Coupling to Phosphorus

In trialkyl phosphanes, the magnitude, whereas the Hp coupling constants are significantly larger. Coupling constants in aryl phosphanes are even more difficult to assign correctly. Table 2.6 shows a few representative examples. 

The coupling constant to the tp o-carbon of the phenyl ring (Tp, ) is frequently smaller than the coupling constant to the ort/to-carbon CJpc)- However, this is by no means a general trend, as there are numerous examples where Hp is smaller than Jp, . 

Note The magnitude of increases dramatically upon loss of the lone pair on phosphorus. 

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Table II lists all pertinent chemical shifts and coupling constants for the known phosphinocarbenes and their respective diazo precursors. The (phosphino)(silyl)carbenes are all characterized by high field chemical shifts for phosphorus ( 24 to 50 ppm) and silicon (-3 to -21 ppm), and low field chemical shifts for carbon (120 to 143 ppm) with large couplings to phosphorus (147 to 203 Hz).

Related to these electronic factors, some examples of stereospecific coupling are known. In 7 the geminal protons on phosphorus are non-equivalent and exhibit slightly different couplings to phosphorus, both of which are temperature and solvent dependent.(1969,51) A clearer example is given by the recent work of Mikolajczyk(1969,52) on the two isomers 8 and 9. 

One example is the set of dynamic processes going on in Ru2(CO)6(fi2-PPh2)(/X2- -C=C-i-Pr). The molecule (fig. 6) is asymmetric in the absence of any dynamics, but near 300 K the alkyne flips back and forth ( windshield wiper motion ) quickly, so that the carbonyls on either side are equivalent. Each carbonyl is coupled to phosphorus, so if the carbonyls do not scramble amongst themselves, there are six lines in the carbonyl... 

Next, consider the phosphonium salt you met at the end of Chapter 14 for use in the Wittig reaction, turning aldehydes and ketones to alkenes. It has a 2/ph of 18 Hz. There is no doubt about this structure—it is just an illustration of coupling to phosphorus. There is coupling to phosphorus in the carbon spectrum too the methyl group appears at 5c 10.6 p.p.m. with a l/pc of 57 Hz, somewhat smaller than typical Vph- We haven t yet talked about couplings to 13C we shall now do so. 

The possibility of using I3C-NMR spectroscopy to assist in assigning the geometry of bis-phosphine transition metal complexes has attracted the attention of several groups (80, 86-90). The examination of the PMR spectra of such complexes and analysis of the resulting AA XnXJ, spectra has proved valuable (91) but has the disadvantage that it is limited to systems in which the phosphorus atom is attached to methyl, methoxy, or methylene or to related groups in which the protons couple to phosphorus but not to other protons. The condition for the observation of a 1 2 1 triplet (usually associated with trans phosphine molecules) in the proton resonance spectrum is... 

MeC CMe, PMe2Ph, CCH2CH2CH26) are tabulated in ref. 153. The CF3 groups absorb in the range -46-9 to —51-3, with no observable coupling to phosphorus. The Pt-CF3 couplings range from 78 Hz for the carbene complex to 143 Hz for the acetone complex, and correlate... 

When chiral amino aldehydes were coupled to phosphorus ylides on a support, the salt-free Wittig procedure caused minimal epimerization [500]. 

The resulting spectrum displays crosspeaks correlating carbon chemical shifts in f2 and proton shifts in f] which are further spread by homonuclear proton couplings in fi. Fig. 6.32 displays a part of the carbon-proton shift correlation spectrum of the palladium complex 6.11. Despite the extensive crowding in the aromatic region, the carbon shifts are sufficiently dispersed to resolve all correlations (note some resonances are broadened by restricted dynamic processes within the molecule and some are split by coupling to phosphorus). 

Note Coupling to phosphorus - and other NMR active heteronuclei - is observed in C-NMR spectroscopy. 

The CAAC framework has been used to stabilize group 15 radicals as well as phosphorus mononitride, PN. A first report (2010) [305] describes the preparation of a phosphinyl radical cation by oxidation of a tetramethylpiperidine capped, monoamino phosphaalkene formed from CAAC A (Scheme 15.32). The radical cation was characterized in the solid state using X-ray crystallography and in solution by EPR spectroscopy. The room temperature EPR spectrum consists of a doublet of multiplets with a large hyperfine coupling to phosphorus and a small coupling to one or two nitrogen atoms (g= 2.007 ( P)=99 G ( N) 4 G), typical of phosphinyl radicals. 

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