P Couplings

Benzyl radicals and a- and 3- substituted derivatives also undergo unsymmetrical coupling through the aromatic ring (Section 2.5). The formation of the (i-o and a—p coupling products is reversible. Consequently, these materials are often only observed as transient intermediates. 

It was subsequently shown that the polymers contain semi-quinonoid structures 47 proposed to arise from a-p coupling of radicals 46 as shown in Scheme 9.15., (M It was also suggested that 47 could be subject to radical-... 

P-coupling occurs in the formation of azophosphonic esters [ArN2PO(OCH3)2] from diazonium salts and dimethyl phosphite [HPO(OCH3)2] (Suckfull and Hau-brich, 1958). P-coupled intermediates are formed in the reaction between diazonium salts and tertiary phosphines, studied by Horner and Stohr (1953), and by Horner and Hoffmann (1956). The P-azo compound is hydrolyzed to triphenylphosphine oxide, but if a second equivalent of the tertiary phosphine is available, phenyl-hydrazine is finally obtained along with the phosphine oxide (Scheme 6-26 Horner and Hoffmann, 1958). It is likely that an aryldiazene (ArN = NH) is an intermediate in the hydrolysis step of the P-azo compounds. 

The P-coupling of 1,2- and 1,4-quinonediazides with phosphines (mainly tri-phenylphosphine) was studied intensively in the 1960s by the groups of Ried and Horner (see summary by Ershov et al., 1981, p. 147). The azo derivatives formed in these reactions are more stable than those of arenediazonium salts because of the stabilization by mesomeric delocalization (6.35 a ++ 6,35 b). 

Anodic oxidation of the diphenylmethylenephosphorane (18) has been shown to give the radical (19). The P coupling constant for the radical shows that little delocalization of the unpaired electron onto the phos-... 

The derivatives 15a-i exhibit characteristic 31P NMR spectroscopic data (see Table I), which are distinctly different from those observed for the P-organo-substituted derivatives 12 and 13 (8(3IP) = 65.8-136.0).27 However, the similar Si chemical shifts and. /(Si, P) coupling constants observed for 12,13, and 15 clearly show the identical electronic nature of the low-coordinate silicon centers in these derivatives. 

The rule in carbon-13 NMR is that sp2-hybridized carbons (carbonyl, aromatic, olefinic) absorb at lowest field, followed by sp-hybridized (acetylenic, nitrile) and sp3 (aliphatic). A first glance leads us to believe we have seven signals, but we must remember that the methine carbon is directly bonded to phosphorus, so that we shall expect a relatively large C-P coupling. The other C-P couplings will probably be very much smaller. 

So the seven signals reduce to six, one obviously being a doublet. If we expand the spectrum we see that another three signals are doublets with a small C-P coupling. 

Figure 15 shows the normal broad-band decoupled and gated decoupled spectra of compound 1 in the latter we can see the multiplets arising from C-H coupling (across one or more bonds) and C-P coupling. The rules for the number of lines in a multiplet and their intensities are the same as for protons, since 13C and 31P are both spin-Vi nuclei. 

In order to correctly predict which ligands occupy which sites in such compounds, one must recognize that, as a general rule, fluorines will always prefer the axial site in a trigonal bipyramidal system, perhaps because of fluorine s small size, but probably also because of its preference to bind to orbitals with as little s-character as possible. The orbitals used by P to make its axial bonds have less s-character than those used to make its equatorial bonds. This is reflected by the larger F—P coupling constants to the equatorial fluorine substituents. 

For simplicity s sake only the products of p-interaction in (106) have been shown above o-interaction can also lead to an o-dihydro isomer of (109), and to both o-/o- and o-/p-coupled isomers of (108). Product mixtures from arylation of aromatic species can thus be quite complex. 

The system (23)/SnCl2, an active intermediate in the catalytic hydroformylation of 1-hexene, has been investigated by 31P NMR spectroscopy and two species are observed at low temperature, in equilibrium with the starting Pt complex (23). One is complex (27), and the other is a species which does not show Sn-P coupling and which has been tentatively attributed to a complex having chloride ions bridging the Pt and Sn metal centers. Formation of the complex (27) does not occur when EtOH is added to the CD2C12 or acetone solutions.91... 

Also, 13C NMR assignments have been published for a recently synthesized phosphorus-containing ring system imidazo[l,2-r-][l,3,2]oxazaphosphinine 49 <1996TL977>. The chemical shifts as well as the /C P coupling constants are shown in Table 2. 

Table 2 13C chemical shifts (with respect to the residual signal of CDCI3 at 77.0 ppm) as well as the Jc-p coupling constants of the imidazo[1,2-c][1,3,2]oxazaphosphinine derivative 49... 

Table 11.1 Selectivity (%) among the individual isomers in the diglycerol class (s,s coupling of two secondary alkanols p,p coupling of two primary alkanols in glycerol) (after [29]). For the synthesis of authentic standards of pure diglycerol isomers see [30],...

The HMQC approach has also been employed in the measurements of 3/(H3 P) and 3/(C4 P) coupling constants using a 2D 31P spin-echo difference constant-time experiment [44] to increase sensitivity by a factor of 1.5-2.4 in a 17 kDa DNA-protein complex. 

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

A sequence suitable for measurement of J(H, P) and J(C, P) couplings is shown in Fig. 7.9a. The pulse sequence is a constant-time [13C, H]-HSQC (heteronuclear single-quantum correlation), in which 31P decoupling is applied in ot, in the first experiment and in co2 in the second. 

Fig. 7.9 Pulse sequence and schematic multiplet to measure both the /(H,P) and J(C,P) coupling...

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