Steric compression shift

Up to this point our discussion of y-gauche effects has demonstrated that the transmission mechanisms are not yet well understood and still open to speculation. The original concept of a steric interaction is highly controversial, and as long as there is no convincing explanation, the use of the deep-rooted term steric compression shift must be discouraged. 

The carbon-13 NMR spectrum of acetone oxime has three resonances, the deriva-tized carbonyl carbon at 154.5 ppm and the two nonequivalent CH3 groups at 21.5 and 14.7 ppm. The difference between the chemical shifts of the two methyl groups, 6.8 ppm, is primarily a steric compression shift. This is clearly indicated in the carbon NMR spectrum of methyl ethyl ketoximes (Scheme 1), where the two oxime substituents are not sterically identical and thus the two isomers are not present in equal amounts. In methyl... 

Conformations A and B are also supported by, 3C-NMR data. The strained conformation A of the more crowded cis-annulated structure is indicated by the increased shielding of the C-5a (3.4-5.1 ppm), C-6 (1.3— 2.5 ppm), C-8 (0.6-2.5 ppm), and C-9 (0.6-2.8 ppm) carbon atoms in the 3-ethoxycarbonyl-10-methyl compounds 92 compared to that in the 3-aryl- 10-methyl analogues 92 and compounds unsubstituted on C-10 (91) this is also true of the field effect or steric compression shift in the shielding of carbon atoms bearing sterically hindered substituents. On the other hand, the signal for C-10 bonded to the unhindered methyl group in A shows a significant downfield shift of 3.6 ppm, unlike that in B. 

A combination of Fourier transform and partially relaxed Fourier transform spectroscopy has been used to determine steric compression shifts in aliphatic amides and oximes, allowing, inter alia, a sensitive study of internal rotation. 

Shifts are also affected by steric compression of any kind on the atom nnder study. The effect on a C atom can... 

Carbon-13. Steric compression by axial P substituents produces a so-called y-effect by their interaction with the two axial protons on the same side of the ring, and there is a resultant upfield shift of the carbon atoms bearing the axial protons. This effect has been noted for a number of dioxaphosphorinans (22)42>43 as well as for phos-phorinans.22 A small but significant shift of the C-4 resonance occurs when the... 

TABLE 2. 31P NMR chemical shifts (ppm) of phosphonium and aminophosphonium halides35,37 (also steric compression was shown to give upfield 31P chemical shifts in cycloalkyltriphenylphosphonium salts)... 

A notable deshielding jS-effect (10 ppm) has been observed in an aminophosphonium salt on the P+NMe 13C chemical shift when the NHMe group is further alkylated to NMe237. Steric compression has been used, with some caution, to explain the relatively shielded / -carbon atoms in triphenylphosphonio cycloalkyl salts when they are compared with their methylcycloalkyl analogues36. 

Table 15 Steric Compression and Group Interaction Effects on 13C Chemical Shifts of Pyrrole Derivatives"...

A dissociation constant of about 1.5 X 10-3 (25°C) was assessed from the intensity ratio. The complex does not show intramolecular fluxional behavior up to +50°C. Of the three methyl resonances the chemical shift of that at position b occurs at highest field owing to the influence of the phenyl ring current. Consistent with a corollary of the X-ray study (132) on Pt(PPh3)2(CH2=C=CMe2), we may infer the existence of a steric compression between the methyl and the phenyl groups which may be primarily responsible for the ready dissociation of TMA. 

Table 4.6 shows the effects of a substituent on linear and branched alkanes. The effect on the a-carbon parallels the electronegativity of the substituent except for bromine and iodine.t The effect at the /3-carbon seems fairly constant for all the substituents except for the carbonyl, cyano, and nitro groups. The shift to the right at the y carbon results (as above) from steric compression of a gauche interaction. For Y = N, O, and F, there is also a shift to the right with Y in the anti conformation, attributed to hyperconjugation. 

Quaternization of imidazoles (55, Figure 14) to imidazolium salts 56 leads to low-field shifts of all the imidazole carbon signals except that of C-4, which is shielded probably by steric compression resulting from N-3 substitution. In contrast, the corresponding imidazole 1-oxides 57 show a high-field shift for all the imidazole carbons. This high-field... 

In 28 the cyclopropyl methylene protons absorb at H3a = 1.13, H3S = 0.88 ppm respectively in the stereoisomer 29, the corresponding chemical shifts are for H3g, 0.28 and for H3s, 1.08 ppm. The unexpected deshielding of H3S in 29 may have its origins in steric compression that overcompensates the shielding by the double bond. It is noteworthy in passing that C(3) of 29 is located approximately over the double bond and accordingly experiences a pronounced shielding, 54.6 compared with 520.0 for C(3) of 28 a broadly similar shift is found for C(2), C(4). ... 

This problem has been addressed using isotope effects. Two different cases are found due to the interaction present. Two typical examples are seen in 42 and 43. In 43 steric twist is observed and in 42 steric compression is found. Spectroscopically these effects are characterized by the pairs of H chemical shifts and deuterium isotope effects on chemical shifts 50H, AX(OD), AX=0(0D), ACH3(OD) and AOH(CD3). The latter is especially useful, but requires that deuterium is incorporated into methyl groups of, e.g., acetyl groups . In 43 the number of intervening bonds are six (H—C8—C8a— Cl—C(0)—C—H) whereas for the steric compression cases (42) the number of bonds is five (0-C2-C3-C(0)-C-H). 

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