Catalysts Containing the Adamantane Scaffold

Author Manuscript Title: Catalysts Containing the Adamantane Scaffold Authors: Kylie Agnew-Francis; Craig Mckenzie Williams This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofrea- ding process, which may lead to differences between this version and the Version of Record. To be cited as: 10.1002/adsc.201500949 Link to VoR: https://doi.org/10.1002/adsc.201500949 REVIEW DOI: 10.1002/adsc.201((will be filled in by the editorial staff)) Catalysts Containing the Adamantane Scaffold Kylie A. Agnew-Francis,a and Craig M. Williamsa* a School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland, Australia, 4072 E-mail: [email protected], Homepage:http://www.scmb.uq.edu.au/homepages/williams/index.html In memory of Paul von Ragué Schleyer Received: ((will be filled in by the editorial staff)) Abstract. The bulky, but symmetrically beautiful, 2. Organometallic Catalysts adamantane ring system is now pervasive throughout 2.1. Aryl Palladium couplings physical, medicinal and synthetic chemistry, since it was first 2.2. C-H activation discovered in 1924 and coined “dekaterpene”. This 2.3. Metathesis fascinating name lived up to its natural product roots when 2.4. Hetero-Diels-Alder Reactions adamantane was isolated from crude oil in 1933, but it was 2.5. Cyclopropanation not until 1957 in a landmark contribution by von Ragué 2.6. Alkyne Addition Reactions Schleyer that adamantane was made readily accessible 2.7 Hydrogenation through synthesis. Beyond the legacy to physical and 2.8 Dendritic Catalysis medicinal chemistry the adamantane moiety has been 3. Organocatalysts quintessential in the development of some of the most 3.1 Alcohol oxidation important catalysts to-date. Considering adamantane’s 3.2 Michael additions impact to catalyst development past, present and future, this subject is for the first time reviewed herein. 3.3 Acyl transfer and lipase mimics 3.4 Reactions of 1,3-dicarbonyls Table of Contents 3.5 Strecker Reactions 1. Introduction 3.6 NHC catalysis 1.1. Stereoelectronic Influences in Ligand Design – 4. Summary and Outlook Why Adamantane? 1.2. Functionalization of Adamantane Keywords: adamantane; adamantyl; metal catalysis; organo catalysis; ligand design 1 Introduction Adamantane (1, Scheme 1), the simplest of the diamondoid structures,[1] is a highly symmetrical and stable tricycloalkane first discovered as natural constituent of fossil fuel, being first isolated from petroleum sources in 1933.[2] Though the structure was first proposed by Decker in 1924 (as dekaterpene),[3] it was not until 1941 that the compound was finally constructed by Prelog and his group.[4] A later more robust synthesis was provided by Schleyer,[5] based on the Lewis acid catalyzed rearrangement of endo-trimethylenenorbornane 2 (Scheme 1). This development sparked substantial physical organic chemistry interest, especially in the 1960’s, and opened the door to functional group Scheme 1. Schleyer’s synthesis of adamantane. incorporation (e.g. bromo 3 and alcohol 4 derivatives; both obtainable in 95% from 1).[6] 1 This article is protected by copyright. All rights reserved Since this time, the adamantane scaffold has seen extensive application. Notably within the context of Kylie A. Agnew-Francis was born this review, the desirable properties that adamantane in Wellington, New-Zealand. She relocated to Brisbane, Australia in provides to catalyst development include, inert 1999. In 2011, she received her hydrocarbon reactivity, rigidity and symmetry, and BSc(Hons) in chemistry from the steric bulk. Not surprisingly, these features have been University of Queensland, receiving the chemistry honors adopted into the design portfolio of those developing research prize for that year. Kylie new catalytically driven reactions. Over the past commenced her PhD under the decade or so, the number of literature examples of supervision of Craig M. Williams successful catalysts featuring the adamantyl moiety and Luke Guddat in 2015, focusing on the synthesis and biological has certainly increased. The review presented herein application of anti-fungal compounds. focuses on the application of the adamantane scaffold to metal and organocatalyst-based ligand design and highlights key organic reactions mediated by these catalytic systems. Craig M. Williams was born in Adelaide, Australia. He received his BSc(Hons) degree in chemistry in 1994 and in 1997 was awarded his PhD in organic chemistry from Flinders University under the supervision of Prof. Rolf H. Prager. He worked as an Alexander von Humboldt Postdoctoral Fellow with 1.1 Stereoelectronic Influences in Ligand Design – Prof. Armin de Meijere at the Why Adamantane? Georg-August-Universität, Göttingen, Germany until 1999 and then took up a postdoctoral fellowship at the Australian Tuning the reactivity of catalysts is an essential National University with Prof. Lewis N. Mander. He has held an academic position, currently Assoc. Professor, at process in optimizing or adapting systems to suit The University of Queensland since 2000 and during this particular reactions. Within this, it follows that ligand time has won a number of awards including a Thieme selection and / or design must play an important role. Chemistry Journals Award in 2007. The primary research Indeed, over the past few decades, a number of focus of the Williams group is the construction and isolation of biologically active complex natural products, ligand selection guides for various reaction types and [7] and designing methodology to assist in this endeavour. The metal centers have been reported. Two primary group also enjoys dabbling in medicinal, physical organic factors used to control the reactivity and stability of and computational chemistry. catalyst systems are steric saturation and electron o [12] richness. In transition metal catalysis, optimizing angle ≥ 160 and the pKa exceeded 6.7. It could be these features are observed to play major roles in said that this key observation paved the way for the controlling key steps of the catalytic cycle, namely generation of many successful catalysts in the through their influence on incoming ligand approach, subsequent decades. ligand dissociation rates and geometric isomerism.[8] Mechanistic implications of added bulk or basicity has been extensively reviewed.[8b, 9] Numerous methods exist for quantifying the stereoelectronic properties of a ligand, however the most commonly used in the case of monodentate Figure 1. a) Tolman cone angle of a phosphine ligand. ligands is through calculating Tolman cone angles (θ) The M-P bond length is set to 2.8 Å by default; b) - defined as the apex angle of a cone emanating from Bite angle of a bidentate phosphine ligand. the metal center towards the van der Waals radii of the outermost ligand atoms (Figure 1.) - and Tolman electronic parameters, based on the Strohmeier Though particularly useful for phosphine ligands, the approach.[10] These and other methods of describing rough approximations used in the Tolman approach ligand properties have been well reviewed.[11] In 1989, mean that it is not as applicable to more complex Osborn and co-workers reported their conclusion that ligands, such as N-heterocyclic carbenes (NHCs) or optimal catalytic activity of Pd-phosphine complexes bidentate ligands. Improvement on the mathematics towards the carbonylation of dichloromethane and for calculating the exact cone angle by Allen and chlorobenzene was only observed where the cone coworkers have alleviated some of these issues,[13] 2 This article is protected by copyright. All rights reserved although alternate means, such as calculation of One consequence of the above can be observed in buried volume and solid cone angles (Θ), have also melting points of hydrocarbons. Caged hydrocarbons been proposed.[14] commonly high melting points compared to other hydrocarbons. The uniqueness of adamantane in this The low cost, ready availability, stereoelectronic regard may be illustrated through comparing its properties and relative ease of functionalization of melting point to other caged hydrocarbons and those adamantane makes it an ideal candidate for of similar molecular weights (Table 2). These data incorporation into catalyst design. Certainly, there indicates that polyhedranes, and adamantane exist many commercially available inexpensive particular, must possess significant intermolecular ligands and catalyst systems with this moiety pre- attractive forces not normally associated with installed.[15] Based on the conclusions highlighted by hydrocarbons. Recently, it has been suggested that the Osborn and in comparing the cone angles of relative strength of the C-H H-C interaction could phosphine ligands containing ligands with various be accounted for by the tertiary nature of the carbons present, the higher number of short H H contacts, amounts of steric bulk (Table 1), it is easy to see why [19] its incorporation into ligand design has seen so much and low level of pyramidality. Overall, this makes success. adamantane (or indeed, other caged systems) an excellent, polarizable electron donor. The introduction of various groups onto ligands is an effective and simple method for tuning the electronic properties and thus, the reactivity of a catalyst system. Table 2. Comparison of the melting point of adamantane Ligands containing groups that are highly electron (1) to other hydrocarbons of similar molecular mass.

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