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Current Topics in Catalysis   Volumes    Volume 14 
New trends in phase-transfer catalysis: N-heterocyclic carbenes
Hanan El-Sharkawy Ali Ahmed
Pages: 25 - 38
Number of pages: 14
Current Topics in Catalysis
Volume 14 

Copyright © 2019 Research Trends. All rights reserved

Agents used as phase-transfer catalysts are onium salts (ammonium and phosphonium salts), macrocyclic polyethers (crown ethers), aza-macrobicyclic ethers (cryptands), and open chain polyethers (polyethylene glycols, PEGS, and their dimethyl ethers, glymres) in the presence of dipolar aprotic solvents like dimethyl sulfoxide (DMSO), N-N-dimethylformamide (DMF), N-methyl-2-pyrrolidone, tetramethylurea, and so on in both soluble and quenched forms. A large number of phase-transfer-catalyzed reactions involving simple displacement reactions are carried out under neutral conditions, although some variants of the typical phase-transfer-catalyzed mechanism are possible. In the general case, the role of the phase-transfer catalyst (Q+X-) is to function as a vehicle to transfer the anion (Y-) of the metal salt (Y-M+) from the aqueous or solid phase into the organic phase where it reacts with the organic substrate RX, giving the desired product RY and regenerating Q+X-, which can continue the phase-transfer-catalyzed cycle. Carbenes are neutral compounds featuring a divalent carbon atom with only six electrons in its valence shell. Considering a prototype carbene -C ¨-, the carbon atom can be either linear or bent, each geometry describable by a certain degree of hybridization. The linear geometry implies a sp-hybridized carbene center with two nonbonding degenerate orbitals (px and py). Bending the molecule breaks this degeneracy and the carbon atom adopts an sp2-type hybridization: the py orbital remains almost un-changed (it is usually called pπ), while the orbital that starts as pure px orbital is stabilized since it acquires some s character (it is therefore called σ). The linear geometry is an extreme case; most carbenes are bent and their frontier orbitals will be systematically called σ and pπ. The energy state has one electron in the σ and one in the pπ, as for state, but with antiparallel spins.
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