Least motion pathway

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... 

The MOs and electronic states of carbene have been discussed in Chapter 7. The orbital and state correlation diagrams for addition of CH2 to ethylene is shown in Figure 14.9. The Walsh bonding picture for the MOs of cyclopropane requires that the and a MOs of the ethylene also be included in the diagram. The a2 and vertical plane of symmetry (as well as the other elements of the C2v point group), and the... 

Bredt s rule. In this way, 1-adamantyl p-methoxyacetophenone 86a was forced to yield only cyclobutanols 87a and 88a as photoproducts [281]. Whereas (benzene) solution phase irradiations of 86a yielded a 2.6 ratio of 87a/88a, the solid state photoproduct ratio, 0.5, favors the more sterically hindered cyclobutanol. X-Ray diffraction studies predict a chair-like y-hydrogen abstraction pathway for 86a (in contrast to the boat-like transition states of 82) in which the C=0-Ha distance is 2.67 A. Other abstractable hydrogens (Hb) are at least 0.3 A farther from the carbonyl oxygen (Scheme 44). If i-BR has a conformation which mimics that of the ketone, its least motion pathway favors formation of the more sterically hindered cyclobutanol 88a. 

The so-called least-motion path has been popular for many years. In the original formulation, the elementary reactions that involve the least change in the atomic and electronic configurations are favored (160,161). This hypothesis has found numerous applications in organic and inorganic chemistry (162-167). However, it is necessary to admit that there exist rather numerous exceptions, primarily due to the fact that the non-least-motion pathway is symmetry allowed while the least-motion path is symmetry forbidden (168,169). Dimerization of singlet carbenes is a typical example. 

The trajectories from TS(90,0) show strong preference for i over r at short times. This also reflects a least-motion pathway. The overall s/a ratio time dependence is dominated by the results of the TS(y/) trajectories. 

Carbene adducts of germanium and tin have also been reported, as shown in Scheme 9.56 58 model for the dimerization of singlet carbenes along a non-least-motion pathway was provided by the reaction of Gel2 with carbene (1) (R = Mes, R = H). The formation of a polarized Ge-C bond, rather than a true double bond, was indicated by the C NMR chemical shift of the carbene atom (5 = 60.88) in the resulting germene-carbene adduct (81). The solid-state... 

In Fig. 5 the values of the packing parameters d and d> are plotted for constant separations R between the reacting atoms Cl and C4. The relevance of the model considerations can be tested using crystal structure data, which have become available recently for a number of reactive and unreactive diacetylene monomers. Reactivity is only observed in a small area of the map. The distribution of the points for highly reactive structures suggest the criterion for which the separation R should be less than 4 A to be a more critical condition than the requirement of a least motion pathway as calculated by Baughman Figure 5 shows that all but one reactive diacetylene... 

Two singlet isomers for the three-coordinate M(I) complexes (cis-1, trans-1) have been identified in the previous section. Addition to either complex of a dihydrogen molecule with the formation of two M-H bonds formally oxidizes the metal atom to M(in). Assuming a least-motion pathway, the cis-1 reactant will lead to the square pyramidal (SQP) product cis-2, reaction (3) similarly, addition of H2 to trans-1 produces the trigonal-bipyramidal (TBP) product trans-2, reaction (4). 

Anti elimination is also favoured because it is the least motion pathway and because the syn pathway can also be disfavoured by steric repulsion between base and leaving group. Despite these a priori reasons why anti elimination is favoured, experimentally, there seems to be a only slight preference for anti elimination, even for classical E2 reactions with central transition states [such as Hofmann eliminations of (acyclic) alkyltrimethylammonium hydroxides]. 

Borden et al. have described a 1,5-sigmatropic rearrangement which leads to inversion at the migrating carbon for both thermal and photoprocesses. This seems to be a case in which preservation of orbital symmetry is of less dominating importance than the adoption of a least motion pathway . Dauben and Olsen have provided other examples of least motion control in the ring-opening of cyclohexa-1,3-dienes. 

The metallacyclobutadiene formation from acetylene and molybdenum carbyne complex CI3M0CH was studied [45] along a postulated least-motion pathway as shown in Fig. 12. The CI3M0CH was brought together with the acetylene molecule such that the three carbon atoms and the metal center remained coplanar throughout the course of the reaction. It has been found... 

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