Endo hydrogen

The preference for endo attack in 7,7-dimethylnorbomene is certainly steric in origin, with the 7-methyl substituent shielding the exo direction of approach. The origin of the preferred exo-attack in norbomene is more subject to discussion. A purely steric explanation views the endo hydrogens at C—5 and C—6 as sterically shieldihg the endo approach. There probably is also a major torsional effect Comparison of the exo and endo modes of reproach shows that greater torsional strain develops in the endo mode of... 

The stability of the liquid carboranes depends on the substituents R at carbon and boron. The axial (endo) hydrogen atom is acidic and involved in 3c2e bonding to one of the basal boron atoms. In the 1H NMR it exhibits a high field shift near S = —1.4. Deprotonation of 55 with potassium or Bu Li leads to the anion (55-H)-, which is isolobal with C5H5. Reactions of 55 and (55-H) with appropriate metal complexes lead to metallacarboranes with sandwich structures [67, 69],... 

In the mechanism preferred by the authors, the observed KIE is the product of the EIE for the reversible formation of the bromonium ion and the KIE for the rate-determining formation of the /3-bromocarbocation (Scheme 3). Because the steric crowding of the C-2, C-2, C-3 and C-3 endo-hydrogens in the bromonium ion would be relieved in going to the /3-bromocarbocation intermediate, one would expect the secondary deuterium KIE for the k2 step of the reaction to be normal, i.e. >1.00. If this is the case, the EIE for the formation of the bromonium ion must be significantly more inverse than the KIE for the k step of the reaction, i.e. the KIE for the formation of the... 

In line with this interpretation, it has been experimentally observed that in the reduction of 9 by UAIH4, preferential attack comes from the left ° This can be easily rationalized if one assumes that the methyl substituent tends to move away from the endo-hydrogens ... 

Steric compression causes pronounced dcshiclding of H nuclei. In the epimeric pentacyclic alcohols 1 for instance, the endo-hydrogen atoms are deshielded by 1.7 to 2.4 ppm relative to corresponding protons in the uncompressed bicyclo[2.2.1]heptane69. 

Only ihc exo hydrogen atoms are shown In Fig. 16.50. The number of bridging and endo hydrogen Moms will vary depending upon whether the species is neutral or ionic, has hclcroaioms (sec below), has a Lewis base coordinated lo it. cic. 

The inductive effect of the donating C—D bonds to the observed large inverse secondary deuterium isotope effect has not been given proper consideration but treated as a rather minor component superimposed on the important steric component caused by larger amplitudes of vibrations of C—H bonds than those of the C—D bonds. 14C KIE have not been studied in this reaction. The C2, C V. C3 , C3, endo hydrogens are separated only by 2.11 A, substantially less than van der Waals radii (2 x 1.2 A)404. 

The molecule is a butterfly structure, H2e and H4e are the equatorial hydrogens of the BH2 groups and H2a and H4a are the axial hydrogens. A more up-to-date terminology would refer to H2e and H4e as exo-hydrogens and H2a and H4a as endo-hydrogens Structural parameters are given in Table 4. 

The electronic structure of 6 has been probed by using the Fenske-Hall quantum chemical method, and the factors that influence the distribution of endo-hydrogen atoms have been examined (30). The preference for M—H—B versus M—H—M interactions may also be rationalized in... 

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