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

Synthesis and Characterization of a [1,2,6]Diazaphosphonine Oxide: An Example of a Photoswitchable Phosphorus-Containing Cyclic Azobenzene

We report herein the synthesis and characterization of a phosphorus-containing cyclic azobenzene as a new photoswitchable scaffold. This backbone reveals high bidirectional photoswitching yields and high thermal stability for both isomers, with t1/2 > 90 days at 60 °C. Both E- and Z-isomers have been characterized by UV-vis spectroscopy and X-ray crystallography.

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.

Theoretical determination of the standard enthalpies of formation of alkyl radicals using the concept of a complete set of homodesmotic reactions

The complete sets of homodesmotic reactions (HDR) for 107 acyclic alkyl free radicals С4-С9 of normal and branched structure were constructed using the graph-theoretic representation and analysis of a tested compound. The absolute enthalpies of the studied compounds and HDR reference structures were calculated using the M062X/cc-pVTZ level of theory. Based on these data, the thermal effects of HDRs were calculated and then applied to determine the standard enthalpies of formation of the studied radicals using the known enthalpies of formation of reference structures. The dissociation energies of BDE C-H and C-CH3 bonds were also calculated. The effect of radical structure on the BDE value is discussed, and a new effect of stabilization of the radical center in the skewed conformation of free radical is established; this effect has not been previously described in the scientific literature.

Ring Strain Energy of Diheteropnictogeniranes El2 Pn (Pn=N, P, As, Sb)- Accurate versus Additive Approaches

Accurate ring strain energy (RSE) values for sixty-six parent pnictogeniranes having two other identical p-block elements, El2 Pn, have been reported. A decrease in RSE was observed to correlate with an increase in the p character of the AO used in endocyclic bonds, which is particularly remarkable on descending the groups 15 and 16. The latter also parallels higher -NICS(1) values, which seems not to be related with an increase in aromaticity, as pointed out by other NICS-related criteria, but to atom-centred diatropic currents mostly arising from the presence of lone pairs. Only in case of pnictogenaditrieliranes Tr2 Pn (Tr=B, Al, Ga), the decrease of -NICS(1) is related to a lower Hückel-type 2π-electron aromaticity on descending group 13. The use of an additive methodology based on atom-strain contributions enables estimation of RSEs for a large majority of all possible three-membered rings containing group 13-16 elements with modest accuracy (RMSE=4.371 kcal/mol), that could be remarkably improved by using bond-strain contributions (RMSE=1.183 kcal/mol) instead.

1,2-Palladasilacyclobutene: The Missing Link in the Pd-Catalyzed Annulation of Alkynes for the Silirene-to-Silole Transformation

The palladium-catalyzed annulation reaction of alkynes enables an attractive approach to siloles. Their access from silirenes and terminal alkynes proved rather general, involving reactive intermediates that have remained elusive to date. Starting from 1,2-bis(3-thienyl)silirene as a source of photochromic siloles, the mechanism of the annulation reaction has been revisited, and palladasilacyclobutenes resulting from the activation of the silirene could be isolated and thoroughly characterized (NMR, X-ray, and DFT). Their role as reactive intermediates and their fate in the course of the reaction were also studied in situ. In combination with in-depth DFT calculations, a clearer picture of the mechanism and the reactive key species is disclosed.

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.

Stability of Carbocyclic Phosphinyl Radicals: Effect of Ring Size, Delocalization, and Sterics

In this computational study, we report on the stability of cyclic phosphinyl radicals with an aim for a systematical assessment of stabilization effects. The radical stabilization energies (RSEs) were calculated using isodesmic reactions for a large number of carbocyclic radicals possessing different ring sizes and grades of unsaturation. In general, the RSE values range from −1.2 to −14.0 kcal·mol^–1, and they show practically no correlation with the spin populations at the P-centers. The RSE values correlate with the reaction Gibbs free energies calculated for the dimerization of the studied simple radicals. Therefore, the more easily accessible RSE values offer a cost-effective estimation of global stability in a straightforward manner. To explore the effect of unsaturation on the RSE values, delocalization energies were determined using appropriate isodesmic reactions. Introducing unsaturations beside the P-center into the backbone of the rings leads to an additive increase in the magnitude of the delocalization energy (∼10, 20, and 30 kcal·mol^–1, respectively, for radicals with one, two, and three C=C bonds in the conjugation). Parallelly, the spin populations at the P-centers also dwindle gradually by ∼0.1 e in the same order, indicating that the lone electron delocalizes over the π-system. Radicals containing exocyclic C=C π-bonds were also investigated, and all of these radicals have rather similar stabilities independently of the ring size, outlining the primary importance of the two exocyclic π-bonds in the conjugation. Among the radicals involved in our study, those with the best electronic stabilization are the unsaturated three-, five-, six-, and seven-membered rings containing the maximum number of conjugated vinyl fragments. The largest delocalization energy of 31.5 kcal·mol^–1 and the lowest obtained spin population of 0.665 e were found for the fully unsaturated seven-membered radical (phosphepin derivative). Importantly, the electronic stabilization effects alone are insufficient for stabilizing the radicals in monomeric forms epitomized by the exothermic dimerization energies (−40 to −58 kcal·mol^–1). Therefore, it is essential to apply sterically demanding bulky substituents on the α-C-atoms. Tweaking the steric congestion enabled us to propose radicals that are expected to be stable against dimerization and, consequently, may be realistic target species for synthetic investigations. The effects contributing to the stability of radicals having sterically encumbered substituents have also been explored.

To systematically evaluate the stabilization effects, the radical stabilization energies of various carbocyclic phosphinyl radicals having saturated backbones or unsaturation(s) in either endocyclic or exocyclic manner have been determined and analyzed. As the electronic stabilization is alone insufficient to hamper the possible dimerization of these species, the effect of several sterically demanding substituents has been explored for the congeners with best electronic stabilizations, thus enabling us to propose synthetically accessible candidates in the future.

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.

Composite Corelated Molecular Orbital Theory Calculations of Ring Strain for Use in Predicting Polymerization Reactions

Strained ring systems play an important role in synthesis and can be characterized by the ring strain energy (RSE). The RSE of 3, 4, 5, and 6 membered saturated and unsaturated ring systems containing N, O, P, and S heteroatoms and H, F, Si(CH 3 ) 3 , and SO 2 (CH 3 ) substituents were calculated at the G3(MP2) composite correlated molecular orbital theory level using up to 5 models to predict the RSE. Generally, the RSE decreased as ring size increased with a substantial decrease from 4 to 5 membered rings. Replacement of a ring CH 2 with P or S reduced the RSE, consistent with less angle strain. The RSE for unsaturated systems were generally greater than for saturated systems due to increased angle strain. No general trends were found with respect to substituent effects. The RSE values suggest that 3-pyrroline and 2-pyrroline may be able to support ring opening metathesis polymerization and warrant further study.

Synthesis of azadiphosphiridine complexes. Theoretical studies on ring formation, the P-to-P' metal shift and the resulting nitrogen geometry

A set of azadiphosphiridine complexes 3a and 4b,c were synthesized in high selectivity using N-H and P-H deprotonation as key steps and RPCl₂ as substrates (R = NⁱPr₂ (a), -^tBu (b), Ph (c)). While complex 3a (P-NⁱPr₂) retained the P-W linkage of the starting material W(CO)₅{Ph₃CP(H)NH}, complexes 4b (P-^tBu) and 4c (P-Ph) revealed that a P-to-P' haptotropic shift of the W(CO)₅ group has occurred. Remarkably, complex 3a, bearing an unligated P-NⁱPr₂ unit, displays a planar ring N geometry while 4b,c showed a pyramidal geometry of the ring nitrogen atom. Theoretical studies on the ring formation including the P-to-P' haptotropic metal shift and the factors influencing the ring nitrogen geometry are reported.

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.

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.

Analysis of Non-innocence of Phosphaquinodimethane Ligands when Charge and Aromaticity Come into Play

Several phosphaquinodimethanes and their M(CO)5 complexes (M=Cr, Mo, W) and model derivatives have been theoretically investigated regarding the quest of non-innocence. Computed structural and electronic properties of the P-Me/NH2 substituted phosphaquinodimethanes and tungsten complexes revealed an interesting non-innocent ligand behaviour for the radical anion complexes with distonic ion character and a strong rearomatization of the middle phenyl ring. The latter was further probed taking also geometric aromaticity (HOMA) and quinoid distortion parameters (HOMQc) into account, as well as NICS(1). Furthermore, the effect of the P-substitution was investigated for real (or plausible) complexes and their free ligands focusing on the resulting aromaticity at the middle phenyl ring and vertical one-electron redox processes. The best picture of ligand engagement in redox changes was provided by representing NICS(1) values versus HOMA and the new geometric distortion parameter HOMQc8.