Steric effects in 3-eliminations

These studies on steric assistance (12) led to a detailed consideration of the possible role of steric effects in a wide variety of systems, such as steric effects in displacement reactions 10, 14), steric effects in elimination reactions 15,16), steric effects in alicyclic systems (77), and steric effects in... 

Brown, H. C. Wheeler, O. H. Steric Effects in Elimination Reactions. IX. The Effect of the Steric Requirements of the Leaving Group on the Direction of Bimolecular Elimination in 2-Pentyl Derivatives J. Am. Chem. Soc. 1956, 78, 2199-2202. Also see Bartsch, R. A. Bunnett, J. F. Orientation of Olefin-Forming Elimination in Reactions of 2-Substituted Hexanes with Potassium frrf-Butoxide-fiprf-Butyl Alcohol and Sodium Methoxide-Methanol /. Aw. Chem. Soc. 1969, 91, 1376-1382. Provide the products expected from the following olefin-forming reactions. (CJH-7)... 

There are also steric effects. In some cases the direction of elimination is determined by the need to minimize steric interactions in the transition state or to relieve steric interactions in the ground state. 

If steric effects in solid-state cluster compounds could be eliminated by the isolation of discrete molecules with the same cluster units as Chevrel phases, a part of the preceding problem would be solved. The structures of the cluster molecules in the isolated state free from the crystal packing effects are most suitable for such discussion. The geometry of free molecules in vacuum cannot be known from X-ray structure determination, and usually it is difficult to measure crystal packing effects in molecular crystals. In the case of metal complexes with sym-... 

As discussed in Section 3.1.11.1, which covers the reductive cleavage of the 3-hydroxy sulfone derivatives to alkenes, the Julia reaction proceeds by the formation of an anion that is able to equilibrate to the thermodynamic mixture prior to elimination. Therefore, there is no inherent advantage in producing the erthyro- or threo-fi-hydroxy sulfone selectively fix>m the keto sulfone. The ( )/(Z)-mixture of alkenes should be the same. This method is used to produce alkenes in cases where the acid derivative is more readily available or more reactive. The reaction of the sulfone anion with esters to form the keto sulfone, followed by reduction with metal hydrides has been studied. The steric effects in the reduction do become important for the reaction to produce vinyl sulfones, which are formed from the anti elimination of the 3-hydroxy sulfone adduct, as mentioned in Section 3.1.11.6.2. Some examples of the use of esters are presented below. 

These differences in selectivity are obviously due to differences in steric effects in the metal-lacyclic intermediates. With 7, -hydrogen elimination is preferred over the cyclopropylmethyl to but-3-enyl rearrangement for statistical and steric reasons. This is in line with the observation that additional increase of the steric bulk around the metal center results in exclusive formation of the open-chain monomer or in greatly diminished reactivity and thus low overall yields of dimerization products. 

Perhaps the best experimental evidence supporting the hypothesis of an inductive effect was educed from the results of the hydrolysis of tetra-hydro-2-methoxypyrans substituted at C-6 (see Table XVI) (see also, Ref. 49). Here, the order of decreasing hydrolysis is in the order of increasing induction, steric effects being eliminated by replacing all other groups with hydrogen atoms. However, it must be pointed out that this experiment does not eliminate the possibility that steric effects may be... 

We can understand the regioselectivity of the Hofmann elimination by comparing steric effects in the E2 transition states for formation of 1-butene and trans-2-... 

Brown s investigation of the addition compounds of trimethyl borane, diborane, and boron trifluoride with amines has provided a quantitative estimation for steric strain effects in chemical reactions. He also investigated the role of steric effects in solvolytic, displacement, and in elimination reactions. His results demonstrate that steric effects can assist, as well as hinder, the rate of a chemical reaction. 

The regioselectivity also results from steric effects in the transition state for the E2 ehmination. A (CH3)3N group has approximately the same van der Waals radius as a tert-huvy group. A trans periplanar arrangement of the proton to be eliminated and the (CH3)3N group is required for an E2 elimination reaction of the quaternary ammonium salt to take place. 

Various factors, including steric effects and the relative acidity of the P hydrogens, have been suggested as being most important in such eliminations. ... 

The scope of heteroaryne or elimination-addition type of substitution in aromatic azines seems likely to be limited by its requirement for a relatively unactivated leaving group, for an adjacent ionizable substituent or hydrogen atom, and for a very strong base. However, reaction via the heteroaryne mechanism may occur more frequently than is presently appreciated. For example, it has been recently shown that in the reaction of 4-chloropyridine with lithium piperidide, at least a small amount of aryne substitution accompanies direct displacement. The ratio of 4- to 3-substitution was 996 4 and, therefore, there was 0.8% or more pyridyne participation. Heteroarynes are undoubtedly subject to orientation and steric effects which frequently lead to the overwhelming predominance of... 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

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