Principle of least-motion

It can be seen that the two bonds whose bond order is 1 are unchanged in the two products, but for the other four bonds there is a change. If the 1,4-diene is formed, the change is 5 + 5 + 5 -l- 5, while formation of the 1,3-diene requires a change of j + j + l + Since a greater change is required to form the 1,3-diene, the principle of least motion predicts formation of the 1,4-diene. This may not be the only factor, because the NMR spectrum of 46 shows that the 6 position has a somewhat greater electron density than the 2 position, which presumably would make the former more attractive to a proton. 

The stereochemistry of the electrocyclic ring opening following the attack of the nucleophile on the vinylic carbon appears to be governed by the principle of least motion . 

The isolation of the -isomer 139 was in fact unexpected, since all tetrasubstituted olefins previously obtained from thiirene dioxide have been assigned the cis-configuration with respect to the two phenyl substituents based on the principle of least motion during the ring opening to olefins ". It might well be, therefore, that the -isomer is obtained through the isomerization of the initially formed Z-isomer. 

Another question that has come up is why the addition of proton to carbanion occurs only at position 6 to give 1,4 diene and not at 2 or 4 to give a 1,3 diene This has been answered on the principle of least motion advanced by Hine according to which those reactions are favoured which involve the least change in atomic position and electronic configuration. 

CH2 reacts with ethylene to form cyclopropane. The symmetrical approach shown below appears to be the simplest. It also respects the principle of least motion, often invoked in the literature. Use FO analysis to show that this mechanism is improbable and to find the best approach. Assume that the carbene has a singlet configuration. 

A tacit assumption in the change from tetrahedron to bipyramid is that the principle of least motion (PLM see a later section) holds. On this basis, the choices in Table 7 were set up. According to the energy criterion supplied by Van Der Voorn and Drago (1966), symmetrical substitution with inversion, i.e. equatorial in—equatorial out, or axial in—axial out, should be the rule for trigonal bipyramidal transitionstates. 

PLM (Principle of Least Motion) favors axial over equatorial entry and departure. 

STEREOSELECTION IN STEPS OP ORGANIC REACTIONS 301 D. Principles of Least Motion (PLM)... 

In contrast to the explanations based on steric considerations, the principle of least motion unambiguously predicts that abstraction of the 3-exo-H is much easier than that of the 3-endo-H in the 2-norbornanone series. Indeed, the parameter EmXn (see above), which expresses the amount of the coordinate variations of the nuclei, is much smaller for 3-exo-H (2.46 A2 for 2-norbornanone) than for 3-endo-H (5.94 A2) (Tee et al., 1974). However, as emphasised by the authors of this work the success of the PLM method does not necessarily imply that least motion of atoms is itself the factor which determines the stereochemical pathway. It may well transpire that the least-motion type of calculation, by virtue of its geometric constraints and... 

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