Ketone carbonyl orbitals

The diphenyldisulfide, shown above, although useful in some applications, is not elEcient enough to be used as an initiator in many commercial apphcations. Aromatic ketone compounds, however, are a common choice. The ketone carbonyl orbitals that are important in photochemistry are shown in Figure 2.1. The picture is similar to the one shown for formaldehyde in Chapter 1. In flie ground state, the n orbital is localized on the carbonyl oxygen. In flic excited state, however, the n orbital is delocalized over the entire... 

In the case of tt complexes of substituted cyclopentadienones, such as the iron tricarbonyl derivatives prepared by Weiss and H libel (30), qualitative molecular-orbital theory (20) predicted a considerable reduction of the ketonic carbonyl bond order. It was observed that the ketonic carbonyl frequency dropped by as much as 65 cm-1, in agreement with theory. A similar explanation can also be provided in terms of valence bond theory (Fig. 14). It has been suggested that n complexing of arenes such as benzene results in loss of aromaticity of the ring in contrast to the dicyclopentadienyl... 

FIGURE 4. (a) Coordinate system for C=0 group, (b) Relevant n-orbital of ketone carbonyl n - tt transition. The vertical XZ plane is a nodal plane for the n-orbital that bisects the R—C—R angle and lies perpendicular to the R(R )C=0 local symmetry plane, (c) Relevant w -orbital of ketone n - tt transition. The horizontal YZ plane is a nodal plane for the tt orbital and lies on the R(R )C=0 local symmetry plane, (d) Carbonyl oxygen, as viewed looking down the Z-axis from O to C, showing circular movement of electron from the n to the tt orbital... 

Related photochemistry has also been examined with other functional groups such as phthalimides, which also abstract nearby hydrogens with the photoexcited carbonyl group. - Furthermore, since the hydrogen abstraction is performed by the half-vacant nonbonding orbital of a photoexcited ketone carbonyl, related chemistry is observed if the electron is removed electrochemically, not just photoexcited into a Tr -orbital. Electrochemical functionalization of nearby carbons has been reported in which, after hydrogen atom abstraction by an oxidized ketone, the resulting radical is electrochemically oxidized further to the carbon cation, which reacts with solvent (Scheme 7). ... 

Finally, Figure 7.40 permits the conclusion that oxetane formation can be considered to be either a nucleophilic attack by a ketone on an electron-poor olefin or an electrophilic attack by a ketone on an electron-rich olefin, corresponding to the predominant interaction of a half-filled carbonyl orbital either with the empty jc MO or with the doubly filled ji MO of the olefin. 

Figure 7.49. Possible mechanisms of the [1.3] shift of /3,y-unsaturated ketones a) concerted, b) concerted with inclusion of carbonyl orbitals, c) biradical intermediate, and d) radical pair intermediate (adapted from Houk, 1976).

For the (n, ) state of a /3,y unsaturated ketone to undergo radical a cleavage or a [1,3] shift, the a bond must be approximately parallel to the Ji orbital, since only then can allyl resonance stabilize the biradical and the carbonyl orbital overlap with the Ji system at the y terminus. These geometrical requirements are not fulfilled for the aldehyde 142, and intersystem crossing dominates a cleavage and [1,3] shift (Baggiolini et al., 1970). 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

A nucleophilic addition reaction to an aldehyde or ketone. The nucleophile approaches the carbonyl group from an angle of approximately 75° to the plane of the sp2 orbitals, the carbonyl carbon rehybridizes from sp2 to sp3, and an alkoxide ion is formed. 

Facial selectivities of spiro[cyclopentane-l,9 -fluorene]-2-ones 30a-30e were studied by Ohwada [96, 97]. The carbonyl tz orbital can interact with the aromatic % orbital of the fluorene in a similar manner to spiro conjugation [98-102]. The ketones 30 were reduced to alcohols by the action of sodium borohydride in methanol at -43 °C. The anti-alcohol, i.e., the syn addition product of the reducing reagent with respect to the substituent, is favored in all cases, irrespective of the substituent at C-2 or C-4 of the fluorene ring (2-nitro 30b syn anti = 68 32), 4-nitro... 

To explain the stereochemistry of the photoaddition, Buchi proposed that the reaction of electron-rich olefins and excited ketone involves an interaction of the electron-deficient carbonyl lone-pair orbital with the electron-rich 7r-olefin orbitals to form a diradical intermediate which could subsequently close to give the observed products. Indeed, reaction to yield the most stable diradical intermediate usually does nicely rationalize the observed product distribution. Examples of this are as follows11005 ... 

Although oxidation potentials of aldehydes and ketones are generally very high, silyl substitution at the carbonyl carbon results in a significant decrease in the oxidation potential [16]. The decrease in the oxidation potentials is attributed to the rise of the HOMO level by the interaction of the C Si cr-bond and the nonbonding p orbital (lone pair) of the carbonyl oxygen (Fig. 9). In the case of a-silyl-substituted ethers, the rotation around the C-O bond is free and,... 

Most UV absorption bands correspond to transitions of electrons from ra->7i, or n o molecular orbitals. Besides aromatic compounds, organic functional groups such as carbonyl, carboxylic, amido, azo, nitro, nitroso, and ketone groups have absorbance in the UV region. 

The ketone which is formed as a result of [1, 3] rearrangement is due to the interaction between the methyl group and a hydrogen atom in a bridging methylene group. The migration with inversion is particularly favoured by overlap with the carbonyl n orbital. 

The carbonyl oxygen of aldehydes and ketones is less basic than that of an alcohol by several powers of 10. We have just seen above that this arises because the lone pair electrons of the carbonyl oxygen are in orbitals that are approximately sp in character, and are more tightly held than the alcohol lone pairs in sp orbitals. The neutral carbonyl group is thus... 

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