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Microwave-Assisted Transition Metal-Catalyzed Synthesis of Pharmaceutically Important Heterocycles
Published in Banik Bimal Krishna, Bandyopadhyay Debasish, Advances in Microwave Chemistry, 2018
Dipti Shukla, Priyank Purohit, Asit K. Chakraborti
The 3-(phenylmethylene)isoindoline moiety is a crucial part of many drugs such as local anesthetic and the AChE inhibitors. Hellal et al. synthesized 3-(phenylmethylene)isoindoline-1-one derivatives by using one-pot copper-free Sonogashira coupling reaction followed by 5-exo-dig cycloisomerization [61]. The protocol involves the reaction of N-substituted 2-halobenzamide 58 with phenylacetylene 59 in the presence of PdCl2(MeCN)2, BINAP as ligand, DBU as base in DMF under the microwave irradiation (120°C for 15 min) to afford different substituted isoindolines 60 with prominent (Z)-isomer in moderate to excellent yields (Scheme 9.23). The methodology was further utilized for the synthesis of pyrrolopyridine derivatives. Chauhan et al. also synthesized similar isoindoline scaffold by using Pd(OAc)2 as catalyst and isocyanides in place of terminal alkyne under microwave irradiation [62].
Elastic Adhesives
Published in A. Pizzi, K. L. Mittal, Handbook of Adhesive Technology, 2017
Johann Klein, Christina Despotopoulou
Similarly to the silicone formulations, an indispensable component of all adhesive formulations is the catalyst. The same applies also to the two-component PU adhesives in the reaction between the OH moiety and the NCO functionality to form the urethane link. This reaction can either be catalyzed with a Lewis acid, by activating the carbonyl functionality, or with a Lewis base, by activating the alcohol. The resulting urethane groups can also act as a Lewis base, so that the use of the catalyst can be minimized or even eliminated [14]. Examples of commonly used Lewis bases are tertiary amines such as 1,8-diazabicyclo[5.4.0] undec-7-en (DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO), 2,2-dimorpholinodiethylether (DMDEE), or triethanolamine. The tertiary amine is frequently incorporated into the polyol component, so as to prevent migration of the catalyst out of the finished bond.
Other Nonnitrogenous Organocatalysts
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
The literature is replete with different heterocyclic precursors to NHCs. Some of the more useful precatalysts for organocatalytic reactions are based on thiazole (35), imidazole (36), imidazoline (38), and triazole (37) scaffolds (Figure 8.21a). These cores are responsible for the reactivity and stability of the carbene itself and have been decorated by all manner of substituents in order to further tune the electronic and steric properties of the catalyst. In a recent review, Rovis has compiled a very comprehensive structural survey of these structures for both chiral and achiral catalysts.46 Most of the organocatalytic reactions that involve NHCs generate the active carbene (e.g., 40) in situ through deprotonating the precatalyst (e.g., 39) with an appropriate base (Figure 8.21b). To be sure, there is some variance in the acidity of the different heterocyclic precursors, but for the most part, the pKa values of the different precatalysts are between ~16 and ~24 in water. This is low enough that bases such as KOt-Bu, K2CO3, Et3N, or 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) can be used to generate the carbene, but this does not need to be a complete deprotonation for catalysis to proceed.47
A review on DBU-mediated organic transformations
Published in Green Chemistry Letters and Reviews, 2022
Shashi Kanth Boddu, Najeeb Ur Rehman, Tapan Kumar Mohanta, Anjoy Majhi, Satya Kumar Avula, Ahmed Al-Harrasi
DBU is a sterically hindered base, which exists in liquid form with a boiling point of 261°C. It is one of the strongest organic base containing a pKa value of 12. The presence of adjacent nitrogen atoms is believed to stabilize the protonated species during its basic action (Scheme 1).