Ehrhard AA, Gunkel L, Jäger S, Sell AC, Nagata Y, Hunger J. Elucidating Conformation and Hydrogen-Bonding Motifs of Reactive Thiourea Intermediates.
ACS Catal 2022;
12:12689-12700. [PMID:
36313523 PMCID:
PMC9594049 DOI:
10.1021/acscatal.2c03382]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2022] [Revised: 09/19/2022] [Indexed: 11/29/2022]
Abstract
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Substituted diphenylthioureas (DPTUs) are efficient hydrogen-bonding
organo-catalysts, and substitution of DPTUs has been shown to greatly
affect catalytic activity. Yet, both the conformation of DPTUs in
solution and the conformation and hydrogen-bonded motifs within catalytically
active intermediates, pertinent to their mode of activation, have
remained elusive. By combining linear and ultrafast vibrational spectroscopy
with spectroscopic simulations and calculations, we show that different
conformational states of thioureas give rise to distinctively different
N–H stretching bands in the infrared spectra. In the absence
of hydrogen-bond-accepting substrates, we show that vibrational structure
and dynamics are highly sensitive to the substitution of DPTUs with
CF3 groups and to the interaction with the solvent environment,
allowing for disentangling the different conformational states. In
contrast to bare diphenylthiourea (0CF-DPTU), we find the catalytically
superior CF3-substituted DPTU (4CF-DPTU) to favor the trans–trans conformation in solution,
allowing for donating two hydrogen bonds to the reactive substrate.
In the presence of a prototypical substrate, DPTUs in trans–trans conformation hydrogen bond to the
substrate’s C=O group, as evidenced by a red-shift of
the N–H vibration. Yet, our time-resolved infrared experiments
indicate that only one N–H group forms a strong hydrogen bond
to the carbonyl moiety, while thiourea’s second N–H
group only weakly interacts with the substrate. Our data indicate
that hydrogen-bond exchange between these N–H groups occurs
on the timescale of a few picoseconds for 0CF-DPTU and is significantly
accelerated upon CF3 substitution. Our results highlight
the subtle interplay between conformational equilibria, bonding states,
and bonding lifetimes in reactive intermediates in thiourea catalysis,
which help rationalize their catalytic activity.
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