Phosphine ligands and nitrogen bases in the solvent-free Heck reaction of butenone with aryl iodides. A highly selective synthesis of benzalacetones

Abstract

The solvent-free palladium-catalyzed reaction of butenone with aryl iodides has been studied. Phosphine ligands have been found to affect the vinylic substitution to hydroarylation (conjugate addition type) ratio. The nature of the nitrogen base also plays a role in controlling the product selectivity. In the presence of Pd(OAc) 2 , tris-(2,4,6-trimethoxyphenyl)phosphine and proton sponge the reaction affords exclusively vinylic substitution products usually in high to excellent yield.

Palladium-Catalyzed Cyclization/Heck- and Cyclization/Conjugate-Addition-Type Sequences in the Preparation of Polysubstituted Furans

“…As noticed above, under the standard cyclization/coupling conditions, the reaction with methyl vinyl ketone departed from the general trend in that it provided 10a as major product in addition to the expected 9a (entry 5, Table ). The tendency of α,β-unsaturated ketones (and aldehydes) to give unwanted hydroarylation (conjugate addition) products with aryl halides or triflates, under typical Pd(0)-catalyzed Heck reaction conditions, is well-known. , In fact, that particular reactivity of methyl vinyl ketone (and related conjugated ketones and aldehydes) has already been exploited with nucleophile-tethered (oxygen- or nitrogen-based) unsaturated derivatives in Pd(II)-catalyzed cyclization/conjugate-addition reactions. , However, in the reported cases a relatively high acidity of the nucleophile (pH ≤ 5) has been found to be a requirement for adequate reactivity, whereas the use of relatively nonacidic alcohol nucleophiles has not been described so far . Therefore, the possibility of taking synthetic advantage of this divergent behavior to perform alternatively Heck- or conjugate-addition-type coupling sequences in the context of furan synthesis was explored next.…”