Oxaphosphirane-Borane Complexes: Ring Strain and Migratory Insertion/Ring-Opening Reactions

Protonation of P-Stereogenic Phosphiranes: Phospholane Formation via Ring Opening and C-H Activation

Protonation of cyclopropanes and aziridines is well-studied, but reactions of phosphiranes with acids are rare and have not been reported to result in ring opening. Treatment of syn-Mes*PCH₂CHR (Mes* = 2,4,6-(t-Bu)₃C₆H₂, R = Me or Ph, syn-1-2) or anti-Mes*PCH₂CHPh (anti-2) with triflic acid resulted in regiospecific anti-Markovnikov C-protonation with ring opening and cyclophosphination of a Mes* ortho-t-Bu group to yield the phospholanium cations [PH(CH₂CH₂R)(4,6-(t-Bu)₂-2-CMe₂CH₂C₆H₂)][OTf] (R = Me or Ph, 3-4), which were deprotonated with NEt₃ to give phospholanes 5-6. Enantioenriched or racemic syn-1 both gave racemic 3. The byproduct [Mes*PH(CH₂CH₂Me)(OH)][OTf] (7) was formed from syn-1 and HOTf in the presence of water. Density functional theory calculations suggested that P-protonation followed by ring opening and hydride migration to C yields the phosphenium ion, [Mes*P(CH₂CH₂Me)][OTf], which undergoes C-H oxidative addition of an o-t-Bu methyl group. This work established a new reactivity pattern for phosphiranes.

The quest for unligated oxaphosphiranes with phosphorus in different coordination numbers

After more than 160 years of oxirane (epoxide) chemistry, the first derivative of a phosphorus analogue, namely oxaphosphirane, has been synthesized. Reactions with borane sulfane, a peroxy compound and elemental sulfur, as well as tetra-chloro-ortho-benzoquinone (TOB) reveal a significant destabilization upon increasing the coordination number at phosphorus from σ³ to σ⁴, thus somehow supporting previous reports on such fleeting species. Theoretical studies provide insight into ring strain energy, ring/ring interconversion and the oxidation pathways.

Ring Strain Energies of Three-Membered Homoatomic Inorganic Rings El3 and Diheterotetreliranes El2Tt (Tt = C, Si, Ge): Accurate versus Additive Approaches

Accurate ring strain energies (RSEs) for three-membered symmetric inorganic rings El₃ and organic dihetero-monocycles El₂C and their silicon El₂Si and germanium El₂Ge analogues have been computed for group 14–16 “El” heteroatoms using appropriate homodesmotic reactions and calculated at the DLPNO-CCSD-(T)/def2-TZVPP//B3LYP-D4/def2-TZVP(ecp) level. Rings containing triels and Sn/Pb heteroatoms are studied as exceptions to the RSE calculation as they either do not constitute genuine rings or cannot use the general homodesmotic reaction scheme due to uncompensated interactions. Some remarkable concepts already related to the RSE such as aromaticity or strain relaxation by increasing the s-character in the lone pair (LP) of the group 15–16 elements are analyzed extensively. An appealing alternative procedure for the rapid estimation of RSEs using additive rules, based on contributions of ring atoms or endocyclic bonds, is disclosed.

Accurate ring strain energies (RSEs) are calculated for saturated three-membered homoatomic inorganic rings El₃ and diheterotetreliranes El₂Tt (Tt = C, Si, Ge). Aromaticity, lone-pair-induced strain relaxation for the group 15−16 elements, and bond stiffness are extensively discussed as main factors affecting the RSE. An attractive alternative procedure for fast estimation of the RSE using additive rules is proposed.

Accurate Ring Strain Energies of Unsaturated Three-Membered Heterocycles with One Group 13-16 Element

High-quality ring strain energy (RSE) data for 1H-unsaturated (CH)₂X parent rings, where X is a group 13–16 element, are reported in addition to the 2H-isomers of the pnictogenirene rings. RSE data are obtained from appropriate homosdesmotic reactions and calculated at the DLPNO-CCSD(T)/def2-TZVPP//B3LYP-D3/def2-TZVP(ecp) level. 1H-Tallirene and 1H-plumbirene have unique donor–acceptor structures between an acetylene π(CC) orbital and an empty p orbital of a metallylene subunit (a Dewar–Chatt–Duncanson description) and therefore cannot be described as proper rings but as pseudocyclic structures. Also, 1H-indirene and 1H-oxirene lack ring critical points and constitute borderline cases of pseudorings. 1H-Unsaturated rings exhibit enhanced RSE compared to their saturated homologues. The mechanism of ring strain relaxation by increasing the s character in the lone pair (LP) of group 15–16 elements is remarkable and increases on descending the groups. Furthermore, RSE is affected by the aromatic character of group 13 rings and certain aromatic or antiaromatic character in group 14 or 15–16 rings, respectively, which tend to vanish on descending the group as shown by NICS(1) values. 2H-Unsaturated rings were found only for group 15 elements (although only 2H-azirine shows a proper cyclic structure) and displayed lower RSE (higher stability) than the corresponding 1H-isomers.

Accurate ring strain energies (RSE) are calculated for three-membered 1H-unsaturated rings containing a 13−16 group element and group 15 2H-isomers. The presence or absence of lone pairs at the heteroatom and the existence of intrinsic or residual (anti)aromaticity were shown to affect the RSE.

Configurational Lability at Tetrahedral Phosphorus: syn/anti-Isomerization of a P-Stereogenic Phosphiranium Cation by Intramolecular Epimerization at Phosphorus

Tetrahedral main-group compounds are normally configurationally stable, but P-epimerization of the chiral phosphiranium cations syn- or anti-[Mes*P(Me)CH₂ CHPh][OTf] (Mes*=2,4,6-(t-Bu)₃ C₆ H₂ ) occurred under mild conditions at 60 °C in CD₂ Cl₂ , resulting in isomerization to give a syn-enriched equilibrium mixture. Ion exchange with excess [NBu₄ ][Δ-TRISPHAT] (Δ-TRISPHAT=Δ-P(o-C₆ Cl₄ O₂ )₃ ) followed by chromatography on silica removed [NBu₄ ][OTf] and gave mixtures of syn- and anti-[Mes*P(Me)CH₂ CHPh][Δ-TRISPHAT]⋅x[NBu₄ ][Δ-TRISPHAT]. NMR spectroscopy showed that isomerization proceeded with epimerization at P and retention at C. DFT calculations are consistent with a mechanism involving P-C cleavage to yield a hyperconjugation-stabilized carbocation, pyramidal inversion promoted by σ-interaction of the P lone pair with the neighboring β-carbocation, and ring closure with inversion of configuration at P.

Accurate Ring Strain Energy Calculations on Saturated Three-Membered Heterocycles with One Group 13-16 Element

Accurate ring strain energy (RSE) data for parent (CH₂)₂X rings are reported, where X are group 13-16 elements (El) in their lowest oxidation state, from the second to the sixth row, with their covalence completed by bonds to H. They are obtained from appropriate homodesmotic and hyper-homodesmotic reactions at different levels up to the CCSD(T) level, thus providing a benchmark of high-quality reference RSE values, as well as acceptably accurate faster lower-level options. Derivatives of indium, thallium, and lead cannot be properly described by a three-member ring connectivity, because they display a unique donor-acceptor structure from an ethylene π(C═C) orbital to an empty p orbital of a metallylene subunit. The RSE of groups 13 and 14 heterocycles increases on descending in the group (except for Ga and Ge), while it decreases for groups 15 and 16. The latter is presumably due to a strain-releasing mechanism favored by the increase of p-character at the sp³-type atomic orbital used by El in the endocyclic El-C bonds, %p(El)_El-C, originated by the tendency of the El lone pairs in groups 15-16 to increase their s-character. This strain-releasing mechanism does not exist in heavier tetrels, which keep almost unchanged the p-character in the endocyclic bonds at El, whereas for triels the p-character is still lower owing to their sp²-like hybridization. Remarkable linear correlations were found between the RSE and either the above-mentioned %p(El)_El-C, the distal C-C bond distance or the relaxed force constants for the endocyclic bond angles.

A synthetic equivalent for unknown 1,3-zwitterions? – A K/OR phosphinidenoid complex with an additional Si–Cl function

Although the chemistry of frustrated Lewis pairs (FLPs) has seen tremendous developments, investigations on anionic, mono-molecular FLPs are still scarce and 1,3-zwitterions are unknown, but a functional K/OR phosphinindenoid complex can be used instead.

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.

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.

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

While 1,2σ5λ5-oxaphosphetanes are well known intermediates from the Wittig-reaction, no 1,2σ3λ3-oxaphosphetanes have been described, so far. Herein, we present the first synthesis of 1,2σ3λ3-oxaphosphetanes derived from their κP-Mo(CO)5 complexes and first investigations towards metal coordination and P-oxidation. Bonding, ring strain energy and potential retro-[2+2] cycloaddition reactions of the 1,2-oxaphosphetane ring were studied by DFT methods.

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.

Going for strain: synthesis of the first 3-imino-azaphosphiridine complexes and their conversion into oxaphosphirane complex valence isomers

Reaction of a Li/Cl phosphinidenoid complex with carbodiimides yielded novel 3-imino-azaphosphiridine complexes, which was converted to the first stable valence isomer of an oxaphosphirane complex.