"Low-coordinate" 1,2-oxaphosphetanes - a new opportunity in coordination and main group chemistry

Deoxygenation of Oxiranes by λ3 σ3 -Phosphorus Reagents: A Computational, Mechanistic, and Stereochemical Study

The deoxygenation of parent and substituted oxiranes by λ3 σ3 -phosphorus reagents has been explored in detail, therefore unveiling mechanistic aspects as well as regio- and stereochemical consequences. Attack to a ring C atom is almost always preferred over one-step deoxygenation by direct P-to-O attack. In most cases a carbene transfer occurs as first step, leading to a phosphorane and a carbonyl unit that thereafter react in the usual Wittig fashion via the corresponding λ5 σ5 -1,2-oxaphosphetane intermediate. Betaines rarely constitute true minima after the first C-attack to oxiranes, at least in the gas-phase. Use of the heavier derivatives AsMe3 and SbMe3 as oxirane deoxygenating reagents was also mechanistically studied. The thermodynamic tendency of λ3 σ3 -phosphorus reagents to act as oxygen (O-attack) or carbene acceptors (C-attack) was theoretically studied by means of the thermodynamic oxygen-transfer potential (TOP) and the newly defined thermodynamic carbene-transfer potential (TCP) parameters, that were explored in a wider context together with many other acceptor centres.

Arbuzov meets 1,2-oxaphosphetanes: transient 1,2-oxaphosphetan-2-iums as an entry point to beta-halo phosphane oxides and P-containing oligomers

Herein, we describe the synthesis of a 1,2σ3λ3-oxaphosphetane from ethylene oxide and its reactions with alkyl halides to form β-halo phosphane oxides in an Arbuzov-type reaction. When methyl triflate was used as a hard electrophile, cationic oligomerisation of 1,2-oxaphosphetanes was observed. DFT calculations indicate 1,2-oxaphosphetan-2-iums as intermediates and reveal differences between the Arbuzov and the potential Perkow reaction pathway.

Formation and reactivity of NHC-boryl radicals: insight into substituent effect from theoretical calculations

Substituent modification effects of N-heterocyclic carbene (NHC) boranes on their hydrogen atom abstraction (HAA) reactions and the chemical reactivities of corresponding NHC-boryl radicals have been investigated by density functional theory calculations. The substituent modification of NHC-boranes may notably affect the HAA reaction, both kinetically and thermodynamically, and shows remarkable substitution position dependence. The multi-site-modification of NHC-boranes is proved to be more effective for reduction of the B-H bond dissociation energy (BDE), promotion of the HAA reaction, and the reactivity regulation of their corresponding NHC-boryl radicals. Computational screening reveals that the spin density and the charge population of the radical boron center have good correlation with the B-H BDEs of NHC-boranes and the chemical reactivities of NHC-boryl radicals, and they can be considered as property and reactivity descriptors of these boron-based systems. The present results and established scaling relationships are beneficial to promote the advancement of design of NHC-boryl radical catalysis.

1,2σ3λ3-Oxaphosphetanes and Their P-Chalcogenides-A Combined Experimental and Theoretical Study

Although 1,2σ⁵λ⁵-oxaphosphetanes have been known for a long time, the “low-coordinate” 1,2σ³λ³-oxaphosphetanes have only been known since their first synthesis in 2018 via decomplexation. Apart from ligation of this P-heterocycle to gold(I)chloride and the oxidation using ortho-chloranil, nothing on their chemistry has been reported so far. Herein, we describe the synthesis of new 1,2σ³λ³-oxaphosphetane complexes (3a–e) and free derivatives (4a–e), as well as reactions of 4a with chalcogens and/or chalcogen transfer reagents, which yielded the P-chalcogenides (14–16a; Ch = O, S, Se). We also report on the theoretical results of the reaction pathways of C-phenyl-substituted 1,2 σ³λ³-oxaphosphetanes and ring strain energies of 1,2σ⁴λ⁵-oxaphosphetane P-chalcogenides.

CHNO isomers and derivatives – a computational overview

A state-of-the-art computational study of CHNO isomers is presented, including substituted derivatives and cyclotrimerization and deprotonation products.

M/X Phosphinidenoid Metal Complex Chemistry

ConspectusLike singlet carbenes and silylenes, transient electrophilic terminal phosphinidene complexes enabled highly selective synthetic transformations, but the required multistep synthetic protocols precluded widespread use of these P₁ building blocks. By contrast, nucleophilic M/Cl phosphinidenoid complexes can be easily accessed in one step from [M(CO)_n(RPCl₂)] complexes. This advantage and the mild reaction conditions opened broad synthetic applicability that enabled access to a variety of novel compounds. The chemistry will be described in this Account, including bonding and mechanistic considerations derived from high-level density functional theory calculations.In 2007, we gained the first strong evidence for the formation of these thermally labile complexes using two different synthetic approaches: P-H deprotonation and Cl/Li exchange; the latter has become the preferred method. Intense studies revealed that steric demand of the P substituents in combination with metal complexation, a donor solvent, and/or the presence of a crown ether are necessary prerequisites for the formation and especially the usability of these intermediates as novel P₁ building blocks. Solution-phase NMR spectroscopy and solid-state X-ray diffraction studies revealed the bonding situation, i.e., a solvent-separated ion pair structure, and typical ³¹P NMR signatures of the anions. To date, we have established the following reactivity patterns for Li/Cl phosphinidenoid complexes: self-condensations (I), electrophilic and nucleophilic reactions (II), 1,1-additions (III), [2 + 1] cycloadditions (IV), ring expansions (V), and redox reactions (VI). For example, self-condensations can yield dinuclear acyclic or polycyclic diphosphane or diphosphene complexes. Their use as nucleophiles and electrophiles can be employed to access functional phosphane ligands with mixed substitution patterns. 1,1-Addition reactions were a puzzling discovery because the resulting products resembled classical P-C π-bond structures but the bonding was more of a donor-to-phosphorus adduct with significant differences in bonding parameters. Into the same category and also surprising fall formal E-H insertion reactions leading to 1,1'-bifunctional phosphane complexes. To date, the most important synthetic impact was achieved in the chemistry of strained P-heterocyclic ligands such as oxaphosphiranes and azaphosphiridines, obtained via [2 + 1] cycloadditions of the title compounds with carbonyls and imines, respectively. Ring expansions have been shown to yield 1,2-oxaphosphetanes and 1,2-thiaphosphetanes, and because of the pool of industrially important epoxides, this provides straightforward and affordable access to these novel P-heterocyclic ligands, which also promise to be of interest in catalytic applications. Recent developments describe redox transformations of Li/Cl phosphinidenoid complexes into new reactive intermediates such as complexes with open-shell P-functional phosphanyl ligands via oxidative single electron transfer reactions or into terminal electrophilic phosphinidene complexes via chloride elimination. The latter is clearly restricted to P-amino derivatives because of their enhanced π-donation capability, as evidenced in a recent study on umpolung of these reactive intermediates. While our efforts to expand M/X phosphinidenoid complex chemistry are ongoing, we want to emphasize that the development of new reactive intermediates not only improves our understanding of bonding and reactivity but also opens new perspectives in organoelement chemistry.

Oxophosphonium-Alkyne Cycloaddition Reactions: Reversible Formation of 1,2-Oxaphosphetes and Six-membered Phosphorus Heterocycles

While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic synthesis, the reactivity of alkynes with isoelectronic main-group R₂E═O compounds is unexplored. Herein, we show that oxophosphonium ions, which are the isoelectronic phosphorus congeners to carbonyl compounds, undergo [2 + 2] cycloaddition reactions with different alkynes to generate 1,2-oxaphosphete ions, which were isolated and structurally characterized. The strained phosphorus-oxygen heterocycles open to the corresponding heterodiene structure at elevated temperature, which was used to generate six-membered phosphorus heterocycles via hetero Diels-Alder reactions. Insights into the influence of the substituents at the phosphorus center on the energy profile of the oxygen atom transfer reaction were obtained by quantum-chemical calculations.

Benchmarking the inversion barriers in σ3λ3-phosphorus compounds: a computational study

Benchmark study of three mechanisms for inversion at phosphorus, including parametric criteria for classification and analysis of factors affecting barriers.

Synthesis of free and ligated 1,2-thiaphosphetanes – expanding the pool of strained P-ligands

The 1,2-thiaphosphetane heterocycle could be synthesized for the first time in the coordination sphere of a metal complex (I) from which the free ligand (II) could be obtained, too.

Quantum Chemical Calculations on CHOP Derivatives-Spanning the Chemical Space of Phosphinidenes, Phosphaketenes, Oxaphosphirenes, and COP- Isomers

After many decades of intense research in low-coordinate phosphorus chemistry, the advent of Na[OCP] brought new stimuli to the field of CHOP isomers and derivatives thereof. The present theoretical study at the CCSD(T)/def2-TZVPP level describes the chemical space of CHOP isomers in terms of structures and potential energy surfaces, using oxaphosphirene as the starting point, but also covering substituted derivatives and COP- isomers. Bonding properties of the P⁻C, P⁻O, and C⁻O bonds in all neutral and anionic isomeric species are discussed on the basis of theoretical calculations using various bond strengths descriptors such as WBI and MBO, but also the Lagrangian kinetic energy density per electron as well as relaxed force constants. Ring strain energies of the superstrained 1H-oxaphosphirene and its barely strained oxaphosphirane-3-ylidene isomer were comparatively evaluated with homodesmotic and hyperhomodesmotic reactions. Furthermore, first time calculation of the ring strain energy of an anionic ring is described for the case of oxaphosphirenide.

Unconventional ionic ring-deconstruction pathways of a three-membered heterocycle

Two different ionic deconstruction reactions of the oxaphosphirane ring in I are reported. One is induced by base and involves displacement of the aldehyde unit forming II whilst acid-initiated extrusion of the ring-carbon with its substituents yielded III. Further insights into the latter ring-deconstruction were obtained by quantum chemical calculations.