Michael reaction reversal

Both C-S bonds are now P to carbonyl groups and so can be discomiected in turn by reverse Michael reactions. 

Hydroxyethyl Undergo reverse Michael reaction readily (lose H2O) /3-Hydroxy ketones... 

With any substrate, when Y is an ion of the type Z—CR2 (Z is as defined above R may be alkyl, aryl, hydrogen, or another Z), the reaction is called the Michael reaction (see 15-21). In this book, we will call all other reactions that follow this mechanism Michael-type additions. Systems of the type C=C—C=C—Z can give 1,2, 1,4, or 1,6 addition. Michael-type reactions are reversible, and compounds of the type YCH2CH2Z can often be decomposed to YH and CH2=CHZ by heating, either with or without alkali. 

Mannich bases (see 16-15) and p-halo carbonyl compounds can also be used as substrates these are converted to the C=C—Z compounds in situ by the base (16-15, 17-12). Substrates of this kind are especially useful in cases where the C=C—Z compound is unstable. The reaction of C=C—Z compounds with enamines (12-18) can also be considered a Michael reaction. Michael reactions are reversible. 

The primary amino group must come from a nitrile (21) and this time the Michael reaction is reversed ... 

Example Compound (21) may not look like a Robinson annelation product, but it is certainly an enone so a,B disconnection gives a 1,5-di-carbonyl compound A reverse Michael reaction disconnecting the ring from the chain gives enone (22),... 

There are two 1,3-diX disconnections (p T 48 ), that is reverse Michael reactions. 

Neither chiral centre in (1) can be epimerised by acid orbase, but a reverse Michael reaction, which should happen in base, provides both a suitable disconnection and a way to epimerise one chiral centre. 

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

Aldol and -Michael reactions can also proceed on the reaction products of retro-Aldol and -Michael reactions. The reverse (direct) Aldol and Michael reaction can also proceed on various intermediates. Hence, these few reactions can already form a very large variety of possible products. They, indeed, account for most of the reactivity of carbohydrates discussed below, being under pyrolysis, hydrolysis or fermentation conditions. 

The reaction is promoted by a variety of bases, usually in catalytic quantities only, which generate an equilibrium concentration of carbanion (92) it is reversible, and the rate-limiting step is believed to be carbon-carbon bond formation, i.e. the reaction of the carbanion (92) with the substituted alkene (91). Its general synthetic utility stems from the wide variety both of substituted alkenes and of carbanions that may be employed the most common carbanions are probably those from CHjfCOjEtlj—see below, MeCOCHjCOjEt, NCCH -COjEt, RCH2NO2, etc. Many Michael reactions involve C=C—C=0 as the substituted alkene. 

Michael reactions are base catalyzed and reversible, and so it is common to use either the nitronate salt of the nitroalkane substrate or the nitroalkane in the presence of a catalytic amount of alkali metal hydroxide, alkoxide or amine base. 

Treatment with a base results in the ehmination of the alcohol at the 5 position by what is in effect a reverse Michael reaction to give the enone (19-1). The additional... 

In general, substituents removed from the ring by two or more saturated carbon atoms undergo normal aliphatic reactions. A notable exception is the reverse Michael reaction which is undergone by p -substituted ethyl compounds such as 2-(p-hydroxyethyl)pyridine (see Section 3.2.3.4.4). 

Compounds of type Het-CH2CH2X where X is a leaving group (e.g. halogen, OR, NR2, SR, etc.) undergo reverse Michael reaction via... 

The stability of an a-silyl carbanion is responsible for the unproved synthetic utility of the Stork annulation over other annulations195,196. These reactions involve the Michael addition of an enolate ion to an enone, and in the absence of a a-silyl substituent suffer drawbacks due to the reversibility of the Michael reaction. However, the addition of enolate ions to a-trimethylsilylvinyl ketones is not reversible, owing to a-silicon stabilization of the canonical form 152 shown in equation 122. 

The intramolecular carbon-carbon bond-forming reactions considered in this section are based on the aldol condensation (see Section 5.18.2, p. 799), the Claisen-Schmidt reaction (see Section 6.12.2, p. 1032), the Claisen ester condensation (see Section 5.14.3, p. 736), and the Claisen reaction (see Section 6.12.2, p. 1032). Since these carbonyl addition reactions are reversible, the methods of synthesis are most successful for the formation of the thermodynamically stable five- and six-membered ring systems. The preparation of the starting materials for some of these cyclisation reactions further illustrates the utility of the Michael reaction (see Section, 5.11.6, p. 681). 

Alternatively, Michael reaction of the nonaol with acrylonitrile yields a nonanitrile, which can be reduced to the nonaamine. This nonaamine was allowed to react with chloro-carbonylmetallocenes and other chlorocarbonyl sandwich complexes to yield nonaamido-metallocenes [61-63] and nonaamido-sandwich compounds [63] (Scheme 20). These metallodendrimers also give rise to only one cyclic voltammetry wave, the intensity of which corresponds to approximately nine electrons using the technique indicated above (the solvent was CH2CI2 for the nonaamidoferrocene and MeCN for the polycationic dendrimers). Chemical reversibility was observed at room temperature, although some adsorption was noted. 

Analysis The electrophile is an enono since a reverse Michael reaction cleaves the C-N bond ... 

Hydroxyethyl Undergo reverse Michael reaction (loss of H20) 3.2.3.4.4 -Hydroxy ketone... 

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