CHAPTER 1: INTRODUCTION
1.3 Controlling Catalysis Through Substrate Activation
Further applications of pincer-crown ether complexes can be envisioned based on the ability of the pendent aza-crown ether moiety to dock cations in close proximity to the metal center of the catalyst. Commonly employed alkali metal, alkaline earth, transition metal, and lanthanide cations exhibit varying degrees of Lewis acidity. Lewis acids have a rich history of promoting a wide array of organic transformations through substrate activation and
organization.89 These electron pair acceptors form a donor-acceptor complex upon coordination
to a Lewis basic site on a substrate (Figure 1.8). This complexation results in a partial charge transfer to the Lewis acid and a resulting polarization of the donor molecule making it more reactive. Lewis acids increase the electrophilicity of carbonyl groups, increase the acidity of hydroxy groups, and make coordinated hydroxy, ether, and thioether functionalities better leaving groups.89
Figure 1.7 General Lewis acid activation of a variety of organic molecules.
Convenient metrics of Lewis acidity include the pKa of [M–OH2]n+ complexes and
fluorescence maxima shifts of 10-methylacridone–metal ion salt complexes (Table 1.1).90-91 The
Lewis acidity of these ions varies widely allowing for a wide range of control over substrate activation. O R R O H R R R R O R O NR2 R N R R R R S R O O R R N R O Mn+ Mn+ Mn+ Mn+ Mn+ Mn+ M n+ Mn+
Table 1.1 Lewis acid induced shifts of 10-methylacridone florescence lmax as a measure of
relative Lewis acidity.91
Lewis Acid lmax Florescence Energy (eV)
Fe(ClO4)3 478 2.59 Sc(OTf)3 474 2.62 Fe(ClO4)2 471 2.63 Cu(ClO4)2 471 2.63 Co(ClO4)2 470 2.64 (C6F5)2SnBr2 470 2.64 Lu(OTf)3 461 2.69 Y(OTf)3 460 2.7 La(OTf)3 458 2.71 Zn(OTf)2 456 2.72 Ph3SnCl 455 2.72 Bu2SnCl2 453 2.74 (C6F5)3SnBr 452 2.74 Mg(ClO4)2 451 2.75 Ca(ClO4)2 449 2.76 Sr(ClO4)2 445 2.79 Ba(ClO4)2 444 2.79 LiClO4 442 2.81 NaClO4 437 2.84 Mn(ClO4)2 435 2.85 10-methylacridone 432 2.87
Cooperation between transition metal catalysts and Lewis acids as substrate or intermediate activators has been realized as a powerful strategy for controlling chemical
reactivity and catalysis in organic synthesis.92-93 In these reactions, generally either the transition
another example, the palladium catalyzed dimerization of vinyl arenes is promoted by an In(OTf)3 Lewis acid cocatalyst.95 The indium trication activates vinyl arenes to nucleophilic
attack by Pd(0) complexes. The resulting proposed zwitterionic Pd+–alkyl–In– intermediate then
inserts an additional vinyl arene and eliminates the product to regenerate the Pd(0) and In3+
cocatalysts.
Figure 1.8 Lewis acid promoted reaction of zirconacyclopentadienes with isocyanates to form iminocyclopentadienes.
Lewis acids can accelerate numerous transition metal-catalyzed reactions. The positive influence of the Lewis acid has, in many cases, been attributed to interactions between the Lewis acid and the metal-bound substrate. For reactions involving carbon monoxide as a substrate, for example, the carbonyl ligand can interact with a Lewis acid during or after C–C bond
formation.96-101 In cases where pendent macrocycles were present as part of a ligand scaffold and
positioned the Lewis acid near the metal center, carbonyl activation or acyl stabilization has been proposed (Figure 1.7).83, 102-103 Pincer-crown ether Ir complexes have previously been shown to
promote lithium- and lanthanum-accelerated migratory insertion and salt-responsive methanol Zr R’N=C=O R R R R No Reaction R’N=C=O LA Cp2Zr O R R R R NR’ LA Cp2Zr ’RN R R R R O LA R R R R NR’ O Cp2Zr LA R R R R R’N
carbonylation catalysis.104-105 Although the role of the Lewis acid in the mechanisms of these
reactions is not fully understood, postulated transition states include those wherein a cation complexed in the pendent crown ether of the molecule interacts with a carbonyl ligand, activating it towards nucleophilic attack by a methyl ligand (Figure 1.7).
Figure 1.9 Select examples of macrocycle containing complexes in which a Lewis acid is proposed to interact with carbonyl or acyl ligands bound to the metal center.
Positioning activated substrates near the active site of a pincer-crown ether complex provides an opportunity for controlling catalysis based on the extent of substrate activation via the Lewis acidity of the cationic additive. If the substrate is involved in the rate-determining steps of the reaction, activation with Lewis acids could lower barriers to make the reactions proceed faster. Additionally, if a given reaction does not proceed with an unactivated substrate, but proceeds in the presence of a Lewis acid additive, switchable bimetallic catalysis or substrate selective reactivity becomes possible.
CO (OC)4M O O O O P P Ph2 O R Li+ Ph2 Re CO OC OC I Ph2P O N O O O Ca2+ I Ir PiPr 2 N C O O O O O CH3 O M+
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