Destabilizing electronic effects

Studies have established that the partition between transition states 3 and 4 depends on the nature of the diol unit bound to boron and on the steric and electronic effects of the a-sub-stituent X23. The data shown below demonstrate that the reactions of2-(l-methyl-2-propenyl)-4,4,5,5-tetramethyl-l,3,2-dioxaborolane proceed with a moderate preference for transition state 3 with the C2 methyl group in an axial position. Selectivity diminishes with 2-(l-methyl-2-propenyl)-l,3,2-dioxaborolane and reverses with dimethyl (l-methyl-2-propenyl)boronale, suggesting that steric interactions (gauche interactions in the case of the tetramethyl-1,3,2-diox-aborolane) between X and the diol unit on boron are capable of destabilizing transition state 4 relative to 3. 

HOMO, destabilizing the transition state. The compound 3-formylcyclobutene provided a test. Steric factors would cause the CHO (an electron-withdrawing group) to rotate outward electronic effects would cause it to rotate inward. The experiment showed inward rotation. 

The authors (162) attempted to explain the stereochemical outcome of the reactions (Schemes 3.169 and 3.170) in the terms used earlier (337), that is, by steric factors, which destabilize the endo approach of a dipolarophile, and the electronic effect (secondary orbital interactions), which is most typical for electron-rich dipolarophiles and can slightly stabilize the endo approach of these olefins. 

Table 3 presents the experimental enthalpies of formation of polynitrobenzenes and Table 4 presents the calculated additivity values and DSEs for these same compounds. Enthalpy-of-formation values have been determined experimentally for all three dinitrobenzene isomers in the gaseous state. The enthalpy-of-formation difference between the meta and para isomers is indistinguishable from 0. Conventional wisdom suggests that the para isomer should be destabilized relative to the meta because of adjacent positive charges in key ionic or polar resonance structures. Thus it seems that electronic effects due to meta/para dinitro substituent position are small. This small enthalpy-of-formation difference is similar to that for the meta and para dicyano, difluoro and dichloro benzenes, but does not mimic the ca 22 kJ mol 1 difference for the phthalic acids with which the... 

Fluorine in organic compounds is associated with a set of electronic effects inductive and mesomeric, stabilizing and destabilizing, pulling or pushing electrons, which are convincingly... 

Heterosubstituted cyclopropanes can be synthesized from appropriate olefins and car-benes. Since cyclopropane resembles olefins in its reactivity and is thus an electron-rich car bo-cycle (p. 76ft). it forms complexes with Lewis acids, e.g. TiCL, and is thereby destabilized This effect is even more pronounced in cydopropanone ketals. If one of the alcohols forming the ketal is a silanol, the ketal is stable and distillable. The O—Si-bond is cleaved by TiCl4 and a d3-reagent is formed. This reacts with a -reagents, e.g. aldehydes or ketals. Various 4-substituted carboxylic esters are available from 1-alkoxy-l-siloxycyclopropanes in this way (E. Nakamura, 1977). If one starts with l-bromo-2-methoxycyclopropanes, the bromine can be selectively substituted by lithium. Subsequent treatment of this reagent with carbonyl compounds yields (2-methoxycyclopropyl)methanols, which can be transformed to /7,y-unsaturated aldehydes (E.J. Corey, 1975B). 

It is possible and probably very likely that both types of electronic effects are occurring in the acetal function. In other words, 2 could be more stable than 2 because 2 is stabilized relative to 2 by a partial electron transfer and 2 is destabilized relative to 2 by electronic repulsions. There is presently no experimental technique to differentiate between the two effects. At the present time, many chemists, including myself, prefer to consider the anomeric effect as a stabilizing rather than a destabilizing effect. The main reason is that the concept of stabilization of a system through electronic delocalization is a well established principle in organic chemistry. The resonance theory is indeed based on this principle. I believe that this concept rather than the dipole - dipole or electron pair - electron pair repulsions allows the organic chemist to rationalize his results better. 

This qualitative model, based on semiempirical MO theory, focuses entirely on the so-called electronic effects, as the A—H bonding orbital interactions are often called. However, steric repulsion (i.e., the destabilizing orbital interactions) between the hydrogen substituents in AH3 is just as important in the interplay of mechanisms that determine whether the molecule adopts a planar or a pyramidal shape. In fact, as will become clear from the following discussion, which is based on a Kohn-Sham DFT study at the BP86/TZ2P level,107 108 steric repulsion turns out to be the decisive factor in determining the pucker of our example.133... 

More sophisticated calculations indicate that cyclic An systems like cyclobutadiene (where planar cyclooctatetraene, for example, is buckled by steric factors and is simply an ordinary polyene) are actually destabilized by n electronic effects their resonance energy is not just zero, as predicted by the SHM, but less than zero. Such systems are antiaromatic [17, 46]. 

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