Christopher C. Cummins

Conspectus: Although phosphate is the thermodynamically stable and overwhelmingly abundant form of phosphorus in Nature, the modern phosphorus chemicals industry still relies heavily on white phosphorus (P4) as its central feedstock. In the thermal process, phosphate rock is reduced with coke and silica at temperatures around 1500 °C to produce P4, which is then converted to intermediates such as PCl3 before further conversion to value-added phosphorus compounds. This reaction sequence is effective, but involves the reduction of phosphate under high temperature, followed by additional redox and functionalization steps before many target phosphorus compounds are obtained. We therefore asked whether phosphate-derived feedstocks could be converted more directly into reduced phosphorus compounds, making the production of P4 as an intermediate unnecessary while retaining the synthetic flexibility needed for producing a large variety of phosphorus compounds. That question led to three areas of investigation. The first exploits the strong thermodynamic driving force of Si–O bond formation to achieve oxygen-abstraction reduction of phosphate-derived substrates. This led to the discovery of the bis(trichlorosilyl)phosphide anion,[P(SiCl3)2]−, a previously unknown P(−3) synthon that can now be accessed from trimetaphosphate, orthophosphate salts, and phosphoric acid. It can be further converted to value-added phosphorus compounds, such as phosphines, hexafluorophosphate, or phosphonium salts. Protonation of [P(SiCl3)2]− gives bis(trichlorosilyl)phosphine,HP(SiCl3)2, which undergoes selective radical hydrophosphination and gives synthetic access to primary phosphines, chlorophosphines, phosphinic acids, phosphonic acids, and phosphonium salts. The second strategy uses mechanochemistry to convert condensed phosphates directly to phosphite through hydride phosphorylation. Ball milling with alkali-metal hydrides provides a solvent-free route to a P(III) product while retaining the PO3 framework of the starting material. This route also works with biologically recovered polyphosphate feedstocks, which is important for the recycling of sewage sludge. The same mechanochemical platform further proved useful for the analogous redox-neutral phosphorylation of acetylides, giving alkynyl phosphonates directly from condensed phosphates. Finally, strongly reducing mechanochemical conditions based on supported alkali-metal dispersions gave access to ortho-phosphite,PO33−, a previously “missing” phosphorus oxoanion. This new anion can be converted further to phosphite salts, tris(trimethylsilyl) phosphite, and alkyl phosphonates. Taken together, these studies show that condensed phosphates and phosphoric acid can be directly used as feedstocks for synthetic phosphorus chemistry, without relying on P4 as an intermediate. This approach enables the identification of reactions that preserve P–O bonds where desirable and creates new opportunities to link phosphorus recovery from waste streams with the synthesis of value-added phosphorus compounds. Determining whether such routes are also more sustainable relative to established manufacturing processes will require a comparative assessment of energy demand, reagent production, waste generation, and process hazards.

The discovery of new borylation strategies remains a major focus in synthetic chemistry, as organoboron compounds serve as highly versatile intermediates that enable a wide range of transformations, while also playing increasingly important roles in small-molecule activation and advanced functional materials. Herein, we report a novel borylation reaction mediated by diazoboranes (RLBN₂). In contrast to conventional borylation processes, which proceed through addition to unsaturated bonds or substitution of leaving groups, diazoboranes, behaving as dicoordinate borylenes, undergo net insertion into olefinic C-H bonds. This diazoborane-mediated borylation displays a broad substrate scope, encompassing monosubstituted and gem-disubstituted olefins, as well as electron-poor and conjugated systems. Most substrates afford the E isomer with excellent selectivity (>99%). In contrast, less sterically hindered olefins exhibit partial formation of the Z isomer. Notably, acrylonitrile delivers the Z-configured product with >99% selectivity. Detailed mechanistic studies reveal that this stereochemical outcome arises from a subtle interplay between orbital interactions and steric effects during the conversion of a cyclic intermediate to the C-H activation product.

Cyclic peroxides are highly reactive oxygen-containing species that play important roles in chemical synthesis, yet, aside from dioxiranes (R₂CO₂), well-defined cyclic peroxides of main-group elements remain rare. In particular, cyclic peroxides featuring boron–oxygen frameworks have not been explored. Here we show that boron peroxides can be prepared directly from accessible boron precursors and molecular oxygen under mild conditions. Specifically, reactions of diazoboranes with triplet oxygen afford a family of cyclic dioxaboriranes, including both neutral and anionic variants. These compounds exhibit distinct reactivity: neutral species behave as electrophilic oxygen-atom donors, whereas the anionic analogue is nucleophilic and reacts with carbon dioxide. Computational studies explain why these boron peroxides form thermally, in contrast to dioxiranes that require strict photochemical control. Finally, we show that cyclic dioxaboriranes can be converted into acyclic boron peroxide derivatives, highlighting opportunities for controlling peroxide structure and reactivity at boron. The alternative text for this image may have been generated using AI.

ConspectusReactive intermediates are valuable and intriguing in synthetic chemistry, but their high reactivity often makes them challenging to handle. Therefore, developing strategies to generate these species in a mild and controlled manner is crucial. One effective approach involves embedding the reactive intermediate within a molecular scaffold. Upon gentle heating, the scaffold undergoes fragmentation, liberating the desired intermediate. Ideally, the resulting byproducts are inert and do not participate in the subsequent reaction. Carpino's hydrazine, H₂N₂A (A = C₁₄H₁₀ or anthracene), thus serves as an excellent scaffold candidate. By attaching a functional group of interest (E) to the hydrazine, the resulting compound EN₂A is expected to undergo fragmentation, releasing E, dinitrogen (N₂), and anthracene.In this account, we describe our efforts to develop a series of molecular precursors featuring the composition EN₂A (E = C, CH₂, SO, RLB, and R₂B). These precursors are expected to be capable of releasing a single carbon atom, methylene, sulfur monoxide, borylene, and boryl anion, respectively. Interestingly, the fragmentation behavior of these hydrazine-based precursors is highly dependent on the substituents at nitrogen. For CN₂A, H₂CN₂A, and OSN₂A, the precursors are stable at room temperature. Meanwhile, for (RLB)N₂A, and (R₂B)N₂A, the precursors are transient intermediate and undergo anthracene extrusion even at low temperatures.While the initial goal was to generate reactive species E, many cases have shown that free intermediates are not necessarily required for group transfer reactions. Instead, the hydrazine precursors often facilitate group transfer through highly selective, associative mechanisms (Type A). Additionally, the diazo intermediates formed via primary fragmentation are of particular interest, as they display reactivity analogous to diazoalkanes (R₂CN₂) or organic azides (RN₃, Type B). Notably, although hydrazine precursors, diazo intermediates, and low-valent species all participate in group transfer reactions, they exhibit distinct electronic structures. Consequently, their reactivity patterns and selectivity vary significantly, underscoring the diverse chemical space accessible through this versatile platform.We believe that continued development of Carpino's hydrazine derivatives holds significant potential for uncovering new reactive intermediates and gaining deeper mechanistic insights. Moreover, the reactivity demonstrated with boron may be extended to other main group elements, potentially enabling access to a broader class of compounds featuring terminal N₂ complexes.

While diazo compounds and organic azides have been extensively studied over the past two centuries, their boron analogs, diazoboranes, were only successfully isolated last year, and their properties remain largely unexplored. This study presents a systematic investigation of the synthesis, properties, and reactivity of diazoboranes. In this work, the diazoborane library is expanded to eight species by varying the carbene ligands coordinated to boron, allowing the identification of key correlations between ligand electronic properties and diazoborane behavior. The results demonstrate that increasing carbene π-acidity weakens the B-N bond while strengthening the N-N bond, significantly decreasing thermal stability. Kinetic studies reveal that less π-acidic carbenes promote a second-order decay mechanism, leading to dibora-azine formation, whereas more π-acidic carbenes favor first-order dinitrogen dissociation, facilitating borylene generation. Electrochemical studies indicate that redox potentials also correlate with carbene π-acidity. Three representative reactivity studies are presented in which the same substrate reacts with different diazoboranes to yield either different products or the same product via different reaction mechanisms. These insights provide a generalizable framework for tuning diazoborane properties, facilitating their use in cycloaddition chemistry and small-molecule activation.

Researchers in the authorʼs laboratory have investigated small-molecule activation reactions using low-coordinate early transition-metal complexes supported by sterically demanding N-hydrocarbyl anilide ligands. Such complexes are potent one, two, or three-electron reducing agents, and early on the focus was on small molecules including N2, NO, and N2O with metalligand multiply bonded systems formed as products of the bond activation reactions. Reactions with P4 (white phosphorus) and As4 led respectively to complexes with metal-phosphorus or metal-arsenic triple bonds. This led to the notion of element or molecule activation using transition metals, to be followed by atom or group transfer to organic molecules. Phosphorus transfer was achieved in a novel phosphaalkyne synthesis method involving transfer of a P4-derived phosphorus atom using a niobium complex. Next, diphosphorus transfer was achieved using first a niobium and then an anthracene-based platform, the latter culminating in a synthesis of diphosphatriazolate, P2N3 −. Triphosphorus group transfer was harnessed in a synthesis of AsP3. This was echoed by the synthesis of HCP3. Anthracene emerged as a privileged two-electron platform for the construction of molecular precursors to reactive intermediates including HCP, P2, and phosphorus mononitride, PN. This account thus describes an arc from the activation of kinetically inert small molecules to the synthesis, transfer, or transient generation of reactive ones.

Reported boryl anions (R₂B^-) often exhibit n-p conjugation between the boron atom and adjacent heteroatoms, with their nucleophilicity being the primary focus. In this work, we present evidence that aryl-substituted boryl anions (Ar₂B^-) can exhibit ambiphilicity. Specifically, the diazoborane anion [K(2.2.2-cryptand)]⁺Dmp(Mes)BN₂^- (3, Dmp = 2,6-dimesitylphenyl, Mes = 2,4,6-trimethylphenyl) has been synthesized via diazo transfer using Carpino's hydrazine as a reagent. Compound 3 exhibits a strong tendency to release dinitrogen, acting as a free boryl anion Dmp(Mes)B^- source. Ligand exchange reactions with carbon monoxide and 2,6-xylyl isocyanide highlight the metallomimetic (ambiphilic) behavior of 3. The ambiphilic nature of 3 further enables its reaction with nitrous oxide to yield an oxoborane anion (R₂BO^-). Compound 3 also reacts with carbon dioxide to form the cycloaddition product R₂B(N₂CO₂)^- (7). Additionally, the boryl-N₂ anion undergoes N₂ release upon heating with LiCl or irradiation, yielding different bond activation products. The underlying mechanisms of these reactions were investigated through in-depth DFT analysis.

This publication delves into a comprehensive exploration of a new synthetic route to functionalized phosphorus-derived compounds. Bis­(trimethylsilyl)­phosphonite HP­(OSiMe₃)₂, prepared in a high yield from H₃PO₂, was found to be an excellent reagent for hydrophosphination of activated, unactivated, and amino acid-derived olefins under UV irradiation and catalyst-, solvent- and glovebox-free conditions. The resulting phosphorus trivalent compounds have been subjected to postfunctionalization, leading to the formation of H-phosphinate, phosphonate, and thiophosphonate end products in good to excellent yields. With a combination of experimental and theoretical calculations, the mechanism for this hydrophosphination reaction has been investigated, revealing a radical process.

We present a method for the generation of boron-containing unsaturated small molecules via hexamethylbenzene elimination.