Carbonyl compounds reactivity observations

The HDAR was also studied on reactive dienes such as cyclopenta-diene. In spite of the propensity of tyclopentadiene to dimerise, its reactivity with carbonyl compounds was observed in water. The capacity of water to accelerate the hetero cycloaddition was sufficient compared to the rate of the dimerisation in these conditions. As mentioned recently by Chisholm, the cycloaddition of cyclopentadiene with aldehydes are uncommon, but can be observed in water. 

The selective intermolecular addition of two different ketones or aldehydes can sometimes be achieved without protection of the enol, because different carbonyl compounds behave differently. For example, attempts to condense acetaldehyde with benzophenone fail. Only self-condensation of acetaldehyde is observed, because the carbonyl group of benzophenone is not sufficiently electrophilic. With acetone instead of benzophenone only fi-hydroxyketones are formed in good yield, if the aldehyde is slowly added to the basic ketone solution. Aldols are not produced. This result can be generalized in the following way aldehydes have more reactive carbonyl groups than ketones, but enolates from ketones have a more nucleophilic carbon atom than enolates from aldehydes (G. Wittig, 1968). 

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

Subsbtuting vinylic hydrogen in a,P-unsaturated carbonyl compounds with vinylic fluonne does not affect their dienophilic character negatively Indeed, 3,3-difluoroacrylic acid is more reactive toward furan than its nonfluonnated counterpart [95] (equation 81) Consistent with this observation is the fact that tctrafluorobenzoquinone forms only a bis-Diels-Alder adduct m 68% yield in its reaction with cyclopentadiene at room temperature [96, 97 ... 

Especially with the ordinary aldol reaction a number of side reactions can be observed, as a result of the high reactivity of starting materials and products. For instance, the a ,/3-unsaturated carbonyl compounds 4 can undergo further aldol reactions by reacting as vinylogous components. In addition compounds 4 are potential substrates for the Michael reaction. 

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

The observations recorded in Tables I and II reveal that no product was isolated when reaction of a sugar was attempted with the following classes of carbonyl-containing compound monocarbonyl compounds a-ketonic acids or esters y-carbonyl compounds and /3-carbonyl compounds yielding a high percentage of enolic form. A possible course (A) has been formulated for the reaction which occurs with appropriate substances this takes into consideration the mobility of the enol and the reactivity of the glyco-... 

A study518 of the mechanism of oxidation of alcohols by the reagent suggested that a reversible, oriented adsorption of the alcohol onto the surface of the oxidant occurs, with the oxygen atom of the alcohol forming a coordinate bond to a silver ion, followed by a concerted, irreversible, homolytic shift of electrons to generate silver atoms, carbon dioxide, water, and the carbonyl compound. The reactivity of a polyhydroxy compound may not, it appears, be deduced from the relative reactivity of its component functions, as the geometry of the adsorbed state, itself affected by solvent polarity, exerts an important influence on the selectivity observed.519... 

It can be noted that solvent has only marginal effect on the Fukui functions and hence local site selectivity. A systematic investigation has been made to study the effect of solvation on the local philicity indices of carbonyl compounds using B3LYP scheme employing direct calculation method [55], It is possible to observe from the results that solvation marginally influences the local reactivity profiles. 

Larger changes in bond lengths, as expected, are observed for more localized carbocations. Most of the structures available are for stabilized systems, such as protonated carbonyl compounds [e.g. the protonated cyclopropyl ketones referred to on page 110 (Childs et al., 1990), and dioxacarbocations (Paulsen and Dammeyer, 1973, 1976 Paulsen and Schuttpelz, 1979 Childs et al., 1986, 1991). It is normal to see one of the atoms of the counterion (in most cases MXJ or MX ) packing in the position expected for addition to the activated C=OH(R)+ system, apparently just within the sum of the van der Waals radii for the neutral centres (Childs et al., 1986). This can happen without significant pyramida-lization, however (Childs et al., 1991), and on both sides of the planar carbon centre it tells us little new about reactivity. 

As far as we are aware, these observations are the first that show that the well-known Norrish Type I reactions of p,7-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. This unusual reactivity may be due to the fact that the TiC-ir, -ir ) excited states of 53 and 55 possess sufficient energy to promote the homolytic allylic bond fission to form the stabilized pentadienyl radical 57. As a result, photodecarbony-lation competes favorably with the ODPM rearrangement. 

Since many of the reactions of C atoms are extremely exothermic, it may be that they proceed without an enthalpic barrier. Thus, selectivities observed in C atom reactions may result from free energy barriers in which entropy considerations are the major factor. In discussions of C atom reactions, we shall see that carbenes are often intermediates. Two ways in which carbenes can be produced in C atom reactions are C—H insertion (Eq. 7) and deoxygenation of carbonyl compounds (Eq. 8). In several cases, the same carbene has been generated by both methods. When this comparison has been made, the reactions will be discussed together even though they represent different aspects of C atom reactivity. 

The ke[ values of photoinduced electron transfer reactions from [Ru(bpy)3]2 + to various nitrobenzene derivatives in the presence of 2.0 mol dm-3 HC104 are listed in Table 1, where the substituent effect is rather small irrespective of electron-withdrawing or donating substituents. A similar insensitivity to the substituent effect is also observed in the acid-catalyzed photoinduced electron transfer from [Ru(bpy)3]2+ to acetophenone derivatives [46,47]. The stronger the electron acceptor ability is, the weaker is the protonation ability, and vice versa. Thus, the reactivity of substrates in the acid-catalyzed electron transfer may be determined by two reverse effects, i.e., the proton and electron acceptor abilities, and they are largely canceled out. Such an insensitive substituent effect shows a sharp contrast with the substituent effect on the acid-catalyzed hydride transfer reactions from Et3SiH to carbonyl compounds, in which the reactivity of substrates is determined mainly by the protonation ability rather than the electron acceptor ability. 

In all cases, superelectorophilic dicationic intermediates3 5 were suggested to be involved in the activation of carbonyl compounds based on the observation that protonated /V-heterocycles significantly enhance the reactivity of adjacent carbo-cationic centers. For example, cyclohexanone and acetophenone are unreactive toward benzene in triflic acid, whereas 4-piperidones252 and acetylpyridines254 react readily. Likewise, 3-pyridinecarboxaldehyde is able to alkylate deactivated... 

---