Carbenes carbene mimics

In terms of ligand development, imino functionalised carbenes mimic the aU nitrogen donor ligands bis-imines (N,N) and bis-imino pyridines (N,N,N) [129] (see Figure 3.44) and can be seen as an extension of a series of somewhat older functionalised carbenes oxazoline functionalised NHC [133], pyrido functionalised NHC [134,135] and bis-carbene pyridines... 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

It has been found that the catalytic activity of PKC is enhanced by a lipid component of the cell membrane, namely phosphatidylserine. This activity is further stimulated by sn-1,2-diacylglycerol. Oleic acid also activates the enzyme in the presence of 1,2-diacylglycerol, and thus it is presumed to mimic phosphatidylserine. In order to identify that modulating binding site for oleic acid on PKC, a photoaffinity analogue was devised. A carbene generating photophore, diazirine was placed in the apolar terminus of the unsaturated fatty acid ligand (30, Fig. 12). The synthesis and the photochemical activation properties were reported by Ruhmann and Wentrup [113]. 

NHCs possess a lone pair of electrons and an accessible vacant orbital. This combination resembles the situation in many transition metal centers and could thus mimic their chemical behavior. While cyclic diaminocarbenes are inert towards hydrogen, it has been demonstrated that cyclic (aIkyl)(amino)carbenes of type 16... 

Silylenes are, in a certain sense, the younger but bigger siblings of car-benes, and their chemistry has been reviewed several times.1 In 1978, Gaspar asked the question Are we to conclude from our present knowledge that nature has designed silylenes to humbly mimic carbenes, or is the situation different 13 Today, we know that silylenes and carbenes have a number of properties in common, but also show significant differences. 

I have been pursuing enzyme mimics, artificial enzymes that perform biomimetic chemistry, since starting my independent career in 1956. In the first work [52-59] my co-workers and I studied models for the function of thiamine pyrophosphate 1 as a coenzyme in enzymes such as carboxylase. We discovered the mechanism by which it acts, by forming an anion 2 that we also described as a stabilized carbene, one of its resonance forms. We examined the related anions from imidazolium cations and oxazolium cations, which produce anions 3 and 4 that can also be described as nucleophilic carbenes. We were able to explain the structure-activity relationships in this series, and the reasons why the thiazolium ring is best suited to act as a biological... 

Abstract N-Heterocyclic carbene (NHC) has become a major ligand class and has proven to be more than just a phosphine mimic . Some important features like electronic and steric properties are discussed and typical examples of NHC are given herein. 

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