Bridging hydrogen systems

These molecules are mentioned here since it is likely that complexes containing bridging hydrogen are important in some catalytic reactions. 

Some catalytic reactions wbidi inrolre hydrogen transfer isomerization reactionsf 

Olefins may be isomerized under homogeneous conditions by a variety of solutions of transition metal compounds. The active catalyst may be the t For a review, see reference [25a]. 

1-Hexene to 2,3-hexene isomers 4-Methylpent-l-ene to isomers - Pt(II) complexes and RhCh aq. HCo(CO)4 (hydrofoni lation [38] 

Olefins to internal isomers fPdOa, NaO in gladal acetic  

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The unusual carbocation structure 53, a linear 5-center 4-electron C-H-C-H-C array can be derived from three anthracenes joined up around a C-H-C-H-C core.73 The GIAO-DFT computed H and 13C NMR chemical shifts for the bridging hydrogens (2.9 ppm) and carbons (112 and 182 ppm) of the 5-center array differ considerably from those found in typical 3-center 2-electron systems. 

In a given molecular system the total number of 3-center bonds will be dictated by the empirical formula with an ideal number expressed within the cage, whereas if extra hydrogens are present they may be expressed as bridge hydrogens. 

In these cases, I-, II-, and III-Al 9, conservation of the 3-center bonds would require five (or six) 3-center bonds to be expressed within the skeleton matched with four (or three) 3-center bridge hydrogens, respectively. There are several examples demonstrating that the nine-vertex arachno system is flexible enough to adapt to both electronic alternatives. The structures for i-BgHjgrLB (X-A19) and i-B9Hi4 (A-VI9) are both known in the encumbered crystalline phases and both must accommodate six 3-center bonds within the skeletal framework. By contrast, i-C2B7Hx3 (II-A19) absorbs only five 3-center bonds within the skeletal framework. 

By examining the size, shape, and orientation of the thermal ellipsoid associated with the bridging hydrogen atom, we concluded that the hydrogen atom in the bent Mo-H-Mo system is described preferably as an effectively symmetric atom oscillating around a single equilibrium point rather than being randomly distributed between two equilibrium positions in the crystal lattice. The thermal... 

Exo-endo additions of organic azides to the norbornene skeleton and other bridged bicyclic systems can be easily differentiated from the NMR spectra of the adducts. The endo protons (exo adducts) of the norbornane do not couple with the bridgehead hydrogens.386 The endo protons in position 3a and 7a of adduct 65 exhibit an AB spectrum. The lack of coupling with the bridgehead protons is a result of a dihedral angle of 82°.25,96 Based on this, the exo form of cycloadduct 66 exhibits symmetrical doublets centered at 8 4.60 and... 

There are a number of characteristic properties of hydrogen-bonded systems. In the first place, the distance between the heavy atoms is expected to be shorter than the sum of their van der Waals radii. Along with formation of the AH B interaction comes a stretch of the bridging hydrogen away from the... 

As an illustration of these principles, consider B5H9 (Figs. 5-3 and 5-5). The 5 BH units and 4 bridging hydrogen atoms supply 10 + 4 = 14 electrons or 7 pairs total. Since n = 5, this is an n+2 electron pair system and hence is predicted to be nido, i.e., a square pyramid, in accordance with its known structure. The B H ... 

Olah and coworkers have carried out reactions of alkanes in superacid solution (SbFs-HF or SbFs-FSOsH), leading to observable (by NMR) tertiary alkyl carbocations (17, 18, 19). This reaction is pictured as a protonation of the alkane to give a pentacoordinated intermediate which then loses H2. Thus, a plausible mechanism for the cyclodecyl -> 9-decalyl cation reaction could involve loss of the bridging hydrogen (as H ) to form decalin, and then reprotonation of the decalin to eventually form H2 + 9-decalyl cation. However, when the cyclodecyl cation 1 was prepared in SbFs-FSOsD solution, there was no evidence of H-D formation in the hydrogen gas that was produced. It was concluded therefore that the H2 gas must come directly from the carbocation hydrogens without any involvement by the superacid system. 

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