Sp3 hybridized C-H bonds

Applications of controlled radical reactions - including oxidation - deal almost exclusively with C=C double bonds. Indeed, a multitude of examples have been reported for the selective transformation of this functional group. Contrasting with this situation, only a very limited number of selective ( stereocontrolled ) radical reactions involving sp3-hybridized C-H bonds are known. Particularly useful functionalizations along these lines include the hydroxylation and the acyloxylation of alkyl chains. The reason for their limited success is of course due to the high stability of the C-H bond compared with that of the olefinic C=C unit most electrophilic reagents which readily add to unsaturated substrates are not able to oxidize a C-H bond. 

Cross-Dehydrogenative Coupling Reactions of sp3-Hybridized C-H Bonds... 

Selectivity among various sp3 hybrid C—H bonds is more difficult to rationalize. For example, the reaction of in situ generated Cp Rh(PMe3) with alkanes generates the product of activation of the stronger primary C—H bonds in preference to weaker secondary (or tertiary) C—H bonds. Other systems have been demonstrated to possess similar selectivity. A possible explanation of such selectivity is a steric inhibition against activation of internal C—H bonds of a linear alkane. That is, on the timescale of the C—H oxidative addition event, the metal center might access only the terminal C—H bonds. However, recent detailed mechanistic studies have revealed this notion to be incorrect, at least for some systems. 

Sabyasachi, B., Rai-Shung, L. (2008). Gold-catalyzed 1,3-addition of a sp3-hybridized C—H bond to alkenylcarbenoid intermediate. Journal of the American Chemical Society, 130,16488-16489. 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

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 ... 

Carbon atoms (1) and (4) use sp3 hybrid orbitals to form four sigma bonds, three by overlap with the hydrogen Is orbitals and one by overlap with an sp2 orbital from the central carbon (2). The two carbon atoms involved in the double bond undergo sp2 hybridization. They form C-H bonds by overlapping with Is orbitals of the H atoms. The C=C double bond is formed similarly to that described in (a). 

Carbon atoms (1) and (2) undergo sp2 hybridization, while carbon atoms (3), (4) and (5) undergo sp3 hybridization. The orbital overlaps are similar to those in 2-butene except carbon atoms (1) and (2) are involved in the double bond and carbon atom (2) is bonded to carbon atom (3) by sp2-sp3 overlap. The overlap of the hybrid orbitals with the Is orbitals of H atoms gives the C-H bonds. 

The left-most C atom (in the structure drawn below) is sp3 hybridized, and the C-H bonds to that C atom are between the sp3 orbitals on C and the Is orbital on H. The other two C atoms are sp hybridized. The right-hand C-H bond is between the sp orbital on C and the Is orbital on H. The c a C triple bond is composed of one sigma bond formed by overlap of sp orbitals, one from each C atom, and two pi bonds, each formed by the overlap of two 2p orbitals, one from each C atom (that is a 2py—2py overlap and a 2pz—2pz overlap). 

For example, in the methane molecule (CH4), the four sp3 hybrid orbitals of the carbon atom overlap end to end with one Is orbital from each hydrogen atom to form four C — H bonds. Those bonds are all o bonds. 

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 sp3 orbitals, for example, stem from only one possible approximate solution of the Schrodinger equation. The s and the three p atomic orbitals can also be combined in many other equally valid ways. As we shall see on p. 12, the four C—H bonds of methane do not always behave as if they are equivalent. 

The axes of the sp3 orbitals point toward the corners of a tetrahedron. Therefore, sp3 hybridization of carbon is consistent with the tetrahedral structure of methane. Each C—H bond is a a bond in which a half-filled Is orbital of hydrogen overlaps with a half-filled sp3 orbital of carbon along a line drawn between them. 

Each carbon in propane is bonded to four atoms and is sp3-hybridized. The C—C bonds are a bonds involving overlap of a half-filled sp3 hybrid orbital of one carbon with a half-filled sp3 hybrid orbital of the other. The C—H bonds are u bonds involving overlap of a half-filled sp3 hybrid oribital of carbon with a half-filled hydrogen Is orbital. 

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