Phosphorus Carbon Multiple Bonds

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 resonance of the phosphorus signal in the P-NMR spectrum can be shifted downfield or upheld by changing the substituent on the carbon atom of the carbon phosphorus triple bond. Table 5.5 shows the influence of the para-substituent on the aromatic ring of a series of Ar—C=P compounds. As the substituent operates 

Note The coupling constants decrease in line with increasing upfield shift of the phosphorus resonance, indicating decreasing s-character of the phosphorus lone pair. 

Note The decrease in the coupling constants has likewise increased in magnitude. 

---

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. 

The coordination chemistry of compounds containing phosphorus-carbon multiple bonds, such as phosphaalkynes, phosphaalkenes, phos-phaallenes, phosphaalkenyls, and phosphaallyls, has been studied extensively (349,350). The chemistry is dominated by the donor properties of the phosphorus atom, and, as far as we are aware, no examples of bimetallic or trimetallic compounds with bridging ligands which link metal centers by a carbon cr bond and a C=P or C P n bond have been recorded. Once the lone pair on the phosphorus atom is involved in bonding, the unsaturated bond becomes a possible site for further coordination. 

The availability of phosphaalkenes R1P=CR22 and phosphaalkynes RC=P has led to the development of another route to phosphiranes and phosphirenes involving the formal addition of carbenes to phosphorus-carbon multiple bonds. When using diazoalkanes as the carbene source, the reaction mechanism involves the series of steps demonstrated by Niecke and co-workers in Scheme 23 <8iAG(E)i3i, 83CC1171) see also <75AG(E)363>. 

Fig. 2.8 P-NMR chemical shift values for phosphorus carbon multiple bonds...

The investigated transients with double bonds to silicon exhibit short effective 1/e lifetimes ranging from a few ms (for H2Si=0) up to ca. 30 ms for 3a. In contrast to these silanes, the kinetic instability of multiply-bonded transient phosphines depends more strongly on the electronegativity of the element involved in the multiple bond to phosphorus. The observed 1/e lifetimes span a substantial range from 8 ms for highly reactive 14a up to stable derivatives with unprotected phosphorus-carbon multiple bonds. FVT coupled with FTIR spectroscopy has proved to be a competitive technique to study robustly bound transients with 1/e lifetimes as short as 8 ms. 

The behavior of phosphorus pentachloride toward carbon-carbon multiple bonds has received considerable attention, and the procedure described represents but one example of a wide variety of derivatives of unsaturated phosphonic acids which are accessible. Indene was the first olefinic compound to be reacted... 

Further changes in hybridization are observed in additions to carbon-carbon multiple bonds adjacent to phosphorus for convenience, these reactions are collected together in a separate section. 

The particular interest here is the reactivity of carbon-carbon multiple bonds when attached to the phosphorus atom. However, there are also reactions of considerable interest in which the phosphorus ultimately participates in reactions which initially occur at carbon-carbon double bonds distant therefrom. 

It is well-known that multiple bonds involving heavier main group elements are unstable, and thus, some stabilizing techniques are needed to prepare compounds with heavy unsaturated skeletons. Kinetic stabilization utilizing steri-cally crowded substituents to stabilize a reactive species is a method to obtain such multiple bonding, and many kinds of kinetically stabilized heavy multiple bonds have been derived so far [Ij. As for phosphorus compounds, in 1978 Bickelhaupt and coworkers reported the first kinetically stabilized phosphorus-carbon double bond I (phosphaethene) [2], and in 1981 Yoshifuji and coworkers reported the first stable phosphorus-phosphorus double bond II (diphosphene)... 

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. 

Figure 4.19 Molecules of phosphorus and sulfur with multiple bonds to carbon, nitrogen, and oxygen.

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

Elimination of trimethylchlorosilane and nitrogen occurs when the (phos-phino)(silyl)diazomethane la is reacted with para-toluenesulfinyl chloride at low temperature. The formation of the four-membered heterocycle 92, obtained in 87% yield, can be rationalized by a multiple-step mechanism involving the formation of the (phosphino)(sulfinyl)carbene 2v. The insertion of the (phosphoryl)(sulfenyl)carbene 91, resulting from a 1,3-oxygen shift from sulfur to phosphorus in 2v, into a carbon-hydrogen bond of a diisopropylamino group readily accounts for the formation of 92.84... 

---