Methylene shielding

Table 13 1 collects chemical shift information for protons of various types The beginning and major portion of the table concerns protons bonded to carbon Within each type methyl (CH3) protons are more shielded than methylene (CH2) protons and meth ylene protons are more shielded than methme (CH) protons These differences are small as the following two examples illustrate... 

Finally, it should be noted that y-gauche effects are not confined to carbon atoms. Analogous shifts have been reported for l9F, 31P (191), and 15N nuclei (206). Distinct shieldings of I70 nuclei caused by y-gauchc-oriented methyl or methylene groups were found, too (207). 

The unusually pronounced shielding effect on the C(7) methylene group of an annelated three-membered ring, as in 85 (202,203), 86 (371), and 250 (372), has already been mentioned (cf. Section III-C) and exists also, although to a somewhat lesser extent, in nortricyclene (251) (373). 

The 1H—NMR spectrum of [2.2]paracyclophane 13-18> consists of two singlets of equal intensity for the methylene (t=6.96) and aromatic protons because of the shielding effect of the two benzene rings, the latter absorb at comparatively high field strength (r =3.70). 

C. Crotyl diazoacetate. A solution of 10.0 g. (0.038 mole) of the />-toluenesulfonylhydrazone of glyoxylic acid chloride in 100 ml. of methylene chloride is cooled in an ice bath. Crotyl alcohol (2.80 g. or 0.038 mole) (Note 7) is added to this cold solution, and then a solution of 7.80 g. (0.077 mole) of redistilled triethyl-amine (b.p. 88.5-90.5°) in 25 ml. of methylene chloride is added to the cold reaction mixture dropwise and with stirring over a 20-minute period. During the addition a yellow color develops in the reaction mixture and some solid separates near the end of the addition period. The resulting mixture is stirred at 0° for 1 hour and then the solvent is removed at 25° under reduced pressure with a rotary evaporator. A solution of the residual dark orange liquid in approximately 200 ml. of benzene is thoroughly mixed with 100 g. of Florisil (Note 8) and then filtered. The residual Florisil, which has adsorbed the bulk of the dark colored by-products, is washed with two or three additional portions of benzene of such size that the total volume of the combined benzene filtrates is 400-500 ml. This yellow benzene solution of the diazoester is concentrated under reduced pressure at 25° with a rotary evaporator, and the residual yellow liquid is distilled under reduced pressure. (Caution This distillation should be conducted in a hood behind a safety shield) (Note 9). The diazo ester is collected as 2.20-2.94 g. (42-55%) of yellow liquid, b.p. 30-33° (0.15 mm.), n T) 1.4853 - 1.4856 (Note 10). 

Not shown in Table I are several adducts obtained by cycloaddition of the acridizinium ion with 5,6-endo-substituted norbomene derivatives. These adducts each have two (imequally shielded) methylene hydrogen atoms which make simple the NMR analysis of mixtures of syn (8) and anti (9). When the 5,6-endo chain (R) was of the type... 

The answer to the cis-trans question is to be found in the methylene carbon spectrum of Fig. 7. If we look at the (61 ppm) and C4 (53 ppm) peaiks for the -78° polymers, —which we recall has an almost exclusively alternating structure—, we see that they are clearly split, but by less than 1 ppm. We might at first think this represents cis and trans structures. However, ejqierience with diene polymer spectra shows that when methylene carbons are involved in a cis structure they shield each other by 8 to 10 ppm. This is due to the operation of the Y steric effect, particularly strong when the carbon bonds actually eclipse each other rather than being merely gauche. In chloroprene units one ejqiects the C4 carbon to shift little between a cis and trans structure because it always sees a bulky substituent across the... 

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