The iminophosphorane Ph(2)P(CH(2)Py)(NSiMe(3)) (1) was treated with deprotonating alkali metal reagents to give [(Et(2)O)Li[Ph(2)P(CHPy)(NSiMe(3))]] (2), [[Ph(2)P(CH(2)Py)(NSiMe(3))]Li[Ph(2)P(CHPy)(NSiMe(3))]] (3) and [[Ph(2)P(CH(2)Py)(NSiMe(3))]Na[Ph(2)P(CHPy)(NSiMe(3))]] (4). We report their coordination behaviour in solid-state structures and NMR spectroscopic features in solution. Furthermore, we furnish experimental evidence against hypervalency of the phosphorus atom in iminophosphoranes from experimental charge-density studies and subsequent topological analysis. The topological properties, correlated to the results from NMR spectroscopic investigations, illustrate that the formal P=N double bond is better written as a polar P(+)--N(-) single bond. Additionally, the effects of metal coordination on the bonding parameters of the iminophosphorane and the related anion are discussed.

Keywords: Oxidation / Tetrahydrofuran / Catalysis / Asymmetric synthesis / Vanadium Vanadium(V) complexes 4 have been prepared from tridentate Schiff-base ligands 3 and VO(OEt) 3 . All vanadium(V) compounds were characterized (IR, UV/Vis, and 51 V NMR spectroscopy, and in selected examples by X-ray diffraction analysis) and were applied as oxidation catalysts for the stereoselective synthesis of functionalized tetrahydrofurans 2 starting from substituted bis(homoallylic) alcohols 1 (mono-or trisubstituted C−C double bonds). Oxidation of secondary or tertiary 1-alkyl-, 1-vinyl-, or 1-phenyl-substituted 5,5-dimethyl-4-penten-1-ols under optimized conditions [TBHP as primary oxidant and 1,2-(amino)indanol-derived vanadium(V) reagent 4g as catalyst] provided 2,5-cis-configured tetrahydrofurans in synthetically useful yields and diastereoselectivities (22−96% de). On the other hand, trans-disubstituted oxolanes (62%−96 de) were obtained from oxidations of 2-or 3-alkyl-and 2-or 3-phenyl-substituted 5,5-dimethyl-4-penten-1-ols bis(homoallylic) alcohols. Treatment of 4-penten-1-ols (i.e. substrates with monosubstituted olefinic π-bonds) with TBHP and catalytic amounts of vanadium(V) complex 4g furnished trans-disubstituted tetrahydrofurans as major products (20−96% de), no matter whether an alkyl or a phenyl substituent was located in position 1, 2, or 3 of the alkenol chain. The mechanism of this reaction has been investigated in detail. Based on re-

No abstract

Monoanionic heteroallylic ligand systems [R−N−E−N−R]- (E = Si(R2), S(R2) or S(R), C(R), and P(R2)) are versatile chelating substituents both in main group and transition metal chemistry as they provide sufficient steric demand and solubility to the products. Their application is only limited by the rigid bite of the ligands as the N···N distance cannot be tuned to the various radii of different metals. In this paper we present the new concept of opening the ligand periphery to additional coordination. The NP(R2)N- chelate in classical aminoiminophosphoranates is extended by additional coordination sites in the organic substituents (e.g., 2-pyridyl (Py) instead of phenyl (Ph)). Py2P{N(H)SiMe3}(NSiMe3) (1) is the starting material for a new class of complexes as deprotonated 1 contains along with the NPN- chelate the pyridyl ring nitrogen atoms to generate a side-selective Janus face ligand. In [(THF)Sr{Py2P(NSiMe3)2}2] (2) and [(4,4‘-bipy)Ba{Py2P(NSiMe3)2}2] n (3) both pyridyl rings are involved in metal coordination but only one imido nitrogen atom. Hence, the classical NP(Ph2)N- chelating ligand is converted into a NP(Py2)N- tripodal ligand. In the coordination to zinc in the complex [Zn{Py2P(NSiMe3)2}2] (4) one pyridyl ring and one imido nitrogen atom is employed in metal coordination. Pyridyl substitution of the P(V) center gives not only access to new coordination modes but also changes the reactivity of aminoiminophophoranes considerably. [Li(Py2PNSiMe3)]2 (5) is a lithiated phosphanylamine derived from the reduction of 1 with lithium di(trimethylsilyl)amide. Reaction of 1 with lithium organics yields [(THF)2Li(Py2P)] (6). Pyridyl substitution facilitates single or even double PN bond cleavage, unprecedented in alkyl- or aryl-substituted aminoiminophosphoranes. This reduction of P(V) species to P(III) compounds supplies easy access to phosphanylamines and secondary phosphanes.

The unusual trigonal prismatic structure of tris(butadiene)molybdenum, reported in 1975 by Skell, has been revisited by extensive quantum chemical calculations and by a low-temperature single-crystal X-ray diffraction study. While a trigonal prismatic coordination arrangement is confirmed by DFT and MP2 structure optimizations, the calculations provide very different bond lengths than earlier crystallographic studies:  Due to appreciable back-bonding, the terminal Mo−C1 bonds are significantly shorter than the central Mo−C2 bonds (ca. 2.29 vs ca. 2.36 Å), and the central (C2−C2A) bonds are actually shorter than the terminal ones (ca. 1.40 vs ca. 1.44 Å), as found previously for substituted complexes. Similar structures have been computed for tris(butadiene)tungsten and for related, substituted systems. A structure redetermination of tris(butadiene)molybdenum at low temperature shows that the erroneous bond lengths obtained previously are due to the presence of a disorder resulting from the superposition of two different orientations of the three butadiene ligands with different site occupation factors. Refinement of this disorder results in a physically more plausible orientation of the anisotropic displacement parameter and gives by a factor of 10 improved estimated standard deviations for the geometrical features. A much better agreement between theory and experiment is attained. It is now obvious that resonance structures involving metallacyclopentene rings contribute significantly to bonding. This conclusion has been confirmed by natural bond orbital/natural resonance theory analyses, which indicate overall larger contributions from metallacyclopentene resonance structures than from traditional resonance structures with π-bonded diolefins. Explanations are provided for the trigonal prismatic structure preferences. MO analyses differ qualitatively and quantitatively from previous work, due to the use of refined structural parameters. Computed ligand NMR chemical shifts agree well with experimental data, provided that they are calculated at the correct structures.