Phosphorus multiple resonance

All compounds with more than one chiral centre yield diastereomeric mixtures. All heteroalkylated species have a chiral centre at the sulphur. In the compounds 6 and 2 the carbon 5 is chiral, too. After phosphorylation there is one more centre at the phosphorus. So the appearance of two diastereomeres b 1 and 6 and four b 2 respectively is expected. This expectation is verified by NMR-results. In l3C-NMR as well as 31P-NMR spectra one can see multiple resonances corresponding to almost all nuclei of the compounds. The 31P-spectrum of 1 and the l3C-spectrum of 6 are the most significant for tbs phenomenon. 

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. 

Alternatively, the rhodium complexes of 4 exhibit enhanced performance relative to the ferrocene diphosphine analog 3 which cannot be explained simply as a switch from one binding mode to another. P NMR experiments on in situ formed rhodium complexes are useful for correlating rhodium binding of different ligands in solution when a single species dominates, but are inconclusive when multiple phosphorus resonances are observed. Preparation and isolation of preformed complexes may provide better systems for study by NMR spectroscopy. 

Complexation of phosphorus-containing multiple-bond systems to transition metals has been investigated extensively in recent years. In this regard, bonding between the phosphaalkene and transition metal carbonyls can be achieved via the free electron pair of the phosphorus (tjl coordination, type A) (149-151) as well as via the n system (r 2 coordination, type B) (152-154). The latter reaction type can be explained by two different electronic resonance structures. Type C is a combination of the two possibilities and can be occasionally observed (155, 156) (Fig. 23). 

Resonance, to some degree, is believed to take place between the various phosphorus-oxygen linkages in nearly all phosphates. The double bond is seldom located wholly in one place as indicated in (5.6a,b), but this formal designation will be used in many places in this book. Even in phosphoryl compounds such as (5.4c) the multiple characters may be shared to some extent with the remaining bonds and be reflected in the bond lengths. 

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