Enantioselective Cobalt-catalysed Transformations

Hélène Pellissier

• Publisher: Royal Society of Chemistry
• Year: 2018
• Print ISBN: 9781788014625
• eBook ISBN: 9781788015158
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

With a foreword from leading organic chemist Professor Paul Wender, this book collects the major developments reported in the past thirty years in the field of enantioselective reactions promoted by chiral cobalt catalysts, illustrating the power of these green catalysts to provide all types of organic reactions from the basic to completely novel methodologies.

The search for new methodologies to prepare optically pure products is one of the most active areas of research in organic synthesis. Of the methods available for preparing chiral compounds, catalytic asymmetric synthesis has attracted the most attention. In particular, asymmetric transition-metal catalysis is a powerful tool for performing reactions in a highly enantioselective fashion. Efforts to develop new asymmetric transformations have previously focused on the use of rare metals such as titanium, palladium, iridium and gold. However, the ever-growing need for environmentally friendly catalytic processes has prompted chemists to focus on the more abundant and less toxic first-row transition metals, such as cobalt, to develop new catalytic systems.

The ability of cobalt catalysts to adopt unexpected reaction pathways has led to an impressive number of enantioselective cobalt-promoted transformations being developed over the past three decades. These have included the synthesis of many different types of products, often under relatively mild conditions and with remarkable enantioselectivities. This book is a useful reference resource for chemists, both academic and industrial, working in organic synthesis and interested in greener or more economical catalytic alternatives.

CHAPTER 1

Enantioselective Cobalt-catalysed [2+1] Cycloadditions

The first chapter of the book deals with enantioselective cobalt-catalysed [2+1] cycloadditions. The first section of this chapter collects together and describes enantioselective cyclopropanation reactions, while the second part of the section deals with aziridination and epoxidation reactions. Concerning cyclopropanations, excellent enantioselectivities have been described using predominantly two types of chiral cobalt complexes, based on salen and porphyrin chiral ligands. In the area of aziridinations, important results have also been described using porphyrin cobalt chiral catalysts, while only rare examples of epoxidations have been developed so far.

1.1Introduction

Reactions forming multiple bonds and stereocentres represent important tools for the efficient assembly of complex molecular structures.¹ Of the many families of reactions discovered over the past century, cycloadditions hold a prominent place in the area of current synthetic methodologies and the research activity in this field shows no signs of abatement.² Among the metals used to catalyse cycloadditions,^1,2a,b,3 cobalt has been found to be highly efficient in enantioselectively promoting the formation of carbo- and heterocycles of different ring sizes and especially three-membered chiral products.

1.2Cyclopropanations

1.2.1Intermolecular Cyclopropanations

1.2.1.1With Salen Cobalt Complexes

Organic chemists have always been fascinated by the strained structure of the cyclopropane subunit,⁴ which is found in a wide variety of naturally occurring compounds, such as terpenes, pheromones, fatty acid metabolites and unusual amino acids.⁵ This fact has inspired chemists to find novel approaches to their synthesis, and thousands of cyclopropane compounds have already been prepared.⁶ In this context, the cyclopropanation of alkenes based on the transition-metal-catalysed decomposition of diazoalkanes has been widely developed.⁷ Indeed, the synthesis of cyclopropanes by transition-metal-mediated carbene transfer from aliphatic diazo compounds to carbon–carbon double bonds is not only a major method for the preparation of cyclopropanes, with them most of the time exhibiting a trans-configuration, but is also among the most developed and general methods available to the synthetic organic chemist.^{7c,e, f} The asymmetric synthesis of cyclopropanes has remained a challenge,^4d,7i,8 but it has been attempted since it was demonstrated that members of the pyrethroid class of compounds were found to be effective insecticides.⁹ Since the first enantioselective copper-catalysed cyclopropanation reported by Nozaki and co-workers in 1966,¹⁰ many groups have tried to find more efficient catalysts, and the most spectacular advances were reported by Aratani et al., who discovered, through extensive evaluation of a large number of ligands, a chiral (salicylaldiminato)copper(II) complex which allowed enantioselectivities of up to 95% ee to be achieved.¹¹ Ever since, other highly effective and stereocontrolled syntheses of functionalised cyclopropanes have been reported, in particular, with catalysts based on copper,¹² rhodium, and ruthenium.¹³ Moreover, cobalt complexes have been shown to be reactive catalysts for α-diazoester decomposition, leading to a metal carbene that could convert alkenes into cyclopropanes. Although the early work in this area established that chiral cobalt(II) complexes were catalytically active, the low levels of diastereo- and enantiocontrol have limited their use in synthesis for a long time.¹⁴ The first highly enantioselective intermolecular cobalt-catalysed cyclopropanation reaction was reported by Nakamura et al. in 1978.¹⁵ It employed 3 mol% of bis[(−)-camphorquinone-α-dioximato]cobalt(II) complex as a catalyst, allowing enantioselectivities of up to 88% ee to be achieved in combination with excellent yields (90–95%), for example in the synthesis of neopentyl trans-2-phenylcyclopropanecarboxylate. Ever since, many other chiral cobalt catalysts have been successfully applied to promote these transformations, often derived from salen or porphyrin chiral ligands. For example, Katsuki et al. introduced novel chiral salen cobalt(III) complexes to induce trans-selective cyclopropanation reactions.^7a,16 The optimal trans-selective cobalt complex was demonstrated to be cobalt(III) catalyst 1. As shown in Scheme 1.1, it promoted the decomposition of tert-butyl diazoacetate in the presence of styrene derivatives to yield the corresponding trans-cyclopropanes with both excellent diastereoselectivities (90–94% de) and enantioselectivities (92–96% ee).

Scheme 1.1 Trans-selective cyclopropanation of aromatic alkenes with tert-butyl diazoacetate.

In 1999, Yamada et al. demonstrated that chiral 3-oxobutylideneaminatocobalt(II) complexes,¹⁷ such as 2 employed at a 5 mol% catalyst loading in THF as solvent at 40–50 °C (Scheme 1.2), were efficient promotors of the same trans-selective reaction of monoaryl-substituted alkenes with tert-butyl diazoacetate.¹⁸ The addition of a catalytic amount of N-methylimidazole (NMI) was found to increase the rate of the reaction, as well as the enantioselectivity. The scope of the reaction was, however, limited to aryl-monosubstituted alkenes, resulting in the formation of the corresponding chiral trans-cyclopropanes in high yields (85–99%) and good trans-diastereoselectivities (64–82% de) combined with excellent enantioselectivities (92–96% ee). Indeed, the reaction of 1,1-disubstituted alkenes led to the corresponding enantiopure trisubstituted cyclopropane derivatives with only low diastereocontrol (6% de). The authors also found that the diastereoselectivity in the cyclopropanation of styrene decreased to 66% de when methyl diazoacetate was used. A theoretical analysis of the reaction pathway using a density functional theory method revealed that the axial donor ligand produced two prominent effects.¹⁹ One was that the activation energy for the formation of the cobalt carbene was reduced and that the activation energy for the cyclopropanation step was increased. The other was that the distance of the carbene carbon above the plane was shortened during the cyclopropanation step. From these results, the axial donor ligand effects, enhancing the reactivity and improving the diastereo- and enantioselectivities, in the 3-oxobutylideneaminatocobalt(II)-catalysed asymmetric cyclopropanation could be explained. In relation to the axial donor ligand effect, the same authors showed that these highly enantioselective cyclopropanations could also be performed in environmentally friendly alcoholic and aqueous solvents.²⁰ Indeed, the tetradentate ligand of the β-ketoiminatocobalt complex produces a rigid square planar structure around the cobalt atom and the structure of the complex is almost independent from the solvent. Hence, the coordination of donor solvent at a vacant axial position would directly lead to the activation of the carbene carbon located at the other axial position. It is generally considered that metal–carbene carbon bonds in carbene complexes for cyclopropanation should be double-bonded; however, the authors reported theoretical and FT-IR analyses revealing that the cobalt–carbon bond of the 3-oxobutylideneaminato or the salen–cobalt–carbene complexes was characterised as a single bond.²¹ Furthermore, dinuclear salen complexes, suc...

Table of contents

• Cover
• Title
• Halftitle
• Catalysis Series
• Copyright
• Contents
• Foreword
• Abbreviations
• Chapter 1 Enantioselective Cobalt-catalysed [2+1] Cycloadditions
• Chapter 2 Other Enantioselective Cobalt-catalysed Cycloadditions
• Chapter 3 Enantioselective Cobalt-catalysed Cyclisations Through Domino Reactions
• Chapter 4 Miscellaneous Enantioselective Cobalt-catalysed Cyclisations
• Chapter 5 Synthesis of Chiral Acyclic Compounds Through Enantioselective Cobalt-catalysed Reduction Reactions
• Chapter 6 Synthesis of Chiral Acyclic Compounds Through Enantioselective Cobalt-catalysed Ring–Opening Reactions
• Chapter 7 Synthesis of Chiral Acyclic Compounds Through Enantioselective Cobalt-catalysed Michael and (Nitro)-Aldol Reactions
• Chapter 8 Synthesis of Chiral Acyclic Compounds Through Enantioselective Cobalt-catalysed 1,2-Nucleophilic Additions to Carbonyl Compounds and Derivatives
• Chapter 9 Synthesis of Chiral Acyclic Compounds Through Enantioselective Cobalt-catalysed Cross-coupling Reactions
• Chapter 10 Synthesis of Chiral Acyclic Compounds Through Enantioselective Cobalt-catalysed Hydrovinylation Reactions
• Chapter 11 Synthesis of Chiral Acyclic Compounds Through Miscellaneous Enantioselective Cobalt-catalysed Reactions
• General Conclusion
• Subject Index