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Chapter 1 Small Molecule Activation by Main VU Research Portal Small Molecule Activation and Capture by Preorganized Frustrated Lewis Pairs Bertini, F. 2013 document version Publisher's PDF, also known as Version of record Link to publication in VU Research Portal citation for published version (APA) Bertini, F. (2013). Small Molecule Activation and Capture by Preorganized Frustrated Lewis Pairs. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. E-mail address: [email protected] Download date: 02. Oct. 2021 Chapter 1 Small Molecule Activation by Main Group Compounds Federica Bertini, J. Chris Slootweg, Koop Lammertsma Abstract: This introductory chapter describes recent spectacular discoveries with respect to the new and fascinating field of small molecule activation by main group compounds. Chapter 1 1.11.11.1.1.1 ... IIIntroductionIntroduction The activation of small molecules like H 2, CO 2, NH 3, P 4 and N 2O among others, and their subsequent utilization for synthetic purposes, is of fundamental importance in chemistry. Creation of new synthetic methodologies based on small molecule activation would create new opportunities for material development from cheap, readily available and renewable feedstock. For example, there is great interest in understanding how to use carbon dioxide as a feedstock in green processes and as a carbon source for the production of more complex molecules,1 while the development of novel strategies for the transformation of white phosphorus (P 4) is of great importance owing to the high demand of organophosphorus compounds and to environmental concerns. 2 Small molecules are generally quite stable thermodynamically and key to their successful utilization is to provide low-barrier reaction pathways, which can be achieved through binding and activation processes typically mediated by transition metal ions. A great amount of fundamental chemistry research has therefore been aimed to understand how metal complexes coordinate to such small and often rather inert molecules, how they modulate their reactivity and how to use the gained knowledge for the development of new catalytic processes. 3 This work has led to a deep understanding that has significantly impacted the fields of organometallic chemistry and catalysis. The terrific success of transition metal-based catalysts for the activation of small molecules perhaps has overshadowed for many years the possibility of the utilization of main group elements for the same purpose. In recent years, there has been an intense drive towards developing “green” chemical processes using more environmentally benign chemicals, reagents, solvents and catalysts. 4 A part of this drive is to avoid or minimize the use of transition metals in chemical reactions, as these are often toxic and difficult to dispose properly in large quantities. Moreover, the difficulty of their separation leaves a chance of their contamination of the product. The presence of a metal, even at the lowest level, in pharmaceutical products is closely regulated. Thus, a transition metal-free process is desired as a part of the requirements for the chemical industry as well as clean environment. 4 Hence, the development of metal-free systems that can replace the use of transition metals is highly desired. The past decade witnessed extraordinary discoveries in the field of small molecule activation by main group species. The first major breakthrough occurred in 2005, when Power and co-workers discovered that the germanium species ArGeΞGeAr (Ar = C 6H3- 2,6(2,6-diisopropylphenyl) 2) reacts with molecular H 2 under mild conditions to give the 5 hydrogenated products Ar(H)Ge=Ge(H)Ar, Ar(H) 2GeGe(H) 2Ar and Ge(H) 3Ar. This finding 2 Small Molecule Activation by Main Group Compounds gained enormous attention in the scientific community since H 2 activation had long been known to occur at transition metal centers, but the reaction of H 2 with a main group compound under mild conditions was unprecedented. In 2006, the group of D. W. Stephan reported on the reversible activation of the dihydrogen molecule under ambient conditions, using a unimolecular phosphine-borane 6 Lewis pair. The ability of such phosphine-borane pair to activate H 2 was attributed to the contemporary presence of unquenched Lewis basic and Lewis acidic centers, which could synergically interact with the dihydrogen molecule, leading to the heterolytic splitting of the H―H bond. Mutual quenching of the reactive phosphorus and boron atoms was prevented by bulky substituents, which inhibited Lewis adduct formation due to steric hindrance. This finding opened a window of new opportunities in the field of metal-free small molecule activation and catalysis and was the starting point from which the chemistry of the so called “frustrated Lewis pairs” (FLPs) rapidly developed. 7 In the same years, it was increasingly recognized that also low valent main group species such as carbenes 8 and their heavier congeners 9 posses the ability to activate small molecules. As in the case of frustrated Lewis pairs, the ability of carbene-like species to split strong bonds heterolytically is due to the to the contemporary presence of an electrophilic vacant orbital and a nucleophilic lone pair of electrons. Consequently to these discoveries, the ability of such main group low valent species to activate small molecules has been extensively studied. This chapter illustrates recent developments with respect to an emergent and fascinating topic in main group chemistry: transition metal-free small molecule activation. This chapter is not intended to furnish a comprehensive treatment of the literature on such a broad topic, but rather to illustrate the most remarkable discoveries, with emphasis on those main group species which gain their ability to activate small molecules owing to the contemporary presence of acidic empty orbitals and basic lone pairs of electrons. We will first describe a few selected examples of small molecule activation by low valent main group species. In particular, we will describe how carbenes, as well as their heavier congeners, can mimic the behavior of transition metals owing to their ambiphilic nature, and we will briefly comment on the chemistry of the related nitrenes and phosphinidenes, which developed differently. Additionally, a few remarkable examples of small molecule activation by phosphenium ions will be also described. Finally, we will illustrate the chemistry of the newly discovered frustrated Lewis Pairs (FLPs), which is the main focus of this PhD thesis. 3 Chapter 1 1.21.21.2.1.2 ... Singlet CCCarbenesCarbenes Carbenes are a family of organic molecules composed of a neutral divalent carbon atom with a sextet of electrons and two substituents. 10 They can be regarded as singlet or triplet species depending upon their electronic structure, which is dictated by the nature of the substituents on the carbene carbon atom.11 Triplet carbenes react as diradicals and are therefore less interesting from a synthetic viewpoint since their reactions are non- stereospecific.12 In contrast, singlet carbenes are closed shell species which possess a lone pair of electrons and an empty orbital and react stereospecifically participating in cheletropic reactions which occur in a single step, as exemplified by the widely documented 1,2-cycloaddition to double bonds to yield cyclopropanes. 10-13 Carbenes have long been known as very reactive and short lived species that could not be isolated and were usually studied by observing the reactions they undergo. However, strategies for the isolation of persistent carbenes have been developed and nowadays stable carbenes, such as the Arduengo-type 14,15 or the Bertrand-type,16,17 are very well known. Key to their stabilization is the presence of one or two heteroatoms (such as nitrogen or phosphorus) adjacent to the carbene carbon atom, which decrease the electron deficiency of the empty orbital by donating electron density through resonance, while stabilizing the lone pair on the carbon by inductively withdrawing electron density. Bulky substituents on the heteroatom(s) provide additional stabilization by steric protection of the reactive carbon atom. Despite the provided stabilization, such persistent carbenes remain fairly reactive species. In 2006, the group of Bertrand discovered that acyclic and cyclic (alkyl)(amino)carbenes (aAACs and cAACs) react with CO to afford the corresponding ketenes 18 (as exemplified by aAAC 1 that reacts with CO to form 2; Scheme 1). This led to the question of whether singlet carbenes could mimic the chemical behavior of transition metals, which appeared reasonable since singlet carbenes possess both a lone pair of electrons and an accessible vacant orbital and therefore resemble, at least to some extent, transition metal centers. Consequently to this discovery,
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