Bridge carbon bond

The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface As the molecular model m Figure 6 3 shows one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst The bottom face of the double bond is more exposed and both hydrogens are transferred from the catalyst surface to that face... 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

Perform an optimization of these two derivatives at the PM3 or RHF/STO-3G level in order to discern which is the more favorable isomer (the latter is a very long job). What are the most dramatic structural features that characterize these two isomers Do the bridging carbons remain bonded in the derivative ... 

The structures are quite similar for all of the model chemistries we are considering. The second isomer is characterized by a very long bridging carbon distance, indicating that the two carbons are no longer bonded (which is why we refer to it as the open form). 

Complexes of the composition RCo (dioximeH)2L (R = alkyl, L = neutral ligand) and their parent complexes with BR2 bridges RCo(dioxime-BR2)2L 127 (Fig. 33) are known as organocobaloximes [173-178] and have received attention being models for vitamin B12 (cobalamines) [183]. A series of related complexes of the composition Fe (dioxime-BR2)LL 128 (Fig. 33) without the metal-carbon bond is also known [179, 180]. 

In the presence of H2, perhydrocarbyl surface complexes loose their ligands through the hydrogenolysis of their metal carbon bonds to generate putative hydride complexes, which further react with the neighbouring surface ligands, the adjacent siloxane bridges (Eqs. 8-9) [46,47]. 

A pyrolylnickel complex 21 also could activate water to afford a bridging hydroxo complex 22 accompanying cleavage of the Ni-carbon bond (Eq. 6.12) [20]. The hydride (hydroxo) species was not observed in the reaction mixture. 

Crosslinked polyethylene consists of molecular chains that are linked at random points to form a network, as shown schematically in Fig. 18.2 f). The crosslinks can consist of carbon-carbon bonds, which directly link adjacent chains, or short bridging species, such as siloxanes, which may link two, three, or four chains. We often refer to these materials as XLPE. 

An attempted synthesis, Scheme 65, of a heterozincate from dimethylzinc and bis (2-pyridyl)methyl lithium gave instead the dimeric [bis (2-pyridyl)methyl methylzinc] complex 84, shown in Figure 43.143 The intermolecular zinc-carbon bond to the bridging carbon atoms is remarkably short (2.269(3) A), while the zinc-methyl bond is slightly elongated (1.974(3) A). 

Although we have described terminal and bridging carbon monoxide, it is known to bond to metals in other ways. Some of the other types of linkages are shown as follows ... 

FIGURE 21.25 The structure of [CqtCty HjOlj] showing the quadruple bond between Cr atoms and the bridging carbonate ions. 

We referred earlier to the significance of reactions at the alkylidyne carbon atoms of the dimetal species. Our studies in this area are in a preliminary stage, but Schemes 1 and 2 summarise some chemistry at the bridged carbon centres for the compounds (1 ) and (3,)(12). It will be noted that protonation of the neutral bridged al ylidyne compounds yields cationic alkylidene species in which one C—C bond of the tolyl group is n2 co-ordinated to tungsten, a feature revealed by both n.m.r. and X-ray diffraction studies. 

The cluster catalyzes hydrogenation (20°C and 3 atm) of dialkyl- and diarylacetylenes to the c/s-olefins via unsaturate routes, likely involving Ni4(CNR)6(RC=CR) and Ni4(CNR)4(RC==CR)3 (391, 392). The acetylenes in the latter complex bridge three nickel centers, and increase of the acetylenic carbon-carbon bond distance is considered to enhance reduction by hydrogen (392, 393). 

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