Silylenes with a Small Chalcogenide Substituent: Tuning Frontier Orbital Energies from O to Te

The general synthesis of heteroleptic acyclic silylenes with a bulky carbazolyl substituent (dtbpCbz) is detailed and a series of compounds with a chalcogenide substituent of the type [(dtbpCbz)SiE16R] (E16R=OtBu, SEt, SePh, TePh) is reported. With the bulky carbazolyl substituent present, the chalcogenide moiety can be very small, as is shown by incorporating groups as small as ethyl, phenyl or tert-butyl. For the first time, the electronic properties of the silylene can be tuned along a complete series of chalcogenide substituents. The effects are clearly visible in the NMR and UV/Vis spectra, and were rationalised by DFT computations. The reactivity of the heaviest chalcogenide-substituted silylenes was probed by reactions with trimethylphosphine selenide and the terphenyl azide TerN3 (Ter=2,6-dimesitylphenyl).

Direct Synthesis of Phosphoryltriacetates from White Phosphorus via Visible Light Catalysis

Organophosphorus compounds (OPCs) are widely used in many fields. However, traditional synthetic routes in the industry usually involve multistep and hazardous procedures. Therefore, it's of great significance to construct such compounds in an environmentally-friendly and facile way. Herein, a photoredox catalytic method has been developed to construct novel phosphoryltriacetates. Using fac-Ir(ppy)3 (ppy=2-phenylpyridine) as the photocatalyst and blue LEDs (456 nm) as the light source, white phosphorus can react with α-bromo esters smoothly to generate phosphoryltriacetates in moderate to good yields. This one-step approach features mild reaction conditions and simple operational process without chlorination.

Formation, Reactivity and Decomposition of Aryl Phospha-Enolates

Two lithium phospha-enolates [RP=C(Siⁱ Pr₃ )OLi]₂ were prepared by reaction of triisopropyl silyl phosphaethynolate, ⁱ Pr₃ SiPCO, with aryl lithium reagents LiR (R=Mes: 1,3,5-trimethyl phenyl; or Mes*: 1,3,5,-tri-tertbutyl phenyl). Monomer/dimer aggregation of the enolates can be modulated by addition of 12-crown-4. Substitution of lithium for a heavier alkali metal was achieved through initial formation of a silyl enol ether, followed by reaction with KO^t Bu to form the corresponding potassium phospha-enolate [MesP=C(Siⁱ Pr₃ )OK]₂ . On addition of water, the enolates are protonated to afford RP=C(Siⁱ Pr₃ )(OH). For the sterically less demanding system (R=Mes), this phospha-enol rapidly tautomerises to the corresponding acyl phosphine MesP(H)C(Siⁱ Pr₃ )(O), which on heating extrudes CO. In contrast, bulkier phospha-enol (R=Mes*) is stable to rearrangement at room temperature and thermally decomposes to RH and ⁱ Pr₃ SiPCO.

Binding, Release and Functionalization of Intact Pnictogen Tetrahedra Coordinated to Dicopper Complexes

The bridging MeCN ligand in the dicopper(I) complexes [(DPFN)Cu₂(μ,η¹ : η¹‐MeCN)][X]₂ (X=weakly coordinating anion, NTf₂ (1 a), FAl[OC₆F₁₀(C₆F₅)]₃ (1 b), Al[OC(CF₃)₃]₄ (1 c)) was replaced by white phosphorus (P₄) or yellow arsenic (As₄) to yield [(DPFN)Cu₂(μ,η² : η²‐E₄)][X]₂ (E=P (2 a–c), As (3 a–c)). The molecular structures in the solid state reveal novel coordination modes for E₄ tetrahedra bonded to coinage metal ions. Experimental data and quantum chemical computations provide information concerning perturbations to the bonding in coordinated E₄ tetrahedra. Reactions with N‐heterocyclic carbenes (NHCs) led to replacement of the E₄ tetrahedra with release of P₄ or As₄ and formation of [(DPFN)Cu₂(μ,η¹ : η¹‐^MeNHC)][X]₂ (4 a,b) or to an opening of one E−E bond leading to an unusual E₄ butterfly structural motif in [(DPFN)Cu₂(μ,η¹ : η¹‐E₄^DippNHC)][X]₂ (E=P (5 a,b), E=As (6)). With a cyclic alkyl amino carbene (^EtCAAC), cleavage of two As−As bonds was observed to give two isomers of [(DPFN)Cu₂(μ,η² : η²‐As₄^EtCAAC)][X]₂ (7 a,b) with an unusual As₄‐triangle+1 unit.

Continuous Flow Acylation of (Hetero)aryllithiums with Polyfunctional N,N-Dimethylamides and Tetramethylurea in Toluene

The continuous flow reaction of various aryl or heteroaryl bromides in toluene in the presence of THF (1.0 equiv) with sec -BuLi (1.1 equiv) provided at 25 °C within 40 sec the corresponding aryllithiums which were acylated with various functionalized N,N-dimethylamides including easily enolizable amides at -20 °C within 27 sec, producing highly functionalized ketones in 48-90% yield (36 examples). This method was well suited for the preparation of α-chiral ketones such as naproxene and ibuprofen derived ketones with 99% ee . A one-pot stepwise bis-addition of two different lithium organometallics to 1,1,3,3-tetramethyurea (TMU) provided unsymmetrical ketones in 69-79% yield (9 examples).

Synthetic Approaches to Triarylboranes from 1885 to 2020

In recent years, research in the fields of optoelectronics, anion sensors and bioimaging agents have been greatly influenced by novel compounds containing triarylborane motifs. Such compounds possess an empty p-orbital at boron which results in useful optical and electronic properties. Such a diversity of applications was not expected when the first triarylborane was reported in 1885. Synthetic approaches to triarylboranes underwent various changes over the following century, some of which are still used in the present day, such as the generally applicable routes developed by Krause et al. in 1922, or by Grisdale et al. in 1972 at Eastman Kodak. Some other developments were not pursued further after their initial reports, such as the synthesis of two triarylboranes bearing three different aromatic groups by Mikhailov et al. in 1958. This review summarizes the development of synthetic approaches to triarylboranes from their first report nearly 135 years ago to the present.

Fragmentation Mechanism of White Phosphorus: A Theoretical Insight into Multiple Cleavage/Formation of P-P and P-C Bonds

Molecular-level understanding of metal-mediated white phosphorus (P₄ ) activation is meaningful but challenging because of its direct relevance to the conversion of P₄ into useful organophosphorus compounds as well as the complicated and unforeseeable cleavage process of P-P bonds. The related study, however, has still rarely been achieved to date. Here, a theoretical insight into the step-by-step process of three P-P bond cleavage/four P-C bond formation for [P₃ +P₁ ]-fragmentation of P₄ mediated by lutetacyclopentadienes is reported. The unique charge-separated intermediate and the intermolecular cooperation between two lutetacyclopentadienes play a vital role in the subsequent P-P/P-C bond breaking/forming. It is found that, although the first P-C formation is involved in the assembly of the cyclo-P₃ [R₄ C₄ P₃ ]^- unit, the construction of the aromatic five-membered P₁ heterocycle [R₄ C₄ P]^- is completed prior to the cyclo-P₃ formation. The reaction mechanism has been carefully elucidated by analyses of the geometric structure, frontier molecular orbitals, bond index, and natural charge, which greatly broaden and enrich the general knowledge of the direct functionalization of P₄ .

Functionalization of Pentaphosphorus Cations by Complexation

The chemistry of polyphosphorus cations has rapidly developed in recent years, but their coordination behavior has remained mostly unexplored. Herein, we describe the reactivity of [P5 R2 ]+ cations with cyclopentadienyl metal complexes. The reaction of [CpAr Fe(μ-Br)]2 (CpAr =C5 (C6 H4 -4-Et)5 ) with [P5 R2 ][GaCl4 ] (R=iPr and 2,4,6-Me3 C6 H2 (Mes)) afforded bicyclo[1.1.0]pentaphosphanes (1-R, R=iPr and Mes), showing an unsymmetric "butterfly" structure. The same products 1-R were formed from K[CpAr ] and [P5 R2 ][GaCl4 ]. The cationic complexes [CpAr Co(η4 -P5 R2 )][GaCl4 ] (2-R[GaCl4 ], R=iPr and Cy) and [(CpAr Ni)2 (η3:3 -P5 R2 )][GaCl4 ] (3-R[GaCl4 ]) were obtained from [P5 R2 ][GaCl4 ] and [CpAr M(μ-Br)]2 (M=Co and Ni) as well as by using low-valent "CpAr MI " sources. Anion metathesis of 2-R[GaCl4 ] and 3-R[GaCl4 ] was achieved with Na[BArF24 ]. The P5 framework of the resulting salts 2-R[BArF24 ] can be further functionalized with nucleophiles. Thus reactions with [Et4 N]X (X=CN and Cl) give unprecedented cyano- and chloro-functionalized complexes, while organo-functionalization was achieved with CyMgCl.

Dithiocarboxylic Acids: An Old Theme Revisited and Augmented by New Preparative, Spectroscopic and Structural Facts

The unstable dithiocarboxylic acids dithioacetic acid, 2-methyl-dithiopropionic acid, 2,2-dimethyl-dithiopropionic acid and dithiobenzoic acid were synthesized and characterized by NMR spectroscopy and GC/MS. The stable dithiocarboxylic acids 2,4,6-trimethyl benzoic acid, 2,4,6-tri-iso-propylbenzoic acid and 2,6-dimesityl benzoic acid were synthesized, isolated and characterized by spectroscopic methods and in parts by mass spectrometry and X-ray crystallography. The new data were used to re-evaluate literature data on the synthesis, spectroscopy and structural data of dithiocarboxylic acids as a fundamental class of organic compounds in general.

Synthesis and Reactivity of an End-Deck cyclo-P4 Iron Complex

Reduction of the Fe^II complex [(^Ph PP₂^Cy )FeCl₂ ] (2) generated an electron-rich and unsaturated Fe⁰ species, which was reacted with white phosphorus. The resulting new complex, [(^Ph PP₂^Cy )Fe(η⁴ -P₄ )] (3), is the first iron cyclo-P₄ complex and the only known stable end-deck cyclo-P₄ complex outside Group V. Complex 3 features an Fe^II center, as shown by Mössbauer spectroscopy, associated to a P₄^2- fragment. The distinct reactivity of complex 3 was rationalized by analysis of the molecular orbitals. Reaction of complex 3 with H⁺ afforded the unstable complex [(^Ph PP₂^Cy )Fe(η⁴ -P₄ )(H)]⁺ (4), whereas with CuCl and BCF, the complexes [(^Ph PP₂^Cy )Fe(η⁴ :η¹ -P₄ )(μ-CuCl)]₂ (5) and [(^Ph PP₂^Cy )Fe(η⁴ :η¹ -P₄ )B(C₆ F₅ )₃ ] (6) were formed.

Functionalization of P4 through Direct P-C Bond Formation

Research on chlorine-free conversions of P4 into organophosphorus compounds (OPCs) has a long track record, but methods that allow desirable, direct P-C bond formations have only recently emerged. These include the use of metal organyls, carbenes, carboradicals, and photochemical approaches. The versatile product scope enables the preparation of both industrially relevant organophosphorus compounds, as well as a broad range of intriguing new compound classes. Herein we provide a concise overview of recent breakthroughs and outline the acquired fundamental insights to aid future developments.

A Tetracyclic Octaphosphane by Successive Addition, Inversion, and Condensation Reactions

An example of an octaphosphane of type R₂ P₈ (R=(DDP)Ga) was isolated by treatment of cage compound (DDP)GaP₄ (2, DDP=(2,6-diisopropylphenyl)(4-((2,6-diisopropylphenyl)imino)pent-2-en-2-yl)amide) with (C₆ F₅ )₂ PBr. The initially formed endo-exo butterfly shaped pentaphosphane 7 rapidly rearranges to the more stable exo-exo isomer 8, which undergoes dimerization to decaphosphane 11. Compound 11 unexpectedly eliminates tetraaryldiphosphane 13 to give tetracyclo[3.3.0.0^2,7 .0^4,6 ]octaphosphane [(DDP)GaBr]₂ P₈ (12). The reaction steps were confirmed by crystal structure analysis of the key intermediates and supported by kinetic studies using NMR techniques.

Metalate-Mediated Functionalization of P4 by Trapping Anionic [Cp*Fe(CO)2(η1-P4)]- with Lewis Acids

The development of selective functionalization strategies of white phosphorus (P4) is important to avoid the current chlorinated intermediates. The use of transition metals (TMs) could lead to catalytic procedures, but these are severely hampered by the high reactivity and unpredictable nature of the tetrahedron. Herein, we report selective first steps by reacting P4 with a metal anion [Cp*Fe(CO)2]- (Cp*=C5(CH3)5), which, in the presence of bulky Lewis acids (LA; B(C6F5)3 or BPh3), leads to unique TM-substituted LA-stabilized bicyclo[1.1.0]tetraphosphabutanide anions [Cp*Fe(CO)2(η1-P4⋅LA)]-. Their P-nucleophilic site can be subsequently protonated to afford the transient LA-free neutral butterflies exo,endo- and exo,exo-Cp*Fe- (CO)2(η1-P4H), allowing controllable stepwise metalate-mediated functionalization of P4.

Rearrangement of a P4 Butterfly Complex-The Formation of a Homoleptic Phosphorus-Iron Sandwich Complex

The versatile coordination behavior of the P₄ butterfly complex [{Cp'''Fe(CO)₂ }₂ (μ,η^1:1 -P₄ )] (1, Cp'''=η⁵ -C₅ H₂^t Bu₃ ) towards different iron(II) compounds is presented. The reaction of 1 with [FeBr₂ ⋅dme] (dme=dimethoxyethane) leads to the chelate complex [{Cp'''Fe(CO)₂ }₂ (μ₃ ,η^1:1:2 -P₄ ){FeBr₂ }] (2), whereas, in the reaction with [Fe(CH₃ CN)₆ ][PF₆ ]₂ , an unprecedented rearrangement of the P₄ butterfly structural motif leads to the cyclo-P₄ moiety in {(Cp'''Fe(CO)₂ )₂ (μ₃ ,η^1:1:4 -P₄ )}₂ Fe][PF₆ ]₂ (3). Complex 3 represents the first fully characterized "carbon-free" sandwich complex containing cyclo-P₄ R₂ ligands in a homoleptic-like iron-phosphorus-containing molecule. Alternatively, 2 can be transformed into 3 by halogen abstraction and subsequent coordination of 1. The additional isolated side products, [{Cp'''Fe(CO)₂ }₂ (μ₃ ,η^1:1:2 -P₄ ){Cp'''Fe(CO)}][PF₆ ] (4) and [{Cp'''Fe(CO)₂ }₂ (μ₃ ,η^1:1:4 -P₄ ){Cp'''Fe}][PF₆ ] (5), give insight into the stepwise activation of the P₄ butterfly moiety in 1.

Reduction of [Cp*Sb]4 with Subvalent Main-Group Metal Reductants: Syntheses and Structures of [(L1 Mg)4 (Sb4 )] and [(L2 Ga)2 (Sb4 )] Containing Edge-Missing Sb4 Units

[Cp*Sb]₄ (Cp*=C₅ Me₅ ) reacts with [L¹ Mg]₂ and L² Ga with formation of [(L¹ Mg)₄ (μ₄ ,η^1:2:2:2 -Sb₄ )] (L¹ =iPr₂ NC[N(2,6-iPr₂ C₆ H₃ )]₂ , 1) and [(L² Ga)₂ (μ,η^2:2 -Sb₄ )] (L² =HC[C(Me)N(2,6-iPr₂ C₆ H₃ )]₂ , 2). The cleavage of the Sb-Sb and Sb-C bonds in [Cp*Sb]₄ are the crucial steps in both reactions. The formation of 1 occurred by elimination of the Cp* anion and formation of Cp*MgL¹ , while 2 was formed by reductive elimination of Cp*₂ and oxidative addition of L² Ga to the Sb₄ unit. 1 and 2 were characterized by heteronuclear NMR spectroscopy and single-crystal X-ray diffraction, and their bonding situation was studied by quantum chemical calculations.

Selective [3+1] Fragmentations of P4 by "P" Transfer from a Lewis Acid Stabilized [RP4 ]- Butterfly Anion

Two [3+1] fragmentations of the Lewis acid stabilized bicyclo[1.1.0]tetraphosphabutanide Li[Mes*P₄ ⋅ BPh₃ ] (Mes*=2,4,6-tBu₃ C₆ H₂ ) are reported. The reactions proceed by extrusion of a P₁ fragment, induced by either an imidazolium salt or phenylisocyanate, with release of the transient triphosphirene Mes*P₃ , which was isolated as a dimer and trapped by 1,3-cyclohexadiene as a Diels-Alder adduct. DFT quantum chemical computations were used to delineate the reaction mechanisms. These unprecedented pathways grant access to both P₁ - and P₃ -containing organophosphorus compounds in two simple steps from white phosphorus.

CpPEt2 As4 -An Organic-Substituted As4 Butterfly Compound

Cp^PEt₂ As₄ (Cp^PEt =C₅ (4-EtC₆ H₄ )₅ ) (1) is synthesized by the reaction of Cp^PEt. radicals with yellow arsenic (As₄ ). In solution an equilibrium of the starting materials and the product is found. However, 1 can be isolated and stored in the solid state without decomposition. As₄ can be easily released from 1 under thermal or photochemical conditions. From powder samples of Cp^PEt₂ As₄ , yellow arsenic can be sublimed under rather mild conditions (T=125 °C). A similar behavior for the P₄ -releasing agent was determined for the related phosphorus compound Cp^BIG₂ P₄ (2; Cp^BIG =C₅ (4-nBuC₆ H₄ )₅ ). DFT calculations show the importance of dispersion forces for the stability of the products.

Stabilization and Transfer of the Transient [Mes*P4](-) Butterfly Anion Using BPh3

The transient bicyclo[1.1.0]tetraphosphabutane anion, generated from white phosphorus (P4) and Mes*Li (Mes*=2,4,6-tBu3C6H2), can be trapped by BPh3 in THF. This Lewis acid stabilized anion can be used as an [RP4](-) transfer agent, reacting cleanly with neutral Lewis acids (B(C6F5)3, BH3, and W(CO)5) to afford unique singly and doubly coordinated butterfly anions, and with the trityl cation to form a neutral, nonsymmetrical, all-carbon-substituted P4  derivative. This reaction path enables a simple, stepwise functionalization of white phosphorus.

E4 Butterfly Complexes (E=P, As) as Chelating Ligands

The coordination properties of new types of bidentate phosphane and arsane ligands with a narrow bite angle are reported. The reactions of [{Cp'''Fe(CO)2 }2 (μ,η(1:1) -P4 )] (1 a) with the copper salt [Cu(CH3 CN)4 ][BF4 ] leads, depending on the stoichiometry, to the formation of the spiro compound [{{Cp'''Fe(CO)2 }2 (μ3 ,η(1:1:1:1) -P4 )}2 Cu](+) [BF4 ](-) (2) or the monoadduct [{Cp'''Fe(CO)2 }2 (μ3 ,η(1:1:2) -P4 ){Cu(MeCN)}](+) [BF4 ](-) (3). Similarly, the arsane ligand [{Cp'''Fe(CO)2 }2 (μ,η(1:1) -As4 )] (1 b) reacts with [Cu(CH3 CN)4 ][BF4 ] to give [{{Cp'''Fe(CO)2 }2 (μ3 ,η(1:1:1:1) -As4 )}2 Cu](+) [BF4 ](-) (5). Protonation of 1 a occurs at the "wing tip" phosphorus atoms, which is in line with the results of DFT calculations. The compounds are characterized by spectroscopic methods (heteronuclear NMR spectroscopy and IR spectrometry) and by single-crystal X-ray diffraction studies.

A Gallium-Substituted Distibene and an Antimony-Analogue Bicyclo[1.1.0]butane: Synthesis and Solid-State Structures

RGa {R=HC[C(Me)N(2,6-iPr2C6H3)]2} reacts with Sb(NMe2)3 with insertion into the Sb-N bond and elimination of RGa(NMe2)2 (2), yielding the Ga-substituted distibene R(Me2N)GaSb=SbGa(NMe2 )R (1). Thermolysis of 1 proceeded with elimination of RGa and 2 and subsequent formation of the bicyclo[1.1.0]butane analogue [R(Me2N)Ga]2Sb4 (3).

Attenuated Organomagnesium Activation of White Phosphorus

Sequential reactions between a 2,6-diisopropylphenyl-substituted β-diketiminato magnesium n-butyl derivative and P₄ allow the highly discriminating synthesis of unusual [nBu₂P₄]²⁻ and [nBu₂P₈]²⁻ cluster dianions.

Low-Temperature Isolation of the Bicyclic Phosphinophosphonium Salt [Mes*2P4Cl][GaCl4]

The reaction of [ClP(μ-PMes*)]2 (1) with the Lewis acid GaCl3 yielded a hitherto unknown tetraphosphabicyclo[1.1.0]butan-2-ium salt, [Mes*P4(Cl)Mes*][GaCl4] (3[GaCl4]), which incorporates a positively charged phosphonium center within its bicyclic P4 scaffold. The formation of the title compound was studied by means of low-temperature NMR experiments. This led to the identification of an intermediate cyclotetraphosphenium cation, which was trapped by reaction with dimethylbutadiene (dmb). All of the compounds were fully characterized by experimental and computational methods.