Lithium Diisopropylamide: Nonequilibrium Kinetics and Lessons Learned About Rate Limitation Russell F

Perspective pubs.acs.org/joc Lithium Diisopropylamide: Nonequilibrium Kinetics and Lessons Learned about Rate Limitation Russell F. Algera, Lekha Gupta, Alexander C. Hoepker, Jun Liang, Yun Ma, Kanwal J. Singh, and David B. Collum* Department of Chemistry and Chemical Biology Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States *S Supporting Information ABSTRACT: The kinetics of lithium diisopropylamide (LDA) in tetrahydrofuran under nonequilibrium conditions are reviewed. These conditions correspond to a class of substrates in which the rates of LDA aggregation and solvation events are comparable to the rates at which various fleeting intermediates react with substrate. Substrates displaying these reactivities, by coincidence, happen to be those that react at tractable rates on laboratory time scales at −78 °C. In this strange region of nonlimiting behavior, rate-limiting steps are often poorly defined, sometimes involve deaggregation, and at other times include reaction with substrate. Changes in conditions routinely cause shifts in the rate-limiting steps, and autocatalysis is prevalent and can be acute. The studies are described in three distinct portions: (1) methods and strategies used to deconvolute complex reaction pathways, (2) the resulting conclusions about organolithium reaction mechanisms, and (3) perspectives on the concept of rate limitation reinforced by studies of LDA in tetrahydrofuran at −78 °C under nonequilibrium conditions. ithium diisopropylamide (LDA), a highly reactive and at −78 °Cconditions that are of singular importance in L selective Brønsted base, stands among the most prominent organic synthesisoccur at nearly the rates at which the reagents in organic synthesis.1 A survey of 500 total syntheses aggregates in Scheme 1 exchange. The resulting mechanistic revealed that LDA is one of the most commonly used reagents.2 complexity proved high even by organolithium standards, and In the world of structural and mechanistic organolithium the effort expended to understand metalations under such chemistry in which solvent-dependent aggregation and mixed nonequilibrium conditions would have been difficult to justify aggregation impart enormous structural and mechanistic for reagents of lesser importance. 8 complexity,3 LDA has appeal for the study of structure− Eight publications form the core of this second review. LDA, reactivity relationships owing to its relative structural simplicity: however, is only one part of a three-part story. Section 1 is a it exists exclusively as disolvated dimers in most coordinating tutorial that delineates the methods and tactics used to untangle solvents.4 That said, tetrahydrofuran (THF)-mediated deag- the interwoven mechanistic pathways, in particular, when rate- gregation of LDA, depicted in Scheme 1 (1−6), exemplifies limiting steps routinely change. These strategies and principles fl are generally applicable to the deconvolution of complex only a few of the many structural forms that can occur eetingly fi in solution at full equilibrium. Include the plethora of possible ensembles of mechanisms. Section 2 summarizes the speci cs mixed aggregates formed from LDA and other lithium salts of LDA-mediated metalations under nonequilibrium condi- (LiX), and it is clear that even the simplest organolithium tions. Rather than readjudicating the cases, we merely reagent offers the potential for breathtaking mechanistic summarize observations that are of potential interest to complexity.5 mechanistic organolithium and synthetic chemists. Section 3 focuses on seemingly simple notions of rate limitation This review of the chemistry of LDA is our second. The first emanating from rate studies that may be counterintuitive and described investigations that probed mechanistic pathways possibly even difficult to accept. We begin by illustrating why nearly a dozen amide−solvent stoichiometries in the rate- this particular subset of LDA-mediated metalations is so limiting transition statesthat become available when LDA strange. solvated by standard coordinating solvents reacts with various 6,7 electrophiles. We thought we were nearing a logical end ■ BACKGROUND point, but that proved to be premature. We had assiduously avoided studying metalations at −78 °Cbasedonthe LDA-mediated metalations as well as all other organolithium misguided notion that dry ice−acetone baths would provide reactions can be described using three scenarios (Figure 1). inadequate temperature control. Reevaluating this bias, we discovered an extraordinary coincidence that forms the Received: December 27, 2016 foundation of this review: LDA-mediated metalations in THF Published: April 3, 2017 © 2017 American Chemical Society 4513 DOI: 10.1021/acs.joc.6b03083 J. Org. Chem. 2017, 82, 4513−4532 The Journal of Organic Chemistry Perspective a Scheme 1. Simplified Deaggregation of LDA Dimer to Monomer aReproduced from ref 8e. Copyright 2011 American Chemical Society. A2Sn(ArH) stoichiometry. LDA/THF-mediated metalations of relatively unreactive substrates are comfortably monitored from −55 °C to room temperature. Substrates necessarily undergo rate-limiting proton transfers, which display large (sometimes very large) primary kinetic isotope effects (KIEs).9 Although the multitude of substrate−solvent combinations have revealed almost a dozen stoichiometrically distinct monomer- and dimer-based mechanisms,6 one or two mechanisms usually dominate for any given substrate−solvent combination. Scenario 2: Aggregate Nonexchange. In the limit that an organolithium reagent reacts with substrate rapidly relative to the rate that aggregates exchange, only the observable aggregates are available to react (Scheme 3). (Solvent exchanges may Figure 1. Abstract depiction of mechanistic complexity in the limit of fast aggregate exchange (scenario 1), aggregate nonexchange (scenario Scheme 3. Reactions of Aggregates That Do Not Exchange 2), and nonequilibrium aggregate exchange (scenario 3). The dimensionless concept of reactivity in Figure 1 can be construed as the reaction temperature required to monitor a reaction on laboratory time scales using standard kinetic methods. Complexity, also a dimensionless entity, will become clear in the forthcoming description of scenario 3. remain rapid on the reaction time scales.) After seminal studies Scenario 1: Fast Aggregate Exchange. 10,11 In the limit that by McGarrity and co-workers using rapid injection NMR all aggregates rapidly equilibrate on the time scales of spectroscopy, Reich12 investigated a number of organolithium subsequent metalations, the mechanistic course of a reaction reactions under conditions in which two or more aggregates are is dictated by the lowest barrier of the LDA-mediated proton observable at the nonexchange limit. The rates were fast, and transfer (Scheme 2). We rely heavily on the shorthand shown the requisite temperatures were typically much lower than in the inset in Scheme 2 to simplify forthcoming discussions. −100 °C. Although the technical challenges of carrying out For example, A2S2 refers to a disolvated dimer such as 1, these reactions were considerable, the rate studies were simple: [A S *] connotes a spectroscopically invisible dimer of 2 n ⧧ they measured the relative rates at which each observable form solvation number n, and [A2Sn(ArH)] corresponds to a disappears. Fleeting intermediates were not germane. transition structure for the metalation of an arene, ArH, of Scenario 3: Nonequilibrium Aggregate Exchange. There is a fateful level of substrate reactivitya narrow Scheme 2. Reactions of LDA via Parallel Pathways temperature range required to monitor reactions on normal laboratory time scalesat which the barriers to LDA aggregate−aggregate exchanges are comparable to the barriers that fleeting structural forms react with substrates. Imagine that the equilibria in Scheme 1 are not fully established on the time scales of a metalation. In this nonlimiting regime, the rate- limiting steps often become poorly defined with an affiliated spike in mechanistic complexity (Figure 1). Moreover, any of the fleeting intermediates could react with substrate via rate- limiting deaggregation, substrate complexation, or proton transfer (Scheme 4). The most viable mechanism for proton transfer could lie behind an insurmountable barrier to deaggregation. 4514 DOI: 10.1021/acs.joc.6b03083 J. Org. Chem. 2017, 82, 4513−4532 The Journal of Organic Chemistry Perspective Scheme 4. Rate Limiting Aggregation, Complexation, or constrain the computational methods to address precise Proton Transfer questions and comparisons. 1.2. Saturation Kinetics. Plots of initial rates or pseudo- fi rst-order rate constants (kobsd) versus the concentration of a substrate or other reagent can show first-order (or higher order) dependence at low concentration and independence at high concentration. These so-called saturation kinetics are illustrated in Figure 2. Saturation behavior emerges in two mathematically interchangeable but chemically distinct ways. With an irony that will be lost on few, this fateful twilight zone corresponding to scenario 3 for LDA/THF-mediated metalations is centered on substrates that react on laboratory time scales (half-lives of minutes) at −78 °C. Under this nonlimiting regime, rate-limiting deaggregations are common- place. Traces of LiX, particularly LiCl, catalyze reactions at parts per million levels with marked changes in mechanisms, rates, and rate laws. LDA generated in situ from LiCl-contaminated n- butyllithium can have reactivities that are

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