Cyclohexanes substituent increments

C Shifts of 4-t-butyl-, 4-t-butyl-2,2,6-trimethyl-, and 4-t-butyl-2,2,6,6-tetramethyl-cyclohexanones all show similar (ot, 21.1 + 0.5 p, 1.0 + 0.5 y, —0.5 + 0.2 p.p.m.) t-butyl induced shifts, and since these values are similar to those found in t-butyl-cyclohexane and trans-1,4-di-t-butylcyclohexane it was considered that each of the 4-t-butylcyclohexanones is mainly in a chair conformation, as different substituent increments are anticipated for twist-conformations. This finding is of relevance in the case of 4-t-butyl-2,2,6,6-tetramethylcyclohexanone on grounds both of an increase in energy of the chair conformer and the finding that related heterocyclic systems show a propensity for twisted conformations. 

Following the trend reported for other substituents (Section 4.5.2), carbon shift increments for equatorial amino groups at noninverting cyclohexane are more positive than those of axial ones, particularly in the y position [340],... 

These parameters show that the rate of the hydrolysis reaction is faster with hydrogen than methyl, and slows as the R group incrementally increases in size to f-butyl (that is, Es becomes more negative). Phenyl, however, comes out surprisingly large in this analysis (Es = -2.55). In Chapter 2 we noted that efforts to characterize the steric size of a group will be strongly context dependent, and this is an example. In an alternative measure of the steric size of substituents—the cyclohexane A value (Table 2.14)—phenyl is much smaller than f-butyl. 

The chemical shift of the parent compound (e.g., 22.9 for cyclobutane, 25.6 for cyclopentane, and 27.1 ppm for cyclohexane) and the same increments as for alkanes (see Chapter 4.1) can be used to estimate the chemical shifts of sp -hybridized carbon atoms of alicyclic compounds. Appropriate use of the conformational correction terms, K (page 79), is especially important with axial and equatorial substituents in cyclohexanes. The additivity rule is, however, not suitable for estimating chemical shifts of substituted cyclopropanes. 

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