Carbonyl compounds addition-elimination reactions

In the reduction of acids there is a tendency for the lithium salt, RCO20Li to separate from the ethereal solution, and thus bring reduction to a halt this can be avoided by first converting the acid to a simple, e.g. Me or Et, ester. In the reduction of the latter, the initial nucleophilic attack by AIH4 results in an addition/elimination reaction—OR is a good leaving group in (40)—followed by normal attack, as above, on the resultant carbonyl compound (41) to yield the primary alcohol (42) ... 

One may assume that reactions of RP(SiMe3)2 with carbonyl compounds of the 0=C=X type afford addition products of the RP(SiMe3)—C(OSiMe3)=X type, or phosphaallenes of the RP=C=X type via an addition-elimination reaction. However, in many cases the reactions are not straightforward, as shown below. 

Moving from rearrangements, condensation reactions were also presented. Condensation reactions occur when two reactive species condense with one another forming a new compound. The first was the aldol condensation (Scheme 8.9). Later, a more complex application of the aldol condensation was presented in the form of the Robinson annula-tion (Scheme 8.10). For both of these reactions, the underlying lessons relate to the ability to induce reactions and incorporate substitutions at carbon atoms adjacent to carbonyl groups. Similar reactivities of such carbon atoms can be utilized for alkylation (SN2) and acylation (addition-elimination) reactions as illustrated in Scheme 8.11. 

Carbonyl Compounds II Nucleophilic Acyl Addition, Nucleophilic Acyl Substitution, and Nucleophilic Addition-Elimination Reactions of a, 6-Unsaturated Carbonyl Compounds... 

Contrary to some reports, electrophilic addition reactions may occur in other multiple-bond systems. In many of the reactions of aldehydes and ketones the first stage involves the addition of some entity across the carbon-oxygen bond, e.g., the formation of oximes, semicarbazones, hydrazones, hydrates (1,1-diols) and their ethers, and the aldol condensation. Most of these reactions entail a subsequent loss (elimination) of a small molecule e.g. water, ammonia, ethanol) and, while one must be careful to determine whether the rate-determining stage involves attack on the carbonyl compound or elimination from the adduct , there are some systems in which it is evident that electrophilic attack is involved in the slow stage of the reaction sequence. Examples of such reactions are the acid-catalysed formation of oximes of aliphatic - and aromatic carbonyl compounds, of furfural semi-carbazone , and of 1,1-diols from aldehydes or ketones . 

A condensation reaction is, therefore, caused due to the combination of two or more molecules to result into the formation of a new molecule with the elimination of a simple molecule, for instance water. In general, the carbonyl compounds undergo condensation reactions initially by the process of addition , and then followed by elimination, usually carried out under basic conditions. In fact, the condensation reactions are specifically of immense utility for the ultimate construction (synthesis) of a complex compound starting from rather simpler organic compounds. 

The addition-elimination reactions just described, because of their mechanistic complexities, do not permit an easy assessment of the relative reactivity of various carbonyl compounds. Such information can be obtained more directly by studying the rates of reactions of nucleophiles that form stable addition products with carbonyl compounds. The hydride reducing agents, in particular sodium borohy-dride, provide a convenient system for such studies ... 

Acyl transfer reactions (Section 17.4) A reaction in which a new acyl compound is formed by a nucleophilic addition-elimination reaction at a carbonyl carbon bearing a leaving group. 

Summary Carbonyl compounds readily undergo addition-elimination reactions ... 

Addition reactions such as addition of enolates to carbonyl compounds, photochemical cydoaddition, radical addition and elimination reactions such as dehydration can be carried out in microflow reactors. Addition-elimination reactions such as Wittig reaction, Homer-Wadsworth-Emmons reaction, Baylis-Hillman reaction. 

The nucleophilic reactivity of the nitrogen atoms in thioureas towards carbonyl compounds has been further exemplified by some recently described addition and addition-elimination reactions of thioureas with aldehydes. The same reactivity also played an important part in recent syntheses of heterocyclic compounds containing the thioureido-grouping. ... 

The radical addition of sulfinates to unsaturated compounds via the iodosulfonylation-dehydroiodination reaction sequence constitutes a general method for the preparation of vinyl sulfones the latter may be rearranged to aUyUc sulfones by treatment with base. The radical addition may be carried out on a, -unsaturated carbonyl compounds as well as alkenes. In the case of unsaturated carbonyl compounds the elimination process can be quite stereoselective, ( )-alkenes being normally formed. For the addition to nonconjugated alkenes, conditions have been described for the preparation of either ( )- or (. -alkenes. ... 

One of the important mechanistic considerations involved in addition and addition-elimination reactions of carbonyl compounds is the precise sequence of events. In particular, a major focus is on whether specific or general catalysis is involved in these reactions. In Chapter 10 we will consistently state whether the reactions are subject to general or specific catalysis. Let s examine the factors under which these various mechanisms operate. Figure 9.13 A shows a two-step process involving nucleophilic addition followed by protonation. The first step is rate-determining. The acid is not part of the kinetic equation, and therefore there is no acid catalysis of any kind, specific or general. This mechanism occurs for strong nucleophiles. As we will see in Chapter 10, the addition of cyanide to an aldehyde is one example. 

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