Olefins, activated, conjugate addition

Some other activated olefins have been employed as acceptors in Michael reactions with enolates under PTC conditions, using different chiral ammonium salts as catalysts. For example, a very recent and elegant report has shown that conveniently substituted heterocyclic moieties can also play the role of suitable electron releasing groups for the activation of an olefin toward conjugate addition. In particular 4-nitro-5-styrylisoxazoles were found to undergo clean... 

The mechanism of the cyanide- and thioazolium ion-catalyzed conjugate addition reactions is considered to be analogous to the Lapworth mechanism for the cyanide-catalyzed benzoin condensation. Thus the cyano-stabilized carbanion resulting from deprotonation of the cyanohydrin of the aldehyde is presumed to be the actual Michael donor. After conjugate addition to the activated olefin, cyanide is eliminated to form the product and regenerate the catalyst. 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

The hydrocyanation of conjugated carbonyl compounds is a related reaction.83 Very often such a conjugated addition is carried out in aqueous conditions. For example, in the pioneer work of Lapworth, hydrocyanation of activated olefins was carried out with KCN or NaCN in aqueous ethanol in the presence of acetic acid (Eq. 10.36).84... 

B-alkylcatecholboranes, prepared by rhodium(I)-catalyzed hydroboration of alkenes, are suitable radical precursors for conjugate addition to activated olefins. This procedure is particularly useful for the control of the regio- and... 

Scheme 37 Radical chain mechanism for the conjugate addition of B-alkylcatecholboranes to activated olefins (R = alkyl group EWG = electron withdrawing group R O =Me0C(0)0, MeO )...

The conjugated addition of aryl bromides to activated olefins has... 

The amount of cobalt complex in this step influences the reaction rate, but not the yields. Indeed, with only 0.3 equivalent of cobalt catalyst, the arylzinc compound is consumed after 24 h instead of 10 h when 1 equivalent was used. An excess of the activated olefin is required to optimize the yield of the conjugate addition. Under these conditions, this process has been studied with various aryl halides (X = Br, Cl) and activated olefins. Yields range from 40 to 80%. 

The most reactive Michael acceptors, such as alkylidene malonates, gem-dicyanoalkenes and nitroalkenes, react with a-halozinc esters in a conjugate fashion. Beautiful examples were offered by two stereocontrolled conjugate additions to piperidinone 102 and pyrro-lidinone 104 leading to optically active bicyclic lactams 103147 (equation 60) and 105 (equation 61)148. With these electron-poor alkenes a Grignard two-step protocol is to be adopted in order to avoid the single electron transfer reactions from the metal to the Michael acceptor, which should afford olefin dimers. The best solvent is found to be a... 

The 1,4-addition (or conjugate addition) of resonance-stabilized carbanions. The Michael Addition is thermodynamically controlled the reaction donors are active methylenes such as malonates and nitroalkanes, and the acceptors are activated olefins such as a,P-unsaturated carbonyl compounds. 

In general, the zinc promoted condensation of alkyl halides with aliphatic aldehydes and ketones gives only poor results.2 In contrast good results are obtained when one of the reactants is unsaturated. Thus the addition of an allylic group to a carbonyl compound, or the conjugate addition of an alkyl moiety to an activated olefin, occur in excellent yields. Some corrections to this schematic view will be given in the following discussion. 

Keywords Organozinc, Organolithium, Grignard reagent, Copper, Conjugate addition, Nucleophilic addition, Alkylation, Activated olefin, Enone, Nitroalkene, p-Substituted carbonyl... 

Suppl.to Chapter 31.1 Conjugate Addition of Organometallics to Activated Olefins... 

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