Carbonyl compound mechanism

Lowry TH, Richardson KS. Reactions of carbonyl compounds. Mechanism and Theory in Organic Chemistry. 2nd ed. New York Harper and Row, 1981 596-598. 

CHAPTER 18 Reactions at the a-Carbon of Carbonyl Compounds MECHANISM FOR THE ALDOL ADDITION... 

The following mechanism of the reaction has been suggested it includes the coordination of the carbonyl compound with the aluminium atom in aluminium sopropoxide and the transfer of a hydride Ion ... 

So far in this section we have combined enolate anions with other carbonyl compounds by direct attack at the carbonyl group. We can expand the scope of this reaction by using a,p-unsaturated carbonyl compounds as the electrophiles. This is the Michael reaction. Remind yourself of tliis by writing out the mechanism of a Michael reaction such as ... 

The use of a reagent bearing a basic center or the addition of a base to the reaction mixture was recognized as necessary to prevent the acid-catalyzed elimination of the elements of water from the intermediates. Since the publication of this work, a number of similar intermediates have been isolated from thioamides and a-halogeno carbonyl compounds (608, 609, 619, 739, 754, 801), and as a result of kinetic studies, the exact mechanism of this reaction has been well established (739, 821). 

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

You have had earlier experience with enols m their role as intermediates m the hydration of alkynes (Section 9 12) The mechanism of enolization of aldehydes and ketones is precisely the reverse of the mechanism by which an enol is converted to a carbonyl compound... 

Study of the mechanism of this complex reduction-Hquefaction suggests that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the ceUulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (46). In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur will not be developed. However, the Hquefaction mechanism probably involves catalytic hydrogenation because carbon monoxide would be expected to form at least some hydrogen by the water-gas shift reaction. 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

The carbonyl compound (43) has also been synthesi2ed, and its fluorescence spectmm has been shown to match the bioluminescence spectmm under equivalent conditions (214). The details of the excitation step are unclear and a dioxetanone mechanism (59,215) may apply to the reaction. 

The mechanism of both syntheses has been studied in detail, and well summarized (44,45). Interesting questions remain for example, in neither of these sequences is it certain whether the carbonyl compound or its Schiff base is undergoing Michael addition. 

In the alcohol oxidations, the sulfonium intermediate (2, nucleophile = R2C(OH)) loses a proton and dimethyl sulfide to give the carbonyl compound (42). The most common mechanism for the decomposition of (2) is attack by a mild base to remove a proton from one of the methyl groups. Subsequent cycHc coUapse leads to the carbonyl compound and dimethyl sulfide (eq. 9) ... 

The synthesis of A -pyrazolines by reaction of a 1,2-disubstituted hydrazine, formalin and a carbonyl compound, known as the Hinman synthesis, probably proceeds by the mechanism shown in Scheme 57 (69BSF3300). 

When two moles of a carbonyl compound are used instead of formalin, the mechanism is different (Scheme 58) (70BSF3147). In one example (80CCC2417) the product of the nucleophilic addition of the hydrazine to the pyrazolinium salt (635 R = = Ph, R = R" =... 

Oxaziridines are generally formed by the action of a peracid on a combination of a carbonyl compound and an amine, either as a Schiff base (243) or a simple mixture. Yields are between 65 and 90%. Although oxygenation of Schiff bases is formally analogous to epoxidation of alkenes, the true mechanism is still under discussion. More favored than an epoxidation-type mechanism is formation of a condensation product (244), from which an acyloxy group is displaced with formation of an O—N bond. 

There have been numerous studies of the rates of deprotonation of carbonyl compounds. These data are of interest not only because they define the relationship between thermodynamic and kinetic acidity for these compounds, but also because they are necessary for understanding mechanisms of reactions in which enolates are involved as intermediates. Rates of enolate formation can be measured conveniently by following isotopic exchange using either deuterium or tritium ... 

The second step in acetal and ketal hydrolysis is conversion of the hemiacetal or hemiketal to the carbonyl compound. The mechanism of this step is similar to that of the first step. Usually, the second step is faster than the initial one. Hammett a p plots and solvent isotope effects both indicate that the transition state has less cationic character than... 

There is another possible mechanism for addition of organometallic reagents to carbonyl compounds. This involves a discrete electron-transfer step. °... 

The distinguishing feature of this mechanism is the second step, in which an electron is transferred fixim the organometallic reagent to the carbonyl compound to give the radical... 

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. 

The mechanism of the reaction between a hydrocarbon organohthium compound and a carbonyl compound is well-documented [69] and has been suggested earlier for fluonne-contaming compounds [32, 68, 70] More recent studies on the mechanism have resulted in a better understanding of this reaction [71] (equauon 37)... 

The mechanism of the original Knorr pyrrole synthesis entails in situ reduction of the oxime moiety to an amine, condensation with the second carbonyl compound, and cyclization with loss of a second molecule of water to give a pyrrole for example, 10 + 11 to 12. Several studies have demonstrated that different pathways and pyrrole products obtain depending on the substrates. 

The Pictet-Spengler condensation of indole bases and carbonyl compounds to form 3-carbolines involves a slightly different mechanism than the isoquinoline... 

The Basic Mechanisms of Cycloaddition Reactions of Carbonyl Compounds with Conjugated Dienes... 

There have been few mechanistic studies of Lewis acid-catalyzed cycloaddition reactions with carbonyl compounds. Danishefsky et ah, for example, concluded that the reaction of benzaldehyde 1 with trans-l-methoxy-3-(trimethylsilyloxy)-l,3-di-methyl-1,3-butadiene (Danishefsky s diene) 2 in the presence of BF3 as the catalyst proceeds via a stepwise mechanism, whereas a concerted reaction occurs when ZnCl2 or lanthanides are used as catalysts (Scheme 4.3) [7]. The evidence of a change in the diastereochemistry of the reaction is that trans-3 is the major cycloaddition product in the Bp3-catalyzed reaction, whereas cis-3 is the major product in, for example, the ZnCl2-catalyzed reaction - the latter resulting from exo addition (Scheme 4.3). 

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

The mechanism for the hetero-Diels-Alder reaction of benzaldehyde 9 with the very reactive diene, Danishefsky s diene 10, catalyzed by aluminum complexes has been investigated from a theoretical point of view using semi-empirical calculations [27]. The focus in this investigation was to address the question if the reaction proceeds directly to the hetero-Diels-Alder adduct 11, or if 11 is formed via a Mukaiyama aldol intermediate (Scheme 8.4) (see the chapter dealing with hetero-Diels-Alder reactions of carbonyl compounds). 

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