Carbonyl double

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

Carbonyl Group Reactions. Mandelonitrile [532-28-5] is formed by the addition of hydrogen cyanide to the carbonyl double bond. 

Sodium bisulfite adds to the carbonyl double bond to give a hydroxysulfonate (bisulfite addition compound). 

A number of groups have criticized the ideas of Dauben and Noyce, especially the concept of PDC. Kamernitzsky and Akhrem, " in a thorough survey of the stereochemistry of addition reactions to carbonyl groups, accepted the existence of SAC but not of PDC. They point out that the reactions involve low energies of activation (10-13 kcal/mole) and suggest that differences in stereochemistry involve differences in entropies of activation. The effect favoring the equatorial alcohols is attributed to an electrostatic or polar factor (see also ref. 189) which may be determined by a difference in the electrostatic fields on the upper and lower sides of the carbonyl double bond, connected, for example, with the uncompensated dipole moments of the C—H bonds. The way this polar effect is supposed to influence the attack of the hydride is not made clear. 

In contrast to the open-chain and dipolar models, which are based on conformations of the carbonyl compound not representing energy minima, Karabatsos proposed a different model assuming an early, reactant-like transition state in which the most stable conformation of the free carbonyl compound is preserved1314. Thus, the C-M bond eclipses the carbonyl double bond and, in order to minimize the energy of the transition state, the nucleophile approaches close to the small substituent on the stereogenic center (Figure 5). 

The quinonoid (as compared to aromatic) character of the ligand is strongly reflected in the inequality of the ring bond distances, two of the bonds being substantially shorter than the other C—C bonds of the ring (see Figs. 3.1-3.3). The carbonyl C—O distance is usually in the range of reported carbonyl double bonds (1.23-1.25 A).6 14 18 25... 

Mannig and Noth reported the first example of rhodium-catalyzed hydroboration to C=C bonds in 1985.4 Catecholborane reacts at room temperature with 5-hexene-2-one at the carbonyl double bond when the reaction was run in the presence of 5mol.% Wilkinson s catalyst [Rh(PPh3)3Cl], addition of the B—H bond across the C=C double bond was observed affording the anti-Markovnikoff ketone as the major product (Scheme 2). Other rhodium complexes showed good catalytic properties ([Rh(COD)Cl2]2, [ Rh(PPh3)2(C O )C 1], where... 

The hydroxycarbene isomer (H)Co(CO)3(CHOH) was also examined. It yielded a complex with molecular electronic energy more than 60 kcal/mole higher on the energy scale. The hydroxycarbene complex is not likely to play a significant role in the catalytic cycle. It is of some interest to inquire why the 18e hydroxycarbene complex (H)(CO) Co(=CH0H) is less stable than the 16e isomer (H)(CO)3C0(CH2O). The results suggest that the formation of the carbonyl double bond makes the critical difference. The electronically delocalized structure (H)(CO)3Co+5-CH2 0" may provide some extra stabilization for the formally unbonded formaldehyde moiety. The resonance form is dipolar and could be further stabilized by polar solvents. 

However, the ready distortion of the ar-electron system provides an additional mechanism whereby the charge dispersal can reach the substituents. The greater substituent effects in ketones compared to the alcohols are therefore equally consistent with the loss of an oxygen nonbonding electron. Unsaturated substituents which can conjugate with the carbonyl double bond do not have the expected large effect in reducing... 

Figure 7. The two choices for heterolysis of the carbonyl double bond, and their representation as points in Figure 8.

Isolated double and triple bonds are reduced readily, whereas conjugated alkenes and aromatic systems are difficult to hydrogenate. Carbonyl double bonds react only very slowly, if at all, so it is possible to achieve selective reduction of C=C double bonds in the presence of aromatic and carbonyl functions. 

Reduction of esters of carboxylic acids is complex and takes place in stages and by different routes. In the first stage hydrogen adds across the carbonyl double bond and generates a hemiacetal (route a), or else it can hydrogenolyze... 

Carbon dioxide is a symmetrical, linear triatomic molecule (0 = C=0) with a zero dipole moment. The carbon-to-hydrogen bond distances are about 1.16A, which is about 0.06A shorter than typical carbonyl double bonds. This shorter bond length was interpreted by Pauling to indicate that greater resonance stabilization occurs with CO2 than with aldehydes, ketones, or amides. When combined with water, carbonic acid (H2CO3) forms, and depending on the pH of the solution, carbonic acid loses one or two protons to form bicarbonate and carbonate, respectively. The various thermodynamic parameters of these reactions are shown in Table I. 

Progress in the polymerization of the carbonyl linkage did not result until there was an understanding of the effect of ceiling temperature (Tc) on polymerization (Sec. 3-9c). With the major exception of formaldehyde and one or two other aldehydes, carbonyl monomers have low ceiling temperatures (Table 5-13). Most carbonyl monomers have ceiling temperatures at or appreciably below room temperature. The low Tc values for carbonyl polymerizations are due primarily to the AH factor. The entropy of polymerization of the carbonyl double bond in aldehydes is approximately the same as that for the alkene double bond. The enthalpy of polymerization for the carbonyl double bond, however, is appreciably lower. Thus AH for acetaldehyde polymerization is only about 29 kJ mol-1 compared to the usual 80-90 kJ mol-1 for polymerization of the carbon-carbon double bond (Table 3-14) [Hashimoto et al., 1076, 1978],... 

Anionic polymerization yields a polymer containing two types of repeating units corresponding to reaction through both the alkene and carbonyl double bonds. 

Isomeric polymers can also be obtained from a single monomer if there is more than one polymerization route. The head-to-head placement that can occur in the polymerization of a vinyl monomer is isomeric with the normal head-to-tail placement (see structures III and IV in Sec. 3-2a). Isomerization during carbocation polymerization is another instance whereby isomeric structures can be formed (Sec. 5-2b). Monomers with two polymerizable groups can yield isomeric polymers if one or the other of the two alternate polymerization routes is favored. Examples of this type of isomerism are the 1,2- and 1,4-polymers from 1,3-dienes (Secs. 3-14f and 8-10), the separate polymerizations of the alkene and carbonyl double bonds in ketene and acrolein (Sec. 5-7a), and the synthesis of linear or cyclized polymers from non-conjugated dienes (Sec. 6-6b). The different examples of constitutional isomerism are important to note from the practical viewpoint, since the isomeric polymers usually differ considerably in their properties. 

Figure G shows some insertion reactions of carbonyl compounds. In the isocyanate and ketene cases, the addition takes place, not to the carbonyl double bond, but to the carbon—nitrogen or the carbon—carbon double bond.

The aldolization, a major synthetic tool involving a nucleophilic addition to the carbonyl double bond, is usually conducted under either acidic or basic conditions (Denmark and Lee, 1992). The yields of the aqueous aldolization logically follow the electrophilicity of the aldehyde, rising up to 82% using p-nitrobenzaldehyde. It has been shown that these aqueous aldolizations can be catalyzed by water-stable Lewis acids such as ytterbium or other lanthanide inflates (Kobayashi and Hachiya, 1992). 

Many papers have been published on the polymerization of methyl methacrylate (MMA) with alkylllthlum. It was reported that In the Initiation step an attack of alkylllthlun on monomer occurred In the first few seconds and took place on both olefinlc and carbonyl double bonds and a considerable amount of oligomer was formed in the early stage of the polymerization (1, 3 4 ... 

In the previous communication(7a) we postulated the initiation mechanism involving the attack of BuLi on the vinyl double bond of the butyl lsopropenyl ketone formed. However, from the results mentioned above we will propose the following mechanism for the polymerization of MMA with BuLi in toluene, which is slightly different from the previous one. On mixing the monomer with BuLi the initiator reacts with both olefinic and carbonyl double bonds of the monomer. The attack on the olefinic double bond produces the MMA anions, which add the monomer to form the growing chains(E). 

The coordination of gold complexes to the —C -system activates them very efficiently in order to attack a nucleophile. The carbonylic double bond can also be activated for nucleophilic addition. 

Carbonyl double bonds can also be photoelectrochemically reduced ZnS sols prepared from cold oxygen-free aqueous solutions ofZnSO4 and NajS induce efficient photodisproportionation of aldehydes, i.e., a photo-Canizzaro reaction Thus, acetaldehyde produces ethanol and acetic acid, together with smaller quantities of biacetyl and acetoin, Eq. (37). 

Although the mechanism of the reaction is unknown, Moore and Waters188 have shown that addition of a carbonyl radical to a carbonyl double bond occurs when benzaldehyde is irradiated in the presence of phenanthraquinone, yielding, ultimately, the hydroquinone monobenzoate. 

The second competing factor is the addition of triiluoromethyl radicals to the carbonyl double bond of the parent ketone.61 The per-... 

The high stability of the carbonyl double bond and the possibility for hydrogen bond formation probably accounts for the 2-hydroxythiophene derivative (101) occurring chiefly in the tautomeric form (102) rather than (103) (equation 24). 

Representative benzaldehyde derivatives [73k] and phenones [294] (Tables 4.30 and 4.31) display carbonyl shifts which are essentially influenced by steric repulsions and intramolecular hydrogen bonding. Steric repulsions by bulky alkyl groups in o, o position of the carbonyl group prevent coplanarity of carbonyl double bond and phenyl ring,... 

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