Phosphorus Heterocycles

Abstract

Figure 23.1 The three isomers of the phosphole system (A-C) and the phospholide ion (D).2072j 23 Phosphorus Heterocycles shown with 1-alcoxyphospholes, which are relatively flat with a negligible delocalization [12]. Contrary to phospholes, the structure of phospholide ions reflects their high aromaticity. For example, the planar 2,3,4,5-tetramethylphospholide with Li(TMEDA) þ as the counterion displays the following parameters [13]: P-C, 1.715 A ; C¼C, 1.396 A ; CÀC, 1.424 A ; <CÀPÀC, 90.5 . Theoretical studies have mainly dealt with two problems, the aromaticity of the ring and the equilibrium between 1H-, 2H-, and 3H-phospholes. It is now well established that the phosphole ring is poorly aromatic, the most recent estimate of its aromatic stabilization energy being 3.2 kcal mol À1 for the parent ring [14]. Conversely, the planar transition state is even more aromatic than pyrrole [15]. As a result, the pyramidal inversion barrier of phospholes is quite low, ca. 16-17 kcal mol À1 , confirming early NMR measurements [16]. As expected, the aromaticity of the phospholide ion is of the same order of magnitude as that of the cyclopentadienide ion [15]. A recent study has shown that 2H-phosphole is more stable than 1H-phosphole by 6.0 and 3H-phosphole by 3.3 kcal mol À1 , thus confirming earlier calculations [17,18]. The barrier between 1H-and 2H-phospholes is very low at 19.6 kcal mol À1 but much higher between 2H-and 3H-phosphole at 30.7 kcal mol À1 .Whereas hydrogen migrates very easily from phosphorus to the a-carbon, the migrating ability varies widely for other groups. Some groups such as alkyl or alkoxy do not migrate under conditions that are compatible with the stability of the phosphole ring, while others migrate under acceptable heating such as aryl, alkynyl, CN, SR, and so on, and others migrate even below room temperature, such as acyl and silyl. These trends have been discussed in a recent review [19]. Another recent study has given an interpretation of the characteristic UV absorption band of alkylphospholes (280-290 nm, e 3.3-3.9), of their PES spectra and correlated the low basicity of phospholes with their strained cyclic structure [20]. Synthesis 23.2.3.

Late-Stage Generation of Bidentate η3-Benzophosphorine–Phosphino Ligands from a Rhodium PCcarbeneP Pincer Complex and Their Use in the Catalytic Hydrosilylation of Alkynes

“…Formally, the allyl motif in 2a possesses an ylide carbon donor, which has been shown to be a strong σ-donor; however, to our knowledge, this is the first instance of η 3 -coordination of an λ 5 -phosphorine. Indeed, although the coordination chemistry of λ 3 -phosphorines and λ 4 -phosphorines is reported, the coordination chemistry of λ 5 -phosphorines is largely unexplored, although their synthesis and utilization outside of coordination chemistry are still an active research area …”

Buchwald–Hartwig Amination of Phosphinines and the Effect of Amine Substituents on Optoelectronic Properties of the Resulting Coupling Products

“…Aromatic compounds that contain a low-coordinate phosphorus atom have recently attracted considerable attention owing to their peculiar electronic and steric properties, which differ from those of classic trivalent phosphines . To date, many λ 3 -phosphinines have been discovered, and their unique properties have been reported, which include their low-lying LUMO level compared to that of the corresponding pyridine derivatives .…”

Phenoxaphosphonium Mixed Ylides in Ring Expansion Reaction

“…2 Sevenmembered phosphorus heterocycles (7MPHs) are among the least explored types of these systems. 3 Despite recent publications highlighting the synthesis and research on πconjugated variants 4 and chiral ligands, 5 access to 7MPHs remains limited. 6 Leveraging phosphorus heterocycles with smaller rings as initial substrates for the synthesis of 7MPHs is an attractive approach, achievable through ring enlargement, 7 and several successful examples of this strategy are documented in the literature.…”

“…1 H NMR (CDCl 3 , 400 MHz) δ 7.67 2H),8H),3H),5.57,4.77 (both d,1H,J = 15.7 Hz,J = 20.7 Hz),(both m,2H), 1.26, 1.05, (both t, 3H, J = 7.2 Hz, J = 7.1 Hz); 13 C{ 1 H} NMR (CDCl 3 , 100 MHz) δ 166.7, 166.6 (both d, J = 3.8 Hz, J = 4.0 Hz), 160.4, 159.8 (both d, J = 2.2 Hz, J = 2.1 Hz), 157.9, 157.8 (both d, J = 4.7 Hz, J = 4.4 Hz), 134.9, 134.8 (both d, J = 6.1 Hz, J = 6.0 Hz), 134.2, 134.2 (both d, J = 1.8 Hz, J = 1.9 Hz), 132.9, 131.4 (both d, J = 9.5 Hz, J = 9.5 Hz), 132.8, 129.9 (both d, J = 107. 3 5-Phenylbenzo [b]phosphindole 5-Oxide (5). The compound was synthesized according to procedure B using ylide 2.…”

“…Their higher rigidity compared to acyclic compounds often results in improved binding affinity to biological receptors . Seven-membered phosphorus heterocycles (7MPHs) are among the least explored types of these systems . Despite recent publications highlighting the synthesis and research on π-conjugated variants and chiral ligands, access to 7MPHs remains limited .…”