Combined C—H bond

Intriguingly, while vinyldiazoacetates are utilized as carbene precursors in the allylic C—H bond insertion reaction catalyzed by Rh complexes, the combined C—H bond functionalization/Cope rearrangement occurs readily. This has emerged as a reliable methodology for the construction of 1,5-diene compounds bearing two vicinal stereogenic centers and will be discussed in detail in Section 1.1.2.6. 

Rh-Catalyzed Combined C—H Bond Functionalization/ Cope Rearrangement... 

Scheme 1.23 Application of the combined C—H bond functionalization/Cope rearrangement in organic synthesis.

Another explanation follows from the above discussion on stereoelectronic factors.145 If overlap between the semi-occupied orbital and the breaking C-H bond favors disproportionation, then substituents which delocalize the free spin will serve to reduce this interaction and disfavor disproportionation. A proposal along these lines was made originally by Nelson and Bartlett148 who also noted that diminisbment of the spin density at Ca could retard combination. Flowever, it is not necessary that the two effects should cancel one another. 

Although the hybrid orbitals discussed in this section satisfactorily account for most of the physical and chemical properties of the molecules involved, it is necessary to point out that the sp orbitals, for example, stem from only one possible approximate solution of the Schrddinger equation. The i and the three p atomic orbitals can also be combined in many other equally valid ways. As we shall see on page 12, the four C—H bonds of methane do not always behave as if they are equivalent. 

Consider first the ethylene molecule. Its geometrical structure is shown in Fig. 5. The s, py and pz atomic orbitals of the carbon atoms are assumed to be hybridized. This sp2 hybridization implies H-C-H bond angles of 120°, approximately in agreement with experimental results. The remaining two px orbitals are thus available to contribute to a -electron system in the molecule. Here again, the two linear combinations of atomic orbitals yield bonding and... 

Figure 1.8 Diastereoisomeric effects predicted to arise for coronene adsorption on Cu l 1 1 from a combination of molecular rotation (curved arrows) within the 2D adsorbate lattice to allow C—H bond interdigitation and the ( 19 X 19)R 23.4° lattice. The black arrows indicate a high symmetry within the molecule bisecting C—H bonds.

A carbon atom combining with four other atoms clearly does not use the one 2s and the three 2p atomic orbitals that would now be available, for this would lead to the formation of three directed bonds, mutually at right angles (with the three 2p orbitals), and one different, non-directed bond (with the spherical 2s orbital). Whereas in fact, the four C—H bonds in, for example, methane are known to be identical and symmetrically (tetrahedrally) disposed at an angle of 109° 28 to each other. This may be accounted for on the basis of redeploying the 2s and the three 2p atomic orbitals so as to yield four new (identical) orbitals, which are capable of forming stronger bonds (cf. p. 5). These new orbitals are known as sp3 hybrid atomic orbitals, and the process by which they are obtained as hybridisation ... 

In the case of CH3CD3, various other dissociation modes exist, including C—C and C—H bond breakage, either separately or in various combinations (Freeman, 1968). A few other examples follow of dissociation of excited inorganic and organic molecules ... 

The combined C-H activation/Cope rearrangement generates a new C-H bond in a highly stereoselective manner and, therefore, has the potential to be a strategic reaction in synthesis. An example of this is the enantiose-lective synthesis of (+)-sertraline as shown in Scheme l.91 The C-H insertion step proceeded smoothly to form 17 with 99% ee. The conversion of 17 to (+)-sertraline could be readily achieved using conventional steps. 

Now, consider what happens if the two fluorines are replaced by two chlorines. In this case, the dominant interactions will be acci-° ci because the C—Cl bond is both a better donor and a better acceptor than the C-H bond. Hence, it is predicted that 1,2-dichloroethane will have a preferred trans conformation which combines in an anti orientation the best donor and best acceptor bonds. 

The Tt orbitals constructed through combination of individual C - H bonds can interact with the unpaired spin in much the same way as seen before for... 

A consequence of the symmetry of the molecule is that states must transform according to representations of the appropriate symmetry group. In terms of coordinates, this implies that one must form internal symmetry coordinates. These are linear combinations of the internal coordinates. For example, denoting in Fig. 6.1 by sx, s2, s3,, v4, j5, s6 the stretching coordinates of the six C-H bonds, the internal symmetry coordinates are linear combinations... 

Palladium salts will attack C-H bonds in functionalised aromatics such as acetoaniline to form palladium-carbon bonds that subsequently undergo insertion of alkenes [31], (3-Hydride elimination gave styryl derivatives and palladium hydride, which requires re-oxidation of palladium by benzoquinone. The reaction can be regarded as a combined Murai reaction (C-H activation, if electrophilic) and a Heck reaction (arylalkene formation), notably without the production of salts as the cross-coupling reactions do. An example is shown in Figure 19.15. 

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