Deshielding. NMR

It was also of interest to generate the related symmetric primary benzylic trication. However, the ionization of the 2,4,6-tris(chloromethyl)mesitylene (112) in excess SbF5/S02ClF at -78°C gave apparently only the dienylic allylic dication Lewis acid-base complex (113). The unionized chloromethyl carbon displayed a relatively deshielded NMR absorption, 35.3, indicative of a weak Lewis acid-base interaction. The terminal methylene carbons of the dienylic system showed a 8 C of 197.7 and the terminal carbons of the allylic system displayed a 8 C of 194.8, quite similar to that of the previously described dienylic allylic dications, 107 and 108. 

NMR Like other carbonyl groups the carbon of the —CO2H group of a car boxylic acid is strongly deshielded (8 160-185) but not as much as that of an aldehyde or ketone (8 190-215)... 

NMR Compared with C—H the carbon of C—O m a phenol is deshielded by about... 

Figure 1.2. H NMR spectrum of ethyl dichloroacetate (CDCI3, 25 °C, 80 MHz). The proton of the CHCh group is less shielded (more strongly deshielded) in comparison with the protons of the CH2 and CHj residues...

The 7/NMR spectrum displays signals of shielded protons = - 2.35, integral level 1) and of deshielded ones = 10.45 and 9.39, integral levels 1 1). This reflects a ring current due to aromaticity as described for annulenes and porphyrins in section 2.5.2. To conclude, the reaction involves an oxidative cyclisatlon of 2,5-bis(2-pyrrolylmethyl)-17/-pyrrole 2 with 47/-trlazole-3,5-dlaldehyde 3 to the corresponding 2,3-diazaporphyrin 4, following the 3-t-l pathway of porphyrin synthesis. Two non-equivalent tautomers may exist these are the diaza[ 18]annulene 4a and the tetraaza[18]annulene 4b. 

The NMR spectra of the product do not show these features. The highest C shift value is Sc = 160.9 and indicates a conjugated carboxy-C atom instead of the keto carbonyl function of an isoflavone (5c =175). On the other hand, a deshielded CH fragment at 5c/<5 = 138.7/7.i52 appears in the C NMR spectrum, which belongs to a CC double bond polarised by a -A/effect. The two together point to a coumarin 4 with the substitution pattern defined by the reagents. 

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded. Normally, the carbon of C=0 is the least shielded one in a NMR spectrum. 

Figure 13.5 The NMR chart. The downfield, deshielded side is on the left, and the upfield,...

Carbon atoms bonded to electron-withdrawing -OH groups are deshielded and absorb at a lower field in the 1,it NMR spectrum than do typical alkane carbons. Most alcohol carbon absorptions tall in the range 50 to 80 S, as the following data illustrate for cyclohexanol ... 

---