Olefins conjugate addition

With a substituent such as a methyl group on the double bond of the olefin, conjugate addition products are isolated in satisfactory yields (equation 73). 

Both 59 and 60 have been prepared efficiently from 2,3 5,6-di-0-isopropylidene-D-glucose using a Wittig olefination/conjugate addition/lactonisation (reminiscent of the strategy described last year by Shing Vol. 26, pp.306-307) to construct the bicyclic ring system. 

A Homer-Wads worth-Emmons (HWE) olefination/conjugate addition sequence was reported by Roush et al. for the synthesis of the bis-THP subunit of spongistatin 1 (Scheme 10) [30]. This example demonstrates how substitution affects product distribution of conjugate additions under thermodynamic control. 

Conjugate addition to nitro-olefins—1,4-diketone synthesis... 

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],... 

Among the olefination reactions, those of phosphonium ylides, phosphonate anions, silylmethyl anions, and sulfone anions are discussed. This chapter also includes a section on conjugate addition of carbon nucleophiles to a, (J-unsaturated carbonyl compounds. The reactions in this chapter are among the most important and general of the carbon-carbon bond-forming reactions. 

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... 

This reaction is extended to the intramolecular ring closure of the intermediate radical 224 with olefinic or trimethylsilylacetylenic side chains [121]. Cu(BF4)2 is also effective as an oxidant (Scheme 89) [122]. Conjugate addition of Grignard reagents to 2-eyclopenten-l-one followed by cyclopropanation of the resulting silyl enol ethers gives the substituted cyclopropyl silyl ethers, which are oxidized to 4-substituted-2-cyclohexen-l-ones according to the above-mentioned method [123]. (Scheme 88 and 89)... 

Michael addition in the absence of any catalytic agent has been reported for dialkyl and diaryl phosphites and thiophosphites with a-cyanoacrylate esters and a-cyanoacrylic acid.444 Yields of the conjugate addition products were moderate to good. The regiochemistry of this process is the opposite of that previously reported for similar additions to ketene acetals, the latter presumably proceeding by initial protonation of the distal olefinic carbon site.445... 

Other interesting electron-deficient olefin substrates for the asymmetric conjugate addition include cr-acet-amidoacrylic ester SI,101 dimethyl itaconate S2,114 ct,/3-unsaturated sulfones S3,115 and alkenylphosphonate S4116 (Figure 8). 

Recently considerable attention has been given to chiral 72-bonded olefin spectator ligands for the asymmetric conjugate addition.122 Hayashi and Carreira independently reported that new chiral diene ligands 83,123 84,124 and 85125 showed high enantioselectivities in Rh(i)-catalyzed conjugate addition of arylboronic acids to enones (Scheme 41). 

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... 

On the basis of the mechanism proposed for the addition of the same TeOj/LiBr/HOAc system to single olefins, the addition to conjugated dienes probably involves the acetoly-sis of a homoallylic and an allylic telluroacetate (or teUiirohaUde) generated by the initial addition of a Te(IV) species and an acetate (or halide) anion to the conjugated system. 

Sodium azide also adds to olefins of this t3rpe to give w-triazoles in fairly good yields. A mechanism involving nucleophilic displacement of the substituent X by azide, followed by cyclization of the vinyl azide in the presence of azide ions, has been suggested. An alternative mechanism involves conjugate addition of azide to the double bond, cyclization of the resulting anion, and aromatization. 

---