Carbenes having aryl substituents, structure

Carbenes having aryl substituents, structure and reactivity of, 22, 311... 

Carbene chemistry, structure and mechanism in, 7, 163 Carbenes generated within cyclodextrins and zeolites, 40, 1, 353 Carbenes having aryl substituents, structure and reactivity of, 22, 311 Carbocation rearrangements, degenerate,... 

Structure and Reactivity of Carbenes having Aryl Substituents... 

Schuster, G.B., Structure and Reactivity of Carbenes Having Aryl Substituents, Adv. Phys. Org. Chem., 22, 311,1986. 

The subject of this chapter is carbenes with aryl substituents (aromatic carbenes). These materials are short-lived reactive intermediates in which the normal tetravalency of carbon is reduced by two. Carbenes have been the object of speculation and investigation for more than 80 years. Nevertheless, there still is considerable uncertainty about their chemical and physical properties. In the last five years the pace of research in carbene chemistry has quickened. This is a consequence of the development of high-speed pulsed lasers that permit, for the first time, direct observation of carbenes under the conditions in which they react. This research has provided new information on the effect of structure on the chemical and physical properties of carbenes. 

The introduction of chirafity into NHCs will therefore follow different strategies than those that have proved to be successful in phosphine-based asymmetric catalysis. For example, N-heterocydic carbene units will not create an edge-to-face arrangement of their aryl substituents, a structural feature common to many chiral diphosphines, such as the derivatives of Diop, Binap, Josiphos, Chiraphos and others. Results obtained in asymmetric catalysis, using chiral phosphine ligands, are therefore not directly transferable to the respective NHC-analogues. 

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