P =N bond

STMP reacts with other nucleophiles such as aqueous ammonia to yield amidophosphates, which contains a P—N bond. STMP is used for the modification of the physical properties of starch and proteins by reaction with the amino and hydroxyl groups. 

One of the most widely used methods for the formation of phosphate esters involves the conversion of a P-N bond of a phosphorus(III) compound to a P-O bond by ROH, catalyzed by l//-tetrazole, followed by oxidation to the phosphorus(V) derivative ... 

The P-N bond is one of the most intriguing in chemistry and many of its more subtle aspects... 

Figure 12.22 Structures of (aj (Cl PNMe) , and (b) jCl(S)PNMc 2- Note the difference in length of the axial P-N and equatorial P-N bonds (and of the axial and equatorial P-Cl bonds) about the trigonal bipyramidal P atoms in (a).

The P-—N and P-N bonds arc equivalent in these compounds and they could perhaps better be written as [X3P" N= PX3], etc. Like the parent phosphorus pcntahalidcs (p. 498). these diphosphazenes can often exist in ionic and covalent forms and they are part of a more extended group of compt unds which can be classified into several general series Cl(Cl2PN) PCl4. [Cl(Cl2PN) PCl3] Cl, ... 

Cowley s group reported in 1987 the condensation of a dichlorophosphine with Collman s reagent to form stable complex 12, for which they obtained an X-ray crystal structure [57]. However, the two nearly equally long P-N bond lengths of 1.777(7) and 1.764(7) indicate that 12 is not an unencumbered species. 

As mentioned, stabilization of neutral hexacoordinated phosphorus via nitrogen donation is possible and this topic has been widely studied in the past few years. As P-N bonds are weaker and longer than those of P-C and P-0, chemists have essentially relied on chelation to enforce their formation. Most structures involve five- and six-membered chelating rings and the compounds that have been reported are described in Schemes 7,8, and 9 and Figs. 7 and 8. 

X-ray crystallographic analyses of the structures show that the P-S bond distance vary over one-half of an Angstrom (2.36-2.88 A). The derivatives were generated using procedures similar to those utilized to form pentaoxyphosphoranes with P-N bonds, that is (i) the oxidation of sulfur containing cyclic chlorophosphines with a quinone or (ii) treatment of phosphites with the sulfur-containing diol in presence of N-chlorodiisopropylamine. Two typical examples of these synthetic protocols are shown in Scheme 10. 

Tervalent organophosphorus compounds containing one single P-N bond with the valency of each atom saturated by protons or carbons (but no other heteroatoms) have been known since their discovery by MichaeUs more than one century ago [ 1 ] and named indistinctly as aminophosphanes, phosphanamines, phosphazanes, or phosphinous amides. This last chemical nomenclature is the one used by the Chemical Abstracts Service (CAS) for indexing these compounds and is also the one that best delimits the scope of this review those species derived from the parent H2P-NH2 (phosphinous amide in CAS nomenclature) by partial or total substitution of protons by hydrocarbon radicals (Table 1). 

The P-N bond in phosphinous amides is essentially a single bond, so the lone pairs on N and P are available for electrophiUc reagents and for donor bonding towards metal atoms. Proton addition to the N atom of HjPNHj has been calculated to loosen the P-N bond, whereas protonation at P renders this bond stronger than in the parent molecule [26]. NH-Phosphinous amides are practically not associated by intermolecular hydrogen bonds [27]. 

Hindered rotation around the P-N bond has been observed at low temperature in tetrasubstituted phosphorus amides [28]. For PhjPNJSiMCjJj, two different Me3Si groups are observed below -65 °C, the calculated activation energy for P-N rotation being 10.2 Real moT [29]. Chiral phosphinous amides with stereogenic phosphorus atoms have been prepared [30,31 ]. 

The reaction temperature varies between -40 and 110 °C, depending on the reactivity of both counterparts, amine and chlorophosphane. As usual, aliphatic amino groups react faster than aromatic and heteroaromatic ones due to their greater nucleophilic strength. These differences in reactivity allow chemose-lective phosphinous amide formation, as that represented in Scheme 2 where the P-N bond is formed exclusively at the aliphatic NH2 group of 2 but not at the heteroaromatic NH2, whose lone pair is extensively delocalized in the electron-withdrawing purine ring [35]. 

An alternative method for preparing phosphinous amides makes a profit on the high affinity between silicon and halogen atoms. This is the driving force of the reactions between halophosphanes and Ar-(trimethylsilyl)anilines, AT-(tri-methylsilyl)amides or AT-(trimethylsilyl)ureas and thioureas, as represented in the Scheme 6. In these processes the desired P-N bond and an halosilane are simultaneously formed [53,58-60]. 

The P-N bond also breaks when the phosphinous amides react with gaseous HCl, producing the expected amine hydrochloride and the corresponding chlorophosphane [14] (Scheme 22). 

Aqueous HCI solutions hydrolyze the P-N bond to give the amine hydrochloride and R2P-OH, which then disproportionates and is oxidized to diphenylphosphinic acid. A free phosphinous amide anion, with the countercation complexed by a crown ether, has been shown to be hydrolyzed and oxidized to the corresponding phosphinite with unusual ease [119]. Formic acid in toluene can be utilized for converting P,P-disubstituted phosphinous amides into their respective phosphane oxides [30]. 

In a similar way to the aminolysis of the P-N bond mentioned above (Scheme 9), alcoholysis of phosphinous amides leads to the alkyl esters of the respective phosphinous acids [30, 121]. This reaction occurs with inversion of the absolute configuration of the phosphorus atom, and has been used in a synthetic sequence leading to optically active tertiary phosphanes 22 [122] (Scheme 23). 

The phosphazene backbone has a particularly high resistance to thermal treatment and to homolytic scission of the -P=N- bonds, possibly due to the combination of the high strength of the phosphazene bond and its remarkable ionic character [456]. As a consequence, the onset of thermal decomposition phenomena (as detected, for instance, by TGA) are observed at considerably high temperatures for poly[bis(trifluoroethoxy)phosphazene], [NP(OCH2CF3)2]n [391, 399, 457], for phosphazene copolymers substituted with fluorinated alcohols of different length [391, 399, 457], for polyspirophosphazenes substituted with 2,2 -dihydroxybiphenyl groups [458], and for poly(alkyl/aryl)-phosphazenes [332]. 

The C1 n.q.r. spectra of ClS02 N=PCl3 have been followed over a range of temperatures, thus enabling barriers to rotation about S-N and P-N bonds to be calculated as 0.940 and 6.3 kcal mol respectively. 

Three independent P-N bond 137 lengths. OPO = 102.7°, exo-cyclic groups twisted at 48° to average plane of phosphazene ring... 

Ring has saddle conformation. 139, Variation in P-N bond lengths 141 consistent with 7r-bonding theory... 

The P n.m.r, parameters have been tabulated for a wide range of P" amino-compounds and P compounds. The value of Sp for compounds with four P—N bonds correlates with the hybridization of the nitrogen atom, moving to higher field in the order p < sp < sp < sp. 

Fig. 25. The effect of metal ion size on porphyrin ruffling. Very small metal ions [P(V) with an ideal P-N bond length of 1.84 A and low-spin Ni(II) with an ideal Ni-N length of 1.90 A in (a) and (b)) cause extensive S4 ruffling. Metal ions close to the right size (M-N = 2.035 A) give planar structures [Zn(II) in (c)]. Metal ions that are too large [Pb(II) at (d) with ideal Pb-N of 2.39 A] are extruded from the plane of the porphyrin and cause it to dome. For clarity, substituents on the porphyrins such as phenyl or ethyl groups have been omitted. Modified after Ref. (77).

The lack of reactivity of (45) and CEP-pyrrole towards alcohols is attributed, at least partly, to the double bond character of the phosphorus-nitrogen bond as evidenced by the crystallographically determined abnormally short P-N bond lengths. 

Restricted rotation in a series of ditert-buty1 phosphines (30 R = H, Aik, Ph etc) has been studied using molecular mechanics calculations.10 Rotation about the P-N bond in phosphonyl acetamides has a barrier of dG 16.3 heal mol 1. 66... 

There has been a HO study of the stereoelectronic effects in methy1phosphines (90). Steric effects were found to concentrate in the HOHO and accounted for half the substituent effects on the pK values, whilst electronic effects on the HOHO was minimal.242 The relative basicities of polymethoxytriarylphosphines have been measured. Tris(2,4,6-trimethoxyphenyl)phosphine was considerably more basic than piperidine.243 The basicities of P-N compounds have been reviewed and their correlation with P-N bond lengths invest igated.2 4 4... 

One of the chief defects of the phosphorus based pyrazolyl (PZ) ligands appears to be the hydrolytic sensitivity of the P—N bond particularly after interaction of the ligand with transition metal ions. The interactions of PhP(0)(3,5-Me2pz)2 and Ph2P(0)(3,5-Me2pz)2 with Pd11 salts accelerates the P—N bond hydrolysis.177... 

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