Acyclic Lateral Substituents

Carbenes [74—76], and in particular A-heterocydie carbenes (NHCs), are today the topics of very intense research [43 8]. Carbenes were originaUy considered as chemical curiosities before being introduced by Doering in organic chemistry in the 1950s [77], and by Fischer in organometallic chemistry in 1964 [5]. Later, it was shown that the stability of carbenes could be dramatically enhanced by the presence of heteroatom substituents. After the discovery of the first stable carbene, a (phos-phino)(silyl)carbene, by Bertrand et al. in 1988 [78], a variety of stable acyclic and cyclic carbenes have been prepared. With the exception of bis(amino)cycloprope-nylidenes [79], aU these carbenes feature at least one amino or phosphino group directly bonded to the electron-deficient carbenic center. 

The degree of unsaturation in the molecule, i.e. the length of the conjugated polyene chromophore, has a substantial effect on retention times. Thus the acyclic hydrocarbons of the biosynthetic desaturation series, namely phytoene, phytofluene, tetrahydrolycopene, neurosporene and lycopene are well resolved, as are compounds having the same substituent group but different levels of desaturation, e.g. the 1-hydroxy-derivatives of these hydrocarbons. The 1,2-dihydrocarotenes, found in Rps. viridis are eluted later than their parent hydrocarbons e.g. 1,2-dihydroneurosporene after neurosporene. 

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