Iron-Catalyzed Contrasteric Functionalization of Allenic C(sp²)–H Bonds: Synthesis of α-Aminoalkyl 1,1-Disubstituted Allenes

Abstract

An iron-catalyzed C–H functionalization of simple monosubstituted allenes is reported. An efficient protocol for this process was made possible by the use of a newly developed electron-rich and sterically hindered cationic cyclopentadienyliron dicarbonyl complex as the catalyst and N-sulfonyl hemiaminal ether reagents as precursors to iminium ion electrophiles. Under optimized conditions, the use of a mild, functional-group-tolerant base enabled the conversion of a range of monoalkyl allenes to their allenylic sulfonamido 1,1-disubstituted derivatives, a previously unreported and contrasteric regiochemical outcome for the C–H functionalization of electronically unbiased and directing-group-free allenes.

Recent Advances in Non-Precious Metal Catalysis

“…Wang and co-workers reported the Fe-catalyzed C−H functionalization of allenes by using an optimized structure of an electron-rich cationic cyclopentadienyliron dicarbonyl complex as the catalyst and N-sulfonyl hemiaminal ether reagents as precursors to iminium ion electrophiles. 99 The optimized conditions indicated in Scheme 50 were identified by screening bases, N-protecting groups, and Lewis acids. A wide range of monoalkyl allenes were converted to their corresponding allenylic sulfonamido 1,1-disubstituted derivatives in good yields.…”

Designed Iron Catalysts for Allylic C−H Functionalization of Propylene and Simple Olefins

“…Electrophilic attack of the resulting η 1 ‐allyliron species with S E 2′ selectivity then affords the product of net α‐C−H functionalization (Scheme 2a). This catalytic mode has been successfully applied to propargylic and allenylic C−H functionalization, and allylic C−H functionalization at electronically activated sites, delivering a diverse range of products containing new C−C bonds [19a,c,e–f] . By contrast, simple unfunctionalized olefins performed with substantially inferior reactivity, even after further optimization of reaction conditions.…”

Allene C(sp2)–H Activation and Alkenylation Catalyzed by Palladium

“…This approach has initially been demonstrated for electronically biased allenyl-esters by Feng employing a gold catalyst . Wang subsequently reported the functionalization of electronically unbiased monosubstituted allenes through iron catalysis . Thus, unlike for olefins and arenes, no catalytic C–H activation process for allenes has been reported.…”

“…21 Wang subsequently reported the functionalization of electronically unbiased monosubstituted allenes through iron catalysis. 22 Thus, unlike for olefins and arenes, no catalytic C−H activation process for allenes has been reported. We anticipated that the development of allene C−H activation and functionalization with acrylates would contribute to the exciting field of C−H activation.…”