Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy

Abstract

Lewis acid-activation of carbonyl-containing substrates is a fundamental basis for facilitating transformations in organic chemistry. Historically, characterization of these interactions has been limited to models equivalent to stoichiometric reactions. Here, we report a method utilizing in situ infrared spectroscopy to probe the solution interactions between Lewis acids and carbonyls under synthetically relevant conditions. Using this method, we were able to identify 1:1 complexation between GaCl 3 and acetone and a highly ligated complex for FeCl 3 and acetone. The impact of this technique on mechanistic understanding is illustrated by application to the mechanism of Lewis acid-mediated carbonyl-olefin metathesis in which we were able to observe competitive binding interactions between substrate carbonyl and product carbonyl with the catalyst. Video Link The video component of this article can be found at https://www.jove.com/video/60745/ We believe this method is of general importance to chemists studying carbonyl-centered reactions catalyzed by Lewis acids. This detailed demonstration aims to help chemists apply this technique to their system of study.

Investigation of Lewis Acid-Carbonyl Solution Interactions via Infrared-Monitored Titration

“…We accounted for the dilution factor caused by titration by normalizing the absorbance value of the λ max of each component by the volume of the solution when the spectrum was collected. 17 When we consider the addition of 1 to GaCl 3 , we observe a linear increase in the signal for the initial species (15) between 0 and 1 equiv 1 added; however, once 1 equiv 1 is present, there is negligible change in 15 (Figure 6A). BF 3 •OEt 2 (2 mmol in 12 mL DCE) with 0−3 equiv 1 (A), 2 (B), 3 (C), and 4 (D ).…”

“…We next examined the amounts of each component present in solution with respect to the equivalents of carbonyl added. We accounted for the dilution factor caused by titration by normalizing the absorbance value of the λ max of each component by the volume of the solution when the spectrum was collected . When we consider the addition of 1 to GaCl 3 , we observe a linear increase in the signal for the initial species ( 15 ) between 0 and 1 equiv 1 added; however, once 1 equiv 1 is present, there is negligible change in 15 (Figure A).…”

“…When we compare the values of carbonyl not detected (C ND ) with the difference between maximum complex (C MAX ) and the observed complex (C COORD ), we observe a linear relationship for the systems containing FeCl 3 (Figure 15). 17 The relationship between 1 and 31 suggests that 3−4 equiv 1 are required to consume 1 equiv 1:1 coordination complex 31 (Figure 15A), 3−4 equiv 2 consume 1 equiv 32 (Figure 15B), approximately 3 equiv 3 are required to consume 1 equiv 33 (Figure 15C), and approximately 4 equiv 4 are required to consume 1 equiv 34 (Figure 15D). We simultaneously examined the conductivity of these FeCl 3carbonyl systems using concentrations identical to our IR investigation.…”

“…In order to define the equilibria shown in Figure , we employed two observational techniques: (1) in situ infrared spectroscopy and (2) solution conductivity . With these detection methods, we performed titrations into DCE containing Lewis acid as either a homogeneous solution (GaCl 3 ) or a heterogeneous slurry (FeCl 3 ).…”

“…In order to define the equilibria shown in Figure 1, we employed two observational techniques: (1) in situ infrared spectroscopy and (2) solution conductivity. 17 With these detection methods, we performed titrations into DCE containing Lewis acid as either a homogeneous solution (GaCl 3 ) or a heterogeneous slurry (FeCl 3 ). These data allowed us to determine that GaCl 3 and simple carbonyls like benzaldehyde produce high affinity Lewis pairs, whereas FeCl 3 forms a Lewis pair under stoichiometric conditions but forms more highly ligated complexes in the presence of superstoichiometric carbonyl.…”