On- and Off-Cycle Catalyst Cooperativity in Anion-Binding Catalysis

Chiral, neutral H-bond donors have found widespread use as catalysts in enantio­selective reactions involving ion-pair intermediates. Herein, a systematic mechanistic study of a proto­typical anion-binding reaction, the thiourea-catalyzed enantio­selective alkylation of α-chloro­ethers, is detailed. This study reveals that the catalyst resting state is an inactive dimeric aggregate that must dissociate and then reassemble to form a 2:1 catalyst–substrate complex in the rate-determining transition structure. Insight into this mode of catalyst cooperativity sheds light on the practical limitations that have plagued many of the H-bond donor-catalyzed reactions developed to date and suggests design strategies for new, highly efficient catalyst structures

Synthesis and Properties of Isomerically Pure Anthrabisbenzothiophenes

The synthesis of three heptacyclic heteroacenes is described, namely anthra[2,3-<i>b</i>:7,6-<i>b</i>′]bis[1]benzothiophenes (ABBTs). A stepwise sequence of aldol reactions provides regiochemical control, affording only the <i>syn</i>-isomer. The ABBTs are characterized by X-ray crystallography, UV–vis absorption, and emission spectroscopy, as well as cyclic voltammetry. Field effect transistors based on solution-cast thin films of ABBT derivatives exhibit charge-carrier mobilities of as high as 0.013 cm²/(V s)

Synthesis and Properties of Isomerically Pure Anthrabisbenzothiophenes

The synthesis of three heptacyclic heteroacenes is described, namely anthra[2,3-<i>b</i>:7,6-<i>b</i>′]bis[1]benzothiophenes (ABBTs). A stepwise sequence of aldol reactions provides regiochemical control, affording only the <i>syn</i>-isomer. The ABBTs are characterized by X-ray crystallography, UV–vis absorption, and emission spectroscopy, as well as cyclic voltammetry. Field effect transistors based on solution-cast thin films of ABBT derivatives exhibit charge-carrier mobilities of as high as 0.013 cm²/(V s)

Diazatetracenes Derived from the Benzannulation of Acetylenes: Electronic Tuning via Substituent Effects and External Stimuli

Functionalized diazatetracenes are prepared using a new two-step sequence. The use of a dichlorobenzaldehyde in a Cu-catalyzed benzannulation of acetylenes provides functionalized dichloronaphthalenes that afford diazatetracenes using Buchwald–Hartwig aminations. This approach provides unique substitution patterns and rapid access to covalently linked dimeric diazatetracenes. Their electronic properties are characterized by UV–vis absorption/emission and cyclic voltammetry, revealing strong effects from both external stimuli by acid and internal substituent effects

Conformational Control of Chiral Amido-Thiourea Catalysts Enables Improved Activity and Enantioselectivity

While aryl pyrrolidinoamido-thioureas derived from α-amino acids are effective catalysts in a number of asymmetric transformations, they exist as mixtures of slowly interconverting amide rotamers. Herein, the compromising role of amide bond isomerism is analyzed experimentally and computationally. A modified catalyst structure that exists almost exclusively as a single amide rotamer is introduced. This modification is shown to result in improved reactivity and enantioselectivity by minimizing competing reaction pathways

Conformational Control of Chiral Amido-Thiourea Catalysts Enables Improved Activity and Enantioselectivity

While aryl pyrrolidinoamido-thioureas derived from α-amino acids are effective catalysts in a number of asymmetric transformations, they exist as mixtures of slowly interconverting amide rotamers. Herein, the compromising role of amide bond isomerism is analyzed experimentally and computationally. A modified catalyst structure that exists almost exclusively as a single amide rotamer is introduced. This modification is shown to result in improved reactivity and enantioselectivity by minimizing competing reaction pathways

Mechanism-Guided Development of a Highly Active Bis-thiourea Catalyst for Anion-Abstraction Catalysis

We describe the rational design of a linked, bis-thiourea catalyst with enhanced activity relative to monomeric analogues in a representative enantio­selective anion-abstraction reaction. Mechanistic insights guide development of this linking strategy to favor substrate activation though the intra­molecular cooperation of two thiourea subunits while avoiding nonproductive aggregation. The resulting catalyst platform overcomes many of the practical limitations that have plagued hydrogen-bond-donor catalysis and enables use of catalyst loadings as low as 0.05 mol %. Computational analyses of possible anion-binding modes provide detailed insight into the precise mechanism of anion-abstraction catalysis with this pseudo-dimeric thiourea

Mechanism-Guided Development of a Highly Active Bis-thiourea Catalyst for Anion-Abstraction Catalysis

We describe the rational design of a linked, bis-thiourea catalyst with enhanced activity relative to monomeric analogues in a representative enantio­selective anion-abstraction reaction. Mechanistic insights guide development of this linking strategy to favor substrate activation though the intra­molecular cooperation of two thiourea subunits while avoiding nonproductive aggregation. The resulting catalyst platform overcomes many of the practical limitations that have plagued hydrogen-bond-donor catalysis and enables use of catalyst loadings as low as 0.05 mol %. Computational analyses of possible anion-binding modes provide detailed insight into the precise mechanism of anion-abstraction catalysis with this pseudo-dimeric thiourea

Isomerically Pure <i>syn</i>-Anthradithiophenes: Synthesis, Properties, and FET Performance

The synthesis of isomerically pure <i>syn</i>-anthradithiophene derivatives (<i>syn</i>-ADTs) is described. X-ray crystallography is used to compare the solid-state arrangement of <i>syn</i>-ADT derivatives <b>2a</b>,<b>b</b> to the analogous mixture of <i>syn</i>- and <i>anti</i>-ADTs. Single-crystal OFETs based on isomerically pure <i>syn</i>-ADTs <b>2a</b>,<b>b</b> display device performance comparable to those based on a mixture of ADT isomers <i>syn/anti</i>-<b>2a</b>,<b>b</b> with mobilities as high as 1 cm²/(V s)