Carbonyl compounds, reactions at a position

Reaction of Ta-alkyne complexes with R R2C=0. The Ta complexes formed from TaCls/Zn with unsymmetrical alkynes react with carbonyl compounds to form two rcgioisomcric allylic alcohols with a ratio depending on the substituents on the alkync (both steric and electronic effects) as well as the size of the substituents in the carbonyl group. The complexes from acetylenic esters react with carbonyl compounds mainly at the position or to the ester group, whereas complexes from acetylenic amides react mainly at the position fi to the amide. 

The third major reaction of carbonyl compounds, alpha substitution, occurs at the position next to the carbonyl group—the alpha (a) position. This reaction, which takes place with all carbonyl compounds regardless of structure, results in the substitution of an a hydrogen by an electrophile through the formation of an intermediate enol or enolcite ion ... 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

Domino transformations combining two consecutive anionic steps exist in several variants, but the majority of these reactions is initiated by a Michael addition [1]. Due to the attack of a nucleophile at the 4-position of usually an enone, a reactive enolate is formed which can easily be trapped in a second anionic reaction by, for example, another n,(5-urisalurated carbonyl compound, an aldehyde, a ketone, an inline, an ester, or an alkyl halide (Scheme 2.1). Accordingly, numerous examples of Michael/Michael, Michael/aldol, Michael/Dieckmann, as well as Michael/SN-type sequences have been found in the literature. These reactions can be considered as very reliable domino processes, and are undoubtedly of great value to today s synthetic chemist... 

Reduction of a range of allyl and benzyl chlorides at a stainless steel cathode in dimethylformamide in the presence of carbonyl compounds and using a sacrificial anode of aluminium or zinc, leads to a Reformatsky-type reaction in 40-80 % yields. Allyl halide give products by reaction at both the a- and y-positions. Tetra-chioromethane and bromotrifluoromethane take part in similar reactions provided a... 

Aluminum ate complexes, (CH3)3SiCH -CHCH2 Al(C2H5)3Li+ (1). The ate complex is prepared by reaction of the lithium anion of allyltrimethylsilane with A1(C2Hs)3. In contrast with the anion of allyltrimethylsilane, which reacts with carbonyl compounds mainly at the y-position, 1 reacts selectively at the a-position (equation T). 

As indicated by the experimental and theoretical results described above, both pathways a and b of reaction of carbonyl compounds with nitriles (equation 77) include two variants of the rearrangement of nitrilium salts having an oxygen function at a-position (285) and a-iminoalkyloxycarbenium ions 288. Both rearrangements lead to A-acyliminium cations 289 see equation 94. 

There are two electrophilic positions in the chloro compound the carbonyl carbon and the carbon a to it, which bears chloride as a leaving group. Nucleophilic reaction at either position by 3-55 is a reasonable reaction, and we will illustrate both possibilities. Reaction at the carbonyl gives 3-56, which can close to a cyclopropane. 

This reaction allows the synthesis of (i) p,p -dienols by oxidation of p-hydroxy-y-alkenyl sel-enides or more conveniently from a-lithioalkyl selenoxides and enones (Scheme 136 and 166) (ii) p,5-dienols from l-lithio-3-alkenyl phenyl selenoxides and carbonyl compounds (Scheme 177) and (iii) 2-(r-hydroxyalkyl)-1,3-butadienes from 1-methylselenocyclobutyllithium and carbonyl compounds (Scheme 178). a,p-Unsaturated alcohols bearing a methylselenoxy or a phenylselenoxy group at the a-position do not lead on thermolysis to propargyl alcohols however, those be ng a (trifluoro-methylphenyl)selenoxy moiety at the a-position are valuable precursors of such con unds (Scheme 179). ... 

In connection with hydride ion transfers, it will be observed that addition should occur at a positive center. Since olefin additions are usually initiated by attack at a negative center, the Meerwein-Ponndorf-Verlcy reduction should occur preferentially at the carbonyl group of unsaturated carbonyl compounds. This is actually the case both in this reaction and in reductions carried out with lithium aluminum hydride.42... 

The indium-mediated reaction of cinnamyl bromide with 5-formyluracil derivatives gives the corresponding homoallylic alcohols. The presence of C-4 carbonyl is essential for high diastereoselectivity owing to complexation with indium (Scheme 8.32) [49]. Pentadienylindium, a vinylog of allylindium, reacts with carbonyl compounds selectively at the y position to give 1,4-pentadiene derivatives (Scheme 8.33) [50]. 

The dianion (62 Scheme 35), prepared from alkenethiols, gives good y-selectivity toward both alkylation and carbonyl addition. The y-products (enol sulfides) may be converted to dimethylacetals by treatment with mercury(II) chloride in methanol. When (62) is associated with magnesium dibromide, the addition of carbonyl compounds produces the a-adducts with regioselectivity greater than 90%. The reaction with methyl vinyl ketone, followed by a Cope rearrangement, results in reattachment at the original y-position. 

Anions derived from the aldimine of tiglaldehyde (14) react with carbonyl compounds preferentially at the a-position under conditions of kinetic control to give adducts (15), but products (16) derived from 7-attack are obtained under equilibrating conditions (Scheme 4). Addition of HMPA to the reaction or adduct mixture is required to promote isomerization of the initially formed a-adduct to the 7-product. There is also an increasing preference for 7-capture of the unsaturated imine anion as the degree of substitution a to the carbonyl function increases as in a-branched aldehydes and ketones (Table 2). Efforts to isomerize the initial a-adduct formed from reaction of the aldimine derived from crotonaldehyde with cyclohexanecarbaldehyde gave complicated mixtures. 

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