Least-motion process

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

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.

The higher reactivity of nickel complexes than the palladium congeners was examined theoretically. Fig. 9.1 illustrates a schematic diagram for the orbital correlation on the ethane elimination from cw-M(CH3)2(PH3)2 complexes (M = Ni, Pd) [8,10]. This scheme presumes the least motion process that maintains the C2v symmetry of precursor complex throughout the reaction. The C-M-C angle becomes narrow while the P-M-P angle is gradually extended as the reductive elimination proceeds. Finally, ethane is eliminated with formation of a linear M(PH3)2 complex. 

This result is explained by a least-motion process in which a C—C stretching vibration results in C-1—C-7 cleavage C-7 simply continues its motion until it is captured by interaction with C-5, which necessarily leads to inversion/ ... 

A tetrahedral intermediate in which the central carbon atom is substituted by two oxygen atoms and a nitrogen atom can also be generated by the addition of water or hydroxide to an imidate salt. Least motion considerations can then affect the two possible modes of decomposition of this intermediate, to ester and amine or to amide and, alcohol. The balance between these two modes of fragmentation will depend on protonation-deprotonation processes as well as any least motion effects. However, despite attempts to rationalise results with various acyclic imidates in terms of interplay between steric and stereoelectronic interactions (Deslongchamps, 1983, pp. 118-147), it is quite clear from the incisive work of... 

A recent kinetic study of the extrusion of ( Bu)3Si-Si-Si( Pr)3 from a silirane suggested that the silylene was formed in a singlet state by a non-least motion, concerted, stereospecific process, but that it reacted from a triplet state via the mechanism of Scheme 7 [53]. 

The dissociation of H2Si according to equation 41, where M is an inert molecule in the reaction mixture, requires a barrier of 55 kcal mol-1 (at 3-21G//3-21G) along the C2v ( least-motion ) path299 . Gordon and coworkers21 studied this process at MCSCF/ 6-31G and found that the xAt and 3Bt states dissociate to Si(1D) + H2(1Zg+) and to Si(3p) + H2(1Zg+), respectively21. 

A theoretical study of the interaction of sulfur atoms with ethylene within the framework of the Extended Hiickel MO theory has been reported by HoflFmann and co-workers (19). Potential surface calculations revealed two minima for the 8( 02) + C2H4 system. The higher corresponds to vinyl mercaptan formation via C-H bond insertion, and the lower, lying about 20 kcal below the former, to the least-motion, symmetry-allowed addition of sulfur across the double bond. The two are viewed as competing concerted processes. Similar calculations for the... 

A simple model is proposed in this note. It is shown to incorporate the possibility of including several potential energy curves in a treatment of scattering processes in a linear geometry based on very limited information from electronic structure calculations. The present demonstration of the feasibility will be followed by more detailed examinations and possible comparisons with more elaborate theoretical work and even experimental findings. Extensions to two and higher dimensions appear to be within reach and in conjunction with a geometrical characterization of the molecular conformation space based on a distance concept derived from a quantification of the Rice-Teller least motion principle TO there opens an avenue towards detailed descriptions of chemical processes from basic principles. 

As noted above, the skeletal rearrangements in the 8, 1 reactions and the stereospecificity in the formation of reaction products are essential criteria of the intermediate formation of nonclassical ions. Nevertheless the results obtained can be explained by assuming the C—J bond ionization to proved with a simultaneous shift of the a-bond C —C in the isomer 350 and of the a-bond C —C in the compound 351. The structures 347 and 348 must be transition states of ionization processes. The authors of , however, consider this assumption less probable because the synchronous process in question is a reaction of the 8, 2 type at the bridged carbon atom and for the bridged atoms of rigid bicyclic systems such processes are known to be strongly hindered. The formation of ion 318 with bicyclo[3,2,I]octadiene skeleton consecutively from ion 347 rather than directly from compound 350 with the geometry of bicyclo[2,2,2]octadiene is also in fuller accordance with the principle of so-called least motion " 29)... 

Addition of the butyl group to sulfur generates the sulfurane and initiates the decomposition process. Least motion collapse expels M-butyl-... 

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