Phosphorus substituent

Table 7.10. Acidities of Some Compounds with Suifiir and Phosphorus Substituents...

The results reported in the previous sections illustrate the importance and versatility of P-centered radicals in organic chemistry. In this last section we would like to mention various reports that show that the presence of a phosphorus substituent close to a radical center can dramatically influence its properties. 

In fact, considering the basic structure of these materials (vide supra), it can be immediately realized that the basic features of poly(organophosphazenes) are the result of two main contributions. The first one is fixed and is basically related to the intrinsic properties of the -P=N- inorganic backbone, while the second is variable and mostly connected to the chemical and physical characteristics of the phosphorus substituent groups. Skeletal properties in phos-phazene macromolecules intrinsically due to the polymer chain are briefly summarized below. 

In this case chain mobility is strongly inhibited by the rigid structure of the side phosphorus substituents and the resulting TgS are exceedingly high for the usual standard of phosphazene macromolecules. 

A new series of properties are expected for polyphosphazenes when the percentage of inorganic elements inherently present in the -P=N- skeleton is artificially enhanced by introducing fluorinated alcohols as side phosphorus substituents. This facilitates their application in different fields. 

In fact, fluorinated polyphosphazenes are usually considered to be extremely stable towards chemical agents and aggressives due to the presence of C-F bonds in the side phosphorus substituents. PTFEP, for instance, appears to be completely insensitive to several, most common, solvents (aliphatic and aromatic hydrocarbons, alcohols and water), to acids (e.g. acetic acid), and to bases (e.g. pyridine and concentrated NaOH solutions), although some decomposition could be observed in triethylamine and in concentrated H2SO4 [41]. Phos-phazene fluoroelastomers, moreover, are known to be completely insoluble in aromatic solvents [533] and petroleum-resistant materials [502-506,552]. 

In conclusion, polyphosphazenes containing fluoroalkoxy groups as side phosphorus substituents constitute one of the most relevant class of macromolecules of this family and have attracted remarkable interest in the past because of their outstanding properties and wide range of applicability, especially in low and high temperature domains, and have received renewed interest in more recent times [399,457]. 

Spectral Transparence Starting from 230 nm HN—< CH3 4-Toluidino p-Naphthoxy Substituent Photoactive from the First Excited Singlet State Polymers Photochemically Active from the First Excited Singlet State of the Phosphorus Substituents - " - "... 

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... 

Push-spectator carbenes of the type 31 (R, R = alkyl) were synthesized and reacted with various Lewis Acids to compare the reactivity of the phosphorus and carbene centers. Two such reactions are shown in Scheme 7.11. From an X-ray structural analysis, the phosphorus substituent was shown to act as a spectator, leaving its lone pair available to react in a Lewis basic manner. When carbene 31 was reacted with BF3, only the carbene adduct 32 was formed. By contrast, when 31 was reacted with the softer Lewis Acid BH3, it was the phosphorus that reacted to yield adduct 33. These types of carbenes exhibited C-NMR shifts in the range of 320-348 ppm, a P-C-N angle of 116.5° a short C-N distance of 1.296 A, and a long C-P distance of 1.856 A. The latter is very similar to that of a typical C-P single bond. 

Interest continues in the application of reagents derived from phosphines by lithiation at carbon of one of the phosphorus substituents. The propynylphosphine (34) is metallated on treatment with butyllithium to give (35), which gives rise to the... 

Although benzenes substituted by six carbon, nitrogen, oxygen, silicon, and sulfur are well known [23-29], such compounds are exceptionally limited in the field of phosphorus chemistry. Benzenes carrying six phosphorus substituents have not been synthesized and only limited compounds such as tetraphosphoryl- [30, 31] or tetraphosphinobenzenes [32], tetraphosphorylquinone [33, 34], tetraphosphoryl-cyclobutadiene complexes [35, 36], and pentaphosphinocyclopentadienyl complexes [37] have been reported (Scheme 20). 

Scheme 20 Cychc 7c-conjugated systems carrying phosphorus substituents [30, 32, 33, 35, 37]...

Thiophenes carrying heteroatoms on all carbons are also of interest. However, such studies on thiophenesare limited ascompared with benzenes. Tetrasilylthiophene 12 has been reported recently [38], We synthesized tetraphosphorylthiophene 13 and selenophenel4 recently [39] and would like to focus on studies on thiophenes carrying two to four phosphorus substituents in this section (Scheme 21). 

Very recent investigations of styrene hydroboration with C2-symmetrical 1,2-diphosphines give promising regio- and stereoselectivities. As with the other ligands described above, the electronic properties of the phosphorus substituents prove paramount in determining the reactivity (Scheme 12). Hence, the combination of (li ,2i )-l,2-bis(diphenylphosphino)cyclohexane (19) with [Rh(COD)2]+... 

The cycloaddition of allenes carrying an electron-withdrawing phosphorus substituent has also been studied [118]. Allenyl phosphine oxide 138 is prepared in a manner analogous to allenyl sulfoxide. The [4+ 2]-cycloaddition reaction of 138 with cyclopentadiene proceeded with a high endo selectivity. 

Aromatic substituent effects due to phosphorus groups have been studied for a number of reactions.47 Thus ester hydrolysis and fluoride-displacement rates, for (56) and (57) respectively, are enhanced by phosphorus substituents (X = O or ), while the rate of hydrolysis of the halide (58) is enhanced for X = , but slowed for X = O.47 A perturbation M.O. analysis of these observations has been presented.48... 

Some research groups have investigated effects of substituents less common in organic chemistry. Quin and co-workers (127,128) determined a-SCSs of tri-and tetravalent phosphorus substituents in n-alkyl and cyclohexyl derivatives (Tables 5 and 6). The difference between -PH2 and -P(CH3)2 effects is easily explained by 3-SCS(CH3) (about 10 ppm for each methyl) this approach, however, is not satisfactory for P(OCH3)2 and PC12, since it would suggest a-SCS values of ca. 16 (found 21.4) and ca. 22 (found 32.5), respectively, for those two substituents. A stereochemical dependence similar to that of X = CH3 and OH seems to exist only for the primary phosphine (X = PH2) and the compounds with tetravalent phosphorus substituents, whereas the a-SCS values of the tri-valent functions are quite similar in many cases, regardless of their stereochemical position. 

The conformational preferences of the P(V)—N bond in four co-ordinated phosphorus compounds, Scheme 3, (11) are less pronounced, and depend more on the type of the other phosphorus substituents. However, conformations close to (12) are foimd in a variety of crystal structures of acyclic phosph(V)azanes. 

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