Endocyclic phosphorus atom

The lithiation of 3a-3y with one equivalent RLi yields the lithiated compounds 4a, 4p, which differ in the configuration of the endocyclic phosphorus atoms. Both isomers were characterized by their P NMR spectra, which show characteristic AMX2 spin systems with shifts in the typical range of lithium silylphosphides(-160 to -210 ppm). 

The effect on the endocyclic phosphorus atoms is less pronounced, as they stay within the conjugated double bond system, but since a negative charge and thus more electron density is introduced, the phosphorus resonance for these two phosphorus atoms is shifted upheld as well, by A< =-48 and AA=-68ppm, respectively, independent of the alkyl group. 

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. 

The effect on the endocyclic phosphorus atoms is only small, but downfield... 

As shown in Fig. 5, although the Wiberg indices (21) of the endocyclic (P N) bonds are large (favoring an important delocalization within the ring), the electron density on the phosphorus atoms is so low that no ring current can exist, hence the delocalization remains localized within the PNP units (79). 

Table 6 Bond lengths (pm) in 1,2,4-triphosphole rings (Y) and endocyclic angles at the phosphorus atoms,...

There are numerous examples of the use of cyclophosphazenes with exocyclic dialkylamino or alkyl/aryloxy substituents on phosphorus as scaffolds for the formation of metal complexes. However, the number of complexes in which the endocyclic nitrogen atoms are involved in adduct formation with electrophilic... 

Phosphole derivatives with CN = 5 and CN = 6 are hypervalent species, which are quite rare. Indeed, the phosphoranes (CN = 5) are stable only when the P-atom bears electronegative substituents such as fluorine atoms or alkoxy groups. The geometry adopted at phosphorus is distorted trigonal bipyramidal, something enforced by the small endocyclic CPC bond angle. Compounds with CN = 6 are anions with octahedral phosphorus atoms. 

Donor/acceptor-substituted phosphole 22 exhibits classical properties, namely the phosphorus atom has a pyramidal geometry and the aromatic character of the heterole is similar to that of cyclopentadiene <2000JOC2631>. Due to the push-pull substitution pattern, significant delocalization of the endocyclic 7t-electron density over the entire system... 

Reaction of this lO-S-3 [279] tetraazapentalene derivative with [Pd(PPhj) ] or [Rh(PPh3)3)Cl] results in the formal substitution of sulfur by the transition metal accompanied by a redox reaction (see Figure 4.93) [280], The endocyclic sulfur atom is transferred to a PPhj ligand (oxidation of phosphorus to PhjP=S). At the same time the transition metal is oxidis (palladium from 0 to +11 rhodium from +1 to +III), which leaves sulfur to be reduced by four electrons (it is -II in Ph I S and thus must have been +II in the tr-sulfurane starting material). It follows from this electron transfer analysis that the rt-sulfurane is indeed better desaibed as the sulfur complex of a doubly amide functionahsed NHC ligand. 

The results of ab initio calculations [34,35] provided a theoretical basis for the explanation of experimental results, and indicated that an attack of nucleophile at phosphorus atom in 13 (and analogously in 14 and 15) occurs from the side opposite to the endocyclic oxygen atom of 1,3,2-oxathiaphospholane moiety (Scheme 5). 

X-ray crystal structures of both 97 (all-tra/is isomer) and 99 trans is,trans isomer) have been obtained [39], and they confirm the relief of angle strain at the acetylenic carbon atoms that the phosphorus atoms provide in these small-ring pericyclynes (actual geometries shown in Fig. 9-28 average phosphorus atom endocyclic bond angles = 96° in 97 and 91° in 99 average acetylene carbon bond angles = 174° in 97 and 163° in 99). 

In both (I14a) and (114b), the phosphorus atoms have distorted tetrahedral bonding environments. The P=S(exocyclic) [1.910(5) A] and P-S(endocyclic) [2.051(5) A] bonds of (114a) differ significantly from the corresponding bonds [1.936(2) A and 2.087(2) A] in (114b). This effect... 

The simplest stable singlet carbene derived from a cyclic precursor is cyclo-propylidene. Like other carbocyclic carbenes, it does not contain a heteroatom within the ring and is therefore not further discussed here. The first cyclic carbene based on a four-membered heterocycle was described in 2004. Deprotonation of iminium salt 87a with KHMDS gave carbene dimer 88a=88a while the same reaction with 87b, bearing even bulkier N,N -substituents, led to the isolation of 88b (Scheme 1.11). Attempts to deprotonate 87a with KOt-Bu resulted in the opening of an endocyclic P-N bond, demonstrating the electrophilic character of the phosphorus atom in this compound. 

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