Phosphorus six-membered rings

Bentrude, W.G. and Hargis, J.H., Conformations of six-membered ring phosphorus heterocycles. I. The ring conformations and phosphorus configurations of isomeric six-membered ring phosphites, /. Am. Chem. Soc., 92, 7136, 1970. 

The reaction of /3-enamino-A5-phosphanes with dimethyl acetylenedi-carboxylate gives l-aza-4A5-phosphinines (244) [89JCS(P1)2273]. Six-membered ring phosphorus-containing heterocycles were obtained by the reaction of enaminophosphines (245) with ethyl azidoformate (87TL2875). 

The inference from stereochemical studies is that pseudorotation is not involved in enzymic phosphoryl and nucleotidyl transfer reactions. However, the involvement of pseudorotation has been postulated in several biological reactions involving six-membered-ring phosphorus species. These are discussed in Section 4. 

Associative nucleophilic substitution at phosphorus contained in a six-membered ring is not a rapid process relative to the analogous reaction of acyclic compounds. Whereas associative reactions of five-mem bered sp>ecies are dominated by the influence of special strain effects, the same is not true for six-membered species. One must therefore bear in mind that other competitive processes may become significant in the reaction of six-membered ring phosphorus compounds with nucleophiles. For example, it has been suggested that the cAMP derivative [92] is hydrolysed by a dissociative Sf l mechanism (Scheme 32) (Bottka and Tomasz, 1985). 

Six-membered Ring Phosphorus Heterocycles 4.4.1 Chromone Derivatives... 

Phosphorus heterocyclic compounds, 1, 493-538 five-membered ring systems, 1, 513-523 nomenclature, 1, 496 six-membered ring systems, 1, 497-513 Photoaromatization oxirenes from, 7, 125-126 Photobleaching chromenes in, 3, 880 Photochemical reactions heterocyclic compound synthesis from, 5, 159 reviews, 1, 56 heterocyclic compounds reviews, 1, 71, 72... 

The heterocycles 13.1 are isolated as purple crystals (R = Mc3SiNH, Ph) or as a thermally unstable, purple oil (R = Me), which decomposes at room temperature to give 13.3 (R = Me, Ph). The structure of 13.1 (R = Ph) consists of a six-membered ring in which the phosphorus atom lies 0.26 A out of the S2N3 plane. The S-N bond lengths fall within a narrow... 

Conformation problems in five- and six-membered rings containing phosphorus continue to attract attention and, using n.m.r., i.r., and dipole-moment measurements, studies have been reported on 1,3,2-dioxaphospholan systems (118), > 1,3,2-dioxaphosphorinane systems... 

N.m.r. spectroscopy has played an important part in determining the stereochemistry of the 1,3-dioxaphosphorinanes (52). The presence of the saturated six-membered ring means that there are usually conformational effects to be unravelled before configurational assignments can be made. The chair conformation is generally dominant. Phosphorus substituents which exhibit shielding effects show that in many P " phosphorinanes this substituent occupies an axial position and Sis( has been used to establish the equatorial conformation of a t-butyl substituent at C(5). Even in P" derivatives the isomer possessing the bulkiest P-substituent in an axial... 

Arsenic modifications with the structures of white and black phosphorus have been described. However, only gray (metallic, rhombohedral) a-arsenic is stable. It consists of layers of six-membered rings in the chair conformation that are connected with each other in the same way as in /ran.v-dccalin (Fig. 11.7). In the layer the atoms are situated alternately in an upper and a lower plane. The layers are stacked in a staggered manner such that over and under the center of every ring there is an As atom in an adjacent layer. In this way every As atom is in contact with three more atoms in addition to the three atoms to which it is bonded within the layer it has a distorted octahedral 3 + 3 coordination. The As-As bond length in the layer is 252 pm the distance between adjacent atoms of different layers is 312 pm and thus is considerably shorter than the van der Waals distance (370 pm). 

Reaction of the 2-amino-substituted thiophene 244 with pyrrolidinone in the presence of phosphorus oxychloride led to efficient formation of the central six-membered ring of tricycle 245 in 57% yield (Equation 67)... 

Ah initio calculations to map out the gas-phase activation free energy profiles of the reactions of trimethyl phosphate (TMP) (246) with three nucleophiles, HO, MeO and F have been carried out. The calculations revealed, inter alia, a novel activation free-energy pathway for HO attack on TMP in the gas phase in which initial addition at phosphorus is followed by pseudorotation and subsequent elimination with simultaneous intramolecular proton transfer. Ah initio calculations and continuum dielectric methods have been employed to map out the lowest activation free-energy profiles for the alkaline hydrolysis of a five-membered cyclic phosphate, methyl ethylene phosphate (247), its acyclic analogue, trimethyl phosphate (246), and its six-membered ring counterpart, methyl propylene phosphate (248). The rate-limiting step for the three reactions was found to be hydroxyl ion attack at the phosphorus atom of the triester. ... 

The structure of the orange 1,3-isomer of the eight-membered ring, 2, resembles that of the six-membered ring in that the NSNSN unit is essentially planar. The phosphorus atoms lie out and on opposite sides of this plane by ca. O.ToS (9.). The structural parameters for 2, are given in Figure 1. 

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