A Study of BF3-Promotedortho Lithiation of Anilines and DFT Calculations on the Role of Fluorine–Lithium Interactions

Triptycene Derivatives: From Their Synthesis to Their Unique Properties

Since the first preparation of triptycene, great progress has been made with respect to its synthesis and the understanding of its properties. Interest in triptycene-based systems is intense; in recent years, advances in the synthetic methodology and properties of new triptycenes have been reported by researchers from various fields of science. Here, an account of these new developments is given and placed in reference to earlier pivotal works that underpin the field. First, we discuss new approaches to the synthesis of new triptycenes. Progress in the regioselective synthesis of sterically demanding systems is discussed. The application of triptycenes in catalysis is also presented. Next, progress in the understanding of the relations between triptycene structures and their properties is discussed. The unique properties of triptycenes in the liquid and solid states are elaborated. Unique interactions, which involve triptycene molecular scaffolds, are presented. Molecular interactions within a triptycene unit, as well as between triptycenes or triptycenes and other molecules, are also evaluated. In particular, the summary of the synthesis and useful features will be helpful to researchers who are using triptycenes as building blocks in the chemical and materials sciences.

Mechanism of Lithium Diisopropylamide-Mediated Ortholithiation of 1,4-Bis(trifluoromethyl)benzene under Nonequilibrium Conditions: Condition-Dependent Rate Limitation and Lithium Chloride-Catalyzed Inhibition

Lithiation of 1,4-bis(trifluoromethyl)benzene with lithium diisopropylamide in tetrahydrofuran at -78 °C occurs under conditions at which the rates of aggregate exchanges are comparable to the rates of metalation. Under such nonequilibrium conditions, a substantial number of barriers compete to be rate limiting, making the reaction sensitive to trace impurities (LiCl), reactant concentrations, and isotopic substitution. Rate studies using the perdeuterated arene reveal odd effects of LiCl, including catalyzed rate acceleration at lower temperature and catalyzed rate inhibition at higher temperatures. The catalytic effects are accompanied by corresponding changes in the rate law. A kinetic model is presented that captures the critical features of the LiCl catalysis, focusing on the influence of LiCl-catalyzed re-aggregation of the fleeting monomer that can reside above, at, or below the equilibrium population without catalyst.

Lithium diisopropylamide-mediated lithiation of 1,4-difluorobenzene under nonequilibrium conditions: role of monomer-, dimer-, and tetramer-based intermediates and lessons about rate limitation

Lithiation of 1,4-difluorobenzene with lithium diisopropylamide (LDA) in THF at -78 °C joins the ranks of a growing number of metalations that occur under conditions in which the rates of aggregate exchanges are comparable to the rates of metalation. As such, a substantial number of barriers vie for rate limitation. Rate studies reveal that rate-limiting steps and even the choice of reaction coordinate depend on subtle variations in concentration. Deuteration shifts the rate-limiting step and markedly alters the concentration dependencies and overall rate law. This narrative is less about ortholithiation per se and more about rate limitation and the dynamics of LDA aggregate exchange.

Lithium diisopropylamide-mediated ortholithiation of 2-fluoropyridines: rates, mechanisms, and the role of autocatalysis

Lithium diisopropylamide (LDA)-mediated ortholithiations of 2-fluoropyridine and 2,6-difluoropyridine in tetrahydrofuran at -78 °C were studied using a combination of IR and NMR spectroscopic and computational methods. Rate studies show that a substrate-assisted deaggregation of LDA dimer occurs parallel to an unprecedented tetramer-based pathway. Standard and competitive isotope effects confirm post-rate-limiting proton transfer. Autocatalysis stems from ArLi-catalyzed deaggregation of LDA proceeding via 2:2 LDA-ArLi mixed tetramers. A hypersensitivity of the ortholithiation rates to traces of LiCl derives from LiCl-catalyzed LDA dimer-monomer exchange and a subsequent monomer-based ortholithiation. Fleeting 2:2 LDA-LiCl mixed tetramers are suggested to be key intermediates. The mechanisms of both the uncatalyzed and catalyzed deaggregations are discussed. A general mechanistic paradigm is delineated to explain a number of seemingly disparate LDA-mediated reactions, all of which occur in tetrahydrofuran at -78 °C.

Mechanism of lithium diisopropylamide-mediated substitution of 2,6-difluoropyridine

Treatment of 2,6-difluoropyridine with lithium diisopropylamide in THF solution at -78 degrees C effects ortholithiation quantitatively. Warming the solution to 0 degrees C converts the aryllithium to 2-fluoro-6-(diisopropylamino)pyridine. Rate studies reveal evidence of a reversal of the ortholithiation and a subsequent 1,2-addition via two monomer-based pathways of stoichiometries [ArH*i-Pr(2)NLi(THF)](double dagger) and [ArH*i-Pr(2)NLi(THF)(3)](double dagger). Computational studies fill in the structural details and provide evidence of a direct substitution without the intermediacy of a Meisenheimer complex.

Autocatalysis in lithium diisopropylamide-mediated ortholithiations

Ortholithiation of 3-fluorophenyl-N,N-diisopropyl carbamate by lithium diisopropylamide (LDA) in THF at -78 degrees C affords unusual rate behavior including linear decays of the carbamate, delayed formation of LDA-aryllithium mixed dimers, and evidence of autocatalysis. A mechanistic model in conjunction with numeric integration methods accounts for the time-dependent changes in concentration. The two critical rate-limiting steps in the model entail (1) an LDA dimer-based metalation of arylcarbamate, and (2) a rate-limiting condensation of the resulting aryllithium with the LDA dimer to form two isomeric LDA-ArLi mixed dimers. One isomer elicits a highly efficient (post-rate-limiting) metalation of aryl carbamate, in turn, regenerating aryllithium. The prevalence and implications of such autocatalysis are discussed.