Diels-Alder reactions of 1-phosphabutadienes: a highly selective route to P[double bond, length as m-dash]C-substituted phosphacyclohexenes

Heavy Heterodendralenes: Structure and Reactivity of Phosphabora[3]dendralenes

The incorporation of phosphorus and boron into [3]dendralenes provides access to heavy heterodendralenes, a new class of main-group precursor to "doped" polycyclic hydrocarbons. [n]Dendralenes are a core class of unsaturated hydrocarbons built from geminally connected polyenes; the resulting arrangement of conjugated C═C bonds enables [n]dendralenes to undergo reactions that allow rapid access to complex polycyclic compounds. The increasing technological and synthetic importance of main-group-containing polycyclic hydrocarbons and their analogues makes new routes to access such systems highly attractive. Here we report the preparation of the first heavy heterodendralenes in the form of phosphorus- and boron-containing [3]dendralenes, prepared by a ring-opening reaction of a 1,2-phosphaborete. We reveal the electronic effect of P/B incorporation and demonstrate that, like their hydrocarbon analogues, phosphabora[3]dendralenes undergo diene-transmissive cycloaddition chemistry.

Activation of the C[double bond, length as m-dash]P bond in phosphanylphosphaalkenes (C[double bond, length as m-dash]P-P bond system) in the reaction with nucleophilic reagents: MeLi, nBuLi and tBuLi

Three reactions of phosphanylphosphaalkene (1) with nucleophiles were performed to activate the diphosphorus monomer. We observed similar results in the reactions with MeLi and nBuLi, in which the P-P bond is cleavaged and triphosphorus systems [P(Me)₂-CH(biph)-CH(biph)-P-(PtBu₂)]^- (1a'') and [P(nBu)₂-CH(biph)-CH(biph)-P-(PtBu₂)]^- (1b''), respectively, are formed depending on the nucleophilic reagent (biph = biphenyl). In the case of tBuLi, the P-P bond remains intact; on the phosphorus atom, only one -tBu group is substituted, and as a result, [(biph)(H)C-P(tBu)-PtBu₂]^- (1c) is generated as a stable carbanion. We additionally investigated the effect of substitution in the phenyl ring in the course of these reactions by involving two other phosphanylphosphaalkenes (3 and 4). All initial reactions were conducted in tetrahydrofuran (THF) solution at ambient temperature.

Thiophosphonium-Alkyne Cycloaddition Reactions: A Heavy Congener of the Carbonyl-Alkyne Metathesis

While the metathesis reaction between alkynes and thiocarbonyl compounds has been thoroughly studied, the reactivity of alkynes with isoelectronic main group R₂E=S compounds is rarely reported and unknown for [R₂P=S]⁺ analogues. We show that thiophosphonium ions, which are the isoelectronic phosphorus congeners to thiocarbonyl compounds, undergo [2 + 2]-cycloaddition reactions with different alkynes to generate 1,2-thiaphosphete ions. The four-membered ring species are in an equilibrium state with the corresponding P=C–C=S heterodiene structure and thus undergo hetero-Diels–Alder reactions with acetonitrile. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.

We show that thiophosphonium ions, which are the isoelectronic phosphorus congeners to thiocarbonyl compounds, undergo [2 + 2]-cycloaddition reactions with different alkynes to generate 1,2-thiaphosphete ions. The four-membered ring species are in an equilibrium state with the corresponding P=C−C=S heterodiene structure and thus undergo hetero-Diels−Alder reactions with acetonitrile. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.

Hetero-Diels–Alder reactions of 2H-phospholes with allenes: synthesis and functionalization of 6-methylene-1-phosphanorbornenes

The phospha-Diels–Alder reaction between 2H-phospholes and arylallenes affords 6-methylene-1-phosphanorbornenes in high yields with excellent regioselectivity. Further functionalization provides a 1-phosphanorbornene modified PCH₂CH₂P skeleton.

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.