Carbene complexes nomenclature

Today, however, carbene complexes covering a broad range of different reactivities have been prepared. Often it is no longer possible to predict whether a carbene complex will behave as an electrophile or as a nucleophile. Thus, a reactivity-based nomenclature would be difficult to apply consistently. For this reason in this book compounds with a carbon-metal double bond will be called carbene complexes or alkylidene complexes , terms not associated with any specific chemical behavior. 

A special note about nomenclature is in order. Both the terms carbene and alkylidene complex have come into use, and attempts have been made to use these terms to differentiate various types of reactivity of these species. However, the term carbene complex is used throughout this chapter, primarily because of the emphasis on cyclopropanation reactivity. 

These compounds are usually called l-alkynyl)carbene complexes or 2-alkyn-l-ylidene complexes, but the nomenclature l-metalla-alk-l-en-3-ynes has been used see Refs. 48 and 122, and Aumann, R. Heinen, H. Chem. Ber. 1987, 120, 537... 

Actually, terminal metal carbene and alkylidene complexes are ubiquitous throughout the transition elements. The nomenclatural distinction between "carbene" and "alkylidene" represents a fundamental difference in reactivity. Metal carbene complexes usually behave as electrophiles, with typical reactions including cycloadditions to un-saturabed bonds (e.g. cyclopropanation of olefins). On the other hand, metal alkylidene complexes are nucleophilic, undergoing Wittig-type alkylations and olefin metathesis. 

The term carbene will be used as the generic term. Particular structures will be designated using the system of nomenclature in which carbenes are treated as derivatives of the parent species methylene (CH2 ). Thus, for example, 5 will be referred to as dimethylmethylene rather than dimethylcarbene (which derives from the now obsolescent carbinol convention) or isopropylidene (which might lead to confusion with stable molecules in more complex organic structures). 

Current IUPAC and Chemical Abstracts nomenclature has been employed in this index with the former given preference. Substitutive nomenclature has been given preference over radicofunc-tional, additive, subtractive, conjunctive or replacement nomenclature, except where this becomes unwieldy. With many bicyclic and polycyclic compounds bearing heteroatoms, standard bicyclic or polycyclic oxa, aza, and thia replacement nomenclature has often been used. With certain functional groups, where the names are rather complex and probably not familiar to most organic chemists, such as ylides, those compounds have simply been named as sulfur, tellurium and arsonic ylides. Metal carbenes have been treated similarly. With more complex functionality and many heterocycles, the Beilstein Commander Crossfire nomenclature system has been used with certain modifications. 

Silver complexes have also been described for the cyclopropanation reaction. When using benzene, the use of Tp Ag(thf) (where Tp "" =hydrotris(3, 5-bis(trisfluoromethyl)pyrazolyl)borate for the rules of nomenclature of Tp" ligands see reE ) provided products derived from the addition of the carbene moiety to the arene ring (Scheme 7a), followed by ring expansion into a cycloheptatriene, in the... 

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