Cyclohexane, dimethyl-, ring inversion

Considering then the series a, b, and d in Table 7 the substitution of one hetero-atom lowers the barrier or has little effect which fits with the monosubstitution effect. The series 19 can be extended to 19e X=dimethyl-silyl 79/, X = selenium and 19g, X = tellurium.72) For 19e the barrier to inversion in 5.4 kcal/mol 3) and for 79/ and 19g, it appears to be much lower than in cyclohexane, or in 19a to 19d, since no changes are seen in the nmr spectrum at —100°. It may be in these cases the low barrier reflects the unusual flatness of the six-membered ring, and perhaps particular ease of rotation about carbon-X bonds, both of which may be consequences of these bonds being relatively long. 4)... 

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