Marie-Paule Simonnin

NMR characterization of zwitterionic nitrogen-bonded σ-complexes: the imidazole/4,6-dinitrobenzofuroxan system

Abstract Imidazole (ImH) and N-methylimidazole react with 4,6-dinitrobenzofuroxan to form the σ-complexes 1a, H + and 1b,H + . In excess of ImH, 1a,H + is deprotonated to give the anionic σ-complex 1a . 1a,H + and 1b,H + are the first zwitterionic nitrogen-bonded σ-complexes to be firmly characterized by spectroscopy.

STRUCTURAL STUDY OF CARBANIONIC SPECIES FROM BIS(DIETHOXYPHOSPHONYL)-METHANE BY IR, RAMAN AND 1H, 31P, 13C NMR SPECTROSCOPIES

Abstract An IR and Raman study of the lithium associated carbanionic species 1 formed from bis(diethoxyphosphonyl)-methane in the solid state and in THF solution, as well as the IR and 1H, 31P, 13C NMR study of the potassium associated one in pyridine and dimethylsulfoxide are reported. A chelated symmetrical structure exists in the solid state and in associating medium. The central carbon is planar or nearly so from the NMR data. However, the negative value of 2 J PH in the anionic species raises the problem of estimating the various factors involved in the two bond coupling constants.

Wittig–Horner reagents: structural and conformational study of five- and six-membered cyclic phosphonates and their carbanions. Evidence of anomeric effect in anionic species

A structural and conformational study of the electron-withdrawing substituted phosphonates 2-oxo-1,3,2-dioxaphosphorinanes (1) and (2) and of the related anionic species (3, K+) and (4, K+) in dimethyl sulphoxide has been made. Phosphonates (1) and (2) exist predominantly in the chair form with the PO bond equatorial. The anion (3, K+) gives rise to three rapidly interconverting species, in the chair form, and (4, K+) to two interconverting chairs, chelated to K+. In each anionic species (3) and (4), the PO axially substituted conformer is significantly populated, due to an anomeric effect. The 2-oxo-1,3,2-dioxaphospholane (5) is very sensitive to moisture, and open-chain compounds can be found in solutions of (5), if the solvent is not carefully dried. The related anionic species (6, K+) is not very stable; however, its 1H and 31P n.m.r. data show some similarities with those of the related acyclic compound.

Signs and magnitudes of 1JP–C, 2JP–C, and 3JP–H in some acetylenic PIII and PIV organophosphorus compounds

Coupling constants display a reversal of sign in acetylenic phosphines, 1JP–C 0, 3JP–H < 0; they are positive in acetylenic phosphine oxides and their values suggest dominance by the Fermi contact interaction.

Structure and conformation of polynitrodiphenylmethanes and related carbanions

1 H and 13C n.m.r. data are reported for a series of ortho and para nitro-substituted diphenylmethanes and related diphenylmethyl anions. In the case of the neutral molecules, a predominance of the skew conformation is suggested for mono- and di-o nitro-substituted derivatives, e.g. 2,4,4′-trinitro- and 2,2′4,4′-tetranitro-diphenylmeth anes, while a perpendicular conformation is preferred by the tri-ortho nitro-substituted derivative, e.g. 2,2′,4,4′,6-pentanitrodiphenylmethane. 13C shifts and 1JcαHα coupling constants indicate that, in all carbanions studied, the exocyclic α-carbon is sp2 hybridized and carries only a small fraction of the negative charge. For carbanions with different substituents on the aromatic rings, e.g. 2,4,4′-trinitro- and 2,2′,4,4′,6-pentanitrodiphenylmethyl carbanions, the negative charge appears to be preferentially delocalized in the more substituted aromatic ring.From arguments based upon the observed n.m.r. and u.v. spectroscopic data, the two rings in diphenylmethyl anions with up to two ortho nitro substituents are essentially coplanar, whereas steric interactions prevent such coplanarity in those with three or four such substituents.

Nitrogen-bonded σ-adducts from amide anions and 1,3,5-trinitrobenzene. A 1H nuclear magnetic resonance structural study

The reactions of several amide and imide anions with 1,3,5-trinitrobenzene (TNB) have been studied in acetonitrile and dimethyl sulphoxide. U.v.–visible and 1H n.m.r. spectra have allowed complete characterization of the N-adducts but the formation of the isomeric O-adducts was not observed. Some N-adducts are found to exist as a mixture of Z and E isomers. In these instances the preferred isomer adopts the E configuration in which the bulky TNB substituent is trans to the carbonyl oxygen. Probable conformations around the N–Csp3 bond of the adducts are discussed in the light of the values of the 3JHNCH coupling constant. Most N-adducts have a thermodynamic stability which is of the same order as that of the TNB–methoxide adducts. This results as well as the failure to observe the O-adducts is shown to be consistent with literature data.

Angular dependence of two-bond coupling constant 2JPNC in the NN-dimethylaminophosphines

Low-temperature carbon-13 n.m.r. spectra of NN-dimethylaminophosphines show the two-bond coupling constant 2JPNC to be large and positive when the CN bond is cis with respect to the phosphorus lone pair, whilst it becomes small and negative in the trans-conformation.

Formation of phosphonoacetate carbanions by lithiated bases or by the conjunction of a lithium salt and an amine: structural study of intermediate species

In THF, the anionic species (2) formed from methyl diethylphosphonoacetate (1b) by the action of Li bases coexist as aggregates (2A) and externally solvated monomeric ion pair (2M), while in CH3CN triple ions (2T) and small amounts of free ions (2–) are also characterized. When (1b) is in slight excess, relative to the lithiated base, these four species coexist in both solvents, together with other species (I). From n.m.r. and i.r. data, it appears that the intermediate species (I) include a neutral phosphonate (1b) moiety, as a Li+ cation bidentate ligand, the two other lithium solvation sites being occupied either by (2–), or by (1b) as a monodentate and a solvent molecule, or else by two solvent molecules. The structure of (1b) in this intermediate species is strongly perturbed as deduced from variations in 31P and 13C carbonyl shifts as well as that of νco. Furthermore, a fast proton exchange takes place between the anionic species (2) and free (1b) as shown by 1H n.m.r., but (I) does not participate in this exchange phenomenon. When (2) is formed by action of diazabicycloundecene (DBU) in the presence of LiCl, aggregates (2A), monomeric ion pari (2M), and small amounts of (2T) as well as two intermediate species (I) are characterized by i.r. or 31P n.m.r. spectroscopy. The role of DBU is manifold: it deaggregates LiCl or (2A), it deprotonates (1b) within (I), and it facilitates proton exchange between (2) and (1b), the relative importance of such effects depending upon the DBU concentration.

Nuclear magnetic resonance structural study of phosphonate anions

It is shown by 31P, 13C, and 1H n.m.r. spectroscopy that BunLi and the phosphonates (1) and (2) in tetrahydrofuran give a single, planar, charge-delocalized species (A), whereas ButOK and ButOLi and (1) give another species (B) in addition to (A); the structure proposed for (B) is analogous to that of the lithium adducts of phosphorus ylides.