Accessing five- and seven-membered phosphorus-based heterocycles via cycloaddition reactions of azophosphines

Heterocycles containing both phosphorus and nitrogen have seen increasing use in recent years in luminescent materials, coordination chemistry and as building blocks for inorganic polymers, yet their chemistry is currently dominated by five- and six-membered derivatives. Seven-membered P/N heterocycles are comparatively scarce and lack general, high yielding syntheses. Here, we explore the synthesis and characterisation of 1,2,5-diazaphosphepines from azophosphines. The mechanism has been probed in detail with both computational and experimental studies supporting a stepwise mechanism to form a five-membered ring, and subsequent ring expansion to the diazaphosphepine. Regioselective synthesis of five- and seven-membered rings is possible using asymmetric alkynes. The Lewis acidic borane B(C6F5)3 could either catalyse the formation of the seven-membered ring (iPr derivative) or trap out a key intermediate via a frustrated Lewis pair (FLP) mechanism (tBu derivative).

Ferrocene tethered boramidinate frustrated Lewis pairs: stepwise capture of CO2 and CO

The synthesis and reactivity of novel ferrocene tethered boramidinate frustrated Lewis pairs (FLPs), capable of the sequential capture of small molecules, is reported. Reactions of 1,1'-dicarbodiimidoferrocenes with different boranes provides access to metallocene tethered FLPs. The reactivity of the boramidinate moieties can be tuned by the nature of the carbodiimido substituents (alkyl vs. aryl) and the borane used in the reduction (9-borabicyclo[3.3.1]nonane [(C₈H₁₄)₂BH]₂vs. bis-pentafluorophenyl borane [(C₆F₅)₂BH]₂). The boramidinate FLP arms do not engage in intramolecular reactions, allowing for independent small molecule capture by each FLP. By careful synthetic control, sequential capture of different gaseous small molecules (CO₂ and CO or CO₂ and CNtBu) by the same bis(boramidinate)ferrocene molecule has been demonstrated.

Formation of Active Cyclic Five-membered Frustrated Phosphane/Borane Lewis Pairs and their Cycloaddition Reactions

The cyclic five-membered frustrated phosphane/borane Lewis pairs 11 a, b featuring the bulky octaethylhydrindacenyl- (Eind) substituent or its mono-bromo derivative (^Br Eind) at phosphorus are monomeric at room temperature. The reactive frustrated Lewis pairs (FLPs) cleave dihydrogen. The cyclic FLP 11 b (^Br Eind) undergoes 1,2-P/B addition to ethylene to give the zwitterionic heteronorbornane derivative 14 b. It reacts similarly with the carbon-carbon double bond of norbornene. With a variety of organic π-reagents, the cyclic FLP 11 b often undergoes reaction sequences reminiscent of the Alder-Rickert reaction: the cycloaddition reaction is followed by rapid cycloreversion to form new five-membered heterocyclic FLP products with extrusion of ethene. Reactions of 11 b with benzaldehyde or with acetylenes follow this reaction pattern.