Least motion

Ehrenson S 1974 Analysis of least motion paths for molecular deformations J. Am. Chem. Soc. 96 3778... 

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. 

In some cases orbital symmetry rules can label the least-motion approach of two reacting fragments as forbidden. Semi-empirical MO calculations, such as EHT ones, can then be used to pick out the minimum-energy path, as outlined in the foregoing section. Another example is given by the reactions 17) ... 

Fig. 8 a shows the preferred mutual orientations at large and small C-C distance, and Fig. 8b the energy variation along the least-motion and minimum-energy paths. 

Fig. 7. Approach of two triatomic fragments, a) Relevant planes of symmetry labels such as AA, SS, AS refer to symmetry or antisymmetry with respect to planes 1 and 2 respectively, b) Level diagram for the least-motion approach of two methylenes, c) Level diagram for the least-motion cis dimerization of HNO.

Fig. 8. a) Non-least-motion approach of two methylenes. The sequence 1-2—3 shows the mutual orientation of the two fragments with decreasing distance, b) Energy variation along the least-motion path (i) and along the least-energy path (2). The abscissa scale is the C-C distance (A). The vertical bar is 1 eV on the ordinate energy scale. 

On orbital symmetry grounds, the least-motion processes [14] and [17] are forbidden, all the others allowed. 

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. 

Scheme 6.25 shows the formulas of the compounds 88 [68], 89 [70], 90 [78], 91 and 92 [70], which emerge from the dimerization of 74, 76, 77 and 79, respectively. The structure is of the same type as that of the dimer 38 of 6 (Scheme 6.10). The configuration has not been determined, but it is assumed to be trans as in the case of 38. Hence the cyclization of the tetramethyleneethane diradicals of type 37 (Scheme 6.10), the immediate precursors of the isolated dimers, should proceed as a least-motion process at the unsubstituted radical centers. Only the trimethylsilyl group causes a predominantly alternative course, since the dimers 91 and 92 were obtained in a ratio of 1 2.

Intramolecular photoaddition of tertiary amine and styrene moieties has been extensively studied by Aoyama29 and Lewis group28,30,31 (equations 4-8). Equations 4 and 5 show that if the intramolecular additions result in the formation of a five- or six-membered ring, the product yields are excellent. Highly regioselective intramolecular proton transfer is proposed to occur via least motion pathways from the lowest energy... 

FLUORESCENCE ENERGY OF ACTIVATION ARRHENIUS EQUATION LEAST MOTION, PRINCIPLE OF MARCUS EQUATION MARCUS RATE THEORY TRANSITION-STATE THEORY Om... 

LEAST-CONDUCTIVE STEP LEAST MOTION, PRINCIPLE OF Least squares fit... 

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