Conjugate addition and elimination sequenc

Scheme 8.15. Conjugate addition and elimination sequence, resulting in overall Sn2 substitution.

Cyclopropanation. The reaction of sulfonium ylides 96 with acrylic esters or methyl vinyl ketone provides rrany-2-arylcyclopropanecarboxylic esters and methyl ketones. Generation of the ylides with EtNP(NMe)2-N=P(NMe)j as base is convenient because dichloromethane can be used as solvent. Another synthesis of cyclopropanecarboxylates proceeds by a conjugate addition and 1,3-elimination sequence. ... 

A new method for a-phenol annulation involving base-induced cycloaromatization of readily available 4-bis (methylthio)-3-buten-2-one 114 was applied to 3,4-dihydro-l-benzothiepin-5(2//)-one 113 (Scheme 14) <2002JOC5398>. Equimolar quantities of the benzothiepinone 113 and 114 in the presence of sodium hydride were stirred in dimethylformamide (DMF) at 25 °C. Treatment of the reaction mixture with/>-toluenesulfonic acid in refluxing benzene furnished the phenol-annulated dihydrothiepine 116 in 62% yield. The reaction sequence involved formation of conjugate addition-elimination adduct 115, followed by intramolecular aldol condensation and cycloaromatization, affording 116. 

Multiple-component difunctionalization reactions of a,/ -unsaturated carbonyl systems have been achieved by catalytic conjugate addition/aldol sequences. As Scheme 8.13 illustrates, an efficient method reported by Montgomery [46] allows regioselective addition of an aryl iodide to the /i-position of an unsaturated ester under nickel catalysis and subsequent trapping with an aldehyde to give / -hydroxyesters (e.g. 33). Significantly, premature termination of the sequence by the /Miydride elimination process that is usually observed in Pd-catalyzed Heck reactions does not occur here. 

In accordance with the generally accepted mechanism ofthe MBH reaction, the aza MBH reaction involves, formally, a sequence of Michael addition, Mannich type reaction, and (3 elimination. A commonly accepted mechanism is depicted in Scheme 13.2. A reversible conjugate addition of the nucleophilic catalyst to the Michael acceptor generates an enolate, which can intercept the acylimine to afford the second zvdtterionic intermediate. A proton shift from the a carbon atom to the P amide followed by P elimination then affords the aza M BH adduct with concurrent regeneration of the catalyst [5]. 

Formally, the MBH reaction involves a sequence of Michael addition, aldol reaction and P-elimination. The commonly proposed mechanism consists in a reversible conjugate addition of the nncleophile to the starting enone 237, generating an intermediate enolate 238. This enolate reacts with the electrophilic aldehyde in an aldol-type process, in which two stereogenic centers are formed, to give 239, which suffers an intramolecnlar acid-base eqnilibrinm to give another enolate 240. From this intermediate, the p-elimination of the nncleophile provides the MBH product... 

---