C₁‐symmetric β‐Diketiminatoiron(II) Complexes for Hydroamination of Primary Alkenylamines

Abstract

The synthesis and solid-state characterization of an array of well-defined low-coordinate C 1 -symmetric b-diketiminatoiron(II) alkyl complexes B 3 -B 6 featuring steric and electronic variations on one of the N-aryl substituents of the bdiketiminate ligand scaffold are reported. All complexes display unique catalytic abilities of promoting the selective exo-cyclohydroamination of unprotected 2,2-diphenylpent-4-en-1-amine (1 a) under mild reactions conditions. The incorporation of a potentially coordinative ortho-methoxy substituent on one of the N-aryl rings of the bdiketiminate skeleton, in conjunction with a more crowded 2,6-diisopropylphenyl group on the other, affords a far more active catalyst (B 3 ) than our previously reported C 2 -symmetric b-diketiminatoiron(II) alkyl complex B (Ar 1 = Ar 2 = 2,4,6-(Me) 3 C 6 H 2 )/cyclopentylamine system. Comparative studies let us postulate that this superior activity of B 3 , when compared with B 4 -B 6 , is likely arising from steric effects and/or the coordinating ability of the ortho-methoxy substituent. The scope and limitations of this novel C 1 -symmetric b-diketiminatoiron (II) alkyl complex B 3 are also presented.In the last decades, the catalytic alkene hydroamination reaction has emerged as an appealing methodology for the synthesis of important N-containing building blocks from relative inexpensive and ubiquitous amines and alkenes as starting materials. [1] The recent interest for sustainable catalysis has prompt the development of hydroamination catalytic systems based on first row late transition metal of high availability, low price and relatively low toxicity. At present, noteworthy advances in this nascent field have be gained by systems derived from iron, [2] cobalt, [3] copper [4] and zinc [5] metal thanks to classical NÀH addition and alternative formal approaches such as electrophilic amination [2e,4b-e] or hydrogen-atom [2f-h,3a] transfer. Although these formal approaches have significantly broaden the reaction scope and unlocked some unresolved issues, the requirement for sophisticated electrophilic amine partners and/or the use of a large excess of reducing agent are detrimental to the step and atom efficiency of the overall process. Therefore there is still a need to tackle the search for efficient earth-abundant, first-row late transition metal catalysts for direct NÀH bond addition of simple amines on alkenes under functional group-free manipulation and co-reagent-free conditions. In this context, our group has reported the first examples of iron(II)and cobalt(II)-mediated alkene exo-cyclohydroamination of electronically unbiased primary amines using structurally-defined C 2 -symmetric low-coordinate bdiketiminatometal alkyl complexes under mild reaction conditions (Scheme 1a) for iron). [2c,d,3b,6] Inspired by our comprehensive mechanistic investigations conducted recently on iron systems [2d] and a precedent DFT study, [7] we have synthesized and herein report well-defined C 1 -symmetric b-diketiminatometal alkyl complexes with signif...

Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions

“…These metrical parameters stand in marked contrast to those reported by Hannedouche for an iron complex with related β-diketiminate ligands, which interact with only one o -OMe group (Fe–O distance = 2.465 Å) to form a five-coordinate complex that has a high-spin ground state based on the metrical data. 20 …”

Alkene Hydroamination via Earth‐Abundant Transition Metal (Iron, Cobalt, Copper and Zinc) Catalysis: A Mechanistic Overview

“…A hydroamination selectivity of 95% is reached at 93% conversion of 1c after only 250 min of reaction with 2b‐Fe whereas a reaction time of up to 48 h was needed to attain such selectivity with 2a‐Fe /cyclopentylamine. The better efficiency of 2b‐Fe may be ascribed to the coordinating ability and/or steric hindrance of the ortho ‐methoxy substituent as suggested by comparative studies …”

“…To bypass this drawback in catalyst reactivity linked to use of the cyclopentylamine additive, an alternative strategy involving rational modification of the β‐diketiminate ligand was recently explored to target high hydroamination selectivity . From their mechanistic DFT investigations and a previous computational study, the authors hypothesized that enhancing the steric demands around the metal should disfavor the competitive β‐H elimination pathway and consequently afford higher selectivity in favor of the hydroamination product.…”

“…From their mechanistic DFT investigations and a previous computational study, the authors hypothesized that enhancing the steric demands around the metal should disfavor the competitive β‐H elimination pathway and consequently afford higher selectivity in favor of the hydroamination product. Investigations of the catalytic behavior of diverse well‐defined C 1 ‐symmetric β‐diketiminatoiron(II) alkyl complexes featuring steric and electronic variations on one of the N ‐aryl substituents of the ligand scaffold highlight that complex 2b‐Fe bearing a potentially coordinative ortho ‐methoxy substituent is a significantly more active and selective catalyst for the cyclohydroamination of primary aminoalkenes (Scheme ) . A hydroamination selectivity of 95% is reached at 93% conversion of 1c after only 250 min of reaction with 2b‐Fe whereas a reaction time of up to 48 h was needed to attain such selectivity with 2a‐Fe /cyclopentylamine.…”

Earth-Abundant 3d Transition Metal Catalysts for Hydroalkoxylation and Hydroamination of Unactivated Alkenes

“…It should be noted that the presence of cyclopentylamine, a non cyclizable primary amine as co-catalyst diminishes the formation of β-H elimination products but also reduces the reaction rate. To overcome this limitation and relying on experimental and computational mechanistic studies, the group subsequently developed the synthesis of a variety of well-defined C1symmetric -diketiminatoiron(II)-alkyl complexes that differ by the nature of one aryl group on the N-atom and studied their reactivities [65]. It was shown that steric effects Last year, Chen, Jiang and co-workers published a general catalytic system, composed of Fe(OTf) 3 and γ-cyclodextrin, for the Markovnikov selective hydroamination of styrenes (and norbornene) with carbazoles and carbazole-fused aromatics with high activity and selectivity under mild conditions (Figure 6, bottom) [63].…”