Phosphorus multiple bond

Note Added in Proof This is nicely illustrated by the landmark publication of Protasiewicz et al. [94] who describe derivative (122), the first conjugated polymers featuring phosphorus-phosphorus multiple bonds (Scheme 33). The di-phosphene-PPV (122) exhibits an extended 71-conjugated system as shown by the optical HOMO-LUMO gap that is close to that of related PPV [94]. 

The silyl migration method involves intramolecular 1,3-silyl migration which is useful for introducing phosphorus multiple bonds. Variations of this 1,3-silyl migration have been classified as the condensation-silatropy method and the addition-silatropy method (Scheme 3)8. Becker reported a reaction involving rearrangement of silyl-substituted acylphosphine for the first time in 1976 (equation 3)9. 

Going one step further, we might ask about the influence that a phosphorus multiple bond may have on the chemical shift. From C-NMR, we know that an olefin resonates downfield from an alkane, and an acetylene is found in between, but closer to the alkane. This is explained by diamagnetic anisotropy and the behaviour of P—C, P=C, and I C bonds should be analogous. 

Turning our attention to molecules with carbon phosphorus multiple bonds, we acknowledge the existence of r-bonding between carbon and phosphorus in these molecules. Since carbon is more electronegative than phosphorus, we would suspect that a carbon phosphorus multiple bond would result in a downfield shift of the phosphorus resonance. Indeed, the P-NMR spectra of simple phosphaalkenes usually show aresonance of <5 p=200-300ppm. 

Going from a phosphaalkene to a phosphaalkyne, we increase the. r-contribution in the carbon phosphorus multiple bond, and would therefore expect a further down-field shift of the phosphorus resonance. However, a glance at the situation in carbon carbon multiple bond systems in particular, alkenes and alkynes, tells us that C-NMR spectra of these molecules show the carbon resonance of alkynes upheld from that of alkenes. This is usually explained by anisotropic effects associated with the linear rod-shaped structure of alkynes versus the bend structure of alkenes. As the geometries of phosphaalkenes andphosphaalkynes are analogous to alkenes and alkynes, respectively, we can assume that the explanation given for the appearance of the carbon resonance in alkynes upheld from that for alkenes in C-NMR spectra is also applicable for the respective unsaturated phosphoms compounds. 

The substituent R determines the reactivity of the isocyanate. Aromatic isocyanates react faster than aliphatic isocyanates, and carbonyl and sulfonyl isocyanates are considerably more reactive than the former. Isocyanate groups attached to oxygen or nitrogen are not stable in their monomeric forms. In cycloaddition reactions, isocyanates react preferentially across their C=N bonds, but additions across the C=0 bonds are also encountered. In this respect, isocyanates resemble ketenes (see Chapter 4, Section 4.1.). Suitable substrates for cycloaddition reactions are carbon multiple bonds (acetylenes, olefins, ketenes, etc.), C=N bond-containing compounds (imines, amidines, ketenimines, azines, carbodiimldes, etc.), C=0 bonds and C=S bond-containing substrates and phosphorus multiple-bond-containing substrates. Cycloaddition reactions of isocyanates across multiple metal bonds are also known. 

Although molecules containing silicon-silicon and phosphorus-phosphorus multiple bonds are known, they are thermodynamically unstable, and exist only when bulky groups attached to the silicon or phosphorus atoms provide a kinetic barrier to conversion into single-bonded polymers. 

Regitz, Chem. Rev. 90, 191-213 (1990). See also M. Regitz and O. J. Scherer, Multiple Bonds and Low Coordination in Phosphorus Chemistry, Georg Thieme Verlag, Stuttgart, (1990). 

E. Mathey, in Multiple Bonding and Low Coordination in Phosphorus Chemistry (M. Regitzand O. J. Seherer, eds.),p. 3.Thieme, Stuttgart, 1990. 

MI1 O. J. Scherer, in Multiple Bonds and Low Coordination in Phosphorus... 

The radius of an atom helps to determine how many other atoms can bond to it. The small radii of Period 2 atoms, for instance, are largely responsible for the differences between their properties and those of their congeners. As described in Section 2.10, one reason that small atoms typically have low valences is that so few other atoms can pack around them. Nitrogen, for instance, never forms penta-halides, but phosphorus does. With few exceptions, only Period 2 elements form multiple bonds with themselves or other elements in the same period, because only they are small enough for their p-orbitals to have substantial tt overlap (Fig. 14.6). 

Driess, Matthias, Silicon-Phosphorus and Silicon-Arsenic Multiple Bonds. 39 193... 

Schoeller WW (1990) In Regitz M, Scherer OJ (eds) Multiple bonds and low coordination in phosphorus chemistry. Georg Thieme, Stuttgart... 

Shah S, Protasiewicz JD (1998) J Chem Soc Chem Commim 1585 Yoshifuji M, Shima 1, Inamoto N, Hirotsu K, Higuchi T (1981) J Am Chem Soc 103 4587 Smith RC, Shah S, Protasiewicz JD (2002) J Organomet Chem 646 255 Smith RC, Ren T, Protasiewicz JD (2002) Eur J Inorg Chem 2779 Shah S, Protasiewicz JD (2000) Coord Chem Rev210 181 Shah S, Yap GPA, Protasiewicz JD (2000) J Organomet Chem 608 12 Schmidpeter A (1990) In Regitz M, Scherer OJ (eds) Multiple bonds and low coordination in phosphorus chemistry. Thieme, Stuttgart... 

A brief history of (3p-2p)7i bonds between phosphorus and carbon followed by an introduction to the methods of phosphaalkene synthesis that are pertinent to this review will be provided. The earliest stable compound exhibiting (3p-2p)7x bonding between phosphorus and carbon was the phosphamethine cyanine cation (1) [33]. An isolable substituted phosphabenzene (2) appeared just two years later [34]. The parent phosphabenzene (3) was later reported in 1971 [35]. These were remarkable achievements and, collectively, they played an important role in the downfall of the long held double bond rule . The electronic delocalization of the phosphorus-carbon multiple bond in 1-3, which gives rise to their stability, unfortunately prevented a thorough study of the chemistry and reactivity of the P=C bond. 

A common theme in the speculated polymerization reactions discussed in this section is that the polymers were generally uncharacterized or were composed of small cyclic oligomers. No evidence for high molecular weight polymers from low-coordinate phosphorus compounds was obtained. Of course, multiple bond formation, not polymerization, was the focus of these studies. 

This new area of chemistry is still at a very early stage of development with most of the breakthroughs occurring in the last couple of years. The future holds promise for more exciting developments in the use of P=C bonds in polymer science and it is very possible that apphcations may be found for these new types of materials. In addition, an exciting prospect for the future is the further expansion of these methodologies, which are so common for C=C bonds, to other phosphorus-containing multiple bonds and other p-block elements. 

Heterophospholes A. Schmidpeter, K. Karaghiosoee on Multiple Bonds and Low Go-ordination in Phosphorus Ghemistr/. in M. Regitz and O.J. Scherer (Ed.) Houben-Weyl, Methoden der Organischen Ghemie, G. Thieme, Stuttgart, New York, 1990, p. 258... 

Silicon-Phosphorus and Silicon-Arsenic Multiple Bonds... 

---