Iron-Catalyzed Coupling of Alkenes and Enones: Sakurai–Michael-type Conjugate Addition of Catalytic Allyliron Nucleophiles

Abstract

The iron-catalyzed coupling of alkenes and enones through allylic C(sp 3 )−H functionalization is reported. This redoxneutral process employs a cyclopentadienyliron(II) dicarbonyl catalyst and simple alkene substrates to generate catalytic allyliron intermediates for 1,4-addition to chalcones and other conjugated enones. The use of 2,4,6-collidine as the base and a combination of triisopropylsilyl triflate and LiNTf 2 as Lewis acids was found to facilitate this transformation under mild, functional group-tolerant conditions. Both electronically unactivated alkenes as well as allylbenzene derivatives could be employed as pronucleophilic coupling partners, as could a range of enones bearing electronically varied substituents.

Modern Organometallic C−H Functionalizations with Earth‐Abundant Iron Catalysts: An Update

“…Soon after, iron-catalyzed coupling of olefines 51 and enones 52 through allylic C(sp 3 )À H functionalization by cyclopentadienyliron(II) dicarbonyl catalyst was reported (Scheme 26). [66] Similarly to the previous reports, the [Cp*Fe(CO) 2 (thf)] + [BF 4 ] À catalyst is able to coordinate and enhance the CÀ H acidity of allyl benzene derivatives 51. Then, a pyridine base (collidine) can be used for the abstraction of the α-proton to deliver a nucleophilic allyliron intermediate.…”

“…Soon after, iron‐catalyzed coupling of olefines 51 and enones 52 through allylic C(sp 3 )−H functionalization by cyclopentadienyliron(II) dicarbonyl catalyst was reported (Scheme 26). [66] …”

A C–H functionalization approach to diverse nitrogenous scaffolds through conjugate addition of catalytic allyliron nucleophiles

“…Instead, we turned to silyl triflates as strong Lewis acids that irreversibly trap the incipient enolate ion intermediates as silyl enol ethers, which could be cleaved in a subsequent workup step to reveal the β-phthalimido ketone. 36 We posited that this initial trapping as the silyl enol ether could also prevent the occurrence of deamination as a side reaction. In preliminary experiments, we found that an acidic workup (in contrast to fluoride-mediated or base-mediated desilylation) led to good yields of the ketone and minimal formation of the deaminated product.…”

“…29–35 We further discovered that these nucleophilic organoiron species will undergo electrophilic functionalization with chalcone derivatives through a 1,4-addition process. 36…”

Synthesis, reactions and application of chalcones: a systematic review

“…Michael addition, a nucleophilic addition of β-carbon, is one of the most efficient methods in organic synthesis for producing C–C and C–X (X = O, N, or S) bonds (1,4-addition reaction). It is noted to be an important contender for producing an enantioselective Michael adduct in organocatalytic asymmetric synthesis 103–105 (Scheme 28).…”