Simple Synthesis of Functionalized 2-Phosphanaphthalenes

Phthalazine as a Diene in Diels-Alder Reactions With P- and As-Containing Anionic Dienophiles: Comparison of Possible Reaction Channels

Phthalazine can behave as a diene in Diels-Alder (DA) cycloadditions, typically at the pyridazine ring, however, its application is somewhat limited because these reactions usually require harsh conditions or sophisticated catalysts. As an unconventional example, phthalazine was reported to undergo cycloaddition with the [PCO]- anion without any catalyst. In this computational study, we scrutinise the mechanism of the DA reactions between phthalazine and the so far known [ECX]- (E: P, As; X: O, S, Se) anions as dienophiles. In principle, the attack of an [ECX]- anion may occur at two different sites of phthalazine, either at the benzene or the pyridazine ring, and both of these possible reaction channels were juxtaposed on the basis of energetic aspects. In all of the investigated cases, the analysis of the energy profiles reveals a clear regioselectivity that favours the attack at the pyridazine ring. As a result, so far unprecedented 2-pnictanaphth-3-olate analogues seem achievable as final products. Comparing the characteristics of these pathways allowed us to clarify the source of this regioselectivity: The pyridazine ring of phthalazine exhibits lower aromaticity than the benzene subring; therefore, in the DA step, the former ring shows a higher affinity toward a dienophile than the latter, leading to lower activation barriers. To further map the electronic and structural features of the cycloaddition steps, the local interactions evolving in the transition states were analysed and compared using global and local descriptors. In most aspects, the characteristics of both pathways were found to be rather similar, in contrast to the markedly differing activation barriers on the two routes.

Understanding the Mechanism of Diels-Alder Reactions with Anionic Dienophiles: A Systematic Comparison of [ECX]- (E = P, As; X = O, S, Se) Anions

While Diels–Alder (DA) reactions involving neutral or cationic dienophiles are well-known, the characteristics of the analogous reactions with anionic dienophiles are practically unexplored. Herein we present the first comparative computational investigations on the characteristics of DA cycloadditions with anionic dienophiles on the basis of the reactions of [ECX]⁻ anions (E = P, As; X = O, S, Se) with 2H-pyran-2-one. All of these reactions were found to be both kinetically and thermodynamically feasible, enabling synthetic access toward 2-phosphaphenolate and arsaphenolate derivatives in the future. This study also reveals that the [ECO]⁻ anions show clear regioselectivity, while for [ECS]⁻ and [ECSe]⁻ anions, the two possible reaction channels have very similar energetics. Additionally, the activation barriers for the [ECO]⁻ anions are lower than those of the heavier analogues. The observed differences can be traced back to the starkly differing nucleophilic character of the pnictogen center in the anions, leading to a barrier-lowering effect in the case of the [ECO]⁻ anions. Furthermore, analysis of the geometries and electron distributions of the corresponding transition states revealed structure–property relationships, and thus a direct comparison of the cycloaddition reactivity of these anions was achieved. Along one of the two pathways, a good correlation was found between the activation barriers and suitable nucleophilicity descriptors (nucleophilic Parr function and global nucleophilicity). Additionally, the tendency of the reaction energies can be explained by the changing aromaticity of the products.

In contrast to the phosphaethynolate [PCO]⁻ anion, the cycloaddition reactivity of the heavier congeners ([ECX]⁻, where E = P, As and X = O, S, Se) is unexplored. In this computational study, the Diels−Alder reaction between the known [ECX]⁻ anions and 2-pyrone was employed to compare the reactivity patterns. The first activation barrier of these reactions correlates with the nucleophilicity of the anions, indicating a barrier-lowering effect. The feasibility of the studied reactions, leading to P and As heterocycles, was also explored.

Non-conventional Behavior of a 2,1-Benzazaphosphole: Heterodiene or Hidden Phosphinidene?

The titled 2,1-benzazaphosphole (1) (i. e. ArP, where Ar=2-(DippN=CH)C6 H4 , Dipp=2,6-iPr2 C6 H3 ) showed a spectacular reactivity behaving both as a reactive heterodiene in hetero-Diels-Alder (DA) reactions or as a hidden phosphinidene in the coordination toward selected transition metals (TMs). Thus, 1 reacts with electron-deficient alkynes RC≡CR (R=CO2 Me, C5 F4 N) giving 1-phospha-1,4-dihydro-iminonaphthalenes 2 and 3, that undergo hydrogen migration producing 1-phosphanaphthalenes 4 and 5. Compound 1 is also able to activate the C=C double bond in selected N-alkyl/aryl-maleimides RN(C(O)CH)2 (R=Me, tBu, Ph) resulting in the addition products 7-9 with bridged bicyclic [2.2.1] structures. The binding of the maleimides to 1 is semi-reversible upon heating. By contrast, when 1 was treated with selected TM complexes, it serves as a 4e donor bridging two TMs thus producing complexes [μ-ArP(AuCl)2 ] (10), [(μ-ArP)4 Ag4 ][X]4 (X=BF4 (11), OTf (12)) and [μ-ArP(Co2 (CO)6 )] (13). The structure and electron distribution of the starting material 1 as well as of other compounds were also studied from the theoretical point of view.

Coordination-induced polymerization of P═C bonds leads to regular (P─C) n polycarbophosphanes

The replacement of carbon in (C─C) n chains of polyolefins by phosphorus leads to polycarbophosphanes (P─C) n , which may possess unique chemical and physical properties. However, macromolecules with a regular (P─C) n chain have never been unambiguously identified. Here, we demonstrate that addition polymerization, a general concept to polymerize olefins, can be extended to P═C double bonds. The polymerization of monomeric 2-phosphanaphthalenes is mediated by copper(I) halides and leads to polycarbophosphanes with an M n of 14 to 34 kDa. Each phosphorus is coordinated to Cu(I), which can be easily removed. Unlike long-term durable polyolefins, the metal-free polymers depolymerize rapidly back to monomers under sunlight or ultraviolet irradiation at λ = 365 nm. The monomers can be recycled for repolymerization, demonstrating a cradle-to-cradle life cycle for polycarbophosphanes.

Finding a soft spot for vanadium: a P-bound OCP ligand

Transmetallation studies with the phosphaethynolate ion, [OCP]^-, have largely resulted in coordination according to classical Lewis acid-base theory. That is, for harder early transition metal ions, O-bound coordination has been observed, whereas in the case of softer late transition metal ions, P-bound coordination predominates. Herein, we report the use of a V(iii) complex, namely [(nacnac)VCl(OAr)] (1) (nacnac^- = [ArNC(CH₃)]₂CH; Ar = 2,6-ⁱPr₂C₆H₃), to transmetallate [OCP]^- and bind via the P-atom as [(nacnac)V(OAr)(PCO)] (2), the first example of a 3d early transition metal that binds [OCP]^-via the P-atom. Full characterization studies of this molecule including HFEPR spectroscopy, SQuID measurements, and theoretical studies are presented.

Heavier pnictogens - treasures for optical electronic and reactivity tuning

We highlight recent advances in organopnictogen chemistry contrasting the properties of lighter and heavier pnictogens. Exploring new bonding situations, discovering unprecedented reactivities and producing fascinating opto-electronic materials are some of the most prominent directions of current organopnicogen research. Expanding the chemical toolbox towards the heavier group 15 elements will continue to create new opportunities to tailor molecular properties for small molecule activation/reactivity and materials applications alike. This frontier article illustrates the elemental substitution approach in selected literature examples.

Making the unconventional μ2-P bridging binding mode more conventional in phosphinine complexes

As compared to the normal η¹-P σ-complexes or η⁶-phosphinine π-complexes, the rare μ²-P bridging binding mode of phosphinines can be tuned by employing electron donating substitute.

Phosphinines, as aromatic heterocycles, usually engage in coordination as η¹-P σ-complexes or η⁶-phosphinine π-complexes. The μ²-P bridging coordination mode is rarely observed. With the aim to study the effect of different electronic configurations of phosphinines on the coordination modes, a series of anionic phosphinin-2-olates and neutral phosphinin-2-ols were prepared with moderate to high yield. Then the coordination chemistry of these two series was studied in detail towards coinage metals (Au(i) and Cu(i)). It is observed that the anionic phosphinin-2-olates possess a higher tendency to take a bridging position between two metal centers compared to the neutral phosphinin-2-ols. Based on these experimental findings bolstered by DFT calculations, some insight is gained on how the unconventional μ²-P phosphinine bridging coordination mode can be made more conventional and used for the synthesis of polynuclear complexes.

The Chemistry of the 2-Phosphaethynolate Anion

In all likelihood the first synthesis of the phosphaethynolate anion, PCO^- , was performed in 1894 when NaPH₂ was reacted with CO in an attempt to make Na(CP) accompanied by elimination of water. This reaction was repeated 117 years later when it was discovered that Na(OCP) and H₂ are the products of this remarkable transformation. Li(OCP) was synthesized and fully characterized in 1992 but this salt proved to be too unstable to allow for a detailed investigation of its chemistry. It was not until the heavier analogues of this lithium salt were isolated, Na(OCP) and K(OCP) (both of which are remarkably stable and can be even dissolved in water), that the chemistry of this new functional group could be explored. Here we review the chemistry of the 2-phosphaethynolate anion, a heavier phosphorus-containing analogue of the cyanate anion, and describe the wide breadth of chemical transformations for which it has been thus far employed. Its use as a ligand, in decarbonylative and deoxygenative processes, and as a building block for novel heterocycles is described. In the mere twenty-six years since Becker first reported the isolation of this remarkable anion, it has become a fascinating reagent for the synthesis of a vast library of, often unprecedented, molecules and compounds.

From zinco(ii) arsaketenes to silylene-stabilised zinco arsinidene complexes

The steric congestion of a N-heterocyclic silylene promotes the formation of a monomeric As-metallated silylene-arsinidene compound with somewhat double bond character through replacement of CO in the LZn–AsCO precursor (L = β-diketiminate).