Abstract
A new family of CF3‐containing para‐quinone methides (CF3‐QMs) was systematically investigated for its suitability in organic synthesis. Addition of different nucleophiles gives access to target molecules with a benzylic CF3‐containing stereogenic center straightforwardly. The electrophilicity parameter E of the prototypical CF3‐QM 2,6‐di‐tert‐butyl‐4‐(2,2,2‐trifluoroethylidene)cyclohexa‐2,5‐dien‐1‐one was determined to be –11.68 according to the Mayr scale, making it one of the most reactive quinone methides known so far.
Highlights
Introduction paraQuinone methides (p-QMs) have emerged as versatile reagents for a variety of different transformations over the last years.[1]
Given the broad interest in CF3-containing organic molecules,[7,8] we reasoned that the introduction and systematic investigation of CF3-based para-quinone methides (CF3QMs) may be a worthwhile task to establish a new platform of versatile prochiral starting materials (Scheme 1B).[8]
Arylidene-based p-quinone methides arylidene-based p-QMs (Ar-QMs) have been studied by Mayr's group several years ago
Summary
Arylidene-based p-quinone methides Ar-QMs have been studied by Mayr's group several years ago. Of KOtBu).[16] As well established for other nucleophile/electrophile combinations before,[2,10,17] the reaction kinetics were determined by employing stopped-flow UV/Vis photometry to follow the fading of the colored 1a upon reaction with a large excess of the carbanions 5 (resulting in absorbance decays that follow first-order kinetics) Based on these measurements, it was possible to calculate the corresponding experimental second-order rate constants for these four reactions.[2,10,16] By using these rate constants together with the known parameters (N, sN) of our reference nucleophiles,[2,17] it was possible to determine the electrophilicity parameter E for CF3-QM 1a being –11.68.[16] By comparing this value with E-parameters for well-known tertbutyl-substituted Ar-QMs[2] (Figure 1), it becomes obvious that the presence of the CF3-group significantly boosts the electrophilicity of 1a, which makes it a very promising building block for further transformations: The CF3-QM 1a should be capable of reacting with various types of nucleophiles provided that. We tested if the SEAr strategy outlined in Scheme 3B may be applicable to the corresponding di-tBu-phenol 9a, but surprisingly in this case the yields were rather low and unpractical (compared to the route outlined in Scheme 3A)
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