Bonding in ketones

Oxidation of Remote CH Bonds in Ketones, Carboxylic Acids, and... 

If our considerations concerning the C2H4 biradical are correct then by microscopic reversibility we should expect that in the similar reaction pictured here the product state is a CH20 biradical. We can estimate that the energy of the C—O double bond in ketones and aldehydes corresponds to about 73 keal. so that the color of the emitted light does indeed correspond to this hypothetical excitation energy. Note that this is not an allowed transition for CH20.f... 

The domain of ion/molecule reactions with acetone and ketones in general is very large and only a sketchy overview of the field can be given. Eyler, Ausloos and Lias434 showed that the C=0 bond in ketones can be split by CF3+, C2F5+ and CC13+, e.g. the reaction... 

Several related examples of transition metal-catalyzed addition of C-H bonds in ketones to olefins have been reported (Table 2) [11-14]. The alkylation of diterpenoid 6 with olefins giving 7 proceeds with the aid of Ru(H)2(CO)(PPh3)3 (A) or Ru(CO)2(PPh3)3 (B) as catalyst [11], Ruthenium complex C, Ru(H)2(H2)(CO) (PCy3)2, has catalytic activity in the reaction of benzophenone with ethylene at room temperature [12]. The alkylation of phenyl 3-pyridyl ketone 8 proceeds with A as catalyst [13], Alkylation occurs selectively at the pyridine ring. Application of this C-H/olefin coupling to polymer chemistry using ce,co-dienes such as 1,1,3,3-tetramethyl-l,3-divinyldisiloxane 11 has been reported [14]. 

Enantioselective Hydrosilylation of C=0 Double Bonds in Ketones. The use of Rh-phosphorane catalyst systems to promote asymmetric hydrosilylation of prochiral ketones with silanes of the type RsSiH has met with only limited success. Thus, hydrosilylation of acetophenone with Ph2SiH2 promoted by [Rh(COD)Cl]2-(S,S)-DIOP catalyst afforded the (S)-(-)-phenylmethylcarbonyl with an optical yield of 32% ee. Similarly, the use of a Rh-NORPHOS catalyst in this reaction proceeded with an optical induction of only 16% ee. S... 

The C=0 bond in ketones can be cleaved by tungsten complexes with formation of a very strong oxo-tungsten multiple bond as has been studied extensively [119]. 

The C-0 multiple bonds in ketones were cleaved in a C-C single-bond-form-ing reaction promoted by vanadium(II) complex in the presence of the allyl bromide or a catalytic amount of molecular oxygen (Eqs. 51,52) [120-122]. 

If this relation is true for ketones, then the dissociation energies of a-C—H bonds in ketones may be estimated. Let DC H in cyclohexane be 89 kcal mole-1, then DC H for the cyclohexanone a-CH2 group will be 89 — AD = 85 kcal mole-1, since AD = 1.8/0.45 = 4 kcal mole-1. If DC H t in isopentane is taken as 85 kcal mole-1 (as in isobutane), then Dq h for methyl i-propyl ketone will be 85 — AQ = 85 — 4 = 81 kcal mole-1. The decrease in strength of the a-C—H bond of ketones in comparison with... 

As an introduction to the next subject, an interesting phenomenon deserves comment. The acidity of the a-C-H bond in ketones (pKa about 20) is usually lower than that of the a-C-H bond in a-halogenated esters (pKa about 17) since halogen atoms enhance C-H acidity. We shall see in the next two examples what happens when both species are expected to react and a strong base is added. 

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