Highly Selective Metal Catalysts for Intermolecular Carbenoid Insertion into Primary CH Bonds and Enantioselective CC Bond Formation

Abstract

Direct functionalization of CÀH bonds is an appealing strategy in organic synthesis [1] but its practical application has so far been difficult to realize. The selective functionalization of primary C À H bonds of alkanes that also contain secondary and/or tertiary C À H bonds is a great challenge, as CÀH bond energy follows an order primary > secondary > tertiary. [1c,d] In seminal works by Bergman, [1b] Jones, [1c] and their respective co-workers, stoichiometric reactions of alkanes with [Cp*(Me 3 P)M] (Cp* = C 5 Me 5 ; M = Rh, Ir) resulted in the formation of C À M bonds by selective activation of primary C À H bonds. Subsequent work by Hartwig and coworkers [1g,i, 2] demonstrated C À B bond formation by stoichiometric and catalytic functionalization of primary CÀH bonds mediated by tungsten, rhodium, or ruthenium complexes. The high selectivity for primary CÀH bond functionalization in these C À M or C À B bond-formation reactions (Scheme S1 in the Supporting Information) is considered to result from kinetic factors or steric interaction between the metal complexes and alkanes. [1i, 3] A well-established process in CÀC bond formation by direct CÀH bond functionalization is the metal-catalyzed intra-and intermolecular carbenoid insertion into C À H bonds, with diazo compounds as the carbene source. [1o, 4] These catalytic C À C bond-formation reactions generally feature lower selectivity for primary CÀH bonds than for secondary and tertiary CÀH bonds. For example, a selectivity order of primary < secondary < tertiary C À H bonds has been observed for the extensively investigated carbene insertion catalyzed by rhodium complexes, [4,5] possibly because of the electron density order of primary < secondary < tertiary CÀH bonds, which renders primary CÀH bonds the least susceptible to attack by electrophilic rhodium-carbene intermediates. [5] By manipulating the steric or electronic properties of the metal catalysts, a selectivity for primary C À H bonds of alkanes comparable to that for secondary or tertiary C À H bonds was observed, [6] with the highest primary/secondary and primary/tertiary ratio per CÀH bond being 1.17:1.0 [6b] and 1.0:0.9, [6c] respectively.Herein we report a highly selective primary CÀH bond functionalization by metal-catalyzed carbenoid insertion into the C À H bonds of alkanes (Scheme 1), which features a primary/secondary selectivity (that is, the primary/secondary ratio per C À H bond) of up to 11.4:1. We have also accomplished highly enantioselective functionalization of secondary C À H bonds with ee values of up to 93 % and product turnovers up to 6100 through metal-mediated carbenoid CÀH bond insertion reactions.Our studies in this work were inspired by previous work from the research groups of Callot [6a,b] and Suslick. [7] In the 1980s, Callot and co-workers reported that the primary C À H bond selectivity for the reaction of linear alkanes with ethyl diazoacetate (N 2 CHCO 2 Et, EDA) catalyzed by [Rh(por)I] (H 2 (por) = meso-tetraarylporphyrin) increases with the size of...

Rhodium(II)‐ or Copper(I)‐Catalyzed Formal Intramolecular Carbene Insertion into Vinylic C(sp2)−H Bonds: Access to Substituted 1H‐Indenes

“…[1] Previous studies on carbene insertion into C À Hb onds have focused on aliphatic C(sp 3 ) À Hb onds.F or example,Davies and co-workers have demonstrated remarkable regio-and stereoselectivity for intermolecular C(sp 3 )ÀH bond insertion reactions with donor-acceptor carbene precursors and chiral Rh II catalysts. [3] PØrez and co-workers reported as ilvercatalyzed carbene insertion into methane CÀHb onds in supercritical CO 2 . [3] PØrez and co-workers reported as ilvercatalyzed carbene insertion into methane CÀHb onds in supercritical CO 2 .…”

Rhodium(II)‐ or Copper(I)‐Catalyzed Formal Intramolecular Carbene Insertion into Vinylic C(sp2)−H Bonds: Access to Substituted 1H‐Indenes

“…[2] With ruthenium(II)porphyrin complexes as the catalysts,C he and co-workers demonstrated selective carbene insertion into primary C À H bonds of alkanes. [3] PØrez and co-workers reported as ilvercatalyzed carbene insertion into methane CÀHb onds in supercritical CO 2 . [4] Tr ansition-metal-catalyzed carbene insertion into C(sp 3 ) À Hb onds is considered au nique method for inert C(sp 3 ) À Hb ond functionalization, which has attracted significant attention in recent years.…”