Carbenes nomenclature

Carbenes are highly reactive species, practically aU having lifetimes considerably under 1 s. With exceptions noted below (p. 289), carbenes have been isolated only by entrapment in matrices at low temperatures (77 K or less). The parent species CH2 is usually called methylene, although derivatives are more often named by the carbene nomenclature. Thus CCI2 is generally known as dichlorocarbene, although it can also be called dichloromethylene. 

Cyclopent-2-en-l-one, 2-hydroxy-3-methyl-synthesis, 3, 693 Cyclopentenone, 4-methoxy-formation, 1, 423 Cyclopenthiazide as diuretic, 1, 174 Cyclopent[2,3-d]isoxazol-4-one structure, 6, 975 Cyclophane conformation, 2, 115 photoelectron spectroscopy, 2, 140 [2,2]Cyclophane conformation, 2, 115 Cyclophanes nomenclature, 1, 27 Cyclophosphamide as pharmaceutical, 1, 157 reviews, 1, 496 Cyclopiloselloidin synthesis, 3, 743 Cyclopolymerization heterocycle-forming, 1, 292-293 6H-Cyclopropa[5a,6a]pyrazolo[l,5-a]pyrimidine pyrazoles from, 5, 285 Cydopropabenzopyran synthesis, 3, 700 Cyclopropachromenes synthesis, 3, 671 Cyclopropa[c]dnnolines synthesis, 7, 597 Cyclopropanation by carbenes... 

According to the nomenclature of Hoffmann, this gives rise to two different classes of carbenes having two singlet states, either or p being the lower energy state. 

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. 

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). 

It is perhaps inopportune to elaborate on the nomenclature, but some of the data reported for the tantalum ylide indicate that there may be a fundamental difference between this transition metal compound and the formally related ylides of the Group Yb elements. The most significant discrepancy is found with the 13C NMR shift of the carbene/ylide carbon atoms, which typically is downfield for the Va element, but upheld for the Vb element derivatives. Ylidic carbon atoms may, therefore, possibly bear a much higher negative charge. 

There is some divergence in the nomenclature various names of these species are used, including silicenium, siliconium, and silylenium ion. According to Barton et al. (19), silylenium ion is the proper name since it is derived in a logical way from the name silylene in analogy to the carbenium ion originating from carbene. 

No agreement on the nomenclature for these neutral, univalent nitrogen intermediates was achieved for a long time. The names used include imene 14>, imine radical 15 16), azene 17>, azacarbene 18J, imin 19> and nitrene 20>. The nitrene nomenclature, which parallels that for carbenes, is commonly used today and will be employed in this article. In this nomenclature H—N is nitrene, CeHg—N is phenylnitrene, CH3—0—C—N is methoxycarbonylnitrene etc. 

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... 

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. 

The nomenclature of the cycloalkenecarbenes follows the same rules as apply for the other carbenes. The correct name is obtained by attach-... 

Carbenes belong to a class of highly reactive carbon intermediates where the carbon atom has two nonbonding electrons. Methylene ( CH2) is the reference structure giving rise to the general nomenclature that characterizes divalent carbon species. These structures are simply named as substituted derivatives of methylene.129... 

The definition, nomenclature, structure and mode of generation of carbenes have been comprehensively surveyed in Houben-Weyl, Vol. El9b, parts 1 and 2. This section will cover the current work on the cyclopropanation of double bonds with methylene and alkylcarbenes. 

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... 

The multiply bonded silicon structures under consideration have been collected in Table 1, which also defines the nomenclature we have adopted. We use the common term silylenes for organic compounds of divalent silicon, analogous to carbenes. The existence of the more awkward but IUPAC-sanctioned term silanediyl for this class of compounds needs to be mentioned. 

Scheme 6.25. Diastereoselective cyclopropanations of vinyl carbenoids [97]. For disubstituted carbenes, cis/trans nomenclature is used to describe relative configuration, referring to Ri relative to the carbonyl moiety, as shown in bold.

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