Recent Developments in the Chemistry of N-Heterocyclic Phosphines

Abstract

This chapter gives a survey on five-and six-membered phosphorusnitrogen heterocyclic compounds whose rings combine a phosphazene (>N-P = N-) or phosphazane (>N-P(X)-N<) unit with an unsaturated C 2 or C 3 building block. Representatives contain structurally diverse species like aromatic 1,3,2-diazaphosphinines and (benzo)-1,3,2-diazaphospholes, cationic counterparts of subvalent main-group carbene analogues like 1,3,2-diazaphospholenium ions and phosphenium-diketiminates, and neutral heterocycles like 1,3,2-diazaphospholenes featuring unusual structures and reactivities. The exploration of these species developed rapidly in the last two decades in the wake of cutting edge research on multiple bonding and low coordination in the chemistry of heavier main-group elements, and the discovery of stable carbenes. This review summarizes the elaboration of synthetic approaches for different types of N -heterocyclic phosphine derivatives, discusses their characterization by physical and computational methods which furnished a thorough understanding of structure and bonding, and finally highlights accomplishments in the exploration of the chemical properties at the border of classical organic heterocyclic chemistry and molecular inorganic chemistry.

Lewis Base Stabilized Oxophosphonium Ions

“…[1] There have been regular contributions [2][3][4][5][6] to the area of N-heterocyclic phosphenium ions A since the first reports 30 years ago. [7,8] The area has seen a recent resurgence [6,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] which may be due to the popularity of the isovalent singlet carbenes and, as such, the use of phosphenium ions as ligands in transition metal chemistry has been exploited. [24][25][26][27][28][29][30] In the context of main group chemistry, [31] phosphenium ions have been involved in the preparation of phosphinophosphonium systems, [32][33][34][35][36][37][38][39][40] as well in reactions with P 4 , [39,[41][42][43][44] and other reagents.…”

Straightforward Diastereoselective Synthesis of P-Chirogenic (1R)-1,8,8-Trimethyl-2,4-diaza-3-phosphabicyclo[3.2.1]octane 3-Oxides: Application as Chiral NMR Solvating Agents

“…The first heterocyclic compounds having both phosphorus and nitrogen atoms were prepared by Liebig and Wöhler FIGURE 1 Examples of chiral organophosphorus compounds. [12]. Enantiopure or enantioenriched heterocycles containing a trivalent or pentavalent P-stereogenic phosphorus atom can be easily generated by cyclization of diamines, amino alcohols, or diols around phosphorus moieties.…”

Synthesis and characterization of a 2-chloro-1,3,2-diaza-phospholidine-4,5-diimine

“…Five-membered phosphorus-nitrogen heterocycles like 1,3,2-diazaphospholidines 1 1 and 1,3,2-diazaphospholenes 2 2,3,4 (Scheme 1) are of central importance for the synthesis of stable phosphenium ions 3, 4 with a cationic, two-coordinate phosphorus atom 5,6,7 which are isoelectronic analogues to neutral diaminocarbenes 5, 6, respectively. Whereas cations 3 with CC-saturated heterocyclic rings were among the first phosphenium ions ever prepared,…”

“…under heterolytic cleavage of the phosphorus-chlorine bonds. 5,6,7 Cations 4 have alternatively been accessed via a [1+4]-cycloaddition route starting from a diazabutadiene and a transient P(I) fragment generated via disproportionation of PI 3 , or via reduction of PCl 3 with SnCl 2 , respectively. 10 In view of this approach we became interested in the synthesis of derivatives 7 featuring both a five-membered phosphorus-nitrogen heterocycle and a diazabutadiene moiety.…”

“…Although all attempts toward isolation of this product remained unsuccessful and its constitution remains therefore still in the dark, tentative assignment as a phosphenium cation salt is, however, definitely ruled out by the observed 31 P NMR chemical shift of 109 ppm which appears at even higher field as the signal of 7a and lies far outside the expected range of 31 P NMR chemical shifts between 200 and 450 ppm for diaminophosphenium cations. 5,6,7 The outcome of the reactions of 7a with electrophiles suggests that the exocyclic imino moieties have an adverse effect on the stability of a diazaphospholidinium cation and its formation is therefore disfavored. This hypothesis is further substantiated by calculated energies of the isodesmic chloride transfer reactions shown in Scheme 3 which allow to estimate the stabilities of phosphenium cations 3b, 4b, and 9b relative to the parent diaminophosphenium ion [(H 2 N) 2 ) of isodesmic chloride transfer reactions at different levels of theory.…”