands. The Pd cluster displays both stability and catalytic activity in the Suzuki-Miyaura reaction. DFT analysis suggests that a mononuclear [η2-RP = S]Pd(PPh3) complex is involved in the formation of the Pd4 clusters.

Generation and Trapping of a 1-Phosphafulvene: An Illustration of the P═C/C═C Analogy.

1-Phosphafulvenes can be easily generated by dissociation of dimers resulting from the reaction of phospholes with aldimines. As electron-rich partners, they act as 4π phosphadienic systems toward alkenes and alkynes in [4 + 2] cycloaddition reactions. As electron-poor partners, they act either as 2π systems toward conjugated dienes in [2 + 4] cycloaddition reactions via their P═C double bond or as 6π systems toward phosphadienes in [6 + 4] cycloaddition reactions.

The Chemistry of 1-Acylphosphirane Complexes: A Phosphorus Analogue of the Cloke-Wilson Rearrangement.

The stabilization of a phosphirane ring by complexation to tungsten pentacarbonyl allows the emergence of the Cloke-Wilson rearrangement in 1-acylphosphirane complexes around 130 °C. Contrary to the cyclopropane case, this transformation of the 1-acylphosphirane to the 1,3-oxaphosphol-3-ene complexes is reversible. It is favored by a 2-phenyl, and even a 2-vinyl substitution. The 1,3-oxaphosphol-3-ene complexes are trapped by conjugated dienes.

The chemistry of parent phosphiranide in the coordination sphere of tungsten.

2-Chloroethylphosphine W(CO)5 complex readily reacts with sodium hydride. With one equivalent of NaH, the parent phosphirane complex is obtained. With more than two equivalents, the phosphiranide complex is exclusively formed. With 1.5 equivalents, a 1 : 1 mixture of and is obtained but readily attacks at the phosphorus atom by splitting of ethylene and by the formation of the P-P complex . In turn, the P-P bond of is split by NaH to yield the phosphide complex . The phosphiranide complex is a good source for a large variety of functional phosphirane complexes . With BrCN, the 1-cyanophosphirane complex is formed. Upon heating it loses its complexing group. Upon hydrolysis, it gives the 1-hydroxyphosphirane complex which dimerizes in basic medium by opening one P-C bond of the ring to give . The reaction of with PhPCl2 yields the triphosphorus complex whose molecular structure has been established by X-ray crystal structure analysis.

Insertion of phosphinidene complexes into the P-H bond of secondary phosphine oxides: a new version of the phospha-Wittig synthesis of P=C double bonds.

Terminal phosphinidene complexes [RP-W(CO)5], as generated at 60 °C in the presence of copper chloride from the appropriate 7-phosphanorbornadiene complexes, react with secondary phosphine oxides Ar2P(O)H to give the insertion products into the P-H bonds. After metalation with NaH, these products react with aldehydes to give the corresponding phosphaalkenes which are trapped by dimethylbutadiene.