From Cyclic Iminophosphoranes to π‐Conjugated Materials

Abstract

The "closed" benzannulated s 4 -l 5 -[1,3,2]diazaphosphole 1 c has been shown to exist in valance tautomeric equilibrium with its "open" form 1 o (Scheme 1).[1] As a result of the small energy difference between these two isomers, the air-and moisture-sensitive compound 1 exhibits reactivity that is not only commensurate with an iminophosphorane (e.g. cycloaddition at the P = N moiety and Lewis acid coordination at N with 1 c), but also with a P,N chelate ligand (e.g. metal coordination of 1 o), and with a 1,2-dihydropyridine (e.g. cyclobuta [b]pyridine formation from 1 c).With a view towards exploring further the unusual multifunctional character of 1, it was of interest to probe its reactivity with an equally versatile substrate. In this regard, s 2 -l 2 -phosphenium ions, R 2 P + , were an attractive choice for study. These dicoordinate cations possess both a phosphoruscentered lone pair and a formally vacant 3p orbital, rendering them "ambiphilic" in terms of their reactivity, something that gives rise to an extensive Lewis acid and cycloaddition chemistry. [2][3][4] Treatment of the equilibrium mixture 1 c/1 o with an equimolar quantity of the phosphenium salt [(iPr 2 N) 2 P]- [OTf] [5] in CH 2 Cl 2 affords a new compound 2 as the only phosphorus-containing product (Scheme 2); complete consumption of both starting materials is evident from 31 P NMR spectroscopic analysis of the reaction mixture. Following purification, compound 2 was isolated in good yield (61 %) as an air-and moisture-stable green solid, which was found to contain both cis-and trans-isomers in a 1:1 ratio by multinuclear NMR spectroscopy (d 31 P = + 51.9 and + 51.4 ppm). [6] No interconversion between the two isomers in solution was observed using selective 31 P NMR inversion experiments at 40 8C ([D 2 ]tetrachloroethane).Recrystallization of cis-/trans-2 from a CH 2 Cl 2 /hexane solution gave crystals of trans-2·2 CH 2 Cl 2 suitable for an X-ray diffraction study. [7] The dicationic component has crystallographic C i symmetry (Figure 1) and is comprised of a 1H,1'H- . X-ray molecular structure of the dication of trans-2 (primed atoms are generated by an inversion centre).[7] H atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50 % probability level. Selected bond lengths [] and angles [8]:

Exploiting Non‐Innocent Ligands to Prepare Masked Palladium(0) Complexes

“…As a result, 1 a has been shown to behave as either a κ 2 ‐ PN or a κ 1 ‐ N ligand depending on the nature of the partner metal, and to exhibit unusual redox behavior 15. 16 In contrast, the diphenyl analogue 1 b behaves only as a bidentate κ 2 ‐ PN Py ligand and exhibits no tautomerism 17…”

Exploiting Non‐Innocent Ligands to Prepare Masked Palladium(0) Complexes

“…As a result, 1 a has been shown to behave as either a k 2 -PN or a k 1 -N ligand depending on the nature of the partner metal, and to exhibit unusual redox behavior. [15,16] In contrast, the diphenyl analogue 1 b behaves only as a bidentate k 2 -PN Py ligand and exhibits no tautomerism. [17] Despite the importance of palladium catalysis in a variety of organic transformations, [17] there have been very few studies of Pd-NIL complexes.…”