Diazaphospholene Chemistry

Abstract

Diazaphospholenes are N‐heterocyclic phosphines that can be derived from N‐heterocyclic carbenes by formally replacing the divalent carbon atom by a P–X unit consisting of a tervalent phosphorus atom and an exocyclic substituent X. This arrangement provides a very peculiar electronic structure that is characterized by an exceptionally strong hyperconjugation between delocalized π‐electrons in the ring and the σ*(P–X) orbital and results in a considerable ionic polarization of the exocyclic bond with a concomitant increase of negative partial charge on X. The effect persists even in unpolar P–X bonds and induces unprecedented chemical reactivity like a marked polarization of homonuclear bonds to phosphorus‐based substituents, which immensely facilitates bond activation reactions, and a reversal of the polarity (“Umpolung”) of the P–H bonds in secondary diazaphospholenes, which gives rise to a hydride behavior unprecedented for phosphines. The emerging exploitation of these features in diverse areas of synthetic chemistry sparked various applications of diazaphospholenes both as stoichiometric reagents and as organocatalysts. It is the aim of this article to give a comprehensive overview on the chemistry of diazaphospholenes, starting with a discussion of their specific electronic structure, and addressing their synthesis, principal reaction patterns, and applications as reagents in stoichiometric reactions and organocatalysts in metal‐free catalytic transformations. A concluding section will focus on ligand‐centered reactions of diazaphospholenes with exocyclic transition metal substituents (“metallo‐diazaphospholenes”) and their application in catalysis.

Enantioselective Imine Reduction Catalyzed by Phosphenium Ions

“…Diazaphospholenes are hydridic reagents and catalysts which have recently been shown to be the strongest neutral metal-free hydride donors on the Mayr nucleophilicity scale. , Diazaphospholenes are attractive catalysts for reductions, because they are tolerant of alkenes, alkynes, and Lewis-basic sites, which may pose issues for metal-based reduction catalysts . Use of diazaphospholenes in asymmetric catalysis is a recent development.…”

The reactivity of N-heterocyclic diphospholenes towards iron triad carbonyls

“…[10][11][12][13] The key feature of these compounds is weakly aromatic character of 1,3,2-diazaphospholene cycle and polarization of exocyclic P-X bond, which makes them convenient precursors of phosphenium cations (NHP + ). [14][15][16][17][18][19][20] These cations are considered analogs of carbenes in coordination chemistry, and the metal derivatives based on them are like Fischer complexes. 16,17,[20][21][22][23][24][25][26] Bonding in these complexes is achieved by two types of interactions between a metal atom and phosphorus in these compounds: dative M-L p-bonding and weaker dative L-M s-bonding.…”

“…[14][15][16][17][18][19][20] These cations are considered analogs of carbenes in coordination chemistry, and the metal derivatives based on them are like Fischer complexes. 16,17,[20][21][22][23][24][25][26] Bonding in these complexes is achieved by two types of interactions between a metal atom and phosphorus in these compounds: dative M-L p-bonding and weaker dative L-M s-bonding. Such complexes are currently known for most transition metals.…”