Phospholes with Reduced Pyramidal Character from Steric Crowding. 2. Photoelectron Spectral Evidence for Some Electron Delocalization in 1-(2,4-Di-tert-butyl-6-methylphenyl)-3-methylphosphole

Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra had shown no difference in ionization energies (IE) for simple phospholes and their tetrahydro derivatives (both around 8.0-8.45 eV). Calculations of the Koopmans IE at the Hartree-Fock 6-31G level for 1-methylphospholane showed that, as is known for nitrogen, planarization at phosphorus markedly reduced the ionization energy value (8.74 to 6.29 eV). A reduction in IE also occurred on planarizing 1-methylphosphole, but to a lesser extent, being offset by increased electron delocalization (8.93 to 7.16 eV). This suggests that experimental comparison of IE for the unsaturated and saturated systems could be used to detect the presence of electron delocalization in the former. The IE experimentally determined for the crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was 7.9 eV, the lowest ever recorded for a phosphole. The corresponding phospholane had IE 7.55 eV. The difference in the values is attributed to electron delocalization in the phosphole. Calculations performed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)phosphole showed that the P-substituent adopted an angle of 55.7 degrees (DFT/6-31G level; 57.6 degrees at the HF/6-31 level) with respect to the C(2)-P-C(5) plane (for P-phenyl, 67.1 degrees and 68.3 degrees, respectively).     

13C and 31P NMR studies of configurational and conformational effects in 1-Phenyl-4-phosphorinanones and their 1-selenides

Carbon-13 and ³¹P NMR data are reported for 1-phenyl-4-phosphorinanone and its 1-selenide, as well as for two anancomeric 1-phenyl-3,5-dimethyl-4-phosphorinanones and their 1-selenides. The conformational free energy of the P-phenyl substituent in 1-phenyl-4-phosphorinanone is estimated to be ΔG° = 0.81 kcal mol^?1 (ca 80% axial) in chloroform, and this result is consistent with both ¹³C NMR shielding and coupling data. The γ effects of a single atom substituent on phosphorus are found to be small in the case of selenium; information in the literature indicates significant downfield-shifting γ_e effects due to equatorial oxygen and sulphur substituents in phosphorinanes. In selenides, the shift for the aromatic C-ipso carbon in the axial isomer is further downfield than in the equatorial isomer, an observation not precedented in the literature. In the discussion of P-C coupling data a ‘second order’ Karplus-like relationship is invoked for ³J(PC), which is dependent both on the dihedral angle and on the orientation of the phosphorus lone pair in phosphines. The one-bond P-C-ipso coupling in selenides is identical for all three selenides studied, regardless of the stereochemistry at phosphorus. Similar lack of substantial differentiation is noted for one-bond P-Se coupling. A possible origin of this phenomenon is discussed in terms of diminution of the phosphorus charge contribution to the Fermi contact term. From ³¹P NMR data a high stereodependence of selenylation shifts is apparent, and greater shifts (by ca 20 ppm) are observed when selenium is bound to phosphorus in the more crowded (axial) position. In anancomeric (conformationally biased) phosphines, the isomer with the axial phenyl group has the ³¹P signal at lower field. This is consistent with observations made previously for rigid 1-phenylphosphorinanes.     

The Effect of Trifluoroethoxy Groups on the Structures of Eight-Membered Ring-Containing Cyclic Phosphites 1

New phosphites 1–4 were synthesized from phosphorus trichloride and an appropriate diphenol followed by the addition of trifluoroethanol in the presence of triethylamine. These phosphites are to serve as precursors in the syntheses of biorelated hypervalent phosphoranes. ¹H, ¹⁹F, and ³¹P NMR spectra were recorded. X-ray analysis of 3 and 4 revealed that the sulfur-containing eight-membered ring was in a syn conformation that a allowed a sulfur donor interaction to the phosphorus atom, whereas for phosphite 2, the eight membered sulfonyl containing ring was in an anti conformation that did not allow a donor interaction to phosphorus from the oxygen atom of the sulfonyl group. Structural comparisons are made with related cyclic phosphites and phosphates having donor atoms in eight-membered rings.     

Stereodynamics of N-isopropyl-N-methylpropargylamine. Dynamic NMR studies. Molecular mechanics calculations.

N-Isopropyl-N-methylpropargylamine (N-isopropyl-N-methyl-2-propyn-1-amine; IMPA) is chiral at the pyramidal nitrogen. Racemization occurs via an inversion-rotation process. Both 13C(1H) and 1H dynamic NMR (DNMR) spectra decoalesce in response to slowing inversion-rotation (delta G++ = 7.7 +/- 0.1 kcal/mol). While aspects of the DNMR spectra suggest the presence of minor conformations, the spectrum at 100 K shows a strong preference for one conformation. The NMR data suggest that the preferred conformation has both the isopropyl methine proton and the ethynyl group anti to the lone pair. Both isopropyl methyl groups are gauche to the lone pair. This conformational preference is in significant contrast to N-ethyl-N-methyl-2-aminopropane in which the population of that conformation having the ethyl methyl group and the isopropyl methine proton both anti to the lone pair is only 5% at 95 K. The NMR data, supported by molecular mechanics (MMX) calculations, suggest a special stabilization for the ethynyl group being oriented anti to the lone pair.     

Phosphorus chemical shift tensors of phosphido ligands in ruthenium carbonyl compounds: (31)P NMR spectroscopy of single-crystal and powder samples and ab initio calculations

The phosphorus chemical shift (CS) tensors of several ruthenium carbonyl compounds containing a phosphido ligand, micro), bridging a Ru [bond] Ru bond were characterized by solid-state (31)P NMR spectroscopy. As well, an analogous osmium compound was examined. The structures of most of the clusters investigated have approximate local C(2v) symmetry about the phosphorus atom. Compared to the "isolated" PH(2)(-) anion, the phosphorus nucleus of a bridging phosphido ligand exhibits considerable deshielding. The phosphorus CS tensors of most of the compounds have spans ranging from 230 to 350 ppm and skews of approximately zero. Single-crystal NMR was used to investigate the orientation of the phosphorus CS tensors for two of the compounds, Ru(2)(CO)(6)(mu(2)-C [triple bond] C [bond] Ph)(mu(2)-PPh(2)) and Ru(3)(CO)(9)(mu(2)-H)(mu(2)-PPh(2)). The intermediate component of the phosphorus CS tensor, delta(22), lies along the local C(2) axis in both compounds. The least shielded component, delta(11), lies perpendicular to the Ru [bond] P [bond] Ru plane while the most shielded component, delta(33), lies perpendicular to the C [bond]P [bond] C plane. The orientation of the phosphorus CS tensor for a third compound, Ru(2)(CO)(6)(mu(2)-PPh(2))(2), was investigated by the dipolar-chemical shift NMR technique and was found to be analogous, suggesting it to be the same in all compounds. Ab initio calculations of phosphorus magnetic shielding tensors have been carried out and reproduce the orientations found experimentally. The orientation of the CS tensor has been rationalized using simple frontier MO theory. Splittings due to (99,101)Ru [bond] (31)P spin-spin coupling have been observed for several of the complexes. A rare example of (189)Os [bond] (31)P spin-spin splittings is observed in the (31)P MAS NMR spectrum of the osmium cluster, where (1)J((189)Os, (31)P) is 367 Hz. For this complex, the (189)Os nuclear quadrupolar coupling constant is on the order of several hundred megahertz.     

SYNTHESIS AND STEREOCHEMISTRY OF PENTACARBONYL(PHOSPHOLE)METAL(0)- AND TETRACARBONYLBIS(PHOSPHOLE)METAL(0) COMPLEXES OF 6B ELEMENTS

Abstract

Pentacarbonylphospholemetal(0) and cis-tetracarbonylbis(phosphole)metal(0) complexes were synthesized from the thermal reaction of M(CO)₃(THF) and M(CO)₄(COD) (M: Cr, Mo, W) with corresponding phosphole (1-phenyl-3,4-dimethylphosphole, 1-phenyl-3-methylphosphole, and 1-phenylphosphole). These complexes were isolated as orange crystals by column chromatography on silicagel at 253 K and crystallization from n-hexane at 223 K and characterized by means of IR and NMR (¹H, ¹³C, and ³¹P). Spectroscopic data shows that the phosphole is coordinated to the transition metal through its phosphorus atom rather than through the conjugated diene unit in the both types of complexes. The tetracarbonylbis(phosphole)metal(0) complexes were found to have cis-arrangement of two phosphole ligands. Comparing ¹³C-NMR chemical shifts of the complexes with the free ligands, one can deduce that the involvement of the phosphorus atom in the ring π-electron delocalization is drastically reduced upon coordination. This is attributed to the stronger [sgrave]-donation but weaker π-accepting ability of the phosphorus atom in the phosphole ligands compared to the carbonyl groups.     

Phosphorus Chemical Shift Tensors of Phosphole Derivatives Determined by (31)P NMR Spectroscopy of Powder Samples

The results of a systematic solid-state (31)P NMR study of 5-phenyldibenzophosphole, DBP, its chalcogenides, and some of its transition metal complexes are reported. Phosphorus chemical shift tensors have been obtained from (31)P NMR spectra of stationary samples and of samples spinning about the magic angle. The spans of the phosphorus chemical shift tensors for DBP and its chalcogenides are comparable to those of the corresponding compounds of triphenylphosphine; however, the asymmetry of the tensors for the DBP series reflects the reduced local symmetry at phosphorus. For the complexes (DBP)M(CO)(5) and cis-(DBP)(2)M(CO)(4), where M is a group 6 transition metal (Cr, Mo, W), the most shielded component of the phosphorus shift tensor is found to be relatively independent of the metal or complex, delta(33) = -41 +/- 8 ppm, and is thought to lie along or close to the P-M bond axis direction. In contrast, delta(11) and delta(22) show considerable variation but decrease systematically on descending the group from Cr to W. Group 10 metal complexes, (DBP)(2)MX(2), have also been investigated, including several trans geometric isomers of nickel, cis and trans isomers of palladium, and cis isomers of platinum. The phosphorus shift tensors are nonaxially symmetric with spans in the range 50 -150 ppm. The phosphorus shift tensors of the two nonequivalent DBP ligands of (DBP)(2)PtX(2) (X = Cl, Br) exhibit quite different principal components. The intermediate component of the shift tensor is thought to lie along the Pt-P bond in these complexes. Some of the complexes exhibit interesting MAS-frequency-dependent (31)P NMR spectra.     

Conformation of secondary amides. A predictive algorithm that correlates DFT-calculated structures and experimental proton chemical shifts

The magnetic deshielding caused by the amido group on CON-CHalpha protons of secondary amides can easily be correlated with DFT-based structures at the B3LYP/6-31G level of theory via a novel algorithm that refines previous models, such as the classical McConnell equation. The shift is given by delta = a + 2.16 cos2(alpha - 35)/d, where alpha denotes the virtual dihedral angle resulting from linking the carbonyl and the alpha-carbons and d is the distance (A) between the shifted proton and the carbonyl oxygen. Notably, in this equation a is a parameter that can be optimized for different solvents, namely, CDCl3, DMSO-d6, and D2O. For the development of these correlations, the preferential conformation of amides is taken from the optimized structures in the gas phase obtained at the DFT level. The deshielding on anti and gauche protons in both rotamers of (Z)-acetamides and E/Z isomers of formamides has been evaluated. This methodology has proved to be highly reliable, allowing us to discard ab initio or DFT conformational arrangements when shifts calculated by the above-mentioned equation differ from the experimental values. Thus, the anti disposition between the CHalpha proton and the N-H bond appears to be the more stable conformation of simple amides. For amides bearing only one proton at Calpha, a local syn minimum can equally be characterized. The rotational barriers around the CON-alkyl bond along with the pyramidalization of the amido group have also been reassessed. As the conformation is taken away from anti or local syn minima, the nonplanarity of the amido group appears to increase.     

Phosphole-Fused Dehydropurpurins via Titanium-Mediated [2+2+1] Cyclization Strategy

A [2+2+1] cyclization strategy of bis(alkynyl)porphyrin using low-valent titanium species, generated in situ, afforded phosphole-fused dehydropurpurins for the first time. The systematic investigations on the electronic properties of the dehydropurpurins revealed their unique features owing to the oxidation states of the phosphorus atom on the fused phosphole ring. The phosphole P=O and P=S derivatives were found to possess high electron-accepting character derived from phosphorus(V) centers without the contribution of 24π antiaromatic character, suggesting their potential utility as electron-accepting materials. In contrast, the phosphorus(III) derivatives revealed different optical and electrochemical properties arising from both 18π aromatic and 24π antiaromatic networks including the lone pair of the phosphorus(III) atom. Overall, the oxidation state of the phosphorus atom has a clear impact on the whole electronic properties, demonstrating the advantages of chemical modifications of the phosphorus center for creating an exotic π-system. The application of titanium-mediated [2+2+1] cyclization to porphyrins is highly promising for expanding a world of heterole-fused porphyrinoids.     

X-ray, 31P CP/MAS,and single-crystal NMR studies,and 31P DFT GIAO calculations of inclusion complexes of bis[6-O,6-O'-(1,2:3,4- diisopropylidene-alpha-D-galactopyranosyl)thiophosphoryl] disulfide: the importance of C-H...S=P contacts in the solid state

Bis[6-O,6-O'-(1,2:3,4-diisopropylidene-alpha-D-galactopyranosyl) thiophosphoryl] disulfide shows a strong tendency to form inclusion compounds. The crystal and molecular structure of eight different solvates was established by X-ray analysis. The results indicate three different types of disulfide arrangements in the crystal lattice. By means of 31P CP/MAS NMR experiments the principal values delta 11, delta 22, and delta 33 of the 31P chemical shift tensor were obtained for each form. The orientation of its principal axes with respect to a molecular frame was investigated by means of 31P CP and single-crystal NMR for the complex with propan-2-ol. The principal axis 1 of both chemically equivalent phosphorus atoms is nearly parallel to the P-S bond and the principal axis 3 is very close to the P=S bond. DFT GIAO calculations of the model compound (EtO)2(S)P1SSP2(S)-(OEt)2 allowed assignment of the experimental chemical shift curves to the magnetically nonequivalent atoms P1 and P2. The maximum difference between calculated angles [symbol: see text] i-P-X)calcd and experimental angles [symbol: see text] i-P-X)exptl is 8.3 degrees and the rms distance 3.8 degrees (i = principal axes 1, 2, 3; X = S, -S-, -O1-, -O2-). The influence of C-H...S weak hydrogen bonding on phosphorus shielding was tested theoretically (31P DFT GIAO) employing the dimethoxythiophosphoryl disulfide.CH4 complex as a model compound. The sensitivity of 31P delta ii parameters to intermolecular forces is demonstrated.     

Three-bond 31P?31P coupling and 31P chemical shift effects in dimers of phospholium salts and phosphole oxides

The chemically nonequivalent ³¹P nuclei of the Diels-Alder dimers of phospholium ions and phosphole oxides are coupled through three bonds by as much as 35–45 Hz. The chemical shift for a phosphorus atom in the 2-phospholene moiety falls in the expected range; the shift for the constricted atom bridging the 6-membered ring is far downfield, making Δδ as much as 43.5 ppm for one phospholium ion dimer.     

NMR study of 2-alkoxy-1,3,2-dioxaphospholanes. II—molecular conformation of alkoxy substituents

The conformation of some 2-σ-1,3,2-dioxaphospholanes (σ=OAlkyl, CI) is studied. The determination of the ²J(POC) and ³J(POCC) coupling constants, which are influenced by the bulk of the alkoxy group, is a means of obtaining information about the rotation around the P? OR bond, which is dependent on steric interactions between the phosphorus lone pair, the alkoxy group and the substituents on the ring. When σ is a tert-butoxy group, a direct ‘through-space’ interaction is found between the phosphorus atom and one of the primary carbons of the OR group. If there is no substituent on the ring, the ³¹P chemical shifts are little affected by changes in OR; a diamagnetic effect is observed, however, in the case of the tert-butoxy group which is enhanced for the 4,4,5,5-tetramethyl derivatives.     

Synthesis and characterization of novel π‐conjugated polymers with phosphole ring derivatives

Novel π‐conjugated polymers ( 8 – 10 ) were prepared by the palladium‐catalyzed Sonogashira coupling reaction of three kinds of phosphole‐ring‐containing monomers with 2,5‐dihexyloxyl‐1,4‐diethynylbenzene. The obtained polymers ( 8 – 10 ) were regioregulated with the 2,5‐substituted phosphole ring in the polymer main chain and characterized with ¹H, ¹³C, and ³¹P NMR and FTIR. Polymers 8 – 10 were found to have an extended π‐conjugated system according to the results of UV–vis absorption spectra. In the fluorescence emission spectra of 8 – 10 , moderate emission peaks were observed in the visible blue‐to‐green region. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2867–2875, 2007     

Determination of 3J(31P31P) and 13C31P coupling constants in 1,6-diphosphatriptycene from carbon-13 n.m.r. lone pairs effects on 13C31P couplings

The ³¹P? ³¹P and ¹³C? ³¹P coupling constants in 1,6-diphosphatriptycene have been obtained from analysis of its proton decoupled ¹³C n.m.r. spectra. More accurate data, however, resulted from simultaneous analysis of the proton decoupled ¹³C spectra and ³¹P(¹³C) satellite spectra. The ¹³C? ³¹P couplings are strongly influenced by the proximity and orientation of the phosphorus lone pair electrons. The first ³¹P? ³¹P coupling in an aromatic diphosphine is reported.     

Determination of 31P,31P coupling constants in cyclotriphosphazenes and their influence on 1H and 13C NMR spectra of phosphorus substituents

¹H and ¹³C NMR spectra of symmetrically substituted cyclotriphosphazenes exhibit second‐order effects. The influence of the ³¹P,³¹P coupling constants between ring phosphorus atoms on these effects was studied. Some values of this coupling constant between phosphorus bearing identical substituents were measured using ¹³C satellites of the ³¹P signals or by introduction of a chiral substituent on the third phosphorus atom. Copyright © 2003 John Wiley & Sons, Ltd.     

Toward the development of a structurally novel class of chiral auxiliaries. Conformational properties of the aldol adducts of oxadiazinones: observation of unusual shielding effects

Asymmetric aldol reactions were conducted with the titanium enolate of N(3)-hydrocinnamoyl-3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one to afford aldol adducts 5a-j. The dominant product of the asymmetric aldol reaction was the non-Evans syn adduct as determined by (1)H NMR spectroscopy and X-ray crystallography. When evaluating the (1)H NMR spectra of adducts 5a-j, a highly shielded signal with an average chemical shift of 0.05 ppm was observed. This signal was readily determined to be the C(5)-methyl group of the oxadiazinone. It is presumed that the overall conformation adopted by the aldol adducts in solution places an aromatic ring of the N(3)-substituent in close proximity to the C(5)-methyl group. An investigation of this conformational preference is conducted employing (1)H NMR spectroscopy, X-ray crystallography, and computational methods.     

Quantum-chemical calculation of NMR chemical shifts of organic molecules: III. Intramolecular coordination effects on the 31P NMR chemical shifts of phosphorylated N-vinylazoles

Theoretical and experimental studies on magnetic shielding of the phosphorus nucleus in trichloro-[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ⁶-pyrazolo-[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide showed that intramolecular coordination of the phosphorus atom in the chlorophosphonium group to the nitrogen atom in the pyrazole ring leads to upfield shift of the phosphorus signal (to δ_P 170 ppm) and that the contribution of the spin-orbital contribution to the ³¹P chemical shift reaches 15%. Relativistic effects and effects of the medium are determining in the theoretical calculation of ³¹P NMR chemical shifts.     

<Superscript>1</Superscript>H NMR spectroscopy in the study of the three-dimensional structure of 7-alkoxyalkyl-3-thia-7-azabicyclo-[3.3.1]nonan-9-ones and some of their derivatives

¹H NMR spectroscopy was used to establish that 7-alkoxyalkyl-3-thia-7-azabicyclo[3.3.1]nonan-9-ones and their decarbonylated derivatives in deuterochloroform solution exist in the double chair conformation. The predominantly formed secondary alcohols isomers have preferred double chair conformation with the hydroxyl group equatorial relative to the plane of the piperidine ring. On the other hand, the epimeric alcohols have predominant boat-chair conformation; the piperidine ring takes the boat form due to intramolecular hydrogen bonding between the unshared electron pair of the nitrogen atom and hydroxyl group proton. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1716–1725, November, 2008.     

Nuclear magnetic resonance of phosphorus compounds: 9—The NMR spectra of phosphirane

The 100 MHz proton and 40.4 MHz ³¹P NMR spectra of phosphirane (1) have been recorded at ?20 °C and analysed iteratively to yield coupling constants and chemical shifts. The 22.6 MHz ¹³C spectrum of 1 was recorded at 0 °C and analyzed. The ³¹P chemical shift of 1 was measured as 40 467 515.97 ± 0.08 Hz relative to TMS as 100 000 000.00 Hz. The geminal P-C-H couplings in 1 are opposite in sign and of different magnitudes (+16.14 and ?2.64 Hz); the P? H coupling (+158.34 Hz) is smaller than that in any other organic phosphine. These observations are discussed and correlated with the geometry of 1. The electronic structure of the strained ring of 1 is discussed in terms of a localized valence bond approach.     

Comparing main group and transition-metal square-planar complexes of the diselenoimidodiphosphinate anion: a solid-state NMR investigation of M[N(iPr2PSe)2]2 (M = Se, Te; Pd, Pt)

A comparison of the square-planar complexes of group 10 (Pd(II), Pt(II)) and 16 (Se(II), Te(II)) centers with the tetraisopropyldiselenoimidodiphosphinate anion, [N((i)Pr2PSe)2](-), is made on the basis of the results of a solid-state (31)P, (77)Se, (125)Te, and (195)Pt NMR investigation. Density functional theory calculations of the respective chemical shift and (14)N electric field gradient tensors in these compounds complement the experimental results. The NMR spectra were analyzed to determine the respective phosphorus, selenium, tellurium, and platinum chemical shift tensors along with numerous indirect spin-spin coupling constants. Special attention was given to observed differences in the NMR parameters for the transition metal and main-group square-planar complexes. Residual dipolar coupling between (14)N and (31)P, not observed in the solid-state (31)P NMR spectra of the Pd(II) and Pt(II) complexes, was observed at 4.7 and 7.0 T for M[N((i)Pr 2PSe)2]2(M = Se, Te) yielding average values of R((31)P, (14)N)eff = 890 Hz, CQ((14)N) = 2.5 MHz, (1) J( (31)P, (14)N) iso= 15 Hz, alpha = 90 degrees , beta = 17 degrees . The span, Omega, and calculated orientation of the selenium chemical shift tensor for the diselenoimidodiphosphinate anion is found to depend on whether the selenium is located within a pseudoboat or distorted-chair MSe 2P 2N six-membered ring. The largest reported values of (1)J((77)Se, (77)Se) iso, 405 and 435 Hz, and (1)J((125)Te, (77)Se)iso, 1120 and 1270 Hz, were obtained for the selenium and tellurium complexes, respectively; however, in contrast a correspondingly large value of (1)J((195)Pt, (77)Se)iso was not found. The chemical shift tensors for the central atoms, Se(II) and Te(II), possess positive skews, while for Pt(II) its chemical shift tensor has a negative kappa. This observed difference for the shielding of the central atoms has been explained using a qualitative molecular orbital approach.