Exocyclic phosphorus atom

Lithiation with subsequent alkylation results in alkylation of the exocycUc phosphorus atom, and formation of an aromatic system in the central heterocycle analogous to a cyclopentadienide. Spectroscopically, the effect is a severing of the exocyclic phosphorus atom from the conjugated ring system, accompanied by a significant upheld shift of the phosphorus resonance by over AS= -300 ppm, dependent on the alkyl substituent. 

Subsequent alkylation of one of the endocyclic phosphorus atoms results in loss of the aromatic ring system and formation of a cyclic phosphadiene. The implications to the phosphorus resonances are a signihcant upheld shift for the alkylated endocyclic phosphorus atom as it is released from the. r-system, and a downheld shift of AS = 50 ppm of the resonance for the remaining endocyclic double bonded phosphorus due to the removal of negative charge from the conjugated. r-electron system. The resonance for the exocyclic phosphorus atom is almost unchanged. 

If the 1,3,6-triphosphafulvene is converted into an ylide by oxidation of the exocyclic phosphorus atom, a small downheld shift of all three phosphorus resonances is observed. Although it is a formal oxidation, it is carried out as a protonation of the anionic compound depicted in Fig. 5.16. We can look at it as a protonation of the lone pair on the exocyclic phosphorus atom and subsequent formation of an intramolecular donor bond from the endocychc carbon atom to the exocyclic phosphorus atom. The 1,3,6-triphosphafulvene system is reformed, bnt now with two additional substituents on the exocyclic phosphorus atom. 

The resonance for the exocyclic phosphorus atom is found at Ap=-23.2ppm for R=Me in the region customary for yUdes, and is shifted to lower field, Ap=-4.5ppm, for R=Bu" as expected. For R=F, the resonance shifts even further downfield, Ap=45.2ppm, in line with the greater electronegativity of flnorine compared to carbon. 

Oxidation of the exocyclic phosphorus atom of the 1,3,6-triphosphafulvene system disturbs the conjugation and shifts all three phosphorus resonances upfield. 

The new family of phospholes with 2,4,6-trialkylphenyl substituent on the phosphorus atom show, in many respects, a special reactivity. Due to the flattening of the P-pyramid, the arylphospholes exhibit aromaticity and hence underwent Friedel-Crafts reactions. The regioselective functionalization through reaction with phosphorus tribromide gave a variety of phospholes with an exocyclic P-moiety. Novel phosphole platinum and rhodium complexes were prepared and a part of them was tested in hydroformylation reactions. 

A similar situation is observed in the case of the cation-radical derived from dimethylhexa-phenyl diphosphafnlveninm dication as a resnlt of redaction of the latter by the sodinm salt of the naphthalene anion-radical (Biaso et al. 2006). According to ESR, x-ray experimental data, and results of DFT calculations, the diphosphafnlveninm cation-radical detains the excess electron density within the exocyclic double bond (see Scheme 3.41). Stabilization is gained by conjngation with the two pentavalent phosphorus atoms. Biaso et al. (2006) rationalize the electronic strnctnre throngh two valence-isomeric fluidic forms of the distonic type. 

Heterocyclic phosphorus ylides are a rather diverse and little known class of compounds. A variety of such structures are now known and in some cases these are of considerable synthetic value. In this chapter we have attempted to review all heterocyclic compounds containing one or more exocyclic phosphorus ylide functions, i.e. of general structure 1. It should be noted that in many cases these exist predominantly in the phosphonium ylide (P+—C ) form but for simplicity they are represented in the ylene form 1. Cyclic ylides 2 and 3 in which the phosphorus atom is within the ring are not included. 

Sulfur-rich binary P-S anions of the type [S2P(p-S )2PS2] form monocyclic structures with sulfide or disulfide bridging units ( = 1, 2) (11.39). Another type of cyclic P-S anion with the general formula [(PS2) ] is formed in the reaction of white phosphorus with polysulfides in a non-aqueous medium these anions have homocyclic structures with two exocyclic sulfur atoms attached to each phosphorus atom, e.g. [P4Ss] (11.40). ... 

I. 0 A out of the plane of the three phosphorus atoms (Figure 11.26a).In contrast, the phenyl derivative, which is obtained from the reaction of PhPCl2 with (Meg 80)28 in C82, is comprised of a five-membered P382 ring with an exocyclic sulfur atom attached to one of the phosphorus centres (Figure... 

Se NMR spectra of selenadiphospholes and diselenaphospholes 44, 45, 78, and 47 were reported (Table 25) <2002CEJ2705>. In compounds 44, 45, 78, and 47, the exocyclic selenium atom is at lower frequency relative to those within the heterocycle. A l/(Se,Se) coupling of 271 Hz is characteristic of compound 45. In molecule 47, the ring atom Se-3 has a 2/(P,Se) coupling to the phosphorus(v) center P-1 of 20 Hz. On the other hand, there is no resolved coupling between the exocyclic Se-1 and the internal phosphorus(m) center P-2. 

Analysis of nmr spectra allowed us to determine the absolute configuration of the phosphorus atom of pentaco-ordinated compounds la, 2a and lb, 2b. Protons P-O-CH-CH-O-P and exocyclic substituents P-H and P-0 X (X = H...DMF, HNEtlj) are in a cis position. The more likely structure of oligomers lji and lb is a sequence of TBP drawn up as helix (fig. 1), while dimers 2a and 2b should present an emetic structure with a pentaco-ordinated phosphorus atom (fig. 2). All these compounds manifest strong optical activity. 

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